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A novel approach to interrogate a fiber grating sensor based on dispersive Fourier transformation method is proposed and simulated. In the system, two chirped fiber Bragg gratings (CFBG), with one used as a reference grating and the other one as sensing grating, are employed to form a Fabry–Perot (FP) interference structure, used as the spectral-shaping filter. The filter can be modeled as a two-tap delay-line filter. An optical pulse from a mode-locked laser source is reflected and spectrally shaped by the FP structure, and then the shaped spectrum is mapped to the temporal domain by a dispersive element. When the sensing CFBG is experiencing a strain, its wavelengths will be changed, leading to different interference spectra. Using Fourier transform, the frequency of the generated microwave waveform can be achieved, from which the wavelength shift of the sensing grating and the applied strain can be demodulated. A mathematical model to describe the microwave waveform generation is developed, by which the interrogation principle is analyzed. Numerical simulations are implemented to verify the proposed approach. The results present that the sensitivity of the system is 0.01 GHz/ μ e . The influences of key parameters in the system on the microwave spectra are investigated and discussed, including grating length, cavity length, chirp rate and index modulation depth.

A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.

We propose and experimentally demonstrate an approach to perform high-speed and high-resolution thermal-insensitive interrogation of a fiber Bragg grating (FBG) strain sensor based on a dual-frequency optoelectronic oscillator (OEO). Two phase-shifted FBGs (PSFBGs) are incorporated in the OEO loop to implement a microwave photonic filter with two passbands based on phase modulation and phase-modulation to intensity-modulation conversion, to generate two microwave signals with their frequencies determined by the center frequencies of the two passbands. When one of the PSFBG is experiencing a strain, a beat frequency between the two microwave signals that is linearly proportional to the strain applied to the sensing PSFBG is obtained. By monitoring the beat frequency using a digital signal processor, the strain is measured. The proposed approach is experimentally demonstrated. High-resolution sensing with a resolution of $0.83~\mu \varepsilon $ that is thermal insensitive is demonstrated.

A novel approach to interrogate a fiber grating sensor based on spectral-shaping and wavelength-to-time mapping is proposed and simulated. In the system, two chirped fiber Bragg gratings (CFBG), with one used as a reference grating and the other one as sensing grating, are employed to form a Fabry-Perot (FP) interference structure, used as the spectral-shaping filter. The filter can be modeled as a two-tap delay-line filter. An optical pulse from a mode-locked laser source is reflected and spectrally shaped by the FP structure, and then the shaped spectrum is mapped to the temporal domain by a dispersive element. When the sensing CFBG is experiencing a strain, its wavelengths will be changed, leading to different interference spectra. Using Fourier transform, the frequency of the generated microwave waveform can be achieved, from which the wavelength shift of the sensing grating and the applied strain can be demodulated. A mathematical model to describe the microwave waveform generation is developed, by which the interrogation principle is analyzed. Numerical simulations are implemented to verify the proposed approach. The results present that the sensitivity of the system is 0.014 GHz/μe.

A microwave photonic filter (MPF) with reconfigurable frequency responses using a phase-shifted fiber Bragg grating (PS-FBG) of two phase shifts is proposed and a two-passband filter is experimentally demonstrated with tunable frequency spacing. The PS-FBG with two phase shifts acts as an optical notch filter to suppress one sideband of the phase-modulated signals. Thus, the phase-modulated signals are converted to intensity-modulated single-sideband signals. When the wavelength of the optical carrier from a tunable laser source is tuned into different frequency ranges, the frequency responses of the MPF having two narrow passbands, one passband and one notch, or two notches can be achieved. For the dual-passband MPF, contrasting to a single-passband MPF, the advantages in sensor applications are discussed.

A switchable dual-wavelength erbium-doped fiber laser is proposed and experimentally demonstrated, which is constructed by a polarization-maintaining fiber Sagnac loop mirror (PMF-SLM) and two FBGs with different wavelengths. Wavelength switching operation is achieved by properly adjusting the polarization controller (PC) in the PMF-SLM. Stable single- or dual-wavelength lasing output can be realized. The maximum amplitude variation for every lasing wavelength is less than 1 dB, and the signal-to-amplified spontaneous emission (ASE) ratio is about 35 dB.

The switchable single polarized Er-doped polarization-maintaining-fiber (PMF) linear cavity laser based on two PMF gratings has been proposed. The gratings are fabricated in Er-doped PMFs, each grating has two single polarized wavelength and they function as the reflective mirror of the laser. So the laser can output switchable single wavelength polarized laser, with the switchable of the wavelength of grating the output of the laser can also be switched. Our experiment result shows that the laser has the OSNR of 48.5 dB, the degree of polarization (DOP) of 98.7%, and the laser linewidth with less than 9.2 pm.

Radiation-mode coupling is stronger and more efficient in tilted fiber Bragg gratings than in other fiber gratings; it has good advantage in such fields as optical communication and optical sensors. A simplified coupled-mode theory (CMT) approach is proposed for what we believe to be the first time, whose validity is demonstrated by comparing its simulation results with that of the complete CMT equations. With the simplified CMT approach, a theoretical spectral analysis of coupling from core mode to counter-propagating radiation modes in reflective tilted fiber Bragg gratings is presented. The influence of grating length, refractive index modulation amplitude and tilt angle is exhaustively investigated on the transmission spectrum characteristics. The different dependences between s-polarized and p-polarized radiation-mode coupling on grating tilt angle are discussed, and the coupling strength of 45°-tilted gratings shows the greatest polarization dependence with the limitation of backward-propagating radiation-mode coupling.

The polarization properties for tilted fiber Bragg gratings (TFBGs) are investigated theoretically based on coupled-mode theory and Mueller matrix method. The expression of wavelength-related polarization-dependent loss (PDL) for TFBGs with different tilt angles is derived and calculated. Simulation results are compared, and the results indicate that the polarization capability of TFBGs with 45° angle is stronger than other TFBGs with smaller angles. The degree of polarization for unpolarized light passing TFBGs is also simulated to further evaluate the polarization properties for various tilt angles. In addition, the relationship between physical parameters of a TFBG and its polarization capability is discussed.

Switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber ring laser based on one polarization-maintaining fiber Bragg grating (PMFBG) is demonstrated. Due to the enhancement of the polarization hole burning (PHB) by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). The stable SLM operation is guaranteed by a compound-ring cavity and a saturable absorber (SA). The optical signal-to-noise ratio (OSNR) is over 45 dB. The amplitude variation in nearly one and half an hour is less than 0.2 dB.

A switchable erbium-doped fiber-ring laser providing dual-wavelength outputs with orthogonal polarizations when operating at room temperature is proposed. One polarization-maintaining fiber Bragg grating (PMFBG) in a Sagnac loop interferometer is used as the wavelength-selective filter. Due to the enhancement of the polarization hole burning (PHB) by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). The optical signal-to-noise ratio (OSNR) is over 42 dB. The amplitude variation over 90 min is less than 0.6 dB for both wavelengths.

We present a simple and stable single-longitudinal-mode (SLM) erbium-doped fiber linear-type laser. It consists of three FBGs directly written in a photosensitive erbium-doped fiber (PEDF) to form asymmetric two cavities with different cavity lengths, which can effectively increase the longitudinal mode spacing and suppress mode hopping, experimentally compared with symmetric two cavities and single-cavity structures. The stable SLM operation is conveniently achieved without accurate control of cavity length, and the laser linewidth of less than 5 kHz is acquired.

A switchable multi-wavelength erbium-doped fiber (EDF) ring laser based on cascaded polarization maintaining fiber Bragg gratings (PMFBGs) in a Sagnac loop interferometer as the wavelength-selective filter at room temperature is proposed. Due to the polarization hole burning (PHB) enhanced by the PMFBGs, stable single-, dual-, three- and four-wavelength lasing operations can be obtained. The laser can be switched among the stable single-, dual-, three- and four-wavelength lasing operations by adjusting the polarization controllers (PCs). The optical signal-to-noise ratio (OSNR) is over 50 dB.

On the basis of the coupled-mode theory, a detailed investigation of the optical spectral characteristics is presented for uniform tilted fiber gratings. Explicit expressions are derived for the spectral parameters of reflection and transmission spectra. Numerical simulations are carried on to show the dependences of grating spectral responses on the structural parameters, such as tilt angle, grating length, index modulation amplitude and polarization states. The effects of these parameters on shaping the grating spectra are discussed comprehensively. The physical mechanism and intuitive phase-matching vector model are provided to explain the unique behaviors of transmission loss spectra. The results are helpful for providing a better understanding of the spectral behavior of the tilted fiber grating.

The characteristics of transmission spectra for the all fiber, multi-cavity Fabry-Perot (FP) configuration based on fiber Bragg gratings (FBG) are theoretically analyzed and modeled. The general transmission matrix function for the structure with any number of cavities is derived, and explicit expression of the power transmission coefficient for symmetrical two-cavity FP is presented. The general conditions for flat-top single resonant peak at the central wavelength in FBG stop band are derived and verified in the numerical simulation section. The transmission peaks of single-cavity and two-cavity FP structures are compared and discussed, and results show that compared to the single-cavity FP, flatness of the top and steepness at the edge of transmission peak can be improved by introducing one more cavity. The resonant transmission peak properties of two-cavity structure are investigated in detail for various values of cavity length and FBGs with different reflection characteristics, and the design guidelines for transmission-type filters are presented. The results show that the steepness of peak slope can be improved by increase of FBG reflectivities, and these kinds of filters can be used as narrow-band single-channel selectors and multi-channel wavelength de-multiplexing by properly choosing the length of cavities and reflectivities of FBGs.

A simple and cost-effective multi-wavelength fiber ring laser based on a chirped Moire fiber grating (CMFG) and a semiconductor optical amplifier (SOA) is proposed. Stable triple-wavelength lasing oscillations at room temperature are experimentally demonstrated. The measured optical signal-to-noise ratio (SNR) reaches the highest value of 50 dB and the power fluctuation of each wavelength is less than 0.2 dB within a 1-h period. To serve as a wavelength selective element, the CMFG possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow passband, and its fabrication method is easy and flexible. The lasing oscillation shows a narrower bandwidth than SOA-based multi-wavelength fiber lasers utilizing some other kinds of wavelength selective components. Methods to optimize the laser performance are also discussed.

A novel fiber-laser-based strain sensor is proposed and experimentally demonstrated. The laser cavity is composed of a high-birefringence Sagnac fiber loop mirror (HiBi-SFLM) and a fiber Bragg grating (FBG) which also acts as a strain-sensing element. In the linear region of the HiBi-SFLM reflection spectrum, when the strain applied on the FBG makes the Bragg grating wavelength shift, the laser output power changes due to reflectivity variation of the HiBi-SFLM. Experimental results show that the laser output power varies almost linearly with the applied strain. The measurement of the output power can be performed by a conventional photo-detector.

We have examined the relationship between the field and atomic squeezing in the thermal Jaynes-Cummings model with an initially coherent atom. Also we have studied the influence of virtual-photon processes or thermal photons on the dynamical symmetry between the field and atomic squeezing shown in the absence of thermal photons.

A compact in-fiber Mach-Zehnder interferometer comb-filter is demonstrated by splicing a section of twin-core fiber (TCF) between two single mode fibers (SMFs). The temperature and strain induced wavelength shifts of the interference fringes are experimentally monitored. Redshift (i.e., wavelength shifts to the longer wavelength side) is observed with sensitivity of about 0.037 nm/°C for increased temperature, whereas blueshift (i.e., wavelength shifts to the shorter wavelength side) is observed with sensitivity of about 0.866 pm/μe for applied strain changes. This device is relatively simple to fabricate and expected to have applications in high temperature or strain fiber optic sensors and the multi-wavelength fiber lasers.

Switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser at room temperature is demonstrated. One fiber Bragg grating (FBG) directly written in a polarization-maintaining and photosensitive erbiumdoped fiber (PMPEDF) as the wavelength-selective component is used in a linear laser cavity. Due to the polarization hole burning (PHB) enhanced by the polarization-maintaining fiber Bragg grating (PMFBG), the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.202 nm by adjusting a polarization controller (PC). The stable SLM operation is guaranteed by a saturable absorber (SA). The optical signal-tonoise ratio (OSNR) of the laser is over 40 dB. The amplitude variation in nearly one and half an hour is less than 0.5 dB for both wavelengths.

Radiation-mode coupling is stronger and more efficient in tilted fiber Bragg gratings than in other fiber gratings; it can be used to good advantage in such fields as optical communication and optical sensors. A simplified coupled-mode theory (CMT) approach to the analysis of radiation-mode coupling is proposed for what we believe to be the first time, whose validity and accuracy is demonstrated by comparing its simulation results with that of the complete CMT equations and the volume current method (VCM). With the simplified CMT approach, a theoretical spectral analysis of coupling from core mode to radiation modes in both reflective and transmissive tilted fiber Bragg gratings is presented. The influence of tilt angle on the transmission spectrum characteristics is comprehensively investigated. The different dependences between s-polarized and p-polarized radiation-mode coupling on grating tilt angle are discussed, and an analysis is performed on how to obtain a high-performance in-fiber polarizer that involves a compromise between polarization extinction and insertion loss.

Switchable dual-wavelength with orthogonal polarizations fiber laser based on semiconductor optical amplifier (SOA) and polarization-maintaining fiber Bragg grating (PMFBG) at room temperature is proposed. Owing to the polarization dependent loss of the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting the polarization controller (PC). The amplitude variation in nearly half an hour is less than 0.1 dB for both wavelengths, which is more stable than that of erbium doped fiber (EDF)-based laser with similar configuration.

Dual-wavelength with orthogonal polarizations erbium-doped fiber ring laser at room temperature is proposed. One polarization-maintaining fiber Bragg grating (PMFBG) in a Sagnac loop interferometer is used as the wavelength-selective filter. Due to the polarization hole burning (PHB) enhanced by the PMFBG, the laser can operate in stable dual-wavelength operation with wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). The optical signal-to-noise ratio (OSNR) is over 52 dB. The amplitude variation in nearly one and half an hour is less than 0.6 dB for both wavelengths.

An improved erbium-doped fiber laser configuration for achieving single-polarization, switchable dual-wavelength of orthogonal polarizations oscillations at room temperature is proposed. For the first time, two fiber Bragg gratings (FBGs) directly written in a polarization-maintaining (PM) and photosensitive erbium-doped fiber (PMPEDF) as the wavelength-selective component are used in a linear laser cavity. Due to the polarization hole burning (PHB) enhanced by the polarization-maintaining FBG (PMFBG), the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). Each lasing line shows a single polarization with a polarization extinction ratio of25 dB under different pump levels. The optical signal-to-noise ratio (OSNR) is greater than 50 dB. The amplitude variation with 16 times scans in nearly one and half an hour is less than 0.5 dB at both operating wavelength.

A novel reflection-type filter composed of microring resonator array and MZI is presented and analyzed. Simulation results show that the devices can be used as reflection-type filters for DWDM system or wavelength-selective reflectors for fixed or tunable lasers by properly choosing the values of coupling ratios.

The all-fiber, multi-cavity, Fabry-Perot passband filters based on fiber Bragg gratings, up to seven, are presented and modeled. The general formulas of the transfer function for the multiple-cavity Fabry-Perot filters are derived with the transfer matrix method. Transmission spectrum characteristics of the filters with different number of cavities are simulated, analyzed and compared. Numerical results show that near-rectangular bandpass shape can be realized by choosing the proper index modulation depths for every forming FBGs. And the simulations clearly demonstrate that the more we increase the number of cavities the more the shape of the central transmission peak is getting rectangular.

The coupled-mode equations corresponding to a novel complex long-period-grating-assisted coupler (LPGAC), which consists of both the periodic refractive index modulation and gain/loss perturbation, is introduced and the close-form analytical solution is obtained, for the first time to our knowledge. And a unique unidirectional and nonreversible filtering characteristic is achieved by adjusting the gain/loss to match with the refractive index modulation. In addition, the impact of deviations in the grating profile is also evaluated, and the results show that the required device performance can be realized by controlling the amplitude and phase deviation

Chirped fiber Bragg gratings are supposed to be cascaded for multi-channel dispersion compensation in DWDM systems. The interaction between them restricts their employment. Gaussian and Super Gaussian apodization are used to reduce the out-of-band reflection so as to suppress the interaction of the cascaded gratings. The increase of the channel spacing can also diminish the interaction.

We fabricated linearly chirped fiber gratings by using uniform phase mask instead of chirped mask. The chirp of the grating is realized by precisely setting the distance between the fiber and the phase mask at every point of the fiber. In experiments we derived linearly chirped fiber grating which has dispersion -1102ps/nm, time delay ripple is 17ps. And also the asymmetry high order apodization method is used successfully to reduce the time delay ripple. The experiment results consistent with the simulation results. We can fabricate gratings with different chirp extent use one uniform phase mask conveniently by only changing the curve function of the fiber.

Switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser at room temperature is demonstrated. One fiber Bragg grating (FBG) directly written in a polarization-maintaining and photosensitive erbium-doped fiber as the wavelength-selective component is used in a linear laser cavity. Because of the polarization hole burning enhanced by the polarization-maintaining FBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.202 nm by adjusting a polarization controller. The stable SLM operation is guaranteed by a saturable absorber. The optical signal-to-noise ratio of the laser is >40 dB. The amplitude variation in nearly 1.5 h is

Switchable dual-wavelength fiber lasers with orthogonal polarizations, based on the semiconductor optical amplifier at room temperature, are proposed. One polarization-maintaining fiber Bragg grating (PMFBG) is used as the wavelength-selective filter in three different laser configurations. Owing to the polarization-dependent loss of the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller in each laser configuration. The amplitude variation over nearly half an hour is less than 0.1 dB for both wavelengths in each laser configuration, which is more stable than that of erbium-doped fiber lasers with similar configuration.

An all-fiber two-cavity Fabry-Perot (FP) configuration based on fiber Bragg gratings (FBGs) is proposed. The characteristics of transmission spectra for the two-cavity FP structure are theoretically analyzed and comprehensively modeled. The explicit expression of the transmission coefficient for the structures is derived. The general conditions for the lengths of two cavities and reflectivities of FBGs are presented to produce the single resonant transmission peak at the central wavelength in the FBG stop band. Based on the theoretical analysis, the transmission spectra of symmetric and asymmetric two-cavity FP structures are simulated, and the simulation results are discussed and explained qualitatively. The design guidelines of the device, including the choices of cavity lengths, grating lengths, and index modulation depths, are concluded. The results show that when the increasing of the cavity length of a single-cavity FP structure results in multiple resonant peaks in the stop band, the two-cavity FP structures of the same length can inhibit the secondary resonant peaks and keep the main peak without degrading the performance through appropriately designing the cavity lengths and FBGs. Finally, the fabrication error tolerances of the structures, including inaccurate cavity lengths and reflectivities of FBGs, are calculated and discussed.