|Publication number||USRE43421 E1|
|Application number||US 10/820,561|
|Publication date||May 29, 2012|
|Filing date||Apr 8, 2004|
|Priority date||May 28, 1993|
|Publication number||10820561, 820561, US RE43421 E1, US RE43421E1, US-E1-RE43421, USRE43421 E1, USRE43421E1|
|Original Assignee||Tong Zhang|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (16), Non-Patent Citations (4), Referenced by (3), Classifications (6), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This is a continuation-in-part of Ser. No. 08/538,868, Filed Oct. 04, 1995, now abandoned after receiving the allowance; also, is a continuation-in-part of Ser. No. 08/043,006, Filed May 28, 1993, now U.S. Pat. No. 5,515,394, granted 96. The former one refers to single-mode operation, frequency conversions and multipass pumping schemes for diode-pumped solid-state lasers, the latter refers to a pump head with a thin gain zone and the pre-narrowband approach for achieving single longitudinal mode operation.
This invention relates generally to laser pump/cavity/amplifier configuration design, and more particularly to laser arrangement for realizing single longitudinal mode operation and intracavity frequency conversions for diode-pumped solid-state lasers, as well as optical multipass constructions for pumping laser media and fiber lasers, and for the use of multipass optical amplifier.
Since the so-called “green problem” was discovered by T. Baer in 1986, it has become well known and has long plagued the stability of the CW intracavity harmonic generation of diode-pumped solid-state (DPSS) lasers. The essential difficulty in solving the “green problem” results from that, there is a persistent obstacle in effectively obtaining single longitudinal mode CW operation due to the spatial hole-burning interference effect in solid-state lasers. The related critical design issues are extremely tough. For the past decade, much research has attempted to solve this problem to obtain stable green light. Almost every effort has been made and nearly every way has been tried. However, none of true CW devices or designs has been successful by far with a regular standing-wave cavity. Only ring or very short cavity configurations have been used for this purpose, but they have appreciable inconveniences and limitations.
Baer, many AMOCO scientists and others did primary works and made some detailed reviews to the “green problem” in their papers and patents, such as the U.S. Pat. No. 5,164,947 (1992) and paper “Intracavity Doubling of CW Diode-pumped Nd:YAG laser with KTP,” IEEE J. QE-28, 1148(1992). Baer recognized the “green problem” and pointed out that there was a fundamental barrier to successful multimode operation of intracavity doubled lasers. (Now, the “multimode operation” should be corrected to be “a few modes operation”.) AMOCO and other scientists examined and worked out several important problems, including minimizing spatial hole burning effect with the “twisted mode” technology and the various polarization related problems, such as modifying the polarization of the laser modes in the doubling crystal to reduce the likelihood of chaotic amplitude fluctuations.
Controlling spatial hole burning can greatly reduce the possibility of amplitude oscillations. However, weak residual spatial hole burning resulted from imperfect “twisted mode” operation can still cause oscillations. In spite of those intense efforts, there remains a determining approach required to achieve dynamically stable single-mode operation with the use of a regular standing-wave cavity when the spatial hole-burning effect is present. What is needed is to provide a powerful form of wavelength selectivity to clamp the peak position of the operating frequency and prevent the laser operation from mode hopping and shifting to wavelengths outside the phase matching curve while controlling appreciable losses to the system.
On the other hand, an etalon within a cavity is commonly used to further control and suppress the harmful mode operation. Etalons typically have the highest spectral mode discrimination. However, the insertion of an etalon often leads to large passive losses and significantly reduces output power. This is especially true, for example, when the etalon is of high-finesse type, or the cavity has a small spatial mode waist and, hence, large beam divergence, and these effects are worse when the etalon is titled. Therefore, as simply inserting an etalon to a laser cavity, these characteristics often lead to the failure of laser operation.
AMOCO scientists realized and considered this key factor and were very close to success. In fact, there was almost one step behind to win the battle of the “green problemn”. Although they did not cross this decisive step, they have demonstrated several important concerns over the unsolved difficulties inherent in the “green problem” under the condition of single-mode operation. Following are the major concerns in their paper.
(1) “The intracavity harmonic generation laser is much more sensitive to component quality and the associated insertion loss than are most other lasers. To build an efficient intracavity harmonic generation laser, one needs to find some forms of mode selectivity with low loss, which is a significantly difficult task. On the other hand, if enough constraints are placed on the cavity without introducing appreciable losses to the system, stable and efficient operation of intracavity harmonic generation lasers is possible.”
(2) “The doubling efficiency is extremely sensitive to the finesse of the laser cavity so that all these controls must be introduced into the laser cavity without adding appreciable loss to the system.”
(3) “The principle difficulty with this design is that combining a polarizer and a highly birefringent element with a relatively small mode radius (w=100 um) can lead to significant losses. It is found that the green and 1064 nm output from cavities containing Brewster plates are often substantially smaller than those in similar cavities without Brewster plates. A 100 um beam has a far-field divergence angle of 3.4 mr; the off-axis components of the beam are appreciably depolarized by the angle-dependent refractive index. The phase shifts are only a fraction of a wave, but in the presence of a polarizer, these correspond to losses on the order of a fraction of a percent.”
In conclusion, their major point focuses on that, a relatively small mode waist can lead to significant insertion losses for the intracavity optical elements, particularly for an inserted etalon or Brewster plate in the present case.
In order to overcome the dominant difficulties in the prior art, the present invention offers two solutions for obtaining a dynamically stable single-mode operation with regular standing-wave cavities. These two different methods can be used separately, or collectively at the same time to be more powerful and effective.
(1) Cavity designs with a beam expander are applied to render a large mode waist and an improved beam divergence, so as to significantly reduce the insertion losses for intracavity optical elements, typically for a tilted etalon and Brewster plate; and
(2) A pump head with a thin gain zone is applied to minimize the spatial hole-burning effect create a circumstance to promote single-longitudinal-mode operation. The effect caused by a thin gain region is equivalent to that caused by short cavity configurations in which longitudinal modes are separated substantially, so that the required resolving-power of a frequency-selective form will be largely relaxed, and it becomes possible to use a spectral filter with low insertion losses, such as a birefringent filter or a low-finesse etalon, in realizing single-mode operation.
Further, the thickness of a thin gain zone is considered a critical factor to minimize the spatial hole-burning effect. To construct a thin gain zone at the end of a solid-state laser medium, there are several practical limitations to the commonly-used pumping schemes. The conventional side-pumping schemes are not capable of producing such a thin gain region. On the other hand, the constraints on the end-pumping scheme result from the need of a laser medium with a very large absorption coefficient and a very limited thickness. Therefore, a need still exists in the art to provide an effective method to serve this purpose. Consequently, in the present invention the multipass waveguide pump head has been developed for producing a thin gain zone within a laser medium.
Besides, once successfully solving the “green problems” for the intracavity second harmonic generation and obtaining a stable green light output, a new and promising way appears for the development of intracavity third and fourth harmonic generations, which can directly and effectively produce CW UV coherent light from one-single-stage cavity with using two or three nonlinear crystals in a serial manner. This offers a much more attractive solution than the external resonant cavity frequency doubling technology in the art. Nevertheless, such a design used to be considered infeasible in the field.
Further, it is not difficult to produce coherent light over wide spectral ranges from infrared to visible by using the OPO technologies. However, the most OPOs developed for the pulsed mode are not appropriate for CW-mode operations. Intracavity frequency mixing, in contrast to the OPO, is capable of producing coherent light with the CW-mode over wide spectral ranges from infrared to ultraviolet. Based on the technologies developed for frequency doubling, several closely related schemes for frequency mixing and high order harmonic generation are demonstrated for extending the utility of the present invention.
Concurrently, the objects of the present invention are presented as follows.
The major object of the invention is to develop a key technology for overcoming the fundamental barrier to intracavity frequency conversions in DPSS lasers, typically to frequency doubling caused by the so-called “green problem”, which will result in all solid-state, CW, green, blue or UV light lasers.
Another object of the invention is to find a form of wavelength selectivity with an acceptable low insertion loss in realizing single-mode operation. Such a form typically is a spectral filter incorporated with a beam expander.
Yet, another object of the invention is to use a low resolving-power spectral filter relative to a low frequency-selective loss under the condition of using a pump head with a thin gain zone that leads to minimizing the spatial hole-burning effect, so as to realize single-mode operation.
Consequently, a further object of the invention is to construct a multipass waveguide pump head for producing a thin gain zone, capable of maximizing the absorption and concentrate pumping power within a regular solid-state laser material of a small volume around or less than 1 mm3.
Another object of the invention is to provide a way for frequency doubling in which it is considered unnecessary to keep a nonlinear crystal, such as KTP, as a full- or half-wave plate at the fundamental wavelength with temperature control.
Yet, another object of the invention is to provide a way rather than the “twisted mode” technology to minimize the spatial hole burning effect so as to allow using a laser medium with the exhibition of birefringences.
Still another object of the invention is to provide a compact cavity design by inserting a beam expander, which offers a large TEM00 mode volume in the laser rod for mode-matched pumping and a high power intensity in the nonlinear crystal for efficient nonlinear optics actions at the same time, and to achieve a compensation of the thermal lens effect, good beam quality and power scaling.
A further object of the invention is to provide a laser-cavity design with a large TEM00 mode volume so that it becomes possible to use the side-pumping scheme including the use of a corner reflector pump head. The end-pumped laser output power is highly sensitive to the losses as may be introduced by the insertion of intracavity elements such as a quarter-wave plate, which is usually used for producing the “twisted mode” operation.
A still further object of the invention is to provide a laser-cavity design to render a large TEM00 mode waist and an improved beam divergence resulting in a great reduction of the insertion loss. This would allow the use of an etalon made from even the green-absorbing material with a relatively large scatter loss. Such an etalon is capable of absorbing the backward-going second harmonic beam so that the intracavity green light feedback effects are minimized.
Yet, another object of the invention is to provide a laser-cavity arrangement to directly and effectively produce a CW UV output from one-single-stage cavity.
Still another object of the invention is to make a laser-cavity arrangement serve as a high-performance intracavity wavelength converter for frequency mixing, which is able to produce true CW coherent light over wide spectral ranges, from infrared to ultraviolet.
Yet, another object of the invention is to provide an approach to center and overlap the bandpasses of several spectral filters and the cavity mode for a hierarchy of frequency-selective set inside a laser cavity so as to stabilize laser operation and maximize the output power.
An additional object of the invention, in accordance with the pump approaches used for the multipass waveguide pump head in producing a thin gain zone, is to develop multipass pumping geometry for pumping laser rods or slabs, or even for pumping optical fiber with rare-earth dopants. The two major approaches are advanced. The first one is characterized by i) the pump cladding which surrounds a much smaller laser material, is designed to have a graded-index or step-index, or ii) using noncircular profile reflector; so that diode bars can directly be coupled for pumping without coupling optics. The second approach is characterized by that the multipass pump head is constructed as an optical waveguide so as to optimize the reflectivity and the pump efficiency.
Another object of the invention is to provide a novel approach in the use of the beam guide input coupler to couple the uncollimated pumping beams from diode bars to a multipass waveguide pump head, resulting in the controlled angles of incidence of the pumping beams so as to accommodate the pumping beams with total-internal-reflection.
Yet, another object of the invention is to provide a novel approach in the use of the beam guide input coupler to couple the collimated pumping beams from diode bars to a multipass waveguide pump head, resulting in the controlled distribution of the angles of incidence of the pumping beam within a small range so as to meet the satisfaction for total-internal-reflection.
Another object of the invention is to provide an approach to protect total-internal-reflection for an optical surface from the so-called “frustrated total-internal-reflection” when it is contacted with other substances.
Yet, another object of the invention is to provide a novel multipass optical-amplifier design.
In the drawings, closely related figures have the same number but different alphabetic suffixes. As to the XYZ directions presented in the figures where a laser cavity is involved, the Z direction is always along and the XY plane is always perpendicular to the optical axis of the laser cavity, respectively. All the drawings are presented with the schematic diagram for illustrative purposes.
REFERENCE NUMERALS IN DRAWINGS
solid-state laser rod/bar
solid-state laser slab
solid-state laser chip
thin gain zone
pump cladding with graded- or step-index
HR coating on the surface of pump cladding
heat sink block/cooler
shaped pumping beams
pump head with a thin gain zone
linear array laser diode har
2-D slacked diode bar
collimating lens/rod lens/cylindrical fiber lens
cavity minor with phase-preserving coating
beam expander/spatial filter
folding mirror/polarizing reflector
radiation at ω1
cavity mirror with phase-preserving coating
input radiation at ω2
output radiation at ω3 = ω1 ± ω2
nonlinear crystal for mixing
prism beam expander
cavity mirror with phase-preserving
collimated pumping beams
lens or lens set
cavity mirror with phase-preserving coating
nonlinear crystal KTP
SHG crystal LBO
THG crystal LBO
FHG crystal LBO
coolant channel block
upper pump cladding with radius R1
center of 91
lower pump cladding with radius R2
center of 93
upper pump cladding
lower pump cladding
upper inner sleeve with radius R3
upper outer sleeve
lower inner sleeve with radius R4
lower outer sleeve
HR coating at pump wavelength
optical fiber with a rare-earth-doped core
double-clad optical fiber with a rare-earth-doped core
optical fiber assembly
Part I Using “Twisted Mode” for SLM Operation with SHG
There are two major goals in the present invention. One goal is to realize dynamically stable single-mode or narrowband operation with a regular standing-wave cavity for diode-pumped solid-state lasers leading to overcoming the difficulty in intracavity frequency conversions, typically in frequency doubling that is often caused by the so-called “green problem”. The other is to provide several similar compact cavity designs to achieve mode-matched pumping, compensation of the thermal lens effect, good beam quality, high power operation and efficient frequency conversions.
Beam expanding cavities are considered to be the most capable of handling the critical design issues caused by intracavity harmonic generations and of accommodating those requirements to these two major goals at the same time. The two different beam expanding cavities for DPSS lasers are discovered in the two original patents of U.S. Pat. Nos. 5,548,608 (1996) and 5,515,394 (1996).
The high efficient frequency conversion requires a high power density which is generally not available from CW-operated lasers. A ready solution for this problem is to place the nonlinear crystal inside the laser resonator. Moreover, the beam cross section inside the nonlinear crystal should be small enough. Concurrently, the beam cross section inside the laser rod must be large enough to utilize the maximum rod volume which can contribute to TEM00 mode oscillation. This generally requires that the beam cross-sectional area inside the laser rod be at least one order of magnitude larger than that inside the nonlinear crystal.
In this part, the “twisted mode” technology in the use of a pair of quarter-wave plates is applied to minimize the spatial hole-burning effect. In order to attain dynamically stable single-mode operation, a beam expander and an inserted etalon, or a hierarchy of frequency-selective set constituted of several spectral filters with low insertion losses are employed.
The schematics of the beam expanding cavity, which involve the use of a two-dimensional beam expander, i.e., a telescope, and internal nonlinear crystal are shown in
Beam expander 23 is formed by an AR coated lens pair, i.e., an eye lens of focal length f1 and an object lens 24 of focal length f2, and an aperture 26 which is placed at the focal plane where a diffraction-limited point occurs. A nonlinear crystal is inserted adjacent to aperture 26 for intracavity frequency doubling or mixing. Beam expander 23 has magnification M=f2/f1. The distance between lens 24 and lens 25 is adjustable thereby to obtain a small defocusing for the compensation of the thermal lens as scaling to a high pump level. The degree of defocusing can be further actively controlled by a control means for different pump/output power levels and for obtaining good stability against thermal lens fluctuations.
Aperture 26 is an internal iris diaphragm whose adjustment allows TEM00 mode operation. Beam expander 23 is hence configured as a spatial filter at the same time to produce an output of an excellent spatial quality.
By way of example, the preferred design parameters are selected as follows: The cavity length is around 15 cm; the magnification M is around 5-10×; focal length f2 is between 6 cm and 8 cm and has to be chosen correctly, depending on the demanded TEM00 mode volume and the nonlinear crystal acceptance angles; and focal length f1=f2/M. The rear mirror, front mirror and folded mirror should be referred to cavity design criteria and polarization requirement. The outer surface of one end cavity mirror serving as an out-coupler is preferably convex for the output beam correction. In addition, the rays shift caused by the walk-off effect in the case of using the KTP needs to be corrected. The two identical pieces of the KTP crystal oriented properly may be applied for the compensation.
Many papers or books have elaborated the use of telescopic resonators. D. C. Hanna et al. Opt. Quant. Electr. 13 (1981), p.493 and Opt. Quant. Electr. 7 (1975), p.115, made a detailed analysis for the choice of design parameters, such as the mode spot sizes, resonator length, telescope magnification and defocusing, mirror curvatures, placements and spacings of all intracavity elements, alignment tolerance of mirrors and so on. Several relevant computer programs or softwares to serve this purpose are also commercially available. A person skilled in the art would be able to carry out an applicable cavity design.
On the other hand, the selection of the magnification M depends to a large extent on the requirement from the inserted optical element such as an etalon. The reason is, the larger the M, the larger is the mode waist, thus the better is an improved beam divergence and the greater is a reduced walk-off loss, therefore leading to a larger decrease of the insertion losses. In addition, the passive bandwidth of the etalon is also improved.
In the following some embodiment examples of the invention are explained in more detail with reference to the drawing, in which
In order to minimize spatial hole burning and attain single-mode operation, a pair of quarter-wave plates (QWP) 29, 30 are positioned adjacent to opposite ends of laser medium 1a for producing the “twisted mode” operation—circular polarization in the gain medium. In most situations, the principal axes of QWPs 29, 30 are rotated by an angle of 45° or 90° with respect to each other.
Nonlinear crystal 27 needs to be set and oriented properly for phase-matching. For KTP type II phase-matching, the E and O axes of the crystal is oriented at 450 to the field direction of the linearly polarized fundamental wave. In general, a frequency doubler with birefringence would cause an additional phase change and then needs to be kept as a full- or half-wave plate at the fundamental wavelength so as to not degrade the “twisted mode” operation. Thus, this leads to a critical requirement for the strict temperature control of KTP crystal 27.
However, in a certain condition this limitation would be released from that as the eigenvector is not affected by the amount of birefringence of KTP 27. This feature resulted in a significant advantage as using type-II doubling and were indicated by D. W. Anthon et al., IEEE J. QE-28, 1148(1992) and H. Nagai et al., IEEE J. QE-28, 1164(1992). In their setup the axis of QWP 29 is rotated by 45° to the axis of QWP 30 and aligned to be parallel to the axis of KTP 27. It can be showed by using standard Jones matrix calculus and proved by a relevant experiment.
Further, for the “twisted mode” operation, the laser transition should be equally stimulated by any transverse polarization of the standing-wave electrical field. Thus it is necessary to employ a nonbirefringent laser medium or a laser medium made and oriented without the exhibition of birefringences.
The residual spatial hole burning can be controlled by a form of wavelength selectivity, preferably an etalon 17. As an alternative, it may be further simplified when QWP 30 is made and acts functionally as an etalon at the same time.
On the other hand, due to KTP 27 being placed at the right side of aperture 26, the green light feedback reflected from etalon 17 is effectively blocked off and minimized. For example, the size of the pin-hole of aperture 26 is around 60 um, etalon 17 tilted by 0.15° will result in a deviation of around 150 um from the center for the feedback light. Moreover, a green-absorbing material for inserted etalon can be selected to absorb the backward-going second harmonic beam. In addition, considering the relatively small space in the laser cavity, it is preferable to assemble aperture 26 and KTP 27 together. In such case, eye lens 25 and object lens 24 are both AR coated at the fundamental and doubling wavelengths; and this principle can also be applied for
As an alternative, KTP 27 can be moved out from beam expander 23 and positioned between eye lens 25 and front mirror 22 to gain a larger space and to favor using long crystals.
It is important to have more than one spectral filter for a gain medium with a broad lasing bandwidth. In such case, one frequency-selective element is usually not sufficient to uniquely determine the laser oscillation frequency. A hierarchy of frequency-selective set inside a laser cavity with a few separate levels of frequency selectivity needs to be used to force the laser to oscillate on a specific longitudinal mode. Lacking this hierarchy of frequency selectivity, the laser may oscillate with many modes, or it may oscillate with a single mode that is unstable to small perturbations and jump from mode to mode as a function of time.
As such, there are two Lyot filters inherently in the case using type-II doubling. The first one is formed by polarizer 18 and KTP 27. The second one is formed by polarizer 18 and a pair of QWPs 29, 30. In addition, the third one may be added which is formed by polarizer 18 and etalon 17 which is made from a birefringent material.
A birefringent etalon has a high resolving power which is the same as that of a etalon and a wide free spectral range which is the same as that of a Lyot filter. It has its own wavelength tuning character which depends on the direction of rotation axis. But sometimes it is the same as that of an etalon in a certain direction. Moreover, in the case using type-I doubling, there are still two Lyot filters left to be used.
In order to work in an optimum way for a composite Lyot filter with one more birefringent elements, the optical path length of each element, i.e., (no−ne)×thickness which produces a phase retardation should be integrally related to the others.
For the present case, a preferred embodiment for a hierarchy of frequency-selective set has three separate levels of frequency selectivity, consisting of two Lyot filters and one etalon. It starts on the least selective end and the largest free spectral range with a Lyot filter which is formed by Brwester plate 18 and a pair of QWPs 29, 30. This is followed by another Lyot filter which is formed by Brwester plate 18 and KTP 27 and, then etalon 17 with progressively higher resolution.
The overlap of the transmittance peaks of the three spectral filters provides a net transmittance bandwidth that must be narrow enough to uniquely determine a single mode for laser oscillation. The resolving-powers of the three levels of frequency-selectivity should be chosen to ensure that the resolution of one level is sufficient to select a unique resolution element of the next higher level of selectivity. Thus, the first Lyot filter has enough resolution to select a specific second Lyot filter mode, the second Lyot filter selects a specific etalon mode, and the etalon has enough resolution to select a specific longitudinal mode of the laser cavity.
For the system to work in an optimum way, the bandpasses of these spectral filters need to be at least roughly centered on a specific cavity mode. In other words, the transmission functions of these elements overlaid with the laser longitudinal mode structure. To fulfill this goal, as an example, first, it can be assumed that the first mode to go over threshold will be the one with the highest gain which occurs around one of a series transmission peaks of first Lyot filter. Second, through the use of a temperature control means, the second filter maximum can be turned to overlap the first one by the temperature change of KTP 27 within the temperature acceptance width of phase matching, as being judged by the maximum output. Then, the frequency doubler must be seriously or actively temperature stabilized. Third, aligning the etalon transmission peak to the cavity mode is realized by the angle or the temperature tuning. A tilted etalon with an angle rotation of a few milliradian can provides enough wavelength tuning to satisfy the requirement on this purpose.
The frequency stability of such single-frequency lasers is now determined by the mechanical stability of the optical elements, the gain medium, and the resonator's superstructure. A preferred stabilization approach for the optical path length of the cavity is to choose distance holders for the cavity mirrors with a zero thermal expansion coefficient at room temperature, or to use a temperature compensation cavity structure for the compensation of the cavity length variation which is caused by the temperature change. In addition, a laser medium with a lower temperature fluctuation in optical path length is preferable to be used.
The dynamically stable single-mode operation can be accomplished by these approaches described above. The related principles are also applied for the embodiment shown in
Same as before, as an alternative, in order to reduce the number of components, QWP 30 may be also made and acts functionally as an etalon. As such, the pair of QWPs 29, 30 has three functions at the same time. One is for the “twisted mode”, second is for the Lyot filter, third is for instead of etalon 17. The success in such combination depends on if an appropriate trade-off can be made between the requirements on thicknesses of the QWP 30 and the etalon 17, particularly when a thick etalon is demanded. However, the trade-off is not difficult for a person skilled in the art. In addition, the effect caused by a titled etalon would be neglect when the titled angle is small enough.
On the other hand, a birefringent etalon always means that there is a common body for both functions, thus, leading to a natural tendency for two transmission peaks to overlap. However, in the present case, the common body relative to a birefringent element is separated into two parts that may causes a slight displacement between two peaks.
As a further alternative, crystal 27 can be moved out from beam expander 23 and positioned after eye lens 25, then the cavity is folded between beam expander 23 and nonlinear crystal 27 and follows the way same as that shown in
Both faces of nonlinear crystal 37 may be AR coated for radiations 31 and 35. Both mirrors 21, 32 have HR coatings for radiation 31. Front mirror 32 is transparent for radiations 35 and 36. Folding mirror 34 is HR coated for radiation 31, and transparent for radiation 36 and optionally for input radiation 35. In addition, as an alternative, the two positions between input radiation 35 and mixing radiation output 36 can be exchanged. As explained, quarter-wave plates 29, 30 and etalon 17 are applied for single-mode operation at the fundamental wavelength.
This arrangement, in fact, is a high-performance intracavity wavelength converter. It is able to produce coherent light over wide wavelength ranges, from infrared to ultraviolet. For example, in order to obtain the difference-frequency operation in mid-infrared region, radiation 31 (ω1) can be selected from 1064/1319 nm (Nd:YAG), radiation 35 (ω2) can be produced from SDL-8630 tunable laser diode, which provides 0.5 W output power within 780-1060 nm region with 25 nm tuning range. Thus, radiation 36 (ω3=ω2−ω1) operating in mid-infrared wavelengths is achieved by frequency mixing:
(780-1060) nm (SDL-8630)−1319 nm (Nd:YAG)=(1.9-5.4) μm. Correspondingly, in order to obtain a large spectral range covered by mixing radiation 36, mixing crystal 37 with a large birefringence is desired.
Similar to that mentioned in
Accordingly, a pump arrangement is shown in
A grating reflector 16 is placed at the opposite side of the pump face of slab 1b. It diffracts the unabsorbed pumping light back to slab 1b with a large diffraction angle so as to maximize the absorption and to optimize the uniform pump in laser slab 1b.
As an alternative, the bottom facet of laser slab 1b with HR coating at the pump wavelength can also be used as a reflecting mirror to replace grating reflector 16, but with a non-uniform pump distribution. However, it can be largely offset by a pair of diode bars 13 which are symmetrically positioned at the two opposite sides of laser slab 1b as shown in
The pump head consists of solid-state laser slab 1b, the pair of diode bars 13, collimating lens 15 and grating reflector 16. PBE 41 consists of four prisms, preferably cut at the Brewster angle otherwise with AR coatings. Thus PBE 41 also serves as a polarizer. A laser rod can also be used instead of laser slab 1b.
Due to dispersive effects caused by the prisms, the backward-going second harmonic beam is separated and deviated from the optical axis of the laser cavity by PBE 41 effectively so that the intracavity feedback effects of the green light are naturally eliminated without a need of an extra approach. Further, multipass dispersive effects in multiple-prism systems are also incorporated, which are advantageous to obtaining narrowband laser operation. Apparently, it is not necessary for PBE 41 to be a discrete-wavelength zero-dispersion prism beam expander such as that used for pulsed dye lasers.
As to the angular wavelength tuning for etalon 44, in order to reduce the etalon walk-off loss, the rotation axis of etalon 44 should be perpendicular to the plane expanded by the prisms. The walk-off loss results from the multiple reflections within the tilted etalon that bounce laser power out of the spatial mode of the laser cavity.
The principles and related approaches described above in
To improve the doubling efficiency, and to obtain a uni-directional output, a folded cavity configuration, as an alternative, can also be used as shown in
In order to obtain output beam 20 with a circular profile, the height of gain region 50 and the magnification of PBE 41 should be selected appropriately with respect to each other. In a folded cavity, this can be done in another way. A curved mirror can be used for folding mirror 48 which will effectively offset an elliptical beam to a circular beam and contribute a well stable cavity design.
Part II Pump Heads for Producing Thin Gain Zone
In this part, two steps, a pump head with a thin gain zone and an approach for the pre-narrowband operation, are advanced to obtain single-mode or narrowband operation. These two steps are discovered in the original patent of U.S. Pat. No. 5,515,394 (1996).
Since the spatial hole-burning effect is minimized by using the pump head with a thin gain zone rather than the “twisted mode,” the cavity arrangements shown in
In the first step for a pump head, when a thin gain zone is created adjacent to a cavity end mirror in a homogeneously broadened solid-state laser medium, the cavity modes overlap spatially within the thin gain zone so as to compete for the available gain leading to minimizing or eliminating the spatial hole-burning effect. In the case of a pump head with a thin gain zone, the corresponding spectral oscillating mode interval under laser operation is largely dominated by the thickness, denoted by ΔT, of the thin gain zone along the optical axis of the laser cavity.
When the thickness ΔT is small enough so that the oscillating mode interval is larger than the FWHM of the lasing bandwidth of the gain medium, single-mode operation occurs directly. When the thickness ΔT is not small enough, a multimode operation with a much larger oscillating mode interval arises. In fact, this effect caused by a thin gain zone here is equivalent to that caused by short cavity configurations in which spectral modes are separated substantially.
Consequently, in the second step for the so-called pre-narrowband approach, the required resolving-power of a frequency-selective form is largely relaxed in realizing single-mode operation as compared with the situation dominated by the spatial hole-burning effect. A spectral filter with a lower resolving-power usually leads to a lower insertion loss, thus resulting in a key advantage for overcoming the dominant difficulty in the prior art. In order to distinguish a narrowband operation under such situation from the regular one, this approach, therefore, is called the pre-narrowband approach or the pre-narrowband operation in the present invention.
As to the first step, in order to produce a thin gain region at the end of a solid-state laser medium, or within a small chip of a solid-state laser medium, several approaches with different pumping schemes have been developed in the present invention. They are projected as follows.
The simplest technique is to use the end-pumping scheme to produce a thin gain region, including the off-axis end-pumping geometry. But this allows few options. Essentially only one approach can be followed for such pump arrangement. That is, a laser medium with a large absorption coefficient at the pump wavelength is end-pumped resulting in a very short absorption depth at the end of laser medium so as to produce a thin gain region. This method was used by G. J. Kintz and T. Baer in their paper of IEEE J. QE-26 (1990)9, 1457. In addition, the thickness of the laser medium along the optical axis of the laser cavity may be 1 mm around or less to facilitate this goal at the sacrifice of pumping power and cooling rate.
Several approaches with different pump-light delivery optics can be applied to deliver high diode power to the pump region with the end-pumped type. One example is to use the combination of the lens duct and cylindrical microlenses as reported by R. Beach et al., CLEO'93, CFM6, p.644. Another example is to use the fiber-optic coupler or a bundle of fibers, as described by Graf and Balmer, Opt. Lett. 18(1993)1317 and by Kaneda et al., Opt. Lett. 17(1992)1003.
The end-pumping scheme is not only limited by the need of a laser medium with a small thickness and a very large absorption coefficient, but also suffers from the loss of pump power and thermal problems in scaling to higher pump levels. To overcome these drawbacks, the following novel side-pumping approaches in
A small chip of a solid-state laser medium 1c is surrounded by a pump cladding 3 in the form of a circular plate. As the convergent nature of a circle geometry reflector, the outside of pump cladding 3 has an HR coating 4 circlewise at the pump wavelength so that the pumping beams, once entering, undergo multiple reflections and multiple passes through laser chip 1c until they are completely absorbed. An AR-coated or un-coated slit 5, i.e., the spectral opening, serves as an entrance for the pumping beams. A fiber bundle 8 which is used for delivering the pumping beams from diode bars, is butt-coupled to slit 5.
The thickness of solid-state laser chip 1c and pump cladding 3 along the Z direction is selected preferably but not necessarily to be the same, such that due to total-internal-reflection, the pumping beams are confined between the two surfaces of the circular plate which acts as a planar waveguide. In other words, a disk plate formed by laser chip 1c and pump cladding 3 actually become a multipass disk-waveguide, or container-like disk, or disk-blackbody. As an alternative, when the absorption loss within a unpumped volume is small, the extent at one end of laser chip 1c in the Z direction may be selected to be longer than that of pump cladding 3, so that the unpumped volume is able to serve as a heat sink to facilitate heat dissipation. And there would be a small escape loss of pump energy from the disk-waveguide by leakage out through the end of laser chip.
The situation of the multi-reflection and multipass geometrical path for the pumping beams is diagrammed in
In order to work well for such a disk-waveguide pump head, the design parameters must be selected properly. The diameters of circular fiber bundle 8, laser chip 1c and pump cladding 3 are denoted by d, D, and Φ, respectively. In the plate plane, the half-angle of the acceptance cone of the disk for the pumping beams outputed from fiber bundle 8 can be estimated as
where the half-angle is denoted by α, the acceptance cone is denoted by Ω, and parameter β in a selected range from 0.9 to 0.5 around is denoted as the acceptance index. Obviously, here α or Ω is directly proportional to D, and inversely proportional to Φ.
As the diameter d of circular fiber bundle 8 is chosen to be 500 um or less, or equal to (¼-⅙)D around, and the size of slit 5 is appropriately larger than the size of fiber bundle 8 so as to relax alignment tolerances, the escape loss of the pumping beams from slit 5 is less than 10%, typically less than 5%. The diameter D of solid-state laser chip 1c is selected from a range around 0.2-3 mm, which depends on the consideration of the mode-matching and power level condition. The choice of the diameter Φ of pump cladding 3 is dependent on the diameter D of solid-state laser chip 1c and the numerical aperture of fiber bundle 8. When the diameter D is close to or less than 1 mm, the graded- or step-index configuration is preferably used which will be given shortly.
In the plane perpendicular to the disk plane, the acceptance cone of the disk for the pumping beams has much larger tolerance and would not have a problem. The thickness ΔT of the thin gain region now is defined by the thickness of the disk plate, which is preferably constructed to be around 0.2-2 mm, typically 1 mm.
On the other hand, the fiber bundle plays an important role in the present pump arrangement. Fiber optics is a well-developed technology capable of low-loss transport of diode laser output. The pumping beams outputed from the end of fiber bundle 8 need to fit within the region of the acceptance cone Ω as shown in
A low N.A. fiber bundle and a collimating lens constitute a fiber-optic coupler for coupling the output from a diode bar. The relevant technologies were reported by Baer et al. in U.S. Pat. No. 5,127,068 (1992) and others. The radiation of a diode bar is focused into an array of fibers via a collimating lens. The collimation is performed in the fast axis of the diode. The N.A. of the butt coupled fiber is chosen to be roughly equal to the low N.A. direction of the diode. At the rear end of the coupler, the fibers are joined to a bundle.
Fiber bundle 8 may have a circular profile with circular cored fibers, or noncircular profile with circular or noncircular cored fibers, such as a rectangular profile with rectangular cored fibers. The later is preferred because the noncircular cored fiber can better match the linear emitting area of the diode laser and a rectangular slit 5. As an alternative, a fiber bundle with a single fiber of a low N.A. can also be used here.
On the other hand, apparently, in favor of the pumping and laser operation, the pump cladding is transparent at both the pump and fundamental wavelengths and its refractive index should be the same as or close to that of the gain medium. A proper index-matching material may be used between them for the joint. To serve this purpose, an option to build a pump cladding is that an active material as dopant is only doped within a small center area of the host material, therefore, the undoped outside section becomes a pump cladding.
A heat sink 6 are bonded to one or two sides of the peripheral section of pump cladding 3, favorable to uniform temperature distribution and heat dissipation. In order to support the disk-waveguide pump head, apparently, the heat sink can be made as the pedestal or part of the pedestal, which mounted and bonded evenly to the outer section of one side of the pump cladding. Then the heat within the heat sink can be dissipated via coolant or TE cooler.
In order to maintain total-internal-reflection within the bonding area, an metal foil 7, such as an aluminum or silver foil, needs to be inserted between pump cladding 3 and cooler 6. When an optical boundary surface of an optical element is used under the condition of total-internal-reflection and in the cases where the surface is contacted with other substances, this approach can significantly be employed to protect the gas-solid or liquid-solid interface from the so-called “Frustrated Total-Internal-Reflection”.
Other methods also can be applied, including i) inserting a thermal film with a lower refractive index than pump cladding 3 between pump cladding 3 and cooler 6 within the bond area; ii) filling up a thermal material with a lower refraction index than pump cladding 3; and iii) the surface of pump cladding 3 or cooler 6 within the bond area may be HR coated at the pump wavelength to reflect the pumping beams.
As a significant alternative, in order to obtain a large acceptance cone, the above disclosed disk-waveguide can be made using the method used for graded-index optics, such as a GRIN fiber or lens. That is, pump cladding 3 has a variable refractive index that is a continuous function n(r) of the radial distance r from the disk center, the refractive index getting progressively lower away from the center. This characteristic causes the pumping beams to be continually refocussed by refraction into the center area, and then to be multi-reflected and to multipass through the gain medium along a diameter repetitively until they are completely absorbed. In order to facilitate the ray centering process and avoid a possible helical ray track, the rate of change of the amount of the refractive index, i.e., dn(r)/dr, should be designed properly.
Similarly, pump cladding 3 can be also constructed to have two sleeves with different refractive indexes similar to a way used for step-index fibers. The inner sleeve has the same refractive index as that of laser chip 1c. The outer sleeve has a lower refractive index. Therefore, a lens effect caused by the difference of refractive indexes between them strongly converges the pumping beams and, by appropriate choice of parameters, results in a large acceptance cone. Here, as a primary consideration, the thickness of the outer sleeve along the radial direction is expected to be approximately equal to the focus length caused by the lens effect. In such a case, the related acceptance cone Ω becomes
α=½Ω˜β tan−1 D′/Φ
where now D′ is the diameter of the inner sleeve of the pump cladding rather than that of the laser chip.
Thus, in the above two cases when using a graded-index disk-waveguide or step-index disk-waveguide, fiber bundles with a large N.A. or commonly-used pumping coupling optics can be employed for delivering pumping beams, or even a laser diode can be directly butt-coupled to slit 5, leading to a much simpler and cost-effective structure.
As an option, a Selfoc lens serving as an interfacing optics may be applied between slit 5 and the end of fiber bundle 8 or pumping coupling optics. In addition, a interfacing optics may be used in setting fiber bundle 8 or pumping coupling optics perpendicular to the disk plate for a compact package.
In conclusion, using a disk-waveguide, the pumping power from the diode bars can be concentrated within a regular solid-state laser material of a small volume around or less than 1 mm3. A pump head of a size like a dime or quarter which is capable of producing a stable high power green light or even UV light now is practicable and will be sampled shortly below in
As a straightforward and significant extension, the present pump approach with the above multipass pumping geometry can significantly be employed for the utility of pumping a regular solid-state laser rod or bar, as shown in
so as to greatly facilitate mode-matched pumping. Here D is the diameter of laser rod 1a, and Φ is the diameter of cylinder reflector 10.
Apparently, a graded-index pump cladding is preferable to be used to obtain a large acceptance cone. The pumping beams, once entering, are continually refocussed by refraction into the center area for pumping solid-state laser rod or bar 1a. In this situation, the construction of a multipass cylinder reflector is about the same as a section of a graded-index fiber.
Similarly, pump cladding 3 can be also constructed to have two sleeves with a step-index. The inner sleeve has a refractive index same as that of laser rod/bar 1a. The outer sleeve has a lower refractive index appropriately. Therefore, a lens effect caused by the difference of refractive indexes between them strongly converges pumping beams resulting in a large acceptance cone. As mentioned before, as a primary consideration, the thickness of the outer sleeve along the radial direction is expected to be approximately equal to the focus length caused by the lens effect. Thus, in the above two cases, linear array laser diode bar 13 can directly be butt-coupled to stripe entrance 5 without collimating lens 15. Then, a multipass cylinder reflector becomes a multipass graded-index or step-index cylinder reflector and its performance is largely enhanced.
As for thermal managements, various cooling and temperature control systems are well known in the art and are widely available. In an exemplary configuration of the preferred embodiment, passageways for coolant flow can be provided through a monolithic housing structure in direct contact with the laser rod, as described by H. Bruesselbach and D. S. Sumida in their paper “69-W-average-power Yb:YAG laser,” Opt. Lett. 21 (1996)7, p.480, or by P. A. Bournes in his U.S. Pat. No. 5,287,371 and by E. A. Stappaerts et al. in their U.S. Pat. No. 5,307,365.
The pump cladding has a hole to provide a tubular sleeve. The sleeve surrounds the laser rod and forms a passageway through which the coolant flows in direct contact with the rod for efficient absorption of the heat generated within the rod. Such a structure is not shown in
While the above described cooling approach can be called as the Direct Cooling Approach, on the contrary, when the cooler or coolant temperature is low enough and the pump cladding has a good thermal conductivity and its size is not too large, as an alternative, the heat can be dissipated via the pump cladding then into coolant or TE cooler. And such cooling approach can be denoted as the Indirect Cooling Approach.
In the case of the step-index pump head, as an alternative, the outer sleeve can also be made from the coolant fluid as shown in
For example, to construct a hollow cylinder reflector pump head, a laser rod/bar has a diameter/size of 0.5-1 mm around, the diameter of a cylinder pump cladding is 5-15 mm around, and the diameter of a cylinder reflector can be selected from a large range, such as from 10 mm to 40 mm around. Once again, as a primary consideration, the space between the cylinder pump cladding 3 and cylinder reflector 10 is expected to be approximately equal to the focus length caused by the lens effect.
In the prior art, side-pumping geometries put very stringent requirements on the diode wavelength tolerance in the use of regular laser materials because of the short absorption lengths available. The absorption efficiency is seriously limited. Beam quality can also be negatively impacted because of axially asymmetric gain profiles that can be imprinted on the output beams. Now, these historic problems relative to side-pumping scheme have been successfully solved with the use of the above multipass reflector pump heads.
Next, as an alternative of the disk-waveguide pump head shown in
The consideration of the related design parameters for the embodiment set forth in
The cross section of solid-state laser bar 1a perpendicular to the optical axis of the laser cavity is square-shaped with a side length denoted by L. On the other hand, two dimensions are denoted by size E and F of the cross section at the end of shaped pumping beams 9. The size E should be constructed to define the required thickness ΔT of thin gain region 2, and size F should match the parameter L.
It is not very difficult to control size E to around 0.3-0.5 mm and size F to around 3-5 mm with laser diode bars by using common optics; an example is shown below in
Therefore, a beam-shaping optics, i.e., a pumping coupling optics, and a relevant arrangement can vary widely and may be employed in any one of a number of different ways. Such as i) a fiber-optic coupler and imaging telescope; ii) a fiber-optic coupler and a Selfoc lens; iii) the beam shaping optics used by Wallace et al., Optics Lett 16(1991)318, in which size E and F were controlled around 0.15 mm and 1.1 mm respectively when using a single 10 W linear array laser-diode bar; iv) the beam shaping optics used by Brown et al., CLEO'93, CFM7, p.644, in which size E and F were controlled around 0.3 mm and 1.6 mm (FWHM), respectively; and v) the lens duct and fiber lens used by Beach, Laser Focus World, March 1994, p.20, in which input pumping beams can be spatially focused to spot sizes down by a factor of 100 times.
As an alternative, the solid-state laser rod with a diameter L can be used instead of solid-state laser bar.
As an alternative, in the case of uneven one-side or 2-side pumping, in order to maximize absorption efficiency, a laser medium with a very large absorption coefficient, such as 70 cm−1, must be employed. Otherwise, a blazed grating reflector is optionally placed on the opposite side of the pump face for diffracting the unabsorbed pumping beams back to laser medium with a large diffraction angle to maximize the absorption and to facilitate the realization of a uniform pump. Further, the facets on the other side of the laser medium may have HR coating at the pump wavelength to confine the diffraction beams. As an alternative, the opposite side of the pump face can be HR coated as a high reflector mirror instead of using the grating reflector. These two concerns are also appropriate for the pump embodiment set forth in
The output of each tier in the 2-D diode bar 14 is collimated by collimating lens 15, preferably a fiber rod lens array, which is mounted parallel to and in a certain spaced relation with one emitter tier of diode bar 14 by precision spacer means. The arrays in 2-D diode bar 14 and fiber rod lens array 15 matched to each other geometrically in a one-to-one manner by tiers. The diameter of fiber rod lens 15 and its refractive index are chosen correctly for collimating. The compilation of collimated pumping beams 51 is directed toward lens or lens set 52.
Here fiber rod lens array 15 serves as a one-dimensional collimating lens to collimate the beam divergence in the plane perpendicular to the diode junction plane. On the other hand, the beam divergence in the plane parallel to the diode junction plane is relatively small around 5°-10°.
As an alternative, a microlense array serving as a two-dimensional collimating lens can be used instead of fiber rod lens array with a much better result for size F. Further, the combination of several linear array laser-diode bars can be used as the pump source instead of the 2-D stacked laser-diode bars.
Finally, as pump absorptions of some solid-state laser materials are strongly depend on the polarization direction, the pump polarization needs to be oriented or adjusted by a half-wave plate to ensure the strong pump absorption in such case.
Part III Using Thin Gain Zone for SLM Operation with SHG
The pump heads with a thin gain zone have been built successfully in
Once again, as was described above in
For single-mode or narrow-band operation, when the thickness ΔT is controlled to be less than λ2/k(FWHM), the single-mode operation may be directly obtained without using the pre-narrowband approach. Here, λ is the lasing wavelength, the FWHM is the lasing bandwidth of the gain medium. The range of the parameter k is around 5 to 10. This value is dependent on the power level, the linewidth and polarization of the pumping light. As an example, while the FWHM is 1 nm, the thickness of a thin gain zone must be controlled to be about a few hundred micrometers. When the thickness ΔT is not small enough, or the FWHM of the lasing bandwidth is too large to satisfy the requirements above, a multimode operation, which has a much larger oscillating mode interval than the free spectral range of the laser cavity, occurs instead of single-mode operation. In such cases, it is necessary to use the pre-narrowband approach for attaining single-mode or narrowband operation.
The rear optical facet of the laser medium and pump cladding within pump head 12 may be coated at the fundamental wavelength instead of rear cavity mirror 21. As the pre-narrowband approach, an etalon 17 is inserted. However, compared with the situation dominated by the spatial hole-burning effect, etalon 17 here has a much lower finesse leading to a much lower insertion loss.
In this case, there are two points worthy to be mentioned. First, it is not necessary to keep nonlinear crystal 27, even in the case of using type-II doubling, as a full- or half-wave plate at the fundamental wavelength with serious temperature control. Second, a laser medium with the exhibition of birefringences is not forbidden. This is in contrast to the prior art “twisted mode” operation. Obviously, these two advantages are very important and unique as compared with other designs in the prior art.
All of the above mentioned cavity arrangements in
A conventional stable resonator, consisting of a rear mirror 61, a cavity mirror 62 and a folding mirror 63, can be applied. A pump head 12 with a thin gain zone 2 is positioned adjacent to rear mirror 61. The rear optical facet of the laser medium and pump cladding within pump head 12 may be coated at the fundamental wavelength instead of rear cavity mirror 61.
As the pre-narrowband approach, a Brewster plate 18 is inserted and combined with a type-II doubler 64, preferably KTP, to form a birefringent filter. When the thickness ΔT of thin gain zone 2 is small enough, a birefringent filter can provide a necessary frequency-selectivity with substantially less excess loss than that of an inserted etalon in obtaining single-mode operation. This is a key concern in the case of a cavity without a beam expander.
As previously stated, folding mirror 63 serving as an output coupler is HR coated at the fundamental wavelength and transparent for the green light for the harmonic output. Cavity mirror 62 has HR phase-preserving coating for the fundamental and harmonic radiations, and is located directly at the end of nonlinear crystal 64 so as to largely enhance the second harmonic output.
In order to offer a relative larger mode size for pump head 12 and Brewster plate 18, cavity design parameters need to be selected properly. Both rear mirror 61 and folding mirror 63 may be a large-radius mirror or flat mirror, and cavity mirror 62 is a short-radius mirror. Or, cavity mirror 62 is a flat mirror and, a curved mirror with a short-radius may be used for folding mirror 63 that gives a simple self-aligning cavity design.
Part IV Intracavity Frequency Conversions
After successfully solving the “green problems” for the intracavity second harmonic generation (SHG) and obtaining stable green light, a similar exercise can significantly be extended and applied to intracavity higher order harmonic generations.
Intracavity third and fourth harmonic generations (THG and FHG) with using two or three nonlinear crystals, can directly and effectively produce CW UV output from one single cavity. They offer a much more attractive solution than the external resonant cavity frequency doubling technology in the art. In fact, intracavity THG and FHG technologies with all of its inherent difficulties have been usually recognized to be practically impossible before.
Beam expanding cavities (BEC) in the present invention provide two important functions for intracavity THG and FHG. First, the BEC provides high power mode-matched pumping operation at the fundamental wavelength. Second, the BEC, in the unexpanded beam portion, provides a long and stable, smaller beam waist for efficient intracavity frequency conversion while avoiding optical damage and resulting in an optimum condition for the use of two or three separated nonlinear crystals positioned serially.
On the basis of SHG technology, intracavity higher order harmonic generations is projected as follows. In the THG process the THG crystal sums the two radiations of the fundamental and second harmonic. The high intensity of the fundamental allows for efficient THG. The same situation appears to the FHG. There are two processes to produce FHG in a FHG crystal. One is the summing interaction between the fundamental and the third harmonic. Another is the doubling interaction for the second harmonic radiation itself. In the absence of resonant harmonic generation, the summing interaction in the FHG crystal with a high intensity fundamental radiation is much more efficient than the doubling.
As an example, a relevant arrangement for the FHG with using three nonlinear crystals is shown in
A crystal LBO 71 is set with type I for the SHG (532 nm), which produces two orthogonal polarizations between a fundamental (1064 nm) and a second harmonic (532 nm). This situation is just suitable for the following summing process within a crystal LBO 72 which is set with type II for producing the THG (355 nm). Finally, a crystal LBO 73 is set with type I for producing the FHG (266 nm), in which two polarization directions of 1064 nm and 355 nm radiations is parallel and oriented orthogonal to the optical axis. To avoid using an extra waveplate to adjust the polarization of the fundamental radiation, this type I-II-I is the only choice on the present purpose, but other nonlinear crystals besides LBO may be used.
It is worth noting that, as an important feature, the laser polarization at the fundamental wavelength is not affected by such type I-II-I arrangement. Therefore, it is not necessary to keep nonlinear crystals as a full- or half-wave plate at the fundamental wavelength with serious temperature control.
A similar arrangement was used by Ruikun Wu for pulsed UV lasers, OSA Ann. Meet. Technical Digest, Advanced Solid-State Lasers, AMG4, p.119, Feb. 7-10, 1994, Salt Lake City. However, such design has more advantages and less cares, and is much more desirable and necessary for true CW devices due to a much lower intracavity power intensity. All-intracavity design with multiple nonlinear elements is much better for CW-mode operation after solving its remaining difficulties.
Further, the resonantly enhanced technology, i.e., the resonant harmonic generation technology, can be applied for producing third and fourth harmonic generations as well as frequency mixing with using two nonlinear crystals in sequence, in which the intensity of second harmonic radiation is largely enhanced due to its resonance within a cavity.
As to the THG, the first crystal is used for doubling a fundamental radiation to a second harmonic radiation, the second is for summing the two radiations of the fundamental and second harmonic. This situation is the same as that shown in
As to the FHG, the first crystal is used for doubling a fundamental radiation to a second harmonic radiation, the second is for doubling the second harmonic radiation to a quadrupling harmonic radiation. The two crystals can be selected from any combination among type-I and type-II.
As to frequency mixing, the first crystal is used for doubling a fundamental radiation to a second harmonic radiation, the second is for mixing the second harmonic radiation and an input radiation. For example, a fundamental radiation is 2067 nm (ω1) produced by Tm/Ho:YLF, an input radiation is 1369.5 nm (ω2) produced by Er:YAP/YAG with frequency doubling, thus, a output radiation ω3=2ω1+ω2=589 nm, which is a very demanded wavelength in some applications. Here, the ω1 and ω2 can be exchanged, also, the resonant harmonic generation can be replaced by the resonantly enhanced input, or both occur simultaneously.
Since the two polarization directions of the fundamental and second harmonic radiations are not parallel to each other under any combination between the two crystals in the case of the THG or FHG, the applicable cavity arrangement should not include the polarizer and be selected from the two as shown in
Once again, here it commonly is not necessary to keep the two nonlinear crystals for producing FHG as a full- or half-wave plate with serious temperature control, even though in which one or two nonlinear crystals with using type-II doubling and the “twisted mode” operation involves, as long as the two crystals are oriented properly similar to the principle and situation as mentioned in the
On the other hand, however, there is an extra problem caused by the dispersion effect, which is similar to the green light feedback effect, resulting in two additional requisites. Thus, in order to make a second harmonic radiation resonance within a cavity, first, the cavity optical path length enables to be adjusted by a cavity distance adjustor, or a phase compensator, shch as optical wedge, so as to obtain in-phase condition at the second harmonic wavelength. Second, the cavity optical path length must be kept constant seriously. If necessary, an automatic electronic servo may be applied.
Moreover, some common issues also need to be attended, such as i) the laser medium has a high transparency to the second harmonic radiation, otherwise, its one side close to nonlinear crystals needs to be HR coated at the second harmonic wavelength and simultaneously HT coated at the fundamental wavelength; ii) the inserted etalon for the fundamental laser operation is AR coated (T>99.5%) for the second harmonic radiation, even so, this requirement is practicable with a low-finesse etalon at the fundamental wavelength; iii) if the operation within UV range, the second nonlinear crystal is suitable to the UV operation, such as LBO, KBBF and SBBO; iv) all optical surfaces of inserted elements, including both faces of crystals, had better to be AR coated at both fundamental and second harmonic frequencies; and v) the beam expander should work properly at the second harmonic wavelength.
Finally, in order to increase the conversion efficiency, the above mentioned resonant harmonic generation technology can be applied for directly producing green light with using one nonlinear crystal as usually.
Part V Noncircular-Profile Pump Heads for Pumping Laser Slab or Rod
In accordance with the principles of the present invention, the pumping arrangements shown in
In order to easily illustrate and understand noncircular-profile, multipass reflectors, they are initially introduced for pumping laser rods or circular gain chips rather than laser slabs. By way of example,
Such a noncircular-profile reflector 81 can also provide the multipass side-pumping specialized in the present invention. But in the most situations, the pumping beams need much more reflections within the reflector to meet a chance to pass through the gain medium.
The most advantageous characteristic in the use of non-circular profile reflectors lies in their ability to provide a very large pump acceptance cone close to 180°, whether or not the gain media are surrounded by the special cladding. Therefore, for the case involving a regular pump head for pumping laser rod, a linear array laser diode bar can directly be butt-coupled to the stripe entrance without using collimating lens. And in the case of a disk-waveguide pump head, fiber bundles with a large N.A. can be employed for delivering pumping beams, or a laser diode can directly be butt-coupled to the slit entrance without using collimating lens.
For example, to construct such a reflector for a regular pump head with diode bars pumping, a laser rod/bar has a diameter/size of 1 mm which may or may not have a normal pump cladding, and the rectangle size is chosen to be 6×40 mm around. There are four pump entrances shown in
A noncircular-profile reflector may be hollow when it is made up of an envelope and two end-plate reflectors which have a hole for lasing passage. Or, a noncircular-profile reflector may be solid when it is made up of a pump cladding with outside coating. Considering the large angles of incidence of the pumping beams, the two end plates of the reflector, or the two end optical facets of the pump cladding, usually would not need to be coated due to total-internal-reflection of the pumping beams. This principle is applicable to
Pump arrangements using noncircular-profile reflectors are exceedingly well suited for pumping laser slabs. As some exemplary configurations of the preferred embodiment,
Before efforts are made to build the multipass reflector, attention is called to two facts. First, the angles of incidence of the pumping beams variate or spread widely. Second, the reflectance of conventional, multilayer dielectric coatings, which usually have the highest reflectivity, is largely dependent on the angle of incidence. Based on these considerations, several approaches can be selected to build the reflector. First is an evaporated metal-coated reflector, such as the use of silver, gold, copper and so on. Second is a diffuse reflector which is usually fabricated from ceramics or compressed powder. Third, the pump cladding is not coated. Due to total-internal-reflection, those pumping beams with their angles of incidence larger than the critical angle, are totally reflected. And an extra reflector with a conventional HR coating, is added to surround the pump cladding, in order to repetitively reflect the remaining pumping beams. Those remaining pumping beams have small angles of incidence. Such a setup is typical of an optical reflectivity higher than 95%, regardless of the angles of incidence. It is named the double-layer reflector and is illustrated in
Part VI Waveguide Pump Heads for Pumping Laser Slab or Rod
In further accordance with the principles of the present invention, in order to use a simple way to obtain an optimum reflection, a reflector also can be built as an optical waveguide as exemplified in
As for cooling, four heat sinks 6 are bonded to two sides of the peripheral section of cladding 3 for the heat dissipation via TE coolers and fans. For maintaining total-internal-reflection as well as a good thermal conductivity, a metal foil 7, such as aluminum foil, should be inserted within the bond area between cladding 3 and heat sink 6. Optionally, the cooling approach described in
Part VII Improvements for Dube's Pump Cavity
In the U.S. Pat. No. 5,619,522, G. Dube devises a novel multipass pumping configuration. His pump cavity is primarily made up of two semi-cylindrical, or two arcuate, or two one-half of a regular polygon optical reflective surfaces. There is a some difference or the same between the two radii of the pair. They are arranged colinearly but not coaxially with each other. Thus, the longitudinal axes of the two reflective surfaces are parallel and off-set from each other. This pumping geometry converge the pumping beams to the center area once entering at the edge of the pump cavity. Afterwards, it diverge from the center area along the original spiral paths, via multiple reflecting by the two reflective surfaces.
Nevertheless, Dube fails to discover a key factor in his invention. That is, it is vital to obtain a high reflectivity in practice in the successful multi-reflecting pump process, based on the consider at that the angles of incidence of the pumping beams on the main/end surfaces within the pump cavity, variate or spread widely, particularly for pumping a laser rod with a small diameter of around 1 mm. It is mostly desired in the DPSS lasers. In such a case, the pumping beams usually need more than tens or a few tens reflections so as to reach the center area and stay long enough within Dube's pump cavities. As mentioned before, the reflectance of conventional, multilayer dielectric coatings, which usually have the highest reflectivity, is largely dependent on the angle of incidence. And the reflectivity of the metallic coatings such as gold or silver drops apparently at a large angle of incidence
In order to solve the key issue that has unsolved in Dube's invention, according with the principles of the present invention, one can build the Dube's pump cavity as an optical waveguide aforespecified, particularly in
The direct or indirect cooling approach, aforespecified, can be applied. The direct cooling approach is illustrated in the drawing. A hole within the pump claddings provides a tubular sleeve and forms a passageway through which coolant flows in direct contact with laser rod 1a. The two caps (not shown) are each clamped, from both sides via O-rings, to the two end optical facets of pump cladding 95 and 96 for holding laser rod 1a and furnishing passages for the entry and exit of cooling fluid 97. An aluminum foil should be inserted within the bond area between the optical facets and the O-rings to maintain total-internal-reflection.
Next, in order to obtain the high reflectivity, one can build the Dube's pump cavity as a double-layer reflector pump head.
Regarding the end management, it should be similar to that shown in
The two refractive indexes of the inner sleeves and cooling fluid 106, are the same or close. They should be properly selected to form a step-index structure associated with pump cladding 3. Also, the related parameters, such as the diameter of pump cladding 3, the radial extent of the coolant channel should be designed appropriately. But the criteria for the design are somewhat different from those for the regular one aforespecified in
Apparently, these pumping configurations shown in
Finally, there are several points worthy to be noted by the following.
1. In comparison, pump heads of different geometry configurations, such as those shown in
2. Alternatively, in order to increase the pump intensity and decrease the pump power leakage from the entrance area, the laser diode pump source can be fiber coupled to the pump head with or without using a beam guide input coupler in the cases shown in
3. The pump source can be derived from diode laser pump sources, or other laser pump sources, including multiple pump sources with a single or multiple pump wavelengths. The laser material can be co-doped with a second active lasing ionic species or sensitizer ions, or consisted of two different lasing components. The latter can be used in particular for purposes of upconversion lasers and lasers with multiple output wavelengths.
4. The polarization status of the pumping beams within the waveguide pump head shown in
5. The above depicted multipass pumping approaches provide slab-shaped gain regions and can be used for several different types of cavity configurations, such as one-dimensional beam-expanding cavities with an optical propagation along a zig-zag path, and multifolded cavities and hybrid cavities. A hybrid cavity functions as a waveguide along the Y dimension and as an unstable resonator on the X dimension, such as that used by Coherent for the model K500 CO2 laser or in their U.S. Pat. No. 5,353,297 for CO2 lasers.
Part VIII Waveguide Pump Heads for Pumping Fiber Laser
In the rapidly growing field of optical signal communication systems, fiber lasers of the type that comprise optically transmissive cores doped with rare earth ions are being increasingly recognized as important components of such systems. Thus, rare-earth optical fiber lasers can be utilized in these systems as optical signal generators, as optical signal amplifiers and as pump lasers for other optical fiber amplifiers. Many efforts have been directed at trying to devise a more effective way of increasing the pumping power that can be delivered to the core of a single-mode fiber amplifier. Several advanced technologies on cladding-pumped optical fiber lasers, commonly referred to as cladding pumping, have been developed. They are described in U.S. Pats. Nos. 5,530,709 and 5,530,710, and in the paper “High power neodymium-doped single transverse mode fiber laser,” Elec. Lett. 29(1993)17, p.1500.
A cladding-pumped fiber presently in use relies on a relatively large, separately light-guiding pump cladding that surrounds a much smaller rare-earth-doped fiber core. Pump light from a diode array is focused into the pump cladding with the end-pumping or quasi-end-pumping schemes, and then confined and guided within the cladding. As the pumping light propagates along the cladding, the light crosses over and is absorbed by the single-mode core, thereby supplying pumping power. The approach enables the absorbed multimode power to be converted into a single-mode laser emission within the fiber core.
In order to further expand the utilization of the present invention to pump optical fiber lasers or amplifiers, the novel side-pumping approaches characterized by the multipass pumping geometry can significantly be applied. There are two major preferred practices. In the first practice, a fiber assembly in a certain shape can be used to replace the laser slab 1b in
As an exemplary configuration of the preferred embodiment,
As an alternative shown in
In the second preferred practice of pumping fiber, one can simply replace laser rod 1a with a double-clad fiber 112 within the cylinder waveguide pump head shown in
Part IX Multipass Optical Amplifier
As a further direct extension in accordance with the principles of multipass pumping geometry of the present invention, the pump approaches for the disk-waveguide pump head shown in
Dube's multipass geometrical construction shown in
Finally, under circumstance that the gain medium is replaced by an absorption cell, or a sample, or a molecular beam, and that the signal beam is supplanted by a laser beam, this multipass apparatus can widely be applied for many spectroscopy purposes.
1. In accordance with the principles of the present invention, through intracavity frequency conversions, the following operation wavelength, as the summary for some illustrative cases, can be obtained:
Tunable laser media, such as Cr:LiSAF and solid dye materials, can also be applied for the laser-cavity designs in the present invention. A spectral tunable element, such as a acousto-optic device, needs to be inserted for wavelength tuning.
SDL-8630 tunable laser diode provides 0.5 W output power within 780-1060 nm region with 25 nm tuning range.
2. As a supplement for the use of a prism beam expander cavity, the prism expander also acts as a Brewster plate or a polarizer, and naturally become a birefringent filter in conjunction with the KTP.
3. As a supplement for the use of the corner reflector pump head by the following:
The invention being thus described, it is obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to those skilled in the art are intended to be included within the scope of the following claims.
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|U.S. Classification||372/22, 372/19, 372/32|
|International Classification||H01S3/10, H01S3/13, H01S3/098|
|Nov 22, 2013||REMI||Maintenance fee reminder mailed|
|Apr 16, 2014||LAPS||Lapse for failure to pay maintenance fees|