Coherent beam combination of seven 1.5  µm fiber amplifiers through up to 1  km atmospheric turbulence: near- and far-field experimental analysis

Abstract

A laser testbed based on active coherent beam combination (CBC) of seven 1.5 µm, 3 W fiber amplifiers was developed for applications requiring high power such as power density deposition on targets or free space laser communication. For the first time to our knowledge, the frequency-tagging locking of optical coherence by single-detector electronic-frequency tagging technique was implemented in the field in real atmospheric turbulence conditions in a target-in-the-loop configuration. Successful combination was achieved after horizontal propagation of 311 m and 1 km, at 1.5 m above the ground, while the estimated average turbulence strength was C n 2 ∼ 4.10 − 14 m − 2 / 3 . We present the CBC laser bench and an embedded near-field interferometer called PISTIL (PISton and TILt) able to measure the relative phase shift of each emitter. We show that this measurement can provide information on relative turbulence-induced phase variation of the combined laser beams. In particular, the far-field beam envelope wandering can be estimated through this diagnosis. Results are supported by an analytical model and confirmed by numerical post-analysis of measured far-field interference. This additional interferometer may improve CBC beam pointing through turbulence.

Phase noise measurements and diagnoses of a large array of fiber lasers by PISTIL

“…Such a process has proven to be noise resilient. Indeed, measurements taken out from previous researches show an accuracy in the hundredth of wavelength range ( < 𝜆/100) for static segmented wavefront sensing [19,20], and a precision < 𝜆/200 RMS (root mean square) allowing it to perform a channel-to-channel phase diagnosis below λ/20 RMS for closed-loop CBC systems [10,21].…”

“…Coherent Beam Combining (CBC) of fiber amplifiers is today in the industrialization process [1][2][3]. Proposed as a method to increase both peak and average powers of a laser beam [4,5], it has undergone important developments in the last fifteen years, allowing more and more combined channels while scaling the peak and average powers with excellent beam quality under various operating regimes [6][7][8][9][10][11].…”

“…The main objective of CBC is to make N beams interfere constructively in the far-field, which requires the phase of each beam being controlled and set equal altogether via a servoloop [10][11][12][13][14]. Applying such an algorithm will minimize the piston phase difference between two or more beams relatively to a reference beam or a reference value.…”

“…In this paper, we propose investigating the phase noise properties of numerous a priori identical laser channels functioning altogether by performing a collective phase measurement of the laser beams by the use of a PISton and TILt interferometric sensor (PISTIL), a referencefree segmented wavefront analyzer which fits with the segmented pupil architecture [10,[17][18][19][20][21]. We then exploit the measurements to highlight identical behaviors or differences in the fiber laser array.…”

Coherent combining of 7 fiber lasers using a multi-plane light converter device

“…The laser beams propagate in free space and interfere constructively in the far field. This configuration enables an easy combination of a large number of fibers amplifiers leading to successful CBC experiments through km-range atmospheric turbulence [3][4] . However, due to a limited aperture fill factor in the near field and sides lobes in the far field, this technique is limited to a theoretical maximum efficiency of 63%.…”