Coherent light generators – Particular temperature control – Liquid coolant
Reexamination Certificate
2000-02-16
2002-01-15
Font, Frank G. (Department: 2877)
Coherent light generators
Particular temperature control
Liquid coolant
C372S072000, C372S075000, C359S333000
Reexamination Certificate
active
06339605
ABSTRACT:
TECHNICAL FIELD
This invention relates to solid-state lasers, and more particularly to an active mirror amplifier (AMA) laser having a generally thin laser gain medium attached by a pressure differential to a rigid substrate cooled by a flow of cooling medium through microchannels incorporated therein, thus enabling a construction of a laser capable of producing high-average power with good beam quality.
BACKGROUND OF THE INVENTION
High-power solid-state lasers are finding increasingly important applications in defense and commercial applications. The most recent growth in solid-state laser business can be attributed to the introduction of diode pumping. Advantages of solid-state lasers are all-electric operation, wavelength suitable for transmission through optical fibers, continuous duty capability, high wall-plug efficiency, and the possibility of engineering a high-power device into a small, lightweight package. For these reasons the commercial market for high-power (i.e., greater than 200 watts) solid-state lasers has grown steadily for the last decade. Potential military applications have also become more important in recent years. Most applications of high-power solid-state lasers require good beam quality. Beam quality (“BQ”) is a measure of how well the laser beam can be focused to a spot. BQ is critically important in laser weapons where concentrated optical energy is required to thermally damage a specific target (e.g., a missile in flight). Other military applications also require good BQ for certain types of laser illuminators and other imaging-related uses. Similarly, laser beams with good BQ are required for industrial applications to produce high quality, deep penetration welds and precision cuts at increased speeds. Furthermore, availability of a low-cost, high-power solid-state laser with good BQ would open the door to extensive new applications, such as welding of aluminum in manufacture of light-weight automotive bodies, and cutting and drilling of aluminum and titanium in the production of aircraft.
Present day solid-state lasers extract coherent light from an inverted population of neodymium, ytterbium, or other suitable ions doped into crystals or glass. Population inversion is achieved by optically exciting dopant ions by absorption of optical radiation at wavelengths shorter than the laser wavelength. This process is commonly referred to as “pumping.” Depending on the excitation source and the laser ions used, much of the optical pump radiation is converted into heat and deposited into the solid-state laser medium. Thus, for continuous operation, waste heat must be removed in real time by cooling selected surfaces of the laser medium. Because solid-state laser media are dielectrics that typically have a low thermal conductivity, a significant thermal gradient is created between the hot interior and the cooled outer surfaces. This causes a change in the index of refraction (thermal lensing), thermal expansion and mechanical stress in the medium, medium depolarization, detuning, and other undesirable effects, with possible consequences of BQ degradation, reduced laser power, and possibly even fracture of the solid-state medium. In particular, optical distortions caused by temperature gradients transverse with respect to the laser beam optical axis are known to reduce BQ.
Consequently, efficient heat removal and reduction of thermal effects caused by temperature gradients across the active area of the laser medium usually dominate design considerations for high-average power continuous wave (CW) solid-state lasers. Recently introduced pumping by narrow band radiation from laser diodes matched to absorption lines of dopant ions greatly reduces the amount of waste heat dissipated in the laser medium. Nevertheless, major heat-related problems in existing solid-state lasers are limiting their operation at high-average power and good beam quality.
With prior art solid-state, high-power lasers, several techniques have been introduced to reduce temperature gradients and/or mitigate their effects on laser operation. Chernoch, in U.S. Pat. No. 4,233,567 (1980), discloses a laser medium configured as a slab cooled on large flat sides and with the laser beam traversing the slab in zigzag fashion, as shown in
FIGS. 1
a
-
1
c
. In this concept, thermal gradients in the transverse direction parallel to the large flat sides of the solid-state medium are essentially eliminated and the gradient in the other transverse direction is reduced. Furthermore, the zigzag path of the laser beam through the slab generally averages out local thermal gradients. However, despite the inherent advantages (at least on a conceptual level) of a zigzag slab to mitigate thermal problems and nearly 20 years of engineering development, the acceptance of this type of system has been slow. The reasons for this include low efficiency, residual distortion (especially near the slab ends) which limit BQ, high cost of fabrication, and power scaling limitations.
Another class of solid-state laser amplifiers known as “active mirror amplifier” (AMA) has been investigated in the prior art. Originally disclosed by Chernoch in U.S. Pat. No. 3,525,053 (1970), large-scale laser systems employing AMA technology have been constructed for inertial fusion research. See for example, J. A. Abate et al.,
Active Mirror: A Large
-
Aperture Medium
-
Repetition rate Nd:Glass Amplifier
, Applied Optics, volume 20, no. 2, pages 351-361, (1981). In the AMA concept, a single large aspect ratio, free-suspended disk is optically pumped and cooled from the back side, and the laser radiation to be amplified enters from the front, as shown in FIG.
2
. The front face of the disk has an anti-reflection coating for the laser radiation, whereas the backside has a dichroic coating, which is highly reflective for the laser radiation and transparent to the pump radiation. Flashlamp pumping is commonly used with the AMA. Advantages of the AMA are:
The pump radiation source can be closely coupled to the laser gain medium;
The laser gain medium is uniformly pumped across the gain profile;
Surfaces receiving the highest heat deposition are cooled by direct contact with flowing liquid;
Double pass configuration compensates for thermally induced birefringence; and
Suitability for circularly polarized beams.
Lasers using AMA were mainly single shot (low-average power) systems (such as used in inertial confinement fusion research) where real-time heat removal was not required. Prior art lasers using AMA are entirely unsuitable for high-average power operation, however, because of several reasons. For one, to ensure structural rigidity, the solid-state disk must be made relatively thick (i.e., several centimeters), which impedes heat extraction. Another reason is that one sided heating of the free-suspended disk causes mechanical distortion resulting in a wavefront error. Yet another reason is that coolant pressure must be low to avoid distortion of the disk, resulting in low flow rates and low heat transfer coefficients. Still further, coolant flow induces vibrations in the disk. Previous attempts to mitigate these problems and increase the average power output of a laser using an AMA have been met with encouraging but limited results.
In recent years, the AMA concept has been revived in the form of a Thin Disk Amplifier (TDA) introduced by Brauch et al in U.S. Pat. No. 5,553,088. The TDA offers significant improvement over prior art lasers using an AMA as it allows operation at significantly higher average power (several hundred watts) and with good BQ. See, for example, H. Hugel and W. L. Bohn,
Solid State Thin Disk Laser
, SPIE Proceedings, volume 3574, pages 15-28, (1998). The TDA, as shown in
FIG. 3
, consists of a thin disk (i.e., a crystal) of suitable solid-state laser medium (e.g., Nd:YAG, Yb:YAG) attached to a heat sink by a thermally conductive bond. The rear face of the disk has an optical coating exhibiting very high reflection at the laser wavelength and the pump radiation wavelengths, whereas the front face has a coating
Font Frank G.
Harness & Dickey & Pierce P.L.C.
Rodriguez Armando
The Boeing Company
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