Ou Xu

Single-polarization, switchable dual-wavelength erbium-doped fiber laser with two polarization-maintaining fiber Bragg gratings

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 of >25 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.

Analysis of radiation-mode coupling in reflective and transmissive tilted fiber Bragg gratings

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.

Compact in-fiber Mach-Zehnder interferometer using a twin-core fiber

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/µε 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.

Proposal and analysis of two-cavity Fabry-Perot structures based on fiber Bragg gratings

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.

Multi-wavelength fiber ring laser based on a chirped moire fiber grating and a semiconductor optical amplifier

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.

Dual-wavelength erbium-doped fiber ring laser based on one polarization maintaining fiber Bragg grating in a Sagnac loop interferometer

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.

Theoretical analysis of polarization properties for tilted fiber Bragg gratings

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 dual-wavelength erbium-doped fiber laser based on one polarization-maintaining fiber Bragg grating in linear cavity

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 <0.5 dB for both wavelengths.

Switchable dual-wavelength fiber lasers based on a semiconductor optical amplifier and a polarization-maintaining fiber Bragg grating

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.

Theoretical analysis of transmission characteristics for all fiber, multi-cavity Fabry-Perot filters based on fiber Bragg gratings

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.