A multi-beam, multi-color, polarized laser transmitter has been developed for mapping applications. It uses commercial off-the-shelf components for a low-cost approach for a ruggedized laser suitable for field deployment.
The laser transmitter design is capable of delivering dual wavelengths, multiple beams on each wavelength with equal (or variable) intensities per beam, and a well-defined state of polarization. This laser transmitter has been flown on several airborne campaigns for the Slope Imaging Multi-Polarization Photon Counting Lidar (SIMPL) instrument, and at the time of this reporting is at a technology readiness level of between 5 and 6.
The laser is a 1,064-nm microchip high-repetition-rate laser emitting energy of about 8 microjoules per pulse. The beam was frequency-doubled to 532 nm using a KTP (KTiOPO4) nonlinear crystal [other nonlinear crystals such as LBO (LiB3O5) or periodically poled lithium niobiate can be used as well, depending on the conversion efficiency requirements], and the conversion efficiency was approximately 30 percent. The KTP was under temperature control using a thermoelectric cooler and a feedback monitoring thermistor. The dual-wave-length beams were then spectrally separated and each color went through its own optical path, which consisted of a beam-shaping lens, quarter-wave plate (QWP), and a birefringent crystal (in this case, a calcite crystal, but others such as vanadate can be used).
The QWP and calcite crystal set was used to convert the laser beams from a linearly polarized state to circularly polarized light, which when injected into a calcite crystal, will spatially separate the circularly polarized light into the two linear polarized components. The spatial separation of the two linearly polarized components is determined by the length of the crystal. A second set of QWP and calcite then further separated the two beams into four. Additional sets of QWP and calcite can be used to further split the beams into multiple orders of two.
The spatially separated beams had alternating linearly polarization states; a half-wave plate (HWP) array was then made to rotate the alternating states of polarization (SOP) so that all of the beams would have the same SOP. The two wavelength beam paths were then recombined using a dichroic filter such that they were co-aligned. A negative lens array following the HWP array was used to provide specific beam divergence. A final lens was used to provide the angular spread of the multiple beamlets.
This work was done by Anthony W. Yu, Luis Ramos-Izquierdo, David Harding, and Tim Huss of Goddard Space Flight Center. GSC-15950-1