2019 Vol. 48, No. 2
column
- Invited paper-“Micro-nano optics”
- Infrared technology and application
- Laser technology and application
- Optical manufacturing and craftwork
- Optical design and simulation
- Photoelectric measurement
- Optical communication and optical sensing
- Advanced optical material
- Advanced optical materials
- Teraherta technology and application
- Terahertz technology and application
- Information acquisition and identification
2019, 48(2): 203001.
doi: 10.3788/IRLA201948.0203001
Recently, terahertz(THz) technology has developed rapidly, showing promising potential in the fields of communication, anti-terrorism, monitoring and biomedicine, etc. In particular, terahertz biosensor has attracted extensive attentions in biotechnology, because many biological molecules and materials have their finger prints in the THz absorption spectra, and the damage by the low power terahertz wave is low. However, the THz wave-matter interaction is relatively weak because of the mismatch between the long wavelength of THz wave and the size of biomolecules, which limits the performance of terahertz sensors. Current research interest is manipulating the spatial and spectral distributions of the electromagnetic fields based on the microstructures to enhance the sensitivity of the sensors. In this review, we are going to introduce the working mechanisms of various microstructure THz sensors and the recent progress, and then discuss their advantages and disadvantages, finally we conclude the major issues to be resolved and predict the future developing trend and potential applications.
2019, 48(2): 203002.
doi: 10.3788/IRLA201948.0203002
Due to the high energy, narrow distribution and breaking the bandgap limitation, plasmon induced hot electrons has been widely applied to extend the photoresponse spectra of the semiconductor, such as realizing the response spectrum of wideband semiconductor and silicon to visible and near-infrared range, respectively. Besides, the response spectrum can be adjusted by changing the plasmonic nanostructures, which has an important advantage for realizing silicon-based near-infrared photodectection.Firstly, the concept and mechanism of surface plasmon and plasmon enhanced internal photoemission were introduced. Then, the recent progress on near infrared hot electron photodetector based on silicon was summarized. The influence of the shape, size, distribution of plasmonic nanostructure on the generation and transportation of hot electrons were also summarized. Finally the challenges and potential future directions of surface plasmon enhanced near-infrared photodetector based on metal/Si Schottky heterojunction were discussed.
2019, 48(2): 203003.
doi: 10.3788/IRLA201948.0203003
In order to couple a terahertz Gaussian beam into a hyperbolic metal waveguide(HMWG), a tapered elliptical-hyperbolic metal waveguide(TEHMWG) was proposed as a connector. The input of the TEHMWG was rectangular, and the output was elliptical-hyperbolic. In the tapered process, the TE01 mode of rectangular metal waveguide was gradually transformed into the elliptical polarized mode of hyperbolic metal waveguide. According to WKB approximation, the slowly tapered waveguide eliminated the reflection and scattering. The coupling efficiency between the TE01 mode and the elliptical polarized mode was 94%. Through this connector, the coupling efficiency between a Gaussian beam and HMWG was improved to 69.1%.
Multiband absorbers comprised of one dimensional periodic Ag metallic thin layer and MoO3/SiO2 dielectric layer on a reflective Ag thick layer were fabricated by thermal evaporation and magnetron sputtering methods. Experimental results show that with the number of unit cell (N) increasing, the number of the absorption peaks increases accordingly and precisely equals N. For our fabricated devices with 14 nm thick Ag layer, 2 nm thick MoO3 layer, and 135 nm thick SiO2 layer, the integrated absorption efficiency over the wavelength range from 400 nm to 900 nm increases from 29.4%when N=1 to 57.2% when N=6, the trend of which is consistent with the calculation results. Moreover, measurements show that the absorption peaks are insensitive to the incident angles and polarizations. The multilayer absorbers were also fabricated on flexible polyethylene terephthalate substrates, which maintained their original absorption performances after bending for 1 000 times. The fabricated absorbers may have potential applications in areas like photovoltaics and thermal emission tailoring.
2019, 48(2): 203005.
doi: 10.3788/IRLA201948.0203005
A terahertz(THz) wave optical modulation system was introduced. The modulation was based on the coupling of THz waves and THz surface plasmon by a simple slit between a razor blade and a semiconductor wafer. The modulation process was realized by changing the illumination intensity on the intrinsic semiconductor surface. With or without the optical illumination, the plasma frequency of the semiconductor was larger or smaller than the frequencies of the surface plasmon. Therefore, the THz surface plasmon propagating on the semiconductor and the THz wave coupled from the surface plasmon can be switched on and off. In comparison with conventional THz modulation approaches, this method has the advantages of wide modulation bandwidth, high speed, low cost and room temperature operation etc., which are favorable to THz wave communication applications. The simulation and experimental results prove the feasibility of the THz wave modulation system.
2019, 48(2): 203006.
doi: 10.3788/IRLA201948.0203006
The optical property of the flashing hind wings of the Papilio maackii were measured by an angle resolved spectrum system. A multilayer reflection structure model was established to explain the mechanism of the structural color of the butterfly wings. The circular dichroism in the microstructure of the hind wings of the butterflies was measured and the polarization properties were verified. Based on the structural color, the sensing experiments of the refractive index of solutions by using butterfly wings were designed and implemented. The results show that the spectra of butterfly wings redshift with the increase of the refractive index of solutions. The research provides a technological platform for polarization and sensing applications based on the microstructures in nature.
2019, 48(2): 204001.
doi: 10.3788/IRLA201948.0204001
To enhance fusion effects of infrared and visible images in detail preservation and information redundancy, a novel fusion method based on Finite Discrete Shearlet Transform (FDST) and dual-channel Pulse Coupled Neuron Network (PCNN) was proposed. Firstly, the original images were decomposed into low-frequency and high-frequency subband images by FDST; Secondly, low-frequency and high-frequency subband images were fused by modified-spatial-frequency motivated dual-channel PCNN with different linking strengths; Finally, the final fused image was reconstructed from fused subband images by inverse FDST. Experimental results indicate that the proposed fusion method can improve the overall visual performance and the image quality. Compared with other fusion methods, the proposed fusion method gets significant improvement in objective evaluation criteria of mutual information, edge information preservation and standard deviation.
2019, 48(2): 204002.
doi: 10.3788/IRLA201948.0204002
Non-uniformity correction based on blackbody calibration is widely used in staring infrared imaging system. But in some cases, blackbody calibration method is not applicable. Two kinds of cases were presented:when the IR imaging system facing to the sky background, or observing normal objects through high temperature optical window in high speed aircraft. Based on the analysis of the response characteristics of infrared imaging system, the concept of non-uniformity with response spectral was proposed. It was pointed out that the correction results of blackbody calibration method depended on the spectral distribution of scene. The principle of non-uniformity with response spectrum was analyzed, which included internal and external reasons. The intrinsic factor was that the quantum efficiency of different pixels of infrared detector varies with spectrum, and the external factor was that the radiation spectrum distribution of application scene was quite different from blackbody. Suggestions were given to solve response spectral non-uniformity.
2019, 48(2): 204003.
doi: 10.3788/IRLA201948.0204003
Cross-linked polystyrene is a kind of thermosetting plastic which has excellent dielectric properties and high breakdown voltage. It has great potential application value in microwave window, electrical engineering and military industry, while the infrared optical property of cross-linked polystyrene was rarely studied. A new cross-linked polystyrene sample was prepared by a novel method of -ray radiation polymerization in this paper. Cross-linked polystyrene and polystyrene were analyzed by Fourier transform infrared spectroscopy(FTIR), and the reflective spectra were measured. Subsequent Kramers-Kronig(K-K) analysis provided the dispersion of complex refractive index across the measured region, and calculation errors were analyzed. The results show that the refractive index of cross-linked polystyrene is similar to that of polystyrene in terahertz frequency, and in infrared region decreased rapidly with respect to the frequency(about 1.1-1.2 in mid-infrared region), which is much lower than that of polystyrene (around 1.5).
2019, 48(2): 204004.
doi: 10.3788/IRLA201948.0204004
In infrared nondestructive testing, the proportion of defects is very different from that of background, and the low contrast region of infrared image has not been completely eliminated after image sequence enhancement, resulting in impaired accuracy of defect segmentation. In order to solve this problem, a defect segmentation method based on robust Otsu algorithm was proposed, which combined the relative threshold idea of local threshold segmentation method. Firstly, the mean value and the total gradient of the neighborhood were used to represent the category and spatial state of the pixels. Secondly, a point-block fusion statistical adjusted model on this basis was established for dynamically adjusting the gray scale values of the infrared image defects and non-defect regions. Finally, the improved two-dimensional histogram and its region division method based on gray value and neighborhood gray deviation was set for calculation of fitness function in genetic algorithm through which the optimal threshold could be determined from the mutative neighborhood size, then segmentation of defects could be achieved. The results show that this method improves the robustness of Otsu and the accuracy of defect segmentation.
2019, 48(2): 206001.
doi: 10.3788/IRLA201948.0206001
To observe the changes of electron non-equilibrium heat transport time with the femtosecond laser beam parameters in the Cu films, the electron and the lattice temperature curves were studied based on the finite element method and the experimental testing. The simulation results show that the time of electron non-equilibrium heat transport in Cu films will rise with the increasing of both the pump beams number and the energy density of the pulses. The time of electron non-equilibrium heat transport by three pump beams heating was three times higher than that by the single pump beam heating. These results were verified by three femtosecond laser beams pump-probe experimental system. The experimental results show that the transient reflectivity of the Cu films appear three peaks when the Cu films are irradiated by three pump beams with a certain delay, and the time of electron non-equilibrium heat transport has been prolonged greatly. Consequently, the thermal barrier of the device processing is eliminated and the quality, accuracy and efficiency of the femtosecond laser processing are improved.
2019, 48(2): 206002.
doi: 10.3788/IRLA201948.0206002
The laser with high repetition rate, high peak power and narrow line width has important application value in the field of laser radar. In the amplification of high repetition frequency and narrow line width laser, in order to achieve both high magnification and high beam quality laser output at high repetition rate, narrow line width of passively Q-switched laser used to be the seed source, high gain amplifier using 888 nm diode laser end pumped Nd:YVO4 crystal, side pumped Nd:YVO4 slab crystal thermal lens effect of two stage amplification scheme were designed. At the repetition rate of 10 kHz, the average power of 31.5 W, and the laser beam quality of M2 1.98 were obtained. The peak power was 5 MW, the line width was 154 pm and the pulse width was 0.6 ns. Thus, the design idea of amplifier with high magnification ratio and high beam quality control individually was verified.
2019, 48(2): 206003.
doi: 10.3788/IRLA201948.0206003
Laser induced ejection mechanism of micron thick copper film was studied using nanosecond Nd:YAG laser pulses. By carrying out the experiments with different energy of laser pulses 10-500 J, three different ejection regimes were revealed:no ejection, stable ejection and sputtering. In the stable ejection regime, the forward and backward ejection were found to be simultaneously induced by a single laser shot. This phenomenon opened a way to the fabrication of microstructures on both the receiving and the donor substrate. The temperature field and the phase transition in the copper film were analyzed using the finite element method, which revealed that the laser-induced ejection was mainly caused by the hydrodynamics behavior of the molten and the evaporated material. The laser energy thresholds for stable ejection were characterized based on the thermodynamics calculations. The laser induced hydrodynamics behavior (bubble dynamics) was well described by the Rayleigh-Plesset equation, and which was solved numerically in the paper. It was discovered that rapid bubble expansion and collapse were the main causes of the forward and backward ejections, respectively. Based on the experimental and numerical findings, the controlling schemes of the laser pulse parameters for the stable ejections were introduced.
2019, 48(2): 206004.
doi: 10.3788/IRLA201948.0206004
Waveform fitting is a key point in data processing of airborne laser bathymetry (ALB), which can provide the foundation data for water depth calculation, submarine sediment classification and water turbidity analysis. Traditional waveform fitting algorithms are often disturbed by noise. In addition, the problem of the present algorithms is that the fitting of complex waveform is not accurate. Therefore, a new waveform fitting algorithm for ALB based on layered heterogeneous model was proposed in this paper. According to the corresponding characteristics of different components of waveform, the ALB waveforms were fitted to a combination of three functions:a Gaussian function for the water surface contribution, a B-spline function for the water bottom contribution, and a Double-exponential function to fit the water column contribution. The performance of the proposed fitting model was verified by the measured data from the South China Sea, compared with three classical waveform processing algorithms:the Double-Gaussian, Generalized-Gaussian, and Richardson-Lucy (RL) deconvolution. The experimental results demonstrate that the proposed fitting model performs best in terms of waveform fitting accuracy and efficiency. The average running time T of the proposed fitting model is 0.019 4 s, saving 0.328 6 s than RL deconvolution. The proposed fitting model performs significantly better than the Double-Gaussian algorithm by reducing 65.11%, 2.83%, 1.01% and 86.61% of their average root mean square error(RMSE), average coefficient of determination(R2), average correlation coefficient(CORR) and average correlation coefficient standard deviation(STD), respectively. The proposed fitting model has the great robustness and can effectively meet the technical requirements of the scientific research and engineering application for ALB.
2019, 48(2): 206005.
doi: 10.3788/IRLA201948.0206005
The interference encoding method of semi-active homing laser-guided system based on multiple sub-beams interference was proposed. The spatial interference pattern was combined with the temporal coded signal to produce a new temporal-spatial coded signal. A coherent emission system of multiple sub-beams was designed, the laser was divided into several equal power sub-beams which could produce an effective spatial interference pattern due to their coherence. Calculation results show that the interference pattern presents special distribution features of alternating brightness and darkness. When the value of (the ratio of diagonal) is between 2.5 to 3.5, the interference pattern is most obvious and pure. When the position offset and angle deviation of the beam splitter are less than 0.1( is the wavelength) and 0.10(0 is the beam divergence angle) in the sub-beams coherent emission system, respectively, the effect of mechanical vibration on the interference emission system could be ignored.
2019, 48(2): 242001.
doi: 10.3788/IRLA201948.0242001
Since the invention of lasers in the 1960s, the pulse duration has been continuously shorten down to the sub-picosecond and even femtosecond regime. It makes the laser processing technology in the ultrashort pulse laser era. Compared with other methods, the ultrashort pulse laser drilling of micro-hole is independent of the size and materials, and it has the advantages of high precision and automation. The advantages, such as the cold processing, breaking the diffraction limit, for ultrashort pulse laser drilling of micro-hole were presented. Three classical models were demonstrated, which were the threshold fluence-hole diameter model, the multi-pulse cumulative model and the single-pulse ablation depth model. The experimental studies for ultrashort pulse laser drilling of micro-hole were analyzed. Furthermore, the challenging issues and prospects were concluded.
2019, 48(2): 242002.
doi: 10.3788/IRLA201948.0242002
In order to reduce the substrate deformation and residual stress during laser direct manufacturing of TC4 components, the influence of different scanning orders on the deformation and residual stress of the parts formed by subarea scanning strategy were studied. The newly-defined outside-in and inside-out scanning orders, and a randomized scanning order were applied and compared. The deformation was measured by the surface structure light measurement system and the residual stress was measured by the X-ray diffraction method. The results show that different scanning orders have significant effects on the deformation and residual stress. The maximum substrate deformation is reduced by 60% by the outside-in scanning order compared with that by the randomized scanning order, while larger residual stress is introduced, which is even up to 392 MPa. Oppositely, the randomized scanning order makes a more uniform residual stress distribution and the maximum residual stress is only 93 MPa. Therefore, it is beneficial to balance the substrate deformation and residual stress of large TC4 forming parts by applying the outside-in and randomized scanning orders in the front and back forming period respectively.
2019, 48(2): 242003.
doi: 10.3788/IRLA201948.0242003
Circular groove bulkhead can be finished by laser circular scanning without mould. Therefore, research of the curved surface deformation rule and characteristics was of important significance for engineering applications. Firstly, the influence of the scanning number n, the circular scanning radius R, the plates' width W and the length from baseline to the free end L on the curved surface deformation were analyzed in detail. Secondly, combining with the effect of various parameters on the surface deformation and surface deformation test measurement data, obtained surface deformation experience function about the parameters. Then the surface deformation change rule and trend were analyzed. Finally, the strategy of laser circular scanning has obtained the circular groove bulkhead samples and its two side wall of the mean curvature radius are 84.51 mm and 86.77 mm respectively. This study lays a solid foundation of both the circular groove structure and the double circular grooved structure by laser circular scanning.
2019, 48(2): 218001.
doi: 10.3788/IRLA201948.0218001
Epoxy is a key bond material in the mirror assembly of nested conical approximation Wolter-I type focusing telescope which is based on ultra-thin glass. The mechanical properties of the telescope is determined by the bonding strength of the adhesive layer with micron-scale thickness. The bonding strength of ultra-thin glasses-F131 epoxy -graphite bonding structure under different curing environments and surface roughness of graphite was studied. The results show that the bonding strength decreases with the increase of the curing humidity of epoxy and increases with the increase of the surface roughness of the graphite. The peel of the graphite was found to be the main type of failure in bonding structure by comparing the surface peeling area ratio of graphite. Finally, the B basis was introduced as an evaluation of bonding strength to improve the accuracy and reliability of the bonding properties for telescope assembly.
2019, 48(2): 218002.
doi: 10.3788/IRLA201948.0218002
In high energy laser weapon systems, as core devices of beam steering control, fast steering mirrors play a key role in improving damage efficiency. In order to enhance the optical direction range in high energy laser weapon system, a fast steering mirror connected by a universal flexure hinge was designed. The influence factor of each component unit in the fast steering mirror was studied, and the feasibility of a mirror driven by every single voice coil actuator in every single degree of freedom was analyzed. On this foundation, the design basis of the voice coil actuator and the optical grating micrometer was proposed, and the parameters were designed, the flexibility expression equation of the fast steering mirror connected by a universal flexure hinge was deduced, then the bending strength of the universal flexure hinge was verified. The results show that the movement stroke can reach to 3.3nd the first resonance frequency can reach to 231.04 Hz in the fast steering mirror system, which satisfies the requirements of long stroke and high bandwidth.
2019, 48(2): 218003.
doi: 10.3788/IRLA201948.0218003
The light field camera is a new camera structure that put a microlens array in front of image sensor. It can not only record the intensity and color of light at different positions, but also record the direction information of light in different positions, so that the depth map and high-order phase map of the target scene can be calculated. This technology is limited by the depth of field and resolution, and the resolution is reduced when a large depth of field is achieved. Its structural characteristics were analyzed and the relationship between the depth of field and the resolution was derived, and the change curves were ploted for selected design parameters. Based on this, a design method of a novel light field camera based on a microlens array with four types of focal lengths was proposed, which can obtain large depth of field and control the decline degree of resolution in an acceptable range. The simulation results show that compared with the three types of focal length microlens arrays, the depth of field of the four types of focal length microlens arrays designed can be increased three times, and the resolution can reach 18.9% of the ordinary cameras.
2019, 48(2): 218004.
doi: 10.3788/IRLA201948.0218004
In order to improve the control accuracy of large-aperture piezoelectric fast steering mirror(FSM) in precision image stabilization system in space telescope, the compound control strategy combining hysteresis feed forward compensation with the optimal PID control algorithm was adopted. According to the problems that the reversibility of Prandtl-Ishlinskii (PI) model based on Play operator was limited by the constraint condition and the estimation error accumulation of model parameters in the inverse process, the PI inverse model based on the generalized Stop operator was proposed to compensate the PZT hysteresis. The optimal PID closed loop controller was added in the control system for the problems of the inverse hysteresis model uncertainty and the poor anti-interference ability of direct feed forward control. The adaptive differential evolution(ADE) was used to optimize the hysteresis inverse model and PID controller parameters and the chaos search mechanism was introduced to improve the performance of ADE. The experimental results show that, compared with the traditional analytic method to obtain the inverse model, the hysteresis inverse model based on the Stop operator can better describe the inverse hysteresis curve, and the fitting precision increases by 78.04% when fitting the hysteresis curve with 1 Hz frequency; the tracking accuracy of compound control algorithm increases by 38.56%,22.92% and 13.5% respectively compared with the direct feedforward control, in the real-time tracking for target swinging displacements of large-aperture piezoelectric fast steering mirror with 1 Hz,10 Hz and 20 Hz frequencies.
2019, 48(2): 218005.
doi: 10.3788/IRLA201948.0218005
In order to meet the requirements of active optics control technology for segmented mirror telescope, an improved active disturbance rejection controller (ADRC) was designed to improve the position tracking performance of the position actuator and the capability of anti-disturbance. Firstly, the mathematical model of the position actuator, and the model of wind disturbance were established. The improved ADRC was described and the method of controller parameter selection was given. Secondly, the feasibility of the controller was demonstrated through a simulation of the control system of the position actuator. Finally, with the wind disturbance mechanism, the wind force was introduced into the position actuator system, and the performance of the improved ADRC, ADRC and the PID controller was compared by experiments. Experimental results show that the stability time of the step tracking of the improved ADRC is 201 ms, the steady-state mean square error is 7.1 nm, and there is no overshoot. In the anti-disturbance experiment, the maximum deviation of the improved ADRC is 38.8 nm and the steady-state mean square is 7.6 nm. These results illustrate that the performance of the improved ADRC is significantly better than those of ADRC and the PID controller, and the improved ADRC improves the performance of the position actuator system.
2019, 48(2): 218006.
doi: 10.3788/IRLA201948.0218006
In practical application, cryogenic optics can effectively improve the detection sensitivity by reducing the thermal noise of the infrared optical system. The lens mount is a key component to ensure the normal performance of the cryogenic optical system at cryogenic temperature. A transmission optical system was designed to work at the operation temperature of 150 K where the pulse tube cryocooler was used as a novel mechanical cryocooler to cool the optics. However, due to the small diameter of the pulse tube cryocooler's cold finger, the temperature difference within the lens was large when it was connected directly with the cold finger. So the lens needs to be cooled via a properly designed mount. Specialized axial support and radial support were designed to prevent from making damages and large distortion of the lens at cryogenic temperature. The heat transfer model between the lens and the pulse tube cryocooler was established to guide the lens mount thermal design. Finally, the cryogenic performance of the lens mount was tested. The experimental results showed that in about three hours, the optical components survived the cool-down process from the initial temperature of 300 K to the operation temperature of 150 K, and the maximum temperature difference within the lens was less than 1 K. The distortion test showed the maximum deformation of the optical surface was less than 1(1=632.8 nm) when the temperature decreased from 300 K to 150 K. The designed lens mount successfully satisfies the requirements of this cryogenic optical system which provides a new choice for the cryogenic optics cooled by mechanical cryocooler.
2019, 48(2): 217001.
doi: 10.3788/IRLA201948.0217001
A measurement method for angular positioning error of a rotary axis based on fiber laser autocollimation was proposed. An error model was established including the motion errors of the rotary axis and the installation errors. By simulating and analyzing the impact of these 23 errors on measuring angular positioning error, the results showed that only four installation errors between the reference and the target rotary axes have influences depending on the rotation angle of the rotary axis. And only the two angular errors need to be made fine adjustments to insure that the influences were less than 0.2. The angular positioning error of an indexing table was measured by the measuring device built. The repeatability value for measuring three times was about 0.9, and the maximum comparison deviation from the optoelectronic autocollimator was about 0.6. The results show that the angular positioning error of rotary axis can be measured high accurately with a full-circle measuring range by using the method and device, and the proposed model is verified.
2019, 48(2): 217002.
doi: 10.3788/IRLA201948.0217002
In order to efficiently and accurately measure the phase retardation of wave plate and the azimuth angle of fast axis at the same time, to realize the integration and automation of measuring system, a wave plate measuring system based on the combination of the photo-elastic modulation and the digital phase-locked technology was designed. The detecting laser was modulated by the photo-elastic modulator. By means of digital phase-locked technologies based on FPGA, the first harmonic term and the second harmonic term of the modulated signals were extracted. The phase retardation and the azimuth angle of fast axis of wave plate were demodulated by optimized algorithm. The stepper motor drives the wave plate to rotate to make the fast axis of wave plate reach the zero position. And the phase retardation was displayed by LCD. The experimental system was set up to measure the quarter-wave plate. The experimental results show that the measuring accuracy of azimuth angle of fast axis of quarter-wave plate is better than 0.31 and the measuring accuracy and repeatability of phase retardation of quarter wave plate are better than 99.47% and 0.14, respectively. The driving signal of the photo-elastic modulator and stepper motor and the processing of the data were controlled by FPGA. The measuring system realized optical, mechanical and electronic integration and the automation of the measuring process.
2019, 48(2): 222001.
doi: 10.3788/IRLA201948.0222001
In order to solve the curvature measurement problems of flexible mechanism such as biomedical intelligent equipment, software robots and complex structures of high-end equipment, major infrastructure, a highly adaptive curvature sensor was proposed. The fiber Bragg grating was encapsulated in the silicone rubber layer, which was fixed on the polyvinyl chloride matrix to form a flexible silicone rubber curvature sensor based on fiber Bragg grating. The fiber-optic sensor demodulation system and standard curvature calibration blocks were used to measure the characteristics of the FBG sensor's reflection spectrum and its variation with the calibration block curvature. The relationship between the FBG wavelength shift and the curvature change was analyzed, as well as the sensor sensitivity and the embedded depth of FBG. The experimental results show that the curvature sensor based on silicone-polyvinyl chloride can measure the real-time change of curvature with the highest sensitivity of 0.329 2 nm/m-1. With the increase of the embedded depth of fiber grating, the sensitivity of the sensor gradually increases with the range from 0.2 nm/m-1 to 0.35 nm/m-1. The sensor has good consistency in repeated measurements and can be used for curvature measurement of flexible mechanisms and complex structures.
2019, 48(2): 222002.
doi: 10.3788/IRLA201948.0222002
A new type of circular photonic crystal fiber supporting orbital angular momentum was proposed. The designed circular photonic crystal fiber is composed of well-ordered rectangular air hole rings in the cladding, a large air-core in the center and the annular high-index region for orbital angular momentum modes transmission. Utilizing the COMSOL Multiphysics software based on the finite element method, propagation properties of orbital angular momentum modes in fibers were discussed in detail. The results show that the designed circular photonic crystal fiber realizes effective segregation and stable transmission of 50 orbital angular momentum modes over the bandwidth from 1.2 m to 2.0 m. Large effective index differences(10-4) guarantee the stable transmission of every mode, confinement losses are lower than 10-9 dBm-1, and nonlinear coefficients are as low as 0.833 km-1W-1. The proposed rectangular holes circular photonic crystal fiber applied in mode-division multiplexing fiber communication systems might greatly improve channel capacity and spectral efficiency.
2019, 48(2): 222003.
doi: 10.3788/IRLA201948.0222003
Based on the measured Brillouin gain spectrum, the suitable objective function was chosen. According to a large number of numerically generated Brillouin gain spectra, the influences of signal-to-noise ratio(SNR), scanned frequency interval, number of scanned frequency, frequency scanning range, spectra key parameters g0, vB and △vB on the accuracy in the parameters extracted from Brillouin gain spectrum were systematically investigated. The results indicate that the errors in the extracted parameters exponentially decrease with increasing SNR. The errors in the extracted parameters decrease with increasing number of scanned frequency or decreasing scanned frequency interval. The error in the extracted vB increases proportionally with △vB. However, △vB has no influence on the accuracy in the extracted g0 and △vB. If the number of scanned frequency remains constant, then too wide or narrow frequency scanning range will results in significant errors. The optimal frequency scanning range is close to 2△vB. This work provides a frame of reference in which you can determine the suitable parameters in Brillouin scattering-based sensing.
A plasmonic triple-wavelength demultiplexing structure based on metal-insulator-metal waveguides side-coupled with nanoring cavities was proposd by studying theoretically and numerically. The structure consisted of three output channels, each of which was sandwiched by two nanoring cavities. By changing the filling medium of the ring and the relative position of the inner and outer circle, the reflection and transmission spectrum of each channel could be dynamically tunable. Finally, according to the reflection and transmission characteristics of three channels, the demultiplex at three telecommunication wavelengths 1 310 nm, 1 490 nm and 1 550 nm with excellent performance was studied. Temporal coupled-mode (CMT) theory and finite-difference time-domain (FDTD) method are applied to simulation and analysis, and this work can find potential applications for on-chip integrated all-optical circuits.
2019, 48(2): 221002.
doi: 10.3788/IRLA201948.0221002
An InP/In0.53Ga0.47As/InP infrared photocathode model was established. The In0.53Ga0.47As absorber layer was designed as a multi-layer structure, the impurities of it were exponentially distributed by doping with different concentrations of the thin layers. The one-dimensional continuity equations and boundary conditions of the photoelectron in the absorber layer and the emissive layer were given and the probability that photoelectrons overcome the launch of the active layer barrier into the vacuum was calculated. The effects of absorber layer thickness, doping concentration and cathode bias voltage on the internal quantum efficiency of the cathode was simulated under the condition of picosecond response time, and then the law of the external quantum yield of the cathode was obtained with the above three factors. The results show that, when the doping concentration of the absorber layer changes within the range of 1015-1018 cm-3, The internal quantum efficiency change is very small; as the thickness of the absorber layer increases within 0.09-0.81 m, the internal quantum efficiency increases. As the external bias voltage increases, the internal quantum efficiency increases first and then tends to be stable. A set of cathode design parameters that could achieve both high quantum efficiency and fast time response were presented. Theoretically, an external quantum yield of 8.4% can be obtained for 1.55 m incident light, and the response time is 49 ps.
2019, 48(2): 221003.
doi: 10.3788/IRLA201948.0221003
Si thin film has certain absorption characteristics in visible and near-infrared bands and can be used for preparing broadband absorption film. Si thin films were deposited on fused silica substrate with different deposition process parameters by ion beam sputtering technology. Based on full spectrum numerical fitting methods with the transmission spectra, reflection spectra, and ellipsometry spectra, optical constants of Si thin films were calculated. The effects of oxygen and nitrogen flow rate on optical properties were also researched. Choosing Si and Ta2O5 thin film as high refractive index materials and SiO2 thin film as low refractive index material, broadband (1 000-1 400 nm) absorption films with the absorption rate of 2% and 10% were designed. Using ion beam sputtering technique, broadband absorption films were deposited on the fused silica substrates. For broadband absorption film (A=2%), the absorption rate at the wavelength of 1 064, 1 200 and 1 319 nm were respectively 2.12%, 2.15% and 2.22%. For broadband absorption film(A=10%), the absorption rate at the wavelength of 1 064, 1 200 and 1 319 nm were respectively 9.71%, 8.35% and 9.07%. The results are of great importance to the calibration of instrument such as absorption measuring instrument and spectroscopic instrument.
2019, 48(2): 225001.
doi: 10.3788/IRLA201948.0225001
According to the three-dimensional structure featuring variety of materials around an anti-parallel pair of planar Schottky diodes barrier, the 3D-circuit around the diode barrier can be calculated as a four-port diode modal S-parameter package, and the S-parameters produced by the electromagnetic field solver connecting to the non-linear model of the Schottky diode barrier were incorporated into the non-linear circuit simulator as the diode's whole model, and by this way the whole monolithic mixing circuit's behavior could be modeled more accurately. The monolithically integrated 330 GHz sub-harmonic mixing circuit was fabricated on a 12m thick GaAs membrane, and it was suspended in a LO and RF reduced height waveguide split block. The MMIC demonstrated performance with a best double-sideband-mixer conversion loss of 10 dB with 5 mW of LO incident power at 330 GHz. It could withstand some mechanic pressure when its outer environment temperature change, because it was mounted via four flexible beam-leads and a fixed end in a waveguide block.
2019, 48(2): 225002.
doi: 10.3788/IRLA201948.0225002
A 220 GHz tripler frequency multiplier using no-substrate and space combined structure was designed and realized based on four anodes in parallel-series GaAs planar Schottky diodes. Four Schottky diodes was adopted and divided into two groups. One group with two Schottky diodes placed in the same direction was flip-chiped on the top surface of the slab waveguide metal film and the other group on the bottom surface composed anti-series structure. The efficiency of the circuit was stimulated combined of EM simulator and circuit simulator. The stimulated efficiency was bigger than 3% over the band of 213 GHz to 229 GHz with the input power of 300 mW. An E-band power amplifier was used to drive the circuit to get the output power of the multiplier. The measured output power was bigger than 5 dBm over the band of 213 GHz to 229 GHz with the input power of 300 mW. The efficiency was 1% to 2%.
2019, 48(2): 226001.
doi: 10.3788/IRLA201948.0226001
The precision and stability of spot centroid location were crucial in many fields. According to the edge blurring principle on the optical system, a new algorithm of spot centroid extraction was proposed. The proposed algorithm, based on the arctangent function, can be used to fit the gray distribution of the light spot by variable substitution, and then the centroid of spot can be obtained. In the process of solution, the Gauss-Newton method was adopted to iterate, and then the least square method was used for optimal estimation. Firstly, the advantage and disadvantages of the proposed algorithm between the traditional algorithms were obvious through comparison analysis, and then the feasibility was validated experimentally. The experimental results show that the location precision of the spot centroid is 0.125 3 pixels, and the angle measurement accuracy of the photoelectric sensor is 0.172 3, superior to the sensor's technical requirement 0.25. The robustness of the proposed algorithm to noise, contrast, aspect ratio and size is superior to the traditional algorithms. The experimental results were stable and reliable, and meet the requirement of the angle sensor.
2019, 48(2): 226002.
doi: 10.3788/IRLA201948.0226002
In the multi-wavelenth temperature measurement system, the spectral emissivity is assumed to be a wavelength or temperature model to solve the true temperature. Because of the uncertainty of the model assumption, spectral emissivity model is inconsistent with the variation rule of the actual spectral emissivity, which will cause larger inversion error. In addition, the poor universality of the mathematical relationship between the emissivity and wavelength or temperature, especially the measured radiator is changed, the relationship will cease to exist. In order to the obtain the true temperature, the optimization idea is introduced into the method for the first time, and a single objective minimization optimization method(SMO) was established to complete the inversion of true temperature. The new method does not need to establish a model between spectral emissivity and wavelength or temperature, and has a certain universality. Compared with the second measurement method(SMM), the inversion speed of the proposed method was increased over 95% in the same initial conditions.
2019, 48(2): 226003.
doi: 10.3788/IRLA201948.0226003
In the study of human activity recognition(HAR) based on the inertial sensor, feature extraction was one of the key points. The stability of discrete data statistical features depended on the window size of feature extraction. Generally speaking, the length of window needed to be greater than one motion cycle. Therefore, compared to the traditional behavior recognition, short-time behavior recognition was more difficult. Thus a novel template matching method was proposed for recognizing the test samples whose durations were shorter than one motion cycle. Firstly, by properly dividing the long sequence samples, a complete short-time activity template library was constructed. The short-time samples to be tested and the samples in the template library were processed and matched. Secondly, in the matching algorithm, the similarity histogram was obtained by utilizing the sum of the F norm distance between the samples and the 2 norm distance of the global gradient vector as the matching metric. Finally, based on the similarity histogram, the final classification recognition results were obtained according to the voting strategy. Experiments show that in the case of using a single sensor to identify short-term behavior, the new algorithm had better performance than traditional algorithms in accuracy and stability, and can be adapted to short-term behavior classification problems under different windows.
