Interference Light Intensity Research Articles

Research on a signal demodulation algorithm for fiber optic acoustic sensors based on the amplitude ratio of harmonic components

Fiber optic sensing technology's performance is crucially influenced by the choice of demodulation algorithms. Traditional intensity demodulation methods rely on precisely controlling the working point to the quadrature point (Q point) by adjusting the laser wavelength. However, issues like laser wavelength adjustment accuracy and laser power variations with wavelength affect demodulation accuracy, limiting its application in complex scenarios. Additionally, this method requires sensitivity calibration and has a limited dynamic range. To address these limitations, this paper proposes an amplitude ratio of harmonic components (ARHC) demodulation algorithm. It analyzes the harmonic components of the interference light intensity output by the sensor to calculate Fabry-Perot cavity length changes, achieving real-time demodulation of dynamic signals. Compared to traditional methods, the ARHC algorithm simplifies system adjustment, has a larger demodulation range and higher accuracy, and is more adaptable to environmental changes. Simulations and experiments demonstrate its effectiveness and high-precision, large-dynamic-range signal demodulation capabilities under different interference conditions. The EFPI ultrasonic sensitivity based on the ARHC algorithm is 2.5 nm/Pa@23kHz, and it can linearly demodulate up to a vibration amplitude of 372.8 nm for the sensitive diaphragm, with a demodulation error of less than or equal to 0.8 nm.

Electro-optic modulation aberration correction algorithm based on phase difference compensation.

In order to achieve wide field-of-view, high-resolution LIDAR, a gating imaging structure combining an electro-optic crystal and an electron multiplication CCD is constructed. According to the index ellipsoid theory, a 3D ray tracing model is established to explore the principle of electro-optic modulation. The field-of-view and interference intensity distribution of the LiNbO3(LN) crystal electro-optic modulation are studied by using the proposed model. In order to solve the problem that the interference light intensity at the edge of the field-of-view of crystal electro-optic modulation is not homogeneous, we study and propose an electro-optic modulation aberration correction algorithm based on phase difference compensation. The corrected interference light intensity at the edge of the field-of-view increased from 0.87 to 0.94. Finally, a LIDAR imaging simulation system is established to image the target at a distance of 2000m. The results show that under the condition of a 1° wide field-of-view, the imaging accuracy of the system is 5mm, and the average imaging error introduced by the crystal electro-optic modulation is less than 2cm.

Equivalent Measurement and Real-Time Compensation of Error Caused by Intensity Change in Deep Sub-Nanometer Displacement Measuring Interferometry

Heterodyne interferometry is playing an increasingly important role in the field of high-end equipment manufacturing. In photolithography, the precision requirement of displacement metrology is increasing to a deep sub-nanometer scale with the decrease in the critical dimension of chips. The error caused by light intensity changes was investigated, and its principle was found to be related to the time difference in photoelectric conversion. On the basis of the analysis of dynamic characteristics of interference light intensity changes in a heterodyne Michaelson interferometer, the influencing factors, and the features of the measurement error, equivalent measurement and real-time compensation methods were investigated and proposed. Experiments revealed that the error was 220 pm using the method of best-gain detection, while it was 4.8 nm using the method of auto-gain detection over a wide dynamic range when the light intensity was reduced by 30%. However, the proposed compensation method successfully reduced the error to less than 40 pm. Therefore, the real-time compensation method based on equivalent measurement can maintain the signal-to-noise ratio while improving the precision of photoelectric conversion, removing the error caused by intensity changes, and helping heterodyne interferometry achieve deep sub-nanometer precision.

Study on Measurement Error Reduction Using the Internal Interference Light Reduction Structure of a Time-of-Flight Sensor

In recent years, the demand for the installation of Time-of-Flight (ToF) sensors in mobile phones and other devices to send and receive distance information has increased. A ToF sensor is a system that measures the distance to an object by calculating the elapsed time of an infrared (IR) light emitted from a vertical-cavity surface-emitting laser to the moment it returns to the single-photon avalanche diode after being reflected by the object. In this study, we analyzed the IR optical path of a ToF sensor structure using optical simulation. In addition, we analyzed the field of view, amount of light source, and intensity of the internal interference light. Moreover, the internal structure of the ToF sensor, which can minimize the intensity of interference light, was optimized and designed. Consequently, the internal interference light intensity decreased by 84% compared to the structure designed using conventional method. Furthermore, a structure was optimized using a polarizing film and quarter-wave plate, the internal interference light intensity of this structure reduced by 88% compared to that of the conventional method, and the distance measurement error of the ToF sensor decreased significantly.

Fiber-optic Michelson interferometer for detecting coolant level and refractive index

Abstract This paper presents an interferometer based on a single-mode fiber-multimode fiber-thin-core fiber (SMF–MMF–TCF) Michelson interference structure that can be used for the measurements of coolant level and refractive index. Because of the different diameters of the cores of the individual fibers, optical excitation and coupling occur at the splicing points of the fibers. The multimode fibers are the couplers in the sensing structure, which allow the exciting light to enter the cladding of the thin-core fibers. The end face of the thin-core fiber is coated with a silver film to enhance the reflectivity of the light. The results show that the interference intensity first increases and then decreases with the length of TCF. When TCF is 4 cm, the interference light intensity is the strongest. The sensitivity of the sensor is 138.091 nm/RIU with the linearity of 0.977 over the refractive index of the coolant in the range of 1.3605–1.3880, and the temperature and time effects on the sensor are small. The proposed sensor has the advantages of simple fabrication, high repeatability, and good stability and it can be applied to the measurements of coolant level and refractive index in automotive engines.

Fast and Calibration-Free Temperature Imaging of Dynamic Flames via Wavelength Modulated Lateral Shearing Interferometry

Temperature distributions of dynamical flames were measured by using a proposed calibration-free wavelength modulated lateral shearing interferometry. A distributed feedback (DFB) laser was modulated in the wavelength as a sawtooth profile. The wrapped flame phase in the images was obtained pixel by pixel by calculating the phase shift between the interference light intensity profiles before and after flame ignition. Phase unwrapping and recovery were adopted to solve the flame phase distribution. The refractive index distribution was reconstructed with the help of Inverse Abel transform. The temperature imaging was further achieved by the ideal gas law and Gladstone-Dale model. Numerical simulations were conducted for performance evaluations. The flame phase, refractive index and temperature distributions were reconstructed with errors of 0.0094, 0.0154 and 0.0958, respectively. Standard deviations were in the magnitude of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> at the presence of Gaussian random noises and showed the noise immunity of the proposed method. In actual experiments, reconstructed temperature distribution of a steady flame generated on a Bunsen burner agreed well with the thermocouple measurements. A dynamical process of flame extinguishment was portrayed at one kilo frames per second. Temporal variation of the flame temperature distribution well reflected the extinguishing process. The entire measurement process did not require prior calibration of the modulated wavelength, and the imaging speed was more than ten times faster than the existing phase modulated interferometer. The proposed method can be used for on-line evaluation of dynamic flames.

Radius of curvature of spherical wave measurement based on vortex beam interference

We proposed a new method for the radius of curvature (ROC) of spherical wave measurement based on vortex beam interference and Fermat's spiral fitting, which can not only provide a large measurement range but also have high precision. Theoretical analysis showed that the maximum distribution of the interference light intensity is in accordance with the Fermat's spiral, and the spiral coefficient is proportional to the ROC of spherical wave. For a concave spherical mirror with a ROC of 899.89mm and a convex spherical lens with a ROC of 59.99mm, the average measurement error of five measurements are 0.080% and 0.083%, respectively. Controlled experiments showed that, our proposed method has a higher measurement accuracy and a larger measurement range than Newton's rings method, it has almost equal measurement accuracy for large and small ROC. This research provides a theoretical basis for the development of a ROC measurement system with large measurement range and high precision.

Effect of polarization mode on interference processing by dual-injection four-beam laser

The four-beam laser interference processing technology has obtained good theoretical and experimental progress in the preparation of drug-loaded microstructures of coronary stent, and further improving the effect of processing has become a hotspot in recent research. This paper analyzes and calculates the light intensity distribution under different polarization combinations of four-beam lasers. Through modeling and simulation, the interference processes and results under three different polarization modes were simulated, and the higher interference light intensity was obtained in TE-TE-TM-TM mode; based on this, a dual-injection four-beam interference lithography system was built, and interference processing experiments were performed on silicon wafer. The laser wavelength used is 1064 nm, the energy of the four lasers is 97 mJ, 80 mJ, 97 mJ, and 78 mJ, all four beams have an incident angle of 45 degrees, the azimuth angles are 0 degrees, 90 degrees, 180° and 270°, the pulse width is 8.2 ns, the repetition frequency is 10 Hz, and the exposure time is 60 s. In the experiment, the microarray structure of the hole array with a period of 1.073 μm was obtained, which is in line with the simulation results. The work in this paper shows that when the combined polarization mode is TE-TE-TM-TM, the dual-injection four-beam interference processing system can achieve the best processing effect.

A Portable and Accurate Phosphate Sensor Using a Gradient Fabry-Pérot Array.

Here, a portable and accurate phosphate sensor using a gradient Fabry-Pérot array (FPA) is proposed. It can form a bidirectional gradient concentration (absorbance) distribution in the gradient FPA, simplifying the complex operations to get a standard curve and saving time. The gradient FPA can effectively filter out the interference (bubbles, light intensity, and salinity) while improving the absorbance, achieving a highly accurate and stable detection. Besides, the smartphone simplifies data processing and makes sensors more portable. In this work, the detection errors of standard solutions (100, 50, and 30 μM) are 0.39, 1.48, and 1.84%, respectively, and it has also been demonstrated with errors of 2.46 (sample 1, seawater), 2.08 (sample 2, lake water), and 1.83% (sample 3, sewage) for natural samples detection, which is more accurate than a traditional analyzer. The sensor has a good performance when affected by bubbles, light intensity, and salinity. In addition, the detection time is shortened to 80 s, which is more time saving compared with traditional devices, and the limit of detection (LOD) is 0.4 μM. It can be predicted that the novel optofluidic sensor is conducive to build a smart nutrient monitoring system and will be applied in the field of biochemistry and environmental chemistry.

Study on the Interaction between Channels of Image Intensifier

The experiment of mutual interference between channels of the UV imager was carried out. The plateform consists of jamming light with large area and target light with small area was established. The curve of the gray value of target light in image intensifier with the intensity of interference light was measured. Experimental results show that there is mutual interference between channels when the UV imager is irradiated by continuous laser and the mutual effects will benefit to suppress the target signal by reducing the gain of the adjacent channels. Theoretical mechanism of this phenomenon was studied. The electrical coupling model between channels was established, and the influence of transverse resistance between adjacent channels on the gain characteristics of adjacent channels was studied by nodal analysis. Numerical simulation results show that the transverse resistance between channels is the main cause of mutual interference between channels, which greatly influence the gain of the adjacent channels.

Methane Optical Sensing System With Polarization Rotation Gas Cell

A methane optical sensing system using a single reflective optical polarization rotation gas cell is developed. The periodic ripple-like variation of interference light intensity in the common single reflective gas cell is analyzed, in terms of the calculated return loss and insertion loss of the cell. A novel single reflective optical polarization rotation gas cell is designed and demonstrated to adequately suppress the light interference effect. The efficacy of the new approach is demonstrated experimentally, showing that the accuracy of the new methane sensor using optical polarization rotation gas cell is better than the sensor using a common single reflective gas cell by 47 %.

Research on the Frequency-Dependent Halfwave Voltage of a Multifunction Integrated Optical Chip in an Interferometric Fiber Optic Gyroscope.

The multifunction integrated optical chip (MIOC) is one of the most critical parts of the interferometric fiber optic gyroscope (IFOG), and research on the halfwave voltage of the MIOC is meaningful for a high-precision IFOG. In this paper, the correlation between the frequency and halfwave voltage, which affects the interference light intensity of IFOG, is presented theoretically. A widespread measurement method for frequency dependence of the halfwave voltage, based on lock-in amplification and sinusoidal modulation, is proposed. Further, the measurement result and the oscillation of interference light intensity in the Sagnac interferometer are presented, which are in great agreement with the theory. This paper proposes the frequency dependence of the halfwave voltage and provides a new error research direction for the improvement of the MIOC in a high-precision IFOG.

Methodology of filter-type multi-dithering phase control for quasi parallel light interference

The quasi parallel light interference is one kind of basic ways to use the energy of interference light to interact with matter. Because the phase of each parallel light beam needs to meet the coherent condition, it is required that the phase of each light beam be controlled timely. There are some kinds of phase control methods, such as the heterodyne phase-locking method, the stochastic parallel gradient descent algorithm, the self-referred and self-synchronous phase-locking method the multi-dithering phase-locking method, etc. Among them, the multi-dithering method needs not the referenc light, it is to load multi-frequency sinusoid signals to the phase modulator, and realize the recognition of phase difference and the output of feedback voltage by multiplying circuit and integrating circuit. In view of the shortcomings of the existing methods, a scheme of filter-type multi-dithering phase control for quasi parallel light interference is proposed, in which the phase differences are identified and corrected by the modulation signals and filtering signals of different frequencies. Theoretical analysis of coherent light intensity for the scheme is made. The principle of filter-type multi-dithering phase control method is put forward, and the numerical analysis and simulation experiment for filter-type multi-dithering phase control method are carried out. In the simulation experiment, the fiber interference light path is used to simulate the light intensity of quasi parallel light interference at one point in space, and the change of photoelectric signal indicates the change of interference light intensity. The phase control feedback loop is composed of photoelectric signal amplifying circuit, bandpass filtering circuit, amplitude measuring circuit, direct current amplifying circuit and adder circuit. The results have shown that the phase difference among light beams can be recognized by the method, and the direct current voltage signal that is proportional to the phase difference of signal can be fed to control the phase modulator. The phase difference can be corrected. The control bandwidth is 2.5 kHz, and the output voltage range of phase control is 0.034.45 V. Compared with the classical multi-dithering method, the method of filter-type multi-dithering phase control has some advantages. Each multiplying circuit in the classical method needs a very small amplitude reference signal, which causes the reference signal to have a very small range of values, and the relationship between integral time and modulation period needs considering. The integral time is usually ten times longer than the modulation period, which causes the control bandwidth of the system to decrease. However, the feedback loop of the filter-type multi-dithering phase control method does not require any reference signal, so each signal does not affect each other, and the increase in the number of beams does not have a significant influence on the control bandwidth either. Therefore the filter-type multi-dithering phase control method is a useful phase-control method.

Distributed fiber optic vibration sensor based on polarization fading model for gas pipeline leakage testing experiment

To design a distributed fiber optic vibration sensor for urban natural gas pipeline leak detection, the light polarization fading transmission model based on Jones matrix is built in this mixed Sagnac/Mach-Zehnder interferometer configuration. The interference light intensity of pipeline with leakage and without leakage cases is compared through spectrum analyzer. When fiber length ranges from 1.5 to 9.5 km, the light power intensity (central wavelength 1550 nm) reduces to −56 dB. In experiment, the leaky spectrum null frequencies are captured and the average relative error of leakage point location varies from 2.3 to 5.3%. When the sensing length is expanded to 9.5 km, the vibration sensor system sensitivity is 0.57 Hz/m.

Quasi first-order Hermite Gaussian beam for enhanced sensitivity in Sagnac interferometer photothermal deflection spectroscopy.

The detection sensitivity of a Sagnac interferometer photothermal deflection spectroscopy was enhanced by changing the probe beam pattern from zero-order to a quasi-first-order Hermite Gaussian (QHG) beam. The nature of the higher order HG mode, where the beam pattern is preserved during propagation with an increased field gradient, is utilized to enhance the measurement sensitivity. In this spectroscopy, the lateral beam deflection due to the photothermal effect is sensitively detected as a change in the interference light intensity. The change in intensity is amplified due to the higher field gradient of the QHG(1,0) beam at the photodetector. This amplification effect was both numerically and experimentally demonstrated to obtain twofold improvement in the signal-to-noise ratio.

Study of key technology of ghost imaging via compressive sensing for a phase object based on phase-shifting digital holography

In this article, the algorithm of compressing sensing is used to improve the imaging resolution and realize ghost imaging via compressive sensing for a phase object based on the theoretical analysis of the lensless Fourier imaging of the algorithm of ghost imaging based on phase-shifting digital holography. The algorithm of ghost imaging via compressive sensing based on phase-shifting digital holography uses the bucket detector to measure the total light intensity of the interference and the four-step phase-shifting method is used to obtain the total light intensity of differential interference light. The experimental platform is built based on the software simulation, and the experimental results show that the algorithm of ghost imaging via compressive sensing based on phase-shifting digital holography can obtain the high-resolution phase distribution figure of the phase object. With the same sampling times, the phase clarity of the phase distribution figure obtained by the algorithm of ghost imaging via compressive sensing based on phase-shifting digital holography is higher than that obtained by the algorithm of ghost imaging based on phase-shift digital holography. In this article, this study further extends the application range of ghost imaging and obtains the phase distribution of the phase object.

An evaluation of a modulated laser encoder

Abstract This paper introduces a new method of interpolation for sub-nanometer-resolution linear encoders. This method, called SPPE (scanning position probe encoder), uses high-order harmonics information obtained by a sinusoidal scanning pickup located on a periodic grating surface. The proposed encoder uses a current-modulated laser diode with diffractive grating optics. Since the electrical current changes the laser-diode wavelength, the interference light intensity is modulated as a sinusoidal scanning pickup on the scale grating. Phase-detection circuits can decode the position information in the pickup signal by using phase-locked loop techniques. The decoder achieves an interpolation rate of over 1/40,000 with interpolation errors of less than ±1 nm. A new interpolation-error measuring system was developed for the encoder. Finally, the evaluation results reveal that the presented encoder shows both high resolution and strong robustness.

A Fiber Splicing-Plane-Inspection Technique Using Lens–Fiber Interference for the Cascaded Fiber Fabrication

In this paper, a novel inspection technique for the cascaded fiber fabrication is proposed to improve the coupling performance of such fibers. Instead of using a direct-monitoring method, an indirect technique with the help of the ray tracing and the lens-fiber interference is used. By calculating the optical-path length of the light rays passing through the fiber, the interference fringes and light intensity can be counted. Then, the splicing plane of the cascaded fiber can be determined by observing the difference between the two fiber sections. Simulations and experiments have been performed to find the splicing plane. Finally, the accuracy of fabrication can be improved from 10 to about 1 mum; thus, the coupling loss due to the fabrication process can be reduced

Membrane disordering induced by chloroform and carbon tetrachloride

We studied effects of chloroform and carbon tetrachloride on bilayer membranes of dimyristoyl-phosphatidylcholine (DMPC) and egg yolk phosphatidylcholine (Egg-PC) by birefringence, dynamic light scattering and fluorescence methods. It is shown that interference light due to the membrane birefringence considerably decreases by addition of the organohalogen compounds for both lipid membranes, indicating a significant decrease in membrane order. In addition, results of dynamic light scattering and turbidity measurements show a rupture of multilamellar DMPC vesicles induced by addition of chloroform at concentrations above 0.2 v/v%. No rupture of the vesicles is observed within the limit of solubility of carbon tetrachloride in water, but excessive addition of carbon tetrachloride (above 0.2 v/v%) induces the vesicle rupture. Chain orientational order was estimated from the interference light intensity at low concentrations of the organohalogen compounds without the occurrence of the vesicle rupture. The estimation shows a monotonic decrease in the chain order with increasing the concentration. The decreases in DMPC chain order by chloroform and by carbon tetrachloride are about 17% at 0.2 v/v% and 23% at 0.05 v/v%, respectively. The reduction in the chain order is correlated with an increase in the membrane fluidity observed by excimer fluorescence of pyrene incorporated to the membrane. Behavior of membrane disordering of Egg-PC is approximately similar with that of DMPC. This implies the strong interaction between the organohalogen compounds and the lipid chains, whether or not the bilayer has the vacancy resulted from unsaturated double bonds and different chains in length. The results of this work suggest that damages of biological membranes by chloroform and tetrachloride are not only induced by a direct attack on proteins but also by a significant membrane disorder.

Theory of photorefractive dynamic grating formulation

The photorefractive holographic recording and two-beam coupling are both dynamic grating formulation process. The interference light intensity of the two coherent beams induces a phase grating though photo-induced refractive index variation and the phase grating changing the intensities of the two beams through beam-coupling take place at the same time. By solving simultaneously the band transport equations and wave-coupled equations, and using the light intensity modulation as the main variable, the analytic solution is obtained, which is valid for any light intensity modulation and constant light excitation efficiency. Here all the mechanics of drift, diffusion and photovoltaic effect are considered. The result shows that the modulation of the dynamic grating varies more slowly compared with that of the linear modulation approximation.