Direct observation of Kramers–Kronig self-phasing in coherently combined fiber lasers

Abstract

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser. The Kramers-Kronig effect in the fiber gain media was observed to induce a phase shift that effectively canceled the applied path length errors, resulting in efficient lasing under all phase conditions. This process was demonstrated to result in robust lasing over a large range of pump conditions.

Uncovering the physical origin of self-phasing in coupled fiber lasers

“…The lasing wavelength of this coherently phased array was 1052 nm. The lasing power versus applied path length error data L. (blue squares) has the characteristics predicted by a cold-cavity theory 8 . Lasing power varies with the applied path length error nearly periodically with a period of π/2.…”

Study on the effect of nonuniform polarization angles on coherently combined fiber lasers

“…Coherent beam combination (CBC) of fiber lasers is a promising technique to solve the power limitation but also maintain good beam quality. Many approaches of CBC have been demonstrated, which can be classified into two categories: active phase locking [3][4][5][6] and passive phase locking [7][8][9]. Active phasing have been demonstrated to have the advantage in combining a large number of beamlets, especially for fiber lasers [6].…”

Efficient passive phasing of an array of 20 ring fiber lasers

“…However a complex system like a coupled array of lasers cannot be fully described without taking into account the intrinsic nonlinearity of the lasers. Thus, subsequent theoretical approaches including gain saturation and resonant phase nonlinearity [12][13][14][15] have shown that passive techniques to phase-lock a laser array could be more efficient than initially considered. In lasers based on a ytterbium doped fiber amplifier (YDFA) in particular, the resonant nonlinearity may be significant (especially on the long wavelength side 1070-1090 nm).…”

“…The resulting gain-phase coupling leads theoretically to a linewidth enhancement factor close to one [12]. In a properly design cavity, the additional phase-shift accumulated during amplification may significantly impact the behavior and in-phase operation of the fiber laser network with scattered lengths [12][13][14][15]. In a recent paper, Chiang et al [15] experimentally demonstrated that the phase-shift due to population inversion in YDFA could actually compensate for the linear phase detuning between the two arms of a double channel fiber laser.…”