Active coherent combining of laser beam arrays by means of phase-intensity mapping in an optimization loop

System design for coherent combined massive fiber laser array based on cascaded internal phase control

Coherent beam combining (CBC) of a fiber laser can scale the output power while maintaining high beam quality. However, phase detection and control remain a challenge for a high-power CBC system with a massive laser array. This paper provides a novel, to the best of our knowledge, cascaded phase-control technique based on internal phase detection and control, called the cascaded internal phase-control technique. The principle of the technique was introduced in detail, and the numerical simulations were carried out based on the stochastic parallel gradient descent (SPGD) algorithm. The results indicated that the cascaded internal phase-control technique was compatible with the massive laser array. Compared with the traditional CBC based on the SPGD algorithm, the control bandwidth could be improved effectively about seven times (120 steps) than the traditional SPGD algorithm (830 steps). Furthermore, the average root mean square of residual phase error was decreased to 0.03 rad (∼λ/209) with a laser array of 259 channels (7∗37), which was 0.36 rad (∼λ/17) in the traditional SPGD algorithm. In addition, the element expanding capacity was analyzed. Since there is no large-aperture optical device in the phase-detection system, this technique has the advantage of freely designing the caliber of the laser emitting system. This paper could offer a reference for the high-power massive laser array system design and phase control.

Scalable all-fiber coherent beam combination using digital control

This paper describes a filled-aperture coherent beam combining (CBC) system based on locking of optical coherence via single-detector electronic-frequency tagging (LOCSET). The sensing and control architecture is implemented using a field-programmable gate array and high-bandwidth electro-optic phase modulators. The all-fiber optical configuration consists of a narrow linewidth 1560 nm seed laser separated into three channels, each containing 7 W erbium-doped fiber amplifiers. The system was demonstrated experimentally, achieving a total stabilized output power of 20 W, a combination efficiency greater than 95%, and an output RMS phase stability of λ/493. As this architecture employs an entirely digital sensing and control scheme based on LOCSET, it presents a highly scalable and cost-effective solution for CBC that is wavelength agnostic and can support an arbitrarily large number of channels.

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

Fiber laser technology has been demonstrated as a versatile and reliable approach to laser source manufacturing with a wide range of applicability in various fields ranging from science to industry. The power/energy scaling of single-fiber laser systems has faced several fundamental limitations. To overcome them and to boost the power/energy level even further, combining the output powers of multiple lasers has become the primary approach. Among various combining techniques, the coherent beam combining of fiber amplification channels is the most promising approach, instrumenting ultra-high-power/energy lasers with near-diffraction-limited beam quality. This paper provides a comprehensive review of the progress of coherent beam combining for both continuous-wave and ultrafast fiber lasers. The concept of coherent beam combining from basic notions to specific details of methods, requirements, and challenges is discussed, along with reporting some practical architectures for both continuous and ultrafast fiber lasers.

Small footprint phase locking system for a large tiled aperture laser array

A phase sensing system fitted to the control of coherent laser beam array of large cross section is experimented. It is based on the use of a fiber bundle that collects a weak part of the synthetic wavefront, that scales it down (1/40) and that reshapes it in a more compact arrangement (2D to 1D array). Then, the reconfigured beam array can be analyzed by a small footprint system making the large laser beam array easier to phase-lock. The discrete laser array wavefront transmitted by the meter long fiber bundle was stabilized thanks to a multiple arm servo loop. Laser array phase locking was further ensured by random scattering through a diffuser, associated to an alternating projection algorithm. Six fiber laser beams constituting a 110 mm diameter synthetic aperture, were phase-locked with λ/16 accuracy.

Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers

Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of λ/104.

Efficient phase-locking of 37 fiber amplifiers by phase-intensity mapping in an optimization loop

Coherent combination of laser beams from 37 fiber amplifiers in a tiled aperture configuration has been achieved thanks to an innovative iterative process. The high efficiency as well as the speed of the phase control demonstrated the relevance of the method for phase locking of a large array of fiber lasers.

Highly scalable femtosecond coherent beam combining demonstrated with 19 fibers

Coherent beam combining in the femtosecond regime of a record number of 19 fibers is demonstrated. The interferometric phase measurement technique, particularly well suited to phase-lock a very large number of fibers, is successfully demonstrated in the femtosecond regime. A servo loop is implemented to control piezoelectric fiber stretchers for both phase and delay variation compensation. The residual phase errors are below λ/60  rms. Nearly 50% of the total energy is contained in the far-field central lobe. After compression, we obtain a combined pulse width of 300 fs identical to the master oscillator pulse width.