EP1719221A1

EP1719221A1 - Solid state laser with hermetically sealed pump diodes

Info

Publication number: EP1719221A1
Application number: EP04818061A
Authority: EP
Inventors: Jan Vetrovec; Michael Savich
Original assignee: Boeing Co
Current assignee: Boeing Co
Priority date: 2003-12-31
Filing date: 2004-12-21
Publication date: 2006-11-08
Also published as: JP2007517402A; WO2005067112A1

Abstract

A module for a solid-state laser module includes a laser diode array adapted to pump optical radiation, a heat exchanger coupled to the laser diode array, a coolant manifold that is formed in part through the heat exchanger and is capable of supplying pressurized cooling fluid to the heat exchanger, a hermetic enclosure that has an interior portion in which the laser diode array, the heat exchanger, and the laser gain medium are enclosed, and is adapted to allow for pressurization of the interior portion at a level at least as high as the pressurized cooling fluid, and a pressurant supply line extending through the hermetic enclosure to pressurize the interior portion. The module may also include a laser gain medium that is adapted and positioned to receive the optical radiation to produce an amplified laser beam from an incident laser beam.

Description

SOLID STATE LASER WITH HERMETICALLY SEALED PUMP DIODES

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of contract number F49620-02-C-0035 awarded by the U.S. Air Force Office of Scientific Research.

TECHNICAL FIELD

[0002] The present invention generally relates to solid-state lasers, and more particularly to solid-state lasers that employ diodes, and methods for preventing laser diode coolant leakage.

BACKGROUND

[0003] A diode-pumped solid-state laser (SSL) typically includes a plurality of semiconductor diodes such as InGaAs or AlGaAs laser diodes. Laser-diode pumping employs semiconductor lasers as a replacement for incoherent optical pump sources such as flash lamps or arc lamps, and is among the most active areas of current laser development. As diodes became less expensive, more powerful and longer lived, diode pumping developments have rapidly increased. Diode pumping results in a relatively compact laser system and provides the advantages of reduced cooling, reduced power consumption, improved wall-plug efficiency, high-average poser (HAP) operating conditions, and very good beam quality.

[0004] Individual laser diode elements generate approximately 1 watt of average optical output. A 10 kW high-average power solid-state laser (HAPSSL) may require between about 20 and about 40 kW of optical pump power, or between about 20,000 and about 40,000 diodes. A HAPSSL device in the 1 to 10 kW range is used globally as a cutting and welding tool in the automotive, aerospace, appliance, and shipbuilding industries, for example. Future 10 to 100 kW industrial HAPSSL devices are expected to enable new applications such as rock drilling for oil and gas exploration. Consequently, the commercial demand for the HAPSSL is growing at an exponential rate.

[0005] With the high demand for the HAPSSL, there is also a continuous need for efficient manufacturing methods for lasers having prolonged working lives. Diodes for pumping high- average power solid-state lasers are usually manufactured in one-dimensional linear arrays. The diode arrays typically comprise at least ten diodes and are equipped with electrical terminals to generate between about 10 and about 100 W of average power. To prevent the diode arrays from overheating, the arrays are typically mounted on water-cooled heat exchangers. The combination of the diode array and the heat exchanger is commonly known as a semiconductor bar or diode bar. FIG. 1 is an isometric view of a common diode bar 14 including a diode array 10 mounted on a heat exchanger 11 having two through holes that serve as ports 12 through which coolant passes. O-rings 13 are provided at the coolant port peripheries to prevent coolant leaks when the ports 12 interface another surface. As will be further discussed below, coolant leakage can reduce the diode working life and, in turn, reduce the working life of the associated HAPSSL.

[0006] Diode bars mounted on heat exchangers can be further stacked to produce two- dimensional arrays, commonly known as stacks, with a multi-kW average power output. FIG. 2 is a cross sectional view of a coolant manifold/power bus 15. The coolant manifold/power bus 15 includes a diode stack 16 sandwiched between two heat exchangers 15a, 15b that have coolant ports 17a, 17b formed therein. The coolant ports 17a, 17b are in communication with the ports 12 in the diode bar 14 to form a coolant manifold. When a diode stack is assembled, the o- rings 13 located at the interface between adjacent diode bars 14 are compressed to prevent coolant loss. A typical 10 kW HAPSSL requiring 40 kW of diode pump power may contain as many as 1,000 40W diode bars and 2,000 o-rings. A diode stacks 16 is expensive, and if the coolant manifold passing through the diode stack 16 is not kept intact, coolant may leak and permanently damage facets of the semiconductor material and render the diodes inoperable.

[0007] One recent approach to reduce the risk of coolant leakage involves constructing diode stacks with the diode bars permanently attached to each other without intermediately-disposed o- rings. However, such diode stacks incorporate between about ten and about twenty diode bars, have a limited structural reliability, and a relatively high electrical failure risk. Because the diode bars are permanently attached to one another, failure of one diode bar requires replacement of the entire stack. The ability to replace a single faulty diode bar in a large stack outweighs the risks of o-ring failure and the resulting diode damage. Nevertheless, there is a need to reduce the number of faulty diode bar failures due to coolant leakage. Further, the HAPSSL is associated with other coolant line connections that are in the proximity of diodes and other sensitive components such as gain medium coatings, laser mirrors, electronic controls, and electronic sensors, all of which are susceptible to significant damage when exposed to coolant, especially water.

[0008] Accordingly, it is desirable to reduce the susceptibility for diodes and other sensitive components to be exposed to coolant in the SSL and diode laser (DL) environment. In addition, it is desirable to provide an apparatus and method for reducing the high pressure differential between the diode coolant and diode bar exterior, and thereby reduce the potential for o-ring failure or other coolant manifold failures. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

[0009] An assembly is provided for a laser diode. The assembly comprises a laser diode array adapted to pump optical radiation, a heat exchanger coupled to the laser diode array, a coolant manifold that is formed in part through the heat exchanger and capable of supplying pressurized cooling fluid to the heat exchanger, a hermetic enclosure that has an interior portion in which the laser diode array and the heat exchanger are enclosed and being adapted to allow for pressurization of the interior portion at a level at least as high as said pressurized cooling fluid, and a pressurant supply line extending through the hermetic enclosure to pressurize the interior portion.

[0010] A module is also provided for a solid-state laser module. The solid-state module comprises a laser diode array adapted to pump optical radiation, a heat exchanger coupled to the laser diode array, a coolant manifold that is formed in part through the heat exchanger and is capable of supplying pressurized cooling fluid to the heat exchanger, a laser gain medium that is adapted and positioned to receive the optical radiation to produce an amplified laser beam from an incident laser beam, a hermetic enclosure that has an interior portion in which the laser diode array, the heat exchanger, and the laser gain medium are enclosed, and is adapted to allow for pressurization of the interior portion at a level at least as high as the pressurized cooling fluid, and a pressurant supply line extending through the hermetic enclosure to pressurize the interior portion.

[0011] An assembly is also provided for a solid-state laser assembly. The assembly comprises a laser diode array adapted to pump optical radiation, a heat exchanger coupled to the laser diode array, a coolant manifold that is formed in part through the heat exchanger and is capable of supplying pressurized cooling fluid to the heat exchanger, a hermetic enclosure that has an interior portion in which the laser diode array and the heat exchanger are enclosed and is adapted to allow for pressurization of the interior portion at a level at least as high as the pressurized cooling fluid, a pressurant supply line extending through the hermetic enclosure to pressurize the interior portion, and a laser gain medium adapted and positioned outside the hermetic enclosure to receive the optical radiation to produce an amplified laser beam from an incident laser beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and

[0013] FIG. 1 is an isometric view of a common diode bar that can be used as part of the present invention, including a diode array mounted on a heat exchanger having two through holes that serve as ports for coolant to pass through;

[0014] FIG. 2 is an isometric view of a coolant manifold/power bus that can be used as part of the present invention, including a diode stack;

[0015] FIG. 3 is a cross sectional view of a laser amplifier module according to an embodiment of the present invention, including a hermetic enclosure in which a diode array is enclosed to provide optical radiation to a laser gain medium;

[0016] FIG. 4 is a cross sectional view of a laser amplifier module according to an embodiment of the present invention, including a diode array providing optical radiation to a laser gain medium, both of which are disposed inside a hermetic enclosure; [0017] FIG. 5 is a cross sectional view of a laser oscillator according to an embodiment of the invention, including a diode array providing optical radiation to a laser gain medium that is placed in a pathway between an end mirror and an outcoupling mirror, all of which are disposed inside a hermetic enclosure; and

[0018] FIG. 6 is a cross sectional view of a diode laser assembly according to an embodiment of the invention that is similar to that shown in FIG. 3, with the exception of having optical radiation produced by a diode array being directed onto a processed material rather than a laser gain medium.

DETAILED DESCRIPTION

[0019] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

[0020] One embodiment of the invention utilizes a hermetically sealed and pressurized pump container that houses a source of optical radiation for a laser, such as a semiconductor laser diode. The diode may be mounted on a substrate that is attached to a heat exchanger, and functions to pump a laser gain medium to a laser transition. The pressurized container greatly reduces the risk of coolant leakage presented by the hundreds or thousands of o-ring joints in coolant channels in an HAPSSL semiconductor diode stack, and can also be used to protect diode arrays in a DL.

[0021] Turning to FIG. 3, a cross sectional view of a laser amplifier module 100 is depicted, including a diode array 66 providing optical radiation 36 to a laser gain medium 26. The diode array 66, as used herein, includes a diode bar but may be any usable ensemble comprising one or more laser diodes. The diode array 66 may be mechanically and/or electrically connected and equipped with electric terminals 32. The laser gain medium 26, as used herein, refers to an optical material having a host lattice doped with suitable ions that are pumped to a laser transition. Although the present invention is not limited to a specific lasing material or a specific pump source, exemplary host lattice materials are yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), gadolinium scandium gallium garnet (GSGG), lithium yttrium fluoride (YLF), yttrium vanadate, phosphate laser glass, silicate laser glass, and sapphire.

[0022] The laser gain medium may be formed to a variety of shapes including but not limited to rods, slabs, disks, and fibers. The laser gain medium may also be a composite construction comprising doped and undoped sections, or sections doped with different ions. Suitable dopants for the lasing medium include but not limited to Ti, Cu, Co, Ni, Cr, Ce, Pr, Nd, Sm, Eu, Yb, Ho, Dy, and Tm. The ions that dope the laser gain medium are pumped using a laser transition, and absorb significant portions of the incident optical radiation 36. The laser gain medium 26 can function in this fashion as an amplifier to an incident laser beam 64.

[0023] The diode array 66 is located in the enclosure 22 which is hermetically sealed and filled with gas. Suitable filler gases include but are not limited to air, nitrogen, helium, argon, and other generally non-reactive gases. To prevent possible water condensation on the diode facets, the filler gas should be largely free of water vapor. One way to reduce the amount of moisture inside the enclosure 22 is to place a suitable desiccant therein.

[0024] The enclosure 22 has a diode radiation window 28 through which optical pump radiation 36 is transmitted from a diode array 66 to the laser gain medium 26. The diode radiation window 28, as used herein, is a component suitable for transmitting optical radiation generated by laser diodes without significant reflection and attenuation. Depending on the wavelength for the transmitted radiation, exemplary materials suitable for fabricating the window 28 include optical glass, fused silica, and sapphire. Faces of the window 28 may be coated with coatings that are anti-reflective at the laser diode wavelength. In the present embodiment, the diode radiation window 28 is made of one or more materials that are highly transparent at the diode wavelength. The diode radiation window 28 is furnished with one or more antireflection coatings to reduce losses by reflection in an exemplary embodiment of the invention. The diode radiation window 28 is sufficiently thick to avoid excessive stresses due to a pressure differential between the enclosure interior and the external environment.

[0025] The diode array 66 is further provided with electric conductors 32 that are inserted through the enclosure wall by means of a suitable feedthrough 34. Semiconductor diode lasers are typically about 30% to about 50% efficient in converting electric energy into optical radiation suitable for laser pumping. The electric energy that is not converted to optical radiation is converted into heat that must be removed to prevent the diode array 66 from overheating. To control the diode array temperature, a coolant is fed into the enclosure 22 by at least one coolant inlet 54 and removed through at least one outlet 56. Each coolant inlet 54 and coolant outlet 56 is inserted through the enclosure wall by means of a suitable feedthrough 58, and is fed into the diode array. The coolant should have a low viscosity, high heat conductivity, and high heat capacity. Suitable coolants may include deionized water, a mixture of water and alcohol, and suitable chlorofluorocarbons such as certain members from the Freon™ family.

[0026] To overcome friction-related resistance to flow, coolant is fed into the diode array 66 under significant pressure, typically ranging between about 10 psi and 60 psi. The gas inside the enclosure 22 is pressurized to generally the same level as the coolant pressure, thereby removing or substantially reducing a pressure differential between the coolant and the enclosure interior. As a result, the possibility of a coolant leakage due to an imperfect or failed joint anywhere within the enclosure 22 is greatly reduced. Optionally, the gas inside the enclosure 22 may be maintained at a higher pressure than the pressure in the coolant manifold. In such a case, an imperfect or failed joint in the coolant manifold within the enclosure 22 would cause the pressurized gas to leak into the coolant manifold. Such a leak can be readily detected and the laser amplifier module 100 can be serviced. A pressure relief mechanism such as a burst disk or pressure relief valve may extend through an enclosure wall as a safety precaution in the event that the enclosure interior reaches a pressure greater than 200 psi, for example. Additional mechanisms can als.o be incorporated to control the interior pressure, such as a gas inlet regulator that prevents the container interior from exceeding a predetermined pressure.

[0027] The gas inside the enclosure 22 may be permanently sealed therein. Alternatively, the enclosure 22 may be in fluid communication with a gas source at a constant pressure via a pressurization line 24. The gas source may be equipped with a pressure regulator to maintain constant pressure in the enclosure 22.

[0028] A second embodiment of the invention that comprises a similar pressurized enclosure will be described next. The enclosure is adapted to further contain the laser gain medium, and is equipped with one or more windows suitable for injecting a laser from outside the enclosure into the laser gain medium, and for passing the amplified laser beam outside the enclosure. FIG. 4 is a cross sectional view of a laser amplifier module 200 according to the second embodiment of the invention. The module comprises the diode array 66 providing optical radiation 36 to the laser gain medium 26, both of which are disposed inside the enclosure 22b. [0029] The enclosure 22b is partially defined by laser windows 24 that allow a laser beam 64 to enter the enclosure 22b, become amplified by the laser gain medium 26, and then exit the enclosure 22b. As used herein, a laser window 24 refers to a component suitable for transmitting the laser radiation amplified by the laser gain medium 26 without significant reflection and attenuation. Depending on the transmitted radiation wavelength, exemplary laser window fabrication materials include but are not limited to optical glass, fused silica, and sapphire. The laser gain window faces may be provided with coatings that are anti-reflective at the laser wavelength. The window thickness is selected to avoid excessive deformations due to the pressure differential between the enclosure interior and exterior, as excessive mechanical deformations may distort the optical wavefront and reduce beam quality.

[0030] The processes of pumping a laser gain medium 26 and extracting laser power therefrom create a significant amount of unused energy that is absorbed and converted to heat energy by the laser gain medium 26. The waste heat can be removed by contacting the laser gain medium 26 with a heat sink, or immersing the laser gain medium in a suitable cooling fluid. FIG. 4 depicts the laser gain medium 26 in contact with a liquid-cooled heat exchanger 72 that is fed by a coolant inlet 74 and drained by a coolant outlet 76. The coolant inlet 74 and outlet 76 are formed through the enclosure wall using suitable feedthroughs 78.

[0031] As in the first embodiment, the enclosure 22b is hermetically sealed and filled with a dry gas. The filler gas pressure is about the same or higher than the coolant pressure in side the diode array 6 and laser gain medium heat exchanger 72. Under such conditions, the potential for a coolant leak into the enclosure interior is greatly reduced.

[0032] A third embodiment of the invention that comprises another similar pressurized enclosure will be described next. The pressurized enclosure is adapted to further contain the laser gain medium and laser oscillator components, and is equipped with one or more windows suitable for passing the amplified laser beam outside the enclosure. FIG. 5 is a cross sectional view of a laser oscillator 300 according to the third embodiment of the invention. The module 300 comprises the diode array 66 providing optical radiation 36 to the laser gain medium 26 placed in a pathway between an end mirror 46 and an outcoupling mirror 44, all of which are disposed inside the enclosure 22c. The enclosure 22c is equipped with a laser window 24 that allows a laser beam 64 extracted from a laser resonator to exit the enclosure. [0033] Finally, a fourth embodiment of the invention will be described, with reference being made to FIG. 6 which is a cross sectional view of a diode laser assembly 400. The fourth embodiment of the invention is similar to the first embodiment, with the exception of the optical radiation 36 produced by the diode array 66 being directed onto a processed material 82 rather than a laser gain medium.

[0034] Also, the fourth embodiment may include a lens 84 that is optionally used to concentrate the flux of optical radiation 36. In addition to the lens 84, a system of lenses or mirrors (not shown) can be employed, and the diode radiation window 28 can be adapted to function as a lens element.

[0035] The processed material 82 will typically at least partially absorb some of the incident optical radiation 36. Such absorption occurs on the processed material surface and/or interior, and the radiation is converted to heat energy. Heating materials by incident optical radiation is a common industry practice for such methods as heat treatment, surface cleaning, material modification, material removal, cutting, and welding, for example.

[0036] According to the fourth embodiment of the invention, the gas inside the enclosure 22d may be maintained at generally ambient pressure rather than matching the pressure inside the cooling circuit, thus allowing for a relatively large pressure differential between the inside and the outside of the coolant manifold. Although there is a larger risk for coolant leakage due to the relatively large pressure differential, dry filler gas reduces the possibility for moisture condensation on diode faces in the diode array 66. The dry filler gas is particularly advantageous when the diode coolant flows at sub-ambient temperatures to improve the diode electro-optical efficiency.

[0037] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.

Claims

1. A laser diode assembly, comprising: a laser diode array adapted to generate optical radiation; a heat exchanger coupled to said laser diode array; a coolant manifold, formed in part through said heat exchanger, and capable of supplying pressurized cooling fluid to said heat exchanger; a hermetic enclosure, having an interior portion in which said laser diode array and said heat exchanger are enclosed, and being adapted to allow for pressurization of said interior portion at a level at least as high as said pressurized cooling fluid; and a pressurant supply line extending through said hermetic enclosure to pressurize said interior portion.

2. The laser diode assembly according to claim 1, further comprising: a diode radiation window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit said optical radiation.

3. The laser diode assembly according to claim 1, further comprising: a diode radiation window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit and concentrate said optical radiation outside said enclosure.

4. A solid-state laser module, comprising: a laser diode array adapted to generate optical radiation; a heat exchanger coupled to said laser diode array; a coolant manifold, formed in part through said heat exchanger, and capable of supplying pressurized cooling fluid to said heat exchanger; a laser gain medium adapted and positioned to receive said optical radiation to produce an amplified laser beam from an incident laser beam; a hermetic enclosure, having an interior portion in which said laser diode array, said heat exchanger, and said laser gain medium are enclosed, and being adapted to allow for pressurization of said interior portion at a level at least as high as said pressurized cooling fluid; and a pressurant supply line extending through said hermetic enclosure to pressurize said interior portion.

5. The solid-state laser module according to claim 4, further comprising: a first laser window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit said incident laser beam to said laser gain medium; and a second laser window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit said amplified laser beam outside said hermetic enclosure.

6. The solid-state laser module according to claim 4, further comprising: a laser gain medium heat exchanger coupled to said laser gain medium; and a second coolant manifold, formed in part through said laser gain medium heat exchanger, and capable of supplying cooling fluid to said laser gain medium heat exchanger.

7. The solid-state laser module according to claim 4, further comprising: first and second mirrors disposed in said interior portion, being adapted and positioned to together form a resonator cavity having a laser pathway in which said laser gain medium is disposed.

8. The solid-state laser module according to claim 4, further comprising: a laser window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit said amplified laser beam outside said hermetic enclosure.

9. A solid-state laser assembly, comprising: a laser diode array adapted to pump optical radiation; a heat exchanger coupled to said laser diode array; a coolant manifold, formed in part through said heat exchanger, and capable of supplying pressurized cooling fluid to said heat exchanger; a hermetic enclosure, having an interior portion in which said laser diode array and said heat exchanger are enclosed, and being adapted to allow for pressurization of said interior portion at a level at least as high as said pressurized cooling fluid; a pressurant supply line extending through said hermetic enclosure to pressurize said interior portion; and a laser gain medium adapted and positioned outside said hermetic enclosure to receive said optical radiation to produce an amplified laser beam from an incident laser beam.

10. The solid-state laser assembly according to claim 9, further comprising: a diode radiation window, formed as part of said hermetic enclosure, and being adapted and positioned to transmit said optical radiation.