SBIR-STTR Award

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. Potential NASA Applications (Limit 1500 characters, approximately 150 words) 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. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words) 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.

The use of a Passive Q-Switch (PQS) in a high power MOPA configuration with multiple tens of watts output will enable a smaller, compact, lightweight lidar transmitter with significantly increased reliability over existing legacy transmitters for space applications. The proposed MOPA transmitter will reduce transmitter size and weight by integrating a Passive Q-switch (PQS) into the master oscillator, which will eliminate the Active Q-switch (AQS) used in high power MOPA’s. Replacing the AQS with a PQS eliminates the high voltage power supply, high voltage AQS driver, and intracavity AQS crystal. This also eliminates the sensitive alignment and locking of the intracavity AQS and mount. Cr:YAG PQS crystals have been used as a separate component in a resonator to produce Nd:YAG and Nd:YVO4 PQS lasers.They can be designed to produce longer output pulse widths around 8-10 nsec and use QCW laser diode pumping to fix the laser rep rate. The MO output can be amplified up to the level required for lidar applications. The proposed transmitter will use a Carbon Fiber (CF) optical bench to reduce weight and increase stiffness in an aluminum housing. The resonator, amplifiers and second harmonic generation optics and mounts will occupy the top side of the optical bench, with the beam expander located on the bottom side. Two amplifiers will be used with the MO to amplify the energy per pulse up to 5-10 mJ at 4.0-5.0 kHz rep rate, depending on application requirements. The MOPA output can be configured with SHG, THG and OPO wavelength conversion options to operate from the UV to IR. For environmental monitoring applications the laser will operate at 1064 nm and 532 nm. We propose the engineering model phase II deliverable laser to be configured with 6 mJ total output energy/pulse at 4 kHz rep rate, with 3 mJ at 1064 nm, and 3 mJ at 532 nm, and a proposed rep rate of 4 kHz and pulse width of 10 nsec. The laser will use Nd:YVO4 laser slabs pumped at 878.6 nm with VBG stabilization. Potential NASA Applications (Limit 1500 characters, approximately 150 words) This effort proposes miniaturization and reliability over existing lidar transmitter configurations for use SmallSats including environmental research and monitoring. The proposed transmitter applications include Earth Sciences lidar systems, such as use in Cloud Physics Lidar systems based at NASA Goddard Space Flight Center at code 610 (Dr. Matthew McGill). The transmitter would enable smaller, lighter, more reliable lidar instruments for space-based systems, particularly in SmallSats where lidar system weight and volume must be minimized. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words) Applications include military lidar for ranging and imaging, particularly with the emerging use of UAV military platforms, and military space based lidar in SmallSats for surveillance and other military applications. Lidar systems are also used on UAV’s for commercial applications. Lidar can also be used in underwater imaging in UUV's at 532 nm wavelength for bathymetry and object detection.