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We present an in-depth analysis of the Kerr effect in resonator fiber optic gyros (R-FOGs) based on triangular wave phase modulation. Formulations that relate gyro output to the rotation rate, the Kerr nonlinearity, and other fiber and gyro parameters are derived and used to study the effect of Kerr nonlinearity on the gyro performance. Numerical investigation shows that the Kerr effect results in a nonzero gyro output even when the gyro is at stationary, which is interpreted as an error in the measurement of rotation rate. This error was found to increase as the frequencies of the two triangular phase modulations deviate from each other, and is not zero even if the intensities of the two counterpropagating beams are exactly the same. For fixed frequencies of the triangular phase modulations, there exists an optimal intensity splitting ratio for the two counterpropagating beams, which leads to zero gyro error. Calculation shows that the measurement error due to the Kerr effect for an R-FOG with a hollow-core photonic bandgap fiber as the fiber loop can be one to two orders of magnitude smaller than an R-FOG with a conventional single mode fiber loop. [c] 2010 Optical Society of America OCIS codes: 060.2370, 060.2800.

A simple system that simultaneously supports active mode-locking and self-seeding schemes for wavelength-tunable optical short-pulse generation is proposed. The system consists of a gain-switched Fabry--Perot laser diode, an erbium-doped fiber amplifier, a tunable optical filter, and two circulators. The mode-locked optical pulses exhibit good stability, a high side-mode suppression ratio of more than 31 dB over a wide wavelength tuning range of 42 nm, and a pulse width of around 35 ps at a repetition frequency of -2.8 GHz.

We report on interferometric noise limitation of fiber-optic gas sensors with highly coherent lasers and wavelength modulation spectroscopy. Interference between signal wave and reflected waves causes signal fluctuation in the output, which limits the performance of the sensing system. Sensor resolution limited by interferometric noise is calculated for a fiber-optic gas sensor with the Q(6) absorption line of methane gas at approximately 1650 nm. The results are useful for system designers of this particular type of gas sensor. Key words: Fiber-optic sensors, gas sensors, interferometric noise.

We report on the use of a frequency-modulated continuous-wave technique for multiplexing optical fiber gas sensors. The sensor network is of a ladder topology and is interrogated by a tunable laser. The system performance in terms of detection sensitivity and cross talk between Sensors was investigated and found to be limited by coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation-scanning coupled with low-pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 parts in 10s for each sensor may be realized. A two-sensor acetylene detection system was experimentally demonstrated that had a detection sensitivity of 165 parts in [10.sup.6] for 2.5-cm gas cells (or a minimum detectable absorbance of 2.1 x [10.sup.-4]) and a cross talk of -25 dB. [C] 2001 Optical Society of America OCIS codes: 060.2370, 060.4230, 060.2630, 300.1030, 300.6340, 300.6380.

A fiber optic probe for the simultaneous measurement of chloride ions and temperature is presented. The Ag/alginate composite film is used as the reflective surface of the Fabry-Perot interferometer (FPI) and is a sensitive film for the adsorption of chloride ions. The experimental results show that the Fabry-Perot (FP) response sensitivity is approximately 1.4689 nm/µM as the chloride ion concentration changes from 1 to 9 µM, but the fiber Bragg grating (FBG) is insensitive to chloride ions. When the temperature is changed from 35°C to 80°C, the response sensitivities of the FP and the FBG are about 0.7 and 0.01115 nm/°C, respectively.

Owing to manufacturing defects of micropolarizer arrays and differences in the pixel response of detectors, division-of-focal-plane (DoFP) polarimeters have severe nonuniformity, which affects the measurement accuracy of the polarimeters and the calculation of the polarization information. This study proposes a calibration method for thermal infrared DoFP polarimeters considering polarizer reflection characteristics. The temperature-controlled adjustable infrared polarized radiation source is calibrated by a division-of-time polarimeter and is, in turn, used to calibrate a thermal infrared DoFP polarimeter. Through laboratory blackbody and external scenes, the performance of the proposed method is compared to that of state-of-the-art techniques. The experimental results indicate that the proposed method effectively avoids overcalibration and improves the accuracy of polarization information.

To enhance the capability of phase-sensitive optical time domain reflectometers ( Φ -OTDR) to recognize disturbance events, an improved adaptive feature extraction method based on NMF-MFCC is proposed, which replaces the fixed filter bank used in the traditional method to extract the mel-frequency cepstral coefficient (MFCC) features by a spectral structure obtained from the Φ -OTDR signal spectrum using nonnegative matrix factorization (NMF). Three typical events on fences are set as recognition targets in our experiments, and the results show that the NMF-MFCC features have higher distinguishability, with the corresponding recognition accuracy reaching 98.47%, which is 7% higher than that using the traditional MFCC features.

Baseline correction is necessary for the qualitative and quantitative analysis of samples because of the existence of background fluorescence interference in Raman spectra. The asymmetric least squares (ALS) method is an adaptive and automated algorithm that avoids peak detection operations along with other user interactions. However, current ALS-based improved algorithms only consider the smoothness configuration of regions where the signals are greater than the fitted baseline, which results in smoothing distortion. In this paper, an asymmetrically reweighted penalized least squares method based on spectral estimation (SEALS) is proposed. SEALS considers not only the uniform distribution of additive noise along the baseline but also the energy distribution of the signal above and below the fitted baseline. The energy distribution is estimated using inverse Fourier and autoregressive models to create a spectral estimation kernel. This kernel effectively optimizes and balances the asymmetric weight assigned to each data point. By doing so, it resolves the issue of local oversmoothing that is typically encountered in the asymmetrically reweighted penalized least squares method. This oversmoothing problem can negatively impact the iteration depth and accuracy of baseline fitting. In comparative experiments on simulated spectra, SEALS demonstrated a better baseline fitting performance compared to several other advanced baseline correction methods, both under moderate and strong fluorescence backgrounds. It has also been proven to be highly resistant to noise interference. When applied to real Raman spectra, the algorithm correctly restored the weak peaks and removed the fluorescence peaks, demonstrating the effectiveness of this method. The computation time of the proposed method was approximately 0.05 s, which satisfies the real-time baseline correction requirements of practical spectroscopy acquisition.

As a kind of III/VI group compound 2D layered material, indium selenide ( I n 2 S e 3 ) has attracted tremendous interest because of its favorable optoelectronic characteristics. Here, magnetron sputtering deposition (MSD) technology was employed to prepare an I n 2 S e 3 -based saturable absorber (SA). The nonlinear optical properties of this SA, whose modulation depth ( Δ T ) is 6.18%, were studied. With the aid of its saturable absorption, a stable two-wavelength Q -switching Er-doped fiber (EDF) laser was established. When pump power was adjusted to 900 mW, the output power was increased to 63.84 mW. The shortest pulse duration and maximum pulse energy were estimated to be 556 ns and 376 nJ, respectively. The signal-to-noise ratio of 70 dB proves this fiber laser has high stability. In comparison with previous works, the laser performance in this study is improved significantly. These results indicate that the I n 2 S e 3 holds promise as an outstanding candidate for high-energy pulse generation and will advance the development of I n 2 S e 3 -based nonlinear photonics devices.

A convenient method to fabricate two-dimensional photonic quasicrystal microstructures was experimentally demonstrated by using a rotatable four-wedge prism. Two-dimensional eightfold symmetric quasicrystal microstructures are formed by two groups of twisted square lattices in a photorefractive crystal. The experimental devices of this method are simple and stable without complicated optical adjustment equipment. Optical-induced quasicrystal microstructures are analyzed and verified by magnified imaging and far-field diffraction pattern imaging. The method can be extended to fabricate more complex quasicrystal and moiré lattice microstructures. We numerically demonstrate that this method can be used to fabricate other complex photonic microstructures by using different multi-wedge prisms and adjusting the rotation angle properly.

Common polarization imaging models are mostly based on an ideal polarizer assumption. This paper proposes a polarization imaging non-ideal model considering the non-ideality of polarizers. The corresponding correction formulas for degree of linear polarization (DoLP) and angle of polarization are also provided. Experiments on linearly polarized light and partially polarized light reflected by a glass plate suggest that when the extinction ratio of polarizers is 100:1, the DoLP relative error of linearly polarized light with the non-ideal model is reduced by 1.87% compared to that with the ideal model; the DoLP relative error of partially polarized light with the non-ideal model is reduced by 1.69% compared to that with the ideal model. Application of the non-ideal model can effectively improve the precision of polarization measurement. In particular, this improvement is more obvious with a low-extinction-ratio (less than 100:1) analyzer.

This paper proposes a simple and practical reflective birefringent fiber interferometer sensor that consists of a polarization beam splitter, a polarization-maintaining transmission fiber, and a sensor unit comprising two segments of birefringent fibers and a thin-film reflector. This sensor could be used for dual parameter sensing. Experiments with different temperatures and lateral loads applied to the sensor unit demonstrated temperature and load sensitivity of ∼0.0010  nm/°C and 0.298 nm/N, respectively. Further study showed that temperature-insensitive lateral load measurement can be achieved by using equal length of birefringent fibers and performing differential wavelength measurement. The sensor is robust against environmental disturbance on the transmission fiber, making it potentially attractive for practical field applications.

We introduce and verify a single-channel time-division filtering low-light-level (LLL) color night vision system (3LCNV). The imaging scheme, comprising a tunable liquid crystal filter, three-generation GaAsP image intensifier, and CMOS camera, achieves LLL color imaging and ensures sensitivity. The image enhancement and color reconstruction algorithm flow suitable for LLL night vision combines overexposure-against white balance, color correction matrix (CCM) color correction, and color image denoising to improve color visibility and reduce color difference and image noise. The proposed night vision system extends the minimum working illuminance to 10 -4    lx and achieves natural and clear color LLL imaging, improving night-time observations.

At the present time, there are many image contrast enhancement methods where the main considerations are detail enhancement, noise suppression, and high contrast suppression. Traditional methods ignore the characteristics of the display or merely consider the display as a whole. However, due to the limited dynamic range of most display devices on the market, the difference between two adjacent grayscales of the display is often below the just noticeable difference of the human visual systems, which causes many image details to be invisible on the display. To solve this problem, we present a preprocessing method for image contrast enhancement. The method combines the characteristics of the human eye and the display to enhance the image by examining the local histogram. When displaying the processed image, the algorithm maintains as much image information as possible, and image details will not be lost due to the limits of the display device. Moreover, this algorithm performs well for noise suppression and high contrast suppression. The algorithm is an image enhancement method and can also be a correction method for images enhanced by other methods when prepared for display.

Polarization imaging technology provides information about not only the irradiance of a target but also the degree of polarization and angle of polarization, which indicate extensive application potential in the field of ocean remote sensing. Natural light can be converted into partially polarized light by the reflection from an interface, and the Fresnel equations can describe the quantitative relationship between the angle of incidence and the degree of polarization of the reflected light. However, the relationship between the angle of polarization and angle of incidence has rarely been studied. In this study, we investigate the polarization state model of reflected light and establish the relationship between the angle of polarization and angle of incidence. This is verified using polarization imaging experiments on a glass plate and calm water surface. The results indicate that the theoretical model agrees well with the experimental results. A method to eliminate the ambiguity of the angle of incidence is proposed based on the model, and its effectiveness and feasibility are verified. It lays the theoretical foundation for imaging detection based on the polarization imaging of transparent media surfaces and sea surface ripples.

This publisher's note amends the author listing in Appl. Opt.57, 3991 (2018)APOPAI0003-693510.1364/AO.57.003991.

Current methane gas leak detection technology uses infrared imaging in the medium wave (MW) or long wave (LW) bands, essentially applying cooled infrared detectors. In this study, a simplified three-layer radiative transfer model is adopted based on methane gas detection theory, considering background radiation, atmospheric infrared absorption, gas absorption, and emission characteristics to analyze the contrast of methane gas thermography in different infrared bands. The analysis results suggest that under certain conditions, the 6.6-8.6 μm LW band provides higher contrast compared to the 3-5 μm MW band. The optimal imaging wavelength band is selected according to imaging contrast advantages and disadvantages, and infrared optical systems and infrared filters are designed and optimized. We build a passive methane gas leak detection system based on uncooled infrared focal plane array detectors. By collecting gas images under different conditions, the imaging detection capabilities for methane gas leaks in the MW and LW bands in a laboratory environment are compared. Finally, the developing trends in methane gas detection technology are analyzed.

The existing methods for shape measurement using polarization of transparent objects are based on two assumptions: (1) the paraxial assumption, assuming that the reflected ray is parallel to the optical axis of the imaging system; and (2) the s-component approximation assumption, which assumes that the s-component of the reflected light is predominant and the p-component is neglected. To overcome limitations posed by these two assumptions, this paper proposes a method based on the polarization characteristics of reflection from a transparent surface and vector operation. To overcome the paraxial assumption, the normal vector of the transparent surface is deduced by vector operation, analyzing the relationships between the direction vector of reflection, the normal vector of the reflection plane, the intersection line of the reflection plane and imaging plane, and the normal vector of the transparent surface. To overcome the limitations of the s-component approximation assumption, the angle between the s-component and the polarization direction of the reflected light is analyzed, which yields improved measurement precision. An experiment was performed with transparent targets (flat glass positioned at different angles), and the results show that the measurement error with this method is significantly less than those of existing methods. Thus, we believe this method overcomes the abovementioned limitations while also improving measurement precision.

A novel method to simulate the polarimetric infrared imaging of a synthetic sea surface with atmospheric Mie scattering effects is presented. The infrared emission, multiple reflections, and infrared polarization of the sea surface and the Mie scattering of aerosols are all included for the first time. At first, a new approach to retrieving the radiative characteristics of a wind-roughened sea surface is introduced. A two-scale method of sea surface realization and the inverse ray tracing of light transfer calculation are combined and executed simultaneously, decreasing the consumption of time and memory dramatically. Then the scattering process that the infrared light emits from the sea surface and propagates in the aerosol particles is simulated with a polarized light Monte Carlo model. Transformations of the polarization state of the light are calculated with the Mie theory. Finally, the polarimetric infrared images of the sea surface of different environmental conditions and detection parameters are generated based on the scattered light detected by the infrared imaging polarimeter. The results of simulation examples show that our polarimetric infrared imaging simulation can be applied to predict the infrared polarization characteristics of the sea surface, model the oceanic scene, and guide the detection in the oceanic environment.

Spatial heterodyne spectroscopy (SHS) is a new spatial interference spectroscopy which can achieve high spectral resolution. The alignment error of the detector array can lead to a significant influence with the spectral resolution of a SHS system. Theoretical models for analyzing the alignment errors which are divided into three kinds are presented in this paper. Based on these models, the tolerance angle of these errors has been given, respectively. The result of simulation experiments shows that when the angle of slope error, tilt error, and rotation error are less than 1.21°, 1.21°, 0.066° respectively, the alignment reaches an acceptable level.

Thermal imaging technology is an effective means of detecting hazardous gas leaks. Much attention has been paid to evaluation of the performance of gas leak infrared imaging detection systems due to several potential applications. The minimum resolvable temperature difference (MRTD) and the minimum detectable temperature difference (MDTD) are commonly used as the main indicators of thermal imaging system performance. This paper establishes a minimum detectable gas concentration (MDGC) performance evaluation model based on the definition and derivation of MDTD. We proposed the direct calculation and equivalent calculation method of MDGC based on the MDTD measurement system. We build an experimental MDGC measurement system, which indicates the MDGC model can describe the detection performance of a thermal imaging system to typical gases. The direct calculation, equivalent calculation, and direct measurement results are consistent. The MDGC and the minimum resolvable gas concentration (MRGC) model can effectively describe the performance of "detection" and "spatial detail resolution" of thermal imaging systems to gas leak, respectively, and constitute the main performance indicators of gas leak detection systems.

We propose an electro-optic mode switch based on an optical waveguide Mach-Zehnder interferometer fabricated with x-cut lithium niobate by the annealed proton exchange process. The device can switch between the fundamental mode and the higher-order mode with a low driving voltage. Our typical fabricated device, which has a total length of ∼24  mm, shows a mode extinction ratio of ∼35  dB and a 20-dB bandwidth of ∼12  nm at the wavelength 1552 nm, when driven at a voltage of 1.7 V at 26°C. High performance can be obtained at any wavelength in the C+L band with a driving voltage varying by no more than 3 V. The proposed mode switch is easy to fabricate and could find applications in reconfigurable mode-division-multiplexing systems.

Currently, most transducers in photoacoustic endoscopy (PAE) are ceramic based, which are complicated to fabricate and are expensive. In this work, we have for the first time presented a hollow structured epoxy lens-focused transducer that was based on a 52 μm thick polyvinylidine fluoride (PVDF) film for the purpose of PAE imaging. Intensive field characteristic tests were performed on transducers with different lens curvatures, and results show that with the 6 mm fixed aperture, a lateral resolution less than 0.5 mm can be obtained with a focal length around 19 mm, which is close to the theoretical calculations. The PAE application of the built transducer was also demonstrated with phantom experiments. Compared with the commonly used ceramic-based transducers, the proposed method has greatly reduced the design and fabrication cost of the hollow structured focused transducer as required in PAE, and facilitated the development of the PAE system in lab conditions. The built transducer may play an important role in the PAE imaging of some relatively large human structures and organs, such as the gastrointestinal tract and the cervical canal.

A numerical simulator of a turbulence phase screen based on turbulence power spectrum density is described in this paper. The low-frequency adding technique used in the fast-Fourier-transform-based method is extended to the whole frequency domain. The frequency range and spatial coordinates are no longer limited by the spatial sampling, so that the phase screens can be applied in the multibeam time-dependent scenario. Several spectrums can be applied in this simulator. The structure function, modulation transfer function, and variance of the Zernike coefficient are calculated with the Kolmogorov model for validation. The simulation results have shown good agreement with the theoretical results.

For research on inhomogeneous cells, we present a simulation method called the dual-medium quantitative (DMQ) measurement simulation method, which is realized by combining phase-shifting digital holography with DMQ analysis. The reliability of this method is confirmed by comparing the simulated phase map with the experimental one by the Hilbert phase microscope [J. Phys. Chem. A 113, 13327 (2009)10.1021/jp904746r], and its ability for studying inhomogeneous cells is demonstrated with measurements of a simulated HeLa cell. The average deviation and the relative deviation of physical thickness and axially averaged refractive index are 0.0339 μm, 0.69% and 0.0013, 0.094%, respectively. This approach can provide good guidance for experimental research on inhomogeneous cells., (© 2011 Optical Society of America)

An efficient approach is presented to restore a motion-blurred image in real time by optoelectronic hybrid processing, by which an image motion vector can be effectively detected and an accurate point spread function is constructed rapidly. A simple Wiener filter algorithm is employed to restore the blurred image. It greatly alleviates the complexity of the restoration algorithm. The proposed approach can apply to arbitrary motion-blurred image restoration. An optoelectronic hybrid joint transform correlation is constructed to verify the efficiency. The experimental results show that the proposed method has distinct advantages of preferable effect and good real time.

A method for testing the physical and optical properties of hydrogel thin films is reported based on using long-period fiber gratings. For humidity levels from 50% to 70% RH, a wavelength decrease of 11.0 nm is observed, with a sensitivity of 0.6 nm/%RH. For humidity levels from 70% to 98% RH, a wavelength increase of 6.3 nm is seen, with a sensitivity of 0.2 nm/%RH. For humidity levels greater than 60% RH, the transmission loss at resonance increases rapidly, showing a high degree of sensitivity of 0.5 dB/% RH. The blueshift of the wavelength was seen to be due to a thickening of the overlay, while the redshift of wavelength is due to the contribution of the drop in refractive index of the overlay. The results match well with what is expected theoretically.

Asymmetric transverse-load characteristics and the polarization dependence of long-period fiber gratings (LPFGs) written by high-frequency CO(2) laser pulses are investigated in detail. It is demonstrated that the resonant wavelength is dependent on the direction of the applied force and on the polarization state of the input light; however, the coupling strength is independent of these parameters. When a transverse load is applied along different orientations of the LPFG, the resonant wavelength may be shifted toward the longer wavelength, the shorter wavelength, or hardly shifted, whereas the absolute value of peak transmission attenuation is linearly decreased with an increase of the applied transverse load, with almost no sensitivity to the load direction. These unique transverse-load characteristics and the polarization dependence are due to the load-induced birefringence that leads to the rotation of optical principal axes in the LPFG.

A double-lens extended fractional Fourier transform (EFRT) is proposed. In the double-lens setup, arbitrary-order EFRTs including real orders and complex orders can be carried out. To verify that the single-lens EFRT and the double-lens EFRT are equivalent, numerical simulations and optical experiments for the real-order EFRT and the complex-order EFRT are performed. Their results indicate that the double-lens EFRT is consistent with the single-lens EFRT. Thus the double-lens setup can be regarded as another basic optical configuration. Compared with the single-lens EFRT, the double-lens EFRT has some advantages.

A long period fiber grating (LPFG) temperature sensor system based on intensity modulation is developed. The LPFG employed is fabricated by the use of a focused CO2 laser beam to carve periodic grooves on the fiber. The temperature measurement resolution of up to 0.1 degrees C has been obtained within the temperature range between 20 degrees C and 100 degrees C. The system uses a simple intensity measurement method and exhibits the advantages of convenient intensity measurement, double temperature sensitivity, high resolution, simple configuration, and low cost.

A method for measuring the length of an optical fiber by use of an optical fiber ring laser pulse source is proposed and demonstrated. The key element of the optical fiber ring laser is a gain-switched Fabry-Perot laser diode operated in a self-injection seeding scheme. This method is especially suitable for measuring a medium or long fiber, and a resolution of 0.1 m is experimentally achieved. The measurement is implemented by accurately determining the pulse frequency that can maximize the output power of the fiber ring laser. The measurement results depend only on the refractive index of the fiber corresponding to this single wavelength, instead of the group index of the fiber, which represents a great advantage over both optical time-domain reflectometry and optical low-coherence reflectometry methods.

Generation of wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes in a parallel configuration is described. The system supports continuous wavelength tuning in a relatively wide range of 42 nm. The side-mode suppression ratio achieved is 35 dB across the entire wavelength tuning range. The system is convenient for continuous wavelength tuning.

A minimum configuration Michelson fiber-optic low-coherence interferometric quasi-distributed sensing system is proposed that permits absolute length measurement in remote reflective sensor arrays. The sensor's reflective signal characteristics have been analyzed, and the relationship between intensities of light and number of sensors is given for evaluation of multiplexing potential. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application may be strain monitoring in smart structures. Experimentally, four-sensor array has been demonstrated.

We report the modeling results of an all-fiber gas detector that uses photonic crystal fiber (PCF). The relative sensitivity of the PCF as a function of the fiber parameters is calculated. Gas-diffusion dynamics that affect the sensor response time is investigated theoretically and experimentally. A practical PCF sensor aiming for high sensitivity gas detection is proposed.

A fiber-optic quasi-distributed strain sensors ring network has been designed based on a Mach-Zehnder optical paths interrogator. The optical paths matching for each sensor are discussed, and the optical power budgetary analysis is performed. The relation between the number of sensors and the intensity of the signals of the ring network is given for evaluation of the multiplexing capacity. Experimentally, a seven-sensor array ring network was realized under the condition of light source power 35 microW at 1310 nm, and the distribution strain test was also demonstrated.

We report on an amplitude-division-multiplexed interferometric sensor array for locating acoustic emission. Preliminary experiments were carried out with a modified Mach-Zehnder interferometer consisting of two sensing arms and a reference arm and demonstrated a one-dimensional location accuracy of a few centimeters. The system can be extended for two- and three-dimensional location of acoustic emissions by the addition of one or two more sensing arms, respectively, in the interferometer.

The results of an investigation of the performance of a time-division-addressed fiber-optic gas-sensor array by means of wavelength modulation of a distributed-feedback (DFB) laser are reported. The system performance is found to be severely limited by the extinction ratio of the optical switch used for pulse amplitude modulation. Formulas that relate the cross-talk level to the extinction ratio of the switch, the modulation parameters of the DFB laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 methane gas sensors with a detection sensitivity of 2000 parts in 10(6) (ppm) (10-cm gas cell) for each sensor may be realized with a commercially available single Mach-Zehnder amplitude modulator (-35-dB extinction ratio). An array of 100 sensors with a 100-ppm detection sensitivity for each sensor may be realized if a double Mach-Zehnder amplitude modulator is used.

Interferometric noise in fiber-optic grating sensors isinvestigated. Interference between a signal wave and reflectedwaves causes signal fluctuation in the output that limits thewavelength detection accuracy of the sensing system. Themeasurement error limited by interferometric noise is calculated forboth reflective-type and transmission-type sensors.

The effect of source (LED) noise on the sensitivity of fiber-optic methane sensors is discussed. Once the source is dominant, the system sensitivity cannot be improved by increasing the source power further.

A novel time-domain method is described in which the dynamic phase modulation index applied to an interferometer is directly measured from the waveform of the output photocurrent of the interferometer. A dynamic phase deviation as high as 78 rad has been experimentally demonstrated with a minimum detectable phase deviation of ~0.3 rad.

Fiber-optic interferometers have been studied extensively for sensing applications. Recently a technique described as the J(1) ... J(4) technique was reported for the linear measurement of dynamic phase changes in a fiber interferometer that requires no phase bias and for which the measurement is independent of random phase fluctuations. However, the implementation of the J(1) ... J(4) technique is limited because only the magnitude of the J(1) ... J(4) Bessel components can be measured on a spectrum analyzer without information available on the sign of the Bessel function. Here a modified signal-processing technique that overcomes the limitations mentioned above is described.