Abstract
The technical embodiment of the Huygens-Fresnel principle, an optical phased array (OPA) is an arrangement of optical emitters with relative phases controlled to create a desired beam profile after propagation. One important application of an OPA is coherent beam combining (CBC), which can be used to create beams of higher power than is possible with a single laser source, especially for narrow linewidth sources. Here we present an all-fiber architecture that stabilizes the relative output phase by inferring the relative path length differences between lasers using the small fraction of light that is back-reflected into the fiber at the OPA's glass-air interface, without the need for any external sampling optics. This architecture is compatible with high power continuous wave laser sources (e.g., fiber amplifiers) up to 100 W per channel. The high-power compatible internally sensed OPA was implemented experimentally using commercial 15 W fiber amplifiers, demonstrating an output RMS phase stability of λ/194, and the ability to steer the beam at up to 10 kHz.
Highlights
Optical phased arrays (OPAs) provide a way to scale optical power beyond the capabilities of conventional narrow linewidth continuous wave fiber lasers, which are typically limited by the onset of non-linear effects such as stimulated Brillouin scattering (SBS) [1]
Assuming that Le can be minimized by design, and that large mode area optical fiber can be used to increase Ae, the remaining strategy is to broaden the linewidth of the laser, which can be achieved by modulating its phase with high-frequency pseudo-random noise to ‘spread’ the energy of the carrier frequency [16, 17]
Free-space acousto-optic modulator (AOM) were placed in the local oscillator and high-power arms to: 1) generate a heterodyne beat-note at the out-of-loop photodetector for direct measurement of the OPA’s output phase stability; 2) allow the OPA to be locked without digitally enhanced heterodyne interferometry (DEHI), providing a useful baseline from which to gauge what kind of effects DEHI has on ΦRMS; and 3) prevent parasitic interference caused by the small fraction of zero-order unshifted light that couples with the first-order shifted light into the optical fiber
Summary
Optical phased arrays (OPAs) provide a way to scale optical power beyond the capabilities of conventional narrow linewidth continuous wave fiber lasers, which are typically limited by the onset of non-linear effects such as stimulated Brillouin scattering (SBS) [1]. In contrast to external sensing, Bowman et al [9] and Roberts et al [10] presented a technique that does not require free-space optics to measure the output phase of the beam, instead relying on the small fraction of light that is reflected back into the fiber at the OPA’s glass-air interface to infer the relative phase of each emitter. This internal sensing technique infers the differential phase between uncommon paths by measuring the phase of the back-reflected light that double-passes each fiber. The OPA described here (shown in Fig. 1) overcomes both of these limitations, enabling it to operate at optical powers restricted only by the damage threshold of the asymmetric fiber couplers, and the onset of SBS in each emitter
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