Hui Hao

A Hybrid Mach–Zehnder Interferometer for Refractive Index and Temperature Measurement

We fabricate and experimentally demonstrate a hybrid structured in-line Mach-Zehnder interferometer (MZI) for simultaneous measurement of refractive index (RI) and temperature. The MZI is composed of an embedded slender air cavity in a micro fiber cascaded to a piece of photonic crystal fiber (PCF). The interferometer is fabricated by fusion splicing and fused tapering. The interference spectra based on the air cavity and the PCF can be extracted through low-pass and band-pass filters, respectively. RI and temperature are interrogated through a coefficient matrix simultaneously by tracing two-dip wavelength shifts of the interference spectra. Novel structure, easy fabrication, simple system, and simultaneous measurement make it appropriate for dual-parameter sensing application.

Displacement measurement using a laser feedback grating interferometer

A novel laser feedback grating interferometer (LFGI) with a phase-generated carrier demodulation technique is proposed in this paper. Laser feedback grating interference occurs when light emitted from a laser is diffracted by the double-diffraction system and re-enters the laser active cavity, thus generating a modulation of the lasing field. In order to improve the displacement measurement resolution, phase modulation is introduced by an electro-optic modulator in the external cavity of the LFGI. Detection of the displacement can be easily achieved by the time-domain orthogonal demodulation, which does not involve any complicated calculation and is insensitive to the sampling error. Experimental results show that the proposed system has a general nanometer-level resolution. It provides a potential displacement sensor with high resolution, simple mechanical structure, and good reliability.

Sinusoidal phase-modulating self-mixing interferometer with nanometer resolution and improved measurement velocity range.

A new signal-processing method based on an electronic frequency down-conversion technique has been introduced into a sinusoidal phase-modulating, self-mixing interferometer. The developed interferometer employs an electro-optical crystal placed in the external cavity of a He-Ne laser to generate the sinusoidal phase modulation with high modulation rate and ultralow insertion loss. Phase quadrature signals which have been amplitude-modulated by the sine and cosine functions, respectively, of the measured displacement can be extracted from the high-density optical fringes through the use of dual-channel multiplier/filter circuits. Therefore, the displacement of the external target can be retrieved from the phase quadrature signals with nanometer resolution and high computational efficiency. Moreover, a much-improved measurement speed from 2.5 to 22  mm/s has been realized owing to the simplified signal-processing method. The performance of the proposed interferometer has been experimentally verified by comparison to an Agilent 5529A dual-frequency laser interferometer. The measurement results from the two instruments agree well, and we therefore expect that our new technique offers a powerful instrument for high-speed metrology sciences.

Micro-vibration measurement using self-mixing interferometer based on temporal carrier phase shifting technique

A new self-mixing interferometer (SMI) based on temporal carrier phase shifting technique is presented. Self-mixing interference occurs in the laser cavity by reflecting the light from a mirror-like target in front of the laser. Triangular phase modulation of the beam is obtained by an electrooptic modulator (EOM) in the external cavity. The phase of SMI signal coming from the photodetector is extracted by the phase shift demodulation algorithm based on sampling technique. Theoretical analysis and simulation calculations are presented and some errors of this method are discussed. The interferometer is applied to measure the Micro-vibration of a high-precision commercial PZT with an accuracy of <10nm.

Self-mixing birefringent dual-frequency laser Doppler velocimeter

A self-mixing birefringent dual-frequency laser Doppler velocimeter (SBD-LDV) for high-resolution velocity measurements is presented in this paper. The velocity information of the object can be accurately extracted from the self-mixing Doppler frequency shift of the birefringent light-carried microwave signal. We generate a virtual stable light-carried microwave by using a birefringent dual-frequency He-Ne laser which further simplifies the structure of the light source. Moreover, the optical configuration based on the laser self-mixing interference brings benefits of compact optical setup, self-alignment, and direction discriminability. Experimentally, we extracted the Doppler beat frequency signal by the low-frequency (millihertz) phase lock-in amplifier, measured the beat frequency precisely in time-domain with a low sampling rate and calculated the magnitude of velocity. Compared with the previous self-mixing LDV, the average velocity resolution of SBD-LDV is improved to 0.030 mm/s for a target with longitudinal velocity, benefiting from the high stability of light-carried microwave. It is of great meaning and necessity because it helps to provide an available velocimeter with high stability and an extremely compact configuration, making a potential contribution to the velocimetry in practical engineering application.

Self-mixing interference effect of VCSEL and the application on microdisplacement measurement

The Self-Mixing Interference (SMI) effect of a Vertical-Cavity Surface-Emitting Laser (VCSEL) is studied in this paper. The analysis and experiment are presented to verify the dynamics of the VCSEL. The phenomenon is observed and contrasted with traditional interference phenomenon. The output property of VCSEL is modulated by the change of cavity length and feedback intensity. An interferometer using VCSEL self-mixing based on temporal carrier phase shifting technique is studied. Theoretical analysis and simulation calculations are presented and some errors of this method are discussed.

Birefringent dual-frequency laser Doppler velocimeter using a low-frequency lock-in amplifier technique for high-resolution measurements

A birefringent dual-frequency laser with a half-intracavity has been used to develop a laser Doppler velocimeter (LDV). The developed LDV utilizes a new signal-processing method based on a lock-in amplifier to achieve high-resolution velocity measurements and the discrimination of positive and negative velocities. Theoretical analysis and simulation results are presented. The velocity measurement experiments by using a high-precision linear stage are performed to verify the performance of the LDV. Compared with the previous dual-frequency LDVs, the average velocity resolution of the developed LDV is improved from 0.31 mm/s to 0.028 mm/s for a target without the rotational velocity. The measurement results show that our new technique can offer a powerful instrument for metrology sciences.

Implementation of real-time displacement precision measurement technology for the sinusoidal phase-shifting laser self-mixing interferometer

An advanced sinusoidal phase-shifting technique and a time-domain phase demodulation method were used to improve the measurement accuracy and realize the real-time measurement speed of the laser self-mixing interferometer in a large range of displacement. An electro-optic crystal modulator (EOM) was used to realize the sinusoidal phase-shifting on the laser beam in the external cavity. The interference signal was demodulated using a time-domain phase demodulation method. The speed requirement could be met by combining the two together in a wide range of displacement measurement processes together with the real-time measurement requirement as an interferometer at the same time. It was experimentally verified that the displacement measurement precision of a sinusoidal phase-shifting laser self-mixing interferometer could reach less than 0.5 μm in the hundred mm large-scale displacement measuring process. In addition, the factors affecting the interferometer’s measurement speed in the real-time displacement measurement process is analyzed and the maximum speed of our system was obtained as well. Keywords: self-mixing interference; phase modulation; time-domain phase demodulation

Note: Simultaneous measurement of in-plane and out-of-plane displacement by using orthogonally polarized self-mixing grating interferometer

In this paper, we present an orthogonally polarized self-mixing grating interferometer (SMGI) for simultaneous measurement of in-plane and out-of-plane displacements. The measurement ranges in both directions are limited only by the length of grating. The orthogonally polarized lights emitted from a birefringent He-Ne laser are separated and enter the grating at ±1st-order Littrow angles. The diffraction beams re-enter the laser cavity and cause self-mixing interference. To differentiate the orthogonally polarized lights and obtain high resolution, phase modulation technique is introduced to extract phases from the orthogonally polarized SMGI signals. The measurement results show that the proposed system can reach a submicron accuracy in the experiment. This work provides a good way to achieve high precision two-dimensional displacement measurement with a robust system configuration.

An optical fiber MEMS pressure sensor using microwave photonics filtering technique

A fiber-optic micro-electromechanical systems (MEMS) extrinsic Fabry-Perot interferometer (EFPI) pressure sensor exploiting microwave photonics filtering technique is firstly proposed and experimentally demonstrated. A single-bandpass microwave photonic filter (MPF) which mainly consists of a spectrum-sliced light source, a pressurized EFPI, a phase modulator (PM) and a length of dispersion compensating fiber (DCF) is demonstrated. The frequency response of the filter with respect to the pressure is studied. By detecting the resonance frequency shifts of the MPF, the pressure can be determined. The theoretical and experimental results show that the proposed EFPI pressure sensor has a higher resolution and higher speed than traditional methods based on optical spectrum analysis. The sensitivity of the sensor is measured to be as high as 86 MHz/MPa in the range of 0–4MPa.