2020 Vol. 49, No. 10
2020, 49(10): 20200267.
doi: 10.3788/IRLA20200267
Optical imaging-based flow measurement techniques, like particle image velocimetry, are vulnerable to optical distortions caused by inhomogeneous refractive index or fluctuating phase boundaries. These distortions can lead to blurred particle images and uncertain tracer particle position assignment, resulting in a degradation of velocity measurement accuracy. In order to improve the measurement accuracy, adaptive optics system can be applied to correct distortions. For imaging metrology in fluid mechanics, the optical distortions have features of large frequency range, high spatial frequency and large dynamic range. Actuator-based approaches are limited by its performances. In our work, a novel intelligent adaptive optic system was applied to flow measurement, a learning-based aberration correction method without wavefront corrector was demonstrated, which was used to correct distortions in imaging-based flow measurement. A particle image velocimetry setup which can measure wavefront aberration was built to generate training and test dataset for deep neural network, and also the distortion caused PIV image degradation model. The correction performance of the trained neural network was quantitatively evaluated by corrected PIV image quality and flow measurement result.
2020, 49(10): 20200273.
doi: 10.3788/IRLA20200273
Aiming at drawbacks of slow convergence rate and multiple measuring on focal or defocus plane by CCD in phase diversity algorithm, a single-frame deep learning phase retrieval algorithm based on defocus grating was proposed. Algorithm used a defocus grating to modulate incident wavefront, far-field intensity distribution of focal and positive/negative defocus plane can be acquired on focal plane of lens at the same time. In addition, convergence rate was improved when algorithm applied CNN to replace multiple perturbation optimization process. Numerical simulations indicate that the proposed method can achieve precise high-speed wavefront reconstruction with a single far-field intensity distribution, root mean square (RMS) of residual wavefront is 6.7% of that of incident wavefront, computing time for algorithm to perform wavefront reconstruction can be less than 0.6 ms.
2020, 49(10): 20200274.
doi: 10.3788/IRLA20200274
SPGD is a control algorithm widely used in wavefront sensorless adaptive optics (AO) systems. The gain is commonly set to a fixed value in the traditional SPGD algorithm. With the increase of the number of DM elements, which can easily lead to the slow convergence speed of the algorithm and the increase of the probability of falling into the local extreme value. Adam optimizer is an optimized stochastic gradient descent algorithm commonly used in deep learning. It has the advantage of achieving adaptive learning rate. The advantages of Adam optimizer adaptive gain and SPGD algorithm were combined to realize adaptive gain for AO system control. The simulation model of wavefront sensorless AO system was established with 32, 61, 97 and 127 elements DM as wavefront correction devices respectively, wavefront aberrations with different turbulence intensities as correction objects. The results show that the optimized algorithm converges faster than basic SPGD algorithm and the probability of falling into local extremum decreases. As the number of DM elements increases and the turbulence intensity increases, the advantages of the optimized algorithm are more obvious. The above research results provide a theoretical basis for the practical application of the SPGD algorithm based on Adam optimization.
2020, 49(10): 20200299.
doi: 10.3788/IRLA20200299
Applying adaptive optics (AO) system to correct aberrations is an effective technical way to improve the performance of optical systems. In order to ensure the long-term, safe and stable operation of the AO system, it is necessary to monitor the operating data of the AO system and identify the instability state of the system to provide decision making suggestions. Based on the above purpose, a set of 127 units AO system instability data simulation platform was established. The abnormal data frames were inserted into the closed-loop operation of the simulation platform, and the abnormal data sets under four kinds of closed-loop instability were obtained. Once the deformable mirror, the core component of AO system, worked abnormally, it will threaten the safety of the system. Based on the deformable mirror control voltage rms index, three machine learning methods were used:
2020, 49(10): 20201042.
doi: 10.3788/IRLA20201042
Up to now, optical phased array (OPA) technology has been developed for more than 70 years. According to different applications, a variety of devices and techniques have been developed, such as liquid crystal, MEMS, optical waveguide, coherent beam combination and so on. Optical phased array technology has been applied in laser radar, space optical communication, high brightness laser generation, synthetic aperture detection and other application fields. By controlling the phase of sub-beam in the beam array, the phase plane of the array beam can be reconstructed and accurately controlled with the optical phased array technology, which has the advantages of small volume and weight, fast response speed and good beam quality. In this paper, the principle of optical phased array was firstly introduced. Secondly, the development status, application areas and development trend of several main techniques were reviewed from two aspects of laser emission and long-distance imaging. Finally, some thoughts and suggestions were given.
2020, 49(10): 20201043.
doi: 10.3788/IRLA20201043
In adaptive optics (AO) system, the traditional Hartmann wave-front sensor can only be used to effectively correct atmospheric turbulence within a small field of view, while the light field camera served as the wave-front sensor has the characteristics of large field of view and multi-angle directional turbulence information obtained by one single exposure, which can replace multiple wave-front sensors in the traditional multi-layered conjugation adaptive optics (MCAO) system and simplify the system and save costs. In this paper, the light field camera module in Seelight, an optical system simulation software independently developed, was used to restore the complete wavefront of the large field of view combining the optical field digital refocusing technology and the mode atmospheric analysis technology. An adaptive optical simulation system was built with a light field camera and a 89 unit deformable mirror. The simulation results show that the optimized AO system can effectively correct the wavefront distortion caused by atmospheric turbulence in a large field of view under the closed loop working mode.
2020, 49(10): 20200333.
doi: 10.3788/IRLA20200333
Retinal optical coherence tomography (OCT) technology uses external low coherence light source to irradiate the fundus of the human eye, and interfere scattered signals of the fundus of the human eye to obtain the sectional image information of the human retina, so as to realize the non-invasive, real-time and in vivo optical biopsy of the human retina. The axial resolution of traditional optical coherence tomography in retinal imaging can reach more than 3 μm, but the transverse resolution of OCT can only reach about 15-20 μm due to individual differences and inevitable aberrations. Adaptive optics, as an advanced technology of wavefront correction, can correct OCT chromatic aberration and aberrations caused by limited field of view and eye movement, so as to improve the transverse resolution of OCT to less than 2 μm. Adaptive optics OCT can realize near diffraction limit imaging of retinal cells and microvessels to timely detect the early lesions in patients with fundus. Based on the introduction of the technical characteristics of adaptive optics and retinal optical coherence tomography, the development status of adaptive optics in retinal optical coherence tomography at home and abroad was reviewed, and the key technologies and future development trends of adaptive optics OCT retinal high-resolution imaging in wide-band light source chromatic aberration correction, eye movement artifact reduction, adaptive optics field of view expansion and wavefront sensing and correction system simplification were summarized, so as to realize high-speed retinal imaging with large field of view, high efficiency, high sensitivity and high resolution, and provide reference for the future research and application of adaptive optics OCT retinal imaging technology.
2020, 49(10): 20200076.
doi: 10.3788/IRLA20200076
In the exploration application of space-based remote sensing, dynamic range and spatiotemporal change rate are the important factors affecting the high-precision detection, tracking and identification of infrared high temperature targets. Generally, influenced by the dark current and circuit noise of the detectors, the dynamic range of a single scene of the detection system is usually less than about 72 dB, and it is difficult to cover the super targets with the dynamic range of about 100 dB, which reduces the recognition ability and quantitative description accuracy of the target. In terms of the problems mentioned above, an adaptive parameter adjustment technique for the detection payloads was proposed. By adjusting the integral time through double exposure and adaptive strategy, the accurate description of target energy could be achieved while maximizing the signal-to-noise ratio of target detection. Simulation results show that the proposed method is simple to operate and can effectively detect and describe targets in the whole dynamic range. This method can provide a useful reference for the research of intelligent detection of space-based objects.
2020, 49(10): 20190131.
doi: 10.3788/IRLA20190131
In order to study the effects of the parts coated by infrared low-emissivity material on the wall temperature and infrared radiation (IR) characteristics of the exhaust system, the wall temperatures and IR characteristics of the axisymmetric and serpentine 2-D exhaust system were measured in three coating schemes, i.e., (1) only central cone was coated; (2) only mixer inner surface was coated; (3) both central cone and mixer inner surface were coated, under the same experimental condition. The experimental results show that the temperature of the surfaces coated with low-emissivity materials increases, and that of the non-coating surfaces is also affected by the coating layer. When high-temperature parts are coated with low-emissivity materials, the IR intensity of the exhaust system can be effectively suppressed in detection angles where the coating surface can be directly detected, while in other angles the IR intensity varies with the coating scheme. It is appropriate to only coat the inner surface of the mixer for serpentine 2-D exhaust system, and coat the central cone and the inner surface of the mixer together for an axisymmetric exhaust system in order to achieve better IR suppression.
2020, 49(10): 20200036.
doi: 10.3788/IRLA20200036
In this paper, the influence models of different diffraction elements on diffraction efficiency were established, and the diffraction efficiency among single diffraction element, harmonic diffraction element and double diffraction element was compared. The advantages of using double diffraction elements in infrared optical system were analyzed. The average diffraction efficiency of different material combinations was calculated. Based on this, a hybrid infrared dual-band and dual-field optical system suitable for airborne platform was designed. The resolution of the large field of view was 1.5 m@16 km. The long and the short focal length were 960 mm and 480 mm respectively. The zoom function was realized by switching the mirror to ensure the optical axis stability. The simulation results show that the MTF curves are smooth and close to the diffraction limit under the large temperature difference of −40 - +60 ℃. The RMS radius is within the radius of airy spots, and the minimum characteristic size of the binary diffraction surface is 6.9 μm. The design results meet the engineering requirements.
2020, 49(10): 20200021.
doi: 10.3788/IRLA20200021
At 1 064 nm, the atmospheric transmittance is high and the sky background radiation is small. The use of this laser to carry out satellite ranging is helpful to improve the observation capability of ranging system, and satellite laser ranging of the 1 064 nm has become one of the important development trends of international laser ranging technology. Based on the filter with a bandwidth of 2.2 nm, the noise of the 1 064 nm ranging system was calculated and tested in the daytime, which verified the suppression effect of this filter on the background noise during the day. With the help of the infrared camera at night, the coincidence between transmitting light path and the mechanical axis was realized based on the circular center method, which ensured the laser pointing accuracy is better than 5″ in all-sky area and solved the problem that the laser pointing at 1 064 nm needs real-time monitoring in the daytime. Based on the 1 064 nm laser with a repetition frequency of 1 kHz and a power of 5 W, Shanghai Astronomical Observatory established the 1 064 nm daytime satellite laser ranging experimental system. At the farthest, the effective echo data of geosynchronous orbit satellite was obtained. The experimental study will lay a technical foundation for the application and development of 1 064 nm laser ranging in long-distance satellite and diffuse reflection laser ranging in space debris.
2020, 49(10): 20200006.
doi: 10.3788/IRLA20200006
High-precision space debris observation data is of great significance for spacecraft collision early warning. Laser ranging technology is the most accurate technology in space target distance measurement at present, but there is no angle reflector device on most of space debris, and the echo signal of space debris laser ranging is weak. Array detection technology can improve the success probability of space debris laser ranging detection with weak echo signal. Yunnan Observatory of Chinese Academy of Sciences started to carry out space debris laser ranging test based on array detection technology in 2015, and successfully applied array detection technology such as array superconducting nanowire single photon detector and multi-channel event timer to laser ranging test system in 2017, 2×2 array laser ranging data were successfully collected in the space debris laser ranging test in March 2017, and 4×4 array laser ranging data were successfully collected in the space debris laser ranging test in March 2018. Among them, the minimum target detected was the space debris with orbit height of about 1 000 km and size of Radar Cross Section (RCS) 0.045 m2; the farthest target detected was the space debris with oblique distance of about 5 000 km and size of RCS 18.25 m2.
2020, 49(10): 20200218.
doi: 10.3788/IRLA.20200218
Due to the first photon bias effect of Gm-APD, there exists range walk error in Gm-APD lidar, which will generate a distortion of depth image of the target. Two methods to restrain the range walk error were presented and verified by experiments. Signal restoration method was used to obtain signal photoelectron distribution histogram (SPDH) from the photon counting distribution histogram (PCDH). A sum of two Gaussian functions were used to fit the SPDH through, and the peak position of the curve was found to calculate the distance. The center-of-mass algorithm method on the SPDH was used to calculate the distance through the second method. The high-precision 3D depth-intensity merged images was caputured using the two methods by experiments with a 6 ns width laser pulse. The relative accuracy of intensity measurement of the two methods were both less than 3%. The signal restoration & Gaussian functions fitting method has range precision of 1.2 cm. The signal restoration & center-of-mass algorithm method has range precision of 0.6 cm.
2020, 49(10): 20200142.
doi: 10.3788/IRLA20200142
With the appearance of hypersonic weapon, it is a coming tendency to measure radiation in wide dynamic range from low temperature to high temperature. At present, the research of wide dynamic range radiometric measurement is relatively few. In this paper, a dual correction of collimators and attenuators method was proposed for wide dynamic radiation measurement to further improve the accuracy of high temperature infrared radiation measurement. The experiment was carried out based on the mid-wave infrared radiation measurement system of 600 mm aperture. The radiation measurement results with attenuators 2%, 10% and 20% were corrected, and the original measurement accuracy was increased by 3.31%, 3.27%, 1.60% respectively. It is proved that the correction method can effectively improve the accuracy of radiation measurement, which has certain engineering practice significance.
2020, 49(10): 20200216.
doi: 10.3788/IRLA20200216
Turbocharger and gearbox were widely used in the precision machinery manufacturing industry. The dimensional accuracy of turbine components and stud standard parts was an important guarantee for the assembly accuracy of turbine components and gearbox, among which coaxiality was a key parameter for the dimensional accuracy of turbine components and stud standard parts. According to the demands of coaxiality measurement for turbine components and stud standard parts, a set of checking fixture was developed, and a software based on LabVIEW was built for the measurement. The coaxiality of stud standard M12 was measured by experiment and the uncertainty of measurement was evaluated. The experimental results show that the coaxiality error obtained from the four measurements is 6.3–6.5 μm, and the extended uncertainty reaches 2.6 μm. The results show that the developed coaxiality measurement system is suitable for the high precision measurement of the coaxiality of turbine parts and stud standard parts.
2020, 49(10): 20200221.
doi: 10.3788/IRLA20200221
A recognition method of mineral Raman spectrum based on all-optical diffraction neural network was proposed. Firstly, the data structure characteristics of the Raman spectra of minerals were analyzed, the similarities and differences between traditional neural network and optical diffractive neural network were compared and analyzed, and the optical diffractive neural network was constructed according to the preprocessed data. Secondly, the cross entropy loss function and Adam algorithm were used to train the optical diffractive neural network, and the optimized network parameters were obtained. Finally, under the simulation conditions, the effects of different grid-height accuracy on the accuracy of mineral recognition were verified and analyzed, and the network accuracy and accuracy loss corresponding to the different grid-height accuracy was given. The test results on the RRUFF mineral Raman spectrum database show that the recognition accuracy of five kinds of minerals is 94.2%, which proves the feasibility of Raman spectrum recognition using optical diffractive neural network. It provides a reference for the application of optical diffractive neural network; the accuracy of five kinds of minerals under the condition of 6 bit grid-height resolution is 93.6%, which proves that grid height discretization can not only ensure the accuracy of network, but also greatly reduce the difficulty of grating fabrication. It provides theoretical support for grating fabrication.
2020, 49(10): 20200046.
doi: 10.3788/IRLA20200046
High precision and stability are demanded for mainboard as a core construction of high resolution space camera, thus, traditional measurement methods could not match the application requirements. According to the structure and specification of the mainboard used in a large aperture camera and the capability of measurement equipment, horizontal placement was adopted for flatness test of mainboard datum plane using gantry coordinate measuring machine (CMM). The concept of quasi-kinematic mount was introduced, the mainboard was supported by 3 inverted Bipods to avoid clamping and assembly deformation, a specialized measurement structure was designed and manufactured as well. The position and magnitude of unloading force were confirmed by numerical simulation based on ABAQUS and Matlab with an index of datum plane's flatness error (PV). The results show that using designed measurement structure datum plane's flatness error is less than 2 μm. This method promotes the capability of CMM for measurement of large precise objects and has widely application prospect.
2020, 49(10): 20200017.
doi: 10.3788/IRLA20200017
Phase retrieval is to recover the original phase information by using the intensity information obtained from observation. Transport of intensity equation (TIE), as a traditional non-interference phase retrieval technique, can compute the losing phase information from only a minimum of two intensity measurements at closely spaced planes by solving the equation. This method usually requires the acquisition of intensity images by moving the object to be tested or CCD, which inevitably results in mechanical errors. A new phase retrieval method called chromatic dispersion-hybrid phase retrieval (CD-HPR) was proposed. The object was imaged at the same position by setting different wavelengths of light after passing through the single-lens system, in-focus and defocus intensity images were obtained without mechanical movement, and the initial phase information of an object was calculated from the phase retrieval technique based on TIE by combining the relationship between the defocus amount and the wavelength. Next angular spectrum iteration was used to improve the initial phase information. In this simulation, the RMSE between the phase recovered by this method and the original phase was 0.1076. At the same time, the phase of the lens array was restored by experiment. The error between the experimental result and the real parameter is 3.4%, which proves the correctness and effectiveness of the proposed method. This method extends the limitation of the traditional method that requires the light source to be monochromatic and improves the calculation accuracy.
2020, 49(10): 20200166.
doi: 10.3788/IRLA20200166
Probe light pulses are injected into the sensing fiber by a distributed optical fiber vibration sensing system based on the phase-sensitive optical time-domain reflectometer (Φ-OTDR), then the vibration can be recognized and located by measuring the Rayleigh-backscattered light generated during the propagation of light pulses, so the probe light pulse has important effect on Φ-OTDR system performance. The research progress of Φ-OTDR in pulse modulation was introduced, including the impact of optical pulse width, frequency, shape, extinction ratio and other parameters on the system mechanism, and the research progress in the optimization of system parameters such as signal-to-noise ratio, alarm rate, spatial resolution, response bandwidth and recovery of vibration signal waveform by pulse modulation. In addition, this paper provides a new idea to ascend multiple performance indicators of the Φ-OTDR system simultaneously by means of optical pulse modulation to promote the engineering applications of Φ-OTDR.
2020, 49(10): 20200056.
doi: 10.3788/IRLA20200056
In order to improve the dynamic tracking performance of periscope laser communication terminal servo system, the control system of two dimensional servo turntable based on permanent magnet synchronous motor was designed in this paper. The field oriented control method was adopted to realize the decoupling control of the motor, the control model was established and the design of each control loop was completed. Then, the iterative learning control (ILC) method was designed for the dynamic target tracking to improve the terminal dynamic tracking performance, the speed step response of the control system was tested and the low speed stability of the communication terminal system was analyzed. Finally, the dynamic tracking experiment of 4.62 km laser communication was set up, and platform jitter was simulated with a six-degree freedom turntable to create disturbance conditions of external platform for dynamic tracking verification test. Experimental results show that the steady-state error of the speed step response for the communication terminal system is 0.02 (°)/s, which shows that speed loop servo system has fast dynamic response and high steady precision, based on the maximum acceleration of 0.219 (°)/s2 sine wave disturbance conditions, the dynamic coarse tracking precision for the two-dimensional servo turntable can reach to 62 μrad, the accuracy of coarse and fine tracking is 2 μrad, the effectiveness of the communication terminal servo system and its dynamic tracking performance is verified, which lay the foundation to further improve the tracking precision of terminal system.
2020, 49(10): 20200125.
doi: 10.3788/IRLA20200125
Aiming at the problem of dynamic time-varying network topology and diversified service types in satellite optical network, the routing technology was studied to guarantee quality of service (QoS) under the framework of software-defined network (SDN), and a multi-service based ant colony optimization wavelength routing algorithm for satellite optical network was proposed. The heuristic function of ant colony algorithm was improved. The wavelength idle rate, time delay, time delay jitter and packet loss rate were taken as the important basis of ant routing, and the optimal path satisfying various QoS was selected for business. The group wavelength assignment strategy (GWAS) was used to distinguish services of different levels. Different sets of wavelengths were assigned to different services. The simulation results show that compared with CL-ACRWA algorithm and Dijkstra algorithm, the average delay, average delay jitter and average packet loss rate of satellite optical network are reduced. The wavelength utilization is improved. The network congestion probability of high-priority services is reduced.
2020, 49(10): 20200019.
doi: 10.3788/IRLA20200019
Photo-elastic modulator (PEM) is a high-Q resonant device. When driven by a high voltage resonant signal, its resonant frequency will drift with its temperature change, and the stability of the interferometer and the accuracy of the rebuilt spectrum are affected. The vibration model and frequency- drifting model of the photo-elastic modulator were first established, and a frequency-tracking and adjustable-amplitude double closed-loop driving control method was proposed. In the method the digital phase-locking technique was adopted to track the changing resonant frequency of the photo-elastic modulator. Based on the change of the maximum optical path difference for the reference laser interferometer, the amplitude of the driving signal was adjusted to realize the stable control of the rebuilt spectrum. At the same time, the calibration of the reconstructed spectra was realized based on the maximum optical path difference. In the experiment the double closed-loop adaptive drive control method was applied to the driving control system of the photo-elastic modulated Fourier transform spectrometer (PEM-FTs), which realized the real-time tracking of the driving-frequency to the resonant frequency for PEM and the amplitude adjustment of the high-voltage amplifier. The maximum optical path difference of the interference diagram was stable at about 0.236 nm, and its precision was 3.3%. The maximum relative error of the reconstructed spectra was 2.5%. The experiment shows that the method can effectively stabilize the spectral resolution of the PEM-FTs.
2020, 49(10): 20200022.
doi: 10.3788/IRLA20200022
Fresnel incoherent correlation holography is a new technology which can record incoherent object holograms and has important applications in biomedical imaging and 3D remote sensing. The problem of image fusion registration in hyperspectral imaging, three groups of double lens phase masks with constant focal lengths at 492 nm, 562 nm and 672 nm were designed and fabricated. The spatial light modulator called the masks of three wavelengths in turn and recorded the holograms of the objects under the corresponding wavelengths, which owed to the spatial light modulator were programmable. Because of the three-color recorded light was modulated by the corresponding wavelength mask, the spot position size of the CCD surface was the same. The reconstructed images had constant lateral magnification, which could improve the image registration accuracy and avoid complicated spatial registration algorithm of spectral images. The system truly achieved high-precision registration and real-time fusion of holographic color imaging. The color 3D image obtained from the dice holograms had high color reconstruction after numerical reconstruction and color fusion.
2020, 49(10): 20200141.
doi: 10.3788/IRLA20200141
The tracking and recognition of the three-dimensional trajectories of cooperative target points is the key to the estimation of the position and attitude of multi-aircraft in an indoor environment. Therefore, a multi-target three-dimensional trajectory tracking and recognition algorithm based on time and space consistent conditions was proposed. This method included two parts of motion trajectory tracking and recognition. For the three-dimensional trajectory tracking of cooperative target points, a data association method based on the consistency of the displacement vector of the moving target was proposed. This method first used the data association probability calculated by the motion smoothness assumption. Combined with the Hungarian algorithm to solve the target data association relationship, and then the three-dimensional trajectory tracking of cooperative target points under the Bayesian filter framework was realized. The three-dimensional trajectory recognition of cooperative target points was divided into two parts: the rank of the motion trajectory Hankel matrix to realize the coarse recognition of the motion trajectory, and the Hausdorff distance between the motion trajectories to realize the fine recognition of the motion trajectory. Eventually the trajectory recognition and registration of each aircraft was realized. Under the experimental conditions of computer vision measurement method and 2 m×2 m×2 m measurement space, the results show that the proposed multi-target tracking algorithm has a three-dimensional trajectory tracking error of less than 4 mm (3σ), and the trajectory recognition accuracy rate is 100%. Therefore, the proposed algorithm can effectively realize the tracking and recognition of the three-dimensional trajectory of cooperative target points on multi-aircraft.
2020, 49(10): 20200040.
doi: 10.3788/IRLA20200040
Based on the Hausdorff distance (HD), a star map recognition method was presented that did not depend on the rotation direction and focal length of the star sensor. When constructing the data point set of Hausdorff distance, the relative Euclidean distance corresponding to norm L2 was used as the set element to solve the influence of star sensor rolling angle on star pattern recognition. On the other hand, due to the influence of the focal length of star sensor, there were errors between the star sensor image and the standard reference image. When constructing standard data point elements, if a data point set contained another data point set, the L2 normal distance between at least two data points between the two data point sets was the same. Therefore, the relative distance was scaled, and the relative spatial distance in each set was divided by the smallest relative spatial distance in the set to form a new set of data points. This method was not necessary to calibrate the star sensor image due to different focal lengths influence. The calculation formula, implementation steps and the simulation results were presented. The experimental results show that the algorithm can obtain the star map recognition results correctly and get the attitude information of star sensor in the case of star sensor rotation, scale transformation, etc.
2020, 49(10): 20190525.
doi: 10.3788/IRLA20190525
In order to realize the high performance detection of CO2 gas isotope, a multi-pass gas cell temperature and pressure control system with high precision and stability was developed in this paper. Flexible PCB was used as heating sheet to cover the cylindrical multi-pass cell. Considering the heating rate of the temperature control system, the outer coating of thermal insulation cotton served as the heat insulation device, which enabled the whole temperature control system to achieve rapid heating and keep the temperature stable for a long time. Platinum resistance PT1000 temperature sensor was utilized to accurately collect the multi-pass gas cell temperature. The main controller regulated the heating power of the flexible PCB heating film through PWM signal, so as to realize the closed loop of temperature control. In terms of pressure control system, the pressure sensor was connected to the front and back end of the multi-pass gas cell to measure the inside pressure of multi-pass gas cell. The main controller regulated the proportional valves that were at the front and back end of the multi-pass gas cell via PWM signal, so as to realize the closed loop of pressure control. The results show that the temperature control range is from 18.48 ℃ to 42 ℃, and the temperature control precision is ±0.08 ℃. When the pressure of the multi-pass gas cell is 60 Torr (1 Torr ≈ 133.322 Pa), the control accuracy is ±0.04 Torr. The system provides reliable guarantee for the high performance measurement of infrared CO2 gas isotope.
2020, 49(10): 20200009.
doi: 10.3788/IRLA20200009
The amplification of continuous-wave backward signal in 1 μm high-power master-oscillator-power-amplifier based Yb-doped fiber laser was investigated using rate equation model. The results show that the backward light power would be amplified significantly by the high-power amplifier. The 100 W backward signal from the output end of the fiber amplifier can be amplified to up to kW level. Meanwhile, the amplification of backward signal can consume the population inversion, saturate laser gain and thus decrease the laser output power seriously. Furthermore, the backward signal amplification would result in a much higher laser intensity at the incident end of the amplifier gain fiber, where the highest pump power existed. The temperature at the incident end of the fiber can be 100 ℃ higher than that without backward signal. The higher laser intensity at the incident end could break the pump absorption saturation, and enhance the rates of pump absorption and stimulated emission a lot, hence increased the thermal load and the temperature significantly. Since the backward signal gain was determined by the saturation of population inversion by the forward seed, the power fluctuation of oscillator caused by the amplified backward signal may aggravate the backward signal amplification, and further increase the risk of damage. Higher forward seed power resulted in stronger saturation of the laser gain in the active fiber, which could suppress the backward signal amplification effectively. However, higher seed power put forwards much complex requirements to laser oscillator, and the thermal load in the active fiber of the laser amplifier would be more concentrated, which made the thermal management more difficult. Furthermore, with higher seed power, the stimulated Raman scattering and thermal induced transverse mode instability are more likely to occur. Therefore, it is important to optimize the seed laser power based on a comprehensive consideration of the above issues, and to prevent the backward light from coupling into the fiber amplifier.
2020, 49(10): 20200014.
doi: 10.3788/IRLA20200014
In the manufacturing process of the backward pump-signal combiner (BPSC) in high-power fiber lasers, the core diameter of the output signal fiber becomes thinner after being tapered, and the mode field mismatch occurs when splicing with the input signal fiber, which decreases the signal light transmission efficiency of BPSCs. To solve this problem, a simulation model was established to simplify the relationship between transmission efficiency and the mode field mismatch. A test system was built to measure the signal light transmission efficiency of backward pump-signal combiners. Finally, a method was proposed to enhance the signal power transmission efficiency of the BPSC through optimizing the parameters of its signal fiber. Accordingly, a (6+1)×1 BPSC using customized 25/400 double-cladding fibers was fabricated with a pre-tapering method, and its signal light transmission efficiency was better than 98% after being tested. Using this combiner, a bi-direction-pumping master oscillator power amplifier (MOPA) fiber laser system was built, achieving a stable output of 3 kW.
2020, 49(10): 20200026.
doi: 10.3788/IRLA20200026
Wavelength switchable and tunable dissipative soliton mode-locking Yb-doped fiber laser based on nonlinear polarization rotation was studied. Due to the in-cavity comb filtering effect induced by NPR, the central wavelength of laser spectrum could switch between 1 042.8-1 050.2 nm, and 1 040.9-1 048.1 nm. The switchable wavelength intervals were 7.4 nm and 7.2 nm, respectively, and the spectral widths were about 5.5 nm and 2.7 nm. At the same time, the wavelength tunable operation was observed from 1 042.77 nm to 1 045.33 nm with the tuning range of 2.7 nm. In addition, stable dual wavelength mode locking and second harmonic mode locking were obtained in the fiber laser. The research of this experiment is helpful to deepen people's understanding of mode-locking dynamics in Yb-doped fiber laser and provide reference for the design of multi-function laser light source.
