Frequency-modulated Continuous-wave Interferometry Research Articles

High-resolution Michelson Salinometer Based on Frequency-modulated Continuous-wave Interferometry

High-resolution Michelson Salinometer Based on Frequency-modulated Continuous-wave Interferometry

An absolute ranging method based on FMCW principle using dual laser interferometry

The FMCW (Frequency Modulated Continuous Wave) laser interferometer can achieve high-precision displacement measurement by calculating the phase of the interference signal. Due to the need for continuous superposition of signal phases, the optical path and power supply cannot be interrupted during the measurement process. To solve this problem, this article proposes a new absolute ranging method—the decimal comparison method. Two distributed feedback lasers with different central wavelengths are used to measure the same target. The digital signal processing technology is used to solve the sampled interference signals, and the initial phase value corresponding to the decimal of interference is taken. Combined with the central wavelength of the two laser sources, the length variation of the F–P cavity corresponding to the number of interference periods is calculated. Finally, the optical path difference of the target is calculated by the established mathematical model. The experimental results show that this method can realize the absolute measurement of laser interferometry and can expand the application range of FMCW interferometry.

Multipoint dispersion spectroscopic gas sensing by optical FMCW interferometry.

We present a novel, to the best of our knowledge, multipoint gas-sensing method based on dispersion spectroscopy using optical frequency-modulated continuous-wave (FMCW) techniques. By taking advantage of the optical FMCW's excellent multiplexing capability with high spatial resolution, the phase noise in the retrieved dispersion signal is efficiently suppressed. As a proof of concept, this method is experimentally demonstrated with three acetylene gas-sensing nodes, achieving a sensitivity of 30 ppm, a sensing spatial resolution of 30 cm, and a linear dynamic range of more than 3 orders of magnitude. Having advantages of high sensitivity, high spatial resolution, large dynamic range, and immunity to light power variation, the proposed method promotes a novel way for the development of long-distance multipoint spectroscopic gas sensors.

Temperature-compensated multi-point refractive index sensing based on a cascaded Fabry-Perot cavity and FMCW interferometry.

We proposed a novel temperature-compensated multi-point refractive index (RI) sensing system by the combination of the cascaded Fabry-Perot (FP) sensors and the frequency modulated continuous wave (FMCW) interferometry. The former is used for simultaneous sensing of RI and temperature, and the latter is used for multiplexing a series of the cascaded FP sensors to realize multi-point sensing. By means of Fourier transform-based algorithms, the interference spectra of each sub-FP sensors can be divided and demodulated independently. Experimentally, three cascaded FP sensors are multiplexed to verify multi-point RI and temperature sensing ability. RI sensitivity up to ∼1200 nm/RIU is obtained within RI range from 1.3330 to 1.3410, and temperature sensitivity up to ∼0.17 nm/°C is obtained within temperature range from 20 °C to 80 °C. The RI precision is as high as 10-5 RIU and the temperature precision is as high as 0.05 °C. In addition, the prospective multiplexing number could reach about 4000 estimated by the minimum detectable light power. The proposed sensing system has potential advantages in the practical applications that require a large number sensing points.

SPADs and SiPMs Arrays for Long-Range High-Speed Light Detection and Ranging (LiDAR).

Light Detection and Ranging (LiDAR) is a 3D imaging technique, widely used in many applications such as augmented reality, automotive, machine vision, spacecraft navigation and landing. Achieving long-ranges and high-speed, most of all in outdoor applications with strong solar background illumination, are challenging requirements. In the introduction we review different 3D-ranging techniques (stereo-vision, projection with structured light, pulsed-LiDAR, amplitude-modulated continuous-wave LiDAR, frequency-modulated continuous-wave interferometry), illumination schemes (single point and blade scanning, flash-LiDAR) and time-resolved detectors for LiDAR (EM-CCD, I-CCD, APD, SPAD, SiPM). Then, we provide an extensive review of silicon- single photon avalanche diode (SPAD)-based LiDAR detectors (both commercial products and research prototypes) analyzing how each architecture faces the main challenges of LiDAR (i.e., long ranges, centimeter resolution, large field-of-view and high angular resolution, high operation speed, background immunity, eye-safety and multi-camera operation). Recent progresses in 3D stacking technologies provided an important step forward in SPAD array development, allowing to reach smaller pitch, higher pixel count and more complex processing electronics. In the conclusions, we provide some guidelines for the design of next generation SPAD-LiDAR detectors.

Asia-Pacific Optical Sensors Conference: focus issue introduction.

This feature issue of Optics Express contains 17 articles expanding on recent advances in optical sensors presented at the eighth Asia-Pacific Optical Sensors Conference (APOS 2019) held in Auckland, New Zealand, from November 19 to 22, 2019. These articles span sensing for real-time positioning, refractive indices, strain, gas, and temperature using a variety of methods including photoacoustic computed tomography, coherent optical frequency-modulated continuous-wave interferometry, enhanced Bragg gratings, and phase-sensitive optical frequency-domain reflectometry.

Multi-point spectroscopic gas sensing based on coherent FMCW interferometry.

We present an innovative spectroscopic method based on coherent optical frequency-modulated continuous-wave (FMCW) interferometry that can realize multi-point gas detection with high spatial resolution, high sensitivity, and high selectivity. This method takes full advantage of the intrinsic capability of spatial localization of the coherent FMCW, meanwhile efficiently decodes the spectral information from the reflected optical signals. Gas sensors are deployed by adopting bus topology, i.e., distributed along a single backbone fiber in the measurement arm of the FMCW interferometer. For validation, a multi-point acetylene gas sensing system with three sensing nodes is experimentally demonstrated. The transmission spectra of the three gas sensors are accurately extracted, and their corresponding gas concentrations are efficiently retrieved with a low crosstalk below -30 dB. The demonstrated system achieves a sensitivity of 55 ppm (noise equivalent absorbance of 0.004) over a distance of 52 m, with a sensing spatial resolution of 30 cm and a spectral resolution of 0.5 GHz. Our proposed method promotes a novel way for the development of multi-point spectroscopic gas sensing systems for challenging applications such as gas leakage detection and gas emission monitoring, where spatially resolved chemical analysis over a large area is required.

Simultaneous measurement of gas absorption spectra and optical path lengths in a multipass cell by FMCW interferometry.

We present a novel method based on optical frequency-modulated continuous-wave interferometry that can realize simultaneous measurement of gas absorption spectra and optical path lengths (OPLs) in the widely used multipass cells (MPCs). This method involves a Fourier transform (FT) and an inverse FT, by which the gas absorption spectrum and the OPL are retrieved in frequency and time domains, respectively. Various OPLs are achieved by retrieving gas absorption spectra associated with different reflection positions in the MPC. As a demonstration, absorption spectra of acetylene around 1520nm are measured using a commercial White type MPC, achieving a noise equivalent absorbance of 0.01 and a spatial resolution of 290μm over 22.5m OPL. Our proposed method shows advantages for MPC-based gas sensing applications in the significant relaxation of OPL calibration demands and flexible extension of measurement dynamic range.

A Portable FMCW Interferometry Radar With Programmable Low-IF Architecture for Localization, ISAR Imaging, and Vital Sign Tracking

This paper presents a portable radar system for short-range localization, inverse synthetic aperture radar imaging, and vital sign tracking. The proposed sensor incorporates frequency-modulated continuous-wave (FMCW) and interferometry (Doppler) modes, which enable this radar system to obtain both absolute range information and tiny vital signs (i.e., respiration and heartbeat) of human targets. These two different operation modes can be switched through an on-board microcontroller. To simplify the system, the proposed radar utilizes the audio card of a laptop to sample the baseband signal. The FMCW mode of the radar uses operational-amplifier-based circuits to generate an analog sawtooth signal and a reference pulse sequence (RPS). The RPS is locked to the sawtooth signal to obtain coherence for the radar system. For the interferometry mode, a low-intermediate-frequency modulation method is implemented to avoid the slow vital signs from being distorted by the high-pass filter of the audio card. Several experiments were carried out to reveal the capability and distinct operational features of the proposed portable hybrid radar. The experiments also showed that the system can easily detect glass, which is usually difficult to identify for optical-based sensors. In addition, 2-D scanning in a complex environment revealed that the proposed radar was able to differentiate human targets from other objects. Moreover, ISAR images were used to isolate moving human targets from surrounding clutter. Finally, the proposed radar also demonstrated its ability to accurately measure vital signs when a human subject sits still.

Simple low-cost refractometer using a disposable optical fiber tip for measurements

A simple but accurate fiber-optic refractometer based on Fresnel reflection in the tip of a single-mode optical fiber is reported. The refractometer can be easily assembled using a distributed feedback laser as a light source and standard low-cost fiber-optic components. The short section of an optical fiber cleaved at right-angle in both tips is used as a disposable needle-type sensor head. The sensing fiber being connected with the angle-cleaved lead fiber forms a stable reflector used for light intensity reference. Signals from the sensing and reference fiber tips, generated by modulating the laser optical frequency with a sawtooth-wave form, are demultiplexed using methods of optical frequency-modulated continuous-wave interferometry. Despite the simplicity of the device, a repeatability and reproducibility of 6×10−4 RIU and 3×10−4 RIU, respectively, were demonstrated in dip-and-read experiments using replaceable disposable fiber tips.

Fourier transform-limited optical frequency-modulated continuous-wave interferometry over several tens of laser coherence lengths.

We report on a versatile optical frequency-modulated continuous-wave interferometry technique that exploits wideband phase locking for generating highly coherent linear laser frequency chirps. This technique is based on an ultra-short delay-unbalanced interferometer, which leads to a large bandwidth, short lock time, and robust operation even in the absence of any isolation from environmental perturbations. In combination with a digital delay-matched phase error compensation, this permits the achievement of a range window about 60 times larger than the intrinsic laser coherence length with a 1.25 mm Fourier transform-limited spatial resolution. The demonstrated configuration can be easily applied to virtually any semiconductor laser.

Ultrafast and Doppler-free femtosecond optical ranging based on dispersive frequency-modulated interferometry

An ultrafast and Doppler-free optical ranging system based on dispersive frequency-modulated interferometry is demonstrated. The principle is similar to the conventional frequency-modulated continuous-wave interferometry where the range information is derived from the beat frequency between the object signal and the reference signal. However, a passive and static frequency scanning is performed based on the chromatic dispersion of a transform-limited femtosecond pulse in the time domain. We point out that the unbalanced dispersion introduced in the Mach-Zehnder interferometer can be optimized to eliminate the frequency chirp in the temporal interferograms pertaining to the third order dispersion of the all-fiber system, if the dynamic range being considered is small. Some negative factors, such as the polarization instability of the femtosecond pulse, the power fluctuation of the optical signal and the nonuniform gain spectrum of the erbium-doped fiber amplifier lead to an obvious envelope deformation of the temporal interferograms from the Gaussian shape. Thus a new data processing method is proposed to guarantee the range resolution. In the experiment, the vibration of a speaker is measured. A range resolution of 1.59 microm is achieved with an exposure time of 394 fs at a sampling rate of 48.6 MHz.

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.