Quasi phasematched domain periods for bulk and waveguide GaAs nonlinear frequency conversion were calculated.
Waveguide beam confinement resulted in a decrease in effective refractive index and the magnitude of this index change
was found to increase supralinearly with wavelength. Domain periodicity required for a 2.056-μm pumped, 10x10 μm2
GaAs core, Al0.5Ga0.5As cladding waveguide OPO at degeneracy decreased from 60.4 μm for bulk GaAs to 57.9 μm for
a rib waveguide and 49.8 μm for a buried waveguide in a GaAs OPO. Careful accounting for waveguide effects is necessary to fabricate a periodic waveguide structure with the desired function.
The power available from narrow-linewidth single-transverse-mode fiber amplifiers is primarily limited by the
onset of stimulated Brillouin scattering. One approach for increasing the SBS threshold that has shown recent
promise is to tailor the acoustic velocity within the fiber cross-section to suppress Brillouin gain. Relating the
SBS threshold to an acousto-optic effective area has yielded a theory which contradicts experimental measurements
that indicate the nonlinear optical effective area of the tested SBS suppressing and Higher Order Mode
(HOM) fibers is of primary importance in the nonlinear process. In this work, we present a new formalism for
determining the Brillouin gain in fibers with inhomogeneous acoustic velocity which may be implemented with a
wide variety of computational methods. We find that the Brillouin gain amplitude and spectrum are independent
of the acousto-optic effective area and that they reduce to the bulk result for conventional step-index fibers.
Implementing a finite-element method, we find that an SBS-suppressing design employing a negative focal length
acoustic lens exhibits a broadened gain spectrum and reduced gain amplitude relative to step-index fibers. The
SBS threshold of this fiber is increased by 8.4 dB relative to a standard large mode area fiber, each with an
identical 6 meter length. Designs that further flatten the Brillouin gain spectrum have the potential to further
increase the SBS threshold leading to higher single-frequency output power from devices incorporating these
fibers.
A review of research into stimulated Brillouin scattering (SBS) phase conjugation and beam cleanup conducted at the
Air Force Institute of Technology is presented. CW phase conjugation was demonstrated using SBS in short lengths of
multi-mode, step-index fiber. A fidelity of 0.8 was achieved using a 0.06-NA fiber of 40 m length and a 0.13-NA fiber
at 15 m length. In modeling and experiment, the fidelity declined as fiber length or numerical aperture increased. A
relationship was established empirically between step-index fiber parameters and the phase conjugation fidelity. In
addition, increased fidelity was related to an increase in the effective Brillouin gain coefficient in step-index fibers.
Beam cleanup was observed in both graded-index and step-index fibers. The Stokes beam generated in long, multimode,
graded-index fibers was analyzed using spatial and phase methods and found to be a low-order mode of the fiber instead
of a phase conjugate of the pump. SBS in long, graded-index fibers was used to combine multiple beams and to improve
the beam quality of a single aberrated beam.
A novel high accuracy all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn't require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays.
We report a novel coherent beam combining technique. This is the first actively phase locked optical fiber array that eliminates the need for a separate reference beam. In addition, only a single photodetector is required. The far-field central spot of the array is imaged onto the photodetector to produce the phase control loop signals. Each leg of the fiber array is phase modulated with a separate RF frequency, thus tagging the optical phase shift for each leg by a separate RF frequency. The optical phase errors for the individual array legs are separated in the electronic domain. In contrast with the previous active phase locking techniques, in our system the reference beam is spatially overlapped with all the RF modulated fiber leg beams onto a single detector. The phase shift between the optical wave in the reference leg and in the RF modulated legs is measured separately in the electronic domain and the phase error signal is feedback to the LiNbO3 phase modulator for that leg to minimize the phase error for that leg relative to the reference leg. The advantages of this technique are 1) the elimination of the reference beam and beam combination optics and 2) the electronic separation of the phase error signals without any degradation of the phase locking accuracy. We will present the first theoretical model for self-referenced LOCSET and describe experimental results for a 3 x 3 array.
Stimulated Brillouin Scattering (SBS) is a polarization-dependent, nonlinear process that is often the limiting factor for high-power fiber laser applications. We report the results of experiments measuring the SBS thresholds and the SBS gain bandwidths in several passive optical fibers. Fibers with nearly identical mode-field diameters and loss coefficients from different manufacturers were selected. Observations from these experiments indicate that the SBS gain coefficient for fibers from different manufacturers varied significantly resulting in a 70% deviation in SBS threshold. Also, polarization-maintaining (PM) fiber exhibited a significant increase in the SBS threshold for a linearly polarized pump beam that is launched into the PM fiber at 45° relative to the fiber's slow axis. This increase in threshold was not mirrored in non-PM fiber. These results suggest that the polarization multiplier in the SBS threshold equation may be highest when a PM fiber is used with the appropriate launch conditions, rather than a non-PM conventional single-mode fiber. We will present the experimental results and a theoretical model demonstrating the polarization dependent gain properties in both PM and non-PM fiber.
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