Coherent beam combining using a 2D internally sensed optical phased array

Abstract

Coherent combination of multiple lasers using an optical phased array (OPA) is an effective way to scale optical intensity in the far field beyond the capabilities of single fiber lasers. Using an actively phase locked, internally sensed, 2D OPA we demonstrate over 95% fringe visibility of the interfered beam, λ/120 RMS output phase stability over a 5 Hz bandwidth, and quadratic scaling of intensity in the far field using three emitters. This paper presents a new internally sensed OPA architecture that employs a modified version of digitally enhanced heterodyne interferometry (DEHI) based on code division multiplexing to measure and control the phase of each emitter. This internally sensed architecture can be implemented with no freespace components, offering improved robustness to shock and vibration exhibited by all-fiber devices. To demonstrate the concept, a single laser is split into three channels/emitters, each independently controlled using separate electro-optic modulators. The output phase of each channel is measured using DEHI to sense the small fraction of light that is reflected back into the fiber at the OPA's glass-air interface. The relative phase between emitters is used to derive the control signals needed to stabilize their relative path lengths and maintain coherent combination in the far field.

Towards solid-state beam steering using a 7-emitter 1550 nm optical phased array

“…These so-called internally sensed OPAs have the advantage of not requiring external sensing optics, allowing them to be constructed entirely within optical fiber. [4][5][6] One disadvantage of internal sensing is, however, the introduction of what is called a π-phase ambiguity caused by light double-passing the same length of optical fiber. If the relative phase of two emitters at the output of the array is π radians, then upon passing through the same length of optical fiber twice it will appear to be 2π.…”

“…The aim of our demonstration system described in this proceeding is to demonstrate observable beam steering using an externally sensed OPA. Previous work at The Australian National University (ANU) by Roberts et al [4][5][6] used a 3-emitter OPA to demonstrate high power handling, internally sensed architecture, and digital phasemeters. By changing the system architecture to a more simple externally sensed arrangement, which is easier to scale up, we implemented a 7-emitter OPA.…”

Fast beam steering and agile wavefront control with an optical phased array

“…By controlling the phase of each emitter in the array, it is possible to steer the beam in the far-field. 1 In this talk we present the design of a scalable digitally controlled OPA. We describe the experimental demonstration of the phase sensing and control system, and progress towards fast (MHz) steering speeds.…”

High power compatible internally sensed optical phased array

“…Almost all existing CBC techniques measure the output phase of the array by sampling the outgoing beam externally using free space optics (e.g., [5][6][7][8]). 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.…”

“…Whilst the OPA design in [10] successfully validated the internal sensing concept, it also highlighted the architecture's incompatibility with in-line fiber amplifiers which do not tolerate back-reflected light. The total combined power of Roberts' OPA was limited to that of the master laser, and unable to support optical powers exceeding the damage threshold of sensitive optical devices (e.g., EOMs, which have a damage threshold on the order of ∼100 mW) in each path.…”