Cooling of high power laser systems

Details Cooling of high power laser systems Cooling of high power laser systems

2001-08-17

2003-12-23

Ip, Paul (Department: 2828)

Coherent light generators

Particular temperature control

Heat sink

Reexamination Certificate

active

06667999

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
N/A
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
Optically pumped lasers are well known in the art and are used as both laser oscillators and laser amplifiers (“pumped laser”). An optically pumped laser converts pump energy at a pump wavelength into a coherent electromagnetic wave (“laser energy”) at a second wavelength either as a free running laser oscillator, or under control of an input signal as a laser amplifier. Pump energy that is absorbed and not converted into laser energy becomes heat and must be removed.
A pumped laser is typically constructed of a laser gain material that has a low thermal conductivity. The action of pumping a laser gain medium with energy produces stimulated emission of laser energy within the laser gain medium. Different lasers have different operating characteristics. For example, lasers can be operated in a continuous wave (CW) mode or in a pulsed mode. Accordingly, as the amount of pump power increases, and the pulse rate increases, progressively larger amounts of heat will need to be removed from the pumped laser. Because the thermal conductivity of the typical pumped solid-state laser material is low, in a high power and/or high pulse rate pumped laser, heat will not be conducted away from the laser gain medium at a sufficient rate. As such, thermal damage to the pumped laser may occur. For example, optical distortion of the laser output energy due to thermal and stress related birefringence effects in the laser gain medium may occur. A high power pumped laser requires a solid-state laser gain medium having a large cross section to limit flux (W/cm
2
) and fluence (J/cm
2
) related damage and non-linear phenomena.
Typically, pumped lasers are surface cooled, that is, heat is conducted away from the surface of the pumped laser gain medium. In a conventional pumped laser configuration, the gain medium is configured as a rod, in which the length of the rod is larger than the diameter of the rod. In this conventional configuration, the thermal time constant, which is the time for the rod to achieve thermal equilibrium across the entire face, i.e., that the temperature of the rod is equal from the center of the rod to the edge of the rod is approximately proportional to the reciprocal of the diameter squared of the rod divided by a thermal diffusivity constant. Thus for high power applications in which larger diameters are required, efficient heat transfer away from the pumped laser gain medium is difficult to achieve. The approach being used currently is to rely on this slab-like cross section and side heat removal to achieve the required cross section and requisite heat transfer respectively. This asymmetry has several disadvantages from a materials standpoint and also from a beam quality standpoint.
Therefore it would be desirable to provide a pumped laser that includes a heat transfer system that is able to cool the pumped laser at high power and/or high pulse rates and energies over large effective cross sections.
BRIEF SUMMARY OF THE INVENTION
A method and apparatus for cooling a high power solid state laser oscillator or amplifier is disclosed in which a plurality of laser gain media that are configured such that the width of the media is greater than the thickness of the media, are sandwiched between pairs of optically transparent heat transfer media having high thermal conductivity. In this way, a shortened thermal path is created from the face surface of the laser gain media axially via the optically transparent heat transfer medium to an external heat exchange system. This provides an increased level of cooling of the interior of the laser gain media with large effective diameters and cross sections and avoids the deleterious effects caused by overheating of the laser gain media during high power and/or high pulse repetition operation. It also effectively avoids the highly asymmetric structures associated with various slab-like geometries.
In particular, an apparatus for cooling a laser gain element includes a laser element having a cross section with first and second surfaces and a longitudinal axis defined therebetween, and third and fourth surfaces and a transverse axis defined therebetween. The laser gain element has a longitudinal dimension and a transverse dimension wherein the transverse dimension is greater than the longitudinal dimension. A pump source is optically coupled to the laser gain element, the pump source providing pump energy at a pump wavelength, wherein when pumped by the pump source, the laser gain element emits laser energy at a laser wavelength. In one aspect of the invention, the pump source is optically coupled to the laser element parallel to the first axis. An optically transparent heat transfer medium (OTH) is optically and thermally coupled to the first surface of the laser element, wherein the OTH is substantially optically transparent at the laser and pump wavelengths of interest. A heat transfer system including a heat transfer medium and/or heat exchanger is thermally coupled to the OTH. In this way, the heat generated within the laser element flows into the OTH from the laser element substantially parallel to the longitudinal axis. In one aspect of the invention, the OTH is a layer of diamond.
In another embodiment, a laser is disclosed that includes a laser cavity having a first reflective surface and a first partially reflective surface. A plurality of laser elements are optically coupled to one another, each having a cross section having first and second axes and first and second surfaces parallel to the first axis. In addition, each of the plurality of laser elements includes first and second dimensions that parallel the first and second axes respectively wherein the first dimension is greater than the second dimension. Also, each of the plurality of laser elements is coaxial along the second axis. In one aspect the laser element is a solid state gain medium, wherein the solid state gain medium is Nd:YAG. The laser also includes a pump source optically coupled to the laser gain medium, the pump source providing pump energy at a pump wavelength. The plurality of laser elements disposed within the laser cavity between the first and second reflective surfaces and configured and arranged to receive pump energy and to generate a laser emission parallel to the second axis. The laser further includes a plurality of optically transparent heat transfer media (OTH), each OTH optically and thermally coupled to the first surface of a corresponding one of the plurality laser elements, wherein the laser emission passes through each OTH, the OTH being substantially transparent at the laser wavelength and the pump wavelength. The laser includes a heat transfer system and/or heat exchanger thermally coupled to the OTH In this way, the heat generated within the laser element flows primarily into the OTH from the laser element substantially parallel to the longitudinal axis, where it is removed and dissipated by the heat transfer system.
This laser can further include an index matching layer disposed between the first surface of the laser element and the OTH, wherein the index matching layer has an index of refraction that is the square root of the product of the index of refraction of the laser element and the index of refraction of the OTH, and wherein the index matching layer is comprised of Ta
2
O
5
.
In another embodiment a laser is disclosed that includes a plurality of laser elements optically coupled to one another. Each of the laser elements has a cross section having a cross section with first and second surfaces and a longitudinal axis defined therebetween, and third and fourth surfaces and a transverse axis defined therebetween. The laser gain element has a longitudinal dimension and a transverse dimension wherein the transverse dimension is greater than the longitudinal dimension. In one embodiment, the laser element is a solid state gain medium, and can be Nd:YAG. The laser also includes a laser cavity having

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