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In this paper, a novel optical rotary sensor based on laser self-mixing interferometry is developed for the full-circle rotation measurement. The proposed sensor is convenient to use for it does not need any contact with the target or a cooperative mirror. A prototype is fabricated and tested. The measured results demonstrate a good performance compared with other optical rotary sensors, in terms of the 0.1 μrad resolution, the 2.33×10-4 linearity and 2 μrad stability over one hour. Additionally, the repeatability error is below 14.66 mrad under 9-group full-circle tests, which exhibits the potential to be instrumentalized reliably. Error analysis and limitation discussion have been also carried out. Although the accuracy needs further improvement compared with the best rotary sensor, this method has its unique advantages of high resolution, non-cooperative target sensing and electromagnetic immunity. Hence, the proposed optical rotary sensor provides a promising alternative in precise rotation measurement, tremor tracing and nano-motion monitoring.In this paper, a novel optical rotary sensor based on laser self-mixing interferometry is developed for the full-circle rotation measurement. The proposed sensor is convenient to use for it does not need any contact with the target or a cooperative mirror. A prototype is fabricated and tested. The measured results demonstrate a good performance compared with other optical rotary sensors, in terms of the 0.1 μrad resolution, the 2.33×10-4 linearity and 2 μrad stability over one hour. Additionally, the repeatability error is below 14.66 mrad under 9-group full-circle tests, which exhibits the potential to be instrumentalized reliably. Error analysis and limitation discussion have been also carried out. Although the accuracy needs further improvement compared with the best rotary sensor, this method has its unique advantages of high resolution, non-cooperative target sensing and electromagnetic immunity. Hence, the proposed optical rotary sensor provides a promising alternative in precise rotation measurement, tremor tracing and nano-motion monitoring.

A compact cantilever-based voice-eavesdropping microresonator is proposed. The dynamic vibration of the cantilever-microresonator system induced by an applied acoustic pressure is converted into a modulation of the cavity resonances. Then, the resonance-shift-induced optical transmission change is utilized to detect the applied acoustic signals. The enhancement of both the vibration and optical response makes the proposed system highly sensitive and have better performance than many other optical acoustic sensors. The noise equivalent pressure of the proposed system at 900 Hz is 52 μPa/Hz1/2. The system has a favorable broad frequency response range covering 0∼6 kHz. Experimental results prove that the system can acquire and reconstruct a voice signal 7 m away with high sensitivity and reliability, which can meet a variety of needs, such as indoor eavesdropping. Furthermore, the system is easy to fabricate, has low power consumption and is fairly tunable in terms of the frequency response range and sensitivity. This microcavity-based novel acoustic sensor is promising for opening up new possibilities in intrusion detection, voice eavesdropping and many other vibro-acoustic analysis applications.

The development of high-temperature photodetectors can be beneficial for numerous applications, such as aerospace engineering, military defence and harsh-environments robotics. However, current high-temperature photodetectors are characterized by low photoresponsivity (6 A/W, which is ~5 orders of magnitude higher than that of state-of-the-art high-temperature photodetectors. Furthermore, our devices demonstrate good flexibility, making it highly adaptive to various shaped surfaces. Our approach can be extended to other 2D materials and may stimulate further developments of 2D optoelectronic devices operating in harsh environments.

In this paper, the phase response of fiber Fabry-Pérot cavity-based fiber optic microphones (FFPC-FOMs) is discussed through an analysis of the results of simulation and experiments. The phase difference of FFPC-FOMs mainly originates from two aspects: different phase lags of the mechanical-acoustic systems and different quadrature working points (Q*) on interference curves. The former is analyzed by an impedance-type analogous circuit, and the simulation results reveal that the change in cavity length and resonance frequency in a large range have an insignificant influence on the phase difference. The latter shows a unique effect on the phase difference and causes the phase of FFPC-FOMs to be either in or out of phase. The phase differences of four samples of FFPC-FOMs with different cavity lengths and resonance frequencies are measured in the frequency range 50 Hz-4 kHz. Experimental results of the phase difference are well consistent with simulation results. All samples of FFPC-FOMs can be divided into two groups: one is near 0° and the other is near 180°. In addition, the FFPC-FOMs in each group have good phase consistency for the array applications.

Frequency-swept interferometry (FSI) is a powerful ranging method with high precision and immunity to ambient light. However, the stand-off distance of the current FSI-based ranging system for non-cooperative targets is relatively short, because the weak echo power cannot provide the needed signal-to-noise ratio (SNR). Here, we report a ranging method combining FSI and the laser feedback technique. Compared with the conventional FSI, the interference between the weak echo signal and the local oscillator occurs in the laser cavity, which makes the signal enhanced spontaneously and then provides a satisfying SNR. In experiments, the detection limit of the echo power is less than 0.1fW, with 230µW output in total. Based on the enhancement from the laser feedback technique, the system achieves a non-cooperative target ranging hundreds of meters away in space without extra optical amplifiers. On the other hand, a large stand-off distance makes the system sensitive to environmental disturbance, which degrades the ranging precision. To correct it, a compensation device is proposed to correct the unwanted optical-path-length drifts. Owing to the high sensitivity and the validity of the compensation, the standard deviation in 10 measurements is better than 0.07mm, targeting an aluminum sheet at about 152m. Generally, hundred-meter range, high relative precision, and low photon consumption predict a novel technical scheme for laser ranging, and demonstrate new capabilities that promise to enable a wide range of scientific and industrial applications.

Xanthoma pathogenesis is speculated to be associated with oxidized low-density lipoprotein (ox-LDL) deposition, although this remains unclear. Most patients with diffuse plane xanthomas present elevated blood lipid levels, and they benefit from treatment with oral lipid-lowering agents. However, there is no available treatment for diffuse normolipemic plane xanthoma (DNPX). In this study, for the first time, we used a topical simvastatin ointment to treat DNPX in three pediatric patients and observed favorable results. Immunofluorescence staining showed that the pyroptotic pathway was significantly attenuated after topical simvastatin application on the skin lesions of the patients. As ox-LDL deposition was observed in the lesions, we used ox-LDL to build a foam cell model in vitro. In the ox-LDL-induced foam cell formation, simvastatin consistently inhibited pyroptotic activation and inflammation in the macrophages. Additionally, the overexpression of nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) or 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase (HMGCR), the known target of statins, reversed the effects of simvastatin. Moreover, gasdermin D (GSDMD) or HMGCR knockdown inhibited ox-LDL-induced pyroptosis. Furthermore, the immunoprecipitation results confirmed the interaction between NLRP3 and HMGCR, and this interaction was inhibited by simvastatin. In conclusion, we demonstrated that topical application of simvastatin ointment might be a promising treatment for DNPX skin lesions and that this therapeutic effect may be related to pyroptosis inhibition via HMGCR inhibition in foam cells. Moreover, xanthoma pathogenesis might be associated with ox-LDL deposition and inflammation.

The development of high-temperature photodetectors can be beneficial for numerous applications, such as aerospace engineering, military defence and harsh-environments robotics. However, current high-temperature photodetectors are characterized by low photoresponsivity (10 A/W) due to the poor optical sensitivity of commonly used heat-resistant materials. Here, we report the realization of h-BN-encapsulated graphite/WSe2 photodetectors which can endure temperatures up to 700 °C in air (1000 °C in vacuum) and exhibit unconventional negative photoconductivity (NPC) at high temperatures. Operated in NPC mode, the devices show a photoresponsivity up to 2.2 × 10

Most of the traditional scanning feedback imaging systems cannot break through the limitations of scanning speed. In this letter, a fast scanning system with changeable scanning sequences by projecting patterns alternatively on the digital micromirror device is proposed. Combined with the laser feedback techniques, the system exhibits high sensitivity to weak signals from targets with low reflectivity. In experiments, the images of handwriting ink patterns on a piece of glass are obtained in 6 seconds, and the resolution of the system is evaluated to be $124.5~\mu \text{m}$ with a 3.24 mm by 3.24 mm field of view. This scanning feedback imaging system can be combined potentially with single pixel imaging techniques to reconstruct phase objects more efficiently.

We analyze the generation of the vibrational coherent anti-Stokes Raman scattering (CARS) signal from the ground level X2Σg+ of N2+ and demonstrate the feasibility for gas-phase thermometry utilizing the vibrational CARS signals of N2+ in high-temperature flames. The vibrational CARS signal of N2+ is obtained using the filamentation-based and two-beam hybrid femtosecond/picosecond CARS system and is employed to extract the fitting temperatures in high-temperature flames. The accuracy and precision of the fitting temperatures from the 1000 consecutive single-shot vibrational CARS spectra of N2+ at a target temperature of 1706 K are 1.23% and 5.76%, respectively, which are similar to the fitting results of N2 in previous filamentation-based CARS thermometry and could also demonstrate that the measured temperature in the filamentation is indeed not affected by the ionization. We further introduce the CARS signal of N2+ in multiple species measurement to obtain more details about the CARS process during filamentation. This work would provide an optional target molecule for CARS thermometry and help researchers further understand the molecular dynamics of N2+ ions during the filamentation.

The ongoing outbreak of the COVID-19 has highlighted the importance of the pandemic prevention and control. A rapid and sensitive antigen assay is crucial in diagnosing and curbing pandemic. Here, we report a novel surface plasmon resonance biosensor based on laser heterodyne feedback interferometry for the detection of SARS-CoV-2 spike antigen, which is achieved by detecting the tiny difference in refractive index between different antigen concentrations. The biosensor converts the refractive index changes at the sensing unit into the intensity changes of light through surface plasmon resonance, achieving label-free and real-time detection of biological samples. Moreover, the gain amplification effect of the laser heterodyne feedback interferometry further improved the sensitivity of this biosensor. The biosensor can rapidly respond to continuous and periodic changes in the refractive index with a high resolution of 3.75 × 10

After a long-term research, the birefringence-Zeeman dual-frequency (BZDF) laser has entered the practical stage, and the BZDF laser interferometer has more powerful displacement and size measurement functions, especially the near-zero nonlinearity. This paper theoretically analyzes the mechanism of the near-zero nonlinear error, including theoretical simulation and experimental measurements. The measurement results show that the nonlinear error of the BZDF laser interferometer is only 0.3 nm, which is one order of magnitude smaller than that of traditional Zeeman dual-frequency (ZDF) laser interferometer. It has broad application prospects in the field of ultra-precision metrology.

We achieve two-beam, 1-kHz, and single-shot N2 vibrational coherent anti-Stokes Raman scattering (CARS) thermometry using hybrid femtosecond/picosecond CARS in high-temperature flames. Ultrabroadband pump/Stokes pulses are generated employing the filamentation of high-power femtosecond pulses and can excite the Raman shift up to ∼3000 cm−1. This configuration can avoid the need of a long hollow-core fiber and an additional supercontinuum compressor to obtain ultrabroadband pulses. The quasi-common-path second harmonic bandwidth compression system is utilized to generate narrow-band and sideband-free picosecond pulses, which are employed as the probe pulses in the CARS process. The accuracy and the precision of the fitting results from 1000 consecutive single-shot spectra at a target temperature of 1706 K are 0.66% and 4.47%, respectively. The results demonstrate that the simplified two-beam system has the potential for dynamic and accurate temperature measurement in practical applications.

A novel, to the best of our knowledge, interferometry based on the laser feedback technique with long transmission distance is proposed. The system has the advantages of high sensitivity for uncooperative targets and a simple structure. Also, the quasi-common path orthogonally polarized light compensation method is designed to compensate for the drift in long-distance transmission and achieves more than 200 times reduction experimentally. Using a copper block as a target, the displacement resolution of 20 nm is demonstrated experimentally with 300 m transmission distance. The sub-microwatt power consumption of the measurement beam indicates the high sensitivity of the interferometry. The performance, in terms of linearity, is also evaluated. Although a lot needs to be improved, the proposed method is promising for further development toward practical applications, like sensors in remote, nuclear radiative, or other harsh conditions.

We propose an improved opposition-based self-adaptive differential evolution (IOSaDE) algorithm for multi-parameter optimization in vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) thermometry. This new algorithm self-adaptively combines the advantages of three mutation schemes and introduces two opposite population stages to avoid premature convergence. The probability of choosing each mutation scheme will be updated based on its previous performance after the first learning period. The IOSaDE method is compared with nine other traditional differential evolution (DE) methods in simulated spectra with different simulation parameters and experimental spectra at different probe time delays. In simulated spectra, both the average and standard deviation values of the final residuals from 20 consecutive trials using IOSaDE are more than two orders of magnitude smaller than those using other methods. Meanwhile, the fitting temperatures in simulated spectra using IOSaDE are all consistent with the target temperatures. In experimental spectra, the standard deviations of the fitting temperatures from 20 consecutive trials decrease more than four times by using IOSaDE, and the errors of the fitting temperatures also decrease more than 18%. The performance of the IOSaDE algorithm shows the ability to achieve accurate and stable temperature measurement in CARS thermometry and indicates the potential in applications where multiple parameters need to be considered.

In this paper, we report a highly effective relative humidity (RH) sensor implemented on graphene oxide (GO) coated long period grating (LPG). The GO nanocolloides bonded onto a cylindrical fibre cladding enables the LPG with strong evanescent waves to absorb more water molecules increasing its RH sensitivity. In an LPG, the phase matching condition occurs when a forward propagating core mode is coupled with the co-propagating lower order cladding modes generating evanescent waves to interact with the surrounding medium. This unique effect of LPGs can be more enhanced with multilayer GO deposition. There is an expansion of GO film with the absorption of more water molecules as RH increases. The absorption of water molecules on GO coating increases the conducting carrier (holes) density on it, thus decreasing the refractive index of GO film. The combined effect of increasing evanescent waves and modulated refractive index makes the GO coated LPGs as effective RH sensors. Our recently achieved results have shown the RH sensitivity of the GO coated LPG is about 0.01 dB/%RH. We have also investigated the effect on GO layer thickness, showing thicker layer increases the RH response of the LPG cladding mode resonances in lower wavelength region.

A remote eavesdropping system based on the laser feedback interferometry is proposed. The diffuse objects neighboring the speaker can work as inconspicuous sensors. The modulated incident beam is scattered back to the laser cavity by the objects, forming the self-mixing interference. The huge amplification caused by the self-mixing interference provides the system with ultra-high sensitivity, even the feedback light whose intensity is -104dB lower than the laser output can still be sensed. The comprehensible voice reconstruction of the object at the distance of 200 m can be realized, although only 0.91 photon in each interference modulation cycle is scattered back into the laser cavity. Additionally, the range of the incident angle of the beam can be up to ±45 deg, and the optimal one is ±30 deg. This system can meet various requirements of acoustic signals detection and be potentially applied in fields as disaster relief and remote surveillance.

The dielectric constant of the microwave dielectric ceramic solid solution is usually predicted by the Clausius-Mosotti equation but the quality factor ( Q ) cannot be precisely calculated. In this paper, it finds that the dielectric constant of the solid solutions also could be well calculated by the Maxwell-Wagner formula, and that the Q of solid solutions can be precisely calculated, by assuming a solid solution as a two or more materials’ mixture.

The effects of Al2O3 addition on microstructure, phase evolution and microwave dielectric properties of Ca0.61Nd0.26TiO3 – x mol Al2O3 (x = 0, 0.025, 0.05, 0.075, 0.1) ceramics have been investigated in this study. With x increasing from 0 to 0.05, the system maintained a single perovskite phase while bulk density decreased rapidly due to grain size degradation. Correspondingly, dielectric constant (ϵr) decreased from 103 to 79.2 and temperature coefficient of resonant frequency (τf) dropped from +247 to +159.3 ppm/°C. Moreover, anti-reduction of Ti4+ gave rise to a nearly doubled Q×f value, from 8600 to 15500 GHz. For 0.075≤x≤0.1, X-ray diffraction results showed a secondary phase whose peaks were consistent with Ca2Nb2O7 phase, resulting in decreased microwave dielectric properties.

We describe a new method for lens thickness and air gap measurement based on the frequency-shifted confocal feedback. The light intensity fluctuation is eliminated by the heterodyne modulation and the detection sensitivity is improved prominently by the frequency-shifted feedback effect. The measurement results for different materials and kinds of lenses are presented in the paper, including K9 plain glasses, fused silica plain glass, and K9 biconvex lens. The uncertainty of the axial positioning is better than 0.0005 mm and the accuracy reaches micron range. It is promising to be applied in the multi-layer interface positioning and measurement area.

Anodic bonding process is widely used in fabrication of micromachined devices. In order to improve the yield and quality of micromachined devices with movable structures, this paper demonstrates a glass-silicon-glass triple-stack anodic bonding strategy by applying multi-step low-bonding voltages. An optimized bonding parameter was analyzed and implemented on a micromachined electrostatic micromotor with a contactless ring-shaped rotor. Experimental results indicate a significant improvement on the yield of the free-rotor micromotor benefiting from such improved bonding process.

The possible causes of measurement offset in the single-spot two-dimensional displacement measurement system are analyzed. Multi-channel feedback crosstalk will cause sinusoidal disturbances, which brings the measurement error.

The influence of feedback level and shifted frequency on laser spectrum and power spectrum is investigated experimentally and theoretically. Due to optical frequency-shifted feedback, the spectrum of Nd:YVO 4 microchip laser is expanded into one with multi equal-spaced sidebands around its initial central frequency. The detail of expanded spectrum depends on both the shifted frequency and relaxation oscillation frequency. Optical feedback level decides the possible maximum spectrum range. Nevertheless, the actual spectrum range depends on both the frequency shift and the feedback level. The power spectrum turns from simple to complex with the increase of feedback level when the shifted frequency is a constant. A rate-equation model with optical frequency-shifted feedback is built to interpret the experimental phenomena. The theoretical results accord with the experiments very well. This letter can help choose appropriate parameters for feedback interferometry systems and promote the accomplishment of optical frequency-shifted feedback diagram, which could be a guide for feedback interferometry applications.

The liquid refractive index measurement has become an important process in many research and industrial applications. However, the measurement results using traditional methods cannot be calibrated, and the corresponding accuracy is limited, since those methods cannot trace to laser wavelength. This letter describes a novel method to measure the liquid refractive index based on the double-beam laser frequency-shift feedback. The two beams are incident on the liquid surface and the mirror under liquid, respectively. Utilizing the high sensitivity of the frequency-shift feedback of solid lasers and high stability of heterodyne interference, the height variation of the liquid surface and the depth variation of the mirror under liquid are measured. Addressing these two variations, the traceable measurement of liquid refractive index is realized. The measurement results of five different liquid samples and NaCl solution with various concentrations are presented, which proved the system has high repeatability of better than 0.00005.

The device of free-space self-interference microresonator coupled by fiber tapers is proposed. Different from sensors with similar structures on-chip and benefit to the combination between microcavity and optical fiber sensing, the coupling regime is adjustable and a separated sensing area from coupling regions is available. This method makes it feasible to optimize coupling efficiency in detection and broaden the scope of application in dissipative sensing. The transmission spectrum exhibits a distinct phenomenon under a long optical path of sensing arm, including quasi-sinusoidal modulation and profile split. Based on that, transmission characteristics are analyzed by theoretical stimulations and the subsequent experiments are in good agreement with the theory. In addition, we estimate the sensing performance of this device and that the sensitivity can reach −4.76 dB/(10−7 RIU) with a 1 m sensing arm theoretically when applied in monitoring the refractive index change, which is almost an order of magnitude larger than the previously reported data. Besides being adjustable, compact, and efficient, this device shows great potential in the precision measurement and expands the applicable measurement field of similar structures, such as pull pressure that cannot be detected using bus straight waveguides.

Laser confocal tomography faces severe challenges in weak signal detection such as biological tissue imaging and low-reflection interfaces measurement. An ultra-high sensitive optical confocal tomography based on laser frequency-shifted feedback imaging technology is presented, whose theoretical magnification can reach 106 notably without using high-gain detectors. In addition, this achievement possesses the same tomographic resolution as conventional laser confocal tomography without the need for spatial filtering devices. In this work, supported by theories and experiments, the lateral and vertical resolution, optical sensitivity, and system repeatability of LFCT and traditional LCT are investigated and analyzed quantificationally. Finally, comparison on the results of measuring the microfluidic device and biological tissue sections is conducted.

An optical sound measurement technology is developed based on microchip Nd:YVO4 laser feedback interferometry. The system can recover the sound signal by measuring the vibration of non-cooperative target, which is located in the sound field.

A laser frequency-shifted feedback imaging configuration is demonstrated whose depth of focus is extended to twice the focus length of the objective lens. Images on any planes can be refocused from one defocus image.

A new optical method based on frequency-shift feedback and laser confocal microscopy is presented to noninvasively measure a microstructure inside a sample. Due to the limit of axial resolution caused by poor signal detection ability, conventional laser feedback cannot precisely measure the microstructure. In this Letter, the light scattered by the sample is frequency shifted before feedback to the laser to obtain a magnification. Weak signals that change with the microstructure can be detected. Together with the tomography ability of laser confocal microscopy, the inner microstructure can be measured with high axial resolution.

A heterodyne Nd:YVO4 microchip laser self-mixing interferometry based on frequency and polarization multiplexing has been demonstrated. By using two orthogonally polarized lights to measure the measurement and reference target, the effect of the acousto-optic crystal thermal creep and air disturbance in interference light path is eliminated. In addition, the measurement error caused by the difference between two identical microchip lasers is compensated for by shifted frequency multiplexing technique. A rate equations model with multi-channel frequency-shifted feedback is established for interpreting the principle of the interferometry. Due to the ultrahigh sensitivity of the microchip laser, the target used in the experiment is a non-cooperative object which is different from the targets in conventional Michelson interferometers. Under typical room conditions, the short-term resolution is better than 2.5 nm, and the long-term zero drift is less than 60 nm within 7 h. The result shows that this self-mixing interferometry system is feasible and robust in the field of displacement measurement.

Dual-frequency solid-state microchip lasers have advantages, such as large frequency differences, high pumping efficiency, high beam quality, and narrow laser linewidth. In addition, they are characterized by simplicity, compactness, stability, long lifetime, etc. However, the frequency difference of the dual-frequency solid-state microchip laser cannot be modulated and is only determined by its internal stress. The stress distribution inside the microchip laser is uneven and the stability of the frequency difference needs to be improved furthermore, which limits its applications. The structure of a Nd:YAG laser resonator, the mechanism of dual-frequency generation, and the output characteristics are studied. The relationship between the polarization direction of the pumping laser and the intensities of the dual-frequency components is theoretically and experimentally researched, realizing the maximum amplitude of the beat signal. Based on the photoelasticity theory, the relationship between the frequency difference and the external force is analyzed and the PID closed-loop frequency difference controlling system based on the piezoelectric transducer is proposed and realized. By applying the closed-loop control, the frequency difference of the Nd:YAG laser can be continuously modulated in the range from 24 to 30 MHz and the fluctuation is less than 450 kHz. The high-performance Nd:YAG dual-frequency solid-state microchip laser with stable and adjustable frequency difference is achieved.

A novel method of ultrasound-modulated optical tomography (UOT) detection based on the laser feedback technology is proposed in this Letter. The system has advantages such as a simple structure, high sensitivity, and reflective configuration. Effective penetration depths of up to 9 cm and 5 cm in phantom and biological tissues, respectively, have been demonstrated experimentally. The detection capability is comparable with the state of the art in the transmission mode but with a much lower photon consumption. Although a lot remains to be improved, the proposed method is promising for further development toward practical applications.

We introduce a new signal processing method based on phase meter into heterodyne microchip Nd:YAG laser feedback interferometer. The nanometer resolution and a higher measurement speed are realized. The factors determining the accuracy are analyzed. The displacements of the Physik Instrumente nanopositioning system and two piezoelectric transducers were measured. Experimental results indicate laser feedback interferometer’s ability of measuring nanoscale displacement and present promising application prospects in noncooperative targets measurement.

Optical-phase-retardation elements are widely used in many fields. Accurate measurement of their phase retardation is crucial to the practical effect of the element’s processing and application. The development and present situation of the methods for optical phase retardation measurement are reviewed, with the wave plate, the most typical phase-retardation element, as an example. The latest research progress in this field is introduced; the principles and characteristics of individual measurement method are summarized and discussed. Three new methods based on laser frequency splitting or laser feedback are presented in detail, in which the laser is not only regarded as a light source but also plays a role of sensor. Moreover, no standard wave plates are needed and arbitrary phase retardation can be measured. Traceability, high precision and high repeatability are achieved as well.

The output responses of phase, amplitude, and polarization of light intensity of a He–Ne laser with the birefringent external cavity are presented. All phenomena are observed in two orthogonal directions related to the birefringent element’s orientation, and are studied under weak optical feedback. Experiments are carried out in five available detecting positions. It is found that laser intensities are modulated in the two orthogonal directions simultaneously, with a phase difference dominated by the phase retardation of the birefringence element. The modulation amplitudes are different in the two directions. A special polarization phenomenon is also observed and discussed. This research may provide a potential novel measuring principle for transparent birefringent materials.

Displacement sensor based on the polarization mixture and the cavity tuning of the orthogonal polarized He-Ne laser 1.15 μm is presented. The power tuning curves of He-Ne laser are irregular, and it is difficult to measure the change in cavity length. The distortion of the curves is caused by the higher relative excitation compared with the He-Ne laser at 633 nm. In view of its potential for the wider displacement measuring range, a new method of displacement sensing is developed. Experiments show that displacement measuring stability based on the method of the polarization mixture is better than that of the power tuning curves. The displacement sensor achieves the measuring range of 100 mm, resolution of 144 nm, and linearity of 7 × 10-6.

The cavity tuning characteristics of orthogonally polarized dual-frequency He-Ne laser at 1.15 \mu m are presented. Vectorial-extension model based on semi-classical laser theory reveals that cavity tuning characteristics are related to beat frequency, relative excitation, and type of Ne isotope. Distortions of cavity tuning curves become moderate with the increase of beat frequency because of the weakening of the crosssaturation effect. Distortions are enhanced with the increase of relative excitation because of the combined action of the self-saturation and cross-saturation effects. By adopting dual-isotope Ne instead of monoisotoplic Ne, distortions are reduced because of the misalignment between peaks of the self-saturation and net gain coefficients. The theoretical calculations are in good agreement with the corresponding experimental results.

The depth of focus extension in optical imaging is of considerable interest. In this paper, a laser frequency-shifted feedback scanning imaging configuration is demonstrated whose depth of focus is greatly extended through numerical filtering. The transmission characteristics of the system are studied. The original image is acquired through a two-dimensional scanning point by point with the target placed on a defocused plane. Filtered in the frequency domain, images on any oriented plane can be refocused. The superior performances are presented by imaging a three-dimensional target, and the process of gradual refocusing is demonstrated. To obtain the maximum extension in the depth of defocus, a series of numerical experiments has been carried out, which reveals its depth of focus is capable of being extended to four times the length of the objective focus length. The fabulous performances can motivate three-dimensional surface profile measurement.

The accuracy of the existing laser feedback interferometry for measuring the remote target is limited to several microns due to environmental disturbances. A novel approach is presented in this paper based on the double-beam frequency-shift feedback of the laser, which can completely eliminate the dead path errors and measure the displacement or vibration with accuracy at nanometer scale even at a far measurement distance. The two beams emitted from one Nd:YVO4 crystal are incident on the measurement target and its adjacent reference surface, respectively. The reference surface could be taken from the nearby stationary object, without the need to put a reference mirror. The feedback paths and shift frequencies of the two beams are the same, so the air disturbances and the thermal effects in the way could be fully compensated. Under common room conditions, the displacement of a steel block at a distance of 10 m is measured, which proved that the system's stability is ±12 nm in 100 s and ±50 nm in 1000 s, the short-term resolution is better than 3 nm, and the linearity within the 300 mm range is 5 × 10-6 and within the 100 μm range is 1 × 10-4.