Heterodyne laser Doppler vibrometers integrated on silicon-on-insulator based on serrodyne thermo-optic frequency shifters

Development and Validation of a New Type of Displacement-Based Miniatured Laser Vibrometers

Developing a miniatured laser vibrometer becomes important for many engineering areas, such as experimental and operational modal analyses, model validation, and structural health monitoring. Due to its compact size and light weight, a miniatured laser vibrometer can be attached to various mobilized platforms, such as an unmanned aerial vehicle and a robotic arm whose payloads can usually not be large, to achieve a flexible vibration measurement capability. However, integrating optics into a miniaturized laser vibrometer presents several challenges. These include signal interference from ghost reflectance signals generated by the sub-components of integrated photonics, polarization effects caused by waveguide structures, wavelength drifting due to the semiconductor laser, and the poorer noise characteristics of an integrated laser chip compared to a non-integrated circuit. This work proposes a novel chip-based high-precision laser vibrometer by incorporating two or more sets of quadrature demodulation networks into its design. An additional set of quadrature demodulation networks with a distinct reference arm delay line length can be used to conduct real-time compensation to mitigate linear interference caused by temperature and environmental variations. A series of vibration measurements with frequencies ranging from 0.1 Hz to 1 MHz were conducted using the proposed laser vibrometer to show its repeatability and accuracy in vibration and ultrasonic vibration measurements, and its robustness to test surface conditions. The proposed laser vibrometer has the advantage of directly measuring the displacement response of a vibrating structure rather than integrating its velocity response to yield the measured displacement with a conventional laser Doppler vibrometer.

All-fiber heterodyne velocity and displacement interferometer based on DPLL Doppler tracking with sub-nanometer per second and picometer sensitivity

Velocity and displacement measurements play an important role not only in the process of industrial production and metrology on the ground but also in satellite gravity measurement in space. A high-precision all-fiber heterodyne velocity and displacement interferometer based on digital phase-locked loop (DPLL) Doppler tracking is proposed in this paper. The target velocity is measured by tracking the heterodyne frequency changes of the beat-note signal, and the displacement is obtained by the integrated phase of the Doppler frequency change. A dual-signal differential optical-path scheme combined with DPLL signal tracking technology enables high-precision and high-linearity measurement of velocity and displacement simultaneously. For integration and compactness, the interferometer uses all-fiber optics that are packaged in a small box with dimensions of 150×150×70m m ³, except for an externally fiber-connected collimator as the sensor head. The experimental results show a velocity sensitivity below 30p m/s/H z ^1/2 in the 0.03-2 Hz band and a displacement sensitivity below 10p m/H z ^1/2 above 0.4 Hz.

Laser Doppler velocimetry for three-dimensional distribution measurement of the velocity component by combining two-dimensional spatial encoding and non-mechanical scanning

We present a differential laser Doppler velocimeter (LDV) for measuring the velocity distribution in a three-dimensional (3D) space. Our conventional research so far has proposed some methods for measuring two-dimensional (2D) distribution measurement. One of the proposed methods was spatial encoding of measurement points arranged on a 2D plane. Besides, we also have proposed laser Doppler cross-sectional velocity distribution measurement based on a non-mechanical scanning method. We propose a 3D method by combining 2D spatial encoding and non-mechanical scanning, in which the measurement points distributed on a 2D plane are spatially encoded with different bias frequencies, and these points are scanned non-mechanically in another direction by changing the wavelength. As a feasibility study of the proposed method, experiments were conducted using a 4×4 channel optical setup and were performed at five wavelengths over 1537-1553 nm. The experimental results indicate that the proposed method could successfully measure the 3D distribution of the velocity component. The spectral peaks of the beat signals for 68 measurement points for the rotational speed of the target of 2.0s^-1 and those for 67 measurement points for the rotational speed of -2.0s^-1 out of 80 measurement points were successfully observed within the measurement error of 2.8%-4.8%.

Miniaturization of Laser Doppler Vibrometers-A Review

Laser Doppler vibrometry (LDV) is a non-contact vibration measurement technique based on the Doppler effect of the reflected laser beam. Thanks to its feature of high resolution and flexibility, LDV has been used in many different fields today. The miniaturization of the LDV systems is one important development direction for the current LDV systems that can enable many new applications. In this paper, we will review the state-of-the-art method on LDV miniaturization. Systems based on three miniaturization techniques will be discussed: photonic integrated circuit (PIC), self-mixing, and micro-electrochemical systems (MEMS). We will explain the basics of these techniques and summarize the reported miniaturized LDV systems. The advantages and disadvantages of these techniques will also be compared and discussed.

Doppler velocimeter and vibrometer FMCW LiDAR with Si photonic crystal beam scanner

In this paper, we propose and demonstrate a frequency-modulated continuous-wave light detection and ranging (LiDAR) with a Si photonic crystal beam scanner, simultaneously enabling scanning laser Doppler measurements. This nonmechanical solid-state device can reduce the size of conventional scanning laser Doppler vibrometers, making LiDAR a multimodal imaging sensor, which can measure the distributions of distance, velocity, and vibration frequency. We fabricated this device using Si photonics process and confirmed the expected operations. Distance and velocity resolutions were less than 15 mm and 19 mm/s, respectively. The detection limit of the vibration amplitude determined by the signal-to-noise ratio was 2.5 nm.

Proposal for an integrated silicon-photonics terahertz gas detector using photoacoustics

A design and multiphysical model is presented for an on-chip gas sensor that transduces terahertz gas absorption through sound generation into a mechanical motion that can be read out externally. The signal is triply enhanced by designing a structure that functions simultaneously as an optical, an acoustical and a mechanical resonator. The structure is made in high-resistivity silicon and can be fabricated using CMOS and MEMS fabrication technologies. The sensor is a purely passive element, so an external THz source and read-out are required. The chip has a footprint of 3 mm². A detection limit of 234 ppb of methanol for a source power of 1 mW and an integration time of 1 ms is predicted.

Effect of oblique retroreflection from a vibrating mirror on laser Doppler shift

A laser Doppler vibrometer (LDV) fails to measure a large out-of-plane vibration of a rotating mirror when the mirror obliquely reflects the laser beam away, causing a signal loss from being detected. To solve this problem, an external retroreflective tape was used to recover the oblique reflection. However, the reading of LDV obtained from the recovered signal is not right because the retroreflection adds extra Doppler frequency shifts to the oblique reflection. Here, we first derive the relationship of Doppler shift to the oblique angle of retroreflection. For the first time with the help of retroreflection, a standard LDV can measure the largely vibrating mirror as well as a high-speed camera, albeit without the need for heavy computation.

Six-beam homodyne laser Doppler vibrometry based on silicon photonics technology

This paper describes an integrated six-beam homodyne laser Doppler vibrometry (LDV) system based on a silicon-on-insulator (SOI) full platform technology, with on-chip photo-diodes and phase modulators. Electronics and optics are also implemented around the integrated photonic circuit (PIC) to enable a simultaneous six-beam measurement. Measurement of a propagating guided elastic wave in an aluminum plate (speed ≈ 909 m/s @ 61.5 kHz) is demonstrated.

Silicon photonic integrated circuit for fast and precise dual-comb distance metrology

We demonstrate an optical distance sensor integrated on a silicon photonic chip with a footprint of well below 1 mm². The integrated system comprises a heterodyne receiver structure with tunable power splitting ratio and on-chip photodetectors. The functionality of the device is demonstrated in a synthetic-wavelength interferometry experiment using frequency combs as optical sources. We obtain accurate and fast distance measurements with an unambiguity range of 3.75 mm, a root-mean-square error of 3.4 µm and acquisition times of 14 µs.

Nonmechanical compact probe for cross-sectional velocity measurement based on differential laser Doppler velocimetry

In this study, we propose and demonstrate a nonmechanical compact probe for cross-sectional velocity measurement based on differential laser Doppler velocimetry. The system introduces a method that combines simultaneous multipoint measurement using spatial encoding and nonmechanical scanning of measurement points, in which spatially encoded measurement points aligned along the transverse direction are scanned in the axial direction by changing the wavelength. The use of a waveguide-type LiNbO₃ phase shifter array for serrodyne frequency shifting is feasible for the system based on fiber optics with an easily handled probe. To miniaturize the probe, a multimode fiber is introduced in the receiving optics and the parameters of the lens system in the transmitting optics are optimized. For the experiment, an eight-channel probe was assembled on an aluminum plate with an 8 cm × 8 cm area size. The experimental results reveal that the cross-sectional two-dimensional velocity distribution was successfully measured using the easily handled compact probe for the first time.

All-Fiber Configuration Laser Self-Mixing Doppler Velocimeter Based on Distributed Feedback Fiber Laser

In this paper, a novel velocimeter based on laser self-mixing Doppler technology has been developed for speed measurement. The laser employed in our experiment is a distributed feedback (DFB) fiber laser, which is an all-fiber structure using only one Fiber Bragg Grating to realize optical feedback and wavelength selection. Self-mixing interference for optical velocity sensing is experimentally investigated in this novel system, and the experimental results show that the Doppler frequency is linearly proportional to the velocity of a moving target, which agrees with the theoretical analysis commendably. In our experimental system, the velocity measurement can be achieved in the range of 3.58 mm/s–2216 mm/s with a relative error under one percent, demonstrating that our novel all-fiber configuration velocimeter can implement wide-range velocity measurements with high accuracy.

Integrated optical frequency shifter in silicon-organic hybrid (SOH) technology

We demonstrate for the first time a waveguide-based frequency shifter on the silicon photonic platform using single-sideband modulation. The device is based on silicon-organic hybrid (SOH) electro-optic modulators, which combine conventional silicon-on-insulator waveguides with highly efficient electro-optic cladding materials. Using small-signal modulation, we demonstrate frequency shifts of up to 10 GHz. We further show large-signal modulation with optimized waveforms, enabling a conversion efficiency of -5.8 dB while suppressing spurious side-modes by more than 23 dB. In contrast to conventional acousto-optic frequency shifters, our devices lend themselves to large-scale integration on silicon substrates, while enabling frequency shifts that are several orders of magnitude larger than those demonstrated with all-silicon serrodyne devices.

Cross-sectional laser Doppler velocimetry with nonmechanical scanning of points spatially encoded by multichannel serrodyne frequency shifting

A laser Doppler velocimeter (LDV) using nonmechanical scanning of multiple points spatially encoded by multichannel serrodyne frequency shifting is proposed for cross-sectional velocity distribution measurement. In the proposed LDV, nonmechanical scanning using wavelength change and simultaneous multipoint measurement using spatial encoding are combined. The use of a LiNbO3 phase-shifter array for multichannel serrodyne modulation makes it possible to simplify the generation of a spatially encoded beam array. An experiment was performed using a sensor probe setup with a six-channel beam array. The results indicate that two-dimensional velocity distribution measurement was successfully demonstrated.

On-chip laser Doppler vibrometer for arterial pulse wave velocity measurement

Pulse wave velocity (PWV) is an important marker for cardiovascular risk. The Laser Doppler vibrometry has been suggested as a potential technique to measure the local carotid PWV by measuring the transit time of the pulse wave between two locations along the common carotid artery (CCA) from skin surface vibrations. However, the present LDV setups are still bulky and difficult to handle. We present in this paper a more compact LDV system integrated on a CMOS-compatible silicon-on-insulator substrate. In this system, a chip with two homodyne LDVs is utilized to simultaneously measure the pulse wave at two different locations along the CCA. Measurement results show that the dual-LDV chip can successfully conduct the PWV measurement.

Homodyne laser Doppler vibrometer on silicon-on-insulator with integrated 90 degree optical hybrids

A miniaturized homodyne laser Doppler vibrometer (LDV) with a compact 90° optical hybrid is experimentally demonstrated on a CMOS compatible silicon-on-insulator (SOI) platform. Optical components on this platform usually have inadequate suppressions of spurious reflections, which significantly influence the performance of the LDV. Numerical compensation methods are implemented to effectively decrease the impact of these spurious reflections. With the help of these compensation methods, measurements for both super-half-wavelength and sub-half-wavelength vibrations are demonstrated. Results show that the minimal detectable velocity is around 1.2 μm/s.