Description: We propose an energy and space efficient high power continuous wave (cw) narrow linewidth broadband fiber Raman amplifier (FRA) with spectrally tunable multi-Watt-level average power output in the near and shortwave infrared (1080 – 2000 nm) that can be used in remote sensing systems on both atmospheric and terrestrial space-borne platforms. The all-fiber amplifier design concept for power amplification of a lower power commercially available tunable laser seed source (master oscillator) uses a single gain stage architecture based on germanosilicate (GeO2-SiO2) fibers specifically designed to suppress stimulated Brillouin scattering (SBS), one of the main factors limiting the maximum output power from narrow linewidth cw fiber amplifiers. The amplifier will be pumped by a fiber Raman laser whose spectral output can be tuned by compressive fiber Bragg grating technology. For this proposal TIPD will demonstrate power levels beyond what has been previously demonstrated for this technique, necessary for pumping the SBS suppressed gain fiber amplifier stage to its maximum potential output power. By implementing techniques for suppressing SBS in highly doped germanosilicate fibers it is anticipated that the amplifier wall-plug efficiency will reach 10%. In addition, the single gain stage architecture is compatible with distortion-free amplification of a phase/amplitude modulated seed source, useful for sensors that rely on sophisticated signal processing for detection. During Phase I, we will validate the broadband FRA proof-of-concept through modeling and benchtop demonstrations of both the power amplifier and tunable pump laser stages. Furthermore, appropriate designs for both the tunable laser source and SBS suppressed germanosilicate gain fibers will be formulated and assessed in terms of performance that best meets the target technical specifications for the FRA for a potential Phase II effort.

Benefits: The broadband fiber Raman amplifier (FRA) proposed is applicable to future NASA missions where tunable high power cw lasers are needed at infrared wavelengths not currently available by other commercially available sources, in support of applications such as LIDAR, spectroscopy, and communications. Specifically, high power tunable laser sources are relevant to trace gas and atmospheric constituent measurements in future planetary exploration (e.g. Mars, New Frontiers missions to Saturn and Uranus) and earth-science (e.g. ASCENDS) missions as outlined in the NRC Decadal Survey Reports "Visions and Voyages for Planetary Science in the Decade 2013-2022" and "Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond" respectively. Such lasers can provide more power for increased measurement sensitivity and can easily be integrated into absorption LIDAR and tunable laser spectrometer instruments (e.g. Mars Curiosity rover) currently using bulkier free-space solid-state lasers. Not only are all-fiber lasers more robust, but they can reduce the overall instrument footprint in the ever-precious real estate of space science payloads, making room for additional instrumentation and adding value to the overall science mission. Broadband FRAs could also be useful in the relay of data at optical wavelengths which provide greater bandwidth than existing RF channels as deemed important in the NRC Planetary Science Decadal Report.

Fiber Raman amplifiers offer the potential to realize high power laser sources at wavelengths beyond those currently found from existing solid state and fiber laser technology. As such, the development of efficient and powerful broadband FRAs will enable new sensors for science, industry and defense. In addition, new multi-wavelength sources can be developed for pumping solid state and fiber lasers where high power semiconductor diodes are currently unavailable. Further development of fiber Raman amplifiers will also be beneficial to the telecommunications industry where increasing demand for bandwidth will require fiber amplifiers beyond what is currently available with erbium doped fiber amplifiers in the C- and L-bands of the optical spectrum. Recent advances in medicine such as photodynamic therapy used in cancer treatments can also benefit from tunable shortwave infrared fiber-laser sources at wavelengths outside of the traditional 1060 and 1550 nm spectral bands. Finally, advances made in terms of SBS suppressed fibers can be valuable to other fiber applications where SBS is a limiting factor. In particular, fiber laser power levels could be significantly increased for industrial and defense applications while maintaining excellent beam quality, a parameter not found in other high power fiber lasers which use large mode area fibers to avoid the onset of SBS.