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METHOD FOR DESIGN AND
CONSTRUCTION OF EFFICIENT,
FUNDAMENTAL TRANSVERSE MODE
SELECTED, DIODE PUMPED, SOLID STATE
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates to lasers in general and in particular to a method of designing and constructing a diode pumped (side pumped) solid state laser that is rugged, efficient, and automatically fundamental transverse mode (temqq) selected.
Previous methods of pumping a solid state laser gain medium include side pumping with diode bars or flash lamps and longitudinally pumping the solid state laser gain medium with another laser or with laser diodes. In a typical side pumped laser, the entire solid state gain medium is pumped and the gain medium is larger (spatially) than the fundamental transverse mode volume of the laser gain medium. In order to ensure operation in the fundamental transverse mode (TEM^), an intra-cavity aperture is required. This intracavity aperture typically induces substantial loss in energy or power, and can reduce the beam quality due to the effects of diffraction around the aperture.
In diode laser end-pumping (longitudinal pumping), the beam from a group of single element cw diodes are circularized and focused down the longitudinal axis of the laser crystal such that there is a high degree of overlap between the region pumped by the diodes and the fundamental transverse mode volume of the crystal. The crystal is typically longer than the absorption depth, leading to almost complete absorption of the pump beam. This fact, combined with the high degree of overlap with the fundamental transverse mode volume in end-pumped geometries, has lead to the highest reported slope efficiency of any laser. Progress in the construction of single element diodes, however, has not yielded the high power diffraction limited beams necessary to make high power end pumped solid state lasers. Currently, semiconductor master oscillator, power amplifiers are the highest power devices available that readily lend themselves to end pumping of solid state lasers. These devices, which rely on an index guided master oscillator are currently commercially available with only 1 or 2 watts average power.
The above constraints have lead laser engineers and researchers to end-pump laser crystals using gain guided, quasi-cw laser diode bars whose peak powers now exceed 100 watts. These laser diode bars are typically 1 cm long with a 1 um junction width leading to beam divergences on the order of 10x45 degrees, respectively. These numbers make it very difficult to collimate and circularize the laser diode beams such that they are smaller than the fundamental transverse mode volume of the solid state laser crystal to be pumped. Also, when quasi-cw diode bars are used to endpump, they require at least two optical elements to obtain suitable beam quality. This leads to an inefficient and less
rugged delivery of the pump light beam to the laser gain medium.
STATEMENT OF THE INVENTION
It is therefore an object of the present invention to provide a method for the design and the construction of an efficient, fundamental transverse mode selected, diode pumped, solid state laser.
This and other objects are achieved by providing a method for designing and constructing a laser diode pumped laser system that is rugged, efficient and automatically fundamental transverse mode (temqo) selected. Our method utilizes the fact that if the gain region is smaller than the Temqq laser mode volume, fundamental transverse mode selection can be ensured. In many cases, the gain region is smaller than the fundamental transverse mode volume in only one axis. It is for this case that we have presented a design to ensure high performance. The resulting compact, efficient, and portable laser systems will have potential medical and industrial uses, as well as immediate applications to NASA's LADAR and LIDAR systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. la is a perspective view of one embodiment of a laser diode pump which is pumping a solid state laser gain element according to the teachings of the present inventive method.
FIG. lb is an end view of the laser gain element of FIG. la.
FIG. 2a is a perspective view of another embodiment of a laser diode pump which is pumping a solid state laser gain element according to the teachings of the present inventive method.
FIG. 2b is an end view of the laser gain element of FIG. 2a.
FIG. 2c is a schematic of the cavity layout used with the laser system embodiment shown in FIG. 2a.
FIG. 3 is a schematic of the cavity layout used with the laser system embodiment of FIG. la.
FIG. 4 is a view showing a multiple laser diode pump configuration which can be used to pump a solid state laser gain element according to the teachings of the present inventive method.
FIG. 5 is a view showing one method of construction of a solid state laser gain element having gain region that is smaller than the fundamental transverse mode volume in one axis for use in a laser system according to the teachings of the present inventive method.
FIG. 6a is a perspective view of one alternate embodiment of a laser diode pump which is pumping a solid state laser gain element according to the teachings of the present inventive method.
FIG. 6b is a side view of the embodiment of FIG. 6a.
FIG. 7 is a perspective view of one alternate embodiment of a laser diode pump which is pumping a solid state laser gain element according to the teachings of the present inventive method.
FIG. 8 is a view showing one method of construction of a solid state laser gain element having gain region that is smaller than the fundamental transverse mode volume in one axis for use in a laser system according to the teachings of the present inventive method.
DETAILED DESCRIPTION OF THE
The present invention presents a way to circumvent many of the problems associated with longitudinal (or end) pumping by side-pumping a "constrained" gain region in the laser gain medium. By "constrained" we mean that although the longitudinal dimensions of the gain region and the TEM^ laser mode volume in the laser medium are the same, the cross-sectional dimensions of the gain region are smaller than the cross-sectional dimensions of the TEM^, laser mode volume in the laser crystal in at least one of the two orthoganal axis; in other words, the gain region dimensions transverse to the direction of propagation of the constrained gain region being smaller in at least one axis than the fundamental transverse mode in the laser crystal. Automatic fundamental transverse mode control in the other of these two axis is accomplished by a careful design of the cavity and the solid state laser gain medium itself. A laser diode pump, its associated beam and its overlap with the Temqq laser mode volume is shown in FIG. la which is a perspective view of one embodiment of a laser diode pump 1 which is pumping a solid state laser gain element 12 according to the teachings of the present inventive method described herein. In this embodiment of laser diode pump 1, a laser diode bar 2 is attached to heat sink 4. One example of a suitable laser diode bar 2 is an SDL 3230TZS 60 W quasi-cw bar. Beam 6, emitted by laser diode bar 2, is collimated by micro-lens 8, and passes through half-wave plate 10 to laser crystal 12, which in one of our experimental models was a Brewster/Brewster Nd:YLF Slab with the dimensions shown. It is not necessary to use a Nd:YLF crystal, although we have found this type of crystal to perform very well. Half-wave plate 10 (which is not necessary to the proper functioning of this invention but is shown because it was used in one experimental model to rotate the polarization of beam 6) can be used to take advantage of the larger absorption cross-section for pump 1 light polarized parallel to the laser crystal 12 c-axis.
As shown in FIG. lb, which is an end view of gain element 12, on the horizontal (X) axis, the fundamental transverse Hermite-Gaussian mode will be automatically selected because the width of the gain region excited by pump beam 6 is smaller than the width of the TEM^, laser mode volume in crystal 12. In the vertical (Y) direction, however, the fundamental transverse mode will be selected by using gain element 12 as an aperture, that is, the fundamental transverse mode is selected in the y axis by the losses induced by the top and bottom of gain element 12. For optimum laser performance, the need for complete pump beam 6 absorption, which requires a deeper crystal 12, must be balanced against the use of the top and bottom of crystal 12 as an aperture to strip higher order modes in that axis.
As shown in FIG. la, one method of ensuring that the width of the gain region in crystal 12 is smaller than the width of the Temqq laser mode volume in the horizontal axis entails the use of a high power, quasi-cw laser diode bar 2 collimated using cylindrical micro-lens 8 developed at Lawrence Livermore National Laboratories. See U.S. Pat. Nos. 5,080,706,5,155,631,5,081,639 and 5,181,224, which are hereby incorporated by reference, for a complete description of these micro-lenses. These lenses reduce the divergence angle of the light beam emitted by diode bar 2 more than an order of magnitude in the fast axis and allows for a very small (<400 um 1/e2 width) and collimated beam 6 at a distance of 1 cm from gain element 12.
As will be understood by one skilled in this art, in order to use gain element 12 as an aperture without suffering
severe losses, the laser cavity must be designed taking into account the physical dimensions of gain medium 12. Thus, gain element 12 must be deep enough in the vertical (Y) direction to absorb a significant fraction of the energy
5 provided by laser diode bar 2, and the cavity must be designed such that the fundamental transverse mode volume under-fills the gain region in the Y direction as shown. The effects of diffraction losses from the top and bottom of gain medium 12 can be minimized by the use of a Gaussian
10 reflectivity mirror (GRM) 38 (shown in FIG. 3). The use of GRM 38 is not necessary, however, and fundamental transverse mode operation has been accomplished without its use. In the simplest embodiment using a GRM 38, the GRM 38 serves as the output coupler. In a GRM 38, the reflectivity
15 decreases as a function of radial distance from the center of the mirror. This has the effect of changing the intensity profile of both the internal cavity mode volume and the output beam. The change in the profile represents a reduction of the energy in the wings (the part of the Gaussian not
20 within the 1/e2 diameter) of the mode. This reduces the effects of diffraction, and yields a flatter intensity profile. The flatter intensity profile is also beneficial in that it increases the efficiency of the extraction of energy from the gain medium. One other benefit of the GRM 38 is that it
25 increases beam pointing stability.
In summary, the advantages of the laser system shown in FIG. la are the facts that it utilizes high power, quasi-cw diodes, automatically selects the fundamental transverse mode, is more efficient (than traditional side pumping),
30 reduces the number of overall optical elements required, yields better beam pointing stability, is compact, and is conductively cooled.
The present inventive method can be utilized in several different types of laser system configurations. One alternate embodiment, described below, is a Q-switched, cavity dumped oscillator. The pump head implementation is shown
40 in FIGS. 2a and 2b and the cavity design is shown in FIG. 2c. In this embodiment, laser diode pump 3 consists of a pair of laser diode bars 2a and 2b, held by a complementary pair of heat sinks 4a and 4b. Beams 6a and 6b are each collimated by their respective micro-lenses 8a and 86 and pass through
45 half-wave plate 10 (not necessary as described above) to laser crystal 12, which in one experimental model was a Brewster/Brewster Nd:YLF slab with the dimensions shown. The experimental laser system produced 4 mJ, 4.5 ns pulses with fundamental transverse mode, 1.1 times diffrac
50 tion limited. In pump 3, two SDL 3230TZS 60 W quasi-cw laser diode bars 2a and 2b, equipped with cylindrical microlenses 8a and 8fc, are used to pump Nd:YLF crystal 12 simultaneously as shown; although Nd:YLF performs well, other suitable crystals can be used in our design. Diode bars
55 2a and 2b were angled at approximately 10° leading to a pump beam propagation angle less than 7° in crystal 12 due to the index of refraction of Nd:YLF (n=1.4). An angle less than 10° is possible by re-configuring the diode package (diode bars 2a, 2b and heat sinks 4a, 4b) to reduce the 6 mm
6o separation between the two laser diode bars 2a, 2b. To take advantage of the larger absorption cross-section for pump 3 light polarized parallel to the Nd:YLF c-axis, half-wave plate 10 was placed between collimated diodes 2a, 2b and laser crystal 12.
65 Pumping with collimated laser diode beams 6a and 6b provides high gain and enables selection of the fundamental transverse mode without the use of an extra aperture in the