Fiber Raman amplifier for O2 integrated path differential absorption measurements near 1.26 micron

As part of a NASA Advanced Component Technology grant, starting in 2009, ITT Geospatial Systems, along with our partners at TIPD Inc., have been developing a high power narrow linewidth Fiber Raman Amplifier (FRA) in the 1.26 µm wavelength region. This work is based on the use of P2O5 doped fibers designed for suppression of the dominant nonlinear process, Stimulated Brillioun Scattering (SBS), while simultaneously optimizing for Raman gain. P2O5 is being explored due to its relatively large Stokes shift of ~1300 cm-1, which enables a single-stage path to the 1.26 µm region using commercially available Yb pump sources operating near 1.083 µm. The amplifier will serve as the transmitter for demonstrating the measurement of surface pressure through an Integrated Path Differential Absorption (IPDA) lidar measurement of molecular oxygen. At the time of this abstract, a first attempt amplifier has been built which demonstrated 1.8 W of average modulated power with <3 MHz linewidth. The first version of the amplifier was integrated with ITT's existing Multifunctional Fiber Laser Lidar (MFLL), which has been developed and operationally validated for CO2 measurements through multiple ground testing and airborne field campaigns since 2004, using a combination of investments by ITT and NASA. An additional part of this work was done by our partners, Atmospheric and Environmental Research, Inc., who have developed a comprehensive analysis software tool set for direct comparison of the MFLL O2 measurements and model IPDA values based on local weather station data and a detailed Line-By-Line Radiative Transfer Model (LBLRTM). The tools and methods for comparisons with measurements will be discussed. Further evaluation of the first generation fiber using a 50 W Yb pump laser, achieved an output power of > 12 W peak at ~1.26 µm for 200 m of fiber, before SBS became significant. Recently, lessons learned during the first high power amplifier development and testing have been applied to the design and manufacturing of a new SBS suppressed P2O5 fiber. With increased SBS suppression, the new fiber is expected to allow much longer fiber lengths to be utilized to achieve the desired power of > 5W average while using lower pump powers, ultimately increasing the efficiency of the Raman stage with the goal of >6% wall-plug efficiency for the amplifier. A second set of atmospheric oxygen measurements are planned to be completed in fall 2010 utilizing this next generation FRA with the improved fiber. The design and performance of the FRA's, integration with MFLL, and comparisons of measured versus model results will be discussed.

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