Heterodyne Optical Phase-locked Loop Research Articles

Simulation and Design of a PIC-Based Heterodyne Optical Phase Locked Loop

In this paper, we report on our simulation and design of a photonic integrated circuits (PIC)-based heterodyne optical phase-locked loop (OPLL). Our simulation reveals that the OPLL operation can be in one of three states, i.e., absolutely stable, metastable, and unstable states, depending on the relative position of the initial phase reversal point to the loop bandwidth. By systematically optimizing all of the loop parameters involved, the loop bandwidth of 247.8 MHz and the residual phase noise variance of 0.012 rad2 are theoretically obtained in such a PIC-OPLL system, which are better than any reported counterparts. In addition, the lowest required power of the master laser is also evaluated, assuming that the largest acceptable residual phase noise variance is 0.02 rad2, and it is found that the lowest master laser power is −54 dBm in our current OPLL system, and this value can be reduced to −56 dBm, providing that the summed linewidth is reduced to 10 kHz.

Resonant Fiber Optic Gyroscope With HOPLL Technique Based on Acousto-Optic Modulation

Backscattering noise is a key factor to restrict the performance of resonant fiber optic gyroscopes (RFOG). In order to improve the accuracy of RFOG, this paper proposes a scheme with heterodyne optical phase-locked loop (HOPLL) technique based on acousto-optic modulation (AOM) to suppress backscattering noise. In this scheme, the main light source is divided into three output lights, two of which are frequency-shifted by the AOM, and then are phase-locked with the third light, using the HOPLL technique. The two channels of frequency-shifted light are respectively used as the clockwise (CW) and counterclockwise (CCW) light of the RFOG, and are locked on the resonance peak. The transmissive fiber resonator can be regarded as a band-pass filter. By adjusting the reference signal in the HOPLL, the carrier of the two frequency-shifted lights can be moved away from the resonance peak. The carrier light intensity of the frequency-shifted light is suppressed by the cavity itself. The suppression ratio (SPR) reaches 15.83dB. The experimental results illustrate that the bias stability of RFOG is 3.8°/h based on the Allan deviation, strongly proving the effectiveness of this scheme in suppressing backscattering noise in practical 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.

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.

Free spectral range measurement using homologous heterodyne optical phase-locked loop based on acousto-optic modulation.

We demonstrate a homologous heterodyne optical phase-locked loop for free spectral range measurement of a fiber ring resonator. In this loop, the frequency noise within the 10Hz bandwidth is reduced by more than 40dB from 147.350 to 0.014Hz, and the power spectral density of the frequency noise reaches 7.69×10-8 Hz2/Hz at 10Hz. Finally, the relative measurement accuracy of 1.39×10-9 is achieved by this loop and the free spectral range coefficient of thermal expansion is measured as -174.1±0.2 Hz/°C with a cavity finesse of 26.65. This work provides a method to measure free spectral range by tracking the resonance modes of the resonator and reducing frequency noise, especially for two signals with frequency offset.

Evolution of Chip-Scale Heterodyne Optical Phase-Locked Loops Toward Watt Level Power Consumption

We design and experimentally demonstrate two chip-scale and agile heterodyne optical phase-locked loops (OPLLs) based on two types of InP-based photonic-integrated coherent receiver circuits. The system performance of the first-generation OPLL was improved in terms of offset-locking range, and power consumption with the use of a power efficient and compact photonic-integrated circuit (PIC). The second-generation PIC consists of a 60-nm widely tunable Y-branch laser as a local oscillator with a 2 × 2 multimode interference (MMI) coupler and a pair of balanced photodetectors. This PIC consumes only 184-mW power in full operation, which is a factor of 3 less compared to the first-generation PIC. In addition, the sensitivity of these OPLLs was experimentally measured to be as low as 20 μw. A possible solution to increase the sensitivity of these OPLLs is also suggested.

Heterodyne locking of a fully integrated optical phase-locked loop with on-chip modulators.

We design and experimentally demonstrate a highly integrated heterodyne optical phase-locked loop (OPLL) consisting of an InP-based coherent photonic receiver, high-speed feedback electronics, and an RF synthesizer. Such coherent photonic integrated circuits contain two widely tunable lasers, semiconductor optical amplifiers, phase modulators, and a pair of balanced photodetectors. Offset phase-locking of the two lasers is achieved by applying an RF signal to an on-chip optical phase modulator following one of the lasers and locking the other one to a resulting optical sideband. Offset locking frequency range >16 GHz is achieved for such a highly sensitive OPLL system which can employ up to the third-order-harmonic optical sidebands for locking. Furthermore, the rms phase error between the two lasers is measured to be 8°.

Towards chip-scale optical frequency synthesis based on optical heterodyne phase-locked loop.

An integrated heterodyne optical phase-locked loop was designed and demonstrated with an indium phosphide based photonic integrated circuit and commercial off-the-shelf electronic components. As an input reference, a stable microresonator-based optical frequency comb with a 50-dB span of 25 nm (~3 THz) around 1550 nm, having a spacing of ~26 GHz, was used. A widely-tunable on-chip sampled-grating distributed-Bragg-reflector laser is offset locked across multiple comb lines. An arbitrary frequency synthesis between the comb lines is demonstrated by tuning the RF offset source, and better than 100Hz tuning resolution with ± 5 Hz accuracy is obtained. Frequency switching of the on-chip laser to a point more than two dozen comb lines away (~5.6 nm) and simultaneous locking to the corresponding nearest comb line is also achieved in a time ~200 ns. A low residual phase noise of the optical phase-locking system is successfully achieved, as experimentally verified by the value of -80 dBc/Hz at an offset of as low as 200 Hz.

Fiber-based femtosecond optical frequency comb stabilized to iodine frequency standard

A fiber-based femtosecond optical frequency comb spanning wavelengths from 1 to 2 μm was stabilized precisely to an iodine frequency standard by means of heterodyne optical phase-locked loops. It enables transfer of frequency stability across electromagnetic spectrum and implementation of compact optical clocks with ∼10-15 long-term instability.

Phase-locking and coherent power combining of broadband linearly chirped optical waves

We propose, analyze and demonstrate the optoelectronic phase-locking of optical waves whose frequencies are chirped continuously and rapidly with time. The optical waves are derived from a common optoelectronic swept-frequency laser based on a semiconductor laser in a negative feedback loop, with a precisely linear frequency chirp of 400 GHz in 2 ms. In contrast to monochromatic waves, a differential delay between two linearly chirped optical waves results in a mutual frequency difference, and an acoustooptic frequency shifter is therefore used to phase-lock the two waves. We demonstrate and characterize homodyne and heterodyne optical phase-locked loops with rapidly chirped waves, and show the ability to precisely control the phase of the chirped optical waveform using a digital electronic oscillator. A loop bandwidth of ~ 60 kHz, and a residual phase error variance of < 0.01 rad(2) between the chirped waves is obtained. Further, we demonstrate the simultaneous phase-locking of two optical paths to a common master waveform, and the ability to electronically control the resultant two-element optical phased array. The results of this work enable coherent power combining of high-power fiber amplifiers-where a rapidly chirping seed laser reduces stimulated Brillouin scattering-and electronic beam steering of chirped optical waves.

Intersatellite laser ranging with homodyne optical phase locking for Space Advanced Gravity Measurements mission

In this paper, we present the scheme and the preliminary results of an intersatellite laser ranging system that is designed for the Earth's gravity recovery mission proposed in China, called Space Advanced Gravity Measurements (SAGM). The proposed intersatellite distance is about 100 km and the precision of inter-satellite range monitoring is 10 nm/Hz(1/2) at 0.1 Hz. To meet the needs, we designed a transponder-type intersatellite laser ranging system by using a homodyne optical phase locking technique, which is different from the heterodyne optical phase-locked loop used in GRACE follow-on mission. Since an ultrastable oscillator is unnecessary in the homodyne phase-locked loop, the measurement error caused by the frequency instability of the ultrastable oscillator need not be taken into account. In the preliminary study, a heterodyne interferometer with 10-m baseline (measurement arm-length) was built up to demonstrate the validity of the measurement scheme. The measurement results show that a resolution of displacement measurement of about 3.2 nm had been achieved.

Phase-Controlled Apertures Using Heterodyne Optical Phase-Locked Loops

In this letter, we demonstrate the use of an electronic feedback scheme using a voltage controlled oscillator (VCO) to control the optical phase of individual semiconductor lasers (SCLs) phase locked to a common reference laser using heterodyne optical phase-locked loops (OPLLs). The outputs of two external cavity SCLs phase-locked to a common reference laser are coherently combined, and the variation in the relative optical path lengths of the combining beams is corrected by dynamically changing the phase of the offset radio-frequency signal fed into one of the OPLLs by means of a VCO. A stable power combination efficiency of 94% is achieved. This inherently different method of phase control, i.e., electronic rather than the use of electrooptic crystals, is deemed essential for new applications involving coherent optoelectronics.

Coherent power combination of two Master-oscillator-power-amplifier (MOPA) semiconductor lasers using optical phase lock loops

Using heterodyne Optical Phase-Locked Loops (OPLLs), two 1W high power 1550 nm master-oscillator-power-amplifier (MOPA) semiconductor lasers operating as current controlled oscillators are phase-locked to a 1 mW reference laser. The signals of the two MOPAs are then coherently combined and their mutual coherence is studied. In each OPLL, the acquisition range is increased to +/-1.1GHz with the help of an aided- acquisition circuit. Control of the phase of a single slave MOPA is demonstrated using a RF phase shifter. The differential phase error between two MOPAs locked to the common reference laser is typically 22 degrees.

Experimental demonstration of an optical phased array antenna for laser space communications

The feasibility of an optical phased array antenna applicable for spaceborne laser communications was experimentally demonstrated. Heterodyne optical phase-locked loops provide for a defined phase relationship between the collimated output beams of three single-mode fibers. In the far field the beams interfere with a measured efficiency of 99%. The main lobe of the interference pattern can be moved by phase shifting the subaperture output beams. The setup permitted agile beam steering within an angular range of 1 mr and a response time of 0.7 ms. We propose an operational optical phased array antenna fed by seven lasers, featuring high transmit power and redundance.

Narrow-linewidth diode laser using a fiber-optic ring resonator

Experimental results of linewidth reduction in AlGaAs diode lasers with frequency-selective Rayleigh scattering from a fiber-optic ring resonator are presented. The intermediate-frequency signal produced by two such lasers is shown to have less than a 100-Hz intermediate-frequency linewidth. Self-homodyning measurements indicate an estimated laser linewidth of 0.6 Hz. Observing the intermediate-frequency beat signal and a Fabry-Perot interferometer, we demonstrate frequency mode stability and tuning. This tuning allowed us to lock a heterodyne optical phase-locked loop successfully. The narrow-linewidth ring laser also produced a stimulated Brillouin scattering wave.

A wideband heterodyne optical phase-locked loop for generation of 3-18 GHz microwave carriers

Experimental results of a wideband heterodyne second-order optical phase-locked loop with 1.5- mu m semiconductor lasers are presented. The loop has a bandwidth of 180 MHz, a gain of 181 dBHz, and a propagation delay of only 400 ps. A beat signal of 8 MHz linewidth is phase locked to become a replica of a microwave reference source close to carrier with a noise level of -125 dBc/Hz. The total phase variance of the locked carrier is 0.04 rad/sup 2/ and carriers can be generated in a continuous range from 3 to 18 GHz. The loop reliability is excellent with an average time to cycle slip of 10/sup 11/ s and an acquisition range of 640 MHz. >

Heterodyne optical phase-locked loop by confocal Fabry-Periot cavity coupled AlGaAs lasers

Two external confocal Fabry-Perot cavity-coupled 0.83- mu m AlGaAs lasers phase-locked to one another by a heterodyne optical phase-locked loop are discussed. The phase error under phase-locking conditions was measured using the square root of the Allan variance. The result confirmed that the heterodyne signal was a full replica of the spectrum of the reference RF signal within the loop bandwidth. The estimated phase-error variance was 0.02 rad/sup 2/, i.e. approximately=8.1 degrees .< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Improved allan variance real-time processing system to measure frequency tracking error of heterodyne optical phase-locked loops

The performance of an Allan variance measuring system was drastically improved by employing time interval analysis incorporating a beat frequency method. It was used to evaluate the performance of a heterodyne optical phase-locked loop with a very low optical frequency tracking error of 0.4 mHz at the integration time of 70 s. Advantages of the system are precise measurement for highly stable frequency sources with good reproducibility and simple structure