We report on a low-cost Brillouin fiber ring laser pumped from an actively stabilized self-injection locked distributed feedback (DFB) laser diode. Locking of the commercial DFB laser to a ~11-m-length high-Q-factor fiber-optic ring cavity leads to ~10,000-fold narrowing of the laser Lorentzian linewidth down to 400 Hz. Such pump laser operation inside the ring cavity forces the cavity to host Brillouin lasing enabling the laser threshold power as low as ~1.5 mW. The laser operation is perfectly stabilized by active optoelectronic feedback driven by a simple microcontroller. The laser delivers radiation at Stokes frequency with the Lorentzian linewidth reduced down to ~75 Hz and a phase noise less than –100 dBc/Hz (<30 kHz). The reported laser configuration is of great interest for many laser applications where a narrow sub-kHz linewidth, simple design and low cost are important.
Low-noise lasers are a powerful tool in precision spectroscopy, displacement measurements, and the development of advanced optical atomic clocks. All applications benefit from lower frequency noise and robust design, however, the generation of microwave signals additionally requires narrowband lasing at two frequencies. Here, we introduce a simple optoelectronic oscillator enabling the generation of a stable ultra-narrow microwave carrier signal with low phase noise based on stimulated Brillouin scattering. A cost-effective sub-kilohertz Brillouin fiber ring laser with stabilized selfinjection locked pump DFB (Distributed Feedback Laser) laser is used for this purpose. The system is supplied by a low-bandwidth active optoelectronic feedback controlled by a low-cost USB-DAQ card. The full-width of generated microwave signal at -3 dB level is approximately equal to 300 Hz with a peak maximum at ~10.946 GHz. The strongest parasitic harmonics shifted from carrier signal peak by ±50 kHz, ±450 kHz, and ±900 kHz are below the main peak by 45-50 dB. A phase noise below −90 dBc/Hz for a frequency offset above 10 kHz from the carrier after passing the 20 km length test fiber has been achieved.
We present novel experimental method for estimation of the light penetration depth (LPD) in turbid media based on the
analysis of the cross-correlation function of speckle patterns. Under certain illumination conditions, the amplitude of the
correlation function is strongly dependent on the penetration depth. Presented theoretical model based on the Bragg
diffraction from the thick holograms allows LPD estimation if only one parameter of the material, namely refractive
index, of the material is known. However, qualitative LPD comparison is possible without knowledge of the material
properties. Feasibility of the method was checked experimentally. Experimental results were additionally verified by
alternative experimental method.
We have demonstrated suppression of low-frequency fluctuations of backscattered Rayleigh radiation in distributed fiber
optical sensors with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by
incoherent optical feedback. Significant decreasing of Rayleigh power variations up to 15-20 dB for 10-1000 Hz
frequency interval was recorded for both chaotic lasers. It was shown that chaotic DFB laser also efficiently restrain
stimulated Brillouin scattering in the test fiber. The results have important consequences for distributed fiber optical
sensors, which utilized Rayleigh signals.
We present an inexpensive technique capable to interrogate multiplexed sensors based on ultra-low-reflective Bragg
gratings written in a long standard telecom fiber. The technique is suitable for distributed detection and localization of
disturbances in security and early warning systems for pipeline monitoring, in linear temperature sensors for fire
detectors, etc. It is based on the correlation OTDR principle measuring cross-correlation between a noise-like probe
signal and the signal reflected back from the sensing fiber. In order to simplify the sensor configuration, an unmodulated
CW DFB diode laser was used as a light source. A noise-like probe signal was generated by conversion of phase noise
of the DFB laser into intensity noise with a help of unbalanced Michelson interferometer. Results of the experimental
verification of the proposed technique are presented.
An optical fiber sensor system for location of a vibrational disturbance along the fiber is presented. The sensor system is
based on a serial array of identical F-P interferometers, formed directly in the single mode SMF-28e fiber by pairs of
fiber Bragg gratings with reflectivity of 0.04 % each. Interferometers were interrogated by a DFB diode laser which was
intensity modulated at 10 kHz. A method for localization of a disturbed interferometer and experimental results for a
serial array of 14 sensors are presented. Simple sensor configuration and the use of low-frequency components make it
potentially inexpensive and suitable for applications where a continuous monitoring of long structures has to be
performed for appearance of vibrations.
We present very simple and sensitive techniques capable to interrogate ultra-weak Bragg gratings written in a long
SMF-28 fiber. The techniques are suitable for distributed detection and localization of alarm conditions in early warning
systems. Also, a high multiplexing capability was demonstrated in a multi-point measuring system utilizing an array of
identical FBGs. This technique is based on measuring correlation between the probe and reflected signal. A DFB laser
operating in a CW regime was used as a light source. We present results of experimental verification of the techniques in
different sensor configurations for static strain and vibration measuring. Multipoint sensor using Bragg gratings with
reflectivity of 0.01% printed in a 3-km long fiber was demonstrated.
We present a novel approach that enables online, real-time and non-contact measurements of thickness of protective coatings. The proposed technique based on spatial filtering of dynamic speckles generated by rapidly deflected laser beam. An advantageous feature of the technique is that it is capable for very fast measurement of coating thickness with accuracy of one micrometer while their roughness is 20 μm or higher. Such a high performance is achieved due to proper consideration of statistical properties of spatially filtered dynamic speckles. In this paper we report experimental study of correlation properties of photodiode responses in different configurations of optical setup. The results are in good agreement with theoretical estimations. Performance of a laboratory prototype of the proposed sensor is demonstrated in application to profile measurements of a tube coated by protective layers of different thickness.
KEYWORDS: Prototyping, Signal attenuation, Connectors, Fiber optics sensors, Error analysis, Temperature metrology, Time metrology, Data acquisition, Rayleigh scattering, Fiber lasers
The paper describes first completely autonomous measurement system based on transmission-reflection analysis (AMSTRA).
The autonomous system utilizes simple optical scheme with 2 mW Fabry-Perot diode laser and original data
acquisition and processing system. The location of the loss region is determined from unique relationships between
normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber.
Accuracy, temporal and thermal stability of the autonomous system with 5.6 km-length test fiber were investigated. The
paper also presents the preliminary results of the autonomous measurement system implementation for gasoline leak
detection and localization.
We report, for the first time to our knowledge, the detection and localization of a loss-inducing perturbation along more than 20-km-long test fiber based on the analysis of transmitted and Rayleigh backscattered powers of an unmodulated continuous-wave light source. Localization of a strong disturbance with an estimated accuracy of 1 m near the source end and 7.5 m near the remote end of the 21.067-km-long single-mode sensing fiber is demonstrated.
We report on the detection of a loss-inducing perturbation with variable localization accuracy along a test fiber based on the analysis of transmitted and reflected powers of an unmodulated continuous-wave light source. The required accuracy of localization is provided by suitable distribution of the differential reflectivity along the fiber. We demonstrate a localization accuracy equal to ±1.0 m along the 3.939-km single-mode test fiber for the strong perturbation and ±5.0 mm along a designated 10-cm fiber part for weak perturbation.
The paper describes a new fiber-optic sensing technique using Bragg grating-based interferometers. This technique joins advantages of the fiber Bragg grating and interferometric sensor. Sensing interferometers have been formed by pairs of identical closely spaced low reflective Bragg gratings imprinted in the core of single mode fiber. A digital signal processing of interference fringes in the sensor reflection spectrum has been developed. It extends multiplexing capabilities of the fiber Bragg grating-based sensor and provides absolute and high-resolution measurements in a wide dynamic range. Experimental results on testing the sensor performance for strain and temperature measurements are presented.
KEYWORDS: Demodulation, Fiber Bragg gratings, Sensors, Signal to noise ratio, Error analysis, Fiber optics sensors, Interference (communication), Detection and tracking algorithms, Signal processing, Refractive index
Theoretical resolution limits of evaluation of centroid shift in fiber optic Bragg grating sensors are analyzed. In the theoretical model, it is assumed that random additive Gaussian noise in a sensor signal acquisition equipment is the main origin of the system error. To provide noise resistant demodulation, a new optimal signal processing algorithm is suggested. It allows accurate detection of the Bragg wavelength shifts within large range and keeps resolution very close to theoretical limit imposed by a random noise. The new demodulation technique is also compared with well known centroid detection algorithm.
A novel type of distributed fiber optic sensor for petroleum hydrocarbon leak detection based on the measurement of transmitted and Rayleigh backscattered power is presented. The sensor structure includes a sensitive polymer, which reversibly expands in hydrocarbon presence and induces the bending losses in the fiber. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of +/- 1 m along a 2.844 km-length single-mode fiber was demonstrated.
For some applications there is no necessity in precise measurement of external influence strength. Sometimes, for early warning and alarm systems, it is sufficient to detect appearance of the monitoring variable within some predetermined interval. A system must generate an alarm signal when the monitored parameter approaches near or crosses the limit of the dangerous zone. For such applications, we propose a simple distributed sensor based on a large array of equal Bragg gratings. An interrogation of the sensor is performed at a fixed wavelength. We discuss a principle of operation of the sensor and present results of experimental demonstration of this technique for distributed detection of gasoline leak.
New digital demodulation algorithm for white-light interferometric sensors with signal processing in spectral domain is presented. The algorithm uses phase calculation from an interferometric pattern in the acquired spectrum from the sensor to evaluate the optical path difference. Suggested demodulation method provides high accuracy with uncertainty close to theoretical limit imposed by system noise. For a fiber-optic sensor with birefringent fiber and a CCD spectrometer for signal acquisition, the algorithm demonstrated the random error of the optical path difference estimate about 0.18 nm.
This paper presents a brief overview of research activities in fiber optic sensors at the Applied Physics Division of the Scientific Center CICESE. Started in 1992, the research was directed to develop a new wavelength scanning technique for multiplexing and interrogation of a serial array of interferometric sensors in polarization-maintaining fiber. A further developing of this technique resulted in a new approach for multiplexing and demodulation of Bragg grating-based sensors. A novel algorithm has been developed for digital demodulation of the twin-grating sensor, which provides absolute high resolution measurements in a wide dynamic range. Recent experiments on testing a twin grating fiber optic sensor demonstrate very promising results. A few other developed sensing techniques are also presented in this paper.
The paper presents distributed fiber optic bending sensor for petroleum hydrocarbon detection based on COFDR technique. Sensitive polymer, which effectively swells under hydrocarbon influence, was employed in order to introduce bending losses in hydrocarbon presence. In this work we used lumped reflectors, namely fiber Bragg gratings, placed between distributed sensitive elements. Proposed design of the sensor utilises the principle of truly distributed detection with discrete localisation of perturbation. We have demonstrated that the COFDR technique with bending based chemical sensor is capable to detect hydrocarbon presence within a few minutes for 20-cm perturbation-length with spatial resolution up to 0.5 meters.
We present experimental results on testing a twin grating fiber optic sensor for measurement of static strain. The sensor is built with two identical Bragg gratings closely spaced in a single mode fiber. It produces a reflection spectrum modulated due to interference. Similar to traditional FBG sensors, strain of the fiber leads to the wavelength shift of the reflection spectrum. This shift can be determined precisely measuring phase changes for corresponding components of Fourier transform of the reflection spectrum. Such an approach allows an absolute strain measurement with an interferometric sensitivity. Resolution of 0.02 degree(s)C and 0.2(mu) (epsilon) was demonstrated experimentally for temperature and static strain measurements using a pair of 0.7mm long Bragg gratings with reflectivity of 1% each.
A novel differential double Bragg grating sensor for temperature-insensitive strain measurement is presented. The sensor consists of two identical weak measuring and reference gratings separated by some distance. The reference grating is placed inside the silica capillary that made it almost strain insensitive. The basic idea in differential double Bragg grating sensor is to measure the energy of the oscillating term in reflection spectrum of double Bragg grating structure. The normalized energy of the interference term depends on the pitch difference of two gratings and does not change when pitch variations of both gratings are equal with temperature variation. Therefore the normalized energy of the interference term can be used for the temperature-insensitive strain measurement.
A new demodulation technique for fiber Bragg grating based sensors is presented. The technique makes a use of pairs of low reflectance Bragg gratings forming a low finesse Fabry-Perot local sensor. The gratings in a pair have different sensitivity to a measurand, and overlapping of their reflectance spectra depends on the external influence. Demodulation is based on measurement of the degree of the grating’s spectra overlapping by means of energy of interference pattern in reflection spectrum of the interferometer. We demonstrate implementation of the proposed technique in experiments with axial strain and temperature measurements.
We present two configurations of new fiber optic Bragg grating sensor developed for petroleum hydrocarbon detection. The sensor includes FBG attached to the special polymer swelling in the hydrocarbon presence. This polymer reversibly strains the fiber section with the Bragg grating inside. The first sensor configuration employs a single grating and standard demodulation technique to determine a strain induced shift of the Bragg wavelength. The second sensor configuration make a use of double grating interferometric sensor consisting of a reference grating free from axial strain and a sensitive one, attached to the polymer. A new demodulation technique is based on monitoring of energy of the interference term in the reflection spectrum. We demonstrated experimentally that such configuration is free from temperature influence. The paper presents the results of the swelling-behavior test of the sensitive polymer material and the results of the experimental investigation of the hydrocarbon sensor performance.
The paper presents the theoretical and experimental results of the strain- and temperature gradient measurement with differential twin Bragg grating sensor. It is shown that this technique could also be used for temperature- insensitive strain measurements.
In this paper, we present a new fiber optic Bragg grating sensor for petroleum hydrocarbon leak detection. The developed sensor includes fiber section with imprinted in the fiber core Bragg grating covered by special polymer material. This polymer reversibly swells under hydrocarbon influence and strains the fiber section with the Bragg grating inside. As a result of the fiber elongation, the Bragg wavelength shifts to aside longer wavelength. Experimentally demonstrated shift of the grating resonant wavelength was more than 2 nm for 20-min gasoline influence, which significantly exceeds a shift due to possible environmental temperature variation and the width of the grating reflection spectrum, which was about 0.5 nm. The paper presents also the results of the swelling-behavior test of the loaded and unloaded sensitive polymer material under liquid and vapor gasoline influence and the results of the theoretical and experimental investigation of the hydrocarbon sensor performance.
A simple procedure is developed for the measurement of the differential quadratic electro-optic coefficient, R33, by two-beam polarization (TBP) interferometry. It is shown that a TBP interferometer can be used for measuring the Kerr coefficient of a thin film with a strong Fabry-Perot effect. The measured values of the differential effective Kerr coefficient, R33 of lead zirconate titanate 52/48 thin film lie inside the interval between -0.5 * 10-18 m2/V2 and +1.7 * 10-18 m2/V2 for the external DC field from -160 kV/cm to 160 kV/cm, in agreement with the known data. The correlation between differential electro-optic coefficients and field-induced birefringence is discussed.
New digital demodulation algorithm for recently announced twin-grating fiber-optic sensor is presented. The sensor is built with two short, identical, low reflectance, closely spaced Bragg gratings imprinted in a telecommunications fiber. Because of such a design the reflection spectrum of this sensor is a sinusoidal wave within an envelope of the reflection spectrum of a single Bragg grating. Similar to traditional sensor based on Bragg gratings, the change of the temperature or strain leads to a shift of the reflection spectrum along the wavelength axis. However the reflection spectrum of the twin-grating sensor has more complicated structure and instead of measuring the position of gravity center of the spectrum we use more sophisticated algorithm to evaluate the measurands. Two computational methods are used to estimate the shift of the spectrum: evaluation of the phase of the sinusoidal wave and calculation of the position of the envelope. This combination of two different methods provides high accuracy of temperature or strain measurements. The ability of this sensor to provide simultaneous measurement of both temperature and axial strain is also demonstrated using computer simulation of twin-grating sensor in a Hi-Bi fiber.
The development of a new fiber Bragg grating multiplexing technique is reported. The technique is based on a twin Bragg grating sensor configuration and exploits a combination of wavelength- and frequency-domain multiplexing. We demonstrate that very weak Bragg gratings with reflexivity of approximately 0.5 percent or less can be efficiently used by this technique. Experimental results presented in the paper and theoretical estimates indicate that the interrogation of more than 100-element serial array sensor system is feasible.
Many different schemes of Bragg grating based quasi-distributed and multiplexed sensors have been reported.1 Most of them utilize wavelength division multiplexing and detect the resonant wavelength for sensor interrogation
A method of interrogation of a quasi-distributed sensor in the frequency domain is discussed. The sensor consists of a series of pairs of the identical Bragg gratings imprinted in the fiber core along its length. Each pair of the Bragg gratings performs as a low reflective Fabry-Perot interferometer producing a cosine modulated reflection spectrum within a grating's main lobe. The detector signal represents the superposition of the modulated reflection spectra which can be decomposed using the fast Fourier transform.
The operation of distributed polarimetric fiber-optical sensors using wavelength scanning technique is discussed along with algorithms for processing of a signal coming form the sensor to retrieve the information about the distribution of external force and field along the fiber. The principle of the sensor operation is based on a change of spectral and polarization characteristics of broad-band radiation, propagating in the fiber, caused by the external influence. The distribution of the external force along the fiber is determined by the computer processing of the output spectrum. The analysis of such sensor and the results of our experiments demonstrate high spatial resolution and sensitivity.
The operation of new quasi-distributed interferometric sensor is discussed. The sensor is based on the array of unbalanced interferometers formed by point polarization couplers along birefringent fiber. Simultaneous interrogation of the sensing interferometers was achieved by using spectroscopic methods and a simple signal processing. Results of investigation of sensor performance and analysis of cross-talk problem are presented.
The design of a low-noise holographic interferometer applied to the study of diffusely scattering objects, is presented. This system is a dual-beam arrangement used for the reconstruction process, the anisotropic self-diffraction effect in Bi12TiO20 photorefractive crystals (PRC). It is shown both theoretically and experimentally that holographic recording and reconstruction of complex wavefronts in PRCs, is different from the case of a plane wavefront. This fact is taken into account to optimize the setup. The main optimization criterion is the enhancement of the signal-to-noise ratio (SNR) of the interferogram. Our theoretical predictions of SNR dependence on the recording beam intensities ratio, are verified by the experimental results here shown. An experimental approach to optimize a holographic interferometer with PRC is also presented.
A new technique for distributed fiber-optic sensors with polarization mode coupling is presented. The technique includes wavelength scanning and optical path compensation, allowing the user to choose the sensitive part at different positions along the fiber. A new digital processing procedure with birefringence dispersion data taken into account makes it possible to measure the coupling distribution along the sensitive part at more than 300 points simultaneously with a spatial resolution of 0.3 cm.
Analog and digital processors, based on fiber-optic logic elements are presented. It is shown that, despite long signal delays in logic elements, the maximum computing rate of fiber-optic processors is limited mainly by the bandwidth of basic elements.
The fiber exhibits a number of features that make it extremely
attractive for use in optical information processing systems. Among
them are the huge bandwidths and high efficiencies of nonlinear pile-
nomena achieved at relatively low powers of laser radiatioui. In addi
tion, the problem of overheating is obviated here because heat is
removed through fairly large lateral surfaces.
The systems suggested so far rely on two nonlinear rfiechanisms.
The principles of the use of SRS were discussed in /1-3/. Prom the
estimates, the pulse repetition frequency in the SRS-device can be
as high as io12 Hz for pulse energies of i012 A variety
of logic elements operating through light-induced refractive-index
variations were also suggested /4-6/. They provide the pulse repetition frequencies up to 1013-i014 Hz.
Lu this paper we discuss different versions of optical signal
processinF systems.
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