Optical Phase Lock Research Articles

Characterization of optical phase-locked two distributed-feedback fiber lasers for 87Rb atom interferometry

We report the optical phase-locking of two 1560 nm distributed-feedback (DFB) fiber lasers converted to 780 nm with a frequency difference of 6.834 GHz for 87Rb. The fiber laser exhibits a narrow linewidth and low-intensity noise, which are advantageous for a Raman pulse laser in an atomic gravimeter. However, the piezoelectric actuator of the DFB laser causes a low feedback rate in an optical phase-locking loop (OPLL). To overcome this limitation, we used an acousto-optic modulator as an additional actuator to increase the modulation bandwidth of the OPLL and achieved a residual phase noise of approximately -127 dBrad2/Hz in the offset frequency range from 10 Hz to 1 kHz. The sensitivity limit estimated using the sensitivity function and experimental parameters of 87Rb atomic gravimeter shows that our OPLL system can be used in state-of-the-art gravimeters.

Remote vectorial vibration sensing based on locally stabilized Mach-Zehnder interferometers using multi-core fiber and optical phase-locking.

We report on remote sensing of vectorial vibration based on locally stabilized Mach-Zehnder interferometers (MZIs) using commercial multi-core fiber (MCF). Hexa-MZIs with a shared common reference arm are constructed by a 7-core MCF to acquire remotely vectorial vibration. A set of corresponding local receivers consisting of optical phase-locked loops (OPLLs) for not only eliminating the impact of environmental perturbations but also maintaining the stable operation and relative stability among the MZIs, allows guaranteed stabilized remote sensing. It moreover ensures a linearized phase detection, and thus an improved sensing sensitivity and dynamic range. This way, by exploiting the symmetrically geometric distribution for the cores of 7-core MCF, the proposed all-fiber design can enable highly precise remote extraction of vibration in a vectorial manner with a simplified remote structure. We achieve vectorial remote sensing for vibrations with ∼0.1076° and ∼0.3603 µm precision for the angle and displacement, respectively, over 10 km.

Generation of Broadband Reconfigurable LFM Waveforms Via Heterodyne-Beating Synchronized Lasers

We propose and demonstrate a novel linearly frequency modulated (LFM) signal generation method using synchronized lasers in a heterodyne-beating scheme, featuring simple operation, large bandwidth and flexible reconfiguration. A hybrid synchronization mechanism combining the advantages of optical phase-locking and digital signal processing mechanisms maintains the phase coherence between lasers when the wavelength is sweeping. The ability of phase coherence maintaining provides attractive benefits in flexible reconfigurability of beat signal parameters and high scalability of signal bands. A dual-laser synchronization system is established, and an LFM waveform with reconfigurable bandwidth, central frequency and pulse duration is produced. Additionally, an ISAR imaging experiment is carried out. Results show the potential of the proposed method in multifunctional/multiband radar applications.

Dual Laser Indium Phosphide Photonic Integrated Circuit for Integrated Path Differential Absorption Lidar

An indium phosphide photonic integrated circuit (PIC) was demonstrated for integrated path differential absorption lidar of atmospheric carbon dioxide (CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The PIC consists of two widely tunable sampled grating distributed Bragg reflector (SGDBR) lasers, directional couplers, a phase modulator, a photodiode, and semiconductor optical amplifiers (SOAs). One SGDBR laser, the leader, is locked to the center of an absorption line at 1572.335 nm using the on-chip phase modulator and a bench-top CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Herriott reference cell. The other SGDBR laser, the follower, is stepped in frequency over ±15 GHz around 1572.335 nm to scan the target CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption line. The follower laser is offset locked to the leader laser with an optical phase lock loop. An SOA after the follower laser generates a pulse at each frequency step to create a train of pulses that samples the target CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption line. The PIC components and subsystem are characterized and evaluated based on target performance requirements. The leader laser demonstrated a 236-fold improvement in frequency stability standard deviation when locked compared to free running and the follower laser frequency stability standard deviation compared to the leader laser was 37.6 KHz at a 2 GHz programmed offset.

Composite optical phase locking technology of chirp synthetic aperture lidar

An accurate chirp synthetic aperture lidar is proven, and the chirp laser signal is generated by an optical filter selecting specific sidebands from infinite sidebands, which are generated by laser signals passing through an electro-optic phase modulator driven by a chirp signal. And the bandwidth and chirp rate of the N-th order sideband is N times as large as the chirp signal by the signal generator. Then, a chirp signal with a modulation bandwidth of 1.2 GHz and a tuning rate of 12 THz/s is obtained by selecting the positive second-order sideband by an optical filter. Next, by adopting composite optical phase-locking processing, the random phase caused by the lidar system will be well offset, and the phase is stable within 0.1rad. Finally, Laboratory imaging results show that the azimuth resolution of synthetic aperture lidar is better than 0.04 m. In the future, by applying the higher-order sideband, higher tuning rates and bandwidths can be expected.

Frequency Synchronization and Coherent Combining of Two Fiber Ring Cavities

Low noise synchronization of the frequency and phase of a DFB fiber laser and two fiber ring cavities was achieved based on a two-channel optical injection lock and phase lock loop. Two-channel modulation signals of different frequencies were adopted to extract the phase noise signals of the two fiber ring cavities. Finally, the lasers from the two fiber ring cavities were coherently combined in the closed loop of the control system owing to the synchronous frequency and phase. The power stability and polarization stability of the combined laser was improved under the optical injection lock and phase lock loop. The phase noise of the two cavities was $6.56 \times 10^{-7}$ rad/Hz1/2 and $1.34 \times 10^{-6}$ rad/Hz1/2 at 10 Hz; and $7.77 \times 10^{-8}$ rad/Hz1/2 and $1.35 \times 10^{-7}$ rad/Hz1/2 at 10 kHz. The total combined power was ~580 mW in the closed loop, and the entire system oscillated under a single frequency, providing excellent power scaling ability.

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.

Dual-frequency Doppler velocimeter based on delay interferometric optical phase-locking

We present a dual-frequency laser Doppler velocimeter (DF-LDV) relying on a DF laser source (DFLS) generated by optical phase-locking two individual lasers to a common unbalanced Mach-Zehnder interferometer, which allows achieving high stability regardless of the DF separation of the lasers. This DFLS is evaluated using an optical frequency comb, testifying to the generation of DFLS with large DF separation up to terahertz with flexible tunability and high stability. Demonstration of DF-LDV using the DFLS of ${\sim}1.024\; {\rm THz}$ separation has achieved $1.62 \times {10^{- 2}}$ mm/s velocity resolution even for a slow velocity of $1.8\; {\rm mm}/{\rm s}$ in a mere 5 s acquisition time, confirming the high resolution and efficient speckle noise suppression enabled by the proposed DF-LDV. Featuring high precision, flexibility, and robustness, this method is particularly attractive from the practical point of view.

Ultra-precise optical phase-locking approach for ultralow noise frequency comb generation

Optical frequency comb (OFC) with low phase noise enables more stringent metrology in classical and quantum systems. To eliminate phase noise, it is essential to extend the feedback bandwidth of the carrier-envelope phase and the repetition rate. Here, we present an approach to the construction of an ultralow-noise OFC. By exploiting different electro-optic modulators as fast actuators, this method can extend the feedback bandwidth to more than 150 kHz at the repetition rate phase locking and carrier-envelope offset phase locking We obtain a residual phase noise of 21.8 mrad (18.1 as) and 86.1 mrad (71.3 as) for the stabilised optical beat signal (fbeat) and carrier-envelope offset frequency (fceo), respectively. We validate this architecture by a measurement the relative linewidth between two combs, which reveals an in-loop beat fractional instability of less than 1.18 × 10−18 and an out-of-loop beat fractional instability of less than 2.31 × 10−15 at a 1-s averaging time. The out-of-loop beat phase noise is 145 mrad (120 as). These results indicate that the combination of Er-fiber laser technology and active feedback with a high bandwidth ensures coherent tracking of an optical reference with ultralow noise. The ultralow-noise OFC offers a universal tool for accurate, high-resolution optical spectroscopy.

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

Fourier and Inverse Fourier Transform Model for Delayed Self-interferometry System

Understanding the effects of laser phase and frequency noise on laser interferometry is significant for evaluating the system performance. To precisely study the performance limit caused by laser frequency noise, here we propose and demonstrate a versatile model based on the Fourier and inverse Fourier transform (FIFT) method. This model, capable of estimating the beat note spectra of different delayed self-interferometry (DSI) with laser sources of arbitrary frequency noise properties, allows for accurate evaluations of the noise performance in a variety of interferometry based systems. Such a model has been experimentally validated using lasers with irregular frequency noise properties such as cavity stabilized fiber laser or laser under optical phase-locking, providing more detailed insight into the evolution of the frequency noise dynamics at different interferometric conditions. With average estimation goodness (AEG) of 0.9716 and computation complexity of O(NlogN), this model offers greater accuracy and lower complexity than conventional methods. It has also been confirmed that this model permits to distinguish the contributions from the laser frequency stability and other noise sources, which could be helpful for the noise analysis and performance optimization of the system.

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.

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.

基于光外差锁相的激光偏振控制与高偏振消光比的光束合成

基于光外差锁相的激光偏振控制与高偏振消光比的光束合成

A modified 160 Gbit/s OTDM system based on QPSK Nyquist pulses and coherent detection

In this paper, we design and implement a 160 Gbit/s orthogonal time division multiplexing (OTDM) transmission system based on quadrature phase shift keying (QPSK) Nyquist pulses and coherent detection. We proposed an optimized Nyquist pulses generation scheme, which produce ultra-flat frequency combs with near-perfect rectangular spectrum and high suppression of out-of-band components by two cascaded Lithium Niobate Mach Zehnder modulators (LN-MZMs). Divided by variable length of optical delay line (Odl), 10GBaud QPSK Nyquist pulses are multiplexed to 80GBaud in a single wavelength. A set of training sequence (TS) which are inserted in front of data are employed to realize the inclusion of optical phase-locking and estimate channel transmission function. In this scheme, we make use of the TS overhead to achieve homodyne detection synchronization between local advanced receiver and Nyquist-pulses generator which is controlled by loop digital signal processing (DSP) module via an optical phase-locking loop. Meanwhile, for the advantage of the TS scheme and DSP, the Nyquist system compensates the residual physical dispersion and improves the transmission quality after 150 km transmission.

Robust offset locking of laser frequency with electronically tunable LC circuits for sub-millihertz uncertainty

A simple scheme for laser frequency offset locking introduced in Cheng et al. (Opt Express 25:2752, 2017) has been improved by the use of a variable-capacitance diode. By employing this scheme with the optical phase lock loop, robust and precise laser frequency stabilization is achieved, and long-term laser frequency drift is expected to be substantially suppressed even in noisy circumstances. The optical frequency uncertainty becomes less than 10 {upmu }Hz for 1000 s averaging.

Optical phase-locking of two extended-cavity diode lasers by serrodyne modulation.

We report the optical phase-locking of two extended-cavity diode lasers with a frequency difference of 6.9 GHz by serrodyne modulation. The bandwidth of the phase-locking loop is extended up to 9.5 MHz. The residual phase noise of the two phase-locked lasers reaches -130 dBrad2/Hz in the offset frequency range of 1.5 kHz to 9 kHz and below -120 dBrad2/Hz in the range of 150 Hz to 350 kHz, respectively. It is expected that the sensitivity limit of atom interferometers will be enhanced when the phase-locked lasers are used.

Optical Phase Lock Loop as High-Quality Tuneable Filter for Optical Frequency Comb Line Selection

This paper describes an optical phase lock loop (OPLL) implemented as an ultraselective optical frequency comb line filter. The OPLL is based on a photonic integrated circuit (PIC) fabricated for the first time through a generic foundry approach. The PIC contains a distributed Bragg reflector (DBR) laser whose frequency and phase are stabilized by reference to an optical frequency comb generator. The OPLL output is a single-mode DBR laser line; other comb lines and noise at the output of the OPLL filter are attenuated by >58 dB below the peak power of the OPLL-filter output line. The OPLL bandwidth is up to 200 MHz, giving a filter quality factor greater than 1,000,000. The DBR laser can be tuned over 1 THz (8 nm), enabling different comb lines to be selected. Locking to a comb line with a frequency offset precisely selectable between 4 and 12 GHz is also possible. The coherence between the DBR laser and the comb lines is demonstrated by measurements of the heterodyne signal residual phase noise level, which is below −100 dBc/Hz at 5 kHz offset from the carrier. The OPLL-filter output can be up to 6 dB higher than the peak power of the comb line to be isolated by the filter. This optical gain is a unique characteristic which can significantly improve the SNR of communication or spectroscopy systems. This OPLL is envisaged to be used for high purity, tuneable microwave, millimetre-wave, and THz generation.

Optical Frequency Tuning for Coherent THz Wireless Signals

THz wireless signals have become of interest for future broadband wireless communication. In a scenario where the wireless signals are distributed to many small remote antenna units, this will require systems which allow flexible frequency tuning of the generated THz carrier. In this paper, we demonstrate experimentally the implementation of two tuning methods using an optical frequency comb generator for coherent optical frequency tuning in THz wireless-over-fiber systems. The first method is based on using a photonic integrated circuit optical phase lock loop (OPLL) subsystem implemented as a high quality optical filter for single comb line selection and optical amplification. The OPLL generates an optical carrier, which is frequency and phase stabilized in reference to one of the optical comb lines with a frequency offset precisely selectable between 4 and 12 GHz. The second method is based on optical single sideband suppressed carrier (SSB-SC) modulation from the filtered comb line using an optical IQ modulator. With this technique, it is possible to suppress the other unwanted optical tones by more than 40 dB. This generated optical carrier is then heterodyned with another filtered optical comb line to generate a tuneable and stable THz carrier. The full system implementations for both methods are demonstrated by transmitting THz wireless signal over fiber with 20 Gb/s data in QPSK modulation. The system performance and the quality of the generated THz carrier are evaluated for both methods at different tuned THz carrier frequencies. The demonstrated methods confirm that a high quality tuneable THz carrier can easily be implemented in systems where dynamic frequency allocation is required.

Influence of Time Delay on Pull-in Behaviour of Dither Optical Phase-Lock Loop

ABSTRACTThis paper analyses the impact of loop propagation delay on the pull-in behaviour of dither optical phase-lock loop (OPLL), neglecting the shot noise and phase noise of the loop. Analytical expressions of pull-in voltage, pull-in range, and acquisition time are presented in terms of all the parameters affecting the loop acquisition. Also, the influence of dither signal on the pull-in performance of dither OPLL is studied in the presence of loop propagation delay. It is found that the time delay associated with the filtering process within the loop gives rise to degradation in acquisition capability. In addition, dither signal allows pull-in from much larger frequency offset, decreases the acquisition time of the loop, and improves the acquisition characteristics of dither OPLL in terms of the pull-in voltage in the presence of loop delay. The tracking performance of dither OPLL regarding phase-error variance is investigated considering laser phase noise, photo-detector shot noise, and loop propagation delay. Simulation results are given in support of the analytical results.