Compact, efficient, high gain amplifiers and laser resonators are needed in a variety of applications in which one requires high output laser powers in the smallest package possible. These applications include high power free space amplifiers for fibre oscillators [1] and amplifiers for short pulsed laser systems as a means to replace complex regenerative amplifiers [2]. The use of high power and high gain amplifiers enables a new class of compact high peak power and high average power laser sources for material processing, laser manufacturing, and medical and dental applications for the highest processing speeds possible.
Solid state laser technology has now advanced up to the point where the average powers and peak power of pulsed sources has reached a threshold where a number of new applications is now possible. These applications have been greatly limited in scope due to the lack of sufficiently compact, low cost, robust laser system technology to move beyond the demonstration phase. If the laser technology can be made robust and more cost effective a number of important applications are on the horizon.
Some of the most important applications involve pulsed laser applications. It is under pulsed conditions that is possible to delivery energy to a material quickly, strongly localized the absorbed energy, and thereby raise the temperature of the material to its highest point to drive ablation processes in reshaping the material to a desired product [3]. The process of laser ablation and cutting, laser marking, etc. are more efficient the more strongly localized the energy is within the material both spatially and temporally.
There have been numerous methods developed over the years to achieve the required power classes [4]. The challenge is achieving sufficiently high gain conditions as prescribed above without compromising laser brightness. As laser gain media are pumped higher and higher to achieve larger gains, by whatever power source, there is an increase in the temperature of the laser gain media as not all the power transferred to the laser gain media is extracted by the laser beam. Some form of cooling is required to prevent thermal damage of the laser gain media. The conditions of pumping and nonuniform cooling lead to a highly aberrated thermal lens inside the gain media as a consequence of the temperature dependence of the index of refraction [4, 5]. This is the so called mirage effect; as things get hot they expand and the index of refraction decreases for most materials.
Due to the nonlinear conditions of laser amplification, these thermal aberrations act in a similarly nonlinear fashion to clamp the attainable laser brightness for any given cooling/pumping condition. The laser beam quickly goes from diffraction limited TEM00 mode to higher order modes that can not be focused as tightly and thereby reduces the source brightness as well as greatly reducing the working distance over which the beam can achieve its tightest focus. This problem is further exacerbated in the pursuit of high gain amplifiers where one must focus the pump light to the smallest area possible to achieve the maximum gain per unit length for any given gain material. This latter condition can be readily understood. There is a particular stimulated emission cross section for any given atomic or molecular species that is responsible for the laser transition/laser action. By confining the excited species to the smallest area (largest number of excited states per unit area) the probability a photon from an incoming laser beam entering the gain medium stimulating a photon emission event (and avalanche amplification) increases accordingly.
The conditions of confinement of the exciting pump light to achieve the highest gain conditions increases the heat deposited per unit area and the associated thermal gradients such that the problem of thermal aberrations becomes compounded. In addition to this consideration, under the high gain conditions considered here, there is also the prospect of spontaneous stimulated emission triggering a photon avalanche that depletes the gain by a process known as accumulated stimulated emission or ASE [4]. This problem not only limits the extractable gain but also destroys pulse quality by adding long tails to the amplified pulse which as explained above is highly deleterious to avoiding accumulated thermal damage in laser processing.
The problem of ASE is most significant when the laser gain medium is being used as an amplifier; within a properly designed laser resonator these stimulated photons contribute to the laser beam rather than behave parasitically. Optimal designs of high gain amplifiers/gain media must specifically address thermal aberrations and ASE issues from diminishing beam quality and efficiency. Of these two issues, the thermal aberrations are most limiting with respect to power scaling and laser brightness.
The various methodologies that have been developed to deal with thermal aberrations can be reduced to a few basic concepts employing two general strategies. One strategy is to try to minimize the magnitude of the thermal aberrations themselves for example by employing designs that lead to faster heat transfer to keep the laser gain medium cooled [4], more uniform cooling to reduce index of refraction gradients in the transverse direction of beam propagation so that all parts of the laser beam experience the same index of refraction spatial profile [6, 7], and the use of cooling to reduce the magnitude of the change in the index of refraction with temperature by reducing the thermal expansion [8]. The other strategy is to use beam configurations for sampling the pumped gain region in such a way as to average out the thermal aberrations as much as possible such that all parts of the laser beam experience the same spatial profile for the index of refraction and thereby remove thermal lensing and aberration effects [9, 10].
Brauch et al. have disclosed the use of thin disc laser gain media in which thermal aberrations are removed for laser beams coming in perpendicular to a cooled surface [6]. Under uniform pumping conditions, there are no transverse components to the index of refraction spatial profile experienced by the laser beam in the gain medium for this configuration.
Wittrock [7] has used the concept of removal of thermal aberrations in which the laser beam to be amplified is brought in at a very small angle to the cooled surface, rather than normal to the surface as in the Brauch et al design [6], to enable cancellation of the thermal gradients upon reflection as first described by Alcock and Bernard [10] in the grazing incidence amplifier design. The relatively large acute angle to the surface normal enables a much longer interaction region with the gain media and fewer passes to achieve the same overall gain as the thin disc concept of Brauch et al.
It is noted that both the Wittrock and Brauch et al. designs require uniform pumping over the entire laser active solid to rigorously provide 1 dimensional cooling. If uniform pumping is not accomplished the concept fails to remove thermal aberrations. Furthermore, the condition of uniform pumping greatly reduces the power extraction efficiency while maintaining high brightness for the laser beam to be amplified.
Laser amplifiers with lower gain require more round trips to extract the same power as high gain systems and thereby experience more loss. In principle, it is possible to have 100% percent reflectors such that a low gain system requiring up to 10 passes or more to extract power has the same efficiency as a much higher gain system that only requires one pass. As a matter of practice, high gain systems are essential to reduce the dimensions, number of optical surfaces involved in beam transport into and out of the laser gain medium, and the overall path length traversed by the laser beam to be amplified to attain maximum stability and efficiency.
The Wittrock design is capable of higher overall gain per pass relative to the Brauch et al design, however it relies on completely uniform pumping of the laser active material. The laser beam to be amplified in this design concept is brought in appreciably perpendicular to the normal of the cooled surface such that the beam experiences all nonparallel components to the isotherms. In this case, the laser beam to be amplified or resonator beam must be significantly smaller in diameter than the uniformly pumped gain region to avoid diffraction losses and nonparallel isotherms from edge effects. The only way to increase the gain of the laser active solid in this design concept is to increase the pumping uniformly.
From a practical standpoint, the temperature rise in typical gain media with intimate contact with a heat sink reach elevated temperatures of tens to hundreds of degrees. The exposed surfaces with only air to dissipate the heat get to extremely high temperatures under these same pumping conditions and thermally fracture. Surface heating is a well known effect that limits laser power and it is for this reason there are patents explicitly covering the use of fused undoped end caps to reduce surface temperatures [11]. Diode pumped laser systems are designed specifically to avoid pump light near surfaces or to use undoped end caps to avoid surface heating and fracture for even modest laser powers and gains.
It would therefore be very advantageous to provide a laser amplifier which avoids the aforementioned difficulties and which can provide high gain and small thermal aberrations.