MOPA Laser Transmitter with Passive Q-Switch for Space LIDAR

Anbsp;flightnbsp;laser transmitter is proposed that will increase reliabilitynbsp;and reduce size and weight for lidar applications by using a passive Q-switch (PQS) in the master oscillator (MO) of a high power solid state Nd:YVO4 master oscillator power amplifier (MOPA) laser configuration. The PQSnbsp;eliminates the use of annbsp;AQS and the associated hardware and electronics. The AQSnbsp;requires a high voltage electronic driver that provides a fast switching speed ofnbsp;a few nanoseconds, and a high voltage power supply. The AQS crystal is located in the master oscillator resonator and requires maintaining accurate alignment to the resonator beam axis. A high powernbsp;intracavity polarizer is also required to provide high losses when the Q-switch isnbsp;in the offnbsp;state.nbsp; These AQS components are all replaced with a single PQSnbsp;crystal with relatively minor alignment requirements to the resonator beamnbsp;axis. Cr:YAG PQS crystals have been used in the past for microchip lasers that are Cr:YAG crystals diffusion bonded to an Nd:YAG crystal with coatings applied to the ends to form a laser resonator.nbsp;These lasers have short pulse-widths (lt;nbsp;1 nsec) and have littlenbsp;control of the rep-rate. More recently Cr:YAG PQS crystals have been used as separate elements in an MOnbsp;resonator to control pulse width, and have been used with Nd:YVO4nbsp;nbsp;crystals, since the wavelengths are about the same. The pulse width can be increased to 8-10 nsec by using a longer resonator, and the laser rep-rate can be controlled by operating the laser diodes in the QCW mode for pumping the Nd:YVO4. The Cr:YAG PQS does not become 100% transmissive and introduces some additional resonatornbsp;losses. The losses can become negligible in a MOPA configuration if the MO is designed for,nbsp;and operated atnbsp;a relatively low power. Several stages of amplification can be used to raise the output pulse energy to the level required for space based lidar applications.nbsp;nbsp;nbsp;