MOPA Laser Transmitter with Passive Q-Switch for Space LIDAR

A flight laser transmitter is proposed that will increase reliability 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 PQS eliminates the use of an AQS and the associated hardware and electronics. The AQS requires a high voltage electronic driver that provides a fast switching speed of 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 power intracavity polarizer is also required to provide high losses when the Q-switch is in the off state. These AQS components are all replaced with a single PQS crystal with relatively minor alignment requirements to the resonator beam 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. These lasers have short pulse-widths (< 1 nsec) and have little control of the rep-rate. More recently Cr:YAG PQS crystals have been used as separate elements in an MO resonator to control pulse width, and have been used with Nd:YVO4 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 resonator losses. The losses can become negligible in a MOPA configuration if the MO is designed for, and operated at 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.

The transmitter is primarily a candidate for use in NASA lidar systems where high energy, high rep rate pulses are required to provide enough return signal for detection at long ranges, such as from space. Lidar applications from SmallSat platforms have this requirement as a result of the long ranges involved. The NASA ER-2 high altitude aircraft could also serve as a platform for lidar with high power transmitters to provide an increased field of view.

Non-NASA applications include military lidar for ranging and imaging, particularly with the emerging use of UAV military platforms, and agriculture and forestry UAV lidar monitoring. Some of these lidar transmitter applications may be required to operate at an eye safe wavelength around 1550 nm, which can be provided by output wavelength conversion to 1550 nm using KTP in a high power OPO.

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