Patent Publication Number: US-10777959-B2

Title: Planar waveguides with enhanced support and/or cooling features for high-power laser systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
     This application is a divisional of U.S. patent application Ser. No. 15/041,909 filed on Feb. 11, 2016, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure is generally directed to high-power laser systems. More specifically, this disclosure is directed to planar waveguides with enhanced support and/or cooling features for high-power laser systems. 
     BACKGROUND 
     High-power laser systems are being developed for a number of military and commercial applications. However, at high average output powers, problems associated with a lasing medium in a laser system are exacerbated. These problems include stress in the mounting of the lasing medium, inefficient and non-uniform cooling of the lasing medium, inefficient coupling of pumplight into the lasing medium, and inefficient coupling of laser beams into and out of the lasing medium. 
     Low-efficiency cooling may cause an unacceptable rise in the average temperature of the lasing medium relative to an inlet temperature of a coolant. Mounting stress and non-uniform heating can create phase distortions in an amplified output beam and can lead to damage or even fracturing of the lasing medium, which can also damage nearby elements of the laser system. Inefficient coupling of pumplight into the lasing medium can lead to overall laser inefficiencies, and stray pumplight can cause damage to the lasing medium or other elements of the laser system. Stray input or output laser beams can damage elements of the laser system, and the loss of power detracts from a total output power and an overall laser efficiency of the laser system. 
     SUMMARY 
     This disclosure provides planar waveguides with enhanced support and/or cooling features for high-power laser systems. 
     In a first embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region, first and second coating or cladding layers disposed on opposite sides of the core region, and one or more endcaps disposed between the first and second coating or cladding layers at one or more ends of the core region. The core region includes at least one material and is doped with at least one active ion species, and each endcap includes the at least one material and is not doped with any active ion species that creates substantial absorption at pump and signal wavelengths. 
     In a second embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region and at least one cladding layer disposed on the core region. The core region includes at least one crystal or crystalline material, and the at least one cladding layer includes at least one glass. 
     In a third embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region and first and second coating or cladding layers disposed on opposite sides of the core region. The apparatus also includes a first cooler disposed on or adjacent to the first coating or cladding layer and configured to cool the planar waveguide and a second cooler disposed on or adjacent to the second coating or cladding layer and configured to cool the planar waveguide. The first and second coolers are different types of coolers. 
     In a fourth embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region and at least one coating or cladding layer disposed on the core region. The apparatus also includes first and second side claddings disposed on opposite sides of the planar waveguide, where the opposite sides represent the longer sides of the planar waveguide. 
     In a fifth embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region and at least one coating or cladding layer disposed on the core region. The apparatus also includes first and second endcaps sealed to the planar waveguide and to a housing and one or more coolers sealed to the housing and configured to cool the planar waveguide. At least one of the one or more coolers is configured to receive a coolant and to direct the coolant through a passageway defined within an interior space of the housing, where the passageway includes a substantially linear passageway along a length of the planar waveguide. 
     In a sixth embodiment, an apparatus includes a planar waveguide configured to receive and amplify optical signals. The planar waveguide includes a core region and first and second coating or cladding layers disposed on opposite sides of the core region. The apparatus also includes a cooler disposed on or adjacent to the first coating or cladding layer and configured to cool the planar waveguide. The second coating or cladding layer is uncooled. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example high-power laser system according to this disclosure; 
         FIG. 2  illustrates an example planar waveguide (PWG) for use in a high-power laser system according to this disclosure; 
         FIG. 3  illustrates an example PWG pumphead assembly of a high-power laser system according to this disclosure; 
         FIGS. 4A and 4B  illustrate an example PWG cartridge for a PWG pumphead assembly of a high-power laser system according to this disclosure; and 
         FIGS. 5 through 12  illustrate example arrangements for supporting and/or cooling a planar waveguide of a high-power laser system according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 12 , described below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system. 
       FIG. 1  illustrates an example high-power laser system  100  according to this disclosure. As shown in  FIG. 1 , the laser system  100  includes a master oscillator  102  and a planar waveguide (PWG) amplifier  104 . The laser system  100  therefore has a master oscillator/power amplifier (MOPA) configuration. 
     The master oscillator  102  generally operates to generate low-power optical signals. The low-power optical signals could denote any suitable optical signals having relatively low power. For example, the low-power optical signals could include optical signals having a continuous wave (CW) output, a continuous pulse train (CPT), a pulse burst, or any of various other waveforms. The master oscillator  102  includes any suitable structure(s) for generating one or more low-power optical signals. In some embodiments, the master oscillator  102  includes a fiber laser. 
     The PWG amplifier  104  receives the low-power optical signals from the master oscillator  102  and pump power. The PWG amplifier  104  generally operates to amplify the low-power optical signals and generate high-power optical signals. For example, the PWG amplifier  104  could amplify a low-power CW or other optical signal into a high-power CW or other optical signal having ten kilowatts of power or more. The received pump power provides the necessary population inversion in the PWG amplifier&#39;s gain medium for this amplification. 
     The gain medium of the PWG amplifier  104  is formed using a planar waveguide. A planar waveguide generally denotes a structure that includes a core region and one or more coating or cladding layers. The core region is doped with an active ion species responsive to optical signals of at least one specific wavelength, and the one or more coating or cladding layers are optically transparent and contact the core region. The indexes of refraction and dielectric constants of the core region and the coating or cladding layer(s) differ and create boundaries that reflect the optical signals. The planar waveguide therefore operates to guide optical signals in its narrower dimension (referred to as the “fast axis” direction) but not in its broader dimension (referred to as the “slow axis” direction). The planar waveguide could be formed from any suitable materials and in any suitable manner. 
     Relay optics  106  direct the optical signals from the master oscillator  102  into a beam controller  108 , and relay optics  110  direct the optical signals from the beam controller  108  into the PWG amplifier  104 . The relay optics  106  and  110  can also alter the cross-sectional dimensions of the optical signals as needed for injection into the beam controller  108  and the PWG amplifier  104 , respectively. Each of the relay optics  106  and  110  includes any suitable optical device(s) for directing or formatting optical signals. 
     The beam controller  108  generally operates to modify the optical signals from the master oscillator  102  before the optical signals reach the PWG amplifier  104 . For example, the beam controller  108  could pre-distort the optical signals from the master oscillator  102  in order to substantially or completely compensate for distortions created within the PWG amplifier  104 . The beam controller  108  includes any suitable structure(s) for pre-distorting or otherwise modifying optical signals in a controllable manner. 
     The high-power output beam generated by the PWG amplifier  104  is directed towards a beam splitter  112 . The beam splitter  112  provides a substantial portion of the high-power output beam to relay optics  114 , which provide that portion of the high-power output beam out of the laser system  100  as a high-power output beam  116 . The beam splitter  112  also provides a small amount of the high-power output beam as samples to a feedback loop. The feedback loop is used to control and modify operation of the master oscillator  102 , the PWG amplifier  104 , and the beam controller  108 . The beam splitter  112  includes any suitable structure(s) for splitting optical signals. The relay optics  114  include any suitable optical device(s) for directing or formatting optical signals. 
     The feedback loop here includes a diagnostics unit  118 , a laser controller  120 , and diode drivers  122 - 124 . The diagnostics unit  118  generally operates to analyze the samples of the high-power output beam from the PWG amplifier  104  and provide diagnostic data to the laser controller  120 . The diagnostics unit  118  includes any suitable structure for identifying one or more characteristics of at least one sample of a high-power output beam. In some embodiments, diagnostic data from the diagnostics unit  118  could be output by the laser controller  120 , such as to one or more external destinations. This could allow the external destinations to monitor the health, status, or safety of the laser system  100 . 
     The laser controller  120  generally operates to control the operation of the beam controller  108  and the diode drivers  122 - 124 . The laser controller  120  includes any suitable structure for controlling operation of a laser system. For example, the laser controller  120  could include one or more processing devices, such as one or more microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits, or discrete logic devices. The laser controller  120  could also include one or more memories configured to store instructions or data used, generated, or collected by the processing device(s). The laser controller  120  could further include one or more interfaces configured to facilitate communications with other components or systems. 
     The diode driver  122  generates electrical drive signals that cause one or more laser diodes (or other light sources) of the master oscillator  102  to generate optical pump power for the master oscillator  102 , which causes the master oscillator  102  to generate desired low-power optical signals. The diode driver  124  generates electrical drive signals that cause laser diodes (or other light sources) of the PWG amplifier  104  to generate optical pump power for the PWG amplifier  104 , which uses the pump power to provide optical amplification. The diode driver  124  could be capable of operation across a range of input voltages and load conditions while protecting expensive laser diode strings from electrical shorts and transients. Each diode driver  122 - 124  includes any suitable structure(s) for driving any suitable arrangement of laser diodes or other light sources. 
     The laser system  100  can incorporate a number of novel features, which are described below. These novel features can be used individually or in any suitable combination. Various other features can also be incorporated into the laser system  100 . For example, the laser system  100  could use the techniques disclosed in U.S. patent application Ser. No. 14/661,828 and U.S. Patent Publication No. 2014/0268309 (which are hereby incorporated by reference) to cool various components of the laser system  100 , including the use of a thermal-optic interface (TOI) material. The laser system  100  could use the techniques disclosed in U.S. Patent Publication No. 2014/0268309 to suppress amplified spontaneous emission (ASE) within a PWG device, including the use of beveled edges on the PWG device. The laser system  100  could use the techniques disclosed in U.S. patent application Ser. No. 14/682,539 (which is hereby incorporated by reference), including the use of symmetric and asymmetric core regions in PWG devices. The laser system  100  could use the techniques disclosed in U.S. patent application Ser. No. 14/749,398 (which is hereby incorporated by reference), including the use of asymmetric two-layer PWG devices. The PWG amplifier  104  of the laser system  100  could be formed using the techniques disclosed in U.S. patent application Ser. No. 14/845,916 (which is hereby incorporated by reference), including the use of radio frequency (RF) magnetron sputtering and other techniques to form PWG-based laser devices. 
     Overall, the laser system  100  combines the benefits of fiber lasers (such as high gain with good beam quality) and bulk solid-state lasers (such as scalability) in an overall laser architecture that supports the generation of high-power laser outputs. The PWG amplifier  104  functions as a lasing element that facilitates high gain over a long amplifier path while simultaneously maintaining near diffraction-limited beam quality in one dimension (the fast axis of the PWG amplifier  104 ). Substantially uniform pumping of the lasing medium and substantially uniform cooling of the core region combine to produce a reasonably high-quality beam in the transverse dimension (the slow axis of the PWG amplifier  104 ). 
     Although  FIG. 1  illustrates one example of a high-power laser system  100 , various changes may be made to  FIG. 1 . For example, any single feature or combination of features described above and/or below could be used in the laser system  100 . Also, the functional divisions shown in  FIG. 1  are for illustration only. Components could be combined, further subdivided, omitted, or rearranged or additional components could be added according to particular needs. 
       FIG. 2  illustrates an example planar waveguide  200  for use in a high-power laser system according to this disclosure. The planar waveguide  200  could, for example, be used in the PWG amplifier  104  of the high-power laser system  100 . 
     As shown in  FIG. 2 , the planar waveguide  200  includes a core region  202  and one or more coating or cladding layers  204   a - 204   b . The core region  202  is doped with an active ion species responsive to optical signals of at least one specific wavelength, and the one or more coating or cladding layers  204   a - 204   b  are optically transparent and contact the core region  202 . The core region  202  could be formed from any suitable material(s), such as doped yttrium aluminum garnet (YAG). Each cladding layer  204   a - 204   b  could also be formed from any suitable material(s), such as glass or YAG. A number of additional features, such as an anti-reflective coating on input or output face(s) of the coating or cladding layers  204   a - 204   b , could be used. 
     A signal beam  206  from the master oscillator  102  is coupled into the core region  202  of the planar waveguide  200 , and pumplight  208  is coupled into one or more of the coating or cladding layers  204   a - 204   b  to provide pump power for optical amplification. The indexes of refraction and dielectric constants of the core region  202  and the coating or cladding layer(s)  204   a - 204   b  differ and create boundaries that reflect the optical signals of the signal beam  206 . The planar waveguide  200  therefore operates to guide the optical signals of the signal beam  208  in the “fast axis” or “guided axis” direction but not in the “slow axis” or “unguided axis” direction. Amplification is provided along the longer axis of the planar waveguide  200  (referred to as the “lasing axis”). 
     The signal beam  206  is confined within the very thin guiding core region  202  that is doped with a lasing species, and the signal beam  206  is amplified via stimulated emission. The pumplight  208  is injected into the coating or cladding layer(s)  204   a - 204   b  on one or both sides of the core region  202 , and the pumplight  208  is confined and efficiently coupled into the core region  202  along the entire length of the planar waveguide  200 . The thickness of the core region  202  and the refractive index difference between the core region  202  and the coating or cladding layer(s)  204   a - 204   b  define a numerical aperture that supports only the lowest-order propagating mode. This helps to ensure high beam quality in the thin dimension (fast axis) of the planar waveguide  200 . 
     In a MOPA configuration, the signal beam  206  from the master oscillator  102  is injected into one end of the active core region  202 , where it is amplified in one or multiple passes through the planar waveguide  200 . Since the signal beam  206  is unguided in the wide dimension (slow axis), the signal beam  206  is susceptible to thermal lensing effects due to non-uniform temperature distributions across the planar waveguide  200 . It is also susceptible to depolarization due to thermally-induced stress birefringence. Stress may further produce wavefront distortions in the signal beam  206 , depending on the strength of the stress-optic tensor in the lasing medium that additionally distorts the signal beam  206 . 
     This disclosure provides various embodiments of planar waveguides that may improve the performance, reliability, and/or cost of a PWG laser. This can be accomplished by providing improved mechanisms for supporting and/or cooling a planar waveguide. Example embodiments providing these supporting and/or cooling mechanisms are described below. 
     Although  FIG. 2  illustrates one example of a planar waveguide  200  for use in a high-power laser system, various changes may be made to  FIG. 2 . For instance, the size, shape, and dimensions of the various elements shown in  FIG. 2  are for illustration only and can vary as needed or desired. As a particular example, the coating or cladding layers  204   a - 204   b  could have substantially different thicknesses, creating an asymmetric planar waveguide. As another particular example, the core region  202  could have a non-uniform thickness along the lasing axis. 
       FIG. 3  illustrates an example PWG pumphead assembly  300  of a high-power laser system according to this disclosure. The pumphead assembly  300  could, for example, include the planar waveguide  200  of  FIG. 2  and be used in the PWG amplifier  104  of  FIG. 1 . 
     As shown in  FIG. 3 , the PWG pumphead assembly  300  includes a PWG pumphead housing  302  and a PWG cartridge  304 . The PWG cartridge  304  includes the planar waveguide  200 , which receives the signal beam  206  from the master oscillator  102  and the pumplight  208 . The pumplight  208  can be provided by any suitable source(s)  306 , such as one or more arrays of laser pump diodes. 
     The PWG cartridge  304  includes two support blocks  308 , and the planar waveguide  200  is located between the blocks  308 . The blocks  308  could denote coolers or other structures used to hold or encase the planar waveguide  200 . The blocks  308  could be formed from any suitable material(s) and in any suitable manner. In particular embodiments, each block  308  could denote a microchannel cooler, multi-jet liquid impingement cooler, cooling plate, ultra-thin liquid thermal-optical interface cooler (such as in U.S. patent application Ser. No. 14/661,828), or other structure for cooling the planar waveguide  200 . 
     A scraper mirror  310  reflects light that is not coupled into the planar waveguide  200 . The scraper mirror  310  could form a part of or be connected to a light pipe, which can be used to couple the pumplight  208  into the cladding layer(s) of the planar waveguide  200 . The light pipe or an endcap of the planar waveguide  200  can also help to homogenize the pumplight  208  prior to injection into the cladding layer(s) of the planar waveguide  200 . The scraper mirror  310  includes any suitable structure for reflecting light. The light pipe includes any suitable optical device for confining, homogenizing, and/or transporting optical signals. 
     In some embodiments, at least the PWG cartridge  304  and optionally other components (such as the pumplight source or sources  306 ) are implemented as one or more replaceable cartridges for the laser system. Among other things, this cartridge-based approach allows different cartridge designs to be installed with common (possibly standardized) mechanical, thermal, electrical, and optical interfaces without making any changes to the pumphead and optical bench. As a particular example, different PWG cartridges  304  could include different planar waveguides and/or cooling mechanisms for the planar waveguides. Also, different design teams can create and test different designs for the various components of the laser system  100 . There may be little or no need for one design team to wait for the development of one component of the laser system  100  to be completed before designing and testing another component of the laser system  100 . 
     In some embodiments, the PWG cartridge  304  receives a cooling fluid through one or more channels, such as channels  312 - 314 . For example, each block  308  could represent a cooler, and one or more of these coolers could operate using a flow of cooling fluid. The cooling fluid could be received through one or more of the cooling channels  312 - 314  and returned through one or more others of the cooling channels  312 - 314 . In this example, the cooling channels  312 - 314  on opposite sides of the planar waveguide  200  are identical, although different arrangements of cooling channels could be used, or cooling channels could be omitted on one or both sides of the planar waveguide  200 . 
     The PWG cartridge  304  is held in place using one or more clamps  316 . A main body of the housing  302  could be formed from an integral piece of material, and a cover  318  can provide access to the interior of the housing  302 . The cover  318  allows a portion of each clamp  316  to extend out of the housing  302  so that the clamp  316  can be adjusted. The clamps  316  push inward (down in  FIG. 3 ) in order to secure the PWG cartridge  304  in place. Each clamp  316  includes any suitable structure for applying force against a PWG cartridge. The cover  318  includes any suitable structure allowing passage of a portion of at least one pusher assembly. 
     Although  FIG. 3  illustrates one example of a PWG pumphead assembly  300  of a high-power laser system, various changes may be made to  FIG. 3 . For instance, the size, shape, and dimensions of the various elements shown in  FIG. 3  are for illustration only and can vary as needed or desired. 
       FIGS. 4A and 4B  illustrate an example PWG cartridge  304  for a PWG pumphead assembly of a high-power laser system according to this disclosure. The PWG cartridge  304  could, for example, include the planar waveguide  200  of  FIG. 2  and be used in the PWG pumphead assembly  300  of  FIG. 3 . 
     As shown in  FIGS. 4A and 4B , the PWG cartridge  304  includes the planar waveguide  200 . The planar waveguide  200  has at least one TOI material  402  placed in thermal contact with the broad clad faces of the planar waveguide  200 . The exposed sides of the TOI material  402  can be bonded or otherwise attached to the blocks  308 , which could represent one or more coolers. The attachment of the planar waveguide  200  to the blocks  308  could occur in any suitable manner, such as by using a thermally conducting epoxy. The blocks  308  include channels  404  that can be used to transport coolant to cool the PWG cartridge  304 . For example, the channels  404  can align with the channels  312 - 314  of the housing  302 . 
     The TOI material  402  includes any suitable material facilitating the transport of heat away from a planar waveguide. Each block  308  includes any suitable structure for removing heat from a planar waveguide. In some embodiments, for example, each block  308  could denote a copper microchannel heatsink module. 
     Although  FIGS. 4A and 4B  illustrate one example of a PWG cartridge  304  for a PWG pumphead assembly of a high-power laser system, various changes may be made to  FIGS. 4A and 4B . For instance, the size, shape, and dimensions of the various elements shown in  FIGS. 4A and 4B  are for illustration only and can vary as needed or desired. 
       FIGS. 5 through 12  illustrate example arrangements for supporting and/or cooling a planar waveguide of a high-power laser system according to this disclosure. The embodiments of the planar waveguides shown in  FIGS. 5 through 12  incorporate various features to reduce the mounting stresses on the planar waveguides and/or features to more uniformly cool the planar waveguides. Each of these implementations or other similar implementations could be used in the laser systems described above. 
     As shown in  FIG. 5 , a planar waveguide  500  includes a core region  502  and one or more coating or cladding layers  504   a - 504   b . As noted above, the core region  502  receives and amplifies a signal beam (such as the signal beam  206 ), and one or more of the coating or cladding layers  504   a - 504   b  receive pumplight (such as the pumplight  208 ). Note that in this example, the coating or cladding layers  504   a - 504   b  are asymmetric, which means the coating or cladding layers  504   a - 504   b  have substantially different thicknesses. In some embodiments, the thicker coating or cladding layer  504   a  may provide a stable structure for the core region  502  and can be substantially matched in its coefficient of thermal expansion to the core region  502  so that minimal stress is placed on the core region  502 . However, the use of an asymmetric design is not required. In other embodiments, the coating or cladding layers  504   a - 504   b  have substantially the same thickness, leading to a symmetric configuration, which could allow the temperature profile and stress to be balanced between the layers  504   a - 504   b.    
     Passive guiding endcaps  506   a - 506   b  are provided at the ends of the core region  502  in the planar waveguide  500 . In some embodiments, the endcaps  506   a - 506   b  can be added to the core region  502  prior to formation or bonding of the coating or cladding layer(s)  504   a - 504   b . The endcaps  506   a - 506   b  could be formed from any suitable material(s) and in any suitable manner. For example, in some embodiments, the endcaps  506   a - 506   b  could be formed from the same material(s) as the core region  502  (such as YAG), and the endcaps  506   a - 506   b  could be doped with at least one dopant to change their index of refraction. However, unlike the core region  502 , any doping of the endcaps  506   a - 506   b  produces negligible absorption at the pump and signal wavelengths, so little or no lasing occurs within the endcaps  506   a - 506   b . In  FIG. 5 , the endcaps  506   a - 506   b  are used at both ends of the planar waveguide  500  (namely an input end and an output end of the planar waveguide  500 ), although only one endcap  506   a  or  506   b  could be used at one end of the planar waveguide  500 . 
     The one or more endcaps  506   a - 506   b  help to provide support for the planar waveguide  500  and reduce mounting stresses placed on the planar waveguide  500 . For example, the endcaps  506   a - 506   b  do not provide lasing and are used for guiding optical signals, which means the endcaps  506   a - 506   b  are not heated as much as the core region  502 . External support structures could therefore be placed in contact with the endcaps  506   a - 506   b , such as to secure the planar waveguide  500  within a PWG cartridge  304 . The reduced heating of the endcaps  506   a - 506   b  compared to the core region  502  can provide a more stable and less stressful mounting of the planar waveguide  500 . 
     As shown in  FIG. 6 , a planar waveguide  600  includes a core region  602  and one or more cladding layers  604   a - 604   b . As noted above, the core region  602  receives and amplifies a signal beam, and one or more of the cladding layers  604   a - 604   b  receive pumplight. Note that the cladding layers  604   a - 604   b  are asymmetric in this example, although a symmetric design could also be used. 
     The core region  602  in this example could be formed from a single crystal, a crystalline ceramic, or other crystalline material. Also, at least one of the cladding layers  604   a - 604   b  could be formed from a glass. The glass may be directly fused to the core region  602 , such as by applying heat after the core and cladding surfaces are precisely contacted. Pressure may also be added during the fusing process to ensure a stable, robust bond. Various other approaches could be used to fuse glass and crystalline components together. 
     The coefficients of thermal expansion of the glass and crystalline components can be closely matched to reduce stresses on the planar waveguide  600  during operation. The index of refraction of the glass cladding layer(s)  604   a - 604   b  can be different from but close to the index of refraction of the core region  602 , and the cladding layer(s)  604   a - 604   b  can have low absorption at the wavelengths of interest. 
     As shown in  FIG. 7 , a PWG assembly  700  includes a planar waveguide  702 , which in this example is implemented using the core region  602  and the cladding layers  604   a - 604   b  of  FIG. 6  (although other planar waveguides could be used). The planar waveguide  702  here is asymmetric, and as a result the thinner cladding layer  604   b  may increase in temperature less than the thicker cladding layer  604   a  during operation of the planar waveguide  702 . 
     The PWG assembly  700  uses two different cooling interfaces for different sides of the planar waveguide  702 . Each cooling interface could be optimized to perform desired functions or achieve specific goals, and those functions or goals need not be identical. For example, the cooling interface on one side of the planar waveguide  702  could be optimized for cooling, while the cooling interface on another side of the planar waveguide  702  could be optimized for minimizing Amplified Spontaneous Emission (ASE). 
     In this particular example, one cooling interface is implemented using a liquid cooling manifold  704 . The liquid cooling manifold  704  could support any suitable direct liquid cooling technique. In this example, a multi jet liquid impingement cooler is used, although other approaches (such as ultra-thin liquid thermal-optical interface cooling or other approaches described above) may be used. A seal  706 , such as an O-ring or other type of seal, helps to prevent leakage of the cooling liquid out of the liquid cooling manifold  704 . Another cooling interface is implemented in  FIG. 7  using a dry conductive cooler  708  that could be thermally coupled to the planar waveguide  702  using a thermal-optic interface (TOI) material  710 . One example of this approach is described in U.S. Patent Publication No. 2014/0268309 (which was incorporated by reference earlier). 
     Note that in this example, the planar waveguide  702  could be replaced by another planar waveguide, such as the planar waveguide  500  of  FIG. 5  or other asymmetrical or symmetrical waveguide. The ability to cool different sides of a planar waveguide, particularly different sides of an asymmetrical planar waveguide, using different cooling technologies helps to improve the overall cooling efficiency in a PWG assembly. 
     As shown in  FIG. 8 , a PWG assembly  800  includes a planar waveguide  802  and endcaps  804 . The endcaps  804  could denote passive guiding endcaps or unguiding endcaps depending on the design on the planar waveguide  802 . The endcaps  804  could be used at an input end and/or an output end of the planar waveguide  802 . Also, side claddings  806  may be bonded or otherwise attached to the planar waveguide  802  along the longer sides of the planar waveguide  802 , allowing the endcaps  804  to be used in conjunction with the side claddings  806  to provide a continuous mounting region. Any doping of the side claddings  806  produces negligible absorption at the pump and signal wavelengths, so little or no lasing occurs within the side claddings  806 . The side claddings  806  could be formed from any suitable material(s) and in any suitable manner. For example, the side claddings  806  could be formed from the same material(s) as the cladding layer(s) of the planar waveguide  802 . To avoid thermal stress under operation, the side claddings  806  may have a substantially similar coefficient of thermal expansion to elements of the planar waveguide  802 , such as the thicker cladding layer of the planar waveguide  802 . 
     A cooling manifold  808  is used here to cool the planar waveguide  802 . The cooling manifold  808  represents any suitable structure for delivering cooling fluid to a planar waveguide, such as a multi jet liquid impingement cooler or other direct liquid cooler such as those described above. A seal  810  (such as an O-ring) is positioned between the PWG assembly  800  and the cooling manifold  808  to help prevent leakage of the cooling fluid. The endcaps  804  and the side claddings  806  can be used here to facilitate a more reliable seal for the cooling manifold  808 . For example, planar waveguides tend to be more delicate structures, and reducing or eliminating contact between the planar waveguide  802  itself and the seal  810  can help to protect various structures in  FIG. 8 . Note, however, that the side claddings  806  could be used without the endcaps  804 . Also note that while not shown, the same cooling interface or a different cooling interface could be used on the top side of the planar waveguide  802  in  FIG. 8 . 
     Other types of cooling interfaces could also be used with a planar waveguide having side claddings. For example, as shown in  FIG. 9 , a PWG assembly  900  includes a planar waveguide  902 , endcaps  904 , and side claddings  906 . These components may be the same as or similar to corresponding components in  FIG. 8 . In  FIG. 9 , one cooling interface includes an aperture plate  908 , which denotes a plate having an aperture (opening) in the central portion of the plate. The aperture plate  908  can be bonded to the side claddings  906  and to the endcaps  904  (if present). The aperture plate  908  provides a robust surface for an O-ring or other seal for a cooling manifold, placing the O-ring farther away from the input signal and high power output beam. The other cooling interface in  FIG. 9  is formed using a solid plate  910 , which could be bonded or otherwise attached to the planar waveguide  902  using the side claddings  906  and the endcaps  904  (if present). Any suitable bonding could be used here, such as an adhesive, epoxy, glass solder, metal solder, hydroxyl bond, or covalent bond. The use of a solid plate  910  can facilitate cooling and structural stiffness on the side opposite the liquid cooling manifold. Both the aperture plate  908  and the solid plate  910  could have a substantially similar coefficient of thermal expansion to element(s) of the planar waveguide  902 . 
     As shown in  FIGS. 10 and 11 , a PWG assembly  1000  includes a planar waveguide  1002  bonded or otherwise secured to two endcaps  1004   a - 1004   b , which can be secured to a housing  1006 . The endcaps  1004   a - 1004   b  include slots through which the planar waveguide  1002  can extend, allowing portions of the planar waveguide  1002  to remain partially exposed. Any suitable mechanism can be used to secure the planar waveguide  1002  to the endcaps  1004   a - 1004   b , such as metal solder, glass solder, or ultraviolet (UV) curing adhesive. The endcaps  1004   a - 1004   b  create a seal around the ends of the planar waveguide  1002 , but the endcaps  1004   a - 1004   b  can be somewhat compliant to allow for thermal expansion. Coolers  1008   a - 1008   b  are also bonded or otherwise secured to the housing  1006 , and seals can be formed around the sides of the housing  1006 . This creates a sealed internal cavity within which the planar waveguide  1002  is substantially located and through which cooling fluid can flow. 
     Each cooler  1008   a - 1008   b  here includes multiple channels  1010 - 1012  and a flow controller  1014 . For each cooler  1008   a - 1008   b , one of the channels  1010 - 1012  can allow passage of a cooling fluid into the interior of the housing  1006 , and another of the channels  1010 - 1012  can allow exit of the cooling fluid from the interior of the housing  1006 . The cooling fluid flows between the channels  1010 - 1012  within the interior of the housing  1006  through a passage defined by the flow controller  1014 . The flow controller  1014  here creates a substantially linear passageway for the cooling fluid to flow past the planar waveguide  1002  along the length of the planar waveguide  1002 . Note, however, that the flow controller  1014  could have any other suitable size and shape. The flow controller  1014  includes any suitable structure for defining a desired passageway for cooling fluid. In this example, the flow controller  1014  has a design in which the flow controller flares outward. 
     In the embodiment in  FIGS. 10 and 11 , the same cooling interface is used on both sides of the planar waveguide  1002 . However, as with previous embodiments, different cooling interfaces could be used on different sides of the planar waveguide  1002 . For example, the cooler  1008   a  could be used on one side of the planar waveguide  1002 , and a slab cooler or other type of cooler could be used on the other side of the planar waveguide  1002 . As with prior embodiments, the materials used in the cooling interfaces could have a substantially similar coefficient of thermal expansion to elements of the planar waveguide  1002 . 
     As shown in  FIG. 12 , a PWG assembly  1200  includes a planar waveguide  1202 , which in this example is implemented using the core region  602  and the cladding layers  604   a - 604   b  of  FIG. 6  (although other planar waveguides could be used). The planar waveguide  1202  here is asymmetric, and as a result the thinner cladding layer  604   b  may increase in temperature less than the thicker cladding layer  604   a  during operation of the planar waveguide  1202 . The thinner cladding layer  604   b  is cooled using the dry conductive cooler  708  and the TOI material  710  of  FIG. 7  (although other cooling techniques could be used with the thinner cladding layer  604   b ). 
     The thicker cladding layer  604   a  in this example is uncooled, meaning there is no active cooling used in conjunction with the thicker cladding layer  604   a . However, due to the refractive index of air on the thicker cladding layer  604   a , a large fluorescence heat load could be created on the cooler  708 . Placing a transparent material  1204  with a proper index of refraction, such as sapphire or fused silica, on the thicker cladding layer  604   a  allows fluorescence to “leak out” of the PWG assembly  1200 , thereby reducing heat loads on the cooler  708  and reducing temperature and stress in the PWG assembly  1200 . Note that any suitable technique could be used to secure the transparent material  1204  to the thicker cladding layer  604   a , such as clamping. 
     Because the cladding layer  604   b  is thinner than the cladding layer  604   a , there will be less of a temperature rise through the thickness of the cladding layer  604   b . Therefore, the surface of the cladding layer  604   b  can be cooled to help maintain the planar waveguide  1202  at a lower temperature than if the surface of the cladding layer  604   a  was cooled. 
     Although  FIGS. 5 through 12  illustrate examples of arrangements for supporting and/or cooling a planar waveguide of a high-power laser system, various changes may be made to  FIGS. 5 through 12 . For example, each figure may include a symmetric or asymmetric planar waveguide of any suitable design. Also, a planar waveguide could include any single instance of the various cooling interfaces shown and described above or any other suitable cooling interface, or a planar waveguide could include multiple instances of one or more of the cooling interfaces shown and described above or any other suitable cooling interfaces. If multiple cooling interfaces are used, those cooling interfaces may or may not be identical. 
     High-power laser systems, such as the ones described above, could be used in a large number of military and commercial applications. The following discussion provides a description of various example commercial applications. However, the following discussion does not limit this disclosure to any particular applications. 
     High-power laser systems could find use in commercial mining applications, such as in drilling, mining, or coring operations. For instance, high-power laser systems could be used to soften or weaken an earth bed prior to drilling through the earth bed using drill bits. This could allow for fewer drill bit changes and extended lifetimes and reliabilities of the drill bits. Here, free-space propagation of a high-power laser beam from an output window of a laser system could be used, allowing deeper penetration at further distances compared to conventional fiber lasers. 
     High-power and high-beam quality laser systems could also find use in remote laser welding, cutting, drilling, or heat treating operations, such as in industrial or other automation settings. The use of a high-power and high-beam quality laser system allows the processing of thicker materials to occur at larger working distances from the laser system while minimizing the heat-affected zone and maintaining vertical or other cut lines. Among other things, this helps to support welding or cutting operations where proximity to the weld or cut site is difficult or hazardous. It also helps to protect the laser system and possibly any human operators from smoke, debris, or other harmful materials. 
     High-power laser systems could further find use in construction and demolition operations. Example operations could include metal resurfacing or deslagging, paint removal, and industrial demolition operations. High-power laser systems can be used to ablate material much faster and safer compared to conventional operations. As a particular example of this functionality, high-power laser systems could be used to support demolition of nuclear reactors or other hazardous structures. Here, the high-power laser systems could be used to cut through contaminated structures like contaminated concrete or nuclear containment vessels or reactors from long distances. This helps to avoid the use of water jet cutting or other techniques that create hazardous waste, such as contaminated water. It also provides improved safety since human operators can remain farther away from contaminated structures being demolished. 
     A number of additional applications are possible. For example, high-power laser systems could find use in power beaming applications, where high-power laser beams are targeted to photovoltaic (solar) cells of remote devices to be recharged. High-power laser systems could find use in hazardous material (HAZMAT) applications, where the laser systems are used to heat and decompose hazardous materials into less harmful or non-harmful materials. 
     In some embodiments, various functions described in this patent document are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer code (including source code, object code, or executable code). The term “communicate,” as well as derivatives thereof, encompasses both direct and indirect communication. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
     The description in this patent document should not be read as implying that any particular element, step, or function is an essential or critical element that must be included in the claim scope. Also, none of the claims is intended to invoke 35 U.S.C. § 112(f) with respect to any of the appended claims or claim elements unless the exact words “means for” or “step for” are explicitly used in the particular claim, followed by a participle phrase identifying a function. Use of terms such as (but not limited to) “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” “system,” “processor,” “processing device,” or “controller” within a claim is understood and intended to refer to structures known to those skilled in the relevant art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. § 112(f). 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.