A novel technique for electronic phasing of high power fiber amplifier arrays

ABSTRACT We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be O /60. Recent results will be reported. To the best of the authors’ knowledge this is the highest fiber laser power to be coherently combined. Keywords: Laser arrays, optical fibers, optical amplifiers, phase locked lasers 1. INTRODUCTION T O achieve the high brightness required for many laser applica tions it is necessary to phase lock multiple element optical arrays. Recently, IPG Photonics has reported a 6-kW broad linewidth single mode fiber with a near diffraction limited optical beam [1]. The intensity and hence the power available from a single-mode optical fiber is limited either by optical surface damage or nonlinear optical effects. These limitations can be overcome by coherent beam combining the power from multiple optical fiber amplifiers. We have demonstr ated a novel coherent beam combining system that offers not only highly accurate and robust phase locking, but in addition, is readily scalable to more than 100 elements. Furthermore, this is the first phased array locking system that doesn’t require an external reference beam. The results of the first experimental demonstration of the LOCSET technique at high powers are presented. Accurate control of the optical phase is required for any phase locked multi-fiber approach. In a master oscillator power amplifier configuration, the optical paths of each of the fibers must be locked to within a fraction of the wavelength in order to coherently combine the individual outputs into a single, high-power beam. As a result of time varying thermal loads and other disturbances, active feedback is required in order to provide for stable coherent addition. There have been a number of experimental and theoretical re search efforts addressing the need for very high brightness fiber laser sources. The technical approaches that have been attempted include the optical self-organized approaches [2-7] and RF phase locking methods [8-16]. To date the electronic phase locking methods have exceeded the performance of the self-organized approaches in beam quality [8-16], in efficiency of coupling power to the central lobe (88%)[13] and in setting the record for phase locked fiber laser power [11]. Prior to the development of LOCSET, all of the electronically phase locked fiber arrays required an referen ce beam that was phase modulate d at an RF frequency [8-11]. The light emerging from each element was then interfered with the light from a refe rence beam at the photodetector or an array of photodetectors. The light from each element must be sent to a spatially isolated photodetector, because the RF phase modulation was impressed solely upon the reference b eam. Good fringe visibilities of > 94% and hence very low phase errors have been consistently achieved using electronic phase locking methods. Table 1 summarizes the current state of the art in fiber beam combining, including active coherent beam combining, passive coherent beam combining, and spectral beam combining. The squares with dark backgounds indicate the best results for a given beam combination method.