Coherent light generators – Particular active media – Active media with particular shape
Reexamination Certificate
2001-05-22
2003-05-20
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Active media with particular shape
C372S034000, C372S072000
Reexamination Certificate
active
06567452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to systems and methods for pumping solid-state slab lasers.
2. Description of the Related Art
Doped-insulator slab lasers are solid state lasers that are used in a variety of applications requiring moderate to high optical output power. The slab lasing material is typically comprised of a host crystal doped with an ion, such as, for example, ytterbium doped yttrium aluminum garnet (Yb:YAG). High power slab lasers employing lasing media with high aspect ratio slab configurations have traditionally been optically pumped through the broad slab faces with one or more linear flashlamps and have been cooled either by forced convection or conduction through the same faces. (Pumping is the process by which an active (lasing) medium is excited to achieve a population inversion. The population inversion is a condition by which energy is stored in the medium with sufficient gain to cause the medium to lase. See
Solid-State Laser Engineering, Second Edition
by Walter Kocchner, published 1988 by Springer-Verlag, Berlin, Germany.)
Face pumping has been necessary due to the low brightness of the flashlamp pumping sources, which have precluded pumping through the smaller area ends and edges of the slab. Face cooling is advantageous in high aspect slab lasers to minimize the conduction path through the lasing medium for thermal energy produced by intrinsic and extrinsic nonradiative processes within the medium (quantum defect, quenching, excited state absorption and/or up conversion). Minimizing the thermal conduction path is important to minimize the average temperature and temperature gradient within the lasing medium, as is discussed later. Because they require optical pumping and cooling through the same slab faces, the traditional flashlamp-pumped slab lasers are necessarily complicated in their design, requiring optically transparent cooling means.
More modern slab lasers are optically pumped by narrow band, high brightness laser diode arrays. The higher brightness of these laser diode pump sources relative to flashlamps allows a high aspect ratio slab to be pumped either through the narrow edges of the slab in directions generally transverse to the laser beam or through the narrow ends of the slab in directions generally collinear with the laser beam. Edge and end pumping of the slab allows the faces to be cooled without constraining the cooling system to also transmit the pump beam into the slab, thereby simplifying the design. The pumping configuration that results in the optimum absorption and distribution of pump energy in the lasing medium is preferred.
A configuration capable of achieving both high absorption and uniform distribution of pump energy in an edge-pumped geometry is described in commonly assigned patents entitled Laser Pump Cavity Apparatus with Integral Connector and Method, issued Apr. 25, 2000 to R. W. Byren et al., U.S. Pat. No. 6,055,260 (Attorney Reference No. PD 970064 and referred to hereinafter as the '064 application) and Laser Pump Cavity Apparatus with Improved Thermal Lensing Control, Cooling, and Fracture Strength and Method, issued Oct. 26, 1999 to R. W. Byren et al., U.S. Pat. No. 5.974,061 (Attorney Reference No. PD 970226 and referred to hereinafter as the '226 application), the teachings of both of which are incorporated herein by reference.
The approach described in the '064 application requires a cladding layer formed in a hyperbolic cylindrical shape that is thicker at the edge of the slab than in the center to obtain the proper optical concentrator performance. If the outer surface of the cladding layer is cooled to a constant heat sink temperature, the difference in thermal conductance across the width of the slab due to the change in the cladding thickness produces a nonuniform temperature gradient within the slab. This, in turn, introduces nonuniform thermal lensing and stress birefringence, which are difficult to compensate.
In addition to improving pump efficiency and uniformity, it is essential to efficiently remove the large amount of heat that is generated within the lasing medium.
An increase in the operating temperature within the lasing medium reduces the population inversion that can be achieved for a given level of pumping, thereby reducing efficiency. Reducing the operating temperature of the laser increases the gain and extraction efficiency. More specifically, reducing the operating temperature increases the stimulated emission cross-section of the active lasing medium. This lowers the saturation fluence of the active lasing region, which makes it easier to extract the stored energy for gain-switched and Q-switched systems, without damaging the optical coatings at the exit surfaces of the bulk lasing material. Similarly, reducing the temperature also lowers the saturation intensity, which makes it easier to extract power for continuous and high pulse rate systems without optical damage.
Temperature gradients cause mechanical stress within the lasing medium. When the medium is stressed, the crystal becomes birefringent, and energy in the laser beam if polarized in a direction that is neither along nor orthogonal to the stress gradient will be converted from the desired polarization to an undesired polarization as the beam propagates along the beam axis through the crystal. This induced birefringence is undesirable for many applications. For example, when the crystal faces are cut at the Brewster angle to extract energy of a desired polarization, energy converted to an orthogonal polarization will be internally reflected, resulting in a loss of output efficiency.
As another example, in a typical multi-pass master oscillator power amplifier laser system that uses a straightforward polarizer and 90° polarization rotation means to separate the master oscillator input beam from the amplified output beam, depolarization of the beam due to thermal stress induced birefringence in the amplifier will cause a portion of the output beam to feed back into the master oscillator, potentially damaging the oscillator components, reducing the output power, and imprinting on the output beam a nonuniform intensity profile which adversely affects beam quality. It is therefore desirable to maintain a one-dimensional temperature gradient within the slab and orient the polarization of the beam to be collinear with or orthogonal to this gradient in order to avoid depolarization due to thermal stress birefringence. Temperature gradients also cause refraction or bending of the laser beam as it enters, propagates through, and exits the lasing medium. Physical distortion of the lasing medium due to nonuniform thermal expansion produces a lensing effect at the entrance and exit surfaces of the lasing medium. The index of refraction of the medium, which is a function of both the temperature and stress within the medium, varies across the beam producing graded-index lensing within the medium. If the temperature gradient is one dimensional within the slab, i.e. isotherms are parallel to slab faces, the thermal lensing effects can be compensated by means available in the present art. For example, conventional cylindrical lenses can be used to provide a first order correction. The beam can also be guided by total internal reflection at the faces, as described in the above-mentioned co-pending applications, minimizing the beam spreading within the slab. It is, therefore, desirable to maintain a one-dimensional temperature gradient within the slab in order to permit thermal lensing compensation by available methods.
In side-pumped laser cavity configurations, heat is removed from the lasing medium by cooling mechanisms applied to the broad faces of the slab. Prior art methods for cooling the broad slab faces include air cooling, liquid cooling systems (forced convection and impingement) and conductive cooling through metal heat sinks. Air cooling is limited to lower power lasers due to relatively poor thermal
Byren Robert W.
Rock David F.
Tsai Cheng-Chih
Alkov Leonard A.
Ip Paul
Lenzen, Jr. Glenn H.
Raufer Colin M.
Raytheon Company
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