Coherent beam combination of 108 kW fiber amplifier array using single frequency dithering technique

Abstract

Coherent beam combination of a 1.08 kW fiber amplifier array has been demonstrated for the first time, to our knowledge. In the experiment, nine fiber amplifiers are tiled into a 3×3 array, and the output power of each amplifier is approximately 120 W. A single frequency dithering algorithm is used to compensate the phase noises between the elements, which runs on a signal processor based on field programmable gate array for phase control on the fiber amplifiers. When the phase control system goes into closed loop, the fringe contrast of the far-field intensity pattern is improved to more than 85%, and the residual phase error is less than λ/15.

Generation of a 481-W Single Frequency and Linearly Polarized Beam by Coherent Polarization Locking

“…The mathematical principle of single frequency dithering technique can be found in Ref. [11] and it is not repeated here to save place. The metric function of the singlefrequency dithering algorithm is reflected by the amplitude of the voltage signal transformed by the PD.…”

“…By calculation, the control signal S 1 is proportional to sin(δ), as is similarly described in Ref. [11] except for the proportional coefficient. At the interval of t 1 to t 2 , the phase control signal S 2 (proportional to sin(−δ)) are applied to the phase modulator in the second channel.…”

“…The robust single-frequency dithering technique with a control band of > 10 kHz is utilized to phase control the phase difference between two amplifier chains [11]. When the system is in the closed loop, the extinction ratio of the combined beam is measured to be 10 dB and the beam quality of the coherently combined beam is less than 1.2.…”

Spatial correlation properties of coherent array beams modulated by space-time random phase

“…Notably, Ji et al first studied the spatial correlation properties and the spectral intensity distributions of focused Gaussian-Schell model (GSM) array beams and further discussed the beam quality of linear and radial GSM array beams [5,6]. It is not a unique instance, however, array beams has also been adopted in the development of high-power fiber laser [11][12][13][14][15]. As a key technology, it is required for coherent array beams to keep the same phase as each other.…”

“…Otherwise, the existence of random phase errors will bring forth speckle and degrade the beam quality of output laser. Nabors discussed detailedly the effects of Gaussian phase errors on rectangle aperture, rectangle grid, coherent emitter array [16]; and elaborate efforts have been made to the research on phase control and compensation [12][13][14][15]. Random phase, which has dual characters, play a positive role in the generation of partially coherent light by liquid space-lightmodulator (SLM) [3,4] and speckle patterns for ghost imaging [17].…”