We report on the results of an experimental and theoretical investigation of the relevant performance attributes of
Yb:YAG thin disk gain elements for use in high brightness resonators. We compare the laser operation, under extremely
high pump and laser power densities, of crystalline thin disks to ceramic disks with undoped, amplified spontaneous
emission (ASE) suppressing caps. The disks were operated with impingement cooling either directly on the back (high
reflecting) surface or on heat spreaders. Although high optical efficiency was generally maintained, we found a marked
difference in both the strength of ASE and the thermo-optical performance of the disks.
A scaleable diode end-pumping technology for high-average- power slab and rod lasers has been under development for the past several years at Lawrence Livermore National Laboratory (LLNL). This technology has particular application to high average power Yb:YAG lasers that utilize a rod configured gain element. Previously, this rod configured approach has achieved average output powers in a single 5 cm long by 2 mm diameter Yb:YAG rod of 430 W cw and 280 W q-switched. High beam quality (M<SUP>2</SUP> equals 2.4) q-switched operation has also been demonstrated at over 180 W of average output power. More recently, using a dual rod configuration consisting of two, 5 cm long by 2 mm diameter laser rods with birefringence compensation, we have achieved 1080 W of cw output with an M<SUP>2</SUP> value of 13.5 at an optical-to-optical conversion efficiency of 27.5%<SUP>2</SUP>. With the same dual rod laser operated in a q-switched mode, we have also demonstrated 532 W of average power with an M<SUP>2</SUP> less than 2.5 at 17% optical-to-optical conversion efficiency. These q-switched results were obtained at a 10 kHz repetition rate and resulted in 77 nsec pulse durations. These improved levels of operational performance have been achieved as a result of technology advancements made in several areas that will be covered in this manuscript. These enhancements to our architecture include: (1) Hollow lens ducts that enable the use of advanced cavity architectures permitting birefringence compensation and the ability to run in large aperture-filling near-diffraction-limited modes. (2) Compound laser rods with flanged-nonabsorbing-endcaps fabricated by diffusion bonding. (3) Techniques for suppressing amplified spontaneous emission (ASE) and parasitics in the polished barrel rods.
This is a review of tl Chemical Oxygen - Iodine Laser (COIL) technology, paying particular attention to historical perspectives in terms of highlighting the unique characteristics of COIL that have allowed it to develop into a high efficiency and a very forgiving system. Chemical pumping of lasing transitions has been around for a long time (HCI Lasers in 1965 and HF/DF Lasers in 1972), but most of the reaction energy ends up as heat in the lasing medium, generating the associated system engineering difficulties. COIL, on the other hand, has a very high efficiency in converting reaction energy to electronic excitation, and that fraction of the reaction energy which is released as heat does not end up in the laser cavity, but remains in the oxygen generator. This has tremendous engineering advantages for high efficiency systems.
Coherent ladar detection which is virtually free of oscillator instabilities (random phasing effects) may be obtained through the use of two local oscillator frequencies and subsequent four-wave mixing for heterodyne square-law detection. Such a stabilized four-wave system will allow for more compact, lighter, simpler, and shorter-wavelength coherent ladars.
A simple model for predicting the small signal gain as a function of flow direction is presented. It basically couples simplified kinetics and fundamental gas dynamics and allows for the heat release produced by the water deactivation of excited oxygen and iodine. The numeric results lead to a temperature rise in the gas, which causes a substantial decay of the small signal gain in flow direction. An analytic solution is also derived and results of both approaches compare favourably with experimental data.
The present status of the chemical oxygen-iodine laser is discussed. The pertinent processes occurring in the chemical O2 generator, the O2(1Delta) transport region, and the nozzle are reviewed. The energy transfer kinetics, laser gain, and the performance and device efficiency are examined.
A simple model for predicting the small signal gain as a function of flow direction will be presented. The small signal gain was measured on the Weapons Laboratory Rotocoil 0<sub>2</sub>/I<sup>*</sup> gain medium. The characteristics observed in the experiment show a decrease in the small signal gain as a function of distance from the nozzle exit plane. Further results indicated that the small signal gain decreased with time and that the gain increased when the cold trap was turned on. All of these effects suggest a temperature dependence of the small signal gain. The approach presented in this paper is to develop a simple model which includes a simplified kinetics model and the gas dynamics for the flowing medium. An analytic solution to the model equations is also derived. These models account for the reduction in small signal gain in the flow direction due to heat release into the cavity when compared to the Rotocoil small signal gain data. The results show that the rise in gas temperature in the flowing 0<sub>2</sub>/I<sup>*</sup> medium is primarily due to water deactivation of the I<sup>*</sup> and the O<sub>2</sub>(1Δ) plus I<sup>*</sup> pooling leading to water deactivation of <sup>1</sup>Σ. Such temperature rise in the flowing medium causes the small signal gain to decay substantially in the flow direction due to the square root of the temperature dependence in the stimulated emission cross section, the shift in the equilibrium constant with temperature and the decrease in density which is inversely proportional to the temperature.