Optical Phase-locked Loop Research Articles

Remote Michelson interferometric phase sensor based on dual-core fiber transmission and linear phase demodulation.

We present a remote Michelson interferometric phase sensor based on dual-core fiber transmission and linear phase demodulation. The former allows for synchronous transmission of both sensing signal and reference lights, enabling efficient suppression for the environmental disturbances along the transmission link and for the incoherent phase noise between the two lights. The latter is conducted by two optical phase-locked loops, one of which consists of a fiber stretcher that is used to eliminate the residual phase noises, thus stabilizing the operation point while the other relies on a phase modulator that is used to track the remote phase changes, thus achieving a highly linearized phase demodulation. A remote phase sensing over a 20 km fiber link with less than 3% nonlinear phase error over 3π range has been readily realized, corresponding to more than 10 times extension in a linear phase demodulation range. The proposed system shows great potential in the field of remote phase sensing for a variety of physical quantities.

Adaptive Doppler compensation method for coherent LIDAR based on optical phase-locked loop

Coherent LIDAR is a potential and effective technology to detect and identify space debris. However, for the high-speed space debris, the large Doppler frequency shift (DFS) generated by high-speed relative motion challenges the bandwidth of the photoelectric detection system and increases the difficulty of datasampling and processing. In this paper, we propose an effective method to compensate this large DFS adaptively by using an optical phase-locked loop (OPLL), and thus the relative velocity between the target and LIDAR platform can be obtained. A laboratory experiment for moving targets with our OPLL method has been conducted, and it revealed that our method worked well to compensate for 64 MHz frequency difference, thus the relative velocity is about 26 m/s based on the compensated DFS. And the beat frequency is always locked to the reference frequency, while the DFS generated by variable speed motion is compensated adaptively. The proposed compensation method could improve the performance of coherent LIDAR significantly, especially when detecting high-speed target.

Hybrid Solution for Joint Transfer Time Signal With Both RF Frequency and Optical Frequency

We demonstrate a hybrid solution for simultaneous transfer of ultra-stable optical frequency, RF frequency and time signal on a 100 km fiber spool with the same wavelength based on coherent demodulation and optical phase-lock-loop (OPLL). The one pulse per second (1PPS) is encoded by phase and RF frequency is encoded with an intensity modulator. The noise caused by fiber link is compensated by the round-trip method. And the bad effect on optical frequency caused by optical phase modulation (PM) can be cleaned using OPLL. When the fiber link is stabilized, the transferred time deviation (TDEV) can be 2.4E-11@1s and 1.9E-12@104s, RF frequency stability can achieve 8.0E-14@1s and 2.5E-17@104s, the modified Allan deviation (MDEV) of cleaned optical frequency can be 1.4E-16@1s and 1.6E-19@104s.

Dynamic Range Enhanced Optical Frequency Domain Reflectometry Using Dual-Loop Composite Optical Phase-Locking

We report on a dynamic range enhanced optical frequency domain reflectometry distributed backscattering interrogator based on dual-loop composite optical phase-locked loop (OPLL). Exploiting simultaneously an acousto-optic frequency shifter based an external modulation loop and a piezo based direct modulation loop, the proposed composite OPLL allows offering a larger loop bandwidth and gain, permitting a more efficient coherence enhancement as well as sweep linearization. A high fidelity frequency sweep of ~8.2 GHz at 164 GHz/s sweep rate is generated with a peak-to-peak frequency error as low as ~ 120 kHz. It leads to a dynamic range enhancement of more than 3 dB for the measured power loss compared to the case when only piezo loop is applied. This corresponds to ~15 km extension for the measurement range of Rayleigh backscattering without any spatial resolution penalties. Fourier transform-limited spatial resolution has been demonstrated at a range window more than about 28 times of the intrinsic coherence length of the adopted fiber laser. The proposed method provides a straightforward optimization of the real-time sweep control and is expected to be a useful tool in industrial and commercial applications.

Study and simulation of heterodyne optical phase-locked loop for inter-satellite laser interferometer

Coherent transmission of optical signal is an important module of inter-satellite laser interferometer. Through heterodyne optical phase-locking technology, the coherent transmission of laser signals from the master spacecraft can be achieved. And in the case of ensuring the stability of the local laser frequency, amplifying of the optical output power is realized. This article introduces and analyzes the principle, modeling and simulation of heterodyne optical phase-locking technology, which provides a reference for the realization of laser signal’s coherent transmission from slave satellite.

Z-domain model procedure for heterodyne digital optical phase-locked loop

Although there are already some simulation tools modeling and s-domain modeling studied for heterodyne optical phase-locked loop (HOPLL), few studies about z-domain model of OPLL are reported. When the loop filter in HOPLL is implemented by digital circuit, the operation of HOPLL is controlled by digital signals which is called heterodyne digital optical phase-locked loop (HDOPLL) in this paper. It is easily deduced that the performance of HDOPLL can be analyzed more accurately by z-domain model. With a deep analysis, the problem about the nonlinear operation of optical related components can be circumvented, if the optical related components are grouped with phase frequency detector (PFD) during z-domain modeling. So the detail about the z-domain model procedure for HDOPLL is presented in the paper. Then for a case study, stability of HDOPLL is analyzed by an established z-domain model, and the correctness of z-domain model is verified by Simulink behavioral model of HDOPLL.

Noise Analysis for Coherent Phase-Modulated RF Fiber-Optic Link

The advantages of optical fiber as a transmission medium for microwave signals have led to an increasing interest in the implementation of RF/photonic links in radar frontends. Such applications often require a large dynamic range that is beyond the reach of conventional intensity modulation-direct detection (IM-DD) fiber-optic links. For a solution, a coherent fiber-optic link employing an optical phase-locked loop (OPLL) demodulation has been proposed promising enhanced spurious-free dynamic range (SFDR) performance. Under the ideal condition, the link should be shot-noise limited. However, the phase modulated (PM) link was shown to be penalized by the presence of other noise sources and environmental perturbations. In this work, we present a detailed analysis of the phase modulated link's different noise sources, their impact to dynamic range performance, and ways of mitigation.

The noise suppression in resonant micro optic gyroscopes based on dual light sources method

In Resonant Micro-Optical Gyroscopes (R-MOG), optical backscatter noise is generally considered to be one of the largest noises which also greatly limits the improvement of accuracy. A suppression method based on dual light sources is designed in this paper, in which two tunable semiconductor lasers are used to provide light for clockwise (CW) and counterclockwise (CCW), respectively. Two independent lasers have been frequency divided and phase locked by Optical Phase-Locked Loop (OPLL). In this way, high-frequency noise caused by interference between the backscatter light and the signal light can be filtered out, completely. The frequency noise for dual-lasers has also been effectively suppressed by adjusting the OPLL bandwidth. Then, intensity modulators with second harmonic feedback have been used to achieve the stabilization of the optical power in the cavity. The results show that, the influence of the interfering light intensity is reduced by about an order of magnitude compared with the single-phase modulation technology in single laser system and the bias stability has stood at 0.00448∘/s.

Optical linear frequency sweep based on a mode-spacing swept comb and multi-loop phase-locking for FMCW interferometry.

We report on the generation of a highly coherent broadband optical linear frequency sweep (LFS) using mode-spacing swept comb and multi-loop composite optical phase-locked loop (OPLL). We exploit a specially designed agile opto-electronic frequency comb as a sweeping reference, whose mode-spacing is capable of arbitrary frequency sweep while preserving a stable phase and power distribution per mode. By locking a continuous-wave (CW) laser to any of its modes using composite OPLL with a large loop bandwidth, it allows the extraction of the optical LFS at high-order modes in a coherent manner with a multiplied sweep range and rate. With such capability, only intermediate frequency LFS with smaller bandwidth is required to yield a broadband LFS while inheriting the coherence and precision from the comb. We achieve optical LFS of 60 GHz at 6 THz/s sweep rate with a nine-folded sweep bandwidth of the driving signal. Fourier transform-limited spatial resolution at more than 80 times of the intrinsic coherence length of the CW laser is demonstrated in an OFMCW interferometry, verifying the high coherence with more than 4 orders of magnitude improvement in spatial resolution. The characteristics in terms of agility, coherence, and precision are discussed together with the potential limitations. The proposed method is capable of generating arbitrary frequency-modulated optical waveforms with a multiplied bandwidth, showing attractive potential in future metrology applications.

Dynamic Free-Spectral-Range Measurement for Fiber Resonator Based on Digital-Heterodyne Optical Phase-Locked Loop

We propose a novel scheme, based on digital-heterodyne optical phase-locked loop with whole-fiber circuit, to dynamically measure the free-spectral-range of a fiber resonator. The optical phase-locked loop is established with a differential frequency-modulation module consists of a pair of acousto-optic modulators. The resonance-tracking loop is derived with the Pound-Drever-Hall technique for locking the heterodyne frequency of the OPLL on the frequency difference between adjacent resonance modes. A stable locking accuracy of about 7 × 10?9 and a dynamic locking accuracy of about 5 × 10?8 are achieved with the FSR of 8.155 MHz, indicating a bias stability of the resonator fiber optic gyro of about 0.1?/h with 10 Hz bandwidth. In addition, the thermal drift coefficient of the FSR is measured as 0.1 Hz/?C. This shows remarkable potential for realizing advanced optical measurement systems, such as the resonant fiber optic gyro, and so on.

Phase locking technology for Raman laser system based on 1560 nm external cavity lasers

The technology of generating Raman laser is not only an important research content in the field of quantum precision measurement, but also a core technology of quantum inertial sensors such as cold atom gravimeter, gyroscope. For <sup>87</sup>Rb atoms, two 780-nm lasers with a frequency difference of 6.834 GHz and a stable phase are needed to generate Raman light. Raman lasers can be generated by optical phase-locked loops of two 780-nm narrow linewidth external cavity tunable semiconductor lasers (ECDL). But the system thus developed is complicated in structure and very poor in environmental adaptability. The other method to generate Raman laser is based on intracavity 1560-nm laser with frequency doubling and electro-optic modulation technology. This system is simple in structure and strong in environmental adaptability, but it will introduce sideband effects and cannot achieve phase lock due to the limit by the linewidth and feedback bandwidth performance of the laser. In view of this, based on two new 1560-nm external cavity lasers and a home-made phase-locked circuit, in this paper the phase lock of the laser is achieved, and a Raman laser with low phase noise is obtained. The phase noise of beat note signal is as low as –95 dBc/Hz at the Fourier frequency in a range from 1 kHz to 10 kHz. A comparison of this system with the phase-locked performance of the 780-nm dual laser and the hybrid dual laser shows that this scheme has a slight advantage. In addition, the effect of the phase-locking performance on the phase noise of the cold atom interferometer through the method of piecewise integration is analyzed in this work. The experimental results given in this work provide a scheme for developing a miniaturized Raman optical system suitable for external fields.

Amplitude randomness of spontaneous emission influence on the coherence properties of a laser.

Based on the randomness of spontaneous emission, the statistical characteristics of phase noise are discussed. A theoretical analysis model, focusing on the amplitude randomness of spontaneous emission, is established to calculate laser phase noise. Then, the coherence of a laser before and after phase-locked control is calculated when an ideal laser and nonideal laser are used as a reference light in an optical phase-locked loop (OPLL). The effects of the amplitude randomness of spontaneous emission on phase-locked laser coherence are analyzed in detail. The results show that phase noise randomness increases with the increase of the expectation or variance of spontaneous emission amplitude, which represents amplitude randomness. When an ideal light is used as reference light, if the expectation and the variance of spontaneous emission amplitude are about 10 and 100, respectively, the time constant of an OPLL should be set as 1 ns, while a favorable noise suppression result can be achieved. However, to achieve a favorable noise suppression result, the time constant of an OPLL should be set as 0.1 ns when a nonideal laser is used as reference light.

Analog Coherent Detection for Energy Efficient Intra-Data Center Links at 200 Gbps Per Wavelength

As datacenters continue to scale in size, energy efficiency for short reach (<; 2 km) links is a major factor for networks that may connect hundreds of thousands of servers. We demonstrate that links based on analog coherent detection (ACD) offer a promising path to simultaneously achieving significantly larger link budgets and improved link energy efficiency. A complete analysis is presented that considers the power consumption of all the photonic and electronic components necessary to realize an ACD link architecture based on 50 Gbaud (GBd) quadrature phase-shift keying (QPSK) signaling combined with polarization multiplexing to achieve 200 Gb/s/λ. These links utilize receivers that incorporate an optical phase-locked loop (OPLL) to frequency- and phase-lock the local oscillator (LO) laser to the incoming signal. QPSK modulation offers compelling advantages both in achievable link budget and in energy efficiency. Indeed, low-complexity electronics based on limiting amplifiers can be used as opposed to the linear front-ends, A/D converters, and digital signal processing (DSP) required for higher-order QAM or PAM formats. Our analysis indicates that links with 13 dB of unallocated budget operating at error rates of <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> can be achieved and is compatible with higher error rates that require forward error correction (FEC). We present a comparison of silicon and InP platforms and evaluate both traveling-wave and segmented modulator designs, providing an illustration of the wide design space before converging on the most promising architectures that maximize energy efficiency and minimize laser power. We establish the theoretical potential to achieve picojoule-per-bit energy efficiency targets.

Design of Real-Time Free Spectral Range Measurement Based on HOPLL Technique

A real-time free spectral range (FSR) measurement system based on heterodyne optical phase-locked loop (HOPLL) is proposed for the first time to test the FSR of a fiber resonator. In this system, HOPLL is used to lock phase between the master and slave lasers, so that the frequency difference between them reach one FSR. In addition, PDH technique is used to establish a resonant tracking loop. By setting the loop control parameters, the master and slave lasers are respectively locked on two adjacent resonance peaks of the cavity. If the ambient temperature changes, the cavity length of the resonant cavity will change accordingly. Then the real-time change of FSR of the resonant cavity can be measured by detecting feedback change of the loop. The experimental results show that the linewidth of beat signal of the master and slave lasers after phase-locking is less than 10Hz, and also the frequency noise of the 10Hz bandwidth is 0.0154Hz which is 60dB lower than that before phase-locking. The researchers find that the FSR coefficient of thermal expansion is −210.35 ± 0.15Hz/°C, which is consistent with theoretical results. The relative measurement accuracy of $1.5\times 10 ^{-9}$ is achieved in this loop. Therefore, the experimental results effectively verify the actual application performance of the real-time FSR measurement based on HOPLL technique.

High-Performance Optical Frequency-Domain Reflectometry Based on High-Order Optical Phase-Locking-Assisted Chirp Optimization

We present optical frequency-domain reflectometry with high performance in terms of an ultralong range window, high spatial resolution, and precision enabled by a high-order optical phase-locked loop (OPLL)-assisted commercial fiber laser. Due to elaborate loop designs that allow matching with the nonlinear frequency modulation response of the laser, the potential tuning capability can be fully exploited, leading to efficient optimization of the frequency chirp range and rate in addition to chirp linearization and an enhancement of the dynamic coherence. A nearly Fourier-transform-limited spatial resolution is realized over multiples of the intrinsic coherence length. Practically, a cm-level spatial resolution is demonstrated along the entire fiber link. We achieved a spatial resolution of ∼3.2 cm and ∼4.3 cm for backscatter no more than 130 km and reflections at 242 km, respectively, corresponding to a high range resolution factor of up to 1.78 × 10−7 in the latter case. The precision of the backscatter measurement is verified to be at least 0.5 dB throughout the entire measurement range. Moreover, the limiting factors resulting in a degeneration of the spatial resolution are carefully investigated. The proposed approach can be readily applied to other lasers, offering a powerful tool for a variety of applications.

Towards a Scaleable 5G Fronthaul: Analog Radio-over-Fiber and Space Division Multiplexing

The introduction of millimeter wave (mm-wave) frequency bands for cellular communications with significantly larger bandwidths compared to their sub-6 GHz counterparts, the resulting densification of network deployments and the introduction of antenna arrays with beamforming result in major increases in fronthaul capacity required for 5G networks. As a result, a radical re-design of the radio access network is required since traditional fronthaul technologies are not scaleable. In this article the use of analog radio-over-fiber (ARoF) is proposed and demonstrated as a viable alternative which, combined with space division multiplexing in the optical distribution network as well as photonic integration of the required transceivers, shows a path to a scaleable fronthaul solution for 5G. The trade-off between digitized and analog fronthaul is discussed and the ARoF architecture proposed by blueSPACE is introduced. Two options for the generation of ARoF two-tone signals for mm-wave generation via optical heterodyning are discussed in detail, including designs for the implementation in photonic integrated circuits as well as measurements of their phase noise performance. The proposed photonic integrated circuit designs include the use of both InP and SiN platforms for ARoF signal generation and optical beamforming respectively, proposing a joint design that allows for true multi-beam transmission from a single antenna array. Phase noise measurements based on laboratory implementations of ARoF generation based on a Mach–Zehnder modulator with suppressed carrier and with an optical phase-locked loop are presented and the suitability of these transmitters is evaluated though phase noise simulations. Finally, the viability of the proposed ARoF fronthaul architecture for the transport of high-bandwidth mm-wave 5G signals is proven with the successful implementation of a real-time transmission link based on an ARoF baseband unit with full real-time processing of extended 5G new radio signals with 800 MHz bandwidth, achieving transmission over 10 km of 7-core single-mode multi-core fiber and 9 m mm-wave wireless at 25.5 GHz with bit error rates below the limit for a 7% overhead hard decision forward error correction.

Realization of Hollow-Core Photonic-Crystal Fiber Optic Gyro Based on Low-Noise Multi-Frequency Lasers with Intermediate-Frequency Difference.

Mainly focusing on the demand for a novel resonator optic gyro based on a hollow-core photonic-crystal fiber (HC-RFOG), we achieve a multi-frequency lasers generation with low relative phase noise via an acousto-optic modulation of light from a single laser diode. We design a homologous heterodyne digital optical phase-locked loop (HHD-OPLL), based on which we realize the low-noise multi-frequency lasers (LNMFLs) with an intermediate frequency difference. The noise between the lasers with a 20 MHz difference is 0.036 Hz, within the bandwidth of 10 Hz, in a tuning range of 120 kHz, approximately 40 dB lower than that produced without the HHD-OPLL. Finally, based on the LNMFLs, an HC-RFOG is realized and a bias stability of 5.8 °/h is successfully demonstrated.

Raman-Laser System for Absolute Gravimeter Based On 87Rb Atom Interferometer

The paper describes a Raman-laser system with high performance for an absolute gravimeter that was based on 87Rb atom interferometer. As our gravimeter is a part of the standard acceleration of gravity of China, the Raman lasers’ characteristics should be considered. This laser system includes two diode lasers. The master laser is frequency locked through the frequency-modulation (FM) spectroscopy technology. Its maximum frequency drift is better than 50 kHz in 11 h, which is measured by home-made optical frequency comb. The slave laser is phase locked to the master laser with a frequency difference of 6.8346 GHz while using an optical phase lock loop (OPLL). The phase noise is lower than −105 dBc/Hz at the Fourier frequency from 200 Hz to 42 kHz. It is limited by the measurement sensitivity of the signal source analyzer in low Fourier frequency. Furthermore, the power fluctuation of Raman lasers’ pulses is also suppressed by a fast power servo system. While using this servo system, Raman lasers’ pulses could be fast re-locked while its fast turning on again in the pulse sequence. The peak value fluctuation of the laser power pulses is decreased from 25% to 0.7%, which is improved over 35 times. This Raman-laser system can stably operate over 500 h, which is suited for long-term highly precise and accurate gravity measurements.

Remote broadband RF signal down-conversion with stable phase and high efficiency using a sideband optical phase-locked loop.

We propose a phase-stable high-efficiency down-conversion approach for broadband radio frequency signals transmitted from a remote site. A high power coherent optical local oscillator signal is used at the local site to increase the conversion gain and the spurious-free dynamic range (SFDR) at the same time. A sideband optical phase-locked loop ensures the suppression of the phase noise induced by the fiber transmission and the relative frequency drift of the remote and local lasers, which are essential for the signal transmission and the down-conversion. We first experimentally demonstrate the down-conversion of a single frequency signal at 16.45 GHz to a 250 MHz intermediate frequency (IF) signal with 3 dB gain and 103 dB/Hz2/3 SFDR after 10 km fiber transmission. Then we show the broadband down-conversion capability by down-converting a 1 GHz wide linear frequency modulated pulse signal centered at 11 GHz to 1 GHz with 3 dB gain. Along with a positive gain, the SFDR of the IF signal down-converted from 5 GHz to 40 GHz has reached 97.6 dB/Hz2/3 on average. This approach is suitable for weak broadband remote signal down-conversion with a simple-structured remote end.

Simple and robust method for rapid cooling of 87Rb to quantum degeneracy**Project supported by the National Natural Science Foundation of China (Grant No. 51275523) and the State Key Laboratory of Aerodynamics Research Fund, China (Grant No. SKLA2019040302).

We demonstrate a simple and fast way to produce 87Rb Bose–Einstein condensates. A digital optical phase lock loop (OPLL) board is introduced to lock and adjust the frequency of the trap laser, which simplifies the optical design and improves the experimental efficiency. We collect atoms in a magneto-optical trap, then compress the cloud and cut off hot atoms by rf knife in a magnetic quadrupole trap. The atom clouds are then transferred into a spatially mode-matched optical dipole trap by lowering the quadrupole field gradient. Our system reliably produces a condensate with 2×106 atoms every 7.5 s. The compact optical design and rapid preparation speed of our system will open the gate for mobile quantum sensing.