To improve the frequency domain resolution of Doppler frequency of the optical frequency difference in the beat frequency–based laser Doppler velocimetry based on self-mixing interference and heterodyne interference, an ultra-low frequency measurement method with long short-term memory (LSTM) is presented. LSTM can predict the signal for the next period based on the characteristics of the original Doppler signal. The characteristics of the predicted signal remain basically consistent with the previous signal, with almost no change in frequency components. This is very beneficial for directly extracting the frequency information of ultra-low frequency Doppler signals with insufficient signal length. Experimentally, LSTM was utilized to measure Doppler signals generated at different velocities, achieving a resolution of more than 10−4 Hz. Compared with other signal extension methods such as zero padding, LSTM can truly enhance the resolution of Doppler frequency measurement without introducing direct current components and uncorrelated components. This method improves the velocity measurement accuracy while simplifying the signal processing for frequency measurement, demonstrating potential application value.
A multichannel laser self-mixing interferometry (SMI) is proposed based on a combination of linear injection current modulation and carrier frequency division multiplexing (FDM) technique. The laser output is split into several beams with each beam directed toward a specific target for measurement. The reflected beams then return to the laser cavity, where self-mixing interference takes place. Since the external cavity length of each measurement channel is different, the SMI signal contains multiple carriers due to the injection current modulation. An algorithm based on FDM is developed for displacement sensing of multiple targets from a single SMI signal. Experimental results show that the relative error of multichannel displacement measurement is <1.4%. This work demonstrates an attractive multichannel SMI for displacement sensing featuring compact configuration and high resolution.
In spite of the robustness and simplicity of the self-mixing interferometry (SMI) concept, interpreting the SMI signal is often complicated in practice, which is detrimental to its measurement value. SMI based on machine learning is presented to extract the phase directly from pure SMI signals. A simple phase decomposition neural network (PDNN) was constructed to realize direct phase extraction. A special feature of the PDNN is that it can use the simulation data directly to train the model, and the trained model can be used directly for measurement. In the training process of the PDNN model, the simulated cosine-like signal and the flag signal were used as inputs, and the simulated phase was used as the label. Thus, the training set was easy to prepare, the model structure was simple, and the training speed was very fast. Experimentally, we measured targets with cosine-like movements directly using the trained model, and the results obtained were consistent with the simulation. This contributes to simplifying the signal processing of interferometry in practical engineering applications.
An enhanced laser self-mixing Doppler velocimeter by fiber Bragg grating (FBG) is proposed. In fiber sensing, FBG is often used as an optical filter to select the wavelength of the light using the narrow reflection spectrum. The Doppler frequency shift from the rotating target can be parsed in the self-mixing interferometry (SMI) signal. The frequency-fluctuant self-mixing signal is transformed into intensity variations by a FBG when the laser wavelength is set to the edge of the FBG steep transmission profile. An experimental comparison between the enhanced and the traditional SMI approaches is made. The magnitude of obtained FBG-enhanced SMI signal is about 40 times stronger than that from traditional SMI. The result of the experiments indicates that enhanced self-mixing Doppler velocimetry by FBG can be applied to velocity sensing. This method has potential application in the engineering industry because the FBG is easy to fabricate and connect to a SMI system and the wavelength of FBG can be changed to match the different lasers for an enhanced SMI signal.
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.
By combining self-mixing interferometer (SMI) and grating interferometer (GI), a self-mixing grating interferometer (SMGI)
is proposed in this paper. Self-mixing interference occurs when light emitted from a laser diode is diffracted by the doublediffraction
system and re-enters the laser active cavity, thus generating a modulation of both the amplitude and the frequency
of the lasing field. Theoretical analysis and experimental observations show that the SMGI has the same phase sensitivity as
that of the conventional GI and the direction of the phase movement can be obtained from inclination of the interference
signal. Compared with the traditional SMI, the phase change of interference signal in SMGI is independent of laser
wavelength, providing better immunity against environmental disturbances such as temperature, pressure, and humidity
variation. Compared with the traditional GI, the SMGI provides a potential displacement sensor with directional
discrimination and quite compact configuration.
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.
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
A kind of relative humidity sensors is presented. The sensors consist of a layer of three dimensional periodic films and an optical fiber bundle. The three dimensional periodic films which are polystyrene opal films, silica inverse opal films and composite photonic crystal films were self-assembled on end facet of the optical fiber bundle end facet. Principle of the sensor is that optical reflection peak of the three dimensional periodic films is sensitive to relative humidity, and optical reflection spectra of the films can be measured from the other end of the fiber bundle.
A novel optical MEMS pressure sensor with a SU-8, microstructure is proposed. SU-8 photoresist is used to form the
high aspect ratio structure on silicon wafer. The advantage of the novel structure mainly lies in the design of separating
sensing membrane deformation with the length change of Fabry–Perot cavity. The principle of the pressure measurement
has been introduced. The mechanical model is analyzed and parameters of SU-8 structure are determined by simulation.
The fabrication process is described. Experimental results demonstrate that the sensor has a reasonable linearity,
sensitivity under micro-pressure measurement range from 0 to 0.1 MPa.
A U-shaped optical fiber sensing system designed to measure the refractive index of liquid had been
proposed. The sensing mechanism of U-shaped optical fiber was discussed. A general single-mode
fiber was bent into U-shaped and partially cladding of U-shaped fiber was corroded by HF acid buffer
solution. Powers of different diameters of U-shaped fibers had been measured by many experiments.
The results showed that the diameter of U-shaped fiber cladding 40 μm and the diameter of U-shaped
was 1 cm were suitable to measure liquid refractive index. Then, this U-shaped optical fiber was
immersed in liquid, such as pure water, ethanol, acetone and isopropanol, respectively. The evanescent
field of the U-shaped fiber should be modulated by the liquid. The optical signal in the U-shaped fiber
was measured with the optical spectrum analyzers(OSA). Finally, the experimental results were
analyzed, and the spectra in the air was selected as a reference. The relative intensity was obtained for
the different liquid. These results showed that the relative intensity of the liquid had a good linear
relationship. This sensing device could accurately demarcate refractive index of liquid. It is simple, low
cost, and it can also be applied in measuring the level of liquid.
This paper investigates the self-mixing interference in erbium-doped fiber ring laser (FRL) and its application for
displacement and velocity sensing. Self-mixing interference in FRL with a parallel dual-channel is proposed. The
characteristics of the intensities of the laser in the dual channels with optical feedback are theoretically deduced. The
experimental results show a good agreement with the theory, and indicate that self-mixing interference with a parallel
dual-channel is an efficient approach for simultaneous multi-channel displacement measurement. Moreover, a new
method for velocity detecting based on laser speckle optical feedback injected erbium-doped FRL is presented. Random
speckle feedback causes changes in both intensity and frequency of the laser. A dynamic speckled-modulated laser
output is observed and processed by FFT analysis. A linear dependant relationship between the velocity of an object and
the mean speckle frequency defined as the ratio of the number of fluctuations is obtained, which indicates that speckle
optical feedback injected erbium-doped FRL is an effective approach for velocity detecting.
Silica microspheres self-assembled on the outer surface of a standard single-mode silica optical fiber are investigated.
The three dimensional colloidal crystals of FCC structure coating on the cylindrical substrate are fabricated through
isothermal heating evaporation induced self-assembly. The resulting photonic crystal cylindrical annulus is characterized
by scanning electron microscopy (SEM). The SEM images illustrate the [111] and [100]-oriented regions, besides the
two zones regularly intercrossed pattern. Qualitative analysis and discussion are carried out for the arrangement of
various oriented regions.
We report the experimental demonstration of colloidal crystal deposited on the end face of fiber, as well as the side
cladding. The colloidal photonic crystals were fabricated through isothermal heating evaporation-induced self-assembly.
The resulted samples were characterized structurally by scanning electronic microscope (SEM) and optically by
transmission spectra analyzer. A moderately transmission dip at 1543 nm and 1546 nm wavelength coincides with the
photonic band gap expected for the 710 nm silica microspheres.
New approaches for multiplexing self-mixing interference (SMI) in fiber ring laser (FRLs) are presented. The expression
of output powers with optical feedback in FRL with parallel dual-channel and multi-channel is analyzed. The gain
competition, the intensity alternation among different channels and the attainable performance are discussed. The
experimental results show good agreement with the theory. Therefore, two-channel and multi-channel sensing or
measurement will be realized simultaneously.
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.
The modulation and demodulation technique of laser self-mixing interference vibrometer is researched in this paper.
Combining with triangular current modulation and DSP demodulation technique, a new-type laser self-mixing
interference vibrometer is designed to achieve non-contact vibration measurement of a target. Theoretical analysis,
simulation results and error evaluation are presented in this paper. The vibration waveform is reconstructed with an
accuracy of 0.325 micron in a wide dynamic range. Experiments results show a good agreement with the simulative
results. The vibrometer is compact, inexpensive, self-aligning and can be applied to various vibration measurements for
its simplicity.
This paper presents a miniature fiber optic pressure sensor based on Fabry-Perot interference fabricated on the tip of
a single mode (SM) fiber. The sensor measures only 125μm in diameter. A Fabry-Perot cavity and a thin silica
diaphragm are fabricated by simple techniques involving only fusion splicing, cleaving, and wet chemical etching.
Interference pattern of the sensor is analyzed and issues in sensor design are discussed. The overall chemical
reaction of the fiber wet etching is specifically represented. Pressure testing system is carried out. By tracing a peak
point in the interference spectrum, the gap length of the sensor can be demodulated. The sensor is made entirely of
fused silica, whose structure has good stability, cabinet, simple for fabrication and low cost. It may also find uses in
medical applications.
A new method of fabricating three-dimensional colloidal crystals microstuucture fiber is
proposed. The cladding of single-mode fiber was eroded to 60nm by HF and The eroded cladding was
observed by microscopy and scanning electron microscopy. Colloidal photonic crystal was
self-assembled onto the eroded cladding through isothermal heating evaporation induced self-assembly,
and the colloidal photonic crystals cylindrical annulus embeded in fiber was characterized by scanning
electron microscopy. The results shows the colloidal photonic crystals well-orderedly self-assembled
onto the fiber cladding, and the results also demonstrate a practical means of coating macro- or
micro-curved surfaces with three dimensional photonic crystals.
In this paper, we present a demodulation of Fabry-Perot pressure sensor method based on radial basis function
network(RBF). RBF network is a kind of three layers frontal feedback neural network with single connotative layer. It is
proved that RBF is able to approach random continuous function with random precision. The cavity length variation is
simulated from 473 to 483 µm with the step of 0.5 µm and the simulation result shows that the relative error of this new
method is less than 0.02% and the maximum absolute error is less than 0.1 µm. The MEMS Fabry-Perot pressure sensor
is also demodulated by the experiment. In the experiment, we change the pressure from 0 to 2 MPa with the step of 0.1
MPa. The experimental result shows that its linearity of the cavity length versus pressure achieves 0.98858 and the
standard deviation between measured pressures and real pressures is less than 0.05 Mpa. By the experiment we can see
that, this RBF network method can obtain upper precision and can reach the practice demand. This new method adapts to
the practice demand with its higher resolution and less calculation time.
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.
The growing three-dimensional nanostructures colloidal crystal on the end face of optical fiber by isothermal heating
evaporation induced self-assembly method is presented. The wet chemical etching technique is used to etch single mode
fiber to obtain a shallow circular cavity between the coating and cladding. The optical fiber with the etched cavity was
immersed upside into solution containing polymethylmethacrylate (PMMA). The PMMA spheres used here had an
average diameter of 690nm; it takes one week for the sphere particles to completely settle. The nanostructure
morphology of the colloidal crystal is examined by using the SEM. The colloidal crystal has a face-centered-cubic
(FCC) structure. The optical characterization of the colloidal crystal is also analyzed. The simulation and experiment
result show that the colloidal crystal formed by PMMA spheres has an obvious photonic band gap in the wavelength
1543nm that is typical wavelength of optical communication. The spectra feature of the optical fiber colloidal crystal is
measured by using optical sensing analyzer. The experimental results show the band gap at the 1543nm, consistent with
the simulation results.
An electrostatically actuated in-situ measuring method for average stress gradient of a MEMS film was proposed based
on pull-in voltages of a set of cantilevers. The key of the measuring method is to realize accurate calculation of pull-in
voltages of the cantilevers. To increase the accuracy of the measurement, bending of the cantilevers along the width
direction due to the stress gradient was considered. Actual simulations indicate that the calculating speed and the
accuracy of the measuring method are ideal, and the method can be applied to in-situ measurement.
A novel pressure sensor with a mesa structure diaphragm based on microelectromechanical systems
(MEMS) techniques is presented. The operating principle of the MEMS pressure sensor is expatiated
by the Fabry-Parot (F-P) cavity model and the relation between pressure and interference light intensity
is deduced in the sensor. The mechanical model of the mesa structure diaphragm is validated by
simulation, which declares that the mesa structure diaphragm is superior to the planar one on the depth
of parallelism. Experimental system is also introduced.
One of the important applications of aspheric mirror is the camera lens. Compared with spherical mirror, aspheric mirror has the advantage of improving the optics imaging quality, decreasing the optics weight, elevating its stability and reducing its cost. Although the process of aspheric mirror is becoming mature, the measurement of aspheric mirror still remains a problem.
The Hilbert Transform is very useful in the communications systems and the digital processing system. In this article, a method is proposed for interferometric phase's demodulation with a discrete Hilbert Transform. First, a Hilbert Transform demodulated phase principle is represented and the phase information of aspheric mirror is evaluated only from one interferogram. The sine and cosine components are calculated from a single interference signal, whose ratio of arc tangent is called interferometric phase demodulation. In addition, wave-front reconstruction is achieved by using Zernike polynomial fitting. Furthermore, the phase-extraction algorithm, error correction and some applications to the measurement of the surface shape are discussed so that on-line measurement of aspheric mirror is realized.
In this paper, a packaged erbium-doped fiber amplifier (EDFA) is integrated into a fiber ring to form an erbium-doped fiber ring (EDFR) laser which is applied to measure the particle size based on the self-mixing Doppler effect. A detailed analysis is addressed to introduce the measurement principle, and the obtained results demonstrate that the diameter of the Brownian particles in water can be measured by detect the variations of the intensity and frequency of the fiber laser. This technique can be used in biology, process engineering and chemistry.
The method of the three-dimensional nanostructures colloidal crystal growth on the end face of optical fiber by gravity
sedimentation method is presented. The end face of optical fiber is lapped by CNFIBERNET optical lapper. The optical
fiber we used is single mode fiber. We found that the cladding and core of optical fiber would be corroded and the
coating of optical fiber would be reserved when the bare optical fiber is taken into the HF buffer solution (HF:NH4F:H2O=3:7:10), and the cavity would be formed by the coating and cladding. This optical fiber is inserted into
the polymethylmethacrylate(PMMA) microspheres colloidal solution from the lower side, and gravity sedimentation
method is used to grow the PMMA colloidal crystal into the cavity. The three-dimensional nanostructures colloidal
crystal onto the end face of optical fiber can be obtained. The nanostructures is face-centered cubic (FCC) by using the
SEM. The RSOFT is used to simulate photonic band gap of FCC structure. Through the theory analysis and simulation,
the PMMA colloidal crystal formed by the diameter 695nm spheres will have an obvious photonic band gap in the
wavelength at 1550nm.
In recent years, active sensing, which emits radiation directed toward the target to be investigated, has attracted more and
more attention. Laser is not only as a light source, but also as a sensitive element viz. sensing source presented in the
active sensing system which based on the theory of self-mixing interference in this paper. This application can make the
sensing system more simple, compact, and less cost. In this paper, we propose the active sensing by the combination of
DBR lasers and the self-mixing interference technique. The self-mixing interference in DBR lasers based on the study of
F-P cavity and coupling wave equations has been analyzed. Three self-mixing interference models in DBR lasers are
presented, and one of them has been taken for instance to simulate the output variations of self-mixing interference. The
influence from different internal or external parameters on the output variations and the inclination degree of waveform
of the active sensing were discussed in simulation also. And thereby good signal of self-mixing interference in DBR
lasers can be obtained. This application can satisfies the request of microminiaturized sensing device and the demand of
interrogation of optical fiber, and predigests the structure of sensing system.
A new approach for velocity detecting based on laser speckle feedback injected erbium-doped fiber (EDF) ring laser is
presented in this paper. 1550nm wavelength light is outgone through an optical fiber and shoots onto a moving object by
a lens. Portion of the light scattered from the object produces dynamic speckle, when it is back-coupled into the same
fiber and injects the EDF ring laser, random intensity feedback causes changes of both intensity and frequency of the
laser. We theoretically analyzed the laser output when speckle feedback injects an EDF ring laser by using model of the
injection-seeded EDF ring laser, and experimentally obtained the dynamic speckle-modulated laser output signal by a
photodiode (PD). The signal is analyzed by way of FFT analysis, and the mean speckle frequency (MSF), which is
defined as the ratio of number of fluctuations in the detected signal to the measurement time, is obtained. We studied the
relationship between the velocities and MSFs by changing the velocity of object, thus a linear dependent relationship
between them is obtained, which indicates that speckle feedback injected EDF ring laser is an effective approach for
velocity detecting, it is promising to develop a new generation of optical fiber active sensor.
A novel pressure sensor based on Fabry-Perot interferometry and micro-electromechanical system
(MEMS) technology is proposed and demonstrated. Basic micro-electromechanical technique has been
used to fabricate the pressure sensor. Fabrication process and packaging configuration are proposed.
The loaded pressure is gauged by measuring the spectrum shift of the reflected optical signal. The
experimental results show that high linear response in the range of 0.2-1.0 Mpa and a reasonable
sensitivity of 10.07 nm/MPa (spectrum shift/pressure) have been obtained for this sensor.
The statistical analysis and experimental results of self-mixing speckle interference in the distributed feedback (DFB) laser are presented in this paper. Dynamics solution of output gain variation in the DFB cavity is deduced on the basis of speckle theory and self-mixing interference in the DFB laser, when external optical feedback comes from a moving rough surface. By numeric simulations and experiments, the dynamic output variations of the DFB laser as well as their probability density functions (PDFs) are analyzed. Both results of simulations and experiments are in agreement with each other. The experimental results show that this speckle signal processing can be used to measure velocity of target.
A new self-mixing interferometer based on sinusoidal phase-modulating technique for micro-displacement reconstruction is presented in this paper. Phase modulation of the laser beam is obtained by an electro-optic modulator (EOM) in the external cavity. Fourier analysis method is proposed to demodulate the phase. The optimum values of the modulation frequency and modulation depth are discussed. Theoretical analysis and simulation results are given. Experimentally, the micro-movement of a high precision commercial PZT has been reconstructed, which can obtain a displacement measurement accuracy of a few nanometers.
Self-mixing interference in DFB-LD for fiber sensing application has been analyzed in this paper. Due to the characteristics of good model and narrow spectrum, the DFB-LD has the application potentiality in the filed of self-mixing interference, and optical communication technique has rapidly driven the development of optical fiber sensing technique. The combination of self-mixing interference technique and optical fiber sensing technique can satisfy the request of microminiaturized sensing device and the demand of interrogation of optical fiber. It is possible to form a novel optical fiber sensing measurement network, and the combination is helpful to the application at the aspects of avigation, industrial automation, medical examination, etc. In this work, based on the theory of coupled wave, the variation of laser output caused by self-mixing interference has been presented. For optical fiber sensing application, the self-mixing interference under the condition of transmitting the external optical signal by fiber has been analyzed. The influence from the variation of attenuation caused by the increment of fiber length and the reflectivity of the remote target to the output signal of self-mixing interference has been discussed in numerical simulation.
The laser feedback interference in gain-coupled DFB laser diodes (Distribute Feedback LD) is analyzed. The threshold gain and frequency variation in the DFB laser diode are theoretically deduced. Considering the experiment in practice, we analyze the laser feedback interference at the condition of multiple external cavities. Some parameters have influences on the feedback interference, e.g., the coupling coefficient, the linewidth enhancement factor of gain-coupled DFB laser, the reflection coefficient of external reflector. We discussed these different parameters value in numerical analysis, including the simulation at the condition of multiple external cavities. Compared with the effect of laser feedback interference on F-P and λ/4 phase-shifted DFB lasers, the gain-couple DFB lasers are more sensitive to optical feedback for a specific κL value (coupling coefficient and L cavity length). These results show that the gain-couple DFB lasers can be used as laser feedback sensors with a high accuracy.
Region logical operation is proposed for segmenting the object from the background or noise. Region logical operation is quite different from ordinary pixel logical operation. The entire object is processed region-by-region rather than pixel-by-pixel when regional logical algorithm is used for pattern recognition. The experimental results show object pre-processing based on region logical operation is simple and effective, it can be widely used in optical pattern recognition and image processing.
A novel algorithm, which is based on creating the variance image of original image, for infrared image enhancement is proposed. Ordinary spatial filtering is powerful of reducing the noise of image, but it also reduces the contrast of image. Compared with spatial filtering, variance image not only reduces the noise but also enhances the image. In this paper, we combine the variance image and histogram equalization to enhance the infrared image blurred by random noise. The experimental results show the algorithm based on creating variance image is simple and effective, it can be widely used in infrared image enhancement.
An optical MEMS pressure sensor based on multi-layer circular diaphragm has been analyzed by utilizing the shell theory and characteristic matrix methods. Finite element methods are used to analyze the deflection of circular diaphragm with the residual stress effect considered. Simulation results are given by using FEM software tools ANSYS. The analytic expressions for the absolute reflectance of multi-layer circular diaphragm structure are derived. The results are valid for the most optical MEMS pressure sensors based on Fabry-Perot interferometer.
In this paper, aspheric surface testing technique with radial shearing interferometer has been developed. Zone plates are proposed to construct radial shearing interferometer because of its stability and simplicity. Zone plates were designed and fabricated by photographic processing, which is used as radial shearing devices. Radial shearing interferogram are detected by a CCD camera and processed by a PC computer. Fringe phase of shearing interferogram is evaluated by Fast Fourier transform (FFT) algorithm. Wave aberration estimation method of aspheric surface is also presented, which is depended on polynomial expansion fitting. Some errors of interferometer are discussed. Finally, theoretical analysis and experimental results are compared, which shows its accuracy is less than λ/20.
The importance of phase and amplitude in the field of image processing is compared by using the method of Fourier transform. The digital image is encoded with only-phase encode and the computer simulation results are given. The research shows that phase is more important than the amplitude, and phase-only code of image is effective for image encryption and we should make full use ofthe phase of the image.
We describe a method for the ultrasonic displacement measurement in a small volume based on self-mixing effect in a laser diode. The interference signal (mode-hop signal) is modulated with injection variation and demodulated by signal analysis. The interference signal due to the vibration of a testing example under the ultrasonic excitation is used for a determination of the sound displacement. Self-mixing interferometer is used, because it is much simper than the conventional interferometers. Many optical elements like the beamsplitter, reference mirror, and external photodetector are not required. Theoretical analysis and experimental method are presented. The vibration of a PZT transducer-driven sample was measured with resolution of sub-micron.
The design of optical array generator is based on the scalar diffraction theory. Optical array diffraction intensity is the same for all diffraction levels. Optical array generator can be modulated by either the amplitude or phase profile, however, phase array generator is done through ion etching. The design and fabrication of highly efficient optical array generator is studied, one and two dimensional optical array generator is designed, which generate 16, 32,64, 256, 1024, 4096 uniform optical arrays separately. The optical array generator has been used in optical pattern recognition and multi-channel imaging, and the experimental results are presented.
Many optical sensors for the displacement measurement in sub-micron have been developed. In this paper, we propose a new type of self-mixing interferometry (SMI) to measure microscopic displacement in microelectromechanical systems(MEMS). The phase of SMI signal is modulated with external cavity length and demodulated by signal analysis technique. The experimental system was developed to have a sensitivity of several nanometers for displacement sensing.
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