This paper describes the physics and properties of a novel optical fiber that would be attractive for building highpower fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses -a capability that is practically nonexistent in conventional fibers. Particularly interesting is the fact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.Mode image, canonical refractive index profile and mode profile of HOM fibers typically used in the LMA designs.
Suppression of stimulated Raman scattering (SRS) is demonstrated in a cladding-pumped fiber amplifier. The Yb-doped amplifier fiber design incorporates a high-index ring that resonantly couples SRS wavelengths out of the gain material, thus filtering the gain. Modeling shows that fiber asymmetry plays an important role in the filtering spectrum.
The bichromatic optical frequency correlation function for Rayleigh backscattering from a pulse of laser light propagating along a single-mode optical fiber has been calculated and measured. It is shown that the optical correlation frequency, Dnu(c) , is equal to the reciprocal of pulse width T(w) . These results are important for the development of wavelength diversity techniques for the reduction of coherent Rayleigh noise in distributed Rayleigh backscattering single-mode optical fiber sensors.
Accurate spectral and radiometric calibrations of IR cameras are needed to interpret infrared imagery properly, to monitor camera performance over time, and to evaluate new imaging radiometers. In this paper, the physical basis for calibrating IR cameras is derived from first principles, and a laboratory setup used to perform both radiometric and spectral calibrations is described. The calibration procedures used with this setup are then demonstrated on a midwave IR focal plane array camera, a midwave IR scanned camera, and a longwave IR scanned camera. The system spectral response and radiometric response of each camera is given and analyzed. An error analysis of approximating a camera's spectral response by an equivalent top-hat responsivity is also given.
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