Power-scalable system of phase-locked single-mode diode lasers

Abstract

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.

Coherent combining of the output of two semiconductor lasers using optical phase-lock loops

“…1,2 Various technologies have been explored to achieve the goal by using semiconductor (SC) lasers and fiber lasers. 1,[3][4][5][6][7][8] In this Letter we report what we believe to be the first coherent power addition of two commercial grade SC lasers by using current-injection optical phase-lock loops [9][10][11][12][13] (OPLLs). Compared with the previous coherent power-combining techniques, including the optical injection OPLLs, the current-injection OPLLs technique can provide individual electronic control of the frequency and phase of each laser in an array without external phase modulators and thus can lead to a big coherent emitting aperture that can be controlled, focused, distortion corrected, scanned in space, and pulsed by using pure electronic control.…”

Phase-Controlled Apertures Using Heterodyne Optical Phase-Locked Loops

“…We propose a novel electronic feedback scheme using voltage controlled oscillators (VCOs) that enables direct electronic control of the optical phase of an SCL. Electronic feedback con- trol eliminates the need for optical feedback or expensive optical components such as optical phase/frequency modulators [2], [3]. Moreover, the VCO functions as an integrating phase shifter, providing a practically unlimited dynamic range for the feedback scheme.…”

Coherent beam combining with multilevel optical phase-locked loops

“…Though single-mode fiber lasers with powers of ϳ1 kW are already commercially available, further increase of the power available from a single-mode fiber will be ultimately limited by nonlinear effects and material damage. Coherent beam combining (CBC) provides a promising approach to obviate this problem and thus further scale up a single beam's power by tens or hundreds of times without degrading the spectral purity and the beam quality [1][2][3][4]. By definition, CBC requires all the beams to have the same frequency and a stable relative phase.…”

“…Among all the factors affecting the combining efficiency, the relative phase remains the most critical and difficult to control. So far CBC has been implemented using common resonator [5], evanescent-wave coupling, self-organizing [4], injection locking [1], and active feedback [6,7] mechanisms. In the active feedback approach described here, the variation of the differential optical path lengths of the combining beams is detected and fed back to a servo system, which maintains a constant relative phase between the beams.…”

“…In the active feedback approach described here, the variation of the differential optical path lengths of the combining beams is detected and fed back to a servo system, which maintains a constant relative phase between the beams. In previously demonstrated work, the phase actuator of the servo system is either an optical phase modulator [1,6], a piezofiber stretcher [8], or an acoustic optical modulator (AOM) [7].…”