Optical communication and optical sensing
2020, 49(S1): 20200207. doi: 10.3788/IRLA20200207
Free space optical communication(FSO) is a communication system that realizes two-way information transmission through laser without optical fiber. It has the characteristics of wide bandwidth, good confidentiality, strong anti-interference, no frequency application, small size and low power consumption. Despite the high security of the FSO communication system, beam overflow still threatens the security of the communication. The use of chaotic encryption in communication can effectively improve the security of the system. In this paper, the differential chaos keying(DCSK) Gamma-Gamma turbulence model was used to improve the security of the FSO communication system, the expression of the system bit error rate was derived. Through the image data transmission of spatial channel, and the statistical histogram, pixel correlation and plaintext sensitivity were analyzed on the received data, the results show that the system has good security performance.
2019, 48(S2): 103-108. doi: 10.3788/IRLA201948.S218001
As the focus of current research on hollow core photonic bandgap fiber, reducing fiber loss is of great importance. In the view of fiber design, taking the 19 cell hollow core photonic bandgap fiber for example, the relationship between structure parameters and loss characteristic was investigated using the finite element method. Simulation results indicate that the confinement loss can be effectively reduced by adjusting the cladding parameters. With the increase of the layer of air holes, the air filling fraction and the fillet diameter at the corners, the confinement loss can be reduced below 10-4 dB/km. While the surface scattering loss, which depends on the coupling between the core mode and the surface mode, increases with the thickness of the core wall as well as the core expansion factor. In addition, the appearance of surface mode also leads to a sacrifice of transmission bandwidth. Limited by the fiber structure, the transmission loss of 19 cell hollow core photonic bandgap fiber is difficult to be reduced to less than 1 dB/km. Further reducing fiber loss can only be achieved by removing more air holes to form a larger hollow core structure. The research achievement provides theoretical basis for the realization of low loss hollow core photonic bandgap fibers.
The inherent advantages of superimposed training method are restricted by superposition data information, power allocation, and direct current bias in channel estimation of atmospheric optical communications under sand-dust weather. A novel scheme was proposed to perform these issues, especially for the channel with sand-dust particles. In the proposal, the data-dependent superimposed training algorithm was utilized to mitigate the influence of data information, the correlation matching algorithm was utilized for direct current bias elimination, and the maximum output signal-to-noise ratio was utilized to perform the optimal power allocation factor. The performance of mean square error, power allocation factor, bit error rate and algorithm complexity were numerically evaluated. The results demonstrate that the proposed method has a better performance than conventional methods with a slightly increased computational complexity.
2019, 48(S2): 117-124. doi: 10.3788/IRLA201948.S218003
It is important to establish the system that is used to evaluate the performance of the inter-satellite laser communication terminals(LCT) under the laboratory conditions before being sent to space. For this reason, beam transmission process of communication was analyzed, and the link simulation based on optical system was also developed in this paper. The relationship between beam transmission and optical magnification with center sampling was studied firstly. Using physical optics principle, the relationship between magnification and transmission distance was established in the next, and on the basis, a system that can be used to test different transmission wavelength and variable transmission distance LCT was designed. Then, the device was tested and calibrated, besides, the relationship between zoom magnification and geometric attenuation was also analyzed. Finally, the precision of transmission distance simulation was discussed. The results indicate that the apparatus can meet less than 100 000 km LCT's simulation with 4% precision of distance simulation and less than 2% stability, which can satisfy the most LCT's testing requirements.
2019, 48(8): 818001. doi: 10.3788/IRLA201948.0818001
For the shortcomings of backlash shaft, friction and clearances in coarse tracking system of satellite optical communication, a scheme of using a flexible shaft to support the terminal was presented. The principle of flexible shaft in satellite optical communication was presented. Then the characteristics of flexibility in working direction and stress in thinnest incision were analyzed. According to the key point deflection, the rotation error angle expression was derived and its influence on capture probability was analyzed. Based on the research of flexible shaft characteristics, the flexible hinge was optimized by genetic algorithm. The results are as follows:the flexibility in working direction is 0.768 rad/Nm, the stress of thinnest incision is 1.768 e+8 Pa and the rotary error angle is 343 rad. Finally, the finite element method was utilized to verify the design consequences, the relative error of the three indexes was less than 3.5%, which proved that the design was reliable. This study can provide some reference for flexible shaft design in satellite optical communication terminal.
2019, 48(8): 818002. doi: 10.3788/IRLA201948.0818002
For the deep space optical communication system based on pulse position modulation (PPM) and photon detection array, a PPM slot synchronization method that directly predicted frequency offset and initial phase offset was proposed. In this method, the arrival time of each photons was measured firstly, and in this way, it can get the offset of every photon's arrival time relative to PPM slot position and draw out the photon distribution diagram of different offsets of the half-frame forward and backward. Then, the frequency offset was evaluated by using the offset of the photon distribution between the half-frame forward and backward(i.e. the cumulative value of frequency offset in the half frame data). After correcting the data of the arrival time of photons according to the frequency estimation value, the initial phase offset can be estimated by calculating the offset between the frequency corrected photon distribution and that of the ideal synchronization. The simulation result shows, when the counting clock frequency in the receiver is equal or greater than 4 times that of the PPM slot, this method can realize PPM slot synchronization.
2019, 48(8): 818003. doi: 10.3788/IRLA201948.0818003
A novel hydrogen sulfide gas sensor based on titanium dioxide membrane-coated coreless fiber was presented. The sensor was fabricated by two different length no-core fibers (NCFs) which were spliced both ends with single-mode fibers (SMFs) and then constructed an interferometer with the structure of SMF-NCF-SMF. Different modes can be excited in the coreless fiber when the light traveled from SMF to NCF, an interferometer based on multimode interference was formed. The titanium dioxide film was coated on the outside surface of NCF which could adsorb hydrogen sulfide gas, then the relation between gas concentration and spectral shift was obtained, and thus the detection of hydrogen sulfide gas was performed. The experimental results show that a high sensitivity of 18.93 pm/ppm and a good linear relationship are achieved in the range of 0 to 60 ppm, and the interference spectra appear red shift with the increasing concentration of hydrogen sulfide. The rising time and falling time of the sensor are about 80 s and 110 s. The sensor has the advantages of simple structure, high sensitivity and easy manufacture, and can be used in the safety monitoring field of tracing hydrogen sulfide gas.
Wavelength division multiplexers/demultiplexers and variable optical attenuators(VOAS) are key devices used in optical communication systems. In order to get their monolithic integrated chip with simple fabrication process and fast time response, and considering the possibility of it to integrate with other different optical devices, a 16-channel 200 GHz arrayed waveguide grating(AWG) multiplexer/demultiplexer was monolithically integrated with electro-absorption variable optical attenuators on a silicon-on-insulator (SOI) platform. The on-chip loss was less than 7 dB and the crosstalk was less than -22 dB. The power consumption of the electro-absorption VOA is 572.4 mW(106 mA, 5.4 V) at 20 dB attenuation. Besides, the device provides fast optical power attenuation, and in a 0-5 V square voltage, the rising/falling time of the VOA is 50.5 ns and 48 ns, respectively.
2019, 48(7): 722001. doi: 10.3788/IRLA201948.0722001
In order to compare and analyze the characteristics of two kinds of structure mixer through simulation design and measurement, the 250 GHz sub-harmonic mixers of suspended microstrip-line and conventional microstrip-line were designed respectively based on the Hammer-Head filter structure, combined with three-dimensional Schottky diode model and three-dimensional electromagnetic model. The results show that in the radio frequency(RF) range of 230-270 GHz, the conversion loss of single sideband converter was 8.6-12.7 dB for suspended microstrip-line mixer, while that was 8.4-11.4 dB for ordinary microstrip-line mixer with RF range of 220-260 GHz. Contrast results showed that the bandwidth of the suspended microstrip-line mixer was larger than that of the conventional microstrip-line mixer, but its flatness was worse. Finally, considering some undesirable factors introduced in the micro assembly processes, the simulation was calculated and compared with the measuring data, it was found that the results matched well.
2019, 48(7): 722002. doi: 10.3788/IRLA201948.0722002
In the system of the spatial light modulator, the response time of the liquid crystal under normal driving voltage is long, and the response speed of the system is slowed down. Considering these, the factors affecting the response time of the liquid crystal, the relaxation characteristics of the liquid crystal and the overdrive principle of the liquid crystal were analyzed. An overdriving method of liquid crystal based on the FPGA was put forward. Among them, phase quantization, overdriving look-up table, and PWM generation were all performed by the FPGA. This method did not occupy CPU resources and can respond to CPU instructions more quickly, and further saving LCD response time in terms of hardware. Finally, the experimental optical path was built. The experimental results show that after using the overdriving method, in a modulation period and under 5 V driving voltage, the response time of the liquid crystal modulation phase rise process is shortened from 500 ms to 35 ms, and the fall response time is shortened from 300 ms to 36 ms. The rapid deflection of the phase of the liquid crystal molecules is achieved, and the response speed of the system is improved by an order of magnitude.
Protective doors of railway tunnel is an important branch of the tunnel disaster prevention and rescue project and has an important influence on safety. A novel innovative protective railway tunnel door based on the micro-structure fiber distributed sensor(MFDS) was studied in this paper, which could be used for the alarm system, the display and recording of alarm parts and alarm data, detection of operational failure and alarm signal remote transmission function. The following research results are reached:(1) the arrangement scheme of protective doors should be taken away from the center line of tunnel; (2) the partition wall of the protective door is made of beton; (3) the door should be opened in the direction of evacuation; (4) when designing the distance between the outside of protective door and central line of adjacent lines, we should consider the frame height of protective door and vehicle clearance. The research results can provide guidance for the design of protective doors for railway tunnel to a certain extent.
2019, 48(7): 722004. doi: 10.3788/IRLA201948.0722004
To improve the decoding performance and transmission efficiency of optical communication link in atmospheric weak turbulence channel, heuristic adaptive rate polar codes were proposed. Adaptive rate polar codes were designed based on nesting property of information bits of polar codes, which could achieve fully polarization and good error-correcting performance in atmospheric weak turbulence channel. To adjust the code rate of adaptive rate polar codes, cyclic redundancy check(CRC) code was introduced as stop signs, which could check decoding result. Transmitting terminal sent a set of codes with gradually lower code rates, when the received codes passed CRC check, transmitting terminal stopped and the rate of such code was the largest rate which can ensure reliable transmission. Simulation results show that, compared to traditional polar codes, adaptive rate polar codes can obtain 1.7 dB to 2.3 dB coding gain at the frame error rate(FER) of 10-8 in different atmospheric turbulence intensity. Decoding latency of adaptive rate polar codes was simulated and the throughput rate of adaptive rate polar codes was calculated. The results show that adaptive rate polar codes can meet the transmission requirements of FSO in atmospheric weak turbulence channel.
2019, 48(6): 622001. doi: 10.3788/IRLA201948.0622001
The existing monitoring technologies of railway tracks mainly use the technologies of electrical sensing. These technologies were easy to be affected by electromagnetic fields and external environments and there were potential security risks. Therefore, the on-line monitoring technology of railway track strain based on an identity weak fiber Bragg grating(wFBG) array was used to monitor the occupancy of the track in the real-time. The sensor's structure of sensing railway strain was designed by finite element simulation and the encapsulation technology of sensors was studied. The strain signal was obtained by detecting wFBG wavelength shift to achieve highly sensitive strain measurement. The identity wFBG arrays were used to verify this sensor and system in the laboratory and field experiments. The results show that the sensor structure can achieve a smaller optical loss, and can ensure that the sensitivity of the sensor reaches 3.4 pm/, the linearity reaches 0.997 82, and the hysteresis error reaches 0.8%. The online monitoring system of railway track can meet the actual needs of railway operation and management.
2019, 48(6): 622002. doi: 10.3788/IRLA201948.0622002
An optical generation scheme of microwave signals with multiple modulation formats based on a polarization modulator(PolM) and a Sagnac loop was proposed. The PolM was driven by a baseband coding signal to generate a polarization shift keying(PolSK) signal and the generation principle was theoretically analyzed. Two Mach-Zehnder modulators(MZM) were embedded in the Sagnac loop to modulate the PolSK signal transmitted clockwise or counterclockwise, respectively. The outputs of amplitude shift keying(ASK), frequency shift keying(FSK) and phase shift keying(PSK) microwave signals were achieved by properly adjusting the driving signals of two MZMs. In the simulation work, a 40 GHz ASK signal, a 20/40 GHz FSK signal and a 20 GHz PSK signal with a bit rate of 2 Gbit/s were produced. In addition, the broadband tunability of the bit rate and carrier frequency was verified. The system stability was improved with Sagnac ring structure. Furthermore, without changing the link configuration, the bit rate and carrier frequency for each modulation format of microwave signals can be tuned flexibly and independently by controlling the baseband coding signal and the RF driven signals of two MZMs.
2019, 48(6): 622003. doi: 10.3788/IRLA201948.0622003
To meet the demand for monitoring the morphing wing aerodynamic shape of morphing aircraft, a flexible composite skin embedded optical fiber shape sensing method for variant aircraft was proposed. The fiber Bragg grating sensor was embedded into the thin layer of silicone rubber, then combining the silicone rubber layer with polyvinyl chloride sheet to form composite skin. The flexible skin shape sensing system was established, and the optical fiber sensing demodulation system was used to test the reflection spectrum characteristics of the fiber Bragg grating in the flexible skin under different airfoil. The morphing curvature of the flexible skin was calculated and the three-dimensional shape of the flexible skin was reconstructed. The contrast test was completed by the digital photogrammetry system. The results show that the measurement error of flexible composite skin deformation optical fiber sensing compared with digital photography is less than 4.62%, and the sensitivity of optical fiber sensing reaches 245.5 pm/m-1. The effectiveness of the embedded fiber sensing method is verified, which provides a reference for monitoring the morphing wing aerodynamic shape of the morphing aircraft.
2019, 48(6): 622004. doi: 10.3788/IRLA201948.0622004
The biggest challenge for free space optical(FSO) communication systems is the attenuation/fluctuation of light intensity caused by influence of atmospheric turbulence in long distance communication, resulting in communication link interruption. A method to calculate the link loss due to atmospheric turbulence, based on lognormal statistics of the received power, was presented. It can be used to evaluate the system parameters in FSO communication system. The effects of different intensity turbulence were simulated, and the relationship between optical communication link loss and transmission distance at 850 nm and 1 550 nm wavelengths at receiving terminals of 2 cm and 20 cm was obtained. Then the simulated analysis results were used to design a FSO communication system with a receiving aperture of 20 cm, which could transmit HD image and video at a distance of about 2 km under strong turbulence conditions. The transmission rate of the FSO communication system was 1 Gpbs which could meet the sharpness and real-time performance of large amount of uncompressed data transmission compared with the 4 G networks.
2019, 48(4): 422001. doi: 10.3788/IRLA201948.0422001
In order to realize the coupling and steering of polysiloxane polymer optical waveguides with a cross-sectional dimension of 50 m50 m, a multilayer etched grating coupler with high refractive index cladding was designed. Firstly, the structural factors affecting coupling efficiency of polymer waveguide grating couplers were analyzed. Then, the coupling efficiency of polymer waveguide grating coupler was improved by etching the high refractive index layer on the grating surface. Next, different grating structures were formed by arranging and combining different periods (range:100-4 000 nm) and different etching depths (range:0-50 000 nm), where all cases were traversed to obtain the diffraction field distribution and its coupling efficiency of different grating structure based on finite-difference time-domain (FDTD) method. Beyond that, the optimization of period and etch depth were found to maximize coupling efficiency. Finally, a multilayer etched grating coupler was designed to further improve the coupling efficiency. The coupling efficiency of the uniform grating coupler with high refractive index layer was approximately 17.2% with 5 000 nm etching depth and the 2 600 nm grating period. The coupling efficiency is approximately 37.4% with multilayer etching and optimized structure. It provides a theoretical reference for the practical application of polysiloxane polymer optical waveguide in optical interconnection.
2019, 48(4): 422002. doi: 10.3788/IRLA201948.0422002
A graded-index multimode fiber (GI-MMF) with reduced core size was designed and fabricated with large effective mode area and low intermodal dispersion for Raman distribution temperature sensor (RDTS) to simultaneously achieve high spatial and temperature resolution. In experiment, the temperature and spatial resolution of the RDTS was measured using different types of fibers under different launch conditions based on a commercially available RDTS system. By using the GI-MMF under the overfilled launch condition, a 1℃ temperature resolution was achieved with a spatial resolution degradation of 0.13 m at the distance of 25 km. The spatial resolution degradation using the standard MMF is 1.58 m in comparison. Moreover, the RDTS using the proposed GI-MMF under the single mode launch condition achieved a temperature resolution of 4.7℃ temperature resolution at the distance of 25 km with a 2.2℃ improvement and no degradation on spatial resolution compared with that using the standard SMF.
2019, 48(4): 422003. doi: 10.3788/IRLA201948.0422003
Fourier ptychography (FP) is an effective approach capable of imaging with both large field-of-view (FOV) and high resolution, the published works have proven that the resolution is limited by the sum of the illumination numerical aperture (NA) and the NA of the objective lens used. A spatial-and spectral-constrained FP (spFP) reconstruction algorithm was introduced to improve the spatial resolution. Unlike the typical unconstrained algorithm, the proposed algorithm incorporated both spatial-and spectral-constraints based on the additional prior information extracted from the typical FP reconstruction, and it did not need any additional hardware or captured images. The proposed approach was based on an assumption that the image was known to be sparse. Both simulation and experimental results show that the spFP reconstruction improves the spatial resolution by~26%, and also improves the contrast and general quality of the reconstructed image.
2019, 48(3): 322001. doi: 10.3788/IRLA201948.0322001
A kind of optical fiber laser displacement sensor with multi longitudinal mode beat frequency was proposed and designed. The sensor could be realized by using the beat frequency signal between different modes in the resonator to monitor the small displacement of the object. The sensor used a strain sensing substrate structure to effectively protect the optical fiber and buffer the deformation caused by stretching and compression, and increase the sensitivity of the sensor. At the same time, the original multi longitudinal mode beat fiber laser was improved, which made the system get higher signal-to-noise ratio. The measuring principle of multi longitudinal mode beat sensor was described, and strain gage was designed and built. In the experiment, four different beat frequency signals were selected as sensing signals. The experimental data show that the frequency shift is basically linear in the range of 0 to 30 mm, and the highest linear fitting degree can reach 0.999 4, which is in agreement with the theoretical results. Because the precision of the stretching platform is limited, the measuring accuracy of the sensor is 110-3 mm. Repeated experiments show that the sensor has good stability.