Coherent beam combination of four holmium amplifiers with phase control via a direct digital synthesizer chip.

We present the coherent beam combination of four 2100 nm holmium amplifiers with their phase controlled through acousto-optic modulators driven by the RF output of direct digital synthesizer chips. Phase alignment was achieved through the use of a field programmable gate array based stochastic parallel gradient descent algorithm.

Bragg grating fabrication with wide range coarse and fine wavelength control.

A Bragg grating fabrication method based on a phase-controlled interferometric approach with both coarse control and fine tuning of Bragg wavelength in the range of 1200-2200 nm is demonstrated by fabricating multiple Bragg reflectors in a single fiber device. With the fine tuning method, we achieved ~113 nm of FWHM bandwidth centered at ~1545 nm and ~174 nm of FWHM bandwidth centered at ~2005 nm, which were limited only by the size of the overlap of the interfering beams and by the Bragg wavelength. We used the wide range of the coarse wavelength control and the precision of the fine wavelength setting to determine the fiber Sellmeier coefficients with a mean squared error <3.