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23 February 2006 Self-referenced locking of optical coherence by single-detector electronic-frequency tagging
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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.
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T. M. Shay, Vincent Benham, Justin Spring, Benjamin Ward, F. Ghebremichael, Mark A. Culpepper, Anthony D. Sanchez, J. T. Baker, D. Pilkington, and Richard Berdine "Self-referenced locking of optical coherence by single-detector electronic-frequency tagging", Proc. SPIE 6102, Fiber Lasers III: Technology, Systems, and Applications, 61020V (23 February 2006);

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