Fiber curvature sensor based on spherical-shape structures and long-period grating

A novel curvature sensor based on optical fiber Mach–Zehnder interferometer (MZI) is demonstrated. It consists of two spherical-shape structures and a long-period grating (LPG) in between. The experimental results show that the shift of the dip wavelength is almost linearly proportional to the change of curvature, and the curvature sensitivity are −22.144 nm/m⁻¹ in the measurement range of 5.33–6.93 m⁻¹, −28.225 nm/m⁻¹ in the range of 6.93–8.43 m⁻ and −15.68 nm/m⁻¹ in the range of 8.43–9.43 m⁻¹, respectively. And the maximum curvature error caused by temperature is only −0.003 m⁻¹/°C. The sensor exhibits the advantages of all-fiber structure, high mechanical strength, high curvature sensitivity and large measurement scales.     

A dual-pass Mach–Zehnder interferometer filter using a TCF loop mirror for double-wavelength fiber lasers

A dual-pass Mach–Zehnder interferometer filter using a section of twin-core fiber (TCF) loop mirror is proposed. The filter is theoretically and experimentally studied for various interferometer arm difference when TCF length is constant. Theoretical results are validated by the experimental demonstration and in good agreement with the experimental results. And then, by using the filter in a ring fiber laser, a stable and switchable dual-wavelength lasing is obtained experimentally. The 3-dB bandwidth and the SMSR of the output laser are 0.015 nm and higher than 62.4 dB, respectively. The peak power fluctuation and wavelength shift are also monitored to be less than 0.04 dB and 0.02 nm over an hour at room temperature. Furthermore, the output laser can be switched between single and dual wavelength by carefully adjusting the PCs. The experimental results show that the filter can suppress mode competition effectively, improve the SMSR availably, and enhance the stability of the output lasing.     

基于侧漏型光子晶体光纤高灵敏度宽线性范围弯曲传感器的研究

基于所研制的侧漏型光子晶体光纤, 提出并研制出一种Sagnac干涉仪型高灵敏度宽线性测量范围的弯曲传感器. 实验研究结果表明, 当侧漏型光子晶体光纤中的线性缺陷与弯曲方向一致时, 采用群双折射和波谷波长偏移量测量弯曲曲率均可获得高的弯曲灵敏度, 但线性测量范围小, 且不能进行小弯曲曲率的测量. 当线性缺陷与弯曲方向垂直时, 以波谷波长偏移量进行弯曲曲率检测, 可获得10.798 nm/m^-1高灵敏度的同时且可实现0–5.03 m^-1的宽线性测量范围, 结合测量矩阵的引入可实现温度和弯曲曲率的同时测量, 进而剔除环境温度变化对弯曲曲率检测的干扰, 实现了高灵敏度宽线性范围的弯曲传感; 而以群双折射进行弯曲曲率检测, 虽然检测灵敏度较低, 但可实现对环境温度不敏感的弯曲传感. 关键词： 弯曲传感器 侧漏型光子晶体光纤 高灵敏度 宽线性范围     

Curvature measurement by using low-birefringence photonic crystal fiber based Sagnac loop

An optical fiber curvature sensor with low-birefringence photonic crystal fiber (PCF) based Sagnac loop is demonstrated experimentally. The low-birefringence PCF of about 40 cm long is inserted into Sagnac loop, and a section of it about 155 mm is used as the sensing element. The Sagnac output spectra under different curvatures are measured and analyzed. The results show that the wavelength shift of the transmission dip has a linear relationship with the curvature. The sensitivity of the curvature measurement of − 0.337 nm is achieved in the range of 0-9.92 m^− 1. And the temperature effect of the proposed sensor is also analyzed.     

Simultaneous measurement of curvature and temperature based on PCF-based interferometer and fiber Bragg grating

A sensor head consisting of a photonic crystal fiber (PCF)-based Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. The MZI fabricated by splicing a short length of PCF between two single-mode fibers with the air-hole structure that completely collapsed near the splicing points, is sensitive to fiber bending and surrounding temperature, while the FBG is only sensitive to the later. Simultaneous measurement of curvature and temperature is therefore obtained. Sensitivities of 4.06 nm/m^− 1 and 6.30 pm/°C are achieved experimentally for curvature and temperature, respectively. And the corresponding resolutions are 5.2 × 10^− 4 m^− 1 and 1.25 °C for curvature and temperature, respectively, based on the wavelength measurement resolution of 10 pm.     

Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber

A new type fiber bending sensor based on a tilted fiber Bragg grating (TFBG) interacting with a multimode fiber (MMF) is presented. The sensing head is formed by insertion of a small section of MMF between a single-mode fiber (SMF) and the TFBG. The average reflective power in the cladding modes decreases with the increase of curvature. The measurement range of the curvature from 0 to 2.5 m⁻¹ with a measurement sensitivity of −802.4 nW/m⁻¹ is achieved. The proposed sensor is also proved as temperature-independent from the experimental investigation.     

In-fiber Mach–Zehnder interferometer based on multi-mode fiber and up-taper for curvature sensing

A new type of curvature sensor comprises a stub of multi-mode fiber and an up-taper is proposed and demonstrated experimentally. The whole fabrication process is quite simple and the sensor head is cost effective. Measurement results show that it has a maximum curvature sensitivity of −61.877 nm/m⁻¹ at 1.1718 m⁻¹ (the highest value of reported papers among in-fiber Mach–Zehnder interferometers) and −9.2115 nm/m⁻¹ from 0.865 m⁻¹ to 1.1172 m⁻¹. Temperature sensitivity of 89.01 pm/°C within the range of 20–80 °C has also been achieved, which implies the possibility for measurement of temperature.     

Nonlinear refractive index of multicomponent glasses designed for fabrication of photonic crystal fibers

The second order nonlinear refractive index n ₂ of various multicomponent glasses was measured at the wavelength of 1240 nm close to the 1.3-μm fiber transmission window. With the refractive index covering the range from 1.45 to 2.3, a comparatively broad range of n ₂ with values from 1.1×10^?20 m²/W for boro-silicate based glass NC21 to 4.3×10^?19 m²/W for lead–bismuth-gallate based glass PBG08 was measured using the Z-scan method. Considering the broad infrared transmission range of multicomponent glasses, these materials pose a great potential for future applications as photonic crystal fiber sources of infrared supercontinuum.     

A Novel Edge Filter Demodulation Technique by Using High Birefringence Fiber Sagnac Loop Mirror

A novel way of edge filter demodulation for fiber Bragg grating sensor is put forward. A high birefringence fiber Sagnac loop mirror is used as edge filter to demodulate FBG sensor wavelength shift in range of about 8 nm.     

Axial strain and temperature sensing characteristics of the single–coreless–single mode fiber structure-based fiber ring laser

This paper experimentally demonstrated a singlemode–coreless–singlemode (SCS) fiber structure-based fiber ring cavity laser for strain and temperature measurement. The basis of the sensing system is the multimodal interference occurs in coreless fiber, and the transmission spectrum is sensitive to the ambient perturbation. In this sensing system, the SCS fiber structure not only acts as the sensing head of the sensor but also the band-pass filter of the ring laser. Blue shift with strain sensitivity of \(\sim\) ?2 pm/με ranging from 0 to 730 με and red shift with temperature sensitivity of \(\sim\) 11 pm/°C ranging from 5 to 75 °C have been achieved. Experimental results also show the proposal has great potential in using long-distance operation. The fiber ring laser sensing system has a optical signal to noise ratio (OSNR) more than 50 and 3 dB bandwidth less than 0.05 nm. The result shows that the coreless fiber has no improvement of the temperature and axial strain sensitivity. However, compared to the common singlemode–multimode–singlemode fiber structure sensors, the laser sensing system has the additional advantages of high OSNR, high intensity and narrow 3 dB bandwidth, and thus improves the accuracy.     

Transmission characteristics of Mach–Zehnder interferometer comb filter based on thermal operation

We propose and experimentally demonstrate switchable and tunable transmission characteristics of a Mach–Zehnder interferometer comb filter based on thermal operation. Its temperature characteristics are investigated to reveal a shift in the peak wavelength position from 0.003 to 0.004 nm/°C and a tunable range of wavelength spacing of 0.76–0.90 nm for maximum and minimum effective lengths, respectively. This configuration provides the unique advantages of an all-fiber structure, tunable wavelength spacing, switchable spectral peaks, independent tuning of the center wavelength and wavelength spacing of the spectral peaks, and low polarization sensitivity. It is relatively simple to fabricate and expected to have applications in temperature fiber optic sensors and multiwavelength fiber laser sources.     

Optical fiber curvature sensor based on few mode fiber

An optical fiber curvature sensor based on interference between LP₀₁–LP₀₂ modes of a circularly symmetric few mode fiber (FMF) is presented. The device consists of two single-mode fiber and a 10-cm FMF. The two single-mode fiber is offset-spliced to each end of the FMF. When the optical fiber is kept straight and fixed, the interference pattern appears in the transmitted spectrum. As the fiber device is bent, the visibility of the interference fringes (at 1530 nm) decreases, reaching values close to 0.3. The dynamic range of the device can be tailored by the proper selection of the length of FMF. The relationship between the fringe visibility and the curvature is linear while the curvature is between 11 m⁻¹ and 16 m⁻¹. The result indicates that the compact sensor can be used in the measurement of large curvature, which is also important in structural health monitoring.     

A stable and tunable linear polarization single longitudinal mode fiber ring laser

A stable and tunable linear polarization single longitudinal mode fiber ring laser is proposed and demonstrated. A high extinction ratio spatial mode interference filter utilizing two-mode graded-index fiber supresses irregular side modes effectively, the combination of fiber polarizer and polarization maintaining fiber act as a polarization dependent comb filter, and tunable bandpass filter is adopted to select proper lasing wavelength. The power fluctuation and wavelength fluctuation at 1549.88 nm are less than 0.05 dB and 0.02 nm in 1 h, respectively. Only one longitudinal mode appears within a free spectral range of 7.5 GHz, and the degree of polarization in percentage is approximately as high as 99.07%.     

Novel bending sensor based on a meniscus shaped beam with LPG

A novel bending sensor based on a long period fiber grating (LPG) is presented. A LPG was glued into a V-shaped groove, which lies on the lower surface of a meniscus shaped beam. It is found that the transmission optical power of the LPG changed linearly with the variation of the bending of the beam. The bending applied on the beam can be measured by detecting the intensity variation of the LPG's resonant dip wavelength. Under a relative large bending measured range from 0 to 7.5 m^?1, the sensitivity of 3.003 dB m^?1 and curvature resolution of 0.001 m^?1 have been achieved for the proposed bending sensor.     

Broadly tunable multiwavelength Brillouin-erbium fiber laser using a twin-core fiber coupler

A tunable multiwavelength Brillouin-erbium fiber laser (MW-BEFL) using a twin-core fiber (TCF) coupler is proposed and demonstrated. The TCF coupler is formed by splicing a section of TCF between two single-mode fibers. By simply applying bending curvature on the TCF coupler, the peak net gain is shifted close to the Brillouin pump (BP), which has advantage for suppressing self-lasing cavity modes with low-BP-power injection. In this work, the dependency of the Stokes signals tuning range on the free spectral range (FSR) of TCF coupler is studied. It is also found that the tuning range of MW-BEFL can exceed the FSR of TCF coupler by adopting proper BP power and 980-nm pump power. Up to 40 nm tuning range of MW-BEFL in the absence of self-lasing cavity modes is achieved.     

基于错位和花生形结构的全光纤马赫-曾德干涉仪的研究

研究了一种基于错位和花生形结构的全光纤马赫-曾德干涉仪,进行了液位和曲率的测量实验,利用错位结构将纤芯模式激发到包层,包层模式经过花生形结构被耦合到纤芯与原有的纤芯模式发生干涉。包层模式对外界物理量如折射率、应力的变化敏感,导致透射光谱漂移。波谷波长的漂移量与液位和曲率的变化成线性关系,利用波谷的漂移实现液位和曲率的测量。在液位实验中,在水位变化范围为1.00～5.00 cm时,波谷向短波方向漂移,灵敏度最高为-0.68 nm·cm-1,线性拟合度为0.995 4。在曲率实验中,曲率的变化范围为0.3～1.2 m-1时,波谷向长波方向漂移,灵敏度最高为22.47 nm·m,线性拟合度为0.986 4,表现出较高的灵敏度。错位结构和花生形结构被用于组成马赫-曾德干涉仪,用普通的光纤熔接机和普通单模光纤即可熔接,结构和制作方法简单,灵敏度高,尤其在曲率的测量中表现出较高的灵敏度。     

基于阵列波导光栅的边缘滤波温度解调系统

为克服传统阵列波导光栅解调系统体积大、价格昂贵等问题,提出了以窄带光源为输入光源,采用边缘滤波和阵列波导光栅相结合的解调方案,实现对增敏封装后的光纤光栅温度传感器进行温度解调实验。以窄带光源作为输入,通过边缘滤波的方法使得温度传感器反射谱的中心波长偏移程度与解调光路输出光强的变化相对应,利用阵列波导光栅的波分复用实现多传感器同时测量,实现了多传感器多通道的分布式测量,实验结果表明:解调系统的波长解调范围为1 545.30 nm～1 560.50 nm,对35 ℃～42 ℃的温度范围进行检测,波长解调精度为±5.34 pm,温度测量误差可达±0.1 ℃。     

Liquid refractive index sensor with three-cascaded microchannels in single-mode fiber fabricated by femtosecond laser-induced water breakdown

Liquid refractive index sensor with three-cascaded microchannels in single-mode fiber was fabricated by femtosecond laser-induced water breakdown. When the liquid is filled in three microchannels across the fiber core, the transmission spectrum of the sensor shows interference peaks. The refractive index of liquid can be sensed accurately based on both the wavelength shift and loss change of interference peaks. The refractive index sensitivities of wavelength shift and loss change reach to as high as ?2,406.1 nm/RIU and ?156.8 dB/RIU, respectively. The sensor is insensitive to the refractive index change of liquid caused by environmental temperature. It can be used for liquid refractive index sensing with high sensitivity, especially for water pollution monitoring.     

Bend-insensitive long-period fiber grating sensors

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO₂ laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m⁻¹ at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.     

Ultra-long-period fiber grating cascaded to a knob-taper for simultaneous measurement of strain and temperature

This study presents a simple Mach–Zehnder interferometer (MZI) to obtain the bimodal characteristics that realize simultaneous measurement of strain and temperature through cascading an ultra-long-period fiber grating and a knob-shaped taper. We obtain the multi-dip feature from the MZI, and the Dips 2 and 5 are selected from 11 interference dips. Experimental results indicated that the wavelength sensitivities of Dips 2 and 5 are ??0.54 nm mε^?1 and 0.058 nm °C^?1, and ??0.53 nm mε^?1 and 0.055 nm °C^?1 to strain and temperature, respectively. The depth sensitivities are ??3.3 dB mε^??1, ? 0.015 dB °C^?1 and ?5.8 dB mε^?1, and 0.06 dB °C^?1 for Dips 2 and 5, respectively. It is concluded that the proposed structure is suitable for simultaneous strain and temperature measurements.