Dongmei Guo

Self-mixing interferometer based on sinusoidal phase modulating technique

A new self-mixing interferometry based on sinusoidal phase modulating 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. Sinusoidal phase modulation of the beam is obtained by an electro-optic modulator (EOM) in the external cavity. The phase of the interference signal is calculated by Fourier analysis method. The interferometer is applied to measure the displacement of a high-precision commercial PZT with an accuracy of <10nm. The measurement range of the system mainly depends on the maximum operating frequency of EOM and the maximum sampling rate of A/D converter.

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.

Development of a sinusoidal phase-shifting self-mixing interferometer for real-time displacement measurement with nanometer accuracy

By combining the principles of sinusoidal phase-shifting technique and self-mixing interferometry, a novel instrument has been developed for real-time displacement measurement with nanometer accuracy. The sinusoidal phase shifting is introduced by an electro-optic modulator and the displacement of the object is measured by analyzing the harmonic components in the interference signal during each phase-shifting period. Theoretical analysis and simulation results are presented and some error sources are discussed in detail. The displacement measurement experiments by using a high precision nano-positioning stage and a long-range linear stage are performed to verify the performance of the interferometer. The measurement results of the interferometer are in good agreement with the data from a commercial dual-frequency interferometer, showing that the development of the interferometer is feasible for implementation in a real-time displacement sensor with high accuracy.

High-accuracy sinusoidal phase-modulating self-mixing interferometer using an electro-optic modulator: development and evaluation

A sinusoidal phase-modulating He-Ne laser subject to weak optical feedback has been used to develop an interferometer that is capable of performing real-time displacement measurement with nanometer accuracy. The principle and the signal processing method are introduced. A commercial dual-frequency interferometer is included in the displacement measurement in both small and large ranges to evaluate the performance of the developed interferometer. Experimental results show that the average errors and standard deviations of the interferometer are in good agreement with data obtained from the commercial interferometer. The resolution and the multiple feedback effect of the interferometer are discussed in detail. These results show that the development of the interferometer is reasonable and feasible.

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 22u2009u2009mm/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.

Compound cavity theory of resonant phase modulation in laser self-mixing ultrasonic vibration measurement

Abstract. The theoretical basis of self-mixing interference (SMI) employing a resonant phase modulator is explored to prove its tempting advantages. The adopted method induces a pure phase carrier without increasing system complexity. A simple time-domain signal process is used to estimate modulation depth and precisely track vibrating trail, which promises the flexibility of measuring ultrasonic vibration regardless of the constraint of the Bessel functions. The broad bandwidth, low speckle noise, compact, safe, and easy operating SMI system obtains the best resolution of a poor reflection environment. Numerical simulation discusses the spectrum broadening and errors due to multiple reflections. Experimental results agree with theory coherently and are compared with laser Doppler vibration meter showing a dynamical error better than 20 nm in ultrasonic vibration measurement.

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.