Solar-pumped, mode-locked, frequency-doubled Nd:YAG laser

One highly promising space laser communication system utilizes a frequency doubled Nd : YAG laser transmitter mode-locked at 500 MHz. In such a laser, intracavity acoustooptic loss modulation is superior to electrooptic phase modulation for achieving narrow modelocked pulses and stable operation, and resonator loss must be minimized for maximum efficiency. We have developed an acoustooptic mode-locker and frequency doubler (AOML/ FD) that uses a single barium sodium niobate (BSN) crystal to both frequency double and acoustooptically mode-lock a Nd:YAG laser. Thus, one intracavity component is eliminated. A typical AOMLIFD unit measures only 4.5 cm x 4.2 cm x 1.9 cm. A BSN crystal was cut in the form of a rectangular parallelopiped 5 mm x 5 mm x 2 mm with the crystal axes parallel to the edges. The 2 mm x 5 mm faces normal to the e-axis were polished flat and parallel to form the acoustic resonator. A longitudinal transducer was bonded to one of the c-faces using a vacuum cold welding technique, and the transducer was mechanically polished for half wave resonant operation at 250 MHz. The thin upper electrode was sputtered on and connected to the RF matching section. The RF matching was accomplished by a n o ~ l transmission line technique involving line sections having different characteristic impedance. The crystal was mounted in a solid copper heat sink fitted with heater windings, a thermistor, and a thermocouple. The Faces normal to the b-axis were AR coated for both 1.06 pm and 0.53 pm. The acoustooptic figure of merit (FOM) for BSN was measured by the method of Dixon and Cohen.’ For longitudinal acoustic waves propagating down the c-axis (001) direction, the FOM relative to fused silica was 2.07 and 7.25 for light polarized perpendicular (“0”) and parallel ( “ E ) to the c-axis, respectively. The longitudik! acoustic velocity down the c-axis was 6250 MI’S, and the temperature coefficient of longitudinal phase velocity was -3.1 x 10-4/0C. For light propagating normal to the c-axis polarized “0” for efficient SHG and with the sound column propagating at an angle 0 to the c-axis in the a-c plane, the angular dependence of the FOM is A cos’ 0 + B sin’ B for longitudinal waves and C sin 20 for shear waves; where A : B : C N 1 :.2 12.5.