Jonathan A. R. Rall

Spectral ratio biospheric lidar

A new active vegetation index measurement technique has been developed and demonstrated using low power laser diodes to make horizontal-path lidar measurements of nearby deciduous foliage. The two wavelength laser transmitter operates within and adjacent to the 680 nm absorption feature exhibited by all chlorophyll containing vegetation. Measurements from early October through late November 2003 are presented and the results are discussed

Test results of a diffraction grating beam combiner

A laser diode beam combiner employing diffraction gratings has been fabricated and tested. The Grating Laser Beam Combiner (GLBC) uses two holographic diffraction gratings to incoherently combine the first order diffraction components of four 35 mW AlGaAs lasers. The grating rhomb design minimizes the transmitter sensitivity to the inherent frequency instability of laser diodes. The overall throughput of the combiner is 74 percent. Each laser was temperature controlled to 0.1 C and modulated with 110 Mbps QPPM data. Two lasers under modulation were coaligned to within 76 microrad with a combined average power of 45 mW.

Automatic weather station (AWS) lidar

A ground based, autonomous, low power atmospheric lidar instrument is being developed at NASA Goddard Space Flight Center. We report on the design and anticipated performance of the proposed instrument and show data from two prototype lidar instruments previously deployed to Antarctica.

Optomechanical Design Of The Grating Laser Beam Combiner (GLBC) Laser Diode Header

A laser diode header has been fabricated for a grating laser beam combiner (GLBC). The laser diode header provides the thermal control, the drive electronics, and the optical system necessary for proper operation of the beam combiner. The diode header is required to provide diffraction limited optical performance while providing correction for worst case defocus aberration, 0.6 mrad excess divergence, and worst case decenter aberration, 1.0 mrad pointing error. The design of the header considered the mechanical design and the optical design together resulting in a small, self-contained header with 0.7 mrad range for focus correction and +1- 2.5 mrad of beam steering. The complete diode header is currently undergoing optical and mechanical performance testing.

Phase-front measurements of an injection-locked AlGaAs laser-diode array.

The phase-front quality of the primary spatial lobe emitted from an injection-locked gain-guided AlGaAs laser diode array is measured by using an equal-path, phase-shifting Mach-Zehnder interferometer. Root-mean-square phase errors of 0.037 +/- 0.003 wave (lambda/27) are measured for the single spatial lobe, which contained 240-mW cw output power in a single longitudinal mode. This phase-front quality corresponds to a Strehl ratio of S = 0.947, which results in a 0.23-dB power loss from the single lobes ideal diffraction-limited power. These values are comparable with those measured for single-stripe index-guided AlGaAs lasers.

Multi spectral lidar (MSL)

We have developed and demonstrated both Ytterbium-doped and Erbium-doped, diode-pumped and seeded, fiber amplifiers at 1064 and 1570 nm, respectively. By pulse pumping a one-stage Erbium amplifier, we have shown greater than 20 W peak output power and high wall-plug efficiency. Our pulse-pumping approach improves energy efficiency up to 80% (at 1 kHz PRF) over the identical CW pumping scheme while suppressing amplified spontaneous emission (ASE). We report on the development of these rare-earth doped fiber amplifiers and the application of multi-stage fiber amplifiers to create a multi-spectral laser transmitter ideally suited for space and planetary lidar investigations.

Automated Geophysical Observatory (AGO) Lidar - post Antarctic deployment performance

A ground-based, autonomous, low-power atmospheric lidar instrument developed at NASA Goddard Space Flight Center has been returned from an extended period on the Antarctic polar plateau. We report on the pre- and post-deployment performance of the AGO Lidar, compare the instrument performance with lidar models, and compare results with the anticipated performance of the new Automatic Weather Station (AWS) Lidar.