|Publication number||USRE40173 E1|
|Application number||US 09/498,254|
|Publication date||Mar 25, 2008|
|Filing date||Feb 3, 2000|
|Priority date||Sep 27, 1996|
|Also published as||DE69736133D1, DE69736133T2, EP1008212A1, EP1008212A4, EP1008212B1, US5715270, WO1998013910A1|
|Publication number||09498254, 498254, US RE40173 E1, US RE40173E1, US-E1-RE40173, USRE40173 E1, USRE40173E1|
|Inventors||Mark S. Zediker, Robert R. Rice, John M. Haake|
|Original Assignee||Mcdonnell Douglas Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (2), Referenced by (5), Classifications (15), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates generally to diode laser array systems. More specifically, the present invention relates to high efficiency, high power direct diode laser systems.
In numerous applications such as laser tracking, laser guidance and laser imaging, it is desirable to produce a high power coherent laser output. Moreover, high power coherent laser systems find applications in such diverse fields as offensive and defensive weapon systems, e.g., non-visible light illuminators for special operation forces and protective laser grids, as well as material processing, e.g., welding, cutting, heat treating and ablating, and medicine, e.g., surgical and diagnostic aides.
In the earliest laser systems, single semiconductor lasers were utilized to provide a coherent source of laser output. These single semiconductor lasers were limited in the amount of power which they could provide due to their structural limitations and limited efficiency. More recently, arrays of semiconductor lasers have also been utilized in which adjacent emitters of the array of semiconductor lasers spaced upon the same substrate are coupled together. One such laser array system was disclosed in commonly assigned U.S. Pat. No. 5,212,707 to Heidel et al., which is incorporated herein by reference for all purposes.
Once the semiconductors laser array 10 has been fabricated, mounted and powered, the output of the semiconductor laser array's emitters 20 must be collimated in order to obtain the desired collimated output. The lens assembly, as shown in
The binary optical element 24 includes a substrate on which a binary optical diffraction pattern 26 is etched. Generally, the materials of the refractive lens 22 and the binary optical element 24 have substantially equivalent refractive indices such that minimal refraction occurs at the interface between the refractive lens 22 and the binary optical element 24. The binary optical element 24 has a back surface 27 positioned adjacent to the front surface 28 of the refractive lens 24 and a front surface 28 on which the binary optic diffraction pattern 26 is etched. Since the binary optical diffraction pattern 26 is produced in accordance with typical binary optic technology, as well known to those of ordinary skill in the art (See U.S. Pat. No. 4,846,552.), further discussion of this technology will not be provided.
The binary optic diffraction pattern 26 is typically an eight phase level structure (although a two, four, or sixteen-phase level structure could also be utilized) which corrects for optical path differences inherent in the divergent output light of an emitter of a semiconductor laser array. Thus, the rays of light which exit the binary optic element 24 will have all travelled equal optical pathlengths, defined as a physical pathlength multiplied by the index of refraction of the material through which the light rays travelled which are equal or varied from that equal optical pathlength by only an integer multiple of the wavelength of the light being emitted. An eight level binary optic diffractive pattern 26 is shown schematically in FIG. 1.
A two-dimensional semiconductor laser array can be fabricated from a plurality of the one-dimensional semiconductor laser arrays 10 shown in FIG. 1. The one-dimensional semiconductor laser arrays 10 are stacked as shown in
Once the one-dimensional semiconductor laser arrays 10 have been mounted within the clamping fixture 70, the collimating lenses are aligned and attached. The fabrication of the collimating lenses is done in a manner identical to that previously discussed such that the refractive lens 22 is cemented to the binary optical element 24 which has been designed to collimate the laser output of each emitter 20. The alignment and attachment of the collimating lenses is accomplished in a sequential fashion for optimum efficiency. The collimating lenses 80a associated with the first one-dimensional semiconductor laser array 10a are positioned as previously described such that the optical axes of each emitter 20 of the semiconductor laser array 10 are substantially aligned with the center of the collimating lens assembly 80a.
The second collimating lens assembly 80b is then placed in front of a second one-dimensional semiconductor laser array 10b and is held in position by means of a vacuum chuck 76 connected by a vacuum line to a vacuum source, as shown in FIG. 3. The two-dimensional semiconductor laser array 10 is then supplied power such that the emitters 20 produce a light output. A transform lens 72 is positioned within the path of the light emitted from the first and second one-dimensional semiconductor laser arrays. The transform lens 72 may be a plano-convex or a biconvex lens, as shown in
The two-dimensional laser array when properly supplied with power produces a single collimated spot of laser output in the far field. By utilizing a plurality of one-dimensional semiconductor laser arrays 10 whose outputs may be combined, the output power of the two-dimensional semiconductor laser array may be quite high. For example, 25 watts of continuous wave laser energy was produced by a two-dimensional semiconductor laser array consisting of twelve one-dimensional semiconductor laser arrays with each one-dimensional semiconductor laser array having twenty one emitters. Additionally, the overall efficiency of the laser array from electrical input to power in the central lobe was approximately 26%.
U.S. Pat. No. 5,299,222 discloses an alternative approach to producing a high power laser diode system that collects and concentrates laser output from a stack of diode laser bars in a form that is useful and flexible for pumping a laser, e.g., a solid state laser. As shown schematically in
Two or more turning mirrors 17A, 17B, 17C and 17D separate mutually exclusive portions of the light beam 11 into non-overlapping light beam components 19A, 19B, 19C and 19D, respectively, and at least one pump light beam component, such as 19E, is optionally defined by a portion of the light beam 11 that does not encounter a turning mirror. Each light beam component 19A, 19B, 19C, 19D and 19E is then focussed by suitable focusing optics 21A, 21B, 21C, 21D and 21E, respectively, into a corresponding multimode optical fiber 23A, 23B, 23C, 23D and 23E, respectively, with the diameters of the fibers being chosen to fully capture the optical beam intended for that fiber. Preferably, the sine of the convergence angle as a light beam arrives at a light-receiving end of a fiber is less than the numerical aperture NA of that fiber. In one embodiment, each optical fiber has a diameter of about 500 μm, but this fiber diameter may be as large as a few mm. Each of the focusing optics 21j (j=A, B, C, D or E) may be a lens with a short focal length, such as f=6.35 mm, and is intended to cause the resulting beam to converge to an entrance diameter, measured at the entrance of the corresponding fiber 23j, that is about 25 percent of the diameter of the portion of the pump light beam 11 that arrives at the focusing optics 21j.
The numerical aperture NA of the multimode fiber 23j lies in the range 0.15-0.3 but may be as high as 0.6. Each optical fiber 23j delivers the component pump light beam propagating therein to a selected position and with a selected angular orientation relative to the laser cavity to be pumped by this collection of component pump light beams. Each optical fiber 23j is provided with an anti-reflective coating at the diode laser wavelength P, and the coating is either applied directly to the fiber end or to a separate glass window that is bonded to the light-receiving end of that fiber. The core material of the fiber 23j may be glass, and the cladding material of the fiber may be glass or plastic, with a smaller refractive index than the core refractive index, which determines by the numerical aperture of the fiber in a manner well known in the art.
It will be appreciated that expansion of the systems discussed immediately above would require both a large amount of real estate and complex optic assemblies to couple the outputs of a plurality of the disclosed output modules to a single spot. For example, the presence of lens 72 in
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a diode laser system which overcomes the above-described deficiencies.
An object according to the present invention is to provide a direct diode laser system generating a high fluence level at a workpiece.
Another object according to the present invention is to provide a direct diode laser system which generates a high power laser beam. According to one aspect of the present invention, the high power laser beam can be focused onto a single spot for interaction with a workpiece. According to another aspect of the present invention, the high power laser beam may be directed into one end of a solid state laser.
A still further object of the present invention is to provide a direct diode laser system which generates a high fluence level at a workpiece using dichroic coupling of multiple frequency collimated laser beams. Advantageously, all of the collimated laser beams can be generated using laser diode arrays.
Yet another object of the present invention is to provide a direct diode laser system which generates a high fluence level at a workpiece using both dichroic and polarization coupling of multiple frequency collimated laser beams. Advantageously, all of the collimated laser beams can be generated using laser diode arrays.
An additional object of the present invention is to provide a direct diode laser system which generates a high fluence level at a workpiece by simultaneously coupling thousands of collimated laser diode outputs into a single fiber via a single lens.
Another object of the present invention is to provide a direct diode laser system which generates a linearly scalable high power level output.
These and other objects, features and advantages of the present invention are provided by a direct diode laser system which includes N laser head assemblies (LHAs) generating N output beams, N optical fibers receiving respective ones of the N output beams and generating N received output beams, and a torch head recollimating and focusing the N received output beams onto a single spot. According to one aspect of the invention, each of the laser head assemblies of the direct diode laser system includes M modules generating M laser beams, wherein each of the M laser beams has a corresponding single wavelength of light, M-1 dichroic filters, wherein each of the M-1 dichroic filter transmits a corresponding one wavelength of the M laser beams and reflects all other wavelengths of the M laser beams, and a fiber coupling device collecting the M laser beams to produce a respective one of the N output beams.
These and other objects, features and advantages of the present invention are provided by a direct diode laser system, including N laser head assemblies (LHAs) generating N output beams, wherein each of the N laser head assemblies includes M first modules generating M first laser beams, wherein each of the M first laser beams has a corresponding single wavelength of light, M-1 first dichroic filters defining a first optical waveguide for directing all of the M first laser beams into a first optical path, wherein each of the M-1 first dichroic filters transmits a corresponding one of the M first laser beams having a respective wavelength and reflects all other wavelengths of the M first laser beams, a fiber coupling device disposed adjacent to the first optical path for collecting the M first laser beams to produce a respective one of the N output beams, N optical fibers receiving respective N output beams and generating N received output beams, and a torch head recollimating and focusing the N received output beams on a single spot.
These and other objects, features and advantages according to the present invention are provided by a method for generating a high energy laser beam, including steps for:
These and other objects, features and advantages of the invention are disclosed in or will be apparent from the following description of preferred embodiments.
These and various other features and aspects of the present invention will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which:
Preferably, the number N of LHA controllers 300 and LHA's 400 can be varied as required to provide a desired output power of the DLS 1. In an exemplary case, 4 LHA controllers 300A, 300B, 300C and 300N providing electrical power to 4 LHAs 400A, 400B, 400C and 400N, respectively, are included in the DLS 1. The block diagram of
In an exemplary embodiment of the DLS 1 of
Referring now to
The output beams of the polarizer 450 are transmitted to the optical fiber 470 via fiber coupling optics 460. Advantageously, fiber coupling optics may include a relay mirror 462, a transform lens 464 and a fiber coupler 466, arranged in that order along the optical path of the LHA 400. Preferably, polarizer 450 and the relay mirror 463 provide 2 axis adjustment while the transform lens 464 provides 5 axis adjustment. In the exemplary case illustrated in
Advantageously, each of the left and right sets of modules 410 produce output beams each having a different single wavelength, the wavelength separation between the output beams being only dependent on the quality of the dichroic filters used in the DLS 1. (M/2)-1 of the modules 410 are disposed behind a respective optical bandpass filter 420 which transmits only the output beam from that module and reflects all other wavelengths of light. Since the module is mechanically independent of the associated dichroic filter 420, the dichroic filters 420 can be aligned separately from the modules 410. After all of the modules 410 are combined in wavelength, then the broadband polarizer 450 is used to combine the output beams from the opposing groups of M/2 modules 410 into a single high brightness beam.
As discussed immediately above and as shown in
It will also be appreciated that the wavelength produced by the modules 410 advantageously can be selected to facilitate use of the DLS 1. For example, a single one of the modules 410 can produce a wavelength in the visible portion of the spectrum so as to provide a guide beam for reasons of safety.
Each of the modules 410 advantageously can be constructed as shown in
It should be noted that the modules 410, while similar to those disclosed in U.S. Pat. No. 5,212,707 in some respects, are significantly different in a number of other respects. The modules described in U.S. Pat. No. 5,212,707 were actually fabricated and tested as part of a 100 watt fiber coupled system that was sold by the assignee in 1993. While these modules produced highly collimated laser diode arrays, there have since been several new developments in technology that have enabled the modules 410 to be enhanced vis-a-vis those disclosed in U.S. Pat. No. 5,212,707. For example, the basic emitters used in the patent were index guided devices, i.e., rib lasers. In contrast, the modules 410 according to the present invention advantageously can be gain guided structures, in particular, 20 micron wide oxide defined stripes. While the laser diode array 414 does not produce the same divergence as the index guided structures described in U.S. Pat. No. 5,212,707, they do produce significantly higher output power levels. Moreover, the additional improvements that have developed since the '707 patent was issued include:
(a) The use of high power index guided devices, such those found in Model No. SDL 5410 by Spectra Diode Labs, Inc.;
(b) The use of a tapered oscillator design which is, in general, an oxide defined stripe but with a diverging wavefront; and
(c) Improved binary optics, whereby it is no longer necessary to use a refractive element to share the power and collimate the light from the emitters. It will be appreciated that this latter improvement alone increases the effective fluence produced by each of the laser diode arrays 414.
Implementing all of these improvements collectively can dramatically increase the brightness of the module 410 over the original design used in the modules described in U.S. Pat. No. 5,212,707.
It should also be noted that the module 410 illustrated in
Another improvement to the basic design of the modules 410 is the use of stackable microchannel coolers to increase the packing density of the laser diodes and consequently reduce the overall size of the system. Advantageously, cooling systems such as that disclosed in U.S. Pat. No. 5,495,490, which patent is incorporated by reference for all purposes, can be used.
As discussed above, the DLS 1 shown in
It should be recognized that the output power of the DLS 1 can be varied in a number of ways. First, the number N of LHAs 400 can be varied. For example, doubling the number N of LHAs 400 would double the combined power of the output beams. Alternatively, the number M of modules 410 and corresponding dichroic filters could be varied to vary the output power level. In an exemplary case, reducing the number M from 12 to 6 would halve the output power of that particular LHA 400. Finally, it should be noted that the output power of the DLS 1 can advantageously be varied by controlling either the number M×N of system modules 410 energized or by controlling the excitation power level to some portion of the M×N modules 410. Although the output power can be adjusted by uniformly adjusting the excitation current to the M×N modules 410, it will be appreciated that control at the upper and lower limits of system power may be difficult. For that reason, selected portions of the M×N modules may be controlled while the remainder of the M×N modules 410 may be either on or off, depending on the desired system output power. It should also be recognized that the output power of the selected M×N modules 410 may be varied in accordance with excitation current in a cw operating mode or may be varied in accordance with duty cycle in a pulsed operating mode.
As discussed above, the output of each respective module is fiber coupled to an optical fiber 470. It should be noted that the transform lens 464 focuses and couples the entire output beam of LHA 400 into fiber 470. Preferably, the sine of the convergence angle as the light beam arrives at the light-receiving end of the fiber 470 is less than the numerical aperture NA of that fiber. Advantageously, the NA of the fiber 470 is less than 0.47. Preferably, the NA of the fiber 470 is ≦0.19 and, most preferably, the NA of the optical power is ≦0.16.
Preferably, the fiber coupling lens 464 is a lens designed specifically for focusing the collection of beams from the wide wavelength band system of LHA 400 into the optical fiber 470. The number of modules 410 shown in the exemplary case illustrated in
Those of ordinary skill in the art will appreciate that the commercial applications range from surgery, to cutting, welding, and heat treating metals. In additional, this DLS 1 will be ideal for paint stripping, curing, cutting and drilling composite materials. Military applications range from an off-gimbal illumination system to a delay denial system for nuclear storage areas.
Another key application for this technology will be as an optical pump for solid state lasers, as discussed above, based on rare earth elements. This configuration facilitates excellent end pumping of a solid state laser rod, rare-earth doped fiber or dye laser. Moreover, this configuration has proven to be the most efficient means yet devised for converting incoherent laser diode pump light into a high quality, high brightness beam.
Although a presently preferred embodiment of the present invention has been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught, which may appear to those skilled in the pertinent art, will still fall within the spirit and scope of the present invention, as defined in the appended claims.
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|U.S. Classification||372/23, 372/101, 372/92|
|International Classification||H01S3/06, H01S5/026, H01S3/08, G02B6/42, H01S3/10, H01S5/40|
|Cooperative Classification||H01S5/4025, H01S5/4012, H01S5/4087, G02B6/4249|
|European Classification||H01S5/40H, G02B6/42C8|