This report covers work carried out under a NASA Phase II SBIR Program, Contract Number NAS5-98060, whose objective was to develop a diode-pumped, short-pulse, energetic, compact and reliable microlaser-amplifier system suitable for NASA Goddard satellite laser ranging (SLR 2000).
In the performance of the effort, we developed, designed and constructed a 1-W-diode end-pumped, Cr:YAG passively Q-switched Nd:YAG microchip laser that generated 3.2-mJ, 400-ps pulses at a 2 kHz rate. The microlaser pulses were amplified up to 335-mJ energies in a cw side-diode-pumped Nd:YVO4 gain module based on a multiple-pass slab design. We then used the 1064-nm output beam for nonlinear conversion to second, third and fourth harmonics, with the average output powers of 400 mW, 240 mW and 66 mW, respectively.
We also presented experimental data and theoretical modeling of passively Q-switched microlasers that we believe, for the first time, clearly demonstrates the influence of pump-light induced bleaching of the saturable absorber on the microlaser performance leading to longer pulses.
Finally, we delivered a breadboard laser system to NASA Goddard which produced 200-mJ of energy, ~ 400 ps-wide pulses at 532-nm wavelength, at a pulse repetition rate of 2 kHz with near-diffraction-limited beam quality.
Conference Papers:
ASSL 2001:
High-power,
passively Q-switched microlaser - power amplifier system
CLEO 2000:
Atypical behavior of
Cr:YAG passively Q-switched Nd:YVO4 microlasers at high-pumping rates
Experimental Details:
A 1-watt, fiber-coupled diode laser operated at a 2 kHz pulse rate was used as a pump source for the Cr:YAG passively Q-switched Nd:YAG micro-chip laser (microlaser.) Pump light emerging from the 100-mm, 0.22 NA pump-laser fiber was collected and focused into the microchip using two AR-coated aspheric lenses. The fiber was positioned at the front focal plane of the first lens, and the microchip at the nominal back focal plane of the second lens. Two different "telescopes" were used to optimize the microlaser output: a 4:3 or a 2:1 reducing telescope. Approximately 98% of the light emerging from the fiber was delivered to the microchip when using either telescope.
Three different monolithic microchip oscillator designs were evaluated, each having a different Cr:YAG layer thickness. The first two designs were made by Synoptics according to Q-Peak specifications. Both employ a 0.5 mm thick layer of 3%-doped Nd:YAG. One chip design has a 0.25-mm-thick layer of Cr:YAG with a nominal unsaturated absorption of 6 cm-1.The output facet of the chip is coated for 80%R. The other chip design is the same, except that the Cr:YAG layer thickness is 0.5 mm. Originally, we intended to try a similar third design, but with a 0.75 mm Cr:YAG layer and a 60%R output facet. However, this chip was not coated properly and was replaced with an off-the-shelf chip designed by Synoptics, which had features close to what we desired. This chip has a 1.25 mm layer of 1.9%-doped Nd:YAG, a 0.75-mm layer of Cr:YAG (6 cm-1), and an 80%R output facet. The pulse durations calculated for the three designs were 304, 204 and 200 ps, respectively. We believe that pump-light-induced bleaching is one of the reasons for increasing pulse durations.
We obtained the following results:
The pulse waveform resulting from microlaser #3 is shown in Figure 1, below.
Figure 1. Oscilloscope trace of pulse from microlaser #3.
In order to amplify the output of the microlaser to appropriate energies, we employed a side-pumped, Nd:YVO4 crystal, with a multi-pass design, as an amplifier. The optical setup is shown below, in Figure 2. The microchip laser output was collected by a spherical 50-mm FL lens. This lens was positioned about 63 mm from the microchip so the beam was gradually focused into the amplifier stage. The beam then passed through a TGG Faraday isolator equipped with input and output Glan-laser polarizers. A half-wave plate positioned before the collimator lens adjusted the polarization angle of the microchip beam as it enters the first polarizer of the isolator. A second half-wave plate adjusted the polarization angle of the beam emerging from the second polarizer. The beam was turned 90° by the first 45°-incidence HR (45 HR), went through a +150 mm FL cylindrical lens that focuses in the vertical plane, and bounced off the second 45 HR, before entering the 3-pass amplifier stage. The cylindrical lens was positioned about 150 mm from the center of the amplifier slab, taking into account the fact that the beam makes three passes through the slab (the separation between the slab assembly’s miniature fold mirrors is about 20 mm, and the slab is about 15 mm long). The beam was back-reflected through the amplifier with a flat HR , and made another 3 passes through the amplifier slab. The back-reflected, double-pass amplified beam passed back through the optical system and into the Faraday isolator. (The plane of polarization at the first polarizer is rotated 90° relative to the microchip laser polarization.) The double-pass-amplified beam was coupled out the system at the first polarizer, and emerged with a polarization vertical to the plane of the paper.
Figure 2. Schematic of oscillator-amplifier system.
We measured the single pass gain of the Nd:YVO4 amplifier, with the results shown in Figure 3, and determined that the unsaturated gain approached 30 at low input-energy levels. The saturated energy output exceeded 0.6 mJ/pulse. With 6.4 mW of microchip laser power, the double-pass amplifier power was about 670 mW at an amplifier current of 31A. The beam quality of the double-pass-amplified beam was measured (by a Spiricon M2 meter) using the "90/10 knife-edge" method. M2 in the horizontal and vertical planes were measured to be 1.38 and 1.28, respectively. The pulse duration decreased to 370 ps as compared to ~ 440-ps from the microlaser.
Figure 3. Single-pass gain and output energy of Nd:YVO4 amplifier as a function of input energy.
We conducted experiments on nonlinear conversion of the amplifier beam. For second harmonic generation (SHG) we used a non-critically-phase-matched, Type I LBO crystal, with dimensions of 3 x 3 x 15 mm, mounted in a 1700 C, temperature-stabilized oven. Third harmonic generation (THG) at 355 nm was accomplished with a room-temperature, 3 x 3 x 12 mm, Type II critically-phase-matched LBO crystal (q = 42.7° , f = 90° ). And, finally, for fourth harmonic generation (4HG) at 266 nm, a Type I critically-phase-matched, room-temperature BBO crystal (q = 47.6° , f = 0°), 3 x 3 x 7 mm crystal was used. The beams were separated using a Pellin-Broca prism. At input power of 670 mW, the output power of SHG, THG and 4HG was 400, 240 and 86 mW, respectively, which corresponds to ~ 60%, 36%, and 13% conversion efficiency.
Packaging of deliverable system:
Once the system design was firm, we designed and built an optical head to mount the entire system. A drawing of the design is shown below, in Figure 4.
Figure 4. Design of optical head for oscillator-amplifier system.
A picture of the entire system, including power supply (left) and head is shown below in Figure 5.
Figure 5. Picture of entire system, including power supply, controller (left) and optical head (right).