Eye-safe Er:YAG lasers for coherent remote sensing.

Abstract

Multi-watt lasers with an output wavelength in the eye-safe band are required for many remote sensing applications, including Doppler or coherent laser radars (CLR’s). Er:YAG lasers at 1617 nm or 1645 nm operating on the ⁴I₁₃/₂ to ⁴I₁₅/₂ transition can potentially satisfy this need. Although this transition has been known for many years, the development of diode pumping makes these lasers practical. Doppler wind-field mapping requires single frequency, diffraction limited pulses at a high pulse repetition frequency (PRF) to provide a spatially dense array of samples, allow signal averaging with minimal loss of temporal resolution and to minimize the time required to scan an extended volume. Pulses with energies >few mJ and pulse durations of >100 ns are essential for these measurements. Such requirements can be satisfied by continuous-wave (CW) pumping of a Q-switched free-space laser. In this thesis I describe the design and development of a single frequency, continuous wave, Er:YAG laser at 1645 nm that uses resonant pumping at 1470 nm. With an intra-cavity polarizer and uncoated etalon, it produces up to 30 mW in a narrow line-width, single frequency, plane polarized, diffraction limited, TEM00 output. The laser is suitable as a master oscillator of a CLR. I also describe the development and characterization of an efficient high power Er:YAG laser that is resonantly pumped using CW laser diodes at 1470 nm. For CW lasing, it emits 6.1 W at 1645 nm with a slope efficiency of 40%, the highest efficiency reported for an Er:YAG laser that is pumped in this manner. In Q-switched operation, the laser produces diffraction-limited pulses with an average power of 2.5 W at 2 kHz PRF, and thus is suitable as the slave oscillator of a CLR. To our knowledge this is the first Q-switched Er:YAG laser resonantly pumped by CW laser diodes. This thesis also presents an experimental investigation of the observed reduction in the average output power of Q-switched Er:YAG lasers at low PRF. The experimental results are compared with the predictions of a theoretical model developed using rate equations so the primary causes can be determined, and thus could be minimized in a future design.