Abstract:
High-power laser diode system offering reduced consumption and inventory of coolant. The invention provides coolant at a very high flow rate to a heat exchanger. A portion of the coolant flow downstream of the heat exchanger is separated and pumped by a fluid-dynamic pump back into the heat exchanger. The fluid dynamic pump is operated by a fresh coolant supplied at high-pressure. Because a substantial portion of the flow leaving the heat exchanger is recirculated back to the inlet, the amount of fresh coolant consumed is substantially reduced compared to a traditional laser diode system. This enables reduced size of coolant lines and results in a more compact and lightweight system. Other uses of the invention include cooling of devices requiring heat rejection at very high heat flux including photovoltaic cells, solar panels, semiconductor laser diodes, semiconductor electronics, and laser gain medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional patent application U.S. provisional patent application U.S. Ser. No. 61/011,691, filed Jan. 18, 2008; U.S. provisional patent application U.S. Ser. No. 61/066,249, filed Feb. 19, 2008; and U.S. provisional patent application U.S. Ser. No. 61/130,419, filed May 31, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to systems for thermal management and more specifically to supplying a fluid to a heat exchanger for thermal management. 
       BACKGROUND OF THE INVENTION 
       [0003]    High-power semiconductor laser diodes are finding ever increasing industrial applications such as pumping of solid-state lasers (SSL) and direct material processing, namely cutting, welding, and heat treating. Frequently, such laser diodes are part of a system installed on a mobile mount such as a translation stage or a robotic arm. Other applications for high-power semiconductor laser diodes include a variety of electro-optical systems for a field use such as LIDAR, target illuminators, target designators, or high-energy lasers that may be operated on a land or air vehicle. In all such instances, it is essential to reduce the weight and volume of the high-power laser diode system. 
         [0004]    As a byproduct of generating optical output, laser diodes produce large amount of waste heat. To avoid overheating, laser diodes may be mounted on a suitable heat sink. Such a heat sink may be constructed as an actively cooled heat exchanger (HEX). Suitable HEX may use microchannels or impingement cooling. To achieve their target heat transfer performance, such HEX operate at high coolant velocities around 2 to 3 m/s. This results in very high coolant consumption. At the same time, the coolant temperature rise in the HEX is only about 2-3° C., which translates to a rather low coolant utilization. 
         [0005]    For high-power applications, multiple semiconductor laser diodes may be mounted on a common semiconductor substrate known as a bar, which is then mounted on a HEX to form a diode bar assembly.  FIG. 1A  shows a diode bar assembly  186  of prior art comprising a laser diode bar  146  with laser diodes  190  mounted on a HEX  182 . The diodes generate optical output  114 . The HEX  182  has a coolant inlet port  154  and a coolant outlet port  156 . Coolant may flow into the inlet port  154 , is conveyed by internal passages inside the HEX to a close proximity of the laser diode bar  146 , removes waste heat from the laser diode bar, and flow out through the outlet port  156 . General path of coolant flow is identified by arrows  116 . Suitable diode bar assemblies may be purchased, for example, from Northrop-Grumman Cutting Edge Optronics (CEO) in Saint Charles, Mo. and from DILAS in Tuscon, Ariz. 
         [0006]    To achieve even higher optical output, multiple diode bar assemblies may be arranged to form a diode bar stack.  FIG. 1B  shows a diode bar stack  130  of prior art comprising multiple diode bar assemblies  186 , an inlet end cap  110 , and an outlet end cap  112 . The end caps have internal passages arranged to align with the inlet and outlet ports of diode bar assemblies  186 . This arrangement allows for a coolant to be fed to the diode bar assemblies  186  in the stack  130  by a single end cap inlet port  192  and drained by a single end cap outlet port  196 . Suitable diode bar stacks may be purchased, for example, from the already noted Northrop-Grumman CEO and DILAS. 
         [0007]    The wavelength of laser diode output light is known to be sensitive to coolant temperature. This creates a design challenge in applications requiring wavelength stability, such as when pumping SSL where the diode wavelength must be precisely matched into an absorption band of a laser crystal. In this situation, coolant feeds to individual high-power laser diode bar assembly in an array cannot be connected in series, but rather must be connected in parallel. As a result, coolant must be supplied to such arrays at very high flow-rates to maintain the diodes at their design temperature. 
         [0008]    Traditional high-power laser diodes employs a cooling system with a forced convection loop that transports waste heat from the diodes to a chiller or a thermal energy storage. When operating with a powerful laser diode array, large quantities of coolant may be circulated between the array and the chiller. In applications where the laser diodes and the chiller are separated by a large distance, this results in long, large size piping and large coolant inventory. In addition, when laser diodes are mounted on a translations stage or a robotic arm, heavy coolant lines present undesirable inertia and impede motion. The traditional cooling system also stresses the volume and weight carrying capacity of mobile platforms such as land and air vehicles. All such applications would greatly benefit from a cooling system operating with low coolant consumption that is lightweight and compact. 
         [0009]    Furthermore, a traditional laser diode systems may require a large amount of coolant inventory. In the event of an accidental coolant release from the system, such a large coolant inventory may pose significant safety, health, and environmental hazards. In addition, a large coolant inventory has a large inertia, which must be overcome during flow start and stop conditions. The above size, weight, energy consumption, coolant inventory, and inertia characteristics of traditional thermal management system may make it less desirable in applications requiring compactness, lightweight, reduced energy consumption, improved safety, and fast startup. 
       SUMMARY OF THE INVENTION 
       [0010]    The subject invention provides a simple, compact, lightweight laser diode system offering reduced coolant inventory and energy consumption. In particular, the subject invention provides coolant at a very high flow rate to a laser diode HEX. A portion of the coolant flow downstream of the HEX outlet is separated and pumped by a fluid-dynamic pump back into the HEX inlet. The fluid dynamic pump is operated by a fresh coolant supplied at high-pressure that may be provided by a pump, a high-pressure tank, or other suitable source. Because a substantial portion of the flow leaving the HEX is recirculated back to the HEX inlet, the amount of fresh coolant consumed is substantially reduced compared to a traditional laser diode system. A portion of the coolant downstream of the HEX that is not recirculated back to the HEX may be fed to the suction port of a pump, or stored in a tank or an accumulator, or it may be released to environment. See, for example, a publication entitled “Improved Cooling for High-Power Laser Diodes,” authored by John Vetrovec in proceedings from Photonics West, San Jose, Calif., Jan. 20-24, 2008, SPIE vol. 6876, and “Lightweight and Compact Thermal Management System for Solid-State High-Energy Laser,” in proceedings from the 21 st  Annual Solid-State and Diode Technology Review, held in Albuquerque, N.Mex., Jun. 3-5, 2008, both of which are hereby expressly incorporated by reference in their entirety. 
         [0011]    If the coolant provided to the driving nozzle of the fluid dynamic pump is substantially in a gas or vapor form, the fluid dynamic pump may be an ejector. If the coolant provided to the driving nozzle of the fluid dynamic pump is substantially in a liquid form, the fluid dynamic pump may be a jet pump. 
         [0012]    In one preferred embodiment of the subject invention, one or more laser diodes are mounted on a HEX, and an external fluid-dynamic pump recirculates portion of the coolant through external passages. 
         [0013]    In another preferred embodiment of the subject invention, diode bar stack is connected to a recirculator containing internal fluid-dynamic pump and recirculation passages. The recirculator, which is connected to a supply of fresh coolant, then feeds coolant to the diode bar stack and drains coolant therefrom, while recirculating a portion thereof. 
         [0014]    In yet another preferred embodiment of the subject invention, fluid dynamic pump and recirculation passages are made integral to a diode bar assembly HEX. 
         [0015]    These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
         [0016]    Accordingly, it is an object of the present invention to provide a lightweight and compact laser diode system. 
         [0017]    It is another object of the invention to provide a laser diode system for reduced coolant inventory. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1A  is an isometric view of a laser diode bar assembly of prior art. 
           [0019]      FIG. 1B  is an isometric view of a diode bar stack of prior art. 
           [0020]      FIG. 2  is a diagrammatic view of a laser diode system according one embodiment of the present invention. 
           [0021]      FIG. 3  is a side cross-sectional view of a laser diode system according alternative embodiment of the present invention suitable for laser diode bar stacks. 
           [0022]      FIG. 4  is a side cross-sectional view of a laser diode system according another alternative embodiment of the present invention suitable for a laser diode bar assembly. 
           [0023]      FIG. 5  is a cross-sectional view  4 - 4  of the laser diode system in  FIG. 4 . 
           [0024]      FIG. 6  is a cross-sectional view  5 - 5  of the laser diode system in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Selected embodiments of the present invention will now be explained with reference to drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses. 
         [0026]    Referring to  FIG. 2  of the drawings in detail, numeral  20  generally indicates a laser diode system generally comprising a fluid-dynamic pump  220 , laser diode  290 , heat exchanger (HEX)  282 , back-pressure valve  252 , return passage  236 , and interconnecting passages  232 ,  238 , and  239 . The HEX  282  is in good thermal communication with the laser diode  290 . The HEX  282  has an inlet port  254  and an outlet port  256 . The fluid dynamic pump  220 , HEX  282 , return passage  236 , and interconnecting passages  232  and  238  form a recirculation loop  224 . In general, the fluid-dynamic pump  220  is arranged to feed a suitable coolant to the inlet port  254  of the HEX  282  and to recirculate a portion of coolant flowing from the outlet port  256  back to the inlet port  254  of the HEX  282 . The fluid-dynamic pump  220  further comprises a driving nozzle  240  and a pump body  234 . The pump body  234  is generally configured as a duct including a suction chamber  228 . The pump body  234  may also include a converging portion, which may be followed by followed by a straight portion, which may be followed by a diverging portion. The suction chamber  228  includes a suction port  262 . The downstream portion of the pump body  234  has a discharge port  264 . The suction port  262  of fluid dynamic pump  220  is fluidly connected to the return passage  236 . The discharge port  264  of fluid dynamic pump  220  is fluidly connected to the inlet port  254  of heat exchanger  282  by means of the passage  232 . The back pressure valve  252  is fluidly connected to the outlet port  256  of heat exchanger  282  by means of passages  238  and  239 . The return passage  236  is also fluidly connected to the outlet port  256  of heat exchanger  282  by means of the passage  238 . The driving nozzle  240  is of fluid-dynamic pump  220  arranged to discharge high-velocity flow (et)  242  into the throat of the pump body  234 . This arrangement is common in fluid dynamic pumps. The driving nozzle  240  is fluidly connected by means of a supply line  248  to a source of high-pressure coolant. The back pressure valve  252  is arranged to provide a flow impedance to coolant flowing therethrough. One advantage of the back pressure valve  252  is its adjustability. In a variant of the invention not requiring adjustability, alternative flow-impeding device such as an orifice or a venture may be used. 
         [0027]    If the heat transfer fluid is gas, the fluid dynamic pump may be an ejector. Suitable ejectors with a single driving nozzle are Series 20A ejectors made by Penberthy, Prophetstown, Pa. Alternative ejectors may have multiple driving nozzles and/or lobed driving nozzles. If the heat transfer fluid is liquid, the fluid dynamic pump may be a hydraulic ejector also known as a jet pump. Suitable hydraulic ejectors with a single driving nozzle are Series 60A ejectors made by Penberthy, Prophetstown, Pa. Alternative hydraulic ejectors may have multiple driving nozzles and/or lobed driving nozzles. 
         [0028]    In operation, the fluid dynamic pump  220 , HEX  282 , return passage  236 , and interconnecting passages  232 ,  238  and  239  are substantially filled with suitable coolant. The laser diode  290  is connected to a source of electric power and generates optical output  214 . As a by-product of generating optical output, the laser diode  290  generates heat that is conducted to HEX  282 . High-pressure coolant is supplied by a stream  275  via the supply line  248  to the driving nozzle  240  where it forms a jet  242  that is directed into the throat portion of the pump body  234 . The jet  242  entrains coolant in the suction chamber  228  and pumps it. Stream  276  containing both the jet flow and the pumped coolant exits the fluid dynamic pump  220  through the discharge port  264  and flows through the passage  232  into the inlet port  254  of HEX  282 . The coolant removes heat from the HEX  282  and exits the HEX  282  through the outlet port  256  as a stream  276 ′ flowing in the passage  238 . A portion of the coolant stream  276 ′ is separated and directed as a recirculating stream  272  into the return passage  236 . The un-separated portion of the stream  276 ′ forms an exit stream  274  that is released from the laser diode system  20  through he back pressure valve  252 . The back pressure valve  252  may be adjusted so that a large portion of the stream  276 ′ is directed in the form of the recirculating stream  272  into the return passage  236 . As a result, a large flow may be maintained through the HEX  282  while the overall consumption of fresh coolant as, for example, measured by the flow in the stream  275  fed to the driving nozzle  240  is substantially smaller. Coolant supplied to the nozzle  240  may be provided at a temperature such that the stream  276  (which is a mixture of nozzle flow and the stream  272 ) fed to the HEX  282  is provided at a predetermined temperature value. In particular, if the coolant is a gas, this gas provided in the line  248  may be chilled in a heat exchanger, a vortex tube, or a turboexpander prior to being fed to nozzle  240 . Temperature of laser diode  290  may be controlled by appropriately adjusting the backpressure valve  252 . An alternative method for controlling the temperature of laser diode  290  may be achieved by appropriately adjusting the pressure of coolant supplied to the nozzle  240 . 
         [0029]    An alternative embodiment of the invention is particularly suitable for use with diode bar stacks. Referring now to  FIG. 3 , there is shown a cross-sectional view of a laser diode system  30  comprising a diode bar stack  330  connected to a coolant saving recirculator  320 . The laser diode system  30  is similar to the laser diode system  20  except that the laser diodes are now arranged into diode bar assemblies  386  installed in a diode bar stack  330 , and the fluid dynamic pump with the backpressure valve and the passages are now integrated into the recirculator  320 . 
         [0030]    The recirculator  320  includes a fluid dynamic pump  320 , return passage  336 , a backpressure valve  352 , and interconnecting passages  332 ,  338 , and  339 . The recirculator may be machined from a block of suitable material (such as metal, plastic, or ceramic) and the fluid dynamic pump, return passage, backpressure valve, and interconnecting passages may be formed therein. The passage  332  of recirculator  330  is arranged to fluidly couple to the end cap inlet port  392 . The passage  338  of the recirculator  330  is arranged to fluidly couple to the end cap outlet port  396 . 
         [0031]    In operation, the fluid dynamic pump  320 , return passage  236 , and interconnecting passages  332 ,  338 , and  339  as well as the internal passages and HEX of the diode bar stack  330  are substantially filled with suitable coolant. The diode bar assemblies  386  are connected to a source of electric power and generates optical output. As a by-product of generating optical output, the diode bar assemblies  386  generate heat that is conducted to HEX  382 . High-pressure coolant is supplied by a stream  375  to the driving nozzle  340  where it forms a jet  342  that is directed into the throat portion of the pump body  334 . The jet  342  entrains coolant in the suction chamber  328  and pumps it. Stream  376  containing both the jet flow and the pumped coolant exists the fluid dynamic pump  320  and flows through the passage  332  into the end cap inlet port  392 , and therefrom to the inlet ports  354  of HEX  382 . The coolant removes heat from the HEX  382  and laser diode bars  346  attached thereto, exits the HEX  382  through the outlet port  356 , and flows out of the diode bar stack  330  through the end cap outlet port  396  as a stream  376 ′ flowing in the passage  338 . A portion of the coolant stream  376 ′ is separated and directed as a recirculating stream  372  into the return passage  336 . The un-separated portion of the stream  376 ′ forms an exit stream  374  that is released from the laser diode system  30  through the back pressure valve  352 . 
         [0032]    Another alternative embodiment of the invention is particularly suitable for use with diode bar assemblies. Referring now to  FIG. 4 , there is shown a laser diode system  40  comprising a diode bar assembly  486 ′ including a laser diode bar  446  attached to a HEX  482 ′ having a coolant inlet  454  and a coolant outlet  456 . The diode bar assembly  486 ′ is similar to the diode bar assembly  186  shown in  FIG. 1A , except that the HEX  482 ′ now comprises two internal fluid dynamic pumps  420 a and  420 b and associated internal coolant passages ( FIGS. 5 and 6 ). 
         [0033]    In particular,  FIG. 5 , which is a cross-section through the diode bar assembly  486 ′ generally in the plane of the fluid dynamic pumps  420   a  and  420   b , shows fluid dynamic pumps  420   a  and  420   b  respectively having nozzles  440   a  and  440   b  each fluidly connected to coolant inlet port  454  and respectively positioned inside suction chambers  428   a  and  428   b . Nozzles  440   a  and  440   b  are respectively directed respectively into the throats of body  434   a  and fluid dynamic pumps  420   a  and  420   b . Discharge ports  464   a  and  464   b  are fluidly coupled into zone  450  that is in a close proximity of the laser diode bar  446  ( FIG. 4 ). The zone  450  may comprise surface extensions, microchannels, or impingement jet coolers to promote heat transfer from laser diode bar  446  into the coolant flowing through zone  450 . 
         [0034]    Referring now to  FIG. 6 , there is shown a cross-section through the diode bar assembly  486 ′ generally in the plane of the passages  438   a  and  438   b . The passages  438   a  and  438   b  respectively fluidly connect the zone  450  to the suction chambers  428   a  and  428   b  via passages  436   a  and  436   b . The passages  438   a  and  438   b  also fluidly connect the zone  450  to the outlet port  456  via passage  439  and the orifice  452 ′. The orifice  452 ′ is used in lieu of a valve and it is sized to provide appropriate impedance to the flow. 
         [0035]    In operation, all of the internal volumes of HEX  482 ′ are substantially filled with coolant. The laser diode bar  446  is connected to a source of electric power and generates optical output  414 . As a by-product of generating optical output, the laser diode bar  446  generates heat that is conducted to at least one wall of the zone  450  of the HEX  482 ′. High-pressure coolant streams  475   a  and  475   b  are supplied by the inlet port  454  to the respective driving nozzles  440   a  and  44   b  where they forms jet directed into the throat portion of the pump bodies  434   a  and  434   b  ( FIG. 5 ). The jets respectively entrain coolant in the suction chambers  428   a  and  428   b , and pump it. Streams  476   a  and  476   b  containing both the jet flow and the pumped coolant exit their respective fluid dynamic pumps  420   a  and  420   b  through their respective discharge ports  464   a  and  464   b  into the zone  450 . After acquiring heat in zone  450 , coolant flows through passages  438   a  and  438   b  respectively as streams  476   a ′ and  476   b ′. At the end of each passage  438   a  and  438   b  each respective flow  476   a ′ and  476   b ′ is divided into respective streams  472   a  and  474   a , and  472   b  and  474   b . Stream  472   a  flows through the passage  436   a  into the suction chamber  428   a , and stream  472   b  flows through the passage  436   b  into the suction chamber  428   b . Streams  472   a  and  472   b  each flow into the passage  439  and through orifice  452 ′ into the outlet port  456 . 
         [0036]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” and “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0037]    HTF suitable for use with the subject invention include 1) liquids such as water, aqueous solution of alcohol, antifreeze, and oil, 2) gases including air, helium, natural gas, and nitrogen, and 3) vapors such water steam, Freon, and ammonia. 
         [0038]    The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
         [0039]    Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. In addition, the term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
         [0040]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the present invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. Thus, the scope of the present invention is not limited to the disclosed embodiments.