Optical Phase-locked Loop Research Articles

Down-conversion RF/photonic link with a monolithically integrated ACP-OPLL phase demodulator

We present a frequency down conversion RF/photonic link with an attenuation-counter-propagating optical phase lock loop (ACP-OPLL) photonic integrated circuit (PIC) receiver. Frequency down conversion was accomplished by nonlinear optical phase modulation in a 2.5mm long transmitter (TX) multi-quantum-well (MQW) phase modulator. The down-conversion link demonstrated RF to IF conversion loss of ~9dB and spurious free dynamic range (SFDR) of ~118dB∙Hz2/3.

Foundry fabricated photonic integrated circuit optical phase lock loop.

This paper describes the first foundry-based InP photonic integrated circuit (PIC) designed to work within a heterodyne optical phase locked loop (OPLL). The PIC and an external electronic circuit were used to phase-lock a single-line semiconductor laser diode to an incoming reference laser, with tuneable frequency offset from 4 GHz to 12 GHz. The PIC contains 33 active and passive components monolithically integrated on a single chip, fully demonstrating the capability of a generic foundry PIC fabrication model. The electronic part of the OPLL consists of commercially available RF components. This semi-packaged system stabilizes the phase and frequency of the integrated laser so that an absolute frequency, high-purity heterodyne signal can be generated when the OPLL is in operation, with phase noise lower than -100 dBc/Hz at 10 kHz offset from the carrier. This is the lowest phase noise level ever demonstrated by monolithically integrated OPLLs.

Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation.

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique's potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications.

Relative stability analysis of optical injection phase-locked loop

In this paper, we analyze the relative stability of optical injection phase-locked loop (OIPLL) in terms of the phase margin considering loop-delay into account. Phase margin of first- and second-order OIPLL are calculated as a function of loop-delay, damping factor and other parameters of the loop. The effects of optical injection on damping factor and phase margin of the loop are shown. It is observed that the OIPLL has better stability than the conventional optical phase-locked loop (OPLL) in presence of loop-delay.

Stability analysis and pull-in behavior of modified dither optical phase-lock loop with loop propagation delay

This paper analyses the stability of a modified dither optical phase-lock loop (OPLL), which contains all the components of a standard dither OPLL in conjunction of an additional electro-optic phase modulator in the phase-locking branch, taking loop delay into account. Applying Nyquist criteria, expression for maximum allowable loop propagation delay of the modified dither OPLL is derived. Relative stability of the modified loop is studied in terms of the Gain margin and Phase margin of the OPLL. Also, Pull-in behavior of modified dither OPLL is analyzed in the presence of loop propagation delay. Expressions are derived analytically for computing the pull-in voltage, pull-in range and acquisition time for a second-order type-II modified dither OPLL that employs an active low pass loop filter. The influences of the dither signal, electro-optic phase modulator and loop propagation delay on the acquisition of the loop are shown. It is found that the electro-optic phase modulator completely eliminates the false locking phenomenon due to loop delay, decreases the acquisition time of the loop and allows pull-in from larger frequency offset. Simulation results are given in support of the analytical results.

Mid-infrared multiheterodyne spectroscopy with phase-locked quantum cascade lasers

Fabry-Pérot (FP) quantum cascade lasers (QCLs) provide purely electronically controlled monolithic sources for broadband mid-infrared (mid-IR) multiheterodyne spectroscopy (MHS), which benefits from the large gain bandwidth of the QCLs without sacrificing the narrowband properties commonly associated with the single mode distributed feedback variant. We demonstrate a FP-QCL based multiheterodyne spectrometer with a short-term noise-equivalent absorption of ∼3 × 10−4/Hz, a mid-IR spectral coverage of 25 cm−1, and very short acquisition time (10 μs) capability. The broadband potential is demonstrated by measuring the absorption spectra of ammonia and isobutane under atmospheric pressure conditions. The stability of the system is enhanced by a two-stage active frequency inter-locking procedure, where the two QCLs are pre-locked with a slow feedback loop based on an analog frequency discriminator, followed by a high bandwidth optical phase-locked loop. The locking system provides a relative frequency stability in the sub kHz range over seconds of integration time. The strength of the technique lies in the ability to acquire spectral information from all optical modes simultaneously and individually, which bodes for a versatile and cost effective spectrometer for mid-IR chemical gas sensing.

Frequency comb assisted two-photon vibrational spectroscopy.

We report a setup for high-resolution two-photon spectroscopy using a mid-infrared continuous wave optical parametric oscillator (CW-OPO) and a near-infrared diode laser as the excitation sources, both of which are locked to fully stabilized optical frequency combs. The diode laser is directly locked to a commercial near-infrared optical frequency comb using an optical phase-locked loop. The near-infrared frequency comb is also used to synchronously pump a degenerate femtosecond optical parametric oscillator to produce a fully stabilized mid-infrared frequency comb. The beat frequency between the mid-infrared comb and the CW-OPO is then stabilized through frequency locking. We used the setup to measure a double resonant two-photon transition to a symmetric vibrational state of acetylene with a sub-Doppler resolution and high signal-to-noise ratio.

A phase-locked laser system based on double direct modulation technique for atom interferometry

We demonstrate a laser system based on phase modulation technology and phase feedback control. The two laser beams with frequency difference of 6.835 GHz are modulated using electro-optic and acousto-optic modulators, respectively. Parasitic frequency components produced by the electro-optic modulator are filtered using a Fabry–Perot Etalon. A straightforward phase feedback system restrains the phase noise induced by environmental perturbations. The phase noise of the laser system stays below −125 rad2/Hz at frequency offset higher than 500 kHz. Overall phase noise of the laser system is evaluated by calculating the contribution of the phase noise to the sensitivity limit of a gravimeter. The results reveal that the sensitivity limited by the phase noise of our laser system is lower than that of a state-of-the-art optical phase-lock loop scheme when a gravimeter operates at short pulse duration, which makes the laser system a promising option for our future application of atom interferometer.

RF Photonic Link With an MQW Phase Modulator Transmitter and an ACP-OPLL Receiver

A phase modulated (PM) RF photonic link with a multiple quantum well (MQW) phase modulator transmitter and an attenuating-counter-propagating optical phase lock loop (ACP-OPLL) receiver photonic IC is presented. The nonlinearity of the MQW phase modulator was used to cancel the residual nonlinear distortions of the ACP-OPLL receiver. This enables the demonstration of the highest dynamic range performance among the PM RF photonic links.

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.

Frequency-stabilized laser system at 1572 nm for space-borne CO2 detection LIDAR

A frequency-stabilized laser system at 1572 nm for space-borne carbon dioxide (CO2) detection LIDAR to realize the precise measurement of the global atmospheric CO2 concentration is presented in this Letter. A distributed-feedback laser diode serves as the master laser (ML) and is wavelength locked to the CO2 line center at 1572.0179 nm using the external frequency modulation technique. The root mean square frequency drift is suppressed to about 50 kHz at an average time of 0.1 s over 8 h. Based on optical phase-locked loops, an online seeder and an offline seeder are offset locked to the reference laser at 1572.024 and 1572.081 nm, respectively, retaining virtually the same frequency stability as the ML.

光锁相环路中声光移频器插入损耗特性研究及优化

光锁相环路中声光移频器插入损耗特性研究及优化

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.

Optical techniques for Rydberg physics in lattice geometries

We address the technical challenges when performing quantum information experiments with ultracold Rydberg atoms in lattice geometries. We discuss the following key aspects: (i) The coherent manipulation of atomic ground states, (ii) the coherent excitation of Rydberg states, and (iii) spatial addressing of individual lattice sites. We briefly review methods and solutions which have been successfully implemented, and give examples based on our experimental apparatus. This includes an optical phase-locked loop, an intensity and frequency stabilization setup for lasers, and a nematic liquid-crystal spatial light modulator.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters.

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.

Coherent receiver design based on digital signal processing in optical high-speed intersatellite links with M -phase-shift keying

We present simulative and experimental investigations of different coherent receiver designs for high-speed optical intersatellite links. We focus on frequency offset (FO) compensation in homodyne and intradyne detection systems. The considered laser communication terminal uses an optical phase-locked loop (OPLL), which ensures stable homodyne detection. However, the hardware complexity increases with the modulation order. Therefore, we show that software-based intradyne detection is an attractive alternative for OPLL-based homodyne systems. Our approach is based on digital FO and phase noise compensation, in order to achieve a more flexible coherent detection scheme. Analytic results will further show the theoretical impact of the different detection schemes on the receiver sensitivity. Finally, we compare the schemes in terms of bit error ratio measurements and optimal receiver design.

Non-destructive shadowgraph imaging of ultra-cold atoms.

An imaging system is presented that is capable of far-detuned non-destructive imaging of a Bose-Einstein condensate with the signal proportional to the second spatial derivative of the density. Whilst demonstrated with application to Rb85, the technique generalizes to other atomic species and is shown to be capable of a signal-to-noise of ∼25 at 1 GHz detuning with 100 in-trap images showing no observable heating or atom loss. The technique is also applied to the observation of individual trajectories of stochastic dynamics inaccessible to single shot imaging. Coupled with a fast optical phase locked loop, the system is capable of dynamically switching to resonant absorption imaging during the experiment.

Highly Linear Optical Phase Modulation Inside a Mixed QW Modulator

We present a highly linear quantum well (QW) optical phase modulator that employs a mixed QW structure. This modulator demonstrated an excellent phase modulation linearity. Its third-order intermodulation intercept point in the optical phase excursion per unit length was measured to be greater than $8\pi $ /mm. The measured optical loss is $\sim 1$ dB/mm. The new phase modulator design should enable a more compact optical phase-locked loop photonic integrated circuit with a wider bandwidth.

In-Line Optical MEMS Phase Modulator and Application in Ring Laser Frequency Modulation

Optical phase modulators are essential building components in a wide range of applications, for instance, optical communication systems, tunable lasers, optical phase locked loops, and optical sensors. The production of in-plane MEMS-based optical phase modulator with self-aligned mirrors, actuator, and fiber grooves enables the low cost and easy integration with fiber-based lasers and sensors or photonic microsystems. In this paper, we report an in-plane transmission type MEMS-based optical phase modulator fabricated by deep reactive ion etching technology on a silicon-on-insulator (SOI) substrate. Detailed optical analysis of the MEMS phase modulator considering the diffraction of the single-mode fiber output beam, the asymmetric truncation of the beam by the limited aperture of the micromirrors and the tilt angle of the deeply etched mirrors is presented. The device layer height of the fabricated SOI wafer is 100 μm, and the sidewalls are etched with verticality that is better than 89.98°. The micromechanical system is characterized experimentally using electrical technique, and the resonance frequency and quality factor are 11.3 kHz and 163, respectively. The MEMS device is integrated into fiber ring laser (FRL) enabling the achievement of low- and high-frequency modulation indices. The frequency modulation of the FRL using the presented phase modulator is supported with numerical analysis and experimental results.

Discussion on the effect of an optical phase-locked loop on the coherence properties of a laser

The statistical characteristics of laser phase noise are analyzed in detail. To restrain the phase noise, an optical phase-locked loop (OPLL) is employed to achieve phase locking of a laser. The working mechanism of OPLL is discussed. By calculating the cross-correlation function between a phase-locked laser and an ideal laser, the feasibility of laser phase locking by means of an OPLL has been certified. Furthermore, the autocorrelation of a phase-locked laser and the cross correlation between an ordinary laser and an ideal laser are compared to show the performance of the OPLL. The quantitative analysis results show that if the linewidth of a laser is about 2 MHz, the time constant of an OPLL should be less than 50 ns to meet the needs of coherent beam combining.