Chinese Optics Letters
Co-Editors-in-Chief
Zhizhan Xu
Contents 1 Issue (s), 12 Article (s)
Vol. 22, Iss.5—May.1, 2024 • pp: 050601- Spec. pp:
Research ArticlesVol. 22, Iss.5-May..1,2024
Fiber Optics and Optical Communications
Robust thin-film lithium niobate modulator on a silicon substrate with backside holes
Mai Wang, Lu Qi, Haohua Wang, Ziliang Ruan, Gengxin Chen, Bin Chen, Shengqi Gong, Kaixuan Chen, and Liu Liu

Recently, Mach–Zehnder modulators based on thin-film lithium niobate have attracted broad interest for their potential for high modulation bandwidth, low insertion loss, high extinction ratio, and high modulation efficiency. The periodic capacitively loaded traveling-wave electrode is optimally adopted for ultimate high-performances in this type of modulator. However, such an electrode structure on a silicon substrate still suffers from the velocity mismatch and substrate leakage loss for microwave signals. Here, we introduce a thin-film lithium niobate modulator structure using this periodic capacitively loaded electrode on a silicon substrate. Backside holes in the silicon substrate are prepared to solve robustly the above difficulties. The fabricated device exhibits an insertion loss of 0.9 dB, a halfwave-voltage–length product of 2.18 V·cm, and an ultra-wide bandwidth well exceeding 67 GHz for a 10-mm-long device. Data transmissions with rates up to 112 Gb/s are demonstrated. The proposed structure and fabrication strategy are compatible for other types of monolithic and heterogeneous integrated thin-film lithium niobate modulators on a silicon substrate.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 050601 (2024)
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Fiber Optics and Optical Communications
Joint modulation format identification and OSNR monitoring based on Stokes vector distribution features for digital coherent optical receivers
Zhengyu Pu, Lin Jiang, Lianshan Yan, Anlin Yi, Haijun He, Wei Pan, and Bin Luo

For joint modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring, a simple and intelligent optical communication performance monitoring method is proposed, and the feasibility is demonstrated by digital coherent optical communication experiments. The experiment results show that for all modulation formats, including 28 GBaud polarization division multiplexing (PDM) QPSK/8-QAM/16-QAM/64-QAM, 100% MFI accuracies are achieved even at OSNR values lower than the corresponding theoretical 20% forward error correction limit, as well as the high accuracies for OSNR monitoring. Furthermore, the proposed scheme has a reasonable monitoring level when chromatic dispersion and fiber nonlinear effects are varied.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 050602 (2024)
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Fiber Optics and Optical Communications
Preparation of panda-shaped photonic crystal fibers with and without silver wire
Junbo Lou, Yonghui Yang, Qiang Qu, and Shuguang Li

A method of preparing panda-shaped photonic crystal fibers (PCFs) based on secondary drawing technology is proposed in this paper. The secondary drawing can not only reduce fiber diameter but also reduce core size by adding a glass sleeve. Silver-filled and non-silver-filled panda PCFs are prepared. The two ends of silver-filled panda PCF are connected with a broadband light source and a spectrometer, respectively, and the surface plasmon resonance phenomenon is detected. The secondary drawing technology provides a meaningful reference for the preparation of PCF in the future, and the prepared silver-filled panda PCF can be prepared into an optical fiber filter.

Chinese Optics Letters
May. 08, 2024, Vol. 22 Issue 5 050603 (2024)
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Fiber Optics and Optical Communications
Coexistence of noise-like pulse and dark pulse in an Er/Yb co-doped fiber laser
Jing Li, Chuncan Wang, and Peng Wang

The coexistence of a noise-like pulse (NLP) and a dark pulse was experimentally demonstrated in a net-anomalous dispersion Er/Yb co-doped fiber (EYDF) laser, for the first time, to our knowledge. The cavity was mode-locked by nonlinear polarization rotation (NPR) technique. Meanwhile, a Sagnac loop with a section of polarization-maintaining fiber (PMF) was used as a comb filter to enable multiwavelength pulse operation. When the PMF length was 0.3 m, an asymmetric two-peak spectrum with central wavelengths of 1565.3 and 1594.2 nm was obtained by adjusting polarization controllers (PCs). It is a composite state of NLP and dark pulse due to the cross-phase modulation between the two different wavelength components along orthogonal polarization axes. The two pulses are synchronized with a repetition rate of 7.53 MHz. By adjusting the PC in the Sagnac loop, the spectral ranges of NLPs and dark pulses can be tuned from 1560 to 1577.8 nm and from 1581.8 to 1605.4 nm, respectively. In addition, the pulse characteristics were investigated by incorporating the PMF with different lengths, where the coexistence patterns can be generated when the PMF lengths were 0.2 and 0.3 m. A longer PMF can lead to a narrowband comb filtering, which causes a larger loss and is not favorable for stable operation of the coexistence regime. This fiber laser demonstrates an interesting operation regime and has significant potential for numerous practical applications.

Chinese Optics Letters
May. 15, 2024, Vol. 22 Issue 5 051403 (2024)
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Instrumentation, Measurement, and Optical Sensing
High-precision alkali-atom density measurement and control methods using light absorption for dual-beam SERF magnetometers
Ziao Liu, Jixi Lu, Zhaohui Hu, Xiaoyu Li, Yifan Yan, Di Zhan, and Jianli Li

The alkali-atom density measurement method based on light absorption is highly suitable for a spin-exchange relaxation-free (SERF) atomic magnetometer because of its high-precision measurement and complete nonmagnetic interference. In this study, the optical rotation angle detection system based on polarization balance detection is utilized to realize the alkali-atom density real-time measurement without affecting magnetic field measurement. We discovered that there exists an optimal frequency detuning of the probe light, which offers the highest sensitivity in alkali-atom density measurement and the lowest susceptibility to temperature fluctuations in terms of the scale factor. In contrast to conventional light absorption measurements based on pump light, this method demonstrated a threefold improvement in alkali-atom density measurement sensitivity while remaining immune to ambient magnetic fields and incident light intensity fluctuations. Furthermore, we utilized this method to achieve closed-loop temperature control with an accuracy of 0.04°C.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 051201 (2024)
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Integrated Optics
Passively stable 0.7-octave microcombs in thin-film lithium niobate microresonators
Zexing Zhao, Chenyu Wang, Jingyuan Qiu, Zhilin Ye, Zhijun Yin, Zhenlin Wang, Kunpeng Jia, Xiao-Hui Tian, Zhenda Xie, and Shi-Ning Zhu

The optical frequency comb based on microresonators (microcombs) is an integrated coherent light source and has the potential to promise a high-precision frequency standard; self-reference and a long-term stable microcomb are the keys to this realization. Here, we demonstrated a 0.7-octave spectrum Kerr comb via dispersion engineering in a thin-film lithium niobate microresonator, and the single-soliton state can be accessed passively with long-term stability over 3 h. With such a robust broadband coherent comb source using thin-film lithium niobate, a fully stabilized microcomb can be expected for massive practical applications.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 051301 (2024)
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Lasers, Optical Amplifiers, and Laser Optics
Large aperture disk standing-wave unstable resonator with intra-cavity adaptive correction
Yi Yu, Jiayu Yi, Liu Xu, Jianli Shang, Jing Wu, Lixin Tong, Qingsong Gao, Chun Tang, Wei Zhang, and Lei Chen

An unstable resonator with seven large aperture ceramic disks and intra-cavity adaptive correction is presented. The composite ceramic disks with absorption rings were adopted to suppress amplified spontaneous emission. An intra-cavity aberration non-conjugate correction based on round-trip wavefront and relaxation iteration was applied in the resonator. After tilt and defocus were corrected in turn, an average output power of 4.5 kW was obtained. The corresponding beam quality factor β was 19.5. After tilt, defocus, and high order aberrations were corrected, the average output power was increased to 5.4 kW, and the beam quality factor β was improved to 6.8.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 051401 (2024)
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Lasers, Optical Amplifiers, and Laser Optics
Dual-comb soliton rains based on polarization multiplexing in a single-walled carbon nanotube mode-locked Er-doped fiber laser
Yue Zhao, Ying Qin, Kailin Jia, Li Chen, Guangwei Chen, Guoqing Hu, Tengfei Wu, Huiyu Li, Jingwen He, and Zhehai Zhou

We experimentally demonstrate tunable dual-comb soliton rains in a polarization multiplexing fiber laser based on a single-walled carbon nanotube. The repetition frequency difference of dual-comb pulses is about 39 Hz, with a maximum extinction ratio of 29 dB. With suitable polarization states, one of the dual-comb pulses switches into soliton rain sequence with chirped isolating soliton trains. The signal-to-noise ratio reaches 61 dB, which is 11 dB higher than that of the normal dual-comb pulses. The intervals between chirped isolating solitons are distributed progressively, and the number of isolating solitons can be flexibly tuned from 2 to 11 by adjusting polarization state or pump power. Our work will provide support for further understanding of interaction dynamics of solitons and give a new route to the application of precision measurement.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 051402 (2024)
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Lasers, Optical Amplifiers, and Laser Optics
Polarization full feedback external cavity of open loop spectral beam combining based on multi-single emitters laser diode
Man Hu, Weiqiao Zhang, Peng Cao, Tongtong Li, Xuyan Zhou, and Wanhua Zheng

In this paper, a spectral beam combining (SBC) structure of multi-single emitters laser diode based on a polarization full feedback (PFF) external cavity is proposed and demonstrated. The maximum combining efficiency is 75.6%, which leads to an output power of 38.48 W, a degree of polarization (DOP) of 99.42%, and electro-optical conversion efficiency of 35.63% under continuous wave operation at a current of 8 A. Compared to the conventional SBC, the output power, the combining efficiency, the electro-optical conversion efficiency, and the DOP of the PFF-SBC structure present improvements of 5.73 W, 11.26 percentage points, 5.3 percentage points, and 7.26 percentage points, respectively. The results show that this SBC method can achieve a high efficiency and linearly polarized laser output of SBC, thereby making the subsequent polarization beam-combining efficiency approach the limit.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 051405 (2024)
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Microwave Photonics
Stability-enhanced RF signal transmission over long fiber-optic links
Zhiqian Yin, Manhang Zheng, Chuanbo Zhang, Shijian Guan, Xin Zhou, Yitong Liu, Zeyu Gang, Jiaqiang Nie, Yunshan Zhang, Xingbang Zhu, Tao Fang, and Xiangfei Chen

We propose a method for optimizing the phase stability of microwave signal transmission over long distances. First, the design of the photon link was modified to reduce the radio frequency (RF) signal’s baseline noise and increase power. Second, a low-noise driver circuit was developed for a two-section distributed feedback (DFB) laser designed using reconstruction equivalent chirp (REC) technology to create an ultra-stable laser, and its performance was characterized through linewidth data. Test results indicate that the DFB laser achieved narrower linewidth, improving system phase stability. When an injection current (30 mA) is applied to the reflection section of the two-section DFB laser, the laser linewidth will be narrower (1.38 MHz), further enhancing the system’s phase transmission stability. At a 1 Hz offset frequency, a residual phase noise of -88.65 dBc/Hz is obtained. The short-term stability with an averaging time of 1 s is 1.60 × 10-14, and the long-term stability over a testing time of 60,000 s is 3.41 × 10-18. Even after incorporating temperature variations, the long-term stability reaches 8.37 × 10-18 at 22 h.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 053901 (2024)
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Nanophotonics, Metamaterials, and Plasmonics
Metalens with tilted structures for high-efficiency focusing at large-angle incidences
Yue Wang, Chen Chen, Shengjie Wu, Xin Ye, Shining Zhu, and Tao Li

The metalens has attracted remarkable attention due to its ultra-thin and ultra-light characteristics, which indicate great potential for compact imaging. However, the limited efficiency at a large angle incidence severely hinders the application of wide-angle focusing and imaging, which is pursued in the fast-developing imaging systems. Therefore, new strategies to improve the lens performance at large incident angles are in demand. In this work, we propose tilted structures for large-angle focusing with improved efficiency. Metalenses based on dynamic phase and geometric phase are designed and systematically characterized by numerical simulations. We show that tilted structures of unit cells significantly improve the lens performance at oblique incidences. In detail, the focusing efficiency of the metalens with tilted structures is increased over 25% at 30° incidence, as well as the modulation transfer function. In addition, we develop a hybrid metalens array achieving highly efficient wide-angle imaging up to 120°. We believe this design provides a feasible route toward wide-field and high-performance imaging applications.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 053601 (2024)
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Quantum Optics and Quantum Information
Polarization-entangled photon pairs with factorable spectra engineered by using an uneven four-stage nonlinear interferometer
Liang Cui, Haoran Chen, Jiamin Li, and Xiaoying Li

We report the generation of polarization-entangled photon pairs in the 1550 nm band by pumping an uneven nonlinear interferometer loop with two orthogonally polarized counterpropagating pump pulses. The uneven nonlinear interferometer, providing a more ideal interference pattern due to the elimination of secondary maxima, consists of four pieces of dispersion-shifted fibers sandwiched with three pieces of standard single-mode fibers, and the lengths of the nonlinear fibers follow the binomial distribution. The mode number of the photon pairs deduced from the measured joint spectrum is ∼1.03. The collection efficiency of the photon pairs is found to be ∼94% (after background noise correction). The directly measured visibility of two-photon interference of the polarization-entangled photon pairs is ∼92%, while no interference is observed in the direct detection of either the signal or idler photons.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 052701 (2024)
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