Full-circle range and microradian resolution angle measurement using the orthogonal mirror self-mixing interferometry

Small-angle measurement system enhanced by an optical phased array

Small-angle measurement can be realized by embedding the laser beam in a reflective sector, within which multiple reflections enlarge the angle between the input and the output beams. However, the maximum detectable angle is limited by the detector aperture at the receiver side. In this work, we propose, to the best of our knowledge, a novel small-angle measurement system enhanced by an optical phased array (OPA), which is loaded on a spatial light modulator (SLM) to increase the maximum measurement range. The experimental results verify the effectiveness of the proposed system, and a wider measurement range with an unaffected measurement accuracy can be obtained. In the proof-of-concept demonstration, the measurement range of the system is enlarged by at least five times compared to the system without OPA, while maintaining the same measurement accuracy.

High-precision rotation angle measurement method based on polarization self-mixing interference

In this study, a high-precision rotation angle measurement method based on polarization self-mixing interference (SMI) is proposed. The higher signal-to-noise ratio SMI signal can be obtained by the differential processing of two polarized SMI signals with opposite phases. In order to reduce the influence of the speckle effect, the envelope signal is used to normalize the SMI signal. The fringe subdivision method is used to improve the accuracy of the rotation angle measurement. The experimental results show that the error of the rotation angle measurement is within ±0.5%, and the measurement range can reach up to 20°.

Sensing by Dynamics of Lasers with External Optical Feedback: A Review

External optical feedback (EOF) has great impacts on the properties of lasers. It influences the stable operation of lasers. However, various applications based on lasers with EOF have been developed. One typical example is self-mixing interferometry technology, where modulated steady-state laser intensity is utilized for sensing and measurement. Other works show that laser dynamics can also be used for sensing, and the laser in this case is more sensitive to EOF. This paper reviews the sensing technology that uses the dynamics of lasers with EOF. We firstly introduce the basic operating principles of a laser with EOF and discuss the noise properties of and intensity modification in lasers induced by EOF. Then, sensing applications using laser dynamics are categorized and presented, including sensing by frequency-shifted optical feedback, relaxation oscillation frequency, and dynamics with self-mixing interferometry signals and laser optical chaos. Lastly, we present an analysis of the transient response waveform and spectrum of a laser with EOF, showing its potential for sensing.

Rotation angle measurement method based on self-mixing interference of a fiber laser

A rotation angle measurement method based on self-mixing interference (SMI) of a fiber laser is proposed. The rotation angle can be calculated indirectly by the displacement measured by SMI. In the experiment, a linear cavity fiber laser with simple structure and high flexibility is used as the optical source for measuring the deflection angle. To improve the measurement accuracy, the SMI signal is filtered by the variational mode decomposition (VMD) algorithm. The filtered SMI signal is normalized by Hilbert transform. The even-power algorithm is used to subdivide the interference fringes, so as to improve the measurement resolution. The experimental result shows that the measurement error of angular shift is less than 1% in the range of 10°.

Suppression of undamped relaxation oscillation in a laser self-mixing interferometry sensing system

Undamped relaxation oscillation (RO) in a laser self-mixing interferometry (SMI) system may occur in some common application conditions, which may impact the stable operation of the system and degrade its sensing performance. In this work, we proposed to suppress the undamped RO by controlling the system operation parameters in a laser SMI sensing system. By numerically solving the famous Lang Kobayashi equations, the stability of a laser SMI system in a 3-parameter space of external cavity length, injection current and optical feedback factor were investigated. Based on the stability analyses, we determined the system operation conditions required for suppressing the undamped RO and derived an analytical expression for describing the relationship between the operation parameters. An experimental SMI system based on a laser diode (Sanyo, DL4140-001s) was implemented and verified the suppressing method. The experimental results showed that the SMI system in a moderate feedback regime can operate in steady state without undamped RO by setting proper operation parameters. This work provides useful guidance to design a stable SMI sensing system for practical applications.

Compact photothermal self-mixing interferometer for highly sensitive trace detection

A self-mixing interferometer combined with the photothermal spectroscopy is utilized as a remarkable sensor for highly sensitive trace detection, featuring the beneficial property of a He-Ne laser with back-mounted photodiode, to the best of our knowledge, acting as an excitation laser, also as a probe laser, and even more, as a detector. Utilizing the novel implementation of the photothermal self-mixing (PTSM) interferometer with an external cavity modulation, the concentration of the sample is directly measured by the PTSM parameter extracted from the PTSM signal. The metrological qualities of the PTSM interferometer were investigated by methylene blue trace detection. For a low excitation power of 5 mW, a 7.7 nM of the limit of detection was achieved with a relative standard deviation of ∼3%. The compact and simple structure with high sensitivity has guiding significance to a robust analytical tool for the analysis of photosensitive compounds and in the detection of aquatic product hazards in aquaculture.

High precision even-power phase modulation method for a self-mixing displacement sensor under the weak feedback regime

A novel method called even-power phase modulation is proposed in a self-mixing displacement sensor to improve measuring accuracy, to the best of our knowledge, which is realized by combining the even-power fast algorithm with the sinusoidal phase-modulation method. By performing the even-power fast algorithm in the self-mixing interference system, the spectrum of harmonic components is broadened. In this case, the extracted first and second harmonic components in the frequency domain contain rich information, and the displacement of the target can be accurately reconstructed. The principle and signal processing approach are introduced in detail, and the simulation results show that the reconstruction error can be effectively reduced compared with the electro-optic modulator phase modulation method. A series of experiments at different vibration amplitudes is conducted to confirm the feasibility and effectiveness of the method. An amplitude of 120 nm is proved to be measurable, and the absolute error is 10 nm, which shows great potential in the field of non-contact nanometer vibration measurement sensors.

Laser Self-Mixing Sensor for Simultaneous Measurement of Young’s Modulus and Internal Friction

The Young’s modulus and internal friction are two important parameters of materials. Self-mixing interferometry (SMI) is an emerging non-destructive sensing method that has been employed for various applications because of its advantages of simple structure, ease of alignment and high resolution. Some recent works have proposed the use of SMI technology to measure the Young’s moduli and/or internal frictions by measuring the resonance frequencies and damping factors of specimen vibrations induced by impulse excitation. However, the measurement results may be affected by frequencies of SMI fringes, and the implementation requires extra signal processing on SMI fringes. In this work, we developed an all-fiber SMI system without SMI fringes to measure the Young’s modulus and internal friction simultaneously. Simulations and experiments were carried out to verify the feasibility of the proposed method. Two specimens of brass and aluminum were tested. The experimental results show that the standard deviations of Young’s moduli for brass and aluminum are 0.20 GPa and 0.14 GPa, and the standard deviations of internal frictions are 4.0×10−5 and 5.4×10−5, respectively. This method eliminates the influences of the SMI fringe frequency on the resonant frequency and requires no signal processing on SMI fringes, contributing to its simplicity as a method for the measurement of the Young’s modulus and internal friction.

Vibration measurement technology based on the reverse point recognition algorithm for laser self-mixing interference

The self-mixing interference (SMI) signal carries the information of the external moving object, which has great physical significance and application prospects for extracting and analyzing the information of the external object. In this paper, we propose a vibration measurement method based on a reverse point recognition algorithm on the SMI laser signal. By extracting and analyzing the hill and valley values of the SMI signal to determine the reverse point, combined with the semifringe counting method, the vibration information of external objects can be accurately extracted. The method we propose simplifies the displacement reconstruction process with high accuracy. The simulation and experimental results show that this method can achieve high-precision measurements of microvibration with an absolute error of less than 19 nm.

Method of reaching a resolution-controllable micro-angle measurement by using a Michelson interferometer

A novel, to the best of our knowledge, method is proposed in this study to permit the controllable resolution of a micro-angle measurement by using a Michelson interferometer. The resolution of the proposed system can be adjusted by changing the distances between a pair of parallel mirrors. Through experiments, it was observed that as the distance was changed from 0 to 6 mm, the corresponding resolution was significantly altered from 22.88 to 14.02 µrad. Compared with other small angle measurement methods, the proposed method can realize the conversion of multiple measurement resolutions more easily and conveniently.

All-fiber laser feedback interferometer using a DBR fiber laser for effective sub-picometer displacement measurement

A fiber autodyne laser feedback displacement sensor based on the effect of the frequency shift is demonstrated. The all-fiber structure enables our system to be easily employed in diverse complex and narrow scenes. By virtue of adopting an ultra-high sensitivity distributed Bragg reflector (DBR) fiber laser as the laser source and the frequency-shift technology to avoid the phenomenon that the measured signal of the low frequency is submerged in the noise, the measurement of the sub-picometer displacement under weak feedback condition has been achieved, which shows a great potential in the field of micro-vibration measurement. Moreover, the proposed system has advantages such as simplicity in system structure, low cost of implementation, and immunity to electromagnetic interference.

Absolute angular measurement with optical frequency comb using a dispersive interferometry

We have demonstrated a simple method to measure high-precision absolute angular displacement using an optical frequency comb (OFC). The dispersive interferometry with parallel configuration can take advantage of its large non-ambiguity range and achieve absolute angular measurement in a large range. The influence factors of the angle accuracy, including the accuracy of optical path difference, the determination of absolute zero position and the correction of sine arm have been analyzed in detail. The angle comparison is performed with the autocollimator and multi-tooth indexing table. The angle accuracy can reach ±2 arcsec (k=2) in the range of 5°, which represents a good agreement with the Monte Carlo simulation. The proposed approach has potential to be extended to multi-degree-of-freedom measurement with a simple structure in future.

All-fiber self-mixing laser Doppler velocimetry with much less than 0.1 pW optical feedback based on adjustable gain

The all-fiber self-mixing laser Doppler velocimetry with adjustable gain is experimentally investigated based on a distributed Bragg reflector fiber laser. In the measurement system, the modulation gain of the injected light in the laser cavity is adjusted by a pair of fiber-coupled acousto-optic modulators (AOMs) in the external cavity, which can change the intensity and frequency of the self-mixing modulation effect. Simultaneously, the minimum feedback intensity from the target to the laser for successful measurements is 0.063 pW. Thus, the all-fiber laser velocimetry can adapt to the detection of ultraweak optical feedback and wide-range velocity measurements in various complex scenes.

Double-grating with multiple diffractions enabled small angle measurement

We demonstrate a small angle measurement setup enabled by the double-grating configuration, where the multiple diffractions are used to magnify the deflection angle of the optical beam. Such small angle measurement setup has characteristics of high sensitivity and compact size. The use of two unparallel blazed gratings with a special included angle can realize multiple diffractions for the incident light, leading to the realization of deflection angle amplification. Experimental results verify that small angle can be accurately characterized and the angular resolution of measurement can be improved more than 40 times by inserting the double-grating configuration into the conventional auto-collimation angle measurement setup. Therefore, the micro-radian angle can be accurately measured with our proposed compact characterization setup.

Design of a multiple self-mixing interferometer for a fiber ring laser

In this Letter, a novel multiple self-mixing interferometer for a fiber ring laser (FRL) is designed by introducing a circular feedback cavity. A system model is established based on an injection-seeded erbium-doped FRL proposed by Dragic. Owing to the reflection of the collimating lens, the multiplied fringes have different depths, which shows two Doppler frequencies in the spectrum of the self-mixing signal. A double-peak frequency identification algorithm is proposed to extract the Doppler frequency from the unique signal. This technique has the potential to improve the accuracy of fiber self-mixing measurement systems, particularly in Doppler velocimeters.