James Nagel

High-Power Narrow-Linewidth Continuous-Wave Raman Amplifier at 1.27

A high-power narrow-linewidth continuous-wave (CW) Raman fiber amplifier at 1.27 μm is demonstrated for remote sensing measurements of atmospheric oxygen (O2). The amplifier uses phosphosilicate fiber in both a standard single-mode design and with the core size varying in the longitudinal direction for stimulated Brillouin scattering (SBS) suppression. The highest reported power level at this wavelength to date, a total peak power of 3 W, has been achieved for online (2.4 W) and offline (580 mW) spectroscopic measurements, in agreement with theoretical predictions. Additionally, it is shown that when 60-kHz sinusoidal amplitude modulation is applied to the amplifier seed signal, the modulated wave form is preserved at the amplifier output with 1.8 W total average power being obtained.

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In 2004 ITT Exelis developed the Multifunctional Fiber Laser Lidar (MFLL) for measuring atmospheric CO2. This lidar relies on high efficiency telecom laser components and Erbium Doped Fiber Amplifiers (EDFA’s) to implement a unique Continuous Wave (CW) Intensity Modulated (IM) differential absorption lidar measurement. This same approach has also been used to measure atmospheric O2 by replacing the EDFA’s with fiber Raman amplifier technology. The use of all fiber coupled components results in a highly reliable, flexible and robust instrument. The general architecture of the MFLL, its implementation for greenhouse gas measurements, and as a pseudorandom noise encoded altimeter system is reviewed. Results from a 2011 flight campaign on the NASA DC-8 aircraft which included CO2, O2, and PN altimetry using a single receiver for all three measurements are also discussed. In addition, an introduction to a novel variation of this approach that will enable greenhouse gas monitoring from a geostationary orbit is given. This paper provides a general overview of a set of applications for fiber lasers in the area of active remote sensing that have been developed by Exelis over the past several years.

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Optical fibres having a radially nonuniform acoustically antiguiding structure produced by codoping their core with alumina and germania have been fabricated and investigated. The influence of the shape of the antiguiding acoustic refractive index profile and fibre core diameter on the stimulated Brillouin scattering (SBS) threshold and spectrum in the fibres has been assessed. An increase in SBS threshold by 4.4 dB with respect to a germanosilicate fibre having the same mode field diameter has been demonstrated.

Applications of fiber lasers for remote sensing of atmospheric greenhouse gases

Germanosilicate and phosphosilicate-based glasses are two of the most common materials used to amplify light in the near and shortwave infrared wavelengths by stimulated Raman scattering in optical fiber, but are limited in total output power and efficiency that can be achieved due to the onset of stimulated Brillouin scattering (SBS). We experimentally investigate both germanosilicate and phosphosilicate Raman gain fibers with selective transverse doping profiles in the context of altering the fiber acoustic waveguide properties for increasing the threshold at which total Raman amplified power is saturated by SBS. It is shown that selective transverse doping offers greater potential for SBS suppression in germanosilicate gain fibers in contrast to those manufactured from phosphosilicate glasses. Initial SBS suppressed germanoslicate fibers are demonstrated with a 5 to 6 dB reduction in the maximum peak Brillouin gain as compared to a standard step-index fiber such as Corning HI-1060, which allows for over 5 dB in SBS suppression and an increase in saturated Raman amplifier gain from 15 dB to greater than 22 dB.

Optimisation of an acoustically antiguiding structure for raising the stimulated Brillouin scattering threshold in optical fibres

We show both experimentally and theoretically a method to increase the stimulated Brillouin scattering (SBS) threshold and output power of narrow linewidth fiber Raman amplifiers. This method employs two or more fibers with varying concentrations of the Raman gain material dopant such as GeO2 or P2O5 in silicate-based glasses. These fibers are then cascaded to form an amplifier gain stage, disrupting the buildup of SBS that normally occurs in single continuous fibers. The numerical model shown is applicable to arbitrary amplifier systems for gain stage optimization and increased power scaling. We give experimental results for phosphosilicate fibers that agree well with simulation predictions that support the numerical model used.

Experimental Investigation of Silicate-Glass-Based Raman Gain Fibers with Enhanced SBS Suppression by Selective Transverse Doping

We report on the development of a Raman amplifier using fibers with longitudinally varying cores. Approximately 3.0W of a 1.26?m narrow linewidth continuous wave signal has been demonstrated for remote sensing of atmospheric oxygen levels.

Cascaded gain fibers for increasing output power and the stimulated Brillouin scattering threshold of narrow linewidth fiber Raman amplifiers

We report on the development of a continuous wave narrow linewidth fiber Raman amplifier for remote sensing of atmospheric oxygen at 1.27μm with a total combined peak power of 3W for online and offline channels.