Coherent Beam Combination System Research Articles

Deep-learning-based phase control method for tiled aperture coherent beam combining systems

We incorporate deep learning (DL) into tiled aperture coherent beam combining (CBC) systems for the first time, to the best of our knowledge. By using a well-trained convolutional neural network DL model, which has been constructed at a non-focal-plane to avoid the data collision problem, the relative phase of each beamlet could be accurately estimated, and then the phase error in the CBC system could be compensated directly by a servo phase control system. The feasibility and extensibility of the phase control method have been demonstrated by simulating the coherent combining of different hexagonal arrays. This DL-based phase control method offers a new way of eliminating dynamic phase noise in tiled aperture CBC systems, and it could provide a valuable reference on alleviating the long-standing problem that the phase control bandwidth decreases as the number of array elements increases.

Efficient coherent beam combining of fiber lasers based on joint multiple access

Coherent beam combination (CBC) is a promising method of obtaining high-power and high-brightness laser output. To further enhance the combination capacity of CBC systems, we present a novel phase-control technique based on joint multiple access. Using this technique, the CBC of eight fiber lasers is successfully demonstrated. When the phase-control system is in the closed loop, the residual phase error is less than λ/36. According to the experimental results, the average phase-control speed of the CBC system is nearly four times as much as that of the single-frequency dithering technique.

Coherent beam combining of fiber lasers using a CDMA-based single-frequency dithering technique

Coherent beam combination (CBC) is a promising method to obtain high-power and high-brightness laser output. In this paper, we present a novel single-frequency dithering technique based on code division multiple access (CDMA). By using orthogonal phase control signals, the number of channels can be increased and the phase control speed in the coherent beam combination system can be boosted. The design method and coding expression of orthogonal signals, as well as the relevant simulations and experiments of the designed signals, have been demonstrated. In the experiment, we design four orthogonal signals and realize the phase locking of five channels. When the phase control system is in the closed loop, the residual phase error is less than λ/30 and the phase control speed of the CBC system is increased to nearly eleven times compared with that of the single-frequency dithering technique.

Development of an All‐Fiber Coherent Beam Combining System toward High Energy Pulse Fiber Lasers

SUMMARYOur present work develops an all‐fiber coherent beam combining (CBC) system to achieve a high energy pulse fiber laser beyond pulse energy limits due to nonlinear effects in rare‐earth‐doped fibers. Coherent beam combining is a power addition technology that obtains high energy density and high beam quality because it integrates multiple laser beams in a monolight wave. However, the CBC using optical fibers is technically difficult because the optical phases and polarization in optical fibers fluctuate due to disturbances. Therefore, we developed a novel all‐fiber CBC system that can precisely control optical path lengths and automatically compensate polarization changes. When the system combined four laser pulses, it achieved a beam‐combining efficiency of 96.6% that was about four times as high as a pulse energy before combining. In addition, the system obtained 97.1% beam‐combining efficiency of CBC using femtosecond pulses that have very low coherences by minimizing the optical path‐length differences between combined two pulses. Moreover, the system successfully regulated the beam‐combining‐efficiency changes to less than 2.0 percentage points in full width by suppressing optical path‐length fluctuations and tracking‐control of the optical phases.

All-fiber phase-control-free coherent-beam combining toward femtosecond-pulse amplification

Our present work is to develop an all-fiber coherent-beam-combining system that achieves a high-energy femtosecond-pulse fiber laser beyond pulse energy limits due to the nonlinear effects in fiber amplifiers. Coherent-beam combining (CBC) using optical fibers is technically difficult because the optical phases and the polarizations in the optical fibers fluctuate due to disturbances. We developed an all-fiber passive CBC system that does not need to control optical phases and polarizations that achieved a beam-combining efficiency of 95.9%. The combined output changes of the passive CBC system are the less than 1.0% in full width.

Novel adaptive fiber-optics collimator for coherent beam combination.

In this manuscript, we experimentally validate a novel design of adaptive fiber-optics collimator (AFOC), which utilizes two levers to enlarge the movable range of the fiber end cap. The enlargement of the range makes the new AFOC possible to compensate the end-cap/tilt aberration in fiber laser beam combining system. The new AFOC based on flexible hinges and levers was fabricated and the performance of the new AFOC was tested carefully, including its control range, frequency response and control accuracy. Coherent beam combination (CBC) of two 5-W fiber amplifiers array with simultaneously end-cap/tilt control and phase-locking control was implemented successfully with the novel AFOC. Experimental results show that the average normalized power in the bucket (PIB) value increases from 0.311 to 0.934 with active phasing and tilt aberration compensation simultaneously, and with both controls on, the fringe contrast improves to more than 82% from 0% for the case with both control off. This work presents a promising structure for tilt aberration control in high power CBC system.

High Power Narrow-Linewidth Nanosecond All-Fiber Lasers and their Actively Coherent Beam Combination

We present theoretical and experimental research on high power narrow-linewidth nanosecond all-fiber lasers and their actively coherent beam combination (CBC). Nonlinear effects limitations of nanosecond pulses, such as stimulated Brillion scattering (SBS), stimulated Raman scattering (SRS), are discussed. SRS limited narrow-linewidth nanosecond all-fiber laser with average power of 913 W is obtained. A novel double-frequency nanosecond SRS nonlinear amplifier is proposed, which can be used to further scale the output power. CBC system for nanosecond fiber lasers is theoretically analyzed in detail. Effects of aberrations on CBC of nanosecond fiber lasers are theoretically discussed. We also experimentally researched CBC of nanosecond lasers with low repetition rate and high average power, respectively. Combined pulses with peak power of 21.5 kW are obtained.

Direct observation of Kramers–Kronig self-phasing in coherently combined fiber lasers

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser. The Kramers-Kronig effect in the fiber gain media was observed to induce a phase shift that effectively canceled the applied path length errors, resulting in efficient lasing under all phase conditions. This process was demonstrated to result in robust lasing over a large range of pump conditions.

Optimal spectral structure for simultaneous Stimulated Brillouin Scattering suppression and coherent property preservation in high power coherent beam combination system

Based on the spectral manipulation technique, the Stimulated Brillouin Scattering (SBS) suppression effect and the coherent beam combination (CBC) effect in multi-tone CBC system are researched theoretically and experimentally. To get satisfactory SBS suppression, the frequency interval of the multi-tone seed laser should be large enough, at least larger than the SBS gain bandwidth. In order to attain excellent CBC effect, the spectra of the multi-tone seed laser need to be matched with the optical path differences among the amplifier chains. Hence, a sufficiently separated matching spectrum is capable at both SBS mitigation and coherent property preservation. By comparing the SBS suppression effect and the CBC effect at various spectra, the optimal spectral structure for simultaneous SBS suppression and excellent CBC effect is found.

Analysis of multiwavelength coherent beam combining effect

The combination effect of multiwavelength active coherent beam combination (CBC) is investigated theoretically. The dependence of the combination effect on the optical path control precision, spectral width, wavelength number, and channel number is revealed. In the case of small optical path variance, the combination effect approximately decreases in quadratic form with wavelength number N, spectral width Δν, and optical path variance σ increasing. In the case of large optical path variance, the combination effect is independent of the optical path variance and the spectral width. The larger the wavelength number is, the smaller the Strehl ratio expectation is, and it finally degenerates to the incoherent combination. The necessity of optical path control is discussed. This study is helpful for multiwavelength CBC system design and the combination effect estimation.

Polarization self-selection in a coherent beam combination system with an all-optical feedback loop

Polarization self-selection in passive phasing of four fiber amplifiers with an all-optical feedback loop is demonstrated. The polarization extinction ratio (PER) of the combined beam is increased, and the polarized direction is selected with the use of a polarization-maintaining (PM) isolator and some non-PM components. The best visibility of the interference patterns is observed at 95.2% and in the largest increment in the PER of the combined beam up to 7.4 dB. Results show that all PM components are unnecessary in the coherent beam combination with an all-optical feedback loop, whereas non-PM components have good potential to achieve high output power.

A coherent beam combination system based on double PZT phase modulators

A novel coherent beam combination system based on double piezoelectric ceramics transducer (PZT) phase modulators is presented and demonstrated for the first time. In this system, two different PZT phase modulators are used for high frequency phase modulation and low frequency phase control, respectively, while in previous demonstrated system, LiNbO3 phase modulators are often employed. The inherent low insert loss and high laser-induced damage threshold of the PZT phase modulator makes the new proposed system more compact and stable. By the way, the experiment of coherent beam combination of two 5-W fiber laser beams based on double PZT phase modulators is done. In the experiment, the PZT phase modulator with 500-kHz frequency response point made in home is used for high frequency phase modulating and another one with 0∼30-kHz linear frequency response range for phase controlling. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved to be more than 90 % from 5 % in open loop.

Impact of Phase Perturbation on Passive Phase-Locking Coherent Beam Combination

The impact of phase perturbation on passive phase- locking coherent beam combination is studied. A ring cavity passive phase-locking coherent beam combination system of an array of two fiber lasers is built. The phase perturbation is experimentally simulated with a phase modulator. Experimental results show that the contrast ratio of central fringe in the far- field beam patterns decreases with an increase of the perturbation frequency or amplitude. Theoretical analysis is given.

Influence of temporal error with different pulse shapes on coherent beam combination system

Coherent beam combination of fiber laser array is an important technology for high-power output fiber laser systems. However, the coherent beam combination is influenced by temporal error, which always exists in the laser array. The coherent beam combination of pulse fiber lasers in the presence of temporal error is investigated in theory in this paper. The effects of pulse shape and temporal error on the coherently combined beam are studied. The characteristics of combined beam, i.e. pulse shape, peak power, far-field pattern and power in barrel (PIB) are calculated and analyzed when the pulse shapes (including square, triangle and sine) and temporal errors are different. It is revealed that for coherently combining square pulse lasers, the pulse shape of the combined beam changes acutely because of temporal error, and the far-field pattern and PIB change proportionally with temporal error increasing; for coherently combining triangle pulse lasers, the pulse shape and the peak power of the combined beam are acutely influenced by temporal error, and the far-field pattern and the PIB change acutely when temporal error is large. It is revealed that coherently combined beam of sinusoidal pulse lasers has good characteristics when two pulse laser with sinusoidal pulse shapes are coherently combined and the time delay between the two pulses is less than 10% of pulse duration.

β因子用于被动相干组束系统光束质量评价的探讨

β因子用于被动相干组束系统光束质量评价的探讨

Influence and simulated correction of tip/tilt phase error on fiber laser coherent beam combination

Near-field models of coherent beam combination (CBC) with different beam channels and fill factors are employed, and the influences of tip/tilt phase errors on CBC are studied. The key device for tilt-error correction, named adaptive fiber optics collimator(AFOC), is introduced. A MOPA-configuration fiber laser CBC system compensating both piston and tip/tilt phase errors is designed. Then, the dynamic courses of tilt-error correction using stochastic parallel gradient descent (SPGD) algorithm in a seven-channel system are simulated, and the effects of CBC at different tilt-error amplitudes and frequencies are investigated. Experimental results show that the tilt-error should be corrected to improve CBC effects, and algorithm's control ability drops off when tilt-error amplitude and frequency increase. The results in this paper present a reference for multi-channel high power fiber laser CBC systems in real atmospheric environments.

单链路光束波前畸变对相干合成系统相位控制的影响

单链路光束波前畸变对相干合成系统相位控制的影响