Journals >Chinese Optics Letters
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2024
Volume: 22 Issue 1
26 Article(s)
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Research Articles
Biophotonics
Laser speckle contrast imaging based on uniting spatiotemporal Fourier transform
Linjun Zhai, Yongzhao Du, Xunxun Wu, Yong Diao, and Yuqing Fu
We propose a laser speckle contrast imaging method based on uniting spatiotemporal Fourier transform. First, the raw speckle images are entirely transformed to the spatiotemporal frequency domain with a three-dimensional (3D) fast Fourier transform. Second, the dynamic and static speckle components are extracted by applying 3D low-pass and high-pass filtering in the spatiotemporal frequency domain and inverse 3D Fourier transform. Third, we calculate the time-averaged modulation depth with the average of both components to map the two-dimensional blood flow distribution. The experiments demonstrate that the proposed method could effectively improve computational efficiency and imaging quality.We propose a laser speckle contrast imaging method based on uniting spatiotemporal Fourier transform. First, the raw speckle images are entirely transformed to the spatiotemporal frequency domain with a three-dimensional (3D) fast Fourier transform. Second, the dynamic and static speckle components are extracted by applying 3D low-pass and high-pass filtering in the spatiotemporal frequency domain and inverse 3D Fourier transform. Third, we calculate the time-averaged modulation depth with the average of both components to map the two-dimensional blood flow distribution. The experiments demonstrate that the proposed method could effectively improve computational efficiency and imaging quality..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011701 (2024)
Diffraction, Gratings, and Holography
Femtosecond laser direct written fiber Bragg gratings with narrow bandwidth and high sideband suppression
Wenping Qiu, Shuang Liu, Guanghua Cheng, Huan Zhan, Guodong Zhang, Guanpin Ren, Zhongrui Sun, and Min Zhang
Aiming for suppressing side-mode and spectrum broadening, a slit beam-shaping method and super-Gaussian apodization processing for femtosecond laser point-by-point (PbP) inscription technology of fiber Bragg gratings (FBGs) are reported here. High-quality FBGs, featuring narrow bandwidth of less than 0.3 nm, high reflectivity above 85%, low insertion loss (0.21 dB), and low cladding loss (0.82 dB), were obtained successfully. By a semi-automatic PbP inscription process, an array consisting of six FBGs, exhibiting almost no side-mode peaks with high suppression ability and narrow bandwidth, was fabricated along three independently developed single-mode fibers with an interval of 20 mm.Aiming for suppressing side-mode and spectrum broadening, a slit beam-shaping method and super-Gaussian apodization processing for femtosecond laser point-by-point (PbP) inscription technology of fiber Bragg gratings (FBGs) are reported here. High-quality FBGs, featuring narrow bandwidth of less than 0.3 nm, high reflectivity above 85%, low insertion loss (0.21 dB), and low cladding loss (0.82 dB), were obtained successfully. By a semi-automatic PbP inscription process, an array consisting of six FBGs, exhibiting almost no side-mode peaks with high suppression ability and narrow bandwidth, was fabricated along three independently developed single-mode fibers with an interval of 20 mm..
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 010501 (2024)
Fiber Optics and Optical Communications
4.096 Tbit/s multidimensional multiplexing signals transmission over 1000 km few mode fiber
Yu Zhang, Chen Wang, Kaihui Wang, Junjie Ding, Bowen Zhu, Lei Shen, Lei Zhang, Ruichun Wang, Changkun Yan, Bo Liu, and Jianjun Yu
We experimentally transmit eight wavelength-division-multiplexing (WDM) channels, 16 quadratic-amplitude-modulation (QAM) signals at 32-GBaud, over 1000 km few mode fiber (FMF). In this experiment, we use WDM, mode division multiplexing, and polarization multiplexing for signal transmission. Through the multiple-input–multiple-output (MIMO) equalization algorithms, we achieve the total line transmission rate of 4.096 Tbit/s. The results prove that the bit error rates (BERs) for the 16QAM signals after 1000 km FMF transmission are below the soft-decision forward-error-correction (SD-FEC) threshold of 2.4×10-2, and the net rate reaches 3.413 Tbit/s. Our proposed system provides a reference for the future development of high-capacity communication.We experimentally transmit eight wavelength-division-multiplexing (WDM) channels, 16 quadratic-amplitude-modulation (QAM) signals at 32-GBaud, over 1000 km few mode fiber (FMF). In this experiment, we use WDM, mode division multiplexing, and polarization multiplexing for signal transmission. Through the multiple-input–multiple-output (MIMO) equalization algorithms, we achieve the total line transmission rate of 4.096 Tbit/s. The results prove that the bit error rates (BERs) for the 16QAM signals after 1000 km FMF transmission are below the soft-decision forward-error-correction (SD-FEC) threshold of
Chinese Optics Letters
- Publication Date: Dec. 29, 2023
- Vol. 22, Issue 1, 010602 (2024)
Imaging Systems and Image Processing
Optimizing depth of field in 3D light-field display by analyzing and controlling light-beam divergence angle
Xunbo Yu, Yiping Wang, Xin Gao, Hanyu Li, Kexin Liu, Binbin Yan, and Xinzhu Sang
A concept of divergence angle of light beams (DALB) is proposed to analyze the depth of field (DOF) of a 3D light-field display system. The mathematical model between DOF and DALB is established, and the conclusion that DOF and DALB are inversely proportional is drawn. To reduce DALB and generate clear depth perception, a triple composite aspheric lens structure with a viewing angle of 100° is designed and experimentally demonstrated. The DALB-constrained 3D light-field display system significantly improves the clarity of 3D images and also performs well in imaging at a 3D scene with a DOF over 30 cm.A concept of divergence angle of light beams (DALB) is proposed to analyze the depth of field (DOF) of a 3D light-field display system. The mathematical model between DOF and DALB is established, and the conclusion that DOF and DALB are inversely proportional is drawn. To reduce DALB and generate clear depth perception, a triple composite aspheric lens structure with a viewing angle of 100° is designed and experimentally demonstrated. The DALB-constrained 3D light-field display system significantly improves the clarity of 3D images and also performs well in imaging at a 3D scene with a DOF over 30 cm..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011101 (2024)
Differential interference contrast phase edging net: an all-optical learning system for edge detection of phase objects
Yiming Li, Ran Li, Quan Chen, Haitao Luan, Haijun Lu, Hui Yang, Min Gu, and Qiming Zhang
Edge detection for low-contrast phase objects cannot be performed directly by the spatial difference of intensity distribution. In this work, an all-optical diffractive neural network (DPENet) based on the differential interference contrast principle to detect the edges of phase objects in an all-optical manner is proposed. Edge information is encoded into an interference light field by dual Wollaston prisms without lenses and light-speed processed by the diffractive neural network to obtain the scale-adjustable edges. Simulation results show that DPENet achieves F-scores of 0.9308 (MNIST) and 0.9352 (NIST) and enables real-time edge detection of biological cells, achieving an F-score of 0.7462.Edge detection for low-contrast phase objects cannot be performed directly by the spatial difference of intensity distribution. In this work, an all-optical diffractive neural network (DPENet) based on the differential interference contrast principle to detect the edges of phase objects in an all-optical manner is proposed. Edge information is encoded into an interference light field by dual Wollaston prisms without lenses and light-speed processed by the diffractive neural network to obtain the scale-adjustable edges. Simulation results show that DPENet achieves F-scores of 0.9308 (MNIST) and 0.9352 (NIST) and enables real-time edge detection of biological cells, achieving an F-score of 0.7462..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011102 (2024)
Infrared and Terahertz Photonics
Terahertz wide-angle metalens with nearly ideal object-image relation
Yu Wang, Jierong Cheng, Yunyun Ji, Fei Fan, and Shengjiang Chang
Metalenses are essential components in terahertz imaging systems. However, without careful design, they show limited field of view and their practical applications are hindered. Here, a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of ±25°. Simulation and experiment results show that the focusing efficiency, spot size, and modulation transfer function of this lens are not sensitive to the incident angle. More importantly, this wide-angle metalens follows the ideal Gaussian formula for the object-image relation, which ensures a wider field of view and better contrast in the imaging experiment.Metalenses are essential components in terahertz imaging systems. However, without careful design, they show limited field of view and their practical applications are hindered. Here, a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 013701 (2024)
Instrumentation, Measurement, and Optical Sensing
Simultaneous transmission of time-frequency and data with co-amplification over urban fiber links
Qian Cao, Zhou Tong, Lei Liu, Jialiang Wang, Kang Ying, Fufei Pang, and Youzhen Gui
We demonstrate a simultaneous transmission of time-frequency and data over a 160-km urban business network in Shanghai. The signals are transmitted through a cascaded optical link consisting of 48 km and 32 km, which are connected by an optical relay. The metrological signals are inserted into the communication network using dense wavelength division multiplexing. The influence of the interference between different signals has been discussed. The experimental results demonstrate that the radio frequency (RF) instability can reach 2.1×10-14 at 1 s and 2.3×10-17 at 10,000 s, and the time interval transfer of one pulse per second (1 PPS) signal with less than 10 ps at 1 s is obtained. This work paves the way for the widespread dissemination of ultra-stable time and frequency signals over the communication networks.We demonstrate a simultaneous transmission of time-frequency and data over a 160-km urban business network in Shanghai. The signals are transmitted through a cascaded optical link consisting of 48 km and 32 km, which are connected by an optical relay. The metrological signals are inserted into the communication network using dense wavelength division multiplexing. The influence of the interference between different signals has been discussed. The experimental results demonstrate that the radio frequency (RF) instability can reach
Chinese Optics Letters
- Publication Date: Dec. 29, 2023
- Vol. 22, Issue 1, 011201 (2024)
New speckle pattern interferometry for precise in situ deformation measurements
Ruyue Zhang, Yu Fu, and Hong Miao
A new electronic speckle pattern interferometry method is proposed to realize in situ deformation measurements. The feature of the method is the combination of a high-speed camera and multiple laser Doppler vibrometers (LDVs) for synchronous measurements. The high-speed camera is used to record and select effective interferograms, while the LDVs are used to measure the rigid body displacement caused by vibrations. A series of effective interferograms with known shifted phase values are obtained to calculate the deformation phase. The experimental results show that the method performs well in measuring static and dynamic deformations with high accuracy in vibrating environments.A new electronic speckle pattern interferometry method is proposed to realize in situ deformation measurements. The feature of the method is the combination of a high-speed camera and multiple laser Doppler vibrometers (LDVs) for synchronous measurements. The high-speed camera is used to record and select effective interferograms, while the LDVs are used to measure the rigid body displacement caused by vibrations. A series of effective interferograms with known shifted phase values are obtained to calculate the deformation phase. The experimental results show that the method performs well in measuring static and dynamic deformations with high accuracy in vibrating environments..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011202 (2024)
Welding depth measurement for different mode lasers using optical coherence tomography
Guanming Xie, Sanhong Wang, Yueqiang Zhang, You Li, Biao Hu, Yu Fu, and Qifeng Yu
Optical coherence tomography (OCT) allows a direct and precise measurement of laser welding depth by coaxially measuring the keyhole depth and can be used for process monitoring and control. When OCT measurement was taken during single-beam laser welding, the keyhole instability of aluminum welding resulted in highly scattered OCT data and complicated the welding depth extraction methods. As a combination of an inner core beam and an outer ring beam, a novel adjustable ring mode (ARM) laser for producing a stable keyhole was applied to the OCT measurement. Different ARM laser power arrangements were conducted on aluminum and copper. The results indicated that the ring beam greatly improved the stability of the core beam-induced keyhole, and smooth welding depth can be extracted from the concentrated OCT data.Optical coherence tomography (OCT) allows a direct and precise measurement of laser welding depth by coaxially measuring the keyhole depth and can be used for process monitoring and control. When OCT measurement was taken during single-beam laser welding, the keyhole instability of aluminum welding resulted in highly scattered OCT data and complicated the welding depth extraction methods. As a combination of an inner core beam and an outer ring beam, a novel adjustable ring mode (ARM) laser for producing a stable keyhole was applied to the OCT measurement. Different ARM laser power arrangements were conducted on aluminum and copper. The results indicated that the ring beam greatly improved the stability of the core beam-induced keyhole, and smooth welding depth can be extracted from the concentrated OCT data..
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 011203 (2024)
High synchronization absolute distance measurement using a heterodyne and superheterodyne combined interferometer
Ziqi Yin, Fangfei Li, Yunke Sun, Yun Zou, Yan Wang, Hongxing Yang, Pengcheng Hu, Haijin Fu, and Jiubin Tan
We propose an absolute distance measurement method that employs heterodyne and superheterodyne combined interferometers to achieve synchronous detection and demodulation of multiwavelengths. Coarse and fine synthetic wavelengths are generated by a dual-longitudinal-mode He–Ne laser and four acoustic optical frequency shifters. Further, to improve phase synchronization measurement for multiwavelengths, we analyze the demodulation characteristics of coarse and fine measurement signals and adopt a demodulation method suitable for both signals. Experimental results demonstrate that the proposed method can achieve high-precision synchronous demodulation of multiwavelengths, and standard deviation is 1.7 × 10-5 m in a range of 2 m.We propose an absolute distance measurement method that employs heterodyne and superheterodyne combined interferometers to achieve synchronous detection and demodulation of multiwavelengths. Coarse and fine synthetic wavelengths are generated by a dual-longitudinal-mode He–Ne laser and four acoustic optical frequency shifters. Further, to improve phase synchronization measurement for multiwavelengths, we analyze the demodulation characteristics of coarse and fine measurement signals and adopt a demodulation method suitable for both signals. Experimental results demonstrate that the proposed method can achieve high-precision synchronous demodulation of multiwavelengths, and standard deviation is 1.7 × 10-5 m in a range of 2 m..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011204 (2024)
Integrated Optics
Experimental demonstration of a flexible-grid 1 × (2 × 3) mode- and wavelength-selective switch using silicon microring resonators and counter-tapered couplers
Dejun Kong, Hao Lu, Pengjun Wang, Qiang Fu, Shixun Dai, Weiwei Chen, Yuefeng Wang, Bohao Zhang, Lingxiao Ma, Jun Li, Tingge Dai, and Jianyi Yang
A flexible-grid 1×(2×3) mode- and wavelength-selective switch which comprises counter-tapered couplers and silicon microring resonators has been proposed, optimized, and demonstrated experimentally in this work. By carefully thermally tuning phase shifters and silicon microring resonators, mode and wavelength signals can be independently and flexibly conveyed to any one of the output ports, and different bandwidths can be generated as desired. The particle swarm optimization algorithm and finite difference time-domain method are employed to optimize structural parameters of the two-mode (de)multiplexer and crossing waveguide. The bandwidth-tunable wavelength-selective optical router composed of 12 microring resonators is studied by taking advantage of the transfer matrix method. Measurement results show that, for the fabricated module, cross talk less than -10.18 dB, an extinction ratio larger than 17.41 dB, an in-band ripple lower than 0.79 dB, and a 3-dB bandwidth changing from 0.38 to 1.05 nm are obtained, as the wavelength-channel spacing is 0.40 nm. The corresponding response time is measured to be 13.64 µs.A flexible-grid
Chinese Optics Letters
- Publication Date: Jan. 02, 2024
- Vol. 22, Issue 1, 011301 (2024)
Inverse design of highly efficient and broadband mode splitter on SOI platform
Junpeng Liao, Ye Tian, Zirong Yang, Haoda Xu, Chen Tang, Yuheng Wang, Xiaowei Zhang, and Zhe Kang
Mode splitters that directly separate modes without changing their orders are highly promising to improve the flexibility of the mode-division multiplexing systems. In this paper, we design a high-performance mode splitter on the silicon-on-insulator platform with a compact footprint of 14 µm× 2.5 µm using an inverse design method based on shape optimization. The fabrication of this mode splitter requires only a single lithography step and exhibits good fabrication tolerances. The experimental results show that the proposed device exhibits state-of-the-art insertion loss (<0.9 dB) and cross talk (<-16 dB) over a broad bandwidth (1500–1600 nm). Furthermore, the shape optimization method used is implemented to design a dual-mode (de)multiplexer, and the experimental results fulfill the design objective, demonstrating the excellent generality of the design method in this paper.Mode splitters that directly separate modes without changing their orders are highly promising to improve the flexibility of the mode-division multiplexing systems. In this paper, we design a high-performance mode splitter on the silicon-on-insulator platform with a compact footprint of 14 µm× 2.5 µm using an inverse design method based on shape optimization. The fabrication of this mode splitter requires only a single lithography step and exhibits good fabrication tolerances. The experimental results show that the proposed device exhibits state-of-the-art insertion loss (<0.9 dB) and cross talk (<-16 dB) over a broad bandwidth (1500–1600 nm). Furthermore, the shape optimization method used is implemented to design a dual-mode (de)multiplexer, and the experimental results fulfill the design objective, demonstrating the excellent generality of the design method in this paper..
Chinese Optics Letters
- Publication Date: Jan. 22, 2024
- Vol. 22, Issue 1, 011302 (2024)
O-band reconfigurable silicon polarization rotator
Yawen Bai, Pengfei Wang, Bo Peng, and Tao Chu
Silicon waveguides typically exhibit optical anisotropy, which leads to polarization correlation and single-polarization operations. This consequently creates a demand for polarization-control devices. This paper introduces a CMOS-compatible O-band reconfigurable TE/TM polarization rotator comprising two symmetrical polarization rotator–splitters and phase shifters. This configuration enables dynamic conversion of any linear polarization to its quadratic equivalent. Experimental results indicate that the reconfigurable polarization rotator exhibits an insertion loss of less than 1.5 dB. Furthermore, the bandwidth for a polarization extinction ratio beyond 15 dB exceeds 60 nm.Silicon waveguides typically exhibit optical anisotropy, which leads to polarization correlation and single-polarization operations. This consequently creates a demand for polarization-control devices. This paper introduces a CMOS-compatible O-band reconfigurable TE/TM polarization rotator comprising two symmetrical polarization rotator–splitters and phase shifters. This configuration enables dynamic conversion of any linear polarization to its quadratic equivalent. Experimental results indicate that the reconfigurable polarization rotator exhibits an insertion loss of less than 1.5 dB. Furthermore, the bandwidth for a polarization extinction ratio beyond 15 dB exceeds 60 nm..
Chinese Optics Letters
- Publication Date: Jan. 19, 2024
- Vol. 22, Issue 1, 011303 (2024)
Lasers, Optical Amplifiers, and Laser Optics
Dual-mode stabilization for laser to radio-frequency locking by using a single-sideband modulation and a Fabry–Pérot cavity
Yibo Wang, Hongwei Zhang, Chenhao Zhao, Gang Zhao, Xiaojuan Yan, and Weiguang Ma
The dual-mode stabilization scheme has been demonstrated as an efficient way to stabilize laser frequency. In this study, we propose a novel dual-mode stabilization scheme that employs a sizable Fabry–Pérot cavity instead of the microcavity used in previous studies and has enabled higher bandwidth for locking. The results demonstrate a 30-fold reduction in laser frequency drift, with frequency instability below 169 kHz for integration time exceeding 1 h and a minimum value of 33.8 kHz at 54 min. Further improvement could be achieved by optimizing the phase locking. This scheme has potential for use in precision spectroscopic measurement.The dual-mode stabilization scheme has been demonstrated as an efficient way to stabilize laser frequency. In this study, we propose a novel dual-mode stabilization scheme that employs a sizable Fabry–Pérot cavity instead of the microcavity used in previous studies and has enabled higher bandwidth for locking. The results demonstrate a 30-fold reduction in laser frequency drift, with frequency instability below 169 kHz for integration time exceeding 1 h and a minimum value of 33.8 kHz at 54 min. Further improvement could be achieved by optimizing the phase locking. This scheme has potential for use in precision spectroscopic measurement..
Chinese Optics Letters
- Publication Date: Jan. 22, 2024
- Vol. 22, Issue 1, 011401 (2024)
Watt-level acousto-optically Q-switched Pr:YLF laser at 639 nm
Yuchen Xue, Ruisong Zhang, Zhengdong Dai, Zhongyu Wang, Huiying Xu, and Zhiping Cai
We present a study on a watt-level acousto-optically Q-switched Pr:YLF laser at three different repetition rates (10 kHz, 20 kHz, and 50 kHz) for the first time, to the best of our knowledge. The corresponding average output powers and pulse widths were measured to be 1.14 W, 1.2 W, and 1.32 W, and 40 ns, 52 ns, and 80 ns, respectively. A maximum pulse energy of 0.11 mJ was obtained, corresponding to a peak power of up to 2.8 kW at a repetition rate of 10 kHz. The simulated dynamics of a fast Q-switched Pr:YLF laser is in agreement with the experiment. The laser’s ability to generate stable pulses with high peak power and short pulse width makes it highly desirable for various practical applications, such as laser machining and material processing.We present a study on a watt-level acousto-optically Q-switched Pr:YLF laser at three different repetition rates (10 kHz, 20 kHz, and 50 kHz) for the first time, to the best of our knowledge. The corresponding average output powers and pulse widths were measured to be 1.14 W, 1.2 W, and 1.32 W, and 40 ns, 52 ns, and 80 ns, respectively. A maximum pulse energy of 0.11 mJ was obtained, corresponding to a peak power of up to 2.8 kW at a repetition rate of 10 kHz. The simulated dynamics of a fast Q-switched Pr:YLF laser is in agreement with the experiment. The laser’s ability to generate stable pulses with high peak power and short pulse width makes it highly desirable for various practical applications, such as laser machining and material processing..
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 011402 (2024)
14.1 W continuous-wave dual-end diode-pumped Er:Lu2O3 laser at 2.85 µm
Yangyang Liang, Tao Li, Baitao Zhang, Jingliang He, Sascha Kalusniak, Xian Zhao, and Christian Kränkel
We present our efforts towards power scaling of Er:Lu2O3 lasers at 2.85 µm. By applying a dual-end diode-pumped resonator scheme, we achieve an output power of 14.1 W at an absorbed pump power of 59.7 W with a slope efficiency of 26%. In a single-end pumped resonator scheme, an output power of 10.1 W is reached under 41.9 W of absorbed pump power. To the best of our knowledge, this is the first single crystalline mid-infrared rare-earth-based solid-state laser with an output power exceeding 10 W at room temperature.We present our efforts towards power scaling of Er:Lu2O3 lasers at 2.85 µm. By applying a dual-end diode-pumped resonator scheme, we achieve an output power of 14.1 W at an absorbed pump power of 59.7 W with a slope efficiency of 26%. In a single-end pumped resonator scheme, an output power of 10.1 W is reached under 41.9 W of absorbed pump power. To the best of our knowledge, this is the first single crystalline mid-infrared rare-earth-based solid-state laser with an output power exceeding 10 W at room temperature..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011403 (2024)
Microwave Photonics
Time-resolution enhanced multi-path OTD measurement using an adaptive filter based incoherent OFDR
Lihan Wang, Lingyun Ren, Xiangchuan Wang, and Shilong Pan
High accuracy and time resolution optical transfer delay (OTD) measurement is highly desired in many multi-path applications, such as optical true-time-delay-based array systems and distributed optical sensors. However, the time resolution is usually limited by the frequency range of the probe signal in frequency-multiplexed OTD measurement techniques. Here, we proposed a time-resolution enhanced OTD measurement method based on incoherent optical frequency domain reflectometry (I-OFDR), where an adaptive filter is designed to suppress the spectral leakage from other paths to break the resolution limitation. A weighted least square (WLS) cost function is first established, and then an iteration approach is used to minimize the cost function. Finally, the appropriate filter parameter is obtained according to the convergence results. In a proof-of-concept experiment, the time-domain response of two optical links with a length difference of 900 ps is successfully estimated by applying a probe signal with a bandwidth of 400 MHz. The time resolution is improved by 2.78 times compared to the theoretical resolution limit of the inverse discrete Fourier transform (iDFT) algorithm. In addition, the OTD measurement error is below ±0.8 ps. The proposed algorithm provides a novel way to improve the measurement resolution without applying a probe signal with a large bandwidth, avoiding measurement errors induced by the dispersion effect.High accuracy and time resolution optical transfer delay (OTD) measurement is highly desired in many multi-path applications, such as optical true-time-delay-based array systems and distributed optical sensors. However, the time resolution is usually limited by the frequency range of the probe signal in frequency-multiplexed OTD measurement techniques. Here, we proposed a time-resolution enhanced OTD measurement method based on incoherent optical frequency domain reflectometry (I-OFDR), where an adaptive filter is designed to suppress the spectral leakage from other paths to break the resolution limitation. A weighted least square (WLS) cost function is first established, and then an iteration approach is used to minimize the cost function. Finally, the appropriate filter parameter is obtained according to the convergence results. In a proof-of-concept experiment, the time-domain response of two optical links with a length difference of 900 ps is successfully estimated by applying a probe signal with a bandwidth of 400 MHz. The time resolution is improved by 2.78 times compared to the theoretical resolution limit of the inverse discrete Fourier transform (iDFT) algorithm. In addition, the OTD measurement error is below
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 013901 (2024)
Sub-Nyquist radar receiver based on photonics-assisted compressed sensing and cascaded dictionaries
Shiyang Liu, and Yang Chen
A sub-Nyquist radar receiver based on photonics-assisted compressed sensing is proposed. Cascaded dictionaries are applied to extract the delay and the Doppler frequency of the echo signals, which do not need to accumulate multiple echo periods and can achieve better Doppler accuracy. An experiment is performed. Radar echoes with different delays and Doppler frequencies are undersampled and successfully reconstructed to obtain the delay and Doppler information of the targets. Experimental results show that the average reconstruction error of the Doppler frequency is 5.33 kHz using an 8-μs radar signal under the compression ratio of 5. The proposed method provides a promising solution for the sub-Nyquist radar receiver.A sub-Nyquist radar receiver based on photonics-assisted compressed sensing is proposed. Cascaded dictionaries are applied to extract the delay and the Doppler frequency of the echo signals, which do not need to accumulate multiple echo periods and can achieve better Doppler accuracy. An experiment is performed. Radar echoes with different delays and Doppler frequencies are undersampled and successfully reconstructed to obtain the delay and Doppler information of the targets. Experimental results show that the average reconstruction error of the Doppler frequency is 5.33 kHz using an 8-μs radar signal under the compression ratio of 5. The proposed method provides a promising solution for the sub-Nyquist radar receiver..
Chinese Optics Letters
- Publication Date: Dec. 29, 2023
- Vol. 22, Issue 1, 013902 (2024)
Long-term ultrastable frequency dissemination via a 50-km spooled fiber link using a two-section DFB laser
Zhiqian Yin, Chuanbo Zhang, Shijian Guan, Xin Zhou, Yaguang Wang, Leilei Wang, Manhang Zheng, Yitong Liu, Yunshan Zhang, Xingbang Zhu, Tao Fang, and Xiangfei Chen
The stable long-distance transmission of radio-frequency (RF) signals holds significant importance from various aspects, including the comparison of optical frequency standards, remote monitoring and control, scientific research and experiments, and RF spectrum management. We demonstrate a scheme where an ultrastable frequency signal was transmitted over a 50 km coiled fiber. The optical RF signal is generated using a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technique. The 3-dB modulation bandwidth of the two-section DFB laser is 18 GHz and the residual phase noise of -122.87 dBc/Hz is achieved at 10-Hz offset frequency. We report a short-term stability of 1.62×10-14 at an average time of 1 s and a long-term stability of 6.55×10-18 at the measurement time of 62,000 s when applying current to the front section of the DFB laser. By applying power to both sections, the stability of the system improves to 4.42×10-18 within a testing period of 56,737 s. Despite applying temperature variations to the transmission link, long-term stability of 8.63×10-18 at 23.9 h can still be achieved.The stable long-distance transmission of radio-frequency (RF) signals holds significant importance from various aspects, including the comparison of optical frequency standards, remote monitoring and control, scientific research and experiments, and RF spectrum management. We demonstrate a scheme where an ultrastable frequency signal was transmitted over a 50 km coiled fiber. The optical RF signal is generated using a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technique. The 3-dB modulation bandwidth of the two-section DFB laser is 18 GHz and the residual phase noise of
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 013903 (2024)
Nonlinear Optics
Ultrabroadband second-harmonic generation via spatiotemporal-coupled phase matching
Yudong Tao, Wentao Zhu, Yanfang Zhang, Jingui Ma, Jing Wang, Peng Yuan, Hao Zhang, Heyuan Zhu, and Liejia Qian
We propose a spatially chirped quasi-phase-matching (QPM) scheme that enables ultrabroadband second-harmonic-generation (SHG) by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave. A “zero-dispersion” 4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane, where the fundamental is quasi-monochromatic locally in picosecond duration, fundamentally canceling high-order phase mismatch. A fan-out QPM grating characterized by a linear variation of the poling period along the transverse direction exactly supports the QPM of the spatially chirped beam. We theoretically demonstrate the SHG of an 810-nm, 12.1-fs pulse into a 405-nm, 10.2-fs pulse with a conversion efficiency of 77%.We propose a spatially chirped quasi-phase-matching (QPM) scheme that enables ultrabroadband second-harmonic-generation (SHG) by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave. A “zero-dispersion” 4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane, where the fundamental is quasi-monochromatic locally in picosecond duration, fundamentally canceling high-order phase mismatch. A fan-out QPM grating characterized by a linear variation of the poling period along the transverse direction exactly supports the QPM of the spatially chirped beam. We theoretically demonstrate the SHG of an 810-nm, 12.1-fs pulse into a 405-nm, 10.2-fs pulse with a conversion efficiency of 77%..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 011901 (2024)
Optical Materials
Toroidal dipole response in rectangular waveguide: used to generate vector beams and vector vortex beams
Hao Luo, Cong Chen, Peng Gao, Yue Feng, Ziyan Ren, Yujia Qiao, and Hai Liu
Toroidal multipole is a special current distribution that has many different characteristics from electric multipole and magnetic multipole distributions. Because of its special properties, the toroidal dipole is a research hotspot in the field of metamaterials and nanophotonics. However, the low scattering of the toroidal dipole moment makes its excitation a challenging task. At present, there are relatively few studies on its specific engineering applications. In this paper, by slotting in the rectangular cavity, the excitation of an equivalent toroidal dipole is successfully achieved over a wide frequency range of 53–58 GHz. Results indicate that under the action of the toroidal dipole, the TE10 mode electromagnetic waves transmitted in the rectangular waveguide are converted into vector beams and are radiated outwards. Further adjusting the spatial distribution of the magnetic dipoles in the toroidal dipoles yields results that indicate that the resonance mode in the slot is still dominated by the magnetic toroidal dipole moment, and the electromagnetic waves radiating outward are vortex beams carrying vector polarization. The scattered energy of each dipole moment inside the antenna is calculated. This calculation verifies that the mass of the vector beam and vector vortex beam is closely related to the toroidal dipole supported by this antenna. The proposed structure can be applied to explorations in vortex filtering, in photon entanglement, and in the photonic spin Hall effect.Toroidal multipole is a special current distribution that has many different characteristics from electric multipole and magnetic multipole distributions. Because of its special properties, the toroidal dipole is a research hotspot in the field of metamaterials and nanophotonics. However, the low scattering of the toroidal dipole moment makes its excitation a challenging task. At present, there are relatively few studies on its specific engineering applications. In this paper, by slotting in the rectangular cavity, the excitation of an equivalent toroidal dipole is successfully achieved over a wide frequency range of 53–58 GHz. Results indicate that under the action of the toroidal dipole, the
Chinese Optics Letters
- Publication Date: Jan. 22, 2024
- Vol. 22, Issue 1, 011601 (2024)
Optoelectronics
Recent progress of parameter-adjustable high-power photonic microwave generation based on wide-bandgap photoconductive semiconductors
Tao Xun, Xinyue Niu, Langning Wang, Bin Zhang, Jinmei Yao, Yimu Yu, Hanwu Yang, Jing Hou, Jinliang Liu, and Jiande Zhang
Radio frequency/microwave-directed energy sources using wide bandgap SiC photoconductive semiconductors have attracted much attention due to their unique advantages of high-power output and multi-parameter adjustable ability. Over the past several years, benefitting from the sustainable innovations in laser technology and the significant progress in materials technology, megawatt-class output power electrical pulses with a flexible frequency in the P and L microwave wavebands have been achieved by photoconductive semiconductor devices. Here, we mainly summarize and review the recent progress of the high-power photonic microwave generation based on the SiC photoconductive semiconductor devices in the linear modulation mode, including the mechanism, system architecture, critical technology, and experimental demonstration of the proposed high-power photonic microwave sources. The outlooks and challenges for the future of multi-channel power synthesis development of higher power photonic microwave using wide bandgap photoconductors are also discussed.Radio frequency/microwave-directed energy sources using wide bandgap SiC photoconductive semiconductors have attracted much attention due to their unique advantages of high-power output and multi-parameter adjustable ability. Over the past several years, benefitting from the sustainable innovations in laser technology and the significant progress in materials technology, megawatt-class output power electrical pulses with a flexible frequency in the P and L microwave wavebands have been achieved by photoconductive semiconductor devices. Here, we mainly summarize and review the recent progress of the high-power photonic microwave generation based on the SiC photoconductive semiconductor devices in the linear modulation mode, including the mechanism, system architecture, critical technology, and experimental demonstration of the proposed high-power photonic microwave sources. The outlooks and challenges for the future of multi-channel power synthesis development of higher power photonic microwave using wide bandgap photoconductors are also discussed..
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 012501 (2024)
Mid-wavelength nBn photodetector with high operating temperature and low dark current based on InAs/InAsSb superlattice absorber
Peng Cao, Tiancai Wang, Hongling Peng, Zhanguo Li, Qiandong Zhuang, and Wanhua Zheng
In this paper, we demonstrate nBn InAs/InAsSb type II superlattice (T2SL) photodetectors with AlAsSb as the barrier that targets mid-wavelength infrared (MWIR) detection. To improve operating temperature and suppress dark current, a specific Sb soaking technique was employed to improve the interface abruptness of the superlattice with device passivation using a SiO2 layer. These result in ultralow dark current density of 6.28×10-6 A/cm2 and 0.31 A/cm2 under -600 mV at 97 K and 297 K, respectively, which is lower than most reported InAs/InAsSb-based MWIR photodetectors. Corresponding resistance area product values of 3.20×104 Ω ·cm2 and 1.32 Ω ·cm2 were obtained at 97 K and 297 K. A peak responsivity of 0.39 A/W with a cutoff wavelength around 5.5 µm and a peak detectivity of 2.1×109 cm·Hz1/2/W were obtained at a high operating temperature up to 237 K.In this paper, we demonstrate nBn InAs/InAsSb type II superlattice (T2SL) photodetectors with AlAsSb as the barrier that targets mid-wavelength infrared (MWIR) detection. To improve operating temperature and suppress dark current, a specific Sb soaking technique was employed to improve the interface abruptness of the superlattice with device passivation using a
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 012502 (2024)
Spectroscopy
Terahertz spectroscopy of water in nonionic reverse micelles
Jiaqi Zhang, Yuyue Yan, Liyuan Liu, and Weili Zhang
The dynamics of water within a nanopool of a reverse micelle is heavily affected by the amphiphilic interface. In this work, the terahertz (THz) spectra of cyclohexane/Igepal/water nonionic reverse micelle mixture are measured by THz time-domain spectroscopy and analyzed with two Debye models and complex permittivity of background with volume ratios. Based on the fitted parameters of bulk and fast water, the molar concentration of all kinds of water molecules and hydration water molecule number per Igepal molecule are calculated. We find that slow hydration water has the highest proportion in water when the radius parameter ω0<10, while bulk water becomes the main component when ω0≥10. The feature radius ratio of nonhydrated and hydrated water to total water nanopool is roughly obtained from 0.39 to 0.85 with increasing ω0.The dynamics of water within a nanopool of a reverse micelle is heavily affected by the amphiphilic interface. In this work, the terahertz (THz) spectra of cyclohexane/Igepal/water nonionic reverse micelle mixture are measured by THz time-domain spectroscopy and analyzed with two Debye models and complex permittivity of background with volume ratios. Based on the fitted parameters of bulk and fast water, the molar concentration of all kinds of water molecules and hydration water molecule number per Igepal molecule are calculated. We find that slow hydration water has the highest proportion in water when the radius parameter
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 013001 (2024)
Ultrafast Optics and Attosecond/High-field Physics
Pulse repetition rate effect on the plasma inside femtosecond laser filament in air
Fukang Yin, Tie-Jun Wang, Yaoxiang Liu, Juan Long, Yingxia Wei, Bin Zhu, Kainan Zhou, and Yuxin Leng
The characteristics of plasmas play an important role in femtosecond laser filament-based applications. Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates. In our experiments, the measured average plasma density of the filament is 1.54×1017cm-3 and the temperature of the plasma is about 5100 K under 100 Hz pulse repetition rate. The plasma density decreases to 1.43×1017cm-3 and the temperature increases to 6230 K as the pulse repetition rate increases to 1000 Hz. The experimental observation agrees with the numerical simulation by solving the nonlinear Schrödinger equations with repetition rate related “low density hole” correction.The characteristics of plasmas play an important role in femtosecond laser filament-based applications. Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates. In our experiments, the measured average plasma density of the filament is
Chinese Optics Letters
- Publication Date: Jan. 08, 2024
- Vol. 22, Issue 1, 013201 (2024)
Efficient terahertz generation from van der Waals α-In2Se3
Shijie Duan, Ming Yang, Suyuan Zhou, Longhui Zhang, Jinsen Han, Xu Sun, Guang Wang, Changqin Liu, Dongdong Kang, Xiaowei Wang, Jiahao Chen, and Jiayu Dai
Two-dimensional (2D) van der Waals materials have attracted tremendous attention due to their versatile physical properties and flexible manipulation approaches. Among the various types of van der Waals materials, α-In2Se3 is remarkable for its intrinsic 2D ferroelectricity and high-performance opto-electronic properties. However, the study of the α-In2Se3 system in terahertz (THz) radiation is scarce, although it is promising for electrically controlled THz field manipulation. We investigate the α-In2Se3 in different thicknesses and report that the THz generation efficiency induced by femtosecond laser pulses can be largely improved by reducing the thickness from the bulk. Furthermore, we reveal the surge current in thin film coupled with THz emission exhibits a different Auger recombination mode, which is helpful in understanding the mechanism and provides insights into the design of 2D highly efficient THz devices.Two-dimensional (2D) van der Waals materials have attracted tremendous attention due to their versatile physical properties and flexible manipulation approaches. Among the various types of van der Waals materials,
Chinese Optics Letters
- Publication Date: Jan. 18, 2024
- Vol. 22, Issue 1, 013202 (2024)
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