Erbium doped YAG is an intriguing laser material which lases directly at 1645nm when pumped at either 1473nm or
1532nm, all of which are in the eye-safe band. However, a laser made from this material is not particularly
straightforward to design. Er:YAG is a quasi-three-level system, which leads to strong temperature dependence.
Perhaps more importantly, a strong up-conversion process, which is dopant concentration dependent, effectively
produces a pump intensity dependence in the saturation intensity and other laser parameters. We present a detailed
study of the absorption coefficient and the gain as a function of the pump intensity, dopant concentration and crystal temperature. The results of this study will allow us to optimally design the laser.
In 2000, Textron Systems Corporation (TSC) initiated the development of an advanced diamond cooled solid-state laser concept suitable for ultra compact medium and high-power lasers. The resulting laser configuration is applicable to laser diode pumping and a wide variety of lasing materials. In order to further improve the performance and determine the limitation of this laser concept, the detailed physical understanding of the interface between diamond and YAG disks was identified as a critical issue. Numerical analyses had been conducted for investigating the thermal-mechanical interaction in the interface between the gain medium and the diamond disks when the lasing process is in progress. Following this analyses, a computer model has been developed to simulate the phenomena of light interaction with the active medium. Subsequently, this computer model has been applied to optimize the laser design, in which the performance in terms of efficiency and compactness for a diamond-cooled laser has shown significant improvements. The understanding of the thermo-mechanical/optical issues at the interface, in general, will be beneficial to a variety of solid-state laser design activities.
Erbium doped YAG, lasing at 1645 nm from a 1532 nm pump, is an intriguing alternative to wavelength shifted 1-micron lasers for eye-safe applications. In this paper, we will report on an end-pumped Er:YAG laser that employs diamond disks for heat extraction. Using an alternating arrangement of diamond and Er:YAG thin disks, heat flows from the gain material to the diamond along the optical axis and is then radially transmitted to a circulating cooling fluid at the perimeter of the diamond disks. This architecture allows larger diameter disks or rods to be used than conventional radial cooling architectures, thus allowing for higher powers through area scaling. This architecture also provides better beam quality for a given pulse energy. We have demonstrated excellent beam quality (M2=1.3) from a 10-Watt Er:YAG laser. We will also report on results of Q-switching and oscillator-amplifier experiments in a study of the well-known up-conversion process for relatively low dopant concentrations (0.5%-1.0% a.w. of Erbium).
In our diamond-cooled approach, thin disks of laser gain material, e.g., Nd:YAG, are alternated between thin disks of single crystal synthetic diamond whose heat conductivity is over 2000 W/m-°K. The gain medium is face-pumped (along the optical axis) by the output of laser diode arrays. This optical configuration produces heat transfer from Nd:YAG to the diamond, in the direction of the optical axis, and then heat is rapidly conducted radially outward through the diamond to the cooling fluid circulating at the circumference of the diamond/YAG assembly. This geometry effectively removes the heat from the gain material in a manner that permits the attainment of high power output with excellent beam quality.
This paper reviews the basic concepts of laser propulsion and summarizes work done to date using a 10 kW device. The paper describes a candidate megawatt class CO2 laser system which can be scaled relatively near-term to multi-megawatt power levels using demonstrated technology. Such a system would potentially be capable of launching micro-satellites into low earth orbits (LEO) at relatively low cost. Our projections indicate that payloads of about 1kg/megawatt are achievable. The long wavelength of a CO2 laser will require the use of a large aperture telescope and/or large effective beam capture area for the lift vehicle. We believe that these limitations, not withstanding, rep-pulsed CO2 in a blow-down configuration lasting 200-300 seconds could achieve the desired propulsion objectives. The laser would use a helium-free, nitrogen/carbon dioxide mixture to provide a very cost effective fuel.
In this paper, we summarize the performance of a diamond cooled diode pumped solid state (DPSS) Nd:YAG laser developed by Textron Systems Corporation (TCS). Over 50% intrinsic extraction efficiency at 50-Watts average power, equivalent to volumetric power extraction of 2000 W/cc, has been experimentally demonstrated. Beam quality (BQ) of less than 1.1 XDL has been measured in an oscillator configuration with the laser operating in TEM00 mode. With a simple passive thermal lens correction, BQ of less than 1.5 XDL was obtained from an oscillator amplifier arrangement at specific pump rates up to 1600 W/cc and 600 W/cc, respectively. By using an intra-cavity acousto-optic modulator, Q-switched laser pulses as short as 6 ns were obtained from the oscillator. This concept is amenable to scaling up to very high power levels. A conceptual design, using our validated database, for a 100-kW class laser is also presented.
The pulse separation and associated range ambiguity of a CO2 laser radar using a pulse burst waveform consisting of 1.3 ns mode-locked pulses spaced 40 ns apart was successfully doubled to 80 ns and 12 m respectively by suppressing every alternate pulse. This was done by sending the mode-locked laser output beam with 40 ns pulse spacing through a resonant Raman-Nath acousto-optic modulator driven at 18.75 MHz which is 1.5 times the drive frequency used to intracavity mode-lock the CO2 laser transmitted. The spatial filtering of the diffracted pulses results in the suppression of the alternate pulses to better than 15 dB. The throughputs for the unsuppressed pulses were greater than 95%. The laser radar capability in detecting large targets,
The axial separation distance between an intracavity resonant Raman-Nath acousto-optic modulator and the secondary mirror of an unstable resonator of a TE CO2 laser has strong effects on the mode-locking efficiency. Experimental measurements showed that the mode-locker laser pulse width was a periodic function of the separation distance with a period of about 18 mm. Preliminary simple analyses using a two-beam interference model yield a periodic sine-squared acousto-optic diffraction intensity distribution as a function of separation distance with the mode-locked pulse width varying as the inverse of sine to the fourth power of the diffraction intensity. The observed pulse width was in good agreement with the calculated value using the above interference theory.
We have demonstrated a 15 dB lidar receiver detection sensitivity improvement by employing an Er3+-doped fiber preamplifier as the input to an InGaAs P-I-N photodiode. This eye-safe, 1550 nm detection system can significantly improve lidar system performance. It is shown that it is comparatively easier at 1550 nm than at 1064 nm to achieve a higher detection sensitivity for a lidar system using a fiber preamplifier in comparison with that using an avalanche photodiode (APD). The reason is that the Er:fiber preamplifier performs better than a Nd:fiber preamplifier, while the InGaAs APD is less sensitive in comparison with a Si APD. In a lidar receiver using a fiber preamplifier, the beam coupling efficiency from free-space mode to a single- mode fiber is the critical parameter. For moderate target velocities, an automatic front-end alignment system using piezoelectric transducers can be effected to yield a good coupling efficiency. The design and preliminary test results of such a lidar receiver are discussed in terms of optimized optical filter bandwidth, optical preamplifier gain and noise figure, and input saturation level.
A large-aperture, high-Q, germanium standing-wave AO modulator for mode locking a pulsed high-energy CO2 laser is described. By operating it in the Raman-Nath regime, optical absorption-induced thermal lensing effects are minimized. Due to a smaller round-trip transit time and desired Bessel function transmission characteristics, only a single set of mode locked pulses are generated.
Symmetrical edge and face cooling geometries have been evaluated as a thermal control technique for a high power acousto-optic modulator employed as an intracavity mode locker inside a pulsed TE, 0.5 atm, mode-locked CO2 laser oscillator with a 0.5 - 1 kW average output power. Both cooling methods have proved effective in minimizing transverse thermal gradients generated in a germanium crystal of the acousto-optic modulator operating in the Raman-Nath regime.
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