Homodyne Laser Research Articles

Measurement Accuracy Improvement of Homodyne Laser Diode Interferometer by Real-Time Nonlinear Error Suppressing

Measurement Accuracy Improvement of Homodyne Laser Diode Interferometer by Real-Time Nonlinear Error Suppressing

A Compensation Method for Nonlinearity Errors in Optical Interferometry.

Optical coherent detection is widely used for highly sensitive sensing applications, but nonlinearity issues pose challenges in accurately interpreting the system outputs. Most existing compensation methods require access to raw measurement data, making them not useful when only demodulated data are available. In this study, we propose a compensation method designed for direct application to demodulated data, effectively addressing the 1st and 2nd-order nonlinearities in both homodyne and heterodyne systems. The approach involves segmenting the distorted signal, fitting and removing baselines in each section, and averaging the resulting distortions to obtain precise distortion shapes. These shapes are then used to retrieve compensation parameters. Simulation shows that the proposed method can effectively reduce the deviation caused by the nonlinearities without using the raw data. Experimental results from a silicon-photonics-based homodyne laser Doppler vibrometry prove that this method has a similar performance as the conventional Heydemann correction method.

Real-time acquisition and enhancement of remote acoustic signals by a free-space monostatic homodyne laser Doppler vibrometer.

This paper systematically presents the design and performance of an extremely sensitive 1.55-µm free-space monostatic laser Doppler vibrometer (LDV) using optical homodyne detection for real-time acquisition and enhancement of the remote acoustic signals. The phase shifts produced by laser light scattered off a remote target carries the extremely tiny vibration displacement information of the target' surface motivated by the acoustic source around and is demodulated using the optical in-phase/quadrature demodulator. The real-time acquisitions of the remote acoustic signals, including the sinusoidal signal and the speech signal at the target distance of 100m, is performed between two buildings. The real-time speech enhancement of remote speech signals is also carried out by the different algorithms based on the short-time spectral magnitude, and the comprehensible speech signals can be reconstructed. The results demonstrate that the designed free-space monostatic homodyne LDV has a low system background noise and can offer high precision for the uncooperative targets in the real-time acquisition of the remote acoustic signal.

Homodyne laser vibrometer modified by an LCVR for measurement at the nanometer level.

The existence of periodic nonlinear error restricts the performance of the homodyne laser vibrometer in sub-fringe amplitude vibration measurement. A homodyne laser vibrometer with nanoscale-amplitude detectability by using a liquid crystal variable retarder (LCVR) is proposed. The LCVR introduces an extra variation of optical path difference larger than the laser wavelength to acquire a full ellipse so that the nonlinearity correction parameters could be pre-extracted. The experiments showed that the nonlinear error could be well suppressed with the correction process based on the pre-extracted parameters, and the detectable minimum amplitude is less than 1 nm. In addition, measurement of vibration with the reflectivity of measured targets down to 0.048% was achieved with an automatic-gain-control module.

Influence of optical amplifiers for on-chip homodyne laser Doppler vibrometers

Photonic integration allows for the development of compact and relatively cheap laser Doppler vibrometers (LDVs). Optical amplification could enhance the performance of these on-chip LDVs, but the intrinsic noise of optical amplifiers could be detrimental for the limit of detection (LOD) of the displacement of the LDV. Recent developments in heterogeneous integration of semiconductor optical amplifiers (SOAs) on the silicon-on-insulator platform allow for compact and relatively cheap development of on-chip LDVs with SOAs. In this paper, we study the influence of an SOA on the LOD of an on-chip homodyne laser doppler vibrometer. We describe the influence of the shot noise and SOA-noise on the LOD of the vibrometer. SOA intrinsic phase noise depends on the optical power in the SOA, causing the position of the SOA to have a large influence on the performance. From the analysis, it is shown that SOAs can improve the performance of an homodyne LDV when there are large losses in the measurement arm. These losses occur due to the coupling of light from the chip towards the target and back into the on-chip waveguides.

Modified homodyne laser interferometer based on phase modulation for simultaneously measuring displacement and angle.

Many researchers from scientific and industrial fields have devoted their efforts to the laser interferometer, aiming to improve the measurement accuracy and extend the practical applications. Here, we present a modified homodyne laser interferometer based on phase modulation for simultaneously measuring displacement and angle. The active sawtooth wave phase modulation enhances immunity of this interferometer to the environmental fluctuations and laser power drift. Based on polarized optic theory and the sinusoidal measurement retro-reflector, a modified Michelson-type interferometer configuration is designed to simultaneously measure displacement and angle. Phase difference between the reference and measurement interference signals can be obtained using the sawtooth wave phase modulation and zero crossing detection technique, where the real-time displacement and angle values can be derived directly. Experimental results demonstrate our proposed interferometer has good static and dynamic performance.

An interferometric vibrometer using phase carrier generated by liquid surface acoustic waves and an improved phase demodulation scheme

A novel phase carrier generation method for a sinusoidal phase-modulating laser homodyne interferometric vibrometer is proposed in this paper. In this homodyne interferometer scheme, phase modulation of the reference arm is achieved with liquid surface acoustic waves, a technique that is generally considered to be low in cost. The mathematical model for vibration detection signals using the proposed interferometer scheme is analyzed. To facilitate application of the new interferometer scheme, an improved phase-generated carrier arctangent (PGC-Arctan) approach algorithm based on real-time normalization of orthogonal interference signal pair is proposed to eliminate non-linear demodulation errors due to changes of interference signal parameters, such as changes of carrier phase delay and signal visibility caused by thermal capillary waves on liquid surface, and changes of phase modulation depth caused by amplitude fluctuations of liquid surface acoustic waves (LSAWs). In this method, the amplitude attenuation of low frequency components in the vibration detection signal after carrier frequency shift is used to estimate the phase modulation depth, and an orthogonal carrier signal is introduced to estimate the carrier phase delay in real time. Finally, an orthogonal interference signal pair is normalized by using the estimated values of the phase modulation depth and carrier phase delay. The feasibility of the proposed method is proved by numerical simulation and vibration measurement experiments. The experimental results show that the average THD is -43.82 dB for the sinusoidal vibration with the frequency of 10 Hz.

Nonlinear signal errors in homodyne laser Doppler vibrometry induced by strong second-order ghost reflections and their mitigation.

A variety of mechanisms can induce distortions in the output signals of a homodyne laser Doppler vibrometer (LDV). In this paper, the nonlinear LDV distortions caused by a strong second-order ghost reflection originating from lens flares are theoretically explained and analyzed. We propose a simple compensation method to mitigate this distortion. The performance and limitations of this method are also explained both in simulation and in experiment.

Calibration of laser Doppler vibrometer and laser interferometers in high-frequency regions using electro-optical modulator

We proposed a new primary calibration method of a laser Doppler vibrometer (LDV) using the optical modulated excitation of an electro-optical modulator and validated this approach by comparing mechanical modulated excitation with homodyne laser interferometer at the National Metrology Institute of Japan (NMIJ). The velocity sensitivity of the LDV was evaluated with the uncertainty budget at a high-frequency range up to 1 MHz, which is indicated as the upper limit of 50 kHz in ISO (International Organization for Standardization) 16063–41. As the results, we confirmed that velocity sensitivities between optical and mechanical modulated excitations are flat within their uncertainty from 100 Hz up to 1 MHz. Moreover, in order to evaluate the accurate phase shift of the accelerometer up to 20 kHz using optical modulated excitation, we also revealed the reliability of two time shifts between homodyne and heterodyne laser interferometers.

Undersampling homodyne I/Q interferometry for ultra-low-frequency vibration calibration

In order to overcome the limitations associated with existing homodyne laser interferometry for ultra-low-frequency vibration calibration, undersampling homodyne I/Q interferometry is proposed in this letter. Limitation of time-consuming quadrature error compensation algorithm is avoided by using hardware error compensation with a specially designed homodyne interferometry optical configuration, while limitation of high Nyquist frequency is avoided by applying Kalman I/Q signal unwrapping with deeply under-sampled fringe phase signals. The effectiveness of the proposed method is verified through simulation and experiment. Experimental results indicate that sampling frequency can be reduced by 30 times for movement measurement at 1 m/s while quadrature error is compensated in real-time. It can, therefore, be concluded that the proposed method is effective for ultra-low-frequency vibration calibration.

Five-channel fiber-based laser Doppler vibrometer for underwater acoustic field measurement.

In this paper, a 1550 nm five-channel all-fiber homodyne laser Doppler vibrometer with high sensitivity and good signal probing probability is presented. Under the anechoic tank, standing on an airborne platform above the water surface 3 m away, the calibration experiments of the designed system are conducted. The minimum detectable sound pressure level is up to 101.73 dB re 1 µPa at 10 kHz under the hydrostatic water surface condition, and the time distribution of the final outputs are consistent with that of the underwater sound transducer. For the hydrodynamic detection capability, with the help of a 1064 nm high-pulse-energy laser whose pulse energy is 6J, pulse duration is about 8 ns, and repetition rate is 1 Hz, the system performance is tested in Qiandao Lake. And the signal probing probability of the whole sensing system is up to 59.77%.

An On-Orbit Dynamic Calibration Method for an MHD Micro-Angular Vibration Sensor Using a Laser Interferometer.

The magnetohydrodynamic (MHD) micro-angular vibration sensor is a significant component of the MHD Inertial Reference Unit (MIRU) and measures micro-amplitude and wide frequency angular vibration. The MHD micro-angular vibration sensor must be calibrated in orbit since the ground calibration parameters may change after lift-off. An on-orbit dynamic calibration method for the MHD micro-angular vibration sensor is proposed to calibrate the complex sensitivity of the sensor in high frequency. An absolute calibration method that combines a homodyne laser interferometer and an angular retroreflector was developed. The sinusoidal approximation method was applied, and the calibration system was established and tested using a manufactured MHD sensor. Furthermore, the measurement principle and installation errors were analyzed, including the eccentric installation error of the retroreflector, the tilt installation error of the retroreflector, and the optical path tilt error. This method can be realized within a rotation range of and effectively avoid the installation error caused by mechanical errors. The results indicate that the calibratable angular vibration frequency range is 25–800 Hz, and the angular velocity range is –7590 mrad/s. The expanded uncertainties of the sensitivity amplitude and phase shift of the calibration system for the MHD micro-angular sensor are and .

Homodyne Laser Vibrometer With Detectability of Nanoscale Vibration and Adaptability to Reflectivity

Homodyne laser vibrometer plays an essential role in the precision vibration measurement, but its application is restricted when measuring subfringe vibration and targets with various reflectivities. In order to achieve the nanoscale vibration measurement and to adapt to various reflectivities, an enhanced homodyne laser vibrometer is presented in this paper. First, an active vibration mechanism (AVM) based on a flexure hinge stimulated by a piezoelectric transducer (PZT) is introduced into the reference arm, with which the parameters for nonlinear error correction can be predetermined, and thereby the measurement of vibration less than quarter wavelength is feasible. Second, an automatic gain module is employed to optimize the intensity of the interference signals, which significantly improves the adaptability to target reflectivity. The experimental results show that the laser vibrometer proposed is capable of measuring vibration with amplitude down to 2 nm and vibration with surface reflectivity down to 0.08%.

Fringe chasing by three-point spatial phase shifting for discrimination of the motion direction in the long-range homodyne laser Doppler vibrometry

In this work, a three-point spatial phase shifting (SPS) method is implemented for chasing of the moving interference fringes in the homodyne laser Doppler vibrometry (HoLDV). By the use of SPS method, we remove disability of the HoLDV in the discrimination of the motion direction for long-range displacements. From the phase increments histogram, phase unwrapping tolerance value is selected, and adequacy of the data acquisition rate and required bandwidth limit are determined. Also in this paper, a detailed investigation on the effect of detectors positioning errors and influence of the Gaussian profile of the interfering beams on the measurements are presented. Performance of the method is verified by measuring a given harmonic vibration produced by a loudspeaker. Also, by the proposed method, vibration of mounting system of a disk laser gain medium is characterized.

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.

Potential Accuracy of Estimation of the Information Parameter of a Homodyne Laser Doppler Vibrometer Signal

This paper describes the analysis of the potential accuracy of estimation of the signal modulation index of a homodyne laser Doppler vibrometer with frequency modulation and known and unknown non-information parameters of the signal. The Rao — Cramer inequality is used to obtain expressions for calculating the lower bound of variance of the modulation index estimate and study its relationship with a signal/noise mixture. Recommendations on the choice of conditions for measuring the amplitude of vibro-displacement by the homodyne Doppler laser vibrometer are given.

Performing accelerometer calibration using homodyne laser interferometry with displacements smaller than a quarter wavelength

<p class="Abstract"><span lang="EN-US">Laser interferometry is a preferred method of realizing the National Measurement Standard for vibration by National Metrology Institutes. Laser interferometry is well documented and its detailed implementation described in ISO 16036, part 11. When employing Homodyne demodulation of the quadrature signals, the minimum peak to peak displacement is limited at a quarter of the wavelength, or half a circle when viewed on a lissajous plot. A novel technique is described to extend the vibration calibration frequency range from 10 kHz up to 20 kHz with an applied acceleration of 200 m/s². </span></p>

Periodic error evaluation system for linear encoders using a homodyne laser interferometer with 10 picometer uncertainty

An ultraprecision system to evaluate the periodic error of linear encoders (PEL) has been developed. Our system is based on a homodyne laser interferometer as a reference scale. Conventional studies which used an interferometer with unknown periodic error resulted in insufficient reliability because the periodic error of an interferometer (PEI) directly overlaps on the evaluated PEL. In this study, we explicitly reveal the homodyne interferometer in the proposed system has a sufficiently small PEI of less than 10 pm. This PEI is determined by analyzing the amplitude variation of interference signals, which we refer to as “self-evaluation method.” We have demonstrated the applicability of the proposed system through an actual evaluation of one of the most precise linear encoders with a PEL of sub-nanometer order.

Establishment of KRISS watt balance system to have high uniformity performance

Watt balance system is the promising equipment to determine Planck constant without accurate measurement of magnetic flux and coil length. To obtain high measurement uncertainty, uniform magnetic flux density and constant velocity control are required during the coil movement. For that KRISS watt balance system incorporates a closed-type cylindrical permanent magnet, vertical linear motor and piston guide. In this paper, the vertical actuating motor including piston guide is controlled to have constant speed and magnet assembly was fabricated to have high uniform magnetic flux density during the vertical coil movement. The velocity of moving coil was controlled within 4 × 10-3 (1σ) at the velocity of 2 mm s-1 using a linear motor and linear encoder feedback. Using 3-vertical homodyne laser interferometer the velocity ripple of coil located inside magnet is measured within the level of 7.35 × 10-3 (1σ) at the same speed. A magnet assembly, which consists of two permanent magnet rings and closed yokes, was fabricated. A flux shunt was designed for the magnet for minimizing the temperature effect. The yokes were divided into three parts: top, center, and bottom yokes. By re-machining of center outer yoke, magnetic flux density uniformity was improved. The uniformity of the magnetic field was within 3.9 × 10-4 over a vertical distance of 20 mm.

Using a heterodyne vibrometer in combination with pulse excitation for primary calibration of ultrasonic hydrophones in amplitude and phase

A high-frequency vibrometer was used with ultrasonic pulse excitation in order to perform a primary hydrophone calibration. This approach enables the simultaneous characterization of the amplitude and phase transfer characteristic of ultrasonic hydrophones. The method allows a high frequency resolution in a considerably short time for the measurement. Furthermore, the uncertainty contributions of this approach were investigated and quantified.A membrane hydrophone was calibrated and the uncertainty budget for this measurement was determined. The calibration results are presented up to . The measurement results show good agreement with the results obtained by sinusoidal burst excitation through the use of the vibrometer and by a homodyne laser interferometer, with RMS deviation of approximately –4% in the frequency range from to . Further hydrophones were characterized up to with this procedure to demonstrate the suitability for very high frequency calibration.