Fiber Bragg grating strain sensor based on fiber laser

A study on fiber Bragg grating (FBG) strain sensor, based on erbium-doped fiber (EDF) laser, is presented. A strain-sensing element, FBG, also acts as the lasing wavelength selecting component. When strain is applied on the FBG, the laser cavity loss changes, leading to a modification of the laser transient. Strain measurements are obtained in the time domain by simply measuring the EDF laser build-up time. Relative variation in the build-up time of up to 190%, for a strain range from 0 με to 2350 με, is achieved with a resolution corresponding to a strain of better than 2.35 με. This study demonstrates a novel fiber sensor concept and the technical feasibility to develop fiber strain measurement.     

Medium power single-mode single-wavelength fiber laser

Medium-power, single-mode, single-wavelength fiber laser working at room temperature using a polarization-maintaining erbium-ytterbium co-doped fiber as the gain medium, and an un-pumped elliptical core erbium-doped fiber as a saturable absorber to reduce linewidth and mode hopping of the lasing wavelength is reported. The effects of length, erbium ion concentration, and polarization-maintaining property of the saturable absorber were explored. The output power of the laser was more than 100 mW and the lasing line was stable for more than 3 h with an intensity fluctuation of less than 0.2 dB. The laser linewidth (FWHM = Full width at half maximum) was 7.5 MHz and the signal to noise ratio was more than 50 dB. The output of the laser was measured using an optical spectrum analyzer (OSA) of resolution 1.25 GHz and a scanning Fabry-Perot spectrum analyzer (SFPSA) of resolution 6.7 MHz.     

Radio-frequency interrogation of a fiber Bragg grating sensor in the configuration of a fiber laser with external cavities

A fiber laser sensor, which consists of two coupled cavities based on three fiber Bragg gratings (two of them acting as sensing elements) and is interrogated via the longitudinal mode beating frequency, is presented. The two resonant cavities have lengths of 4250 m and 4297 m, respectively. Their beating frequency is of the order of 24 kHz, and its shift as a function of the variation of the period of one (or both) of the sensing gratings, induced by strain or temperature changes, can be measured by a radio-frequency analyzer. The system is suitable for long-distance sensing with high spatial resolution and high sensitivity.     

基于超长周期光纤光栅的高灵敏度扭曲传感器

利用高频CO₂激光脉冲写入的周期达数毫米的超长周期光纤光栅(ULPFG)，实验研究了这种新型ULPFG的扭曲特性，发现它的某些高阶谐振波长漂移与扭曲率之间具有良好的线性关系和方向相关性，其灵敏度可达0.2244nm/(rad/m)，是高频CO₂激光脉冲写入法写入的普通LPFG扭曲灵敏度的4倍.初步的理论分析表明，新型ULPFG横截面折变的非对称性以及导模与高阶包层模之间发生的耦合使得扭曲具有方向相关性和很高的灵敏度.基于这种ULPFG独特的扭曲特性，设计了一种可 关键词： 光纤传感 光纤光栅 2激光')" href="#">CO₂激光 扭曲测量 双折射     

Theoretical study on instantaneous linewidth of Fourier-domain mode-locked fiber lasers

We have numerically simulated the operation of the Fourier-domain mode-locked (FDML) fiber laser based on the wavelength reconstruction method instead of numerical solving the nonlinear Schrödinger equation. We studied the influences of the filter bandwidth and the relative time delay caused by the fiber chromatic dispersion on the instantaneous linewidth of the FDML fiber laser. The results show that the instantaneous linewidth broadens as the filter bandwidth and the relative time delay increase. When the filter has the bandwidth of 0.02 nm, the narrowest and broadest instantaneous linewidths are 0.024 and 0.042 nm, respectively. We give an understanding for the oscillation of the instantaneous linewidth of FDML. The presented result can be used to evaluate the performance achievable in the FDML fiber lasers.     

Measurements of the linewidth of a continuous-wave distributed feedback quantum cascade laser

Two-sample (Allan) variance with a modified algorithm was applied to the determination of the experimental linewidth of a thermoelectrically-cooled continuous-wave distributed feedback quantum cascade laser at a wavelength of 4.333 μm. From successive laser transmittance scans over the CO₂ ν₃, (01¹1 − 01¹0) P(33) absorption line at 2307.653 cm^− 1, two-sample variances were calculated for the laser frequency difference between different points on the sides of the absorption peak. From the minimum two-sample variance of the laser frequency difference between two adjacent points (5 μs between the points) on the same side of the absorption line the experimental laser linewidth was estimated to be less than 36(7) kHz at a laser power of ~ 25 mW, a laser current of 976 mA and a laser temperature of + 19.5 °C.     

Waveband-switchable multiwavelength mixed-cascaded phosphosilicate raman fiber lasers

In this paper, the mixed-cascaded Raman scattering has been developed to investigate multiwavelength phosphosilicate Raman fiber lasers (MRFLs). With a tunable Yb³⁺-doped double-clad fiber laser (YDCFL) as the Raman pump source, we propose a compact and waveband-switchable (from the O- to U-band) MRFL using two- or three-mixed-cascaded Raman scattering of both SiO₂/GeO₂ and P₂O₅ in a P-doped fiber. We also confirm experimentally the feasibility of the proposed mixed-cascaded MRFL. When a 1064 nm YDCFL was used to pump a spool of 1-km P-doped fiber, the compact linear-cavity MRFLs in the O- and L-band operation were obtained, respectively, based on the two- and three-mixed cascaded Raman scattering. Up to 16-wavelength stable oscillation around 1320 nm has been observed with a spacing of 0.40 nm and an extinction ratio >30 dB. 12 lasing lines around 1601 nm have also been achieved with a spacing of 0.58 nm. The multiwavelength output powers as high as 108 and 138 mW were obtained in the O- and L-band operations, respectively. The wavelength spacing of the MRFLs is flexibly adjustable, and the peak wavelength of each lasing line is continuously tunable over the wavelength spacing. In addition, the important characteristics of the mixed-cascaded MRFLs, including the linewidth broadening, the signal-to-noise ratio and the conversion efficiency, are discussed.     

Multiwavelength Erbium doped fiber ring lasers

We demonstrate a multiple wavelength laser, employing wavelength dependent cavity losses, with up to four lasing wavelengths on the ITU-grid in the C-band and report on its stability characteristics. The linewidth of each wavelength was estimated to be less than 4 pm and we observed antiphase oscillations around 3 kHz and relaxation oscillations around 25 kHz. The antiphase oscillations in the multiwavelength laser lie on a limit cycle in a relative phase diagram. We present an analytic lumped model to determine the output power and relaxation oscillation frequency of the multiple lasing wavelengths in the Erbium doped fiber. The model is validated against our implementation of a multiwavelength ring laser.     

Tunable microwave generation based on a Brillouin fiber ring laser and reflected pump

A simple approach for photonic generation of a tunable microwave signal is proposed and successfully demonstrated. By mixing the output from a single-longitudinal-mode Brillouin fiber ring laser with the reflected laser pump at photodetector, a microwave signal at the Brillouin shift can be obtained. Since the Brillouin shift can be changed by tuning the pump wavelength, tunable microwave can be generated. As a result a microwave signal tunable from 10.39 to 10.67 GHz has been achieved, with linewidth less than 600 KHz.     

Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber

A stable and narrow wavelength spacing multiwavelength erbium-doped fiber laser is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with a narrow wavelength spacing of less than 0.1 nm via using a single fiber Bragg gratings written in polarization-maintaining (PM) fiber. By adjusting polarization controller, the wavelength spacing of dual-wavelength lasing oscillations can be tuned to as small as 0.032 nm. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The room-temperature operation principle is based on the polarization hole burning and deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL). The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.     

Discrimination of strain and temperature based on a polarization-maintaining photonic crystal fiber incorporating an erbium-doped fiber

A simple sensing method for simultaneous measurement of temperature and strain is investigated by using a Sagnac fiber loop mirror composed of a polarization-maintaining photonic crystal fiber (PM-PCF) incorporating an erbium-doped fiber (EDF). Amplified spontaneous emission created by a pumped EDF is transmitted to a Sagnac fiber loop mirror. The interference between two counter-propagating signals in a Sagnac fiber loop mirror generates a periodic transmission spectrum with respect to wavelength. When external temperature is increased, the transmission peak power reduces because the amplified spontaneous emission of the EDF is decreased by the applied temperature change (0.04 dB/°C). The peak wavelength is shifted into the shorter wavelength because of the negative temperature dependence of the birefringence of the PM-PCF (0.3 pm/°C). As the applied strain increases, the peak wavelength of the transmission spectrum of the Sagnac loop mirror incorporating the EDF shifts into a longer wavelength (1.3 pm/με) because the phase change of the proposed sensing probe is directly proportional to the applied strain. The transmission peak power, however, is not changed by the applied strain. Since the source and the sensing probe are integrated, the overall system configuration is significantly simplified without requiring any additional broadband light source. Therefore, it is possible to simultaneously measure temperature and strain by monitoring the variation of transmission peak power and peak wavelength, respectively.     

All-optical synchronization of a 40 GHz self-pulsating distributed Bragg reflector laser to return-to-zero 10, 20 and 40 Gbit/s data streams

This paper presents experimental investigations of the all-optical synchronization of a distributed Bragg reflector (DBR) laser self-pulsating at 40 GHz on various injected bit-rate signals. Even though there is no modulation applied to this laser, it exhibits a modulation of its output emission, measured at 39.7 GHz with a linewidth of 30 MHz. Such performance is exploited in all-optical clock recovery for a return-to-zero data stream at 40 Gbit/s. The SP-DBR laser wavelength and the injected signal wavelength are 10 nm apart. All-optical synchronization is demonstrated at 40 Gbit/s with a linewidth of less than 20 MHz for injected signals at 10 and 20 Gbit/s, respectively. Thus the SP-DBR laser proves to be very versatile and can be synchronized on various bit-rate data signals.     

Highly sensitive torsion sensor based on triangular-prism-shaped long-period fiber gratings

We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity. This sensor is configured by a triangular-prism-shaped long-period fiber grating, which is fabricated by the high frequency CO_2 laser polished method. The unique design of the triangular-shaped structure breaks the rotational symmetry of the optical fiber and provides high sensitivity for torsion measurement. In preliminary experiments, the torsion response of the sensor achieves a good stability and linearity. The torsion sensitivity is 0.54 nm/(rad/m), which renders the proposed structure a highly sensitive torsion sensor.     

Wavelength tunability of L-band fiber ring lasers using mechanically induced long-period fiber gratings

We report on oscillation wavelength control in erbium-doped fiber ring lasers by adjusting the period of a mechanically induced long-period fiber grating (LPFG) inserted into the fiber ring resonator. Pump light is provided by a 974 nm laser diode (LD), the emission of which is coupled into the fiber ring resonator through a wavelength-division multiplexing coupler. Laser oscillation occurs with a threshold pump LD current of 40 mA, corresponding to a threshold pump power of 5 mW. When a periodic pressure of 0.81 N/mm is applied to form the LPFG, the fiber ring laser exhibits the tunable range of 40.9 nm, i.e., from 1563.1 to 1604 nm, by changing the grating period.     

Fabrication and characterization of solid-core photonic crystal fiber with steering-wheel air-cladding for strong evanescent field overlap

We report the fabrication and characterization of a photonic crystal fiber with solid-core and steering-wheel pattern air-cladding (SW-PCF). Specifically, SW-PCF is fabricated using sol-gel casting technique. Attenuation spectrum shows the fiber losses of 0.14 dB/m, 0.087 dB/m, and 0.0 32 dB/m at wavelengths of 850 nm, 1000 nm, and 1500 nm, respectively, while the cut-off wavelength for single-mode operation is 1360 nm. Near-field imaging is used for the determination of mode-field diameter. The fiber presents the anomalous dispersion expending to short wavelength range with high non-linearity. Tapered SW-PCFs in the transverse geometries are numerically calculated, which suggests that the tapering of fiber holds a significant promise for the enhancement of power overlap in air holes. Properly designed and fabricated SW-PCF can thus be utilized as attractive platform for evanescent field sensing and detection.     

Lasing of a single crystal Nd fluoride fiber

We present laser action in a single crystal fluoride fiber obtained by the micro-pulling-down technique. The 700 μm diameter and 1 cm long Nd:LiYF₄ fiber was pumped by a beam shaped diode bar emitting at 806 nm. A peak power of 300 mW was achieved with a slope efficiency of 12%. When considering the pump power fraction absorbed by the laser mode, a slope efficiency of 37% was achieved. To the best of our knowledge, this is the first report of a Nd: LiYF₄ fiber laser.     

Incorporate, switchable dual-wavelength fiber laser with Bragg gratings written in a polarization-maintaining erbium-doped fiber

A stable, incorporate and switchable dual-wavelength fiber laser with two fiber Bragg gratings written in a photosensitive and polarization-maintaining erbium-doped fiber directly, that is, without splices in the laser cavity, is proposed and demonstrated. Simultaneous dual-wavelength oscillation is achieved at room temperature with a wavelength spacing of 0.343 nm. The power fluctuation and wavelength shift of single-wavelength oscillations are measured to be less than 0.24 dB and 0.013 nm over 2 h. The wavelength switchability between single- and dual-wavelength oscillations is realized by altering the voltage upon the electrostrictive ceramic actuator.     

可调谐掺铥光纤激光器线宽压缩及其超光谱吸收应用

可调谐二极管激光吸收光谱技术是一种应用非常广泛的吸收光谱测量技术.利用宽带可调谐窄线宽光源进行吸收光谱测量的超光谱吸收技术可以在单次扫描中获取一段连续光谱的所有吸收数据,可大大提高可调谐二极管激光吸收光谱技术的数据信息容量和光谱诊断能力.分析了在2μm波段对水进行超光谱吸收测量时对激光器输出线宽的具体要求.利用掺铥光纤在2μm波段较宽的发射谱,采用可调谐法布里-珀罗滤波器和光纤可饱和吸收体相结合的技术方案搭建了一台宽带调谐窄线宽的2μm光纤激光器.获得了1840—1900 nm约60 nm范围的调谐光谱输出,激光器静态线宽仅为0.05 nm.利用该光源对空气中水在2μm波段的吸收光谱数据进行了超光谱吸收测量,在1856—1886 nm约30 nm的光谱范围内分辨了35条水的吸收谱线.通过对不同线宽条件下1870—1880 nm范围内的理论吸收光谱数据进行对比发现,测量数据无法有效分辨分别位于1873 nm和1877 nm处与强吸收线相邻的两条吸收谱线,且测量结果与激光线宽在0.08 nm条件下的HITRAN2012光谱数据库最为接近.这表明,在动态扫描过程中激光器的线宽得到了展宽.     

Wavelength tunable optical time-domain reflectometry based on wavelength swept fiber laser employing two-dimensional digital micro-mirror array

We demonstrate a cost effective wavelength tunable optical time-domain reflectometry (OTDR) for wavelength division multiplexing passive optical networks (WDM-PON). In order to realize the unique wavelength tunable optical time-domain reflectometry (OTDR), the wavelength swept fiber laser was developed by a digital micro-mirror array device (DMD), and the correlation OTDR (COTDR) technique was used. We successfully detected the fault location at the remote node fibers with 20 m resolution and fast wavelength setting speed of ∼15 μs in conventional band (C-band).     

多模光纤用于激光线宽零拍测量法的原理和实验研究

从理论上对零拍测量法测量激光线宽系统的分辨率进行了计算,并在理论和实验结果中证明多模光纤也可用于光纤延时零拍系统。实验建立起了1064nm、窄线宽、单纵模激光二极管泵浦Nd∶YVO4激光器的延时零拍多模光纤线宽测量系统,并通过这一系统观测和研究了激光的功率谱分布。