In this paper, we propose a method of stable frequency transfer over a 400 km link without an external power supply by combining distributed Raman amplifiers (DRAs) and remote optical pump amplifiers (ROPAs). We elaborate on the advantages of these technologies and their physical limitations in unrepeatered frequency transfer. In this study, by integrating a Bi-ROPA and a first-order DRA in a single-fiber bidirectional configuration over a 400 km ultralong-haul unrepeatered frequency link (with an average loss of 0.2 dB/km), a frequency instability of 8 x 10-14 at 1 s integration time and 2 x 10-16 at 10,000 s integration time has been achieved.

Single-shot spectroscopy enables the capture of an entire transient event within a single measurement, providing critical insights into rapid and non-repetitive processes across diverse fields such as biomedical diagnostics, environmental monitoring, material stress analysis, advanced manufacturing, and telecommunication network fluctuation detection. Despite its promise, current single-shot spectroscopic systems are fundamentally constrained by an inherent incompatibility between spectral bandwidth, spectral resolution, and acquisition speed, limiting their versatility and broader adoption. Here, we introduce a reconfigurable single-shot spectroscopy method that offers full programmability over spectral bandwidth, resolution, and acquisition speed, enabling programmable navigation of the trade-off. By selecting optimal configurations, our approach compensates for the traditional limitations in spectral resolution, achieving a record-breaking resolution of 86 femtometers in the 1.5 μm band-comparable to state-of-the-art Michelson-based Fourier transform spectrometers. Leveraging this technique, we successfully capture rapid dynamic spectral fluctuations of phase-shift fiber Bragg gratings induced by both continuous (9 kHz) and sudden (0.1 ms) vibrations. This innovation broadens the application landscape and paves the way for the commercialization of single-shot spectroscopy.

Photonic time-stretch microwave channelization enables simultaneous multi-channel monitoring using a single optical channel and shows strong potential for military electronic spectrum detection. However, its resolution and accuracy have been limited compared to mature parallel approaches. We present an improved system that overcomes these shortcomings by employing pulse picking to adjust the repetition rate of a mode-locked laser, introducing a pre-modulation dispersion of -829 ps/nm, and using a second dispersion compensation fiber stage with optical amplification to further stretch RF-modulated optical pulses in the time domain. After photodetection and sampling, digital signal processing performs spectral transformation, slicing, and analysis. The enlarged optical time window allows fine channel discrimination and precise frequency estimation. Experiments demonstrate a record-breaking resolution of 73 MHz and accuracy of f 2 MHz in serial photonics channelization.

Laser synchronization is a technique of stabilizing the frequency difference between two lasers so that their frequencies change in synchronization. This technique is vital for both scientific and industrial applications. Conventional synchronization systems, whether digital or analog, exhibit inherent limitations in terms of accuracy and bandwidth. Current hybrid synchronization systems are usually based on hybrid digital and analog phase-locked loops (PLLs), which can partially address this limitation. However, since they are structured serially, their bandwidth remains constrained by the design of the analog component. This paper presents a hybrid “digital + analog” laser synchronization system with a parallel structure. The system's digital and analog locking loops operate independently, providing long-term stability and locking accuracy, respectively. This approach addresses the accuracy and bandwidth limitations present in serial hybrid digital and analog PLLs. We synchronized a single-frequency laser (SFL) to a mode-locked laser (MLL), achieving a frequency offset fluctuation of less than 2.5 Hz over 24 hours. Additionally, by synchronizing two SFLs to a repetition-frequency locked MLL, we achieved indirect synchronization between SFLs with a frequency offset of 10.6 GHz and a fluctuation of less than 5 Hz in 24 hours. These results demonstrate the robust long-term and short-term stability of the system. Based on this synchronization system, we proposed a photonic-assisted microwave frequency identification scheme with a detection error of less than 0.6 MHz. The high performance of the synchronization system enables the proposed frequency identification scheme to achieve high measurement accuracy and a theoretically extensive frequency range.

In this paper, the optical delay information is magnified by the photonics time stretch technique. The precision of distance measurement has been improved by 10 times experimentally. (C) 2024 The Author(s)

We study a relatively excellent BHD(Balanced Homodyne Detector)with large bandwidth and low noise pre-amplification, which is different from the specific BHD implementation scheme and can better adapt to the subsequent cascade scheme, thereby achieving the implementation of BHD with specific needs. Specifically, we optimize our BHD performance by compromising various parameters of BHD, focusing on optimizing and increasing BHD bandwidth, and choosing a more reasonable BHD implementation solution. Judging from the test results, the produced BHD has good performance in extracting effective signals. The bandwidth reaches about 2GHz, which meets the requirements of detection applications. It has a high signal-to-noise ratio of 10 dB and a common-mode rejection ratio (CMRR) of 20 dB near the 2 GHz frequency point.

Long-distance radio frequency (RF) synchronization through fiber link is attracting more attention in recent years. The repeater becomes increasingly important with the increase of the transmission distance. In this article, we analyze the three main relay methods principally including single-span, signal relay, and cascade connection frequency transmission and compare their frequency instability. The proof-of-concept experiments are also conducted along a 3009.8 km fiber link in the laboratory environment. The system used for comparison is a combination of phase conjugation and phase-locked loop (PLL) designed for long-distance frequency transmission. The best transfer preference can be obtained by using the cascade connection, whose frequency instabilities of 8.8x10(-14)@1sand8.4x10(-17) @ 10 000 s have been measured for phase-stable RF signal. Our study can be useful for the calibration and comparison of ultra-long-distance atomic clocks.

Since the advent of Fourier Domain Mode-Locked (FDML) lasers, they have demonstrated outstanding performance in several fields. They achieve high-speed, narrow-linewidth laser output with the new mode-locking mechanism, which has been intensively researched in the past decades. Compared with conventional wavelength-scanning light sources, FDML lasers have successfully increased the scanning rate of frequency-sweeping lasers from kHz to MHz. They are widely used in optical coherence tomography, spectral analysis, microscopy, and microwave photonics. With the deepening research on FDML lasers, several performance metrics have been optimized and improved, offering superior performance for FDML laser-based applications. This paper reviews the principles and key performance indicators of FDML lasers, as well as the recent progress made in some important applications, and highlights further research directions for FDML lasers in the future.

We present a single longitudinal mode erbium-doped fiber ring cavity laser with a long-term mode-hop-free operation by employing sub-cavity tracking feedback control method. Continuous mode-hop-free operation for 17.2 hours are achieved.

A single-longitudinal-mode (SLM) narrow-linewidth Brillouin erbium-doped fiber laser (BEFL) is proposed and demonstrated experimentally. The erbium-doped fiber (EDF) is employed to act as both the linear gain and Brillouin gain medium, which makes it easy to excite the stimulated Brillouin scattering (SBS) in the EDF by cooperating with the ring cavity structure. In order to realize stable SLM and narrow-linewidth laser output, the fiber saturable absorber (SA) and the self-injection feedback structure are added to BEFL for the first time, which can suppress the multimode phenomenon effectively and obtain a stable SLM status. The wavelength stability is less than 1.32 pm over 45 minutes and the linewidth is as narrow as 283 Hz.

Phase-locked loop (PLL) is a core component to achieve stable frequency dissemination over long-distance fiber links, because of its signal regeneration and noise filtering. Simply calculating the bandwidth of the PLL from the transmission delay exacerbates the asymmetry due to the dispersion in the fiber, which ultimately leads to reduced frequency instability. A theoretical analysis to describe the relationship of the PLL bandwidth and the frequency instability is proposed. Optimization strategy for the PLL bandwidth is proposed based on the fiber parameters, and an adaptive fourth-order PLL is designed to evolve the system. In experiment, we transmit a 2.4 GHz frequency signal over a 2500 km optical fiber. The frequency instability of 4.16 x 10(-14) @ 1 s and 5.54 x 10(-17) @ 10,000 s is demonstrated, which is eligible for extra long-distance atomic clock comparison and continental-scale frequency synchronization network construction.

Fiber-optic frequency transfer systems enable high-precision transmission of frequency signals.However,in long-distance frequency transmission,the signal optical power is constantly lost,which needs to be compensated by u-sing an optical amplifier.Based on the good performance of two-stage amplifier,a two-stage bidirectional erbium-doped fiber amplifier(Dual-stage Bi-EDFA)that can be used for fiber-optic frequency transfer is designed,and the scheme is ex-perimented and analysed.The results show that the measured gain of the scheme is up to 27.5dB,the noise figure is 3.56dB,and the gain flatness is less than 0.3dB.The scheme is applied to a 160km frequency transfer system with 1.09 X10-14@1s and 2.36X 10-17@10000s frequency transmission stability.

We proposed a Raman and erbium-doped fiber amplifiers (EDFA) hybrid bidirectional optical amplifier (HBOA). The hybrid amplifier consists of a fiber Raman amplifier (FRA) and two EDFA, which are ingeniously combined by four WDMs. Gain and noise figure (NF) are the metrics used to evaluate the performance. The effects of Raman and EDFA pump on the gain and NF of the proposed hybrid amplifier are explored, resulting in a maximum gain exceeding 40 dB and an NF below 0 dB. Furthermore, when the input signal power is small (-35 dBm), the proposed hybrid amplifier, exhibits a total gain increased of similar to 14 dB, and NF reduction of similar to 4 dB across the wavelength span of 1545 to 1565 nm, as compared with EDFA-only. Finally, The Allan deviation can reach 1.41x10(-14) when HBOA was used, significantly better than the stability of 2.62x10(-14 )obtained by Bi-EDFA, which confirmed the feasibility of the proposed amplifier for radio frequency synchronization systems.

A narrow-linewidth all-polarization-maintaining (PM) Brillouin erbium-doped fiber laser (BEFL) is proposed and demonstrated experimentally, which employs erbium-doped fiber (EDF) acting as both the linear gain and the Brillouin gain medium. In order to realize a single longitudinal mode (SLM) operation and a narrow linewidth fiber laser output, the saturable absorber (SA) and the optical self-injection feedback structure are employed in the BEFL for the first time. In this experiment, three sets of comparative experiments are conducted to demonstrate the effectiveness of the SA and the self-injection feedback structure. The experimental result shows that the SA structure can eliminate transient multimode phenomena and narrow the linewidth, and the self-injection feedback structure can act as an effective mode filter and a compound-cavity to make the intracavity mode purer. A stable SLM operation of the BEFL is verified by the delayed self-heterodyne system, which thanks to the adding of the SA and the self-injection feedback structure. The BEFL's wavelength stability detected by is less than 0.0045 nm over 14 hours. In addition, an ultra-narrow linewidth of approximately 224 Hz is obtained.

Bandwidth and noise are fundamental considerations in all communication and signal processing systems. The group-velocity dispersion of optical fibers creates nulls in their frequency response, limiting the bandwidth and hence the temporal response of communication and signal processing systems. Intensity noise is often the dominant optical noise source for semiconductor lasers in data communication. In this paper, we propose and demonstrate a class of electrooptic modulators that is capable of mitigating both of these problems. The modulator, fabricated in thin-film lithium niobate, simultaneously achieves phase diversity and differential operations. The former compensates for the fiber’s dispersion penalty, while the latter overcomes intensity noise and other common mode fluctuations. Applications of the so-called four-phase electrooptic modulator in time-stretch data acquisition and in optical communication are demonstrated.

The experimental data and primary simulation results.

We report an optical single shot Fourier transform spectroscopy with tunable resolution capability by using a programable optical loop. The proposed method will broaden the applications of single shot spectroscopy.

High-resolution jitter measurement is essential for the next generation of electronic communications and sensor systems. However, most electrical timing jitter measurement equipment has a low resolution because of the limitations of electronic processing accuracy. Meanwhile, photonics-based jitter measurement methods have a higher resolution but cannot measure the widely used electrical signals. This work proposes a potential high-resolution jitter measurement method for electrical signals based on the photonics time stretch technique. The jitter information can be magnified in the optical domain and then measured by electrical equipment. The experimental results demonstrate that the jitter of an electrical pulse is magnified from 59.02 ps to 663.29 ps when the magnification factor is 11.24.

The laser frequency locking system with high robustness, high accuracy, and good anti-noise performance is usually used in laser synchronization and optics communications, continuous-variable quantum key distribution, and ultra-stable laser wavelength stabilization. In such systems, the analog or digital method is usually used to implement feedback control. Digital systems are more widely used than analog systems based on phase-locked loops due to their simplicity, flexibility, and robustness. The proportional-integral-derivative (PID) algorithm is the most typical algorithm well-developed in the digital locking system. However, due to the nonlinearity of the error signal induced by frequency variation, the PID algorithm will have a deviation in compensation for the transient response. This article demonstrates a neural network assisted laser frequency locking system using predicted values to decrease the nonlinearity. Compared to the PID algorithm locking performance in the same locking system, both the peak-to-peak deviation and the root-mean-square error of the neural network assisted locking system are reduced by half.

A passive dual-comb laser can generate two optical frequency combs with different repetition frequencies. These repetition differences have high relative stability and mutual coherence through passive common-mode noise suppression without complex tight phase locking from a single-laser cavity. The comb-based frequency distribution requires the dual-comb laser to have a high repetition frequency difference. This paper presents a high repetition frequency difference bidirectional dual-comb fiber laser based on an all-polarization-maintaining cavity configuration and a semiconductor saturable absorption mirror with single polarization output. The proposed comb laser has a standard deviation of 69 Hz and an Allan deviation of 1.17 × 10-7 at τ = 1 s under different repetition frequencies of 12.815 MHz. Moreover, a transmission experiment has been conducted. Owing to the passive common-mode noise rejection capability of dual-comb laser, after passing an 84 km fiber link, the frequency stability of the repetition frequency difference signal is improved by two orders of magnitude than the repetition frequency signal at the receiver side.