Proceedings Volume 12278

2021 International Conference on Optical Instruments and Technology: Optical Communication and Optical Signal Processing

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Proceedings Volume 12278

2021 International Conference on Optical Instruments and Technology: Optical Communication and Optical Signal Processing

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Volume Details

Date Published: 26 July 2022
Contents: 8 Sessions, 32 Papers, 0 Presentations
Conference: 2021 International Conference on Optical Instruments and Technology 2022
Volume Number: 12278

Table of Contents

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Table of Contents

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  • Front Matter: Volume 12278
  • Optical Communication and Optical Signal Processing I
  • Optical Communication and Optical Signal Processing II
  • Optical Communication and Optical Signal Processing III
  • Optical Communication and Optical Signal Processing IV
  • Optical Communication and Optical Signal Processing V
  • Optical Communication and Optical Signal Processing VI
  • Poster Session
Front Matter: Volume 12278
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Front Matter: Volume 12278
This PDF file contains the front matter associated with SPIE Proceedings Volume 12278, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.
Optical Communication and Optical Signal Processing I
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Millimeter-level high range resolution radar imaging enabled by optically injected semiconductor lasers
Fangzheng Zhang, Guanqun Sun, Shilong Pan
To cope with the bandwidth limitation of traditional electronic radars, many photonic radar signal generation and processing methods have been demonstrated, through which the range resolution has been improved to several millimeters. In recent years, the period-one oscillation of a semiconductor laser under optical injection has been proposed to generate frequency-modulated radar signals. Advantages of this method include large bandwidth, reconfigurable capability, and compact structure. In this report, we introduce the recent progress on radar signal generation and high resolution imaging based on period-one dynamics of semiconductor lasers. The generated frequency modulated signal bandwidth is as large as 18.5 GHz (1.5-20 GHz). The corresponding range resolution is measured to be 8.1 mm. Based on this signal generation scheme, high-resolution through the wall radar imaging is successfully achieved.
Chaotic physical security scheme based on variational auto-encoder for OFDM-PON
This paper proposed a novel chaotic physical security scheme based on Variational Auto-Encoder (VAE) for optical frequency division multiplexing-passive optical networks (OFDM-PON). We adopt the deep generative model VAE to generate chaotic sequences for the encryption of OFDM symbols. Different chaotic security schemes are included to improve the key space and sensitivity of chaotic models, thus enhancing the security of the OFDM-PON system. With the training materials of different chaotic security schemes, VAE can learn the complex structure of data distribution in various chaotic models and finally has the ability to generate the key group with a large space. Meanwhile, the benchmark performance of the OFDM system is experimentally investigated in terms of the bit error rate (BER). Moreover, owing to the parallel computing of GPU, the time consumed for training of VAE can be reduced to a large extent, and the time for generation of chaotic sequences via VAE is only 1.38% of that via repeated iteration of equations, which highlights the remarkable reduction in complexity of the chaotic physical security scheme.
Optical Communication and Optical Signal Processing II
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Bayesian filter-based dynamic joint estimation of carrier frequency offset and linear phase noise for CO-OFDM systems
Shuai Liu, Xinwei Du, Pooi-Yuen Kam, et al.
Coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is a promising technique due to its high spectral efficiency and dispersion tolerance. However, CO-OFDM is sensitive to the carrier frequency offset (CFO) which is caused by the frequency difference between the transmitter laser and local oscillator, and linear phase noise (LPN) which is closely related to the laser linewidth. The existing of CFO and LPN can introduce the inter-carrier interference (ICI) and cause the rotation of constellations, thus degrade the system performance. In this paper, we propose to dynamically and jointly track and compensate the CFO and LPN by utilizing the Gaussian particle filter (GPF) and extended kalman filter (EKF). Firstly, the GPF and EKF can dynamically track the CFO and LPN in a real-time manner, thus can achieve accurate estimation result at even high phase noise variance. Secondly, GPF can successfully prevent the particle impoverishment (PI) problem of the conventional sequential importance resampling (SIR) PF, by recursively updating the mean and variance of the particles based on the weights computed from the posterior density and the designed importance sampling function. Thirdly, the joint estimation of CFO and LPN utilizes merely one OFDM symbol, which ensures the effective data transmission efficiency. Also, EKF can convert the nonlinear problem into linearity which helps to reduce the computation complexity. Simulations are carried out to verify the accuracy, robustness and efficiency of the proposed approach by considering the estimation error variance with the Cram´er-Rao lower bound (CRLB), the convergence speed of the GPF and EKF, and the real-time dynamic tracking errors.
Single-lane 200G+ high speed optical transmission using single-DAC for data center interconnects
Propelled by bandwidth-hungry cloud services, the ongoing growth of intra-datacenter traffic drives the development of high-speed short-reach transceivers, which calls for next generation optical interfaces of 800-GE or even 1.6-TbE. Conventional intensity-modulation direct-detection (IM/DD) systems still dominate the market for high speed short reach optical interconnects due to its simplicity and low cost compared with coherent solutions. Several advanced techniques to achieving net data rates around 200∼250 Gbps have been demonstrated. Effective digital signal processing (DSP) for signal recovery are always used in these systems, including digital pre-distortion, digital timing recovery, feed-forward and decision feedback equalization (FFE/DFE) and stronger forward error correction. Probabilistic shaping (PS) has been introduced for 200G+ per lane IM/DD systems. Semiconductor optical amplifier (SOA) and PS can be potentially used for 200G+ per lane IM/DD systems at O-band over 10 km SMF. There are two main transmission impairments: the nonlinear impairments from the nonlinear region of the electro-optical components, and linear impairments from the bandwidth constraint of the optoelectronic devices and chromatic dispersion. Single-lane 200G+ transmission is difficult to realize due to the nonlinear impairments and the strong bandwidth constraint of optoelectronic devices. In recent years, we have experimentally demonstrated several 200G+ per lane IM/DD short-reach transmission system, making it a promising scheme for data center short-reach applications.
Demonstration of real-time full-duplex photonic-assisted mm-wave communication in Ka-band based on the narrow-band envelope detector
S. Liang, Y. C. Cai, J. Zhang, et al.
With the large-scale commercialization of 5G and the continuous evolution of the ultra-high-definition video industry, the next five years will be a period of strategic opportunities for the technological development and achievement transformation of the ultra-high-definition video industry. 5G mm-wave such as 28GHz in Ka-band will drive the rapid development of ultra-high-definition video industry applications. In this paper, we experimentally demonstrate a real-time full-duplex photonic-assisted 28GHz mm-wave communication system for video services. The experimental results show that our system supports real-time data error-free transmission of 1.25Gbaud and bit error rate level of 10-12 at 2.125 Gbaud under the case of 5km fiber and 1.6m wireless distances. Additionally, we also demonstrate the real-time full-duplex transmission of 1080p uncompressed video with the overall bandwidth of 1.485Gbps. It means this system can enable at least 8 channels of 8K video or 20 channels of 4K video to be live and on-demand at the same time after using video compression techniques. According to the above results, we believe that this system can promote the development of 5G mm-wave real-time ultra-high-definition video services for indoor or outdoor scenarios.
The performance research of ADMIN detection algorithm for MDL-impaired mode division multiplexing fiber transmission
We investigate the performance and complexity of minimum mean square error (MMSE) and alternating direction method of multipliers based infinity-norm (ADMIN) for mode dependent loss (MDL) impaired mode division multiplexing (MDM) systems. Our result shows the performance of ADMIN is better than that of MMSE in medium and small MDL. In QPSK modulation mode, the computational complexity of ADMIN detection algorithm is lower than that of MMSE. In addition, we studied how to continue to improve the performance of the ADMIN detection algorithm. We found that the increase of the β parameter will significantly improve the performance of ADMIN in the MDL communication system without increasing the computational complexity. When theβ parameter is increased by 20 times, the BER performance changes from 10-4 to 10-5, which is an order of magnitude improvement.
Optical Communication and Optical Signal Processing III
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Accurate Resistance Measurement of Composite Materials for Flexible Devices
Composite materials have been widely applied in flexible devices for years. Many researchers focused on the resistance properties of these materials but usually induced or ignored the impact of contact resistances or even Schottky barriers. In this paper, we propose a modified four-contact method to accurately measure the resistivities or sheet resistances of flexible composite materials. By measuring the resistance between each pair of electrodes with different voltage polarities, either ohmic or Schottky resistances can be eliminated through calculation. At the same time, this method is easily integrated into flexible devices, so the actual values of material resistances under pressure can be tested.
High-power DFB semiconductor laser array
Yuechun Shi, Yuxin Ma, Ziming Hong, et al.
We have experimentally demonstrated a 16-wavelength high-power DFB laser array with 200 GHz (1.6 nm) channel spacing based on the asymmetric equivalent π phase shift. Good single-longitudinal-mode (SLM) operations are obtained by introducing asymmetric equivalent π phase shifts. The effect of random phase on the high-reflective (HR) coating facet also is weakened by introducing asymmetric equivalent π phase shifts which are implemented at the 1/5 laser cavity close to the facet with HR coating. The average channel spacing is 1.62 nm, which deviated 0.02 nm from our design under the same injection current (300 mA) of each laser. The output power of 16 channels is above 100 mW at the bias current of 400 mA and the average slope efficiency is 0.41 W/A at 25 °C. Good single-longitudinal-mode are obtained for all the 16 channels with side mode suppression ratios of above 50 dB. Besides, the relative intensity noise at an injection current of 200 mA is below -157 dB/Hz.
Optical Communication and Optical Signal Processing IV
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A rotated QAM based DMT scheme for underwater optical wireless communication system
In the paper, a rotated quadrature amplitude modulation (QAM) based discrete multi-tone (DMT) is proposed and experimentally demonstrated in underwater optical wireless communication (UOWC). After transmission over 2.4-m water channel and 10-m free space, the experimental results show that, compared with conventional 16-QAM DMT, the received optical power (ROP) is improved 3.3dB at the bit error rate (BER) of 10-5 using the rotated QAM based DMT. In addition, for the rotated 32-QAM DMT, the receiver sensitivity can enhance 3.5dB compared with conventional 32 QAM DMT. And compared to DMT with bit loading, it is improved 1dB at the BER of 10-3.
Gsnake: A lightweight SNR optimization algorithm for practical optical camera communication
Pinpin Zhang, Chao Hu, Yimao Sun, et al.
Exploring light-to-camera wireless communication (a.k.a. Optical Camera Communication (OCC)) has been a hot research direction in recent years, the breakthrough and commercialization of its key technology can not only solve the spectrum crunch in traditional radio frequency based communication but also revolutionize the next generation of green communications. However, OCC is still trapped by several serious challenges in realistic complex application scenarios (such as exhibits and museums), where they need reflected OCC, but the reflective surfaces are not regular and smooth. Furthermore, the communication performance of the reflected OCC is seriously degraded by the color and texture of the reflective surface in realistic scenarios. To this end, many researchers, including us, have done systematic and deep research to address above issues. In this work, we propose a novel algorithm namely Gsnake to overcome the bottleneck problem of OCC technology in complex scenarios and further improve the performance and practicability of OCC. Essentially, the proposed scheme can search and select pixels in the high SNR (Signal-to-Noise Ratio) region projected by the LED emitter so as to maximize the received signal strength, whereas the conventional ones normally only select a fixed row of pixels. By deploying the algorithm in a real-world application, the experimental results verify the efficacy and usability of the proposed scheme, which is superior to existing algorithms.
Optical Communication and Optical Signal Processing V
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Highly coherent linearly-swept fiber laser based on composite optical phase-locked loop
Yinxia Meng, Weilin Xie, Yuxiang Feng, et al.
We present a dual-loop composite optical phase-locked loop (OPLL) composed of an acousto-optic frequency shifter based external modulation loop and a piezo based direct modulation loop for the generation of highly coherent swept-frequency fiber laser. It allows offering a larger loop bandwidth and gain, permitting an efficient linearization and coherence enhancement. We have verified experimentally a highly coherent swept-frequency fiber laser with a high fidelity and linearized frequency sweep with sweep range of ~8.2 GHz at ~164 GHz/s sweep rate, accompanied with a peak-to-peak frequency error as low as ~130 kHz. The in-band noise of the coherent beat note has been effective suppressed by almost 60 dB within a loop bandwidth up to ~80 kHz . The proposed linearly swept-frequency fiber laser provides a straightforward optimization of the real-time sweep control and far distance ranging in various scenarios, which is supposed to be a beneficial tool in both industrial and commercial applications.
Optical Communication and Optical Signal Processing VI
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Photonic microwave generation based on semiconductor laser and its applications
Semiconductor lasers (SLs) under external disturbances can be driven into diverse nonlinear dynamical states such as period-one, period-two, multi-period, and chaos. Based on the period-one nonlinear dynamical state in an optically injected semiconductor laser, tunable single-tone microwave signal, ultra-broadband microwave frequency combs, and frequency-modulated continuous wave can be generated. Moreover, through introducing optical feedback, a SL under pulsed current modulation can output pulsed chaotic signal, which can be applied in anti-interference radar.
Poster Session
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Generation of random-hopped LFM signal based on optical injection in a DFB laser
Chuanqi Fang, Yansong Bai, Hao Chen, et al.
With an increase in applications of the photonics RADAR, managing the interference and spoofing becomes crucial for reducing false detection, increasing accuracy on range-resolution, and high-resolution imaging that directly impacts safety and security. One of the solutions to deal with intruders and interferers is to modify the transmitting signal, which is usually an FMCW on the present RADAR system. This paper proposes and experimentally demonstrates photonically generated random-hopped linear frequency modulated (RLFM) signal generation using an optical injection in a distributed-feedback (DFB) laser. The basic principle of the LFM signal is the redshift phenomena in the self-oscillating mode of the DFB laser with a change in the injected beam power. To generate an RLFM signal, a random waveform from an arbitrary waveform generator (AWG) is used rather than the linear waveform such as sawtooth, triangular waveforms. For the proof of the concept demonstration, the conventional LFM signal of 8 GHz bandwidth is divided into four equal sub-interval with the bandwidth of 2 GHz (LFM1: 8-10 GHz, LFM2: 14-16 GHz, LFM3: 12-14 GHz, LFM4: 10-12 GHz). Even though each generated LFM signal has a lower bandwidth of 2 GHz, the range resolution can be maintained at the receiver to that of the conventional LFM signal by concatenating the four beating signals in time domain.
10 Gbps PAM4 transmission for fiber wireless access in Ka-band based on long short-term memory neural network equalizer
In this paper, 28GHz millimeter wave (MMW) in n257 band which is one of the recommended frequency bands for beyond 5G, has been demonstrated by experiment for optical wireless access. The MMW antenna with the bandwidth of 26.5 ~ 29.5GHz and an envelope detector with a 3dB bandwidth of about 500MHz are used to enable 4-ary pulse amplitude modulation (PAM4) signal transmission over 5km fiber and 1.6m wireless distances in our experiment. In order to compensate the linear and nonlinear impairments of the optical wireless links, the long short-term memory (LSTM) neural network nonlinear equalizer is adopted in the receiver DSP. Additionally, the traditional linear equalizer (LE) and Volterra equalizer (VE) are also conducted for comparison. The results show that neither the performances of LE nor VE can reach the 7% hard-decision forward error correction (HD-FEC) threshold (3.8×10-3) in the case of 5 Gbaud PAM4 transmission over 5km fiber and 1.6m wireless distances. Instead, after adopting the LSTM equalizer, the bit error rate can be reduced to approximate 1×10-3, which reveals a noticeable performance improvement. Moreover, the performances of the three kinds of equalizers at different transmission rates are further studied. We find that the LSTM can help improve the system capacity from below 9Gbps to above 10Gbps at 7% HD-FEC threshold, which means more than 10 percent improvement has been achieved. According to the above results, we believe that the LSTM equalizer will facilitate the large-capacity communication for the upcoming 5G MMW in fiber wireless access networks.
Research on dual-wavelength DFB laser with grating reflector
We proposed a dual-wavelength (DW) DFB semiconductor laser with an integrated grating reflector (GR). The proposed laser is based on the reconstruction-equivalent-chirp technique which can be realized easily with high precision. The GR section can provide reflection for the DW-DFB laser. Utilizing the proposed method, higher output power, lower threshold current and larger sidemode suppression ratio (SMSR) can be achieved.
Optimization of EDFA position for high speed short reach PAM4 optical fiber communication systems
In this paper, the position of erbium-doped fiber amplifier (EDFA) in an intensity-modulation and direct-detection (IM/DD) optical fiber communication system is optimized to suppress a part of chromatic dispersion (CD) caused distortions and alleviate the burden of digital signal processing. The results demonstrate that, for a system with a certain signal rate and launch optical power, the transmission fiber length is longer with optimizing the position of the EDFA. If a 20% overhead soft-decision forward error correction threshold of 2.7×10−2 is considered, the CD-uncompensated transmission fiber length of a 50 Gb/s four-level pulse amplitude modulation (PAM4) system can be increased by 34.78% compared with the non-optimized system. Moreover, the launch optical power of the optimized system can be decreased by 6 dB.
Unsupervised-learning neural network for fiber nonlinearity compensation
Pinjing He, Feilong Wu, Meng Yang, et al.
A fiber nonlinearity compensation scheme based on an unsupervised-learning neural network is proposed. In the proposed scheme, labels in the training data and weights of the neural network are iteratively updated until converging. To validate the proposed scheme, a 3200 km dual-polarization 16-QAM simulation link and an 1800 km single-polarization experimental link were carried out. Simulation and experiment results validate that the proposed method can achieve the same equalization performance as the supervised-learning-neural-network-based scheme, without any pre-defined training data.
Extended Kalman-particle filter-based phase noise compensation for CO-OFDM systems
Huiyun Deng, Xinwei Du
Due to longer symbol time duration, the coherent optical orthogonal frequency-division multiplexing (COOFDM) system is more sensitive to the linear phase noise (LPN) than that of the single carrier system. This paper proposes to compensate the LPN in a real-time and dynamic manner based on the extended Kalmanparticle filter (EKPF). The phase noise estimation performance converge to the steady state by utilizing a pilot OFDM symbol, and then decision feedback algorithm is applied for further estimation and detection. Simulations results show that the proposed algorithm can achieve more accurate phase estimation and BER performance compared to the conventional Bayesian filters.
A novel probabilistic shaping method based on optimized chaotic sequence bit-operation
Probabilistic shaping (PS) as an effective method for approaching the Shannon limit further has got much attention nowadays. A novel chaotic sequence bit-operation based probabilistic shaping scheme is proposed in this study, and improvement in bit error rate and channel capacity is demonstrated experimentally. When the optical power is -12 dBm, the maximum bit error rate is improved by 3.61 dB compared to conventional 16-CAP. In addition, the generalized mutual information performance is ameliorated under low received optical power. And the feature of chaotic sequences, which will be further studied in the future, offers potential for improving safety.
Design of multi-channel piezoresistive measurement system based on time division multiplex transmission for neural network training
The gridless planar flexible sensors based on BP neural network have attracted wide interests in the design of smart sensing devices due to their high accuracy and low cost. In order to obtain the effective measurement data from the multichannel electrodes in a gridless planar sensor simultaneously, and to avoid the mutual crosstalk between the electrodes, a time-division multiplex piezoresistive measurement system based on serial port communication function is proposed in this work. A proportional amplification circuit is designed in this system to convert resistance signals into potential signals, and a STM32 is used as the main control chip to control two multiplexers to realize time-division multiplex transmission. The measured potential signals are converted by ADC and sent to computer with the help of USART. A PCB is designed and fabricated accordingly, and the fast and accurate multi-channel data synchronization acquisition is achieved.
Adversarial learning-based knowledge distillation for optical performance monitoring in 7 core fiber
An adversarial learning based knowledge distillation optical performance monitoring scheme based on has been proposed for 7 core fiber in this paper. Adversarial learning-based knowledge distillation simplified the architecture of the neural network for optical performance monitoring, including modulation format recognition (MFR) and optical signal-to-noise ratio (OSNR) estimation, in spatial division multiplexing (SDM) fiber transmission systems. On account of the knowledge distillation technologies, the knowledge in large teacher model is transferred to lightweight student model to reduce the complexity of the neural network and the difficulty of deployment. In addition, the adversarial learning is applied to the teacher-student architecture in order to enhance the generalization ability of the student model. After adversarial learning-based knowledge distillation, the student model is suitable for the deployment of the services in optical nodes. Experimentation results indicate that the student model has the 100 % modulation format recognition success rate for QPSK, 8QAM and 16QAM while the RMSE of optical signal-to-noise ratio (OSNR) estimation is below 0.1 dB. Due to its excellent performance and being easy to implement, the proposed scheme has the potential for the next-generation multiple core fiber based optical network.
Photonics-aided 0.3-THz wireless transmission based on digital sub-carrier multiplexing
To combine the merits of fiber communication and wireless communication, photonics-aided terahertz-wave (THz-wave) technology has become a popular technology in recent years. In this paper, a 92 Gbit/s photonics-aided 0.3-THz wireless transmission system based on digital sub-carrier multiplexing (DSCM) is proposed and simulated. In order to compensate the phase noise, both Viterbi-Viterbi and maximum likelihood (VV&ML) algorithm and decision directed digital phase lock loop (DD-PLL) method are considered and compared. According to the simulation results, DSCM based scheme can provide better performance than single carrier (SC) based scheme in the case of low received optical power (ROP) or input optical power (IOP) while there is no advantage in the case of high ROP or IOP. For SC based scheme, DD-PLL has 1 dB sensitivity gain compared with VV&ML when BER drops from 10-3 to 10-4, while 2 dB sensitivity gain was obtained for DSCM based scheme for ROP. For SCM based scheme, DD-PLL has 1 dB sensitivity gain compared with VV&ML when BER drops from 10-3 to 10-4. On the whole, DD-PLL outperforms VV&ML in terms of performance in high ROP case for both SC and SCM based scheme. Besides, DD-PLL has lower computation complexity than VV&ML.
Research on the effect of key system parameters on linewidth tolerance of space coherent optical communication
You Guo, Yijun Jiang, Xue Wang, et al.
In order to investigate the influence of different system parameters on linewidth tolerance in space coherent optical communication system, we give the ensemble average bit error rate (BER) model of binary phase-shift keying (BPSK) modulation and homodyne detection space coherent optical communication system. The BER model also considers the laser linewidth induced phase noise and pointing error. Based on the BER model, the numerical simulation is conducted to investigate the relationship between linewidth tolerance and different system parameters of divergence angle, receiving aperture, zenith angle, and transmitted optical power. Through our numerical simulation, it is found that the linewidth tolerance will change from several kHz to several hundred kHz. Besides, linewidth tolerance will decrease with the increase of divergence angle and zenith angle. While with the increase of receiving aperture and transmitted optical power, the linewidth tolerance will increase. Also, we find that the linewidth tolerance changes more obviously with the variation of the zenith angle. This paper has a good reference value for the selection of laser linewidth of the space coherent optical communication system under different system parameters.
Continuously bandwidth-tunable nonlinear polarization rotation mode-locked fiber laser
Developing ultrafast lasers with controllable flexible pulses, such as wavelength and pulsewidth tunable lasers are desired for various applications like fiber telecommunication and optical sensing. To achieve wavelength or/and pulsewidth tunability in saturable absorber (SA) based mode-locked fiber lasers, some technologies including tunable band pass filter, fiber Bragg grating, diffraction grating mirror, 45°tilted fiber grating, and tunable Lyot filter are proposed to provide the direct spectral manipulation of the pulses. In this paper, we demonstrate a simple bandwidth-tunable ultrashort pulse generation scheme for a nonlinear polarization rotation mode-locked laser. By utilizing a polarization-maintaining-fiber-pigtailed inline polarizer and a polarization controller, a nonlinear polarization evolution (NPE) mode-locking effect as well as a bandwidth-tunable Lyot filter are enabled in the laser cavity when the intracavity polarization settings adjusted. The Lyot filter formed by the inline polarizer and its birefringence of the two fiber pigtails with lengths of 0.3 m could introduce a spectral filtering effect. By only tuning the intracavity polarization controller, the spectral bandwidth is continuously tuned in the range of 7.8 to 3.5 nm. We attribute the lower limit of the spectral bandwidth to the nonlinear self-phase modulation requiring narrow pulses in nonlinear polarization rotation mode-locked lasers. These results provide a simple way for generating subpicosecond pulse with variable spectral bandwith or pulse duration without using a saturable absorber.
Microwave photonics beamforming for 5G
This paper reviewed and summarized microwave photonics beamforming architecture which could directly apply in 5G communications in recent years. However, those research still have some limits. We investigated the connection of radio frequency (RF) links and antenna elements (AEs) at present 5G multi-antenna systems and propose a microwave photonics beamforming architecture with dynamic subarray by combing reconfigurable optical routing networks with independent optical true time delay units. Optical routing networks enable distributing signals to AEs which are any number and at any position, to accomplish the dynamic combinations of those AEs. Independent optical true-time delay units enable the array could transmit or receive the RF beams towards or from different directions simultaneously. We simulated the beam pattern of the proposed system in various communication situations to validate our concept can effectively expand the application field of microwave photonics beamforming systems and could bring the advantages of microwave photonics technology, such as large bandwidth, low loss, immunity to electromagnetic interference, and non-beam squint effect.
Investigation on the linewidth tolerance of the space coherent laser communication system under atmospheric turbulence
Quadrature Phase Shift Keying (QPSK) is an important digital signal modulation method, which has the advantages of high spectrum efficiency and strong anti-interference. Coherent laser communication has the advantages of high speed, large capacity, light and small equipment as well as high sensitivity. In order to better introduce the QPSK modulation method into the space downlink coherent laser communication system, we give a bit error rate (BER) of QPSK modulation model with laser linewidth suitable for space downlink coherent laser communication system with QPSK modulation under atmospheric turbulence. Based on the model, we simulate and analyze the effects of atmospheric wind speed, zenith angle, gain of Erbium doped optical fiber amplifier (EDFA) and communication rate on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation. The results show that the linewidth tolerance of the system increases when the zenith angle decreases or the gain of EDFA increases or the wind speed decreases. And the effects of zenith angle, gain of EDFA and communication rate on the linewidth tolerance of the space downlink coherent laser communication system with QPSK modulation are large. The effect of wind speed on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation is smaller. And the larger the wind speed is, the smaller the effect of increasing wind speed on the linewidth tolerance. For the communication rate, the linewidth tolerance increases and then decreases with the increase of the communication rate. This work can be a reference for the design of space downlink coherent laser communication system with QPSK modulation.
Wavelength switchable mode-locked fiber laser using optical switch
Programmable and fast wavelength-switchable ultrashort pulses have important applications in optical communication, optical sensing, micro-wave photonics, and other fields. Multi-wavelength mode-locked lasers are commonly developed to achieve wavelength tunability by using tunable optical filters, fiber Bragg gratings, Lyot filters providing the direct spectral manipulation of the pulses. Actually, manual control is essential in process of wavelength switching, which is very time-consuming and sometimes it is difficult to find the appropriate state of polarization for a certain mode-locking state when utilizing polarization adjustment. So a programmable mode-locked laser which can automatically switch to different mode-locking states is desired. Here, we demonstrate a fast electrically-controlled wavelength-switching scheme in ultrafast nonlinear polarization rotation mode-locked lasers. It is achieved by simply introducing an inline polarization beam splitter (PBS) with two output ports and followed by a 1×2 electrically-controlled optical switch in a fiber ring laser. By utilizing the polarization-maintaining-fiber-pigtailed inline PBS and a polarization controller, a nonlinear polarization evolution (NPE) mode-locking effect as well as a tunable Lyot filter are enabled in the fiber cavity. The optical switch could select the optical path with different polarization state for mode-locking at different wavelength.
Three-wavelength mode converter based on three-cascaded tilted sampled Bragg gratings
By expanding the reconstruction-equivalent-chirp (REC) technique into two-Dimensional (2D) grating, we designed a three-wavelength mode converter with three-cascaded tilted sampled Bragg gratings (3C-TSBG). Theoretical simulation results show that the presented 3C-TSBG can realize three-wavelength conversion between the fundamental transverse electric mode (TE0) and the first-order transverse electric mode (TE1). In addition, the mode-conversion wavelength can be tuned by changing the period and tilted angle of the cascaded sampled gratings. More importantly, compared with the cascaded tilted Bragg gratings (C-TBG), the 3C-TSBG can improved the error tolerance and wavelength control accuracy. Therefore, the proposed 3C-TSBG has potential applications in reconfigurable mode-division-multiplexing communication systems and other areas where programmable mode conversion is required.
A multi-chirped lidar system for simultaneous detection of distance, velocity and direction
A lidar system for simultaneous detection of distance, velocity and direction of a moving target is proposed and experimentally demonstrated based on an optical multi-chirped LFM signal generator. The optical multi-chirped linearly frequency-modulated (LFM) signal generator is realized by an optical frequency shifting loop (OFSL) that is driven by an electrical LFM signal. After the OFSL, a sequence of optical LFM signals with incremental chirps are generated. The signal is then emitted into the space and captured again after being reflected by a target. A lidar system is thus realized. An experiment is carried out. A sequence of LFM signals with chirps incremented from 0.389 to 2.713 GHz/s are generated. A distance of 19.1 m and velocities of -2.203 m/s and 4.170 m/s are measured based on a lidar system built based on the LFM signal.
Microwave angle-of-arrival measurement utilizing photonic real-time Fourier transformation
Zihao Wang, Dan Zhu, Bowen Zhang, et al.
A microwave angle-of-arrival (AOA) measurement system based on the photonic real-time Fourier transformation (RTFT) is proposed and demonstrated. In the proposed system, photonic RTFT is implemented utilizing the temporal-spectrum convolution system, which is composed by an optical pulse source, an electro-optical modulator and a pair of dispersive elements with complementary dispersion values. The time delay of the linearly frequency-modulated (LFM) signals can be obtained, which are used to calculate the time-difference-of arrival (TDOA) and the corresponding AOA. A proof-ofconcept experiment is taken. The AOA measurement range from 25° to 155° is achieved with a measurement error of less than 3.4°. In this scheme, the problem of phase ambiguity in traditional AOA measurement systems is overcome.
High-resolution and wideband optical vector analysis using optical FMCW and fixed low-frequency detection
Meihui Cao, Lihan Wang, Shilong Pan
We propose a wideband optical vector analyzer (OVA) with high frequency resolution using optical frequency-modulated continuous-wave (FMCW) and fixed low-frequency detection. In the proposed OVA, an optical FMCW is divided into two portions. One portion propagates through an acousto-optic modulator (AOM) to introduce a fixed frequency shift, and the other undergoes a variable optical delay line (VODL) to match the delay of the two branches. Then, the two optical FMCWs with a fixed frequency interval are combined as the probe signal. After passing through an optical device under test (DUT), the probe signal carrying the response of DUT is transmitted into a low-speed photodetector to perform frequency downconversion. Hence, an intermediate photocurrent (IF) with a fixed frequency is generated and sampled by a low-speed analog-to-digital converter (ADC). Finally, the frequency response of the DUT can be obtained by utilizing a short-time Fourier transform (STFT) algorithm. The proposed OVA is low-cost with a simple structure and has a wide measurement range. Besides, by applying the optical FMCW with a large chirp rate, the proposed FMCW-based OVA can realize a high measurement speed and a high frequency resolution. An experiment is performed in which the chirp rate of the optical FMCW is 1000 nm/s. As a result, the measurement speed achieves 2 ns/point and the frequency resolution is 0.22 MHz within the measurement range of 17 nm.