Engineering of Coherently Combined, High‐Power Laser Systems

Abstract

In recent years, much effort has been expended toward scaling electric lasers to CW power levels on the order of 100 kW or greater [1]. The key challenge in such scaling is maintaining near-diffraction-limited (DL) beam quality (BQ) to enable tight focusing onto a distant target. Despite the maturation of scalable, diode-pumped laser amplifier technologies such as zigzag slabs [2] or fibers [3], thermal effects or optical nonlinearities currently limit near-DL output from single lasers to an order of magnitude lower power, around 10 kW.Actively phase-locked coherent beam combination (CBC) of N laser amplifiers seeded by a common master oscillator (MO) represents an engineerable approach toward scaling laser brightness B (loosely defined here as B $ power/BQ 2 ) beyond the limits of the underlying single-element laser technology. Ideally, the combined output behaves as if it were a single beam, and B is thereby increased by a factor of N over an unphased array or by a factor of N 2 over any individual laser [4].A compelling architectural advantage of CBC systems in comparison to singleaperture lasers of comparable power is the graceful degradation in response to failure of any gain element. This feature can be elucidated from the scaling of B $ N 2 , so the relative rate of change in brightness as individual lasers fail is 1/B(dB/dN) ¼ 2/N. Hence, for large arrays, the drop in brightness is gradual. For example, failure of 1 out of N ¼ 100 lasers would still allow a CBC system to continue operating at 98% of its original brightness.Active CBC with servo-based phase locking can be straightforwardly engineered for very high channel counts and for very high-power laser gain elements. Recently, Northrop Grumman Aerospace Systems adopted an actively phaselocked approach to combine seven 15 kW Nd:YAG slab amplifier chains to demonstrate the world's first 100 kW electric laser with record-setting brightness [5]. As of this writing, work is underway to extend this technology to achieve similar power levels in a CBC array of fiber lasers with improved BQ and efficiency as well as reduced size and weight [6,7].

RF-Photonic Spatial-Spectral Channelizing Receiver

“…A patented technique [22] allows the distributed antenna aperture to coherently up-convert an incident RF field to the optical domain for passive beam-space processing. This preservation of an incident phase front through the up-conversion process enables beam-space processing of the incident field using free-space interferometry of optical beams [23], [24]. A series of bi-convex lenses take the Fourier transform of the optical field at the input of the free-space optics ("Optical Processor") while distributed Bragg reflector (DBR) filters isolate the first lower sideband of the optical signal generated by the phase modulation of the incident light, while the rejected optical carriers are used to maintain spatial coherence among the array elements as described in [22]- [24]…”

Coherent combining of high brightness tapered amplifiers for efficient non-linear conversion

“…It requires a proper and stable phase relationship of the sampled gain medium. Different approaches have been demonstrated: active phase locking of amplifiers seeded by a single frequency laser split into several beams or passive phase locking of emitters in an extended cavity [14]. Impressive results for CBC using a limited number of high power fiber amplifiers (P > 100W per channel) show the potential of power scaling by CBC reaching kW level powers [15,16].…”

Laser Coherent Combination With Circular Array of Airy Beams

“…For decades, laser coherent combination has been proved to be an efficient way to combine multiple laser beams together for achieving high power output due to its advantage of maintaining good beam quality. And many studies have demonstrated filling factor [4]- [6] is an important parameter to affect coherent combination efficiency [7]- [9]. To increase beam coherent combination, people need to reduce beams space and increase the filling factor of coherent combination array.…”