Dual-comb Interferometry Research Articles

Mode-resolved optical frequency comb fixed point localization via dual-comb interferometry

Mode-resolved optical frequency comb fixed point localization via dual-comb interferometry

Graphics card-based real-time processing for dual comb interferometry.

The technique of performing interferometry with two optical frequency combs is used by an increasing number of research groups and even for field deployed commercial applications. Real-time interferogram acquisition, correction, and averaging are, however, still not broadly accessible. This limits the deployment and wider adoption of this high resolution, high sensitivity technique. We herein introduce and describe a freely available correction software performing real-time processing on a graphics processing unit.

Relative timing jitter compression in a Fabry–Pérot cavity-assisted free-running dual-comb interferometry

Relative timing jitter compression in a Fabry–Pérot cavity-assisted free-running dual-comb interferometry

Transient long-range distance measurement by a Vernier spectral interferometry

Rapid and long-range distance measurements are essential in various industrial and scientific applications, and among them, the dual-comb ranging system attracts great attention due to its high precision. However, the temporal asynchronous sampling results in the tradeoff between frame rate and ranging precision, and the non-ambiguity range (NAR) is also limited by the comb cycle, which hinders the further advancement of the dual-comb ranging system. Given this constraint, we introduce a Vernier spectral interferometry to improve the frame rate and NAR of the ranging system. First, leveraging the dispersive time-stretch technology, the dual-comb interferometry becomes spectral interferometry. Thus, the asynchronous time step is unlimited, and the frame rate is improved to 100 kHz. Second, dual-wavelength bands are introduced to implement a Vernier spectral interferometry, whose NAR is enlarged from 1.5 m to 1.5 km. Moreover, this fast and long-range system also demonstrated high precision, with a 22.91-nm Allan deviation over 10-ms averaging time. As a result, the proposed Vernier spectral interferometry ranging system is promising for diverse applications that necessitate rapid and extensive distance measurement.

Integer-locking condition for stable dual-comb interferometry in situations with fluctuating frequency-comb repetition rates

To make dual-comb interferometry usable in a wide range of applications, it is important to achieve reproducible measurement results even in non-ideal environments that affect the repetition-rate stability. Here, we consider dual-comb interferometry based on a pair of fully referenced optical frequency combs (OFCs) and investigate the impact of fluctuations in the OFC repetition frequencies on the peak position of the center burst in the interferogram. We identify a phase-locking scheme that minimizes the impact of these fluctuations through choosing a special combination of phase-locked frequencies, and the resulting type of operating condition is termed integer-locking condition. Under the integer-locking condition, the number of sampling points in each interferogram remains constant regardless of repetition-rate variations, and this enables more stable phase-resolved measurements in non-ideal environments. We demonstrate the application of this approach using absolute path-length measurements and discuss the accuracy limit imposed by the integer-locking condition. Our findings offer a strategy for robust dual-comb interferometry outside metrology laboratories.

Platicon dynamics in photonic molecules

Coherent dissipative structures known as platicons can be reliably generated in photonic molecules, resulting in deterministic and reproducible microcombs derived from a continuous-wave pump. However, the supermode spectrum of standard photonic molecules displays numerous avoided mode crossings, distorting the spectral envelope of platicon microcombs. Here, we obtain a platicon microcomb using a photonic molecule configuration based on two coupled microcavities, whose size differs by an order of magnitude. This results in an engineered microcomb spectrum that closely resembles the one generated in an ideal single microresonator with just one frequency mode shift. We observe the coupling between the repetition rate of the platicon microcomb with the frequency of the pump laser, an effect originating from the dispersive-wave recoil induced by mode crossings. Using two identical platicon microcombs, we make use of such coupling to realize dual-comb interferometry. These results contribute to understanding dissipative structures in normal-dispersion microresonators and offer an alternative to applications such as spectroscopy and metrology.

Entanglement-enhanced dual-comb spectroscopy

Dual-comb interferometry harnesses the interference of two laser frequency combs to provide unprecedented capability in spectroscopy applications. In the past decade, the state-of-the-art systems have reached a point where the signal-to-noise ratio per unit acquisition time is fundamentally limited by shot noise from vacuum fluctuations. To address the issue, we propose an entanglement-enhanced dual-comb spectroscopy protocol that leverages quantum resources to significantly improve the signal-to-noise ratio performance. To analyze the performance of real systems, we develop a quantum model of dual-comb spectroscopy that takes practical noises into consideration. Based on this model, we propose quantum combs with side-band entanglement around each comb lines to suppress the shot noise in heterodyne detection. Our results show significant quantum advantages in the uW to mW power range, making this technique particularly attractive for biological and chemical sensing applications. Furthermore, the quantum comb can be engineered using nonlinear optics and promises near-term experimentation.

Linear dual-comb interferometry at high power levels.

Detector non-linearity is an important factor limiting the maximal power and hence the signal-to-noise ratio (SNR) in dual-comb interferometry. To increase the SNR without overwhelming averaging time, photodetector non-linearity must be properly handled for high input power. Detectors exhibiting nonlinear behavior can produce linear dual-comb interferograms if the area of the detector's impulse response does not saturate and if the overlap between successive time-varying impulse responses is properly managed. Here, a high bandwidth non-amplified balanced photodetector is characterized in terms of its impulse response to high intensity short pulses to exemplify the conditions. With a 23.5 mW average power on each detector in a balanced pair, nonlinear spectral artifacts are at least 40 dB below the spectral baseline. Absorption lines of carbon dioxide are measured to reveal lines discrepancies smaller than 0.1% with HITRAN. A spectral shape independent formulation for the dual-comb figure of merit is proposed, reaching here 7.2 × 107 Hz1/2 limited by laser relative intensity noise, but corresponding to an ideal, shot-noise limited, figure of merit for an equivalent 0.85 mW average power per comb.

Dual-comb interferometry using frequency shifting loops

Dual-comb systems have demonstrated their potential for metrology, e.g. spectroscopy vibrometry or ranging [1], However, the implementation of dual-combs is often complex and generally requires substantial optical and optoelectronic hardware Here, we propose a simple and compact architecture based on a bidirectional frequency shifting loop, that provides more than I 00 mutually coherent comb lines The system makes use ofa CW laser and a slow electronic detection chain (10 MSa/s) We have implemented two configurations enabling dynamic multi-heterodyne interferometry at 20kHz: the first one makes use of acousto-optic frequency shifters, and allows highly sensitive distributed acoustic sensing along a fiber, The second one involves electro-optic frequency shifters, and enables ranging with a sub-mm resolution.

Ultrafast optical phase-sensitive ultrasonic detection via dual-comb multiheterodyne interferometry

Highly sensitive and broadband ultrasound detection is important for photoacoustic imaging, biomedical ultrasound, and ultrasonic nondestructive testing. The elasto-optical refractive index modulation induced by ultrasound arouses a transient phase shift of a probe beam. Highly sensitive phase detection with a high Q factor resonator is desirable to visualize the ultraweak transient ultrasonic field. However, current phase-sensitive ultrasonic detectors suffer from limited bandwidth, mutual interference between intensity and phase, and significant phase noise, which become key to limiting further improvement of detection performance. We report a phase-sensitive detector with a bandwidth of up to 100 MHz based on dual-comb multiheterodyne interferometry (DCMHI). By sensing the phase shift induced by the ultrasound without any resonators in the medium, the DCMHI boosted the phase sensitivity by coherent accumulation without any magnitude averaging and extra radio frequency amplification. DCMHI offers high sensitivity and broad bandwidth as the noise-equivalent pressure reaches 31 mPa / √Hz under 70 MHz acoustic responses. With a large repetition rate difference of up to 200 MHz of dual comb, DCMHI can achieve broadband acoustic responses up to 100 MHz and a maximum possible imaging acquisition rate of 200 MHz. It is expected that DCMHI can offer a new perspective on the new generation of optical ultrasound detectors.

Hybrid Integrated Dual-Microcomb Source

Dual-comb interferometry is based on self-heterodyning two optical frequency combs, with corresponding mapping of the optical spectrum into the radio-frequency domain. The dual comb enables diverse applications, including metrology, fast high-precision spectroscopy with high signal-to-noise ratio, distance ranging, and coherent optical communications. However, current dual-frequency-comb systems are designed for research applications and typically rely on scientific equipment and bulky mode-locked lasers. Here we demonstrate a fully integrated power-efficient dual-microcomb source that is electrically driven and allows turnkey operation. Our implementation uses commercially available components, including distributed-feedback and Fabry-Perot laser diodes, and silicon-nitride photonic circuits with microresonators fabricated in commercial multiproject wafer runs. Our devices are therefore unique in terms of size, weight, power consumption, and cost. Laser-diode self-injection locking relaxes the requirements on microresonator spectral purity and $Q$ factor, so that we can generate soliton microcombs resilient to thermal frequency drift and with pump-to-comb sideband efficiency of up to 40% at mW power levels. We demonstrate down-conversion of the optical spectrum from 1400 to 1700 nm into the radio-frequency domain, which is valuable for fast wide-band Fourier spectroscopy, which was previously not available with chip-scale devices. Our findings pave the way for further integration of miniature microcomb-based sensors and devices for high-volume applications, thus opening up the prospect of innovative products that redefine the market of industrial and consumer mobile and wearable devices and sensors.

Real-time distance and velocity measurement based on the dual-comb system.

With the development of laser metrology, the dual-comb system has natural superiority in the measuring fields. Specifically, distance and velocity represent a basic state for the target in space. We propose an application mode of the dual-comb interferometry integrated into the field programmable gate array. A high-speed parallel processor truly gives full play to the benefit of the data processing rate. The algorithm of the peak extraction and the address matching also bring an efficient working mode into the whole scheme. To verify the performance of this system, we devise a series of experiments for distance and velocity, respectively. The data processing rate of the distance is 425 Hz and that of the corresponding average velocity is 0.425 Hz, which is flexible for different measuring conditions. The experimental results show that the difference can be well within 252.8 µm at 5 m range and 284.9 µm/s over 0.5 m/s.

Correcting spectral baseline fluctuations in dual-comb interferometry.

A method to measure and correct for spectral baseline fluctuations in dual-comb interferometry is presented. Fluctuations can be measured from the amplitude of beat notes between combs and a continuous wave laser or from a separate measurement of the combs' repetition rates, filtered around the spectral region of interest. Amplitude-dependent spectral variations are characterized using low-resolution Fourier transforms around the centerburst of several interferograms, and a nonstationary filter is applied to properly account for the combs' variations during the measurement. This allows removal of this source of statistical, as well as systematic, errors.

Understanding photodetection nonlinearity in dual-comb interferometry

The impact of photodetector nonlinearity on dual-comb spectrometers is described and compared to that of Michelson-based Fourier transform spectrometers (FTS). The optical sampling occurring in the dual-comb approach, being the key difference with FTS, causes optical aliasing of the nonlinear spectral artifacts. Measured linear and nonlinear interferograms are presented to validate the model. Absorption lines of H13CN are provided to understand the impact of nonlinearity on spectroscopic measurements.

Correcting photodetector nonlinearity in dual-comb interferometry.

Photodetector nonlinearity, the main limiting factor in terms of optical power in the detection chain, is corrected to improve the signal-to-noise ratio of a short-time measurement in dual-comb spectroscopy. An iterative correction algorithm minimizing out-of-band spectral artifacts based on nonlinearity correction methods used in classical Fourier-transform spectrometers is presented. The exactitude of the nonlinearity correction is validated using a low power linear measurement. Spectroscopic lines of H12CN are provided and the increase in absorption depth of 24% caused by the saturation of the detector is corrected yielding residuals limited by the measurement noise.

Single-Pixel Photon-Counting Imaging Based on Dual-Comb Interferometry.

We propose and experimentally demonstrate single-pixel photon counting imaging based on dual-comb interferometry at 1550 nm. Different from traditional dual-comb imaging, this approach enables imaging at the photon-counting regime by using single-photon detectors combined with a time-correlated single-photon counter to record the returning photons. The illumination power is as low as 14 pW, corresponding to 2.2 × 10−3 photons/pulse. The lateral resolution is about 50 μm. This technique paves the way for applying dual-comb in remote sensing and imaging.

Dual-comb spectroscopy resolved three-degree-of-freedom sensing

Precise and fast determination of position and orientation, which is normally achieved by distance and angle measurements, has broad applications in academia and industry. We propose a dynamic three-degree-of-freedom measurement technique based on dual-comb interferometry and a self-designed grating-corner-cube (GCC) combined sensor. Benefiting from its unique combination of diffraction and reflection characteristics, the absolute distance, pitch, and yaw of the GCC sensor can be determined simultaneously by resolving the phase spectra of the corresponding diffracted beams. We experimentally demonstrate that the method exhibits a ranging precision (Allan deviation) of 13.7 nm and an angular precision of 0.088 arcsec, alongside a 1 ms reaction time. The proposed technique is capable of precise and fast measurement of distances and two-dimensional angles over long stand-off distances. A system with such an overall performance may be potentially applied to space missions, including in tight formation-flying satellites, for spacecraft rendezvous and docking, and for antenna measurement as well as the precise manufacture of components including lithography machines and aircraft-manufacturing devices.

A Traceable High-Accuracy Velocity Measurement by Electro-Optic Dual-Comb Interferometry

An electro-optic dual-comb Doppler velocimeter for high-accuracy velocity measurement is presented in this paper. The velocity information of the object can be accurately extracted from the change of repetition frequency, which is in the microwave frequency domain and can be locked to an atomic clock. We generate two optical combs by electro-optic phase modulators and trace their repetition frequencies to the rubidium clock. One functions as the measurement laser and the other the reference. Experimentally, we verify its high accuracy in the range of 100–300 mm/s with a maximum deviation of 0.44 mm/s. The proposed velocimeter combines the merits of high accuracy and wide range. In addition, since the repetition frequency used for the measurement is traceable to the rubidium clock, its potential superiority in traceability can be utilized in velocity metrology.

Self-Correction Limits in Dual-Comb Interferometry

Self-sufficient correction algorithms have recently been developed to estimate and compensate the relative frequency noise between two free-running frequency combs using only their dual-comb interferograms (IGMs). However, such algorithms are limited by the relatively low observation rate of the frequency noise, equal to the IGM repetition rate. In this paper, we analyze the effectiveness of estimation methods based on the cross-correlation function and the ambiguity function. We derive estimators and obtain their theoretical variances as a function of the IGMs’ measurable parameters to later establish criteria that verify the success rate of each method for the correction and coherent averaging of successive IGMs. Finally, we present different cases of both real and simulated dual-comb systems and assess their compatibility with these estimation methods.

Dual-comb spectrally encoded confocal microscopy by electro-optic modulators

We demonstrate an electro-optic (EO) based dual-comb spectrally encoded confocal microscopy (DC-SECM). With the capability of large comb spacing (∼8.1-GHz) and comb-line to pixel mapping, only thirteen comb lines are used to sample a spatial point of resolution size. Such spatial sampling effectively lessens the required average focal power for a given signal-to-noise ratio (SNR) and contributes to a higher power-efficiency, which alleviates the risk of photodamage. Furthermore, the tunable beating frequency (0–6 MHz) of dual comb interferometry (DCI) renders an achievable ultrafast imaging speed up to MHz level. The image performance was evaluated using a standard resolution target, indicating a lateral resolution of 2.76-μm, and further demonstrated with biological samples. Compared with time-stretch spectrally-encoded microscopy, the DC-SECM can not only achieve low-focal-power (∼30 μW) imaging but also function under minimum settings of both electrical bandwidth (hundreds-of MHz) and digitization sampling rate (1 Gs/s), while simultaneously maintaining a frame rate on the order of MHz.