Adaptive Fiber-optics Collimator Research Articles

Cantilevered adaptive fiber-optics collimator based on piezoelectric bimorph actuators.

A novel, to the best of our knowledge, cantilever construction design of an adaptive fiber-optics collimator (AFOC) based on piezoelectric bimorph actuators for tip/tilt control is introduced. With this new cantilever structure, an AFOC with a diameter of only 6 mm was developed, and the output laser beam deviation angle and resonance frequency of the device were measured. The experimental results show that this new AFOC can provide more than 1 mrad deflection angle at a 20 V driving voltage, and the first resonance frequency is about 500 Hz. Further, in order to verify whether the cantilever structure can be used in a high-power fiber collimator, a high-power X-Y positioner with an 8 mm diameter fiber end cap was developed. The experimental results show that the high-power X-Y positioner can output more than 2 kW laser power and provide about 330 µm displacement of the fiber end cap in the X direction and about 770 µm in the Y direction at a 150 V driving voltage.

Predictive optimization algorithm for beam combination systems based on adaptive fiber optics collimators

Beam combining of fiber lasers based on adaptive fiber optics collimator (AFOC) plays an important role in the laser propagation applications, whereas the system performance is severely affected by the target occlusion during the beam’s long-range atmosphere transmission. In this paper, the sources, characteristics, and potential impacts of the target occlusion were analyzed for the first time, and the predictive stochastic parallel gradient descent (P-SPGD) algorithm was proposed to solve this problem. The basic principle of P-SPGD was to monitor the target occlusion in real-time, and then dynamically choose whether to use the observed gradient or the predicted gradient to update the control voltages of AFOCs. Beam combining experiments based on three AFOCs were conducted in the free space with a distance of 40 m. The results showed that when the moving target was obscured for 3 s, the AFOCs based on the traditional optimization algorithm had lost track of the target, whereas the P-SPGD algorithm can detect this emergency with a probability of 98.8%, and accurately model the motion state of the target. As a result, the AFOCs can still achieve stable target tracking during the period of target occlusion, and keep the combined beams from divergence.

Experimental Demonstration of Adaptive Optics Correction of the External Aberrations for Distributed Fiber Laser Array

A novel method of pre-compensating the phase distortions outside the fiber laser array based on active integrated wavefront sensing is proposed and demonstrated experimentally. The scheme and the working principle are introduced. Pre-compensation experiment of the fiber laser array based on a 7-element adaptive fiber optics collimator (AFOC) array with filled factor of 0.875 is presented. Residual error RMS of $0.11~\mu \text{m}$ is obtained while sensing lower order aberrations of a glass plate is with initial RMS of $0.62~\mu \text{m}$ . Metric of far-field power-in-the-bucket (PIB) is improved about 3.5 times after the phase distortions are pre-compensated. This method could promote the transmission efficiency of the fiber laser array in atmosphere, without limits of array scale and transmission light delay. This method would be upgraded to correct the aberrations located in the whole optical link from laser source to the target in the future. Such method would highly promote the applications of fiber laser array based systems like free space laser communication.

High-resolution beam scanning technique with microlens array and adaptive fiber-optics collimator.

Conventional beam scanning systems employing a microlens array (MLA) suffer from the problem that only discrete diffraction angles can be addressed because of the periodic structure of the MLA. In this paper, an adaptive fiber-optics collimator (AFOC) that continuously adjusts the position of light source (optic fiber output) is used in front of the periodic structure as a moving linear phase shifter to overcome this discrete scanning angle problem. By introducing the AFOC into the beam scanning system employing MLA, a beam scanning system with continuous scanning capability and high resolution is fulfilled. Theoretical simulations and experimental results both demonstrate the continuous high-resolution scanning capacity of the beam scanning system employing both MLA and AFOC. The proposed beam scanning system is expected to find wide applications in space optical communication, optical interconnection, power projection, and coherent beam combining.

Fiber Positioner Based on Flexible Hinges Amplification Mechanism

Due to the nonlinear effect and the thermal effect, a single optical fiber has limitations in the output power. A beam combination of laser arrays based on adaptive optics can both improve the output power and ensure higher beam quality. This article puts forward one novel fiber positioner structure based on the flexible hinges amplification mechanism, which is the adaptive fiber optics collimator (AFOC), to correct tip-tilt aberration. The theoretical model was established, and the amplification ratios between the output displacement and device’s structure parameters were calculated. The first 6 orders of the mode shape of the vibration of amplification mechanism was obtained by using a modal analysis. Analysis results revealed an excellent performance for the flexible hinges amplification mechanism. The novel fiber positioner has good prospects for applications in laser beam combination systems for ideal tip-tilt control.

Wavefront sensing based on fiber coupling in adaptive fiber optics collimator array.

A new method of wavefront sensing based on fiber coupling in the adaptive fiber optics collimator (AFOC) array has been proposed. The scheme and the recovery process of this sensor are introduced. An experiment of recovering static aberrations utilizing such method with a 7-element AFOC array with filled factor of 0.875 is presented. Experimental results reveal that the wavefront recovery residual error RMS is less than 0.116 μm, under the conditions where the original aberration is with RMS of 0.674 μm and mainly includes Zernike modes of low orders. This method aims at providing abilities for active wavefront sensing and efficiently correcting outside aberrations to the future laser transmission systems based on fiber collimator arrays.

Highly efficient coherent conformal projection system based on adaptive fiber optics collimator array

Coherent beam combination (CBC) of fiber laser array, which is a promising approach to overcome the power scaling limitations of the single fiber laser and to achieve high-brightness laser with good beam quality, can be employed in coherent conformal projection system (CCPS) for beam projection and free space laser communication system with enormous potential. To date, all the demonstrated CBC system through several kilometers atmospheric turbulence have employed collimated beams, which significantly decrease the final CBC effect because of the inefficient wavefront overlap. On the other hand, focused CCPS has lots of advantages over collimated projection system since the wave front of the projected beam is closer to that of a convergent spherical wave. In this paper, we design and manufacture focused CCPS based on an adaptive fiber optics collimator array for the first time, which is achieved by introducing controllable spherical aberration through the adaptive fiber optics collimator. A CBC system is setup to evaluate and validate its performance. Results show that compared with the collimated CCPS, the energy portion of the central lobe could be increased by a factor of 44%, which agrees well with the theoretical analysis.

Control Bandwidth Promotion of Adaptive Fiber-Optics Collimator and Its Application in Coherent Beam Combination

Adaptive fiber-optics collimator (AFOC) plays an important role in the coherent beam combining (CBC) systems. To improve the control bandwidth, the frequency characteristics of AFOC are measured, and an accurate transfer function model is established. Through the analysis of this model, we find that the resonance and the inherent response delay are the main limiting factors. To solve these two problems, an active resonance suppression method based on biquad filters and an improved control algorithm named precise-delayed stochastic parallel gradient descent (PD-SPGD) is proposed. The resonance suppression experiment shows that when the frequency is within 10 kHz, the fluctuation in the AFOC's amplitude-frequency characteristic is suppressed within ±3 dB by the biquad filters, and the phase-frequency characteristic is compensated to be similar to a pure delay element (with inherent response delay of about 88 μs). The two-channel CBC experiment shows that the algorithm's iteration rate increased from 1.6 to 6 kHz and the convergence time decreased from 5.63 to 1.83 ms when the biquad filters and PD-SPGD are used together, which indicate that the AFOC's actual control bandwidth is about 3.1 times higher than before.

Wavefront sensing based on fiber coupling of the fiber laser array

A new method of wavefront sensing based on fiber coupling in the fiber laser array has been proposed. The scheme and the recovery process of this sensor are introduced. Numerical simulations of detecting the turbulence-induced aberrations utilizing such method and experiments of recovering static aberrations with 7-element adaptive fiber optics collimator (AFOC) array are presented. Numerical results show that such sensor could effectively recover the wavefront with turbulence-induced aberrations. For hexagonal array with different units, the optimum reconstructed Zernike mode is also different. Smaller array filled factor leads to larger recovery residual error. Compared with array filled factor of 1.0, value of 0.8 is easy to obtain and brings in recovery residual error increment less than 10%. Experimental results reveal that RMS less than 0.075 μm of the recovery residual error is obtained when detecting the static aberration with 7-element AFOC array with filled factor of 0.875. The aberration is with RMS of 0.433 μm and mainly includes Zernike modes of low orders like defocus. Results here validate the effectiveness of the wavefront sensing method proposed here. Such method would get further application in systems like laser array propagating and turbulence aberrations correcting.

Realization of large energy proportion in the central lobe by coherent beam combination based on conformal projection system

In this paper, we experimentally validate a tiled-aperture conformal projection system with the largest array filling factor and element beam truncation factor to the best of our knowledge. The conformal projection system, which is made up of a hexagonal adaptive fiber-optics collimator (AFOC) array with the proximate ideal intensity distributions, is fabricated and the performance of output beam is tested and evaluated properly and carefully. Both of the active phase-locking control and precise tip-tilt control are implemented successfully in the CBC of the hexagonal seven-beam-array. Experimental results show a large energy proportion (47%, which increases by over 10% comparing with the previously demonstrated largest value) in the central lobe is achieved and the residual phase error is lower than λ/27. When the AFOC array performs, the precise tilt control makes the combining beams overlap well and the average normalized metric value is improved from 0.336 without control to 0.947 with both of active piston and tip-tilt phase controls while the fringe contrast increases from 19% to more than 91% correspondingly. This work presents a promising structure for the achievement of large energy proportion in the central lobe in high power fiber laser CBC systems.

Experimental Demonstration of Coherent Combining With Tip/Tilt Control Based on Adaptive Space-to-Fiber Laser Beam Coupling

A novel approach of tip/tilt control through optimizing laser beam coupling efficiencies from space to polarization-maintaining-fibers with stochastic parallel gradient descent (SPGD) algorithm for coherent beam combining (CBC) applications is proposed and demonstrated experimentally using a seven-element adaptive fiber-optics collimator (AFOC) array for the first time to the best of our knowledge. Compared with the normal target-in-the-loop SPGD method with just a single cost function, the tip/tilt control here is independent of the phase-locking control and parallel for each cell of the AFOC array. Such characteristic gives the AFOC array capacities of real-time correction of fast-changing turbulence induced tip/tilt-type phase errors. Piezoelectric-ceramic-ring fiber-optic phase compensator and AFOC are developed to correct the piston- and tip/tilt-type aberrations, respectively. The power-in-the-bucket (PIB) metric is used for phase-locking. Parallel tip/tilt control is implemented via maximizing the received power in each subaperture with fixed two-channel SPGD control. The average of normalized coupling efficiencies of seven AFOCs increases from 0.76 without tip/tilt control to 0.94 under control. In CBC, the PIB metric increases by 4.6 times, and the phase residual error is less than λ /15.

Large deflection angle, high-power adaptive fiber optics collimator with preserved near-diffraction-limited beam quality.

We report on the development of a monolithic adaptive fiber optics collimator, with a large deflection angle and preserved near-diffraction-limited beam quality, that has been tested at a maximal output power at the 300 W level. Additionally, a new measurement method of beam quality (M2 factor) is developed. Experimental results show that the deflection angle of the collimated beam is in the range of 0-0.27 mrad in the X direction and 0-0.19 mrad in the Y direction. The effective working frequency of the device is about 710 Hz. By employing the new measurement method of the M2 factor, we calculate that the beam quality is Mx2=1.35 and My2=1.24, which is in agreement with the result from the beam propagation analyzer and is preserved well with the increasing output power.

Co-Aperture Transceiving of Two Combined Beams Based on Adaptive Fiber Coupling Control

We demonstrate the reception and transmission of fiber laser beams using two-element adaptive fiber-optics collimator (AFOC) array under simulated turbulences. Coupling efficiency of laser beam from free space into polarization maintaining fibers is optimized with tip/tilt aberration correction. According to the principle of optical reciprocity, tip/tilt errors of the phase-conjugated collimating laser beams are compensated simultaneously. The average of normalized coupling efficiency of one AFOC increases from 0.61 without control to 0.85 under control and 0.57 to 0.85 for another. The average of diffraction-pattern centroid’s deviation of two outgoing combined beams in the far-field drops from $17.5~\mu $ rad to $2.35~\mu $ rad, and mean-square-error value drops from 6.59 to $1.35~\mu $ rad. Control bandwidth of 30 Hz for tip/tilt correction is achieved with computer-based stochastic parallel gradient descent controller with update rate of 1.15 kHz. Results indicate the potential value of the AFOC array in fiber coupling, laser communication and beam projection applications.

Experimental research of boresight error correction using returning signals from the illuminated targets through atmosphere

The relationship between boresight error and returning signal from the illuminated target is studied theoretically. The operating principle of a new-style adaptive-optics device, which is named adaptive fiber-optics collimator with the abilities to correct both laser beam transmission error and pointing error, is introduced. The laser beam pointing experimental setup with a laser propagation distance of 200 m through horizontal atmosphere is established. Based on returning signals from the illuminated two-dimensional and three-dimensional targets, the beam pointing closed loop using stochastic parallel gradient descent algorithm (SPGD) algorithm is achieved under different initial boresight errors. The experimental results indicate that the evaluation parameters of the residual boresight errors are less than 6% and 10.8% for the two-dimensional target and three-dimensional target, respectively, which are within the tolerance of theoretical analysis. Finally, the influence of SPGD parameter on moving-target pointing is analyzed.

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.

Adaptive fiber optics collimator based on flexible hinges.

In this manuscript, we present a new design for an adaptive fiber optics collimator (AFOC) based on flexible hinges by using piezoelectric stacks actuators for X-Y displacement. Different from traditional AFOC, the new structure is based on flexible hinges to drive the fiber end cap instead of naked fiber. We fabricated a real AFOC based on flexible hinges, and the end cap's deviation and resonance frequency of the device were measured. Experimental results show that this new AFOC can provide fast control of tip-tilt deviation of the laser beam emitting from the end cap. As a result, the fiber end cap can support much higher power than naked fiber, which makes the new structure ideal for tip-tilt controlling in a high-power fiber laser system.

Experimental demonstration of using divergence cost-function in SPGD algorithm for coherent beam combining with tip/tilt control

A novel approach of tip/tilt control by using divergence cost function in stochastic parallel gradient descent (SPGD) algorithm for coherent beam combining (CBC) is proposed and demonstrated experimentally in a seven-channel 2-W fiber amplifier array with both phase-locking and tip/tilt control, for the first time to our best knowledge. Compared with the conventional power-in-the-bucket (PIB) cost function for SPGD optimization, the tip/tilt control using divergence cost function ensures wider correction range, automatic switching control of program, and freedom of camera's intensity-saturation. Homemade piezoelectric-ring phase-modulator (PZT PM) and adaptive fiber-optics collimator (AFOC) are developed to correct piston- and tip/tilt-type aberrations, respectively. The PIB cost function is employed for phase-locking via maximization of SPGD optimization, while the divergence cost function is used for tip/tilt control via minimization. An average of 432-μrad of divergence metrics in open loop has decreased to 89-μrad when tip/tilt control implemented. In CBC, the power in the full width at half maximum (FWHM) of the main lobe increases by 32 times, and the phase residual error is less than λ/15.

Coherent beam combination of adaptive fiber laser array with tilt-tip and phase-locking control

We present an experimental study on tilt-tip (TT) and phase-locking (PL) control in a coherent beam combination (CBC) system of adaptive fiber laser array. The TT control is performed using the adaptive fiber-optics collimator (AFOC), and the PL control is realized by the phase modulator (PM). Cascaded and simultaneous controls of TT and PL using stochastic parallel gradient descent (SPGD) algorithm are investigated in this paper. Two-fiber-laser-, four-fiber-laser-, and six-fiber-laser-arrays are employed to study the TT and PL control. In the cascaded control system, only one high-speed CMOS camera is used to collect beam data and a computer is used as the controller. In a simultaneous control system one high-speed CMOS camera and one photonic detector (PD) are employed, and a computer and a control circuit based on field programmable gate array (FPGA) are used as the controllers. Experimental results reveal that both cascaded and simultaneous controls of TT using AFOC and PL using PM in fiber laser array are feasible and effective. Cascaded control is more effective in static control situation and simultaneous control can be applied to the dynamic control system directly The control signals of simultaneous PL and TT disturb each other obviously and TT and PL control may compete with each other, so the control effect is limited.

Experimental study on coherent beam combination of fiber amplifiers using adaptive power-in-the-bucket cost function

Beam combination (BC) of fiber amplifiers based on master-oscillator-power-amplifier configuration is an outstanding way to generate high power and high beam quality laser output, where the tip/tilt aberrations among beamlets are considered as a serious influence factor to BC effects. In this paper, a new tip/tilt control method based on adaptive power-in-the-bucket (PIB) cost function is proposed to solve the problem of restricted tip/tilt correcting value in current BC systems. Experimental setup of coherent BC of two-channel 2 W fiber amplifier array is established. Home-made piezoelectric-ring fiber-optic phase-modulator and adaptive fiber-optics collimator are employed to correct piston-and tip/tilt-type aberrations, respectively. The feasibility of proposed tip/tilt control method using adaptive-PIB is demonstrated and nice effect of coherent BC is achieved.

Experimental investigation on coherent beam combination of a three-element fiber array based on target-in-the-loop technique

A kind of target-in-the-loop (TIL)technique which promises the coherent combination of multi-element laser beams in far field is experimentally demonstrated. The physical model of coherent beam combination (CBC)based on TIL is established. A kind of adaptive fiber optics collimator (AFOC)correcting optical path and tip/tilt errors simultaneously is employed to compensate phase errors between beamlets. The CBC of a 3-element fiber array based on TIL is successfully achieved and a perfect effect of CBC is captured. In this paper, we present a reference for long-distance transmission of multi-element coherent beams in real atmospheric environment.