A Bessel function method incorporating carrier modulation is introduced, which accurately determines the phase shift amplitude of vibration signals by analyzing the amplitude relationship between the carrier’s main frequency and its harmonics. Experiments with a programmable signal generator and interferometer probe confirmed its feasibility and superiority over traditional Bessel function methods in weak signal calculations. In the experiment, this method can precisely calculate phase shifts down to 3.9 mrad and achieve a minimum detectable phase shift of 22 μrad, making it suitable for signal calibration in weak signal scenarios and analysis of phase carrier modulation signals.
In the process of signal acquisition by laser microphones, the high frequency components of speech are missing due to nonadditive distortion. In this paper, we proposed an end-to-end speech bandwidth extension (BWE) approach to recover narrow-band speech acquired by laser microphones. Our preliminary research showed that speech enhancement algorithms based on log-magnitude spectrogram in the frequency domain could not achieve satisfactory performance for this task. Therefore, we designed a speech BWE model in time domain, this model was modified by Wave-U-Net structure, we introduced the time convolution module (TCM), the dilation of convolution is helpful to increase receptive field, improves speech long-range correlation, at the same time introduced the multi-resolution loss function (LMSTFT) instead of the mean square error (MSE), the time-domain Wave-U-Net method avoided the decoupling of magnitude and phase in the frequency domain. The results showed that the signal-to-noise ratio (SNR) of speech was improved significantly compared with approach in the frequency domain, and obtained elaborate high-frequency components than frequency-domain convolutional recurrent network (CRN). We chose laser speech to test the model in an actual scene, which further verifies the practicability of the structure through the speech spectrum analysis, and had better performance and generalization ability than the original Wave-U-Net model.
The Laser Doppler Vibrometry (LDV) system can measure parameters such as amplitude, velocity and acceleration of micro-vibrating objects. However, in the actual measurement process, in addition to the probe beam and the reference beam, as the surface roughness of the measured object increases and the diffuse reflectance decreases, the echo energy of the laser will decrease rapidly. This phenomenon can be extended to heterodyne detection systems. Due to the limitation of optical devices, the third beam (ie stray beam) caused by parasitic reflection is unavoidable. At the same time, the third beam has a certain intensity and phase distribution, which seriously affects signal demodulation of micro-vibrating objects. This paper analyzes the effects of multi-beam interference competition in heterodyne demodulation systems. Mathematical analysis shows that the interference competition depends on the signal amplitude, the interferometric light intensity ratio and the relative phase difference. Through mathematical derivation and simulation verification, the following conclusions can be drawn: when the relative phase difference is π rad, when the ratio of light intensity between the probe light and the stray light is controlled to tend to one, the simple harmonic motion with an amplitude of 1 nm is demodulated, and the amplitude can be observed to be nearly 200 times of the enhancement effect. For micro vibration with different amplitudes, the simulation shows that when the amplitude coefficient is π, the demodulation amplitude does not have amplification effect, and the amplitude coefficient is π can be used as the limit of demodulation amplitude amplification. This method has great application value in the high-sensitivity measurement direction of micro-vibrating objects.
Laser Doppler Velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, the signal of LDV is weaker in most applications. One challenging issue is how to effectively discover the weak characteristics from the signal submerged in noise. To address the issue, a method based on Stochastic Resonance is proposed in this paper. Using the SR method, the noise may be weakened and weak characteristics highlighted, and therefore velocity can be is obtained. FFT and the method are adopted to process simulated laser Doppler signals in circumstance of MATLAB. The input and the output signal-to-noise ratios of the bistable system demonstrate the effectiveness of the SR method.
The deep-sea hydrothermal velocity measurement system based on laser Doppler is mainly used for in-situ accurate measurement of deep-sea hydrothermal velocity. Due to the high-pressure environment of the deep-sea hydrothermal area, the optical window of the measurement system will be deformed under pressure, attenuating the energy of the scattered light received by the system. The system selects sapphire as the compressive window material, uses finite element analysis to simulate the compressive deformation of the optical window at a sea depth of 4000 meters (40MPa), and fits the deformed optical window to the optical design software. The simulation result shows the deep-sea high pressure will reduce the energy utilization rate of the measurement system by 43%. Through the optimized design of the optical system, the laser intensity utilization rate has been increased by 52.2% to meet actual application requirements.
A novel quasi-distributed fiber temperature sensor based on the cascaded quantum dot fibers (QDFs) is proposed in this paper. The cascaded QDFs are fabricated by the 3D printing technology and can be divided into two parts QDF1 and QDF2. When the excitation light is coupled into the fiber, the QDF1 emits the 630nm fluorescence and the QDF2 emits the 530nm fluorescence. Because the fluorescence peaks will change with the temperature linearly, it can be used as the fiber temperature sensor. In the experiment, by controlling the temperature at each QDF, the sensor realizes the temperature measurement at different position. The sensitivity of the sensor at different position is 0.15nm/°C and 0.153nm/°C, respectively. The results verify the feasibility of the structure for distributed temperature sensing. The spatial resolution is 1.8mm, which is limited by the length of the printed QDF.
An adaptive polarized interferometer is proposed for optical free form surfaces metrology without auxiliary devices such as the wavefront sensors and deflectometry systems for deformable mirror (DM) monitoring. In this method, the DM surface monitoring and free form surface measurement are achieved simultaneously in only one interferometer by the polarizing design. The polarizing design divide the interferometer into two partial common path interferometric system, which provide the null test for tested free form surface and non-null test for the DM surface. The final figure error of the free form surface is extracted by ray tracing. Experiments proving the feasibility of this interferometer is shown.
A printing method of quantum dots (QDs) optical fiber is presented in this paper. The printing ink with suitable viscosity is composed of the UV adhesive and the CdSe/ZnS quantum dot. By adjusting the pressure and waveform parameters of inkjet printer, a stable droplet is formed. A segment of QDs optical fiber is printed on the organic polymer material substrate subjected to viscous treatment by controlling the spacing between adjacent droplets. When the printed QDs optical fiber is aligned to multi-mode optical fiber which is used to transmit the excitation light, strong fluorescence of the QDs fiber is detected by the optical spectrum analyzer (OSA). Using the printed QDs optical fiber as the senor, the temperature measurement is realized. The sensitivity of the luminescent peak with the temperature is about 115.0pm/°C.
Based on the principle of Solc interferometer, an optical fiber sensor which can realize torsional direction and torsion angle measurement simultaneously is proposed in this paper. The sensor is consisted of a segment of single mode fiber (SMF), polarization maintaining fiber (PMF) and two polarizers. When the light of the broad band source is transmitted in the sensor, the interference spectra can be observed at the output of the sensor. The interference spectra of the sensor can be changed when the sensor is twisted clockwise or counter-clockwise. Because the peaks position and dips position in interference spectra are reversed when clockwise and counter-clockwise torsions are applied, the torsional direction can be judged conveniently. With the increase of the torsion angle, the extinction ratio (ER) of interference spectra will change significantly. By measuring the changes of the ER, torsion angle can be calculated easily. The highest sensitivity can reach to 0.79dB/° in the range of [-50,52°].
A dynamic acceleration measurement system based on a high sensitivity compliant cylinder fiber optic probe and an all-fiber configuration Distributed Bragg Reflector (DBR) fiber laser with self-mixing technique is presented. The compliant cylinder fiber optic probe consists of a mass-block and an elastic cylinder twined with single mode fiber and can be regarded as being close to that of a simple mass spring system. Experimental results show that the signal to noise ratio (SNR) can achieve 50dB at 800 Hz under 1.5 mg acceleration stimulation, voltage sensitivity is 6.04 V/g and minimum detectable acceleration of the measuring system is 4.7μg/sqrt Hz at 800Hz. Moreover, the material of elastic cylinder is polyurethane (PU) which has better temperature stability and can be applied widely in the field of vibration measurements.
KEYWORDS: Demodulation, Data acquisition, Reflectometry, Photodetectors, Linear filtering, Bragg cells, Data storage, Electronic filtering, Heterodyning, Digital filtering
We propose an electrical I/Q demodulation scheme to decrease the sampling rate and computational cost in coherent phase-sensitive Optical Time-Domain Reflectometry systems. The IF signal from the photon detector and the local-oscillator from the acoustic-optic modulator driver are splited into two parts, respectively. One of local-oscillators is 90°shifted, then the signals and LOs are cross-mixed and the outputs are low-pass filtered to obtain the I/Q signal. Besides of the save of computational cost, the sampling rate and data storage is reduced at least 1.6 times. At last, a quantitative measurement of a 50 Hz vibration is successfully demonstrated in the experiment.
As a kind of distributed optical fiber sensor, Optical Frequency-Domain Reflectometry (OFDR) can realize high spatial resolution distributed strain/temperature measurement. A method of measuring Rayleigh backscatter spectrum shift by cross-correlation calculation is widely adopted in OFDR sensor system. The other approach is based on the phase shift induced by the strain/temperature variation. In this paper, we propose a digital demodulation method to achieve it. Firstly the output of the photon detector is Fourier transformed and the phase information is obtained. The cross-correlation method and phase demodulation method are compared based on the theoretical and numerical analysis. The result shows that the spatial resolution (SR) of strain/temperature sensing is decided by the sweep range of the tunable laser source, while this parameter is much larger in traditional scheme. However, better stability can be achieved in cross-correlation scheme for sharp varying strain/temperature.
A modified phase generated carrier (PGC) demodulation algorithm for interferometric sensor is presented in this letter. Compared with the differential-cross-multiplying measure (PGC-DCM algorithm), the effect of light intensity disturbance (LID) is eliminated. Additionally, the harmonic distortion of arctangent measure (PGC-arctan algorithm) is well suppressed. In the experiment, while the simulated LID frequency is settled to 50 Hz, the signal-to-noise of the improved PGC algorithm respectively receives an increase of 10.3 dB and 18.2 dB over PGC-DCM and PGC-Arctan algorithms. The system has a dynamic range of 45.9 dB at 600 Hz by employing the improved PGC demodulation algorithm.
An adaptive optics based non-null interferometry (ANI) is proposed for optical free form surfaces testing, in which an open-loop deformable mirror (DM) is employed as a reflective compensator, to compensate various low-order aberrations flexibly. The residual wavefront aberration is treated by the multi-configuration ray tracing (MCRT) algorithm. The MCRT algorithm based on the simultaneous ray tracing for multiple system models, in which each model has different DM surface deformation. With the MCRT algorithm, the final figure error can be extracted together with the surface misalignment aberration correction after the initial system calibration. The flexible test for free form surface is achieved with high accuracy, without auxiliary device for DM deformation monitoring. Experiments proving the feasibility, repeatability and high accuracy of the ANI were carried out to test a bi-conic surface and a paraboloidal surface, with a high stable ALPAOTM DM88. The accuracy of the final test result of the paraboloidal surface was better than 1/20 Μ PV value. It is a successful attempt in research of flexible optical free form surface metrology and would have enormous potential in future application with the development of the DM technology.
We proposed an all-fiber tunable distributed Bragg reflector (DBR) laser rangefinder, which realized high-accuracy measurement of the absolute distance in the range of 1.88 to 3.33 m based on the self-mixing effect. The theoretical analysis of the all-fiber tunable DBR laser shows that wavelength tuning range and frequency of the laser can influence the measurement range and resolution. In addition, the experimental results indicate that the influence factors of measurement accuracy and sensitivity in absolute distance measurement are modulation response linearity, experimental equipment, and circuit bandwidth, which is in good agreement with theory analysis. Moreover, increasing the linear wavelength tuning range of the tunable DBR laser is an essential method to improve the measurement accuracy and sensitivity of the absolute distance measurement system.
In this paper, a simple structure, low-cost all-fiber laser self-mixing vibration sensing system is designed for monitoring the health status of thin-walled structures which are workpieces in machining, for resolving the interferential problems with the existing vibration measurement system, caused by the cruel environment such as electromagnetic interference, humidity. In this system, a distributed feedback all-fiber laser with ultra-short cavity length is used as the light source, and the real-time monitoring of vibration state of plate under different positions of forced vibration sources is observed. The experimental results show that when the forced vibration source located at the free end or the restricted end of the plate, the measurable self-mixing signals (signal-to-noise ratio up to 30dB) is available at any position. Moreover, it will help us to analysis the specification and the physical mechanism of the forced vibration, by studying this all-fiber laser self-mixing vibration sensor in detail.
A novel fiber-optic Fabry–Perot accelerometer (FOFPA) based on a spherical air bubble cavity is demonstrated. The cavity is fabricated by corroding a fiber end to form a groove and then arc discharging several times to form a spherical bubble. The bubble diameter and wall thickness are about 130 μm and 5 μm, respectively. Experimental results show that the resonance frequency of the accelerometer is 1520 Hz, the signal to noise ratio (SNR) is 40 dB at 706 Hz under 2 mg acceleration excitation, the voltage sensitivity is 14.16 V/g, and the minimum detectable acceleration of the measuring system is 20 μg/sqrt Hz at 706 Hz. Moreover, the accelerometer has a very low temperature sensitivity of 1.7 pm/°C, which means that FOFPA is basically immune to environmental temperature variation. Thus with advantages of miniaturization and simple structure, the FOFPA can be applied widely in the field of vibration measurements.
The phase-shifted sensitivity of an interferometer can be enhanced by increasing the group index. In this paper, we experimentally demonstrate a slow light sensor by placing an active fiber Bragg grating (FBG) in one arm of the Michelson’s interferometer. A 25 KHz AC voltage was applied to a piezoelectric (PZT) set nearby the active FBG. Once the wavelength is varied to near the FBG band edge, the maximum phase-shifted amplitude appears, which is about 1.8 rad and is 4 times greater than that when wavelength is near the center of the reflection band. The active FBG is pumped by a 980 nm laser diode, which can help us to stabilize the system works in the slow light regime to obtain the maximum phase shift. It provides a very simple approach to increase the phase-shifted sensitivity, which is likely to have important applications for strain and acoustic sensors.
A compact high birefringence polarization maintaining fiber (PMF) sensor for simultaneous strain and temperature measurement is proposed. This sensor is a modal interferometer (MI) sensor which is composed of a segment of high birefringence polarization maintaining fiber. One side of the fiber is spliced to the pigtail of a polarization beam splitter (PBS) with core-offset and axes alignment, the other end is spliced to the standard single-mode fiber (SMF) with core alignment and forms a fiber bubble. In experiments, an optical switch and the PBS are used to generate two orthogonal linear polarized lights. When the two orthogonal linear polarized lights enter the sensing fiber respectively, two different interference spectra will be achieved and own different response to the strain and temperature. Using these properties, the sensor realizes the simultaneous strain and temperature measurement. For 0.01 nm wavelength resolution, the strain and temperature resolution of the sensor are 10 uε and 0.285°C, respectively.
A long distance voice transmission system based on a visible light communication technology (VLCT) is proposed in the paper. Our proposed system includes transmitter, receiver and the voice signal processing of single chip microcomputer. In the compact-sized LED transmitter, we use on-off-keying and not-return-to-zero (OOK-NRZ) to easily realize high speed modulation, and then systematic complexity is reduced. A voice transmission system, which possesses the properties of the low-noise and wide modulation band, is achieved by the design of high efficiency receiving optical path and using filters to reduce noise from the surrounding light. To improve the speed of the signal processing, we use single chip microcomputer to code and decode voice signal. Furthermore, serial peripheral interface (SPI) is adopted to accurately transmit voice signal data. The test results of our proposed system show that the transmission distance of this system is more than100 meters with the maximum data rate of 1.5 Mbit/s and a SNR of 30dB. This system has many advantages, such as simple construction, low cost and strong practicality. Therefore, it has extensive application prospect in the fields of the emergency communication and indoor wireless communication, etc.
In this paper, a method for non-contact detecting the acoustic signal of tank gas leakage based on low-coherence optical fiber interferometer is presented. Vibration signals which caused by acoustic field of gas leakage are detected by low-coherence interference. The experimental results show that the vibration signals are wideband signals (0 Hz~90 KHz). While increasing the internal pressure, high-frequency components of the frequency spectrum have an obvious increasing trend, the amplitude and energy of the acoustic signal will both increase. The minimum detectable internal pressure of tank is 0.12MPa. The sensor is simple and reliable, and has a good practicability.
A novel fiber relative humidity (RH) sensor is demonstrated in this paper. The sensor is composed of a fiber Michelson modal interferometer (MMI) and the ZnO nanorods which grown on the fiber to improve the sensitivity of the sensor. Two standard single mode fibers are spliced to form the MMI, misaligned splicing program is used at the spliced point. Relative humidity sensing experiment shows that the intensity of interference spectrum changes linearly with relative humidity. With the relative humidity increasing in the range from 30% to 85%, the intensity of the dip in the interference spectrum linearly increases higher than 50%. The relative humidity response of the sensor is induced by the interference between core mode and cladding mode. The ZnO nanorods with high surface to volume ratio grown outside of the fiber cladding enhance the sensitivity of the sensor.
In this paper, we described a distributed strain measurement scheme in one-dimensional. The sensing information
of FBG is demodulated by a CCD spectrometer, the discrete strain is achieved by fitting and processing discrete
signal demodulated utilizing labVIEW virtual instrument technology. Then it could be achieved by Using
polynomial fitting method to one-dimensional discrete strain distributed detection. Experimentally, measurement
was implemented in Cantilever to prove the system performance. The experimental result shows that the system
can reflect the strain distribution in one-dimensional and an order strain modal characteristics of cantilever
accurately. The detection system can achieve real-time and dynamic measurements, the response time for 2kHz,
the response accuracy for 4μ(epsilon).
The orthogonal interferometer based on 1/8 wave plate generates orthogonal signals by optical
configuration. It is widely applied to interference system because of simple optical path and passive
demodulation scheme for easy operation. But in the process of demodulation, the DC component of the
signal needs to be eliminated. Because of the instability of light source and optical structure, the DC
component of the signal may change over time. Therefore the circuit subtracting a constant can not
thoroughly eliminate the DC component, causing the demodulation results are inaccurate. A improved
demodulation algorithm is proposed in this paper. Theoretical analysis and experimental results show
that, the demodulation method is almost not affected by the size of the DC component. When small
deviation in DC component occurs, the demodulation results keep stable and accurate.
A novel all-fiber Mach-Zehnder modal interferometer (MZMI) for temperature and strain measurements is demonstrated. The interference between the core and the cladding modes in the single mode fiber (SMF) is utilized. To excite the cladding modes, a microcavity is embedded in the SMF. After propagating a distance in the cladding, the cladding modes will couple back to the core mode at the other point fabricated by core-offset splicing. The MZMI exhibits a high temperature sensitivity (<0.1nm/ °C) in the range of 200-600 (°C). The strain response of the MZMI is also investigated. This compact, simple and cost-effective MZMI owns potential applications in temperature and strain measurements.
A fiber-optic flexural disk microphone is developed to detect acoustic signals in the air. It consists of a Mach-Zehnder interferometer with an optimized sensing arm of 7.93 m. The disk’s resonance frequencies and their influence on the microphone’s sensitivity are investigated. The microphone’s frequency response is measured in the frequency range from 100 Hz to 5 kHz and the average phase sensitivity is about -120.7 dB re 1rad/μPa.
In order to improve the resolution of laser Doppler interferometer, the noise of the system need to be suppressed. Through the analysis of the influence of laser relaxation oscillation mixing with signals in phase generated carrier (PGC) demodulation, we can get a conclusion that the low-frequency part of the laser intensity noise have a greater impact on the signal to noise ratio of PGC demodulation. By using Optoelectronic negative feedback method, we can achieve effective suppression of the low-frequency laser intensity noise and laser relaxation oscillation peak, which reduce the influence of the mixing of the light source intensity noise, and the system signal to noise ratio (SNR) is improved.
We demonstrate a tunable erbium-doped fiber ring laser based on an all-fiber filter. The filter consists of an optical
circulator, a fiber loop mirror(FLM) and a polarizer which is spliced to a segment of polarization maintaining fiber (PMF)
at the angle of 45° with respect to the fast axis of the PMF. The laser can be tuned by adjusting two polarization
controllers (PC) in the laser. The tuning range can reach to 6.08nm (1553.92-1560.1nm) with side mode suppression
ratio (SMSR) over 38 dB. The stability of the laser is verified at room temperature in one hour, and the power fluctuation of the laser is less than 0.5 dB.
In this study, an innovative temperature-insensitive and no adhesive package for FBG pressure sensor was designed. We presents an package construction to compensate for temperature deviation of a FBG by mechanical properties of different metal materials; the package realize no adhesive utilizing combined technique of electroless plating NI-P and electroplating NI and laser spot welding technology. On the basis of the study on the Material structural properties and the compensation principles analysis of the FBG, the finite element method is used to analysis of the feasibility of the construction and optimize the construction parameters. The results show that this compensation construction can effectively reduce the temperature sensitivity of the FBG.
In this paper, by utilizing two series connected Lyot-Sagnac filter, we propose a switchable erbium-doped fiber laser
(EDFL) which can be switched between single-wavelength and multi-wavelength in the experiment. The laser can
realize stable single-wavelength, dual-wavelength, triple-wavelength output at room temperature by adjusting
polarization orientations of two polarization controllers(PCs) in the two series connected Lyot-Sagnac filter. The optical
signal-to-noise ratio (OSNR) of the laser is over 30 dB, and the peak fluctuation over 30 minute is less than 0.5 dB at
room temperature.
A switchable dual-wavelength erbium-doped fiber laser is proposed. The proposed fiber laser, which utilizes a two-taper
Mach-Zehnder interferometer filter as the wavelength-selective component, realizes single-, dual-wavelength switching
by adjusting the curvature of two-taper. The side-mode suppression ratios (SMSRs) is over 36 dB and the power
variation for each wavelength is less than 0.5dB in one hour under room temperature.
In this paper, numerical simulation and analysis of the sensitivity of Strength-based Optical Fiber Sensors (SOFS) have
been reported .The results simulated show that suitable operating point can improve sensitivity of the sensing system
greatly even up to 3.6%. This conclusion can provide theoretical support for optimizing actual strength-based optical
fiber sensors with higher sensitivity.
In this paper, a simple and all-fiber frequency comb generator by stimulated Brillouin scattering (SBS) in highly
nonlinear fiber (HNLF) is demonstrated. The optical resonator is composed of a segment of highly nonlinear fiber and
two optical loop mirrors. Besides these, a segment of 6m-long erbium-doped fiber was placed in the resonator to
provided linear gain. A tunable laser and a 980-nm laser diode were used as the Brillouin pump and the EDF pump
respectively. When the wavelength and the power of pump lasers are adjusted correctly, dozens of comb lines with comb
spacing in 0.075 nm (9.37 GHz) are obtained.
Laser Doppler vibration measuring system has been widely used in many fields, in order to eliminate the phase fading,
phase generated carrier (PGC) homodyne demodulation technology has been applied usually, and dual-optical-channel
balanced detection technology has been used to suppress the intensity noise of the light source. However, theoretical
analysis reveals that the intensity noise of the light source cannot be eliminate totally due to the modulation, which is still
the main noise in the system. In this paper, we consider the influence of the intensity noise of the light source and the
shot noise of the detectors. By numerical simulating, we analyze the origin of the noise, especially the influence of
remnant intensity noise to signal-to-noise ratio of the system, and find the optimal parameters of the system.
Because of a series of advantages such as high sensitivity, non-contact measurement, interferometric vibration sensors
have attracted interest from a lot of researchers in vibration sensing field. In this paper, a novel Mach-Zehnder
interferomtric vibration sensor which utilizes quadrature detection technology is proposed. In our system, non-polarized
light source and 1/4 wave plate is used to obtain two in-phase and quadrature-phase (I/Q) signals. Compared with
previous methods, this sensor system has a simple optical configuration and more reliable stability. Theoretical analysis
indicates that this sensor can measure the vibration displacement accurately.
Fiber Bragg grating (FBG) is an important optical passive device and widely used in optical fiber communication
system. The application of normal organic polymer coating FBG used in some area is limited because of its easy
annealing and aging at high temperature. Employing chemical methods, we obtain certain thickness of uniform and
compact surface silver layer by controlling etching conditions and reaction time. Since the interference of the external
stress, the experiment result shows that the sensitivity of temperature keeps good linear response characteristic in the
meanwhile. It is proved that FBG with metal silver coating effectively improved the characteristic in its sensing area.
KEYWORDS: Demodulation, Interferometry, Signal processing, Algorithm development, Vibrometry, Signal detection, Sensors, Wave plates, Signal generators, Analog electronics
The interferometric vibration sensor with quadrature detection can generate orthogonal signals by optical configuration.
The influence of the DC component existing in the interferometric orthogonal signals is discussed in this paper. The
signals were processed by time derivation and arctangent algorithm to demodulated the analog signals. Theoretical
analysis and experimental results prove that this method can be used to eliminate the DC component in the signal,
meanwhile, the micro-vibration signals can be measured accurately.
We present a theoretical analysis and a comparison to experimental results on self-mixing interference. From the theoretical analysis and experiment, the error of the velocity measurement, which is caused by a speckle pattern, has been investigated and optimization of the laser diode velocity system by selecting suitable experiment parameters and proper signal processing methods has been reported. The actual measurement error is <3.1% over a wide velocity range (5.2-479 mm/s).
The phenomena of homodyne and resonating in optical fiber rings (OFRs) are experimentally investigated. The beat noise characteristics in passive OFRs and active OFRs are revealed. The beat noise spectra of OFRs and homodyne interferometer with the same length of delay fiber line are compared. A substitute homodyne
interferometer configured by band-reject OFR is proposed.
The saturation effect of EDFA on frequency filtering characteristics of active fiber ring resonators is studied theoretically and experimentally in this paper. When input signal power is large enough, it would enter the saturation regime and lower EDFA gain, thus, the finesse of active fiber ring resonator descends as the same when lower pump power is injected, all of which are confirmed by comparing the output signal spectrums with different input signal power and different pump power in our experiments. By using the atom rate equations of EDFA and the multi-light interference theory, the experiment results are well explained. On considering this EDFA saturation effect, the optimization of active fiber ring resonator filter is also discussed. To obtain both narrower linewidth light and larger output power, one should use input coupler with small coupling ratio and output coupler with large coupling ratio to construct an active fiber ring resonator filter, making sure that saturation effect of EDFA can not occur.
A kind of connector utilizing self focusing optical fibers is designed to couple Lambertian point source to multimode optical fibers. According to the emission pattern of the source and transmission characteristic of self focusing optical fiber, our calculation result shows that such a coupling scheme can improve coupling efficiency and tolerance.
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