WO2002084825A1

WO2002084825A1 - High-power, high beam quality slab waveguide laser

Info

Publication number: WO2002084825A1
Application number: PCT/US2002/010482
Authority: WO
Inventors: Hong Po
Original assignee: Optical Power Systems Inc.
Priority date: 2001-04-12
Filing date: 2002-04-05
Publication date: 2002-10-24

Abstract

High-power, high beam quality slab waveguide lasers and associated systems are disclosed. The gain material (102) is optically pumped by a laser diode array (114).

Description

HIGH-POWER, HIGH BEAM QUALITY SLAB WAVEGUIDE LASER

Field of the Technology

The technology relates generally to solid-state laser systems and more specifically to methods and apparatus for producing high power, high beam quality solid-state lasers.

Background of the Technology

A typical solid state laser includes a laser cavity which may be formed by two opposing reflectors, a solid state gain medium located within the laser cavity and an optical pump source used for pumping the gain medium. The gain medium is also referred to as the lasing material. One type of solid-state laser is pumped by means of one or more laser diodes. Multiple laser diodes may be formed into laser diode arrays for pumping. In general, a laser diode pumped solid state laser generates laser light by pumping the lasing material located within the resonator with light from one or more laser diodes. The pumping excites atoms in the lasing material to support laser operation in the laser cavity. Instead of opposing reflectors, the ends of the lasing material can be polished and covered with a highly reflective coating. This allows light to be reflected back and forth through the gain media or lasing material.

The laser cavity geometry determines the volume of a laser mode propagating therein. Typically, incoherent pump energy from a laser diode array is directed into the cavity using various techniques. Laser efficiency is the proportion of pump energy which is converted to coherent output energy. Pump energy directed into the cavity that is not converted to output energy is emitted as wasted incoherent energy or converted into heat. The more pump energy that is wasted, the lower the efficiency of the laser. The desire for greater efficiency generally requires that light from the laser diode array is directed to the useful part of the gain media. This is required since only light impinging on the gain media can excite the atoms within it. Typical side-pumped solid-state lasers are relatively inefficient since much of the pump light is wasted in pumping the entire gain media, instead of just the useful part of the gain media, which contributes to the laser output. The concentration of pump light from the array to the gain media can be problematic, however. For example, the excess heat generated by the pump can locally heat the pumped portion of the gain media. This is especially a problem for higher power lasers on the order of several watts or more. The localized heating severely degrades beam quality by thermal lensing. Thermal lensing can distort the output beam and lead to an unstable energy profile and erratic spot size. Additionally, the heat can distort the flatness of the end faces of the laser cavity, causing the end faces to act like lenses, thereby changing the optical characteristics of the laser cavity. Also, the heat can cause undesirable non-homogeneous refractive index changes in the gain media. These refractive index changes can distort the optical wavefront propagating in the cavity.

Two conventional solid state geometrical crystal shapes have been developed to provide effective cooling, the thin disk laser, and the slab laser. In one example of the disk laser configuration, the heat is removed by attaching a heat sink to one side of the disk, while pump light is coupled through the opposite side. For effective pumping of a disk laser, a source that produces a round beam is desirable. However, diode bars are typically used for pumping a disk laser. Since diode bars generate a line-shaped beam, extensive beam shaping is required in order to transform the line-shaped source into a round beam. In addition, multiple mirrors are used to form the resonator cavity as well as to reflect the residual pump beam back onto the pump spot for multiple pass absorption. The multiple mirrors require precise alignment.

In the case of a slab laser, one configuration entails side-pumping in a transverse direction with incoherent laser diodes along the length of the gain medium. Generally, side-pumped lasers are relatively inefficient. This is due to the entire gain medium volume being pumped, but only a portion contributing to laser gain. The pump energy is dispersed throughout the gain medium, instead of being concentrated within the volume of the lasing mode. In this configuration, heat can be dissipated through the top and bottom flat sides of the slab, for example. Unlike disk laser pumping, slab laser pumping is relatively easy to accomplish, requiring only a simple cylindrical lens to collimate the beam perpendicular to the junction of the emitter. In general, additional optics are required to -achieve a good quality beam.

One major drawback to both disk and slab lasers, however, is that neither configuration provides structural mode confinement in the narrow dimension of the cavity. This mode confinement is important for generating a very high quality output beam.

Typically, end-pumped configurations have improved energy conversion efficiency. In end-pumped lasers, incoherent pump energy is longitudinally focused through the entrance and/or exit mirrors into the central portion of the gain medium within the diameter of the resonating laser mode. The technique has important drawbacks, however. First, it has limited access to the gain media and therefore it is not easily scalable in power. Second, in the case of pumping a disk laser, complex beam shaping is required. Third, heat removal is problematic.

The wavelengths at which a laser will operate are determined by the resonant frequencies of the optical resonator cavity and the frequency range over which stimulated emission can provide sufficient gain. The first of these requirements involves the phase, i.e., the round trip phase within the resonator must be a multiple of 2π, and the second requirement involves the net gain for the amplitudes of the waves propagating in the cavity. The phase condition for oscillation is satisfied at or near resonant frequency. Secondary effects will move the frequency slightly away from its natural resonant frequency. If the phase condition is satisfied at multiple frequencies, then all of these for which there is adequate gain will oscillate. Each such oscillation is known as a longitudinal mode. A typical optical resonator cavity has multiple longitudinal modes. The resonant frequencies of these modes can be represented by:

f₀ = - (Hz) (1)

2Ln where m is any integer, c is the speed of light in a vacuum, L is the cavity length, and n is the index of refraction. Therefore, Equation (1) teaches that there are an infinite number of resonant frequencies, spaced at intervals of c/2Ln.

Many solid state lasers tend naturally to have many modes. For many applications, however, a single-mode laser is preferred. In second harmonic generation, single mode or narrow bandwidth generally results in greater conversion efficiency.

What is needed is an efficient, compact, high power, solid-state laser which does not suffer from the drawbacks of current solid-state lasers.

Summary

In one embodiment, the present technology relates to an efficient, compact, high- power, solid state laser having good beam quality and includes the following advantages:

1) The gain region of the laser is very thin in one dimension, making it relatively easy to extract heat, as well as to structurally confine only the lowest order transverse mode in that dimension;

2) The gain region is well matched to the output of edge emitting diode arrays, facilitating easy coupling of pump light;

3) The pump wavelength and the dopant concentration are chosen for effective pump light utilization;

4) The lasing_.output is substantially in a single transverse mode (i.e., high beam quality); and

5) The structure is power scalable to on the order of tens of Watts.

More specifically, the technology relates to apparatus and methods for producing solid state lasers. In one embodiment, the apparatus includes a gain material, a reflector in optical communication with the gain material, and at least one pump source configured to irradiate the gain material thereby generating an amplified optical signal in the gain material. In one embodiment, the pump source is a diode array. The apparatus further includes a single mode waveguide with the gain material. The single mode waveguide is adapted to output the amplified optical signal substantially in a fundamental transverse mode.

In another embodiment, the technology relates to a solid state laser including a self-imaging cavity which contains a gain material, a reflector with the self-imaging cavity, and at least one pump source configured to irradiate the gain material thereby generating an amplified optical signal in the self-imaging cavity. In one embodiment, the pump source is a diode array. The apparatus further includes a single mode waveguide with the self-imaging cavity. The single mode waveguide is adapted to output the amplified optical signal substantially in a fundamental transverse mode. In yet another embodiment, the apparatus uses the principals of multimode interference to generate efficient gain in the laser cavity. In yet another embodiment, the single mode waveguide includes a grating. The grating generates feedback into the cavity at a specific wavelength thereby limiting the wavelength range where lasing can occur. In yet another embodiment, the single mode waveguide i^ integraljo the self-imaging cavity.

Another embodiment of the apparatus includes employing a cascaded configuration of a plurality of self-imaging cavities. This embodiment increases the gain and improves the efficiency of the apparatus.

In yet another embodiment of the technology, narrowline or single frequency operation is achieved. A narrowline solid state laser according the present technology includes a self-imaging cavity having a high reflector and including a gain material. The apparatus further includes at least one pump source adapted to irradiate the gain material. The apparatus further includes a single mode waveguide in optical communication with the cavity. In another embodiment, the single mode waveguide is integrated into the cavity. The illustrative apparatus further includes a collimating lens, a polarizer, and an interferometer. Alternatively, the apparatus includes a section of polarizing fiber, and a Y-branch fiber splitter. The Y-branch fiber splitter includes gratings configured to behave like an interferometer. In alternative embodiments, the interferometer is a Fox- Smith interferometer, a Michelson interferometer, or a Fabray-Perot interferometer.

In another embodiment of the technology, frequency conversion such as frequency doubling is achieved. A frequency doubled solid state laser according to the present technology includes a self-imaging cavity having a high reflector and including a gain material. The apparatus further includes at least one pump source adapted to irradiate the gain material. The apparatus further includes a single mode waveguide in optical communication with the cavity. In another embodiment, the single mode waveguide is integrated into the cavity. The illustrative apparatus further includes a collimating lens, a polarizer, and an interferometer. The apparatus further includes a dichroic mirror, a quasi-phase matched device, and a high reflectance mirror. In one embodiment, the quasi-phase matched device is a nonlinear crystal.

It will be understood by skilled artisans that in various embodiments, the slab waveguide laser of the present technology can be fabricated of glass, crystal, or a combination of glass and crystal, depending on the desired results. In one embodiment, an up-conversion scheme is realized. For example, a co-doped Pr ⁺ + Yb ⁺ in a ZBLAN or YLF host is co-pumped with two wavelengths 800nm and 910nm, with one wavelength on each side of the slab to create an efficient up-converted red laser at 635nm, as demonstrated in a ZBLAN fiber laser. In other embodiments, blue (Tm³⁺) and green (Er³⁺) up-conversions in a variety of hosts have been contemplated.

Brief Description of the Drawings

The above and further advantages of the technology may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 A is a block diagram of one configuration of a conventional laser diode pumped solid state laser; FIG. IB is a cross-sectional view of the lasing medium illustrating the small resonator mode of the lasing medium of FIG. 1A;

FIG. 2 illustrates a side-pumped configuration of a slab waveguide laser;

FIG. 3 illustrates the slab waveguide of FIG. 2 showing modes in the slab;

FIG 3A illustrates the intensity distribution of the single transverse mode of the slab waveguide of FIG. 3;

FIG 3B illustrates the intensity distribution of the multimode distributions of the slab waveguide of FIG. 3;

FIG. 4 illustrates the multimode interference region of a self-imaging device;

FIG 4A illustrates the intensity distribution of the multiple channels of the self- imaging device of FIG. 4;

FIG 4B illustrates the intensity distribution of the multiple channels of the self- imaging device of FIG. 4;

FIG. 5 is a conceptual block diagram depicting a self-imaging device according to an illustrative embodiment of the technology;

FIG. 6 is a conceptual block diagram depicting a slab waveguide laser according to an illustrative embodiment of the technology;

FIG. 7 is a conceptual block diagram depicting self-imaging devices in a cascaded configuration according to an illustrative embodiment of the technology;

FIG. 8 is a conceptual block diagram depicting a single frequency modification of the illustrative slab waveguide laser of FIG. 6;

FIG. 9 is a conceptual block diagram depicting another single frequency modification of the illustrative slab waveguide laser of FIG. 6; and FIG. 10 is a conceptual block diagram depicting a frequency-doubling modification of the illustrative single frequency slab waveguide laser of FIG. 8.

Detailed Description

FIG. 1A illustrates a solid state laser 100 including a laser medium 102 (e.g. neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal) having a side-pumping geometry. The laser 100 further includes laser diode array 114 which includes laser diodes 116. In one example, an AlGaAs/GaAs laser diode array emits incoherent radiation at a wavelength λ of 0.81 μm. The laser diodes 116 are located along the length of the laser medium 102 and the pumping occurs perpendicularly to the direction of propagation of the laser resonator mode volume 104. If more power is required, more laser diodes can be added along and around the laser rod. The resultant conversion is an inefficient incoherent-to-coherent laser conversion. The inefficiencies of the side- pumped geometry for a solid-state laser result from a small absorption length, relatively large pumped volume (resulting in low pumping density), and a small cross section of the resonator mode volume 104. Additionally, there are pumped regions of the total volume where energy is wasted because of the mode mismatch.

The resonator configuration is planoconcave including one end 106 having a mirror 110 coated for high reflection at the lasing wavelength (λ=l .06μm for Nd:YAG) and the other end 108 having a mirror 112 coated for approximately 95% reflection at that wavelength. The balance allows for transmission of the output beam 118.

FIG. IB illustrates the end view of the lasing medium 102. The total pumped volume 102, pumped by the laser diodes 114 is indicated by the legend, as is the lasing mode volume 104. Only a fraction of the pump light is directed into the mode volume.

FIG. 2 illustrates components of a slab waveguide laser 200 according to one embodiment on the technology. One example of the slab 202 includes three layers. The middle layer 204 is referred to as the guiding or core layer. The outside layers are referred to as the cladding layers 206 and 208. In one illustrative embodiment, the core layer 204 is of a slightly higher refractive index than the cladding layers 206 and 208. The laser 2-00 also includes opposing arrays 210 and 212 of laser diodes. Each of the arrays 210 and 212 contain at least a single row of diodes 214 positioned to emit optical radiation toward slab 202. In alternate embodiments, the arrays 210 and 212 contain a plurality of diodes, and can include up to one hundred diodes each or more. It will be appreciated by those skilled in the art that arrays 210 and 212 are scalable. As the length of the slab 202 increases, so too can the number of diodes 214 in the arrays 210 and 212. In an alternate embodiment, diode arrays 210 and 212 are mounted to heat sinks 216 and 218, respectively. The heat sinks 216 and 218 remove heat generated by the diodes 214 during operation. In another alternate embodiment (not shown), integrated laser diode bars replace the laser diode arrays 210 and 212.

A lens 220 is positioned between array 210 and side 222. In the illustrative embodiment, the lens 220 is cylindrical. The lens 220 directs optical radiation emitted by laser diode array 210 into the gain medium 228 of slab 202. By proper selection of the radius of curvature of the lens 220 and the distance 230 between the array 210 and the lens 220, the radiation from array 210 can be collimated. In another embodiment, the radiation converges onto the gain medium 228. Skilled artisans will appreciate that by adjusting the radius of curvature of the lens 220 and/or adjusting the distance 230, the vertical mode-matching characteristics of the laser 200 is improved independent of the dimensions of the slab 202.

The lens 224 is similarly positioned between the array 212 and the side 226 of the slab 202. The lens 224 is substantially cylindrical and pre-collimates the radiation from the array 212 while directing the collimated radiation toward the slab 202. The lenses 220 and 224 are generally manufactured from fused silica or other glass material, and are generally as long as the slab 202. This ensures that a substantial portion of the radiation emitted by the diode arrays 210 and 212 reaches the slab 202.

In one embodiment (not shown), optional heat sinks are attached to the top and bottom of the slab 202. Another embodiment includes air cooling, for example. Skilled artisans will appreciate that other cooling systems can be employed without deviating from the spirit and scope of the technology.

FIGS. 3, 3 A and 3B illustrate the mode structure 302 of the gain media 228 within the slab 202 of FIG. 2. The sides 222 and 224 can be coated to provide redirection of the radiation emitted by the diode arrays 210 and 212. The radiation directed by the lenses 220 and 224 impinges on the gain medium 228. This results in an excitation region 302 having approximately a rectangular cross section. "Transverse modes" (TEMs) 304 reside within the excitation region 302. By diode pumping the sides 222 and 224 of the slab 202 with arrays 210 and 212, respectively, and pre-collimating the optical radiation using lenses 220 and 224, the device 200 is substantially matched to the TEM₀₀ mode in the vertical axis. Thus, the TEM₀₀ mode is confined in the vertical direction. By contrast, the width 308 of the excitation region 302 is substantially greater than that of the fundamental mode and results in many transverse modes propagating simultaneously. Thus, the lowest order modes are unconfined in this dimension.

To ensure single transverse mode propagation, the technology, in the illustrative embodiment uses properties of "multimode interference" (MMI) phenomenon. MMI couplers depend on the predictable evolution of the modes within a multimode waveguide and hence the predictable reconstruction of them at various propagation distances along the length of the waveguide. For example, FIGS. 4, 4 A and 4B illustrate a block diagram of 1-by-l self-imaging of multiple channels according to an embodiment of the technology. The device 400 includes an input port 402, a multimode interference region 404 having a length L, and an output port 406. The multimode waveguide 400 has a sufficient width W to support many transverse optical modes. The profile of the multimode waveguide 400 is selected such that symmetric self-images of a number of single point spots at the input are developed at fixed distances along the propagation length L of the multimode waveguide 400. A self-image of the input waveform is observed at the output port 406.

FIG. 5 is an illustrative embodiment of a device 500 which exploits the properties of the MMI structure 400 of FIG. 4. In this embodiment, the cavity length of the waveguide is reduced by one-half in comparison with the cavity length of the waveguide 400 of FIGT 4. A reflector 510 is placed on the end of the resonator 500 as shown in FIG. 5. In one embodiment, the reflector 510 reflects substantially all radiation impinging upon it. In another embodiment, the reflector 510 is substantially planar. In yet another embodiment, the reflector 510 is a high reflector deposited on the end face 512 of the waveguide 500. Skilled artisans will appreciate that other embodiments of reflectors 510 can be used without deviating from the spirit and scope of the technology.

In operation, an input waveform 508 traverses the multimode interference region 504. As the waveform encounters reflector 510, it is reflected back onto itself and a fold- back image propagates back down the waveguide 500 toward the input port 502. In the illustrative embodiment, the input port 502 and the output port 506 are substantially the same as depicted in FIG. 5. The illustrative embodiment of the technology uses this configuration to create an efficient slab waveguide laser.

FIG. 6 illustrates a highly schematic top view of a slab waveguide laser 600 according to the present technology. The slab waveguide 500 includes the multimode interference region 504. The multimode interference region.504 includes gain medium 228. The laser diode arrays 210 and 212 along with lenses 220 and 224, respectively, pump light into the gain medium 228. The length of the multimode interference region 504 is expressed as L/2. In the illustrative embodiment, the waveguide 500 includes a reflector 510 located at the end face 512 of the waveguide 500. The waveguide 500 further includes a single mode waveguide 602 abutted to the end face 606 of the waveguide 500. In one embodiment, the single mode waveguide 602 is an integral structure to the waveguide 500. In another embodiment, wedges 610 are used to provide suppression of multimode lasing. In the illustrative embodiment, the single mode waveguide 602 includes a grating 604 disposed within it. The grating 604 provides optical feedback to overcome losses and drive the system over the lasing threshold. In one embodiment, the grating 604 is a longitudinal grating. In another embodiment, the grating 604 is formed within the core of the single mode waveguide 602. In one embodiment, the grating 604 provides feedback to the slab waveguide 500. Heat is efficiently removed from the slab waveguide using various methods. In alternate embodiments, heat is removed from the broad flat sides and/or the end face 512 of the slab waveguide 500 using heat sinks. Alternatively, thermo-electric devices are used to remove heat. In other examples, liquid or water cooling is also used to remove heat from the slab waveguide 500.

In operation, the slab waveguide laser 600 of the present technology provides high power and high beam quality. Diode arrays 210 and 212 provide pump light to the gain medium 228. If more pump power is required, higher power diode arrays can be used. Alternatively, additional pump diodes 214 or diode arrays 210 and 212 can be positioned relative to the slab waveguide 500. Once the gain medium 228 is pumped beyond threshold, a population inversion of the energy states occurs and amplification is observed. Amplification remains possible as long as the population inversion is maintained. Consequently, lasing of the gain medium 228 occurs. The multimode waveguide 500 has sufficient width to support multiple transverse modes. The waveguide 500 is matched to the single mode waveguide 602, and thus develops a symmetrical self-image of an input waveform at a distance defined by the length L. The self-image repeats in multiples of L, e.g. nL. In the preferred embodiment, the length of the multimode waveguide 500 is L/2, or (n + ^l/₂)L and hence, the self-image is imaged at the junction 608 of the single mode waveguide 602 and the multimode waveguide 500. Since only the fundamental transverse mode will propagate in the single mode waveguide, all higher order modes are attenuated. Thus, the bulk of the energy will be directed into the fundamental mode.

The energy in the multimode waveguide 500 is efficiently guided into the single mode waveguide 602, which supports radiation in the fundamental mode only. Higher order modes are not guided by the single mode waveguide 602 and are therefore attenuated by the single mode waveguide 602. The amplified radiation in the slab waveguide 500 propagates through the single mode waveguide 602 and is emitted at the output port 612. Thus, the laser 600 emits an optical signal having only the fundamental mode of the single mode waveguide 602. Skilled artisans will appreciate the advantages of the solid state laser 600 of the present technology. For example, the laser 600 utilizes high power laser diode arrays. Additionally, the laser 600 efficiently couples the output of the diode arrays 210 and 212 to the gain medium. Furthermore, the laser 600 emits a high quality beam in the fundamental transverse mode. Also, the laser 600 is suitable for efficient 3-level or 4- level lasing. In addition, the design is such that sufficient cooling is achieved through the use of heat sinks, conductive cooling, air cooling, thermo-electric cooling, liquid cooling, or the like. An additional advantage of the laser 600 is its compact size.

Another embodiment of a multimode interference waveguide 700 is shown in FIG. 7. This embodiment illustrates a cascaded configuration of a first multimode waveguide 704 and a second multimode waveguide 708. By properly choosing the lengths and the widths of the waveguides 704 and 708, self-imaging is advantageously implemented. The cascaded system 700 includes a single mode waveguide 602. The system 700 also includes a grating 604. In one embodiment, the grating 604 is formed within the single mode waveguide 602. The system also includes a high reflector 510. In another embodiment, the cascaded structure embodying the system 700 is integrally formed. In one embodiment, the self-imaging properties exploited by the technology impose requirements on the length of the waveguides 704 and 708. For example, in order to generate a power splitting function of multiple spots at the junction 706, the length of the waveguide 704 is set to Li. In order to generate a self-image at the junction 706 through the waveguide 708, the length of waveguide 708 is set to L₂. Skilled artisans will appreciate that the value of Li and L₂ are subject to experimental optimization.

In operation, a single mode 710 propagates in the single mode waveguide 602. It will then propagate in the multimode waveguide 704. As it reaches the junction 706, the mode has power split to generate two single mode spots 712. This power split occurs at the distance represented by length L_\. The two single modes 712 then propagate in the multimode interference waveguide 708. The length of the waveguide 708 is set to L₂ and the waveguide 708 includes a high reflector 510. The high reflector 510 ensures that the self-image of the two single modes 712 is imaged back onto the junction 706. In one embodiment, the cascaded system 700 provides greater pump light absorption than conventional side-pumped slab designs. In another embodiment, the waveguide 704 is passive while the waveguide 708 is active. In yet another embodiment, the doping concentration in the active layer as well as the width of the active layer are adjusted to maximize pump light absorption. The lengths Li and L₂ of the respective waveguides 704 and 708 are adjusted to match the pump beam configuration.

The foregoing embodiments are effective for high power, single mode output. However, for many applications it is desirable to generate a beam having narrow spectral output. Applications, such as in the fields of spectroscopy and "second harmonic generation" (SHG), for example, require substantially narrow spectral radiation. FIG. 8 illustrates an embodiment of the technology which achieves an output beam having single frequency operation.

The narrow-line system 800 of FIG. 8 includes the slab waveguide laser 600 as described above with respect to FIG. 6. In the illustrative embodiment, the grating 604 is absent from the system 800, and any residual reflection is suppressed. This configuration provides no feedback into the waveguide 602. The system 800 also includes a collimating lens 802 for collimating the output beam 814. In one embodiment, the collimated beam 814 next passes through a polarizer 804. The polarizer 804 linearly polarizes the collimated beam 814. Next, the polarized beam 816 traverses a frequency selection device such as a Fox-Smith interferometer 818. Skilled artisans will appreciate that other interferometric devices can be used such as Fabry-Perot or Michelson interferometers. In one embodiment, the device 818 includes a holographic grating 806, a tunable mirror 808, and an output coupler 810. The polarized beam 816 passes through the holographic grating and is incident on the output coupler 810. The tuning mirror 808 is adjusted to select the desired output frequency. The difference in the path length between path 820 and path 822 and the optical characteristics of the reflecting surfaces determine the spectral properties of the interferometer 818. By proper choice of these parameters, the system 800 is adjusted to deliver single frequency operation. In alternative scenarios, it is desirable to provide single frequency operation without using discrete optical components in free space. The system 900 of FIG. 9 illustrates such an embodiment using a complete optical fiber solution. The laser 600 is coupled to a single polarization fiber 902. In one embodiment, the grating 604 is removed from the laser 600. The polarization fiber 902 linearly polarizes the output beam from the laser 600. The polarization fiber 902 is coupled to a Y-branch splitter 904. In one embodiment, the Y-branch splitter 904 is also known as a fiber power splitter. In another embodiment, the gratings 908 and 912 are burned into the arms 906 and 910, respectively, of the power splitter 904. Illustratively, the lengths of the arms 906 and 910 of the power splitter 904 are uneven. This configuration behaves like a Fox-Smith interferometer. The gratings 908 and 912 are chosen such that the desired frequency is delivered to the output port 914.

Frequency conversion is useful for extending the utility of high-power lasers. It generally utilizes the nonlinear optical response of an optical medium in intense radiation fields to generate new frequencies. One method used to accomplish this is to use a „ nonlinear optical conversion process. An example of an optical conversion process is harmonic generation. In one embodiment, it is desirable to use second harmonic generation techniques to provide frequency doubling of the signal. In an illustrative embodiment, this is accomplished by way of the system 1000 of FIG. 10.

In one embodiment, the system 1000 includes the subsystem 800 of FIG. 8. In the illustrative embodiment, the output coupler 810 is replaced by the frequency doubling device 1002. The subsystem 800 provides a narrow-line, linearly polarized beam which is ideal for second harmonic generation. Skilled artisans will appreciate that the polarizer 804 is configured so as to orient the polarization of the beam in the desired plane. In one embodiment, it is desirable to couple all of the harmonic power into a single output beam. In the illustrative embodiment, a dichroic mirror 1004 replaces the output coupler 810. The dichroic mirror 1004 deflects the fundamental frequency beam 1010 into a double pass frequency conversion arrangement using a "quasi-phase matched" (QPM) device 1006 together with a high reflectance mirror 1008. In one embodiment, the QPM device is a nonlinear crystal. The frequency doubled beam 1012 is outputted through the dichroic mirror 1004.

In operation, the fundamental frequency beam 1010 from the subsystem 800 is incident on the dichroic mirror 1004. The dichroic mirror 1004 reflects the fundamental frequency beam 1010 through the nonlinear crystal 1006 towards the high reflectance mirror 1008. The second harmonic beam is generated in both directions by the nonlinear or frequency-doubling crystal 1006. The frequency-doubled beam 1012 has a polarization which is rotated 90° with respect to the polarization of the fundamental beam 1010. The dichroic mirror 1004 is designed to completely reflect the fundamental beam 1010 and to transmit virtually the entire orthogonally polarized frequency-doubled beam 1012. In the illustrative embodiment, the high reflector 1008 completely reflects the fundamental beam 1010 and the frequency-doubled (2ω) beam 1012.

Having described and shown the preferred embodiments of the technology, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used and that many variations are possible which will still be within the scope and spirit of the disclosure. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments. What is claimed is:

Claims

1. - A laser system, comprising: a gain material; a reflector in optical communication with the gain material; and at least one pump source configured to irradiate the gain material to generate an amplified optical signal in the gain material.

2. The laser system of claim 1, wherein the at least one pump source comprises a diode array.

3. The laser system of claims 1 or 2, further comprising a single mode waveguide.

4. The laser system of claim 3, wherein the single mode waveguide is adapted to output the amplified optical signal substantially in a fundamental transverse mode.

5. The laser system of any of the preceding claims, wherein the gain region is thin.

6. The laser system of any of the preceding claims, wherein the gain region is well matched to the output of edge emitting diode arrays.

7. The laser system of any of the preceding claims, wherein the gain region comprises a dopant, and the dopant concentration and the pump wavelength are selected for effective pump light utilization.

8. The laser system of any of the preceding claims, wherein the lasing output is substantially in a single transverse mode.

9. The laser system of any of the preceding claims, wherein the system is power scalable.