Coherent Beam Combination Research Articles

Phase Alignment of an Array Optical Telescope System Using Balanced Detection

Differential phase shift keying (DPSK) modulation and multi-aperture receiving are effective means for suppressing flickering, deviation, and fragmentation of the light spot by atmospheric turbulence. What is challenging in coherent beam combination of such an array receiver system is to detect and compensate for phase deviation of sub-apertures. In this paper, a method of phase alignment of an array optical telescope system using balanced detection was proposed and demonstrated. The improved Mach Zehnder Interferometer (MZI) can demodulate the digital signal and recover the phase difference at the same time. It also brings a 3 dB gain to the receiver and improves the detection sensitivity of the system. Adequate simulations with OptiSystem and MATLAB were carried out to show that the power value remains near the ideal state of 2.75 mW, and the bit error rate is less than 10-9 after phase compensation, which indicates the effectiveness and accuracy of the proposed method. Furthermore, taking the communication interruption difference of ninety degrees as an example, the system bit error rate was reduced from 1 to 10-35, and communication was established again.

1.2-kW all-fiber Yb-doped multicore fiber amplifier.

We have demonstrated a record-high 1.2 kW, all-fiber multicore amplifier using a six-core single-mode Yb-doped fiber and a multicore pump-signal combiner (PSC). The output power is limited by the pump power of 1.9 kW. We have developed double-clad six-core fibers and PSCs for this demonstration. Each of the six Yb-doped cores has a 17-µm mode-field diameter (MFD) with a trench index profile and is capable of kW-class operation. The potential power scaling to the 10-kW level in a single amplifier with high brightness should be feasible with advanced thermal management and coherent beam combination.

Study of high power CBC fiber laser systems with non-equal splitting ratio beam-splitters

High power coherent beam combination (CBC) with non-equal splitting ratio beam splitters (BS) is studied. A BS-based two-channel CBC laser system has been built, and experiments with 50:50 BS and 30:70 BS has been carried. It shows that, when the inject laser power are the same, the combining efficiency decreases obviously as the ratio of the BS increases. The perfect combining efficiency can be resumed by adjusting the power ratio to be the same as the splitting ratio of the BS. A theoretical model has been established to further analyze the effect of the BS ratio quantitatively. For the condition of two equal inject power, the deviation of splitting ratio should be less than 9% to maintain 1% allowance for combining loss. The work can provide useful guidance for high power BS-based CBC laser sources.

Controlled generation of picosecond-pulsed higher-order Poincaré sphere beams from an ytterbium-doped multicore fiber amplifier

Higher-order Poincaré sphere (HOPS) beams with spatially variable polarization and phase distributions are opening up a host of unique applications in areas ranging from optical communication to microscopy. However, the flexible generation of these beams with high peak power from compact laser systems remains a challenge. Here, we demonstrate the controlled generation of HOPS beams based on coherent beam combination from an Yb-doped multicore fiber (MCF) amplifier. Using a spatial light modulator to adaptively adjust the wavefront and polarization of the signals seeded into the individual cores of the MCF various structured beams (including cylindrical vector beams and first- and second-order vortex beams) were generated with peak powers up to 14 kW for ∼ 92 ps pulses.

Intense vortical-field generation using coherent superposition of multiple vortex beams

Coherent beam combining technology applied to multiple vortex beams is a promising method to generate high-power vortex beams. We utilize the coherent combination of multiple Laguerre-Gaussian beams at the waist plane and propose theoretically a practical generation system for a high-power beam carrying orbital angular momentum by considering oblique incidence. The results demonstrate that the orbital angular momentum distribution of the combined field is similar to that of a single Laguerre-Gaussian beam within the Rayleigh length. Moreover, the combined field has relativistic intensity local spots that exhibit stable spatial propagation. The proposed system may potentially be applied to intense vortical fields, large scale nuclear fusion device, such as suppressing stimulated Raman scattering and filamentation when a laser beam propagates in plasma.

Coherent combination of micropulse tapered amplifiers at 828 nm for direct-detection LIDAR applications.

We report on the design of a compact master oscillator power amplifier (MOPA) diode laser architecture at 828 nm suitable for direct-detection LIDARs, specifically applied to water vapor differential absorption LIDARs. Coherent beam combination of two pulsed high-brightness tapered amplifiers (1 μs, 10 kHz), seeded by a DBR laser diode, is demonstrated. The phase dynamics during the pulses have been thoroughly investigated. The main limitation to the CBC efficiency is quantified. The maximum combined pulse energy reaches 10.3 μJ with combining efficiency above 82% ± 5%.

Controllable customization of optical vortex lattices with coherent laser array

The customization of optical vortices with controllable structures is of fundamental importance for promoting various applications such as materials processing, optical communication, and optical tweezers. Here, a model of coherent beam combining (CBC) architecture was proposed to customize the exotic light beams array with controllable structures. The simulated results revealed that the optical vortex lattice (OVL) with multi-modes can be generated in the far field, and a proof-of-concept experiment was carried out for further verification. The experiment results were in a high degree of fidelity with the simulated results, indicating that the controllable structures could be generated efficiently by using the CBC architecture. The phenomenon of generated hollow beam having no orbital angular momentum (OAM) and the OVL copying can be observed. Furthermore, the phase detection experimental results verified the OAM states, the topological charge (TC) could be modulated from −2 to + 2. This work may present a new understanding of generating and manipulating the structured light fields.

HIBISCUS: Simulator for the development of control strategies for coherent beam combining lasers with PISTIL technique

CBC (Coherent Beam Combining) is a key technology for the realisation of intense lasers. In this context, PISTIL (PISton and TILt interferometry), a precise metrology tool for measuring segmented wave surfaces, has been developed and used in particular to characterise and diagnose CBC ultrafast and digital laser in the framework of the XCAN (X Coherent Amplification Network) project at the École Polytechnique. We propose here to use it in a way to help the optimisation of control techniques by including PISTIL in an XCAN type CBC laser simulator. This will allow an easy tuning of the control laws, outside the clean rooms in which these large lasers are deployed and without the need to start them up.

Coherent polarization beam combination of two ultrafast laser channels based on fiber stretcher phase locking

Coherent polarization beam combination of two ultrafast laser channels based on fiber stretcher phase locking

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 (120steps) than the traditional SPGD algorithm (830steps). Furthermore, the average root mean square of residual phase error was decreased to 0.03rad (∼λ/209) with a laser array of 259 channels (7∗37), which was 0.36rad (∼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.

Optimizing the spectrum of high power narrow linewidth fiber amplifier through the same complex degree of coherence

The spectra of narrow linewidth fiber amplifiers are closely related to both the stimulated Brillouin scattering (SBS) threshold in power scaling process and the combining efficiency in coherent beam combining (CBC) system. In this manuscript, the SBS thresholds of fiber amplifier with different spectral distributions (Gaussian, sinc2 and rectangular) have been compared under the same spectral complex degree of coherence (CDC), which could promise the same combining efficiency in CBC system. A SBS dynamic model is established to analyze the SBS process in fiber amplifier and a comparing experiment is also performed by measuring the SBS thresholds of different spectra that have the same CDC set to be 0.96. The FWHM linewidths of Gaussian, sinc2 and rectangular spectra are adjusted to be 1.1 GHz, 0.5 GHz and 1.06 GHz, respectively. The corresponding SBS thresholds are measured to be 108 W, 77 W, and 135 W. By contrast, the rectangular spectra could have most excellent capacity on improving SBS threshold in fiber amplifier under the same combining efficiency in CBC system. Overall, it could provide a feasible method on spectra designing in high power narrow linewidth fiber amplifiers used in CBC system.

Free-space remote detection of a spinning object using the combined vortex beam.

The rotational Doppler effect (RDE) associated with orbital angular momentum (OAM) has been used for remote sensing of a spinning object. However, one of the challenges of long-range detection stems from the low echo signal power. In this paper, we propose a new detection scheme that uses the combined vortex beam (CVB) generated by coherent beam combining (CBC) technology as the probe beam to enhance the echo signal power. Furthermore, we establish a rotational speed remote sensing model based on RDE, the emitted power and emission diameter of the probe beam are investigated in detail. The results show that, compared with the superposition vortex beam (SVB) generated by a single laser beam, the CVB detection scheme can significantly enhance the echo signal intensity and detection distance. The measuring range and accuracy of rotational speed are also studied in detail. And finally, we present the first experimental demonstration of the RDE arising directly from the interaction of the CVB with a rotating rough surface. The scheme proposed in our paper offers a good reference for practical application of the remote detection based on RDE.

Generation of off-axis phased Gaussian optical array along arbitrary curvilinear arrangement

Vortex beams can potentially be used in optical communication, imaging, and processing applications, herein, we propose a method to generate a phased Gaussian optical array (PGOA) by arranging an aperture-bounded Gaussian optical array along arbitrary curves and using a discontinuous phase to form a controllable vortex phase. The phase difference between adjacent sub-beams is constant, and the total phase increment is 2πl. The experimental limitations of this technique are discussed, including the aperture radius, off-axis distance, number of sub-beams, and maximum topological charges. The coaxial interference characteristics of the PGOA in free space were examined numerically by using a split-step Fourier transform algorithm. We theoretically and experimentally proved the feasibility of a coherent beam combination scheme to generate a PGOA, which is of particular significance for particle manipulation and free-space optical communication applications.

Far-field phasing method based on deep learning for tiled-aperture coherent beam combination

Deep learning, which is a branch of machine learning, has gained increasing attention in the field of optical measurement owing to its excellent recognition ability. In this study, we applied a deep learning method to measure the phase error from the far-field interference fringe pattern in the two-beam tiled-aperture coherent combination system. We performed both numerical simulations and experiments to verify the validity of this method. The RMS accuracy of the phasing method in the simulation was λ/60, whereas in the experiments, it was λ/30. As an extension, the measurement accuracy of the four-beam simulation reached least λ/40. It satisfies the phase synchronization requirements at 95% combining efficiency in the coherent beam combination system.

Deep-turbulence phase compensation using tiled arrays.

Tiled arrays use modulo-2π phase compensation and coherent beam combination to correct for the effects of deep turbulence. As such, this paper uses wave-optics simulations to compare the closed-loop performance of tiled arrays to a branch-point-tolerant phase reconstructor known as LSPV+7 [Appl. Opt.53, 3821 (2014)10.1364/AO.53.003821]. The wave-optics simulations make use of a point-source beacon and are setup with weak-to-strong scintillation conditions. This setup enables a trade-space exploration in support of a power-in-the-bucket comparison with LSPV+7. In turn, the results show that tiled arrays outperform LSPV+7 when transitioning from weak-to-strong scintillation conditions. These results are both encouraging and informative for those looking to tackle the branch-point problem in adaptive optics.

Coherent beam combining array arrangement with a large number of elements

Improving the beam quality is the constant pursuit of the coherent beam combining (CBC), and CBC is developing toward a large number of elements. In this work, we employed the Fermat spiral array (FSA) in CBC and investigated the CBC effects, compared with traditional periodic arrays. With the element number increasing, the filling factor was increased for FSA, while it was decreased for the hexagonal and ring arrays. In particular, the achieved power in the bucket (PIB) of the CBC with FSA was higher than that of the CBC with hexagonal and ring arrays. Furthermore, the CBC with FSA shows the same tolerance to piston phase error as the CBC with the periodic array, according to numerical calculations. These results provide an effective approach to obtaining better CBC effects, as the CBC develops toward a large number of elements.

Ultrafast Fiber Technologies for Compact Laser Wake Field in Medical Application

Technologies, performances and maturity of ultrafast fiber lasers and fiber delivery of ultrafast pulses are discussed for the medical deployment of laser-wake-field acceleration (LWFA). The compact ultrafast fiber lasers produce intense laser pulses with flexible hollow-core fiber delivery to facilitate electron acceleration in the laser-stimulated wake field near treatment site, empowering endoscopic LWFA brachytherapy. With coherent beam combination of multiple fiber amplifiers, the advantages of ultrafast fiber lasers are further extended to bring in more capabilities in compact LWFA applications.

Watt-level beam combined diode laser systems in a chip-scale hybrid photonic platform.

Scaling up the power of on-chip diode lasers is of great importance for many emerging applications, such as integrated nonlinear optics, remote sensing, free space communication, infrared countermeasure, and light detection and ranging (LIDAR). In this manuscript, we introduce and demonstrate photonic integrated circuits (PIC) based beam combining methods to create power scalable, integrated direct diode laser systems. Traditional laser beam combining, including coherent beam combining (CBC) and wavelength beam combining (WBC), usually requires free space or fiber optical components, leading to bulky and complex systems. Instead, PIC based beam combining methods can greatly reduce the cost, size, weight, and power consumption (CSWaP) of next generation direct diode laser systems. We experimentally demonstrate four channel chip-scale CBC and WBC with watt-level on-chip power by using III/V-Si3N4 hybrid integration. Our results show that PIC based beam combining is very suitable for power scaling in a chip-scale platform.

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.

Internal phase control of coherent fiber laser array without ambiguous phase based on double wavelength detection.

High-power fiber lasers have been widely utilized in manufacturing, medical care, and many other fields. Due to mode instability, nonlinear effects, and so on, the output power of a monolithic fiber laser is limited. Coherent beam combining (CBC) of fiber lasers is a promising way to obtain higher output power. An all-fiber CBC structure with internal phase detection has a compact construction and potential for a larger fiber laser array. For the existing internal active phase control of an all-fiber structure, π phase ambiguity always occurs because of double passing the fiber path. Additional compensation is needed under this condition, and the compactness of the system will decrease. In this paper, internal phase control of an all-fiber structure based on double wavelength detection without π-ambiguity is proposed. By adding a beacon laser with a different wavelength, phase locking of a coherent fiber laser array can be achieved internally without π-ambiguity. A corresponding math model is established, and a phase matched condition is derived. The spectral width of the beacon laser is analyzed, and the result shows that it can reach tens of nanometers with a proper optical path difference. Simulations of seven, 19, and 37 beams are carried out, and the results show that the structure proposed in this paper has the ability to achieve phase control with good robustness. The control bandwidth in the simulation is better than 1 kHz. By properly designing elements, the structure is expected to achieve high-power CBC of an all-fiber structure experimentally.