Coherent beam combination of fiber lasers is considered to be a promising technology to obtain high-brightness laser. In tiled-aperture coherent beam combination, in order to obtain the best combination effect, a high fill factor is required for the beam array output from the fiber collimators. Thus, the collimator array must be closely arranged and each single-mode Gaussian beam must be truncated, which brings great difficulties to design a fiber collimator. We propose a scheme of fiber collimator based on rod lens, which compares to the fiber collimator with the thin lens, has the following advantages. Firstly, rod lenses for beam collimation can be arranged in an array without a frame to get a high fill factor of fiber collimators. Secondly, the single-mode Gaussian beam from fiber can be truncated in any proportion to get a good result of coherent beam combination. Thirdly, almost all of the laser energy output from a fiber include the stray light with the large NA is emitted into space outside the collimator by the rod lens. Fourth, the structure of fiber collimator with the rod lens is simple, which makes it easy to build a large number of fiber lasers array. In this paper, we have developed a rod lens collimator and carried out the preliminary experiments, the results show that the rod lens collimator can emit a collimated beam with the good beam quality, and all stray light with the large NA in the collimator can be exported to space.
Coherent Beam Combining (CBC) can scale the overall output power while maintaining high beam quality. In recent years, this technique has also been employed to generate the high-power Optical Vortex Beam (OVB). In this work, we designed a coherent phased array, and an Optical Vortex Beam Array (OVBA) was generated via the CBC technique. Numerical simulation was studied, and an experimental setup was set up. The results showed that the OVBA could be generated in the far field. Meanwhile, the phase detection results indicated that the OVBA included several phase singularities. This work can present a new idea for manipulating the high power structured optical fields.
Optical signal receiver is a crucial element in Free Space Optical Communication (FSOC) system. Single-Mode Fiber (SMF) is commonly utilized in receivers for connecting with the subsequent optical fiber communication devices. However, the conventional SMF’s tiny core size and the random drift of the signal laser’s optical axis cause small field of view, and the optical signal acquisition and coupling issues was limited. Herein, a wide field of view optical signal receiver via pump and signal combiner was proposed and demonstrated. The experimental results show that the detection field of view of the pump and signal combiner with a 20/400 μm double-clad fiber increased about 40 times, compared with using a 6/125 μm SMF. Additionally, with the home-build adaptive fiber tip controller, the wide detection field of view can further improve the stability of optical signal receiving in the dynamic optical signal receiving system.
In this paper, a 1 kW fiber laser with phase-change cool storage technique is proposed and demonstrated. The phase change materials (PCMs) containing copper foam are filled into the heat sink of the fiber laser. The waste heat can be quickly diffused into the PCMs due to the high thermal conductivity and enormous surface area of copper foam. The fiber laser has a maximum output power of 1.2kW, an electro-optical efficiency of roughly 38%, a weight of roughly 17kg, and a dimension of 330×370×86 mm3 . The fiber laser can operate at maximum power for more than 5 minutes. The power-to-weight ratio of the fiber laser is around 70W/kg (not include power supply equipment).
In this paper, an innovation coherent beam combining (CBC) architecture to generate the structured light beams array was proposed and experimented. The simulation and experimental results reveal that the optical vortex beams array (OVBA) with multi-modes can be generated effectively in the far field. The OVBA is composed of multiple sub-OVB in the intensity distribution. Furthermore, the number of OVBs can be modulated by changing the fill factor of the laser array in the near field. In particular, the performance of a OVBA copier was observed, which may deepen the understanding of creating the structured light fields by CBC technique. The experiment results were in excellent agreement with the simulation results. This work could provide valuable and practical reference on generation and manipulation of high power structured light beams
KEYWORDS: Education and training, Photonics, Digital signal processing, Electronics engineering, Machine learning, Laser applications, Computer science, Atmospheric optics, Signal processing, Signal detection
We will show that investigation of coherent beam combining provide good multidisciplinary education and training environment for graduate students. Knowledge including optics and photonics, electronics engineering, computer science, and so on, can be learned and mastered.
In this letter, a two-stage phase control technique is proposed to increase the control bandwidth of the target-in-the-loop (TIL) system. In this technique, the first stage phase control is enabled by LiNbO3 phase modulator to compensate the phase noises in the fiber amplifiers, and the second stage phase control is enabled by the liquid crystal (LC) to compensate the phase noises induced by the atmospheric turbulence. We built a TIL coherent beam combining system with 3-channel coherent fiber lasers over a 40 m atmospheric propagation path. In our experiment, the stochastic parallel gradient descent (SPGD) algorithm was employed for phase control. When the phase control system was in the close loop, the performance of laser beam projection was significantly improved, and the phase locking bandwidth for transmitter side phase distortions reached 1 kHz. This method can be used for applications such as energy transmission and free-space optical communication.
In order to improve the efficiency of LiDAR point cloud object recognition and reduce the computational overhead, a new feature descriptor, Hemispheric Unique Shape Context (HUSC), is presented in this paper by using an improved neighborhood determination method. Firstly, the normal vector and tangent plane at key point are estimated and the local reference frame is established. Then a hemispherical neighborhood is constructed based on the tangent plane and divided into bins according to azimuth, polar angle and radial direction. Finally, the points in each bin are counted and the local feature descriptors of key points are obtained. HUSC feature descriptor can not only ensure the discriminability of descriptors, but also improve the efficiency of object recognition by reducing the number of free bins. Experiments on Bologna dataset and 3DMatch dataset show that HUSC feature descriptor with hemispheric neighborhood is robust to noise, occupying less memory and operating faster.
In this paper, a two-stage phase control method was proposed to increase the control bandwidth of the target-in-the-loop coherent beam combining (CBC) system. Firstly, the principle of the target-in-the-loop CBC system based on two-stage phase control was introduced. In order to verify the feasibility of two-stage phase control technology, then a 7-channel fiber laser array beam combining system was established. The experimental research showed that when the phase noise in the fibers and on the transmission path from the collimators was controlled by two phase controllers respectively, the laser array coherently combined in the far field stably. The power in bucket was 11.4% in the close-loop, which was 70.1% of the theoretical value. The normalized mean voltage detected by the photoelectric detector increased from 0.107 to 0.648, with an increase of 6.1 times. This experiment initially verified the feasibility of the two-stage phase control method, which will be helpful for the control bandwidth increasing in the target-in-the-loop CBC system.
A self-calibration method for camera using two views of unknown-structure planar scene is introduced. The planar scene is common in the environment and can be easily identifiable outside the lab. Firstly, two orientation- and scale-covariant features, which can be provided by the SIFT feature detector, is used to estimate the homography of two views. Then the homography is decomposed into the camera parameters. A RANSAC scheme is adapted to cope with the outliers of SIFT correspondences. Finally, the camera parameters are optimized with a non-linear parameter optimization using the inliers of two views. This method calibrates the camera parameters and recovers the planar scenes simultaneously. Real scene data experiment demonstrates that the proposed method is easy to operate and provides the reliable calibration results for non-expert users.
We proposed a design of coherent fiber-optics-array collimator (CFAC) which is mainly composed of a single unitary collimating lens and prism. The CFAC system can be regarded as a “sub-aperture” of the whole fibers array in the tiledaperture scheme to expand combining channels efficiently due to its simple and compact structure. Then, we setup an experiment to verify the feasibility of the CFAC system with seven fiber lasers arranged in two dimensions and the CBC in 1064nm wavelength using single-frequency dithering algorithm is successfully achieved. By careful calculation, the residual errors among the laser beams are suppressed below λ/20 through an active-piston-phase control.
Transceiver collimator is an advanced component in high power fiber lasers long distance transmission. Here, based on fiber weak taper technology, an all-fiber laser emitting and target detecting transceiver collimator is proposed. Meanwhile, a (18+1)-channel optical fiber bundle detection experiment was succeed build.With a home-made signal processing circuit, the on-line dynamic tracking image display was achieved, and the target detection and transceiver collimator was experimental demonstrated.
The pulse shape from the pulsed amplifier always distorts because of gain saturation effect, an effective method is active control the output pulse shape by reshaping the pulse shape of the laser seed. We demonstrated a new method for active temporal pulse shape control of fiber amplifier with adaptive proportional control. We numerically researched three proportional control methods, including static proportional control, adaptive proportional control and piecewise adaptive proportional control. The results show that proportional control can generate arbitrary temporal pulse shape with high accuracy and less iterations even if parameters of the fiber amplifier are unknown.
A novel method for active coherent beam combining by Particle Swarm Optimization (PSO) algorithm is demonstrated in this paper. The principle of this method is introduced, and its advantages are presented in detail. In the simulation, 37 fiber lasers are coherently combined by employing stochastic parallel gradient descent algorithm with 91 steps, and then, the combined beams are combined by PSO algorithm with only 30 steps. And the result shows that the more laser elements, the more remarkable PSO algorithm is in the CBC system. Because of the high control ability of PSO algorithm in coherent beam combining combined with traditional algorithm, it is scalable to phase-locking system in a large number of fiber lasers.
We present the laboratory experiments of phase locking of a 37-channel tiled fiber array using a stochastic parallel gradient descent (SPGD) feedback controller. The experimental setup comprises a hexagonally close-packed array of thirty-seven 23-mm-diameter fiber collimator sub-apertures. The fraction of the output laser power within a solid angle of 1.22λ/D was measured in open loop state and then in closed loop state. The results show that the power in the bucket was increased from 0.028 in open loop to 0.890 in closed loop. When the 37 beams were phase-locked, the residual phase error was λ/23. The power ratio in the bucket (PIB) was 28% that improved 32 times than the open loop state.
Stimulated Brillouin scattering (SBS) suppression in high power, narrow linewidth and all polarization-maintained amplifiers with near diffraction limited (NDL) beam quality by rectangular optical spectrum is demonstrated. Rectangular spectrum is generated by using cascaded phase modulations. In the preliminary experiment, output power of 509 W with spectral linewidth of ~ 3 GHz is obtained. At maximal output power, the polarization extinction ratio (PER) is measured to be ~ 14 dB and the beam quality (M2 factor) is M2 ~1.2. The technique presented give useful reference to control the spectral linewidth in high brightness fiber amplifiers. By combining with other linewidth controlling techniques, several kilowatt-level output power with spectral linewidth of < 10 GHz could be expected.
Stimulated Brillouin scattering (SBS) management in kilowatt level, high brightness, narrow spectral width, and all polarization-maintained amplifiers by using laser gain competition is demonstrated. This method can increase the SBS threshold of the fiber amplifier by more than twice without changing the linewidth. By further incorporating phase modulation technique, the SBS threshold is scaled to be as high as 32.4 times compared with single frequency amplification. Output power of 1082 W is achieved with polarization extinction ratio (PER) of ~ 14 dB, linewidth of <7.6 GHz and beam quality (M2 factor) of ~M2 =1.14 in a preliminary validation experiment. This kind of fiber source has great potential in the applications of high power coherent beam combining (CBC) or spectral beam combining (SBC) systems.
We have made simulations for a 3*1 photonic lantern and the latter experiments were carried out to test characters of the lantern we designed. The three core lantern with 0.004 rad taper angle has a 15 mm taper length which begins with a 125 μm outer diameter and ends with a 10.4 μm outer diameter(core diameter to outer diameter ratio is 2/25), while the NA of single mode fiber is 0.08 and for 25/400 μm core/cladding diameter few mode fiber it is 0.1. In order to obtain preferable LP01 mode shape, we used a phase control system to lock the phase of each channel staying consistent. The consequence demonstrated that the whole phase distribution of single mode fibers was controlled to an extent. Compared to amplitude, photonic lantern is more sensitive to phase fluctuation. We made simulations using Rsoft to test different variables of light propagating in 3*1 photonic lantern, and the results were in agreement with the conclusion. Thus, coherent beam combination based on dithering phase locked technique was studied systematically and we applied it to 3*1 photonic lantern. As a result, the beam received by light spot analyzer showed a stable mode shape but still existing some differences compared to expected LP01 mode or LP11 mode, which put forward reflection about the basic theory inside photonic lanterns (the beam propagating through tapered region didn’t make sense if using coherent beam combination calculation). At last, we attempted to explore the beam propagation through the lantern and to give any better evaluation function in phase control algorithm so as to improve the phase locked process.
Structured optical field (SOF), which includes vortex beams, non-diffraction beams, cylindrical vector beams and so on, has been under intensive investigation theoretically and experimentally in recent years. Generally, current research focus on the extraordinary properties (non-diffraction propagation, helical wavefront, rotation of electrical field, et al), which can be widely applied in micro-particle manipulation, super-resolution imaging, free-space communication and so on. There are mainly two technical routes, that is, inner-cavity and outer-cavity (spatial light modulators, diffractive phase holograms, q-plates). To date, most of the SOFs generated from both technical routes involves with single monolithic beam. As a novel technical route, SOF based on array beams has the advantage in more flexible freedom degree and power scaling potential. In this paper, research achievements in SOF generation based on array beams are arranged and discussed in detail. Moreover, experiment of generating exotic beam by array beams is introduced, which illustrates that SOF generated from array beams is theoretically valid and experimentally feasible. SOF generated from array beams is also beneficial for capacity increasing and data receiving for free-space optical communication systems at long distance.
KEYWORDS: Fiber lasers, Numerical simulations, Computer simulations, Signal detection, Optical simulations, Modulation, Detection and tracking algorithms, Plutonium, Collimators, Control systems
Beam combination of fiber laser array is an effective technique contributed to improve the brightness of fiber lasers. In order to realize high-efficiency CBC, challenges like phase distortion (mainly including piston and tilt phase aberrations) should be taken into consideration. Resent years, tilt phase aberrations control has been come true by adaptive fiber optics collimator using the stochastic parallel gradient descent (SPGD) algorithm. However, the convergence rate of tilt control system still cannot satisfy the needs of practical application. In order to increase the tilt control bandwidth, a new idea is put forward that applying the orthogonal single frequency dithering (OSFD) technique into tilt control, and numerical simulation has been completed. A hexagonal laser array with 7 elements has been simulated, and each element has a pair of initial tilt angles in horizontal and vertical direction. The initial tilt angles comply with normal distribution. In the same condition, tilt phase control has been realized through SPGD and OSFD individually, and the convergence steps (defined as the iteration steps that improve the normalized PIB above 0.9) with appropriate parameters are respectively about 20 (SPGD) and 7 (OSFD). Furthermore, tilt phase control of large number hexagonal array is simulated, and the results are as follows: for 19/37 elements, the least convergence steps are about 80/160(SPGD) and 19/55(OSFD). Comparing with SPGD algorithm, it is obvious that the OSFD has higher convergence rate and greater potential for tilt control application in large number coherent fiber laser array.
KEYWORDS: Signal processing, Mode locking, Projection systems, Fiber lasers, Collimation, Control systems, Collimators, Wavefronts, Detection and tracking algorithms, Signal detection
A focused conformal projection system (FCPS) has lots of advantages over parallel projection system, especially in near-field coherent beam combining situation. A home-made FCPS based on fiber laser array with adaptive fiber optics collimator is setup and its performances are investigated. First, the coherent beam combining based on the system is successfully achieved with simultaneous end-cap/tilt control and phase-locking control. Then performances of collimated conformal projection system (CCPS) are examined experimentally and consistent with the theoretical results with efficiencies above 80%. At last, we test the feasibility of FCPS in improving coherent combining efficiency. Results show that comparing with CCPS, the CBC efficiency of FCPS improves about 43.5%, from 59.0% to 84.7%, with perfect fitness to the ideal situation.
In this paper, we present an experimental comparison of coherent beam combining (CBC) effect with different truncation factors based on a triangle fiber laser array for the first time to our best knowledge. First, we fabricate a triangle fiber laser array based on adaptive fiber optics collimators with the fixed focusing length of 0.18m and clear aperture of 50mm. Two output fiber arrays (6/125 fiber array and 20/400 fiber array) with different numerical apertures (0.12 and 0.065, respectively) are used to generate different truncation factors. The direct measurement method is used to measure the intensity distribution of the two collimated beams with different sizes. Results show that the beam diameters are 14.5mm and 27.6mm for 20/400 output fiber and 6/125 output fiber, separately. This means that two fiber laser arrays with truncation factors of 0.29 and 0.55 are achieved. Then we numerically calculate the CBC efficiencies of two situations with different truncation factors. The analytical results show that the CBC efficiency improves from 0.144 with truncation factor of 0.29 to 0.413 with truncation factor of 0.55. At last, a CBC experiment platform is set up. Throughout the whole experiment, single frequency dithering algorithm and SPGD algorithm are separately used to perform the phase-locking control and the tilt control. Two CBC experiments of triangle fiber laser arrays are achieved successfully both with residual phase errors about λ/15. By analysis the experimental results, we get the CBC efficiencies are 0.099 (69% of 0.144) and 0.264 (64% of 0.413) for the two fiber arrays. The experimental results identify the importance of truncation factor on CBC efficiency and provide an important reference on the selection of fiber array parameters in order to achieve the largest energy proportion in the central lobe.
In this manuscript, we establish a model and theoretically investigate the novel structure of AFTP designed by ourselves. We analyze each sub-structure of the new type of AFTP and firstly use the software of ANSYS to simulate the deformation of the flexible hinge under the external force. The result shows that the deformation of the flexible hinge is mainly from and almost linear to the middle part. Further, after considering the influence of the levers and piezoelectric actuators, we setup the theoretical model in which the displacement is only relative to the ratio of the lever R. With the optimal value of R, we can get the relative largest displacement of the end cap when the other parameters are confirmed. As the maximal voltage applied on the piezoelectric stacks actuators (PSA) is finite, the largest displacement of the end cap is restricted. Neglecting the influence of the effective friction force (Ff) of inner-system, the relationship between the largest displacement of the end cap and the ratio (R) is derived numerically. From the calculated results, we get the largest displacement is about 67 μm with R of 6.9. This work provides a reference for structure optimization of AFTP based on flexible hinges and levers.
KEYWORDS: Actuators, Fiber optics, Collimators, Fiber lasers, Collimation, High power lasers, Laser systems engineering, Beam splitters, Signal detection, Plutonium
Beam combining (BC) of fiber lasers based on master oscillator power amplifier (MOPA) configuration has been
considered as a promising way to achieve high power laser output. In recent years, it has been demonstrated that tip-tilt phase errors impact the combining effect seriously, especially the beam quality in coherent beam combining even if all the beamlets are phase-locked. Adaptive fiber-optics collimator (AFOC) is an effective way to compensate the tip-tilt aberrations in fiber laser systems. As the piezoelectric bimorph actuators used in the AFOCs of traditional type provide very weak force (0.1~1N level), they can only actuate the naked fiber. So the application of traditional AFOCs in high power level is limited by the structure. When the output power is scaled up-to several kW, a coreless end cap is usually spliced on the output side as the end of fiber. Because of the end cap, the expansion of the beam reduces the extractable fluence and avoids fiber facet damage. Then the AFOCs that can be used in high-power situations become the direction of research and development.
In this paper, a new structure of AFOC based on flexible hinges is presented for the first time to our knowledge. It
utilizes two piezoelectric stacks actuators for X-Y displacement of the fiber end cap placed in the focal planes of the
collimating lens. Also the new type of AFOC based on flexible hinges has been developed and demonstrated
experimentally. The thrust of zero displacement of the piezoelectric stacks actuators is 330 N. The maximum tip/tilt
deviation angle of the collimated beam is 180μrad in X direction and 150 μrad in Y direction for a chosen focal length of 0.05m. The first resonance-frequency of this device is about 700 Hz and the bandwidth of this device is 500Hz. This work provides a reference for beam combing and beam pointing controlling in high power conditions.
A cascaded all-fiber amplifier was set up to investigate stimulated Brillouin scattering (SBS) suppression with
phase-modulation (PM) in our experiment. With a modulation index of π / 2 , the SBS threshold power of the modulated
amplifier can be enhanced with a factor of 3.5 compared with the unmodulated single frequency amplifier. In high power
level, we demonstrate a 275 W all-fiber amplifier, where the SBS threshold enhancement factor is larger than 2.2
compared with the unmodulated case with SBS threshold power of 126 W. Further enhancing the output power is limited
by available gain fiber and pump powers in this case.
KEYWORDS: Gaussian beams, Fractional fourier transform, Laser optics, Chemical species, Fourier transforms, Commercial off the shelf technology, Plutonium, Defense technologies, Signal processing, Analytical research
The fractional Fourier transform (FRFT) is applied to a radial Gaussian beam array. Analytical formula is derived
for the irradiance distribution of coherent and incoherent radial Gaussian beam array in FRFT domain using Collins
integral formula. It is revealed that the irradiance pattern can be tailored to be controllable dark-hollow, flat-topped and
Gaussian beam pattern by changing of the fractional order of FRFT and the coherent state of the laser array.
A remarkable conversion efficiency from laser to protons is demonstrated by particle simulations in a laser-foil
interaction. The total laser-proton conversion efficiency becomes 16.7% in an optimized multi-hole target, though a
conventional plane foil target serves a low efficiency. When an intense short-pulse laser illuminates the thin foil target,
the foil electrons are accelerated around the target by the intense laser. The hot electrons generate a strong electric field,
which accelerates the foil protons, and the proton beam is generated. In our previous study, we found that multihole thin-foil
target was efficient for the energy conversion from laser to protons [Phys. Rev. E 78, 046401 (2008)], and the energy
conversion efficiency was 9.3%. In this paper the results clarify the role of the target hole thickness and depth in the
laser-proton energy conversion. The optimized multi-hole foil target provides a remarkable increase in the laser-proton
energy conversion efficiency.
Multiwavelength seed laser can suppress stimulated Brillouin scattering (SBS) and improve the ultimate output power of
the fiber laser amplifier. Coherent combining of multiwavelength lasers/amplifiers is a promising way to get much
higher total output power than coherent combining of the single frequency lasers/amplifiers. Coherent beam combining
of stimulated Brillouin scattering based multiwavelength fiber lasers is proposed and demonstrated. Multiwavelength
laser is generated using stimulated Brillouin scattering effect by seeded a 10 kilometer single mode fiber laser with a
single frequency laser and phase locking is achieved using stochastic parallel gradient descent (SPGD) algorithm.
Experiment results shows that more than 15 wavelengths are generated for the laser. With active phase control, mean
power of the main-lobe in close-loop is 1.37 times of that value in open-loop and the visibility of the long exposure
interference pattern is 0.37. Scale up this architecture to higher power involves introduction of power amplifiers,
increasing channel number of amplifiers and power of each individual amplifier.
In this paper, a system of containing dual deformable mirrors (DMs) is proposed to adaptively conversion of input
beam with wave front distortion into near-diffraction-limited flattop beam based on the stochastic parallel gradient
descent (SPGD) algorithm. In the analysis, the wave front distortion of the input beam is chosen as the Zernike
representation of Kolmogoroff spectrum of turbulence. The whole shaping system is controlled by the SPGD algorithm.
One DM adaptively redistributes the intensity of the input beam and the other adaptively compensates the wave front of
the output beam. The near-diffraction-limited flattop beams with different parameters are realized by this technique. The
near-diffraction-limited square flattop beam retains an flattop intensity distribution without significant diffraction peaks
for a working distance of more than 60cm in the near field.
Thulium-doped fiber laser (TFL), which emitted near 2 μm laser beam, has become the latest revolution in highpower
fiber laser technology. Further increasing the output power will face great challenges induced by nonlinear effects;
coherent beam combining of TFL can increase laser output power while simultaneously maintaining beam quality.
In this manuscript, we will present our detailed investigation on coherent beam combining of TFLs. Three different
approaches, i.e., interferometric array, mutual injection locking and active phasing based on multi-dithering technique,
are employed. In the interferometric array scheme, coherent combining is realized by using an intracavity fiber coupler
in an all-fiber laser array configuration. Efficient coherent combining can be achieved by providing sufficient loss
discrimination. High combining efficiency of 85% for two fiber laser has been obtained. In mutual injection locking
scheme, mutual coherence between the two fiber lasers is established by means of mutual coupling through two 3dB
couplers. High combining efficiency of 99% for two fiber laser has been obtained, and the fringe contrast of the intensity
pattern at the receiving plane is as high as 93%. In active phasing scheme, when the phase control system is in the closed
loop, the fringe contrast of far-field intensity pattern is improved by more than 75 % from 10 % in open loop, and the
residual phase error is less than λ/20.
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