High-frequency CO2 Laser Pulses Research Articles

A Low-Cost High-Temperature Sensor Based on Long-Period Fiber/Microfiber Gratings by Local Fictive Temperature Modification

A high temperature-sensitive long-period fiber grating (LPFG) sensor fabricated by the local fictive temperature modification is proposed and demonstrated. High-frequency CO2 laser pulses scan standard single-mode fiber (SMF), and the modification zones extended to the core of SMF. Experimental results demonstrate that the LPFG temperature sensors with 600 μm grating period and 32 period numbers offer the average sensitivity of 0.084 nm/C in the temperature range of room temperature (RM) to 875°C. The LPFGs fabricated here show exponential change in terms of the spectral wavelength shift versus changes in temperature. In addition, the maximum temperature sensitivity of 0.37 nm/C is achieved by employing long-period microfiber grating (LPMFG), fabricated by the microheater brushing technique and the local fictive temperature modification. LPMFG sensor exhibits better temperature characteristics due to a thinner diameter.

A micro MMF layer embedded in LPFG for simultaneous measurement of curvature and temperature

Abstract A micro multimode fiber (MMF) layer embedded in long period fiber grating (ME-LPFG) has been developed and experimentally investigated for the simultaneous measurement of curvature and temperature. The LPFG is written by using high-frequency CO2 laser pulses. The transmission spectrum of the novel LPFG can show the wavelength shift and splitting characteristics relative to the original LPFG. The wavelengths of the two resonant peak changes are used to monitor the variations of the curvature and temperature due to different responses. The experimental results show that the curvature sensitivities of the two peaks are −7.056 nm / m - 1 and −21.080 nm / m - 1 , respectively. The curvature sensitivity is bigger than that of the common LPFG. And the temperature sensitivities of the two peaks reach 60 pm/°C and 58 pm/°C, respectively. Therefore the ME-LPFG has the potential to measure the curvature and temperature simultaneously with the resolutions of 4.7 × 10−5 m−1 and 0.3 °C.

Fabrication of twisted long period fiber gratings with high frequency CO2 laser pulses and its bend sensing

We experimentally demonstrated a simple and effective method to write long period fiber gratings (LPFGs) with low fiber grating birefringence using high frequency CO2 laser pulses. Such LPFGs were fabricated by introducing twist strains to the fiber and then exposing the fiber to the laser. We theoretically analyzed the influence of the twist strains on the spectra of the LPFGs, and examined bend characteristics of twisted long period fiber gratings (T-LPFGs). Experimental results showed that T-LPFGs exhibited clear spectra, low insertion loss, and low polarization-dependent loss (PDL) compared with conventional LPFGs exposed to a single-side CO2 laser beam. The slope of the linear equation of the distance between the two split resonance peaks in the bending experiment is 20.55 nm/m−1, which makes T-LPFGs sensitive bending sensors.

Simultaneous measurement of temperature and refractive index using a simplified modal interferometer composed of tilted long-period fiber grating and fiber bitaper

A simplified long-period fiber-grating-based Mach–Zehnder interferometer (MZI) for simultaneous temperature and refractive index (RI) measurement is presented. The sensing element is constructed by connecting a waist-enlarged fiber bitaper after a tilted long-period fiber grating written by high-frequency CO2 laser pulses, which has higher writing efficiency than the conventional long-period fiber grating (LPFG). By employing the proposed MZI, the transmission bandwidth for each resonant wavelength of the fiber grating could be conveniently reduced without the presence of the LPFG pair. Experimental results indicate that the MZI has good temperature sensitivity (70.42 pm °C−1) and RI sensitivity (−37.43 nm/RIU, in the RI range from 1.3425 to 1.3750). Besides, the extinction ratio of interference fringes could reach 20 dB at the resonant wavelength. With several advantages such as narrow bandwidth, large extinction ratio, simple structure and ease of fabrication, the proposed device would be a promising candidate for precise large-capacity dual-parameter sensing application.

Long-Period Fiber Grating Within D-Shaped Fiber Using Magnetic Fluid for Magnetic-Field Detection

A magnetic sensor utilizing long-period fiber grating (LPFG) written by high frequency CO2 laser pulses in a D-shaped fiber and the magneto-optical effect of magnetic fluid is proposed. Because of the high evanescent field of the grating structure in D-shaped fiber, by immersing the LPFG with a period of 685 μm into a water-based magnetic fluid within a capillary tube, the redshift as high as 33.5 nm of the resonant wavelength of the grating is observed when the external magnetic-field intensity is 189.7 mT, resulting in a sensitivity of 176.4 pm/mT. The hysteresis effect is observed and explained by the characters of magnetic fluid. Such kind of sensors would find potential applications in magnetic sensing fields.

OFS research over the last 10 years at CQU & UESTC

This article reviews my new optical fiber sensing (OFS) research activities in China for the last ten years at Chongqing University and University of Electronic Science and Technology of China, since I returned from UK in 1999. The research progress in long period fiber gratings (LPFGs), distributed fiber sensing systems and microfiber sensors is introduced. For LPFGs, the processing method with high-frequency CO2 laser pulses types of LPFGs fabricated and the related applications for both optical sensing and optical communication are described. For distributed fiber sensing systems, the fiber-optic polarization optical time domain reflectometer (POTDR), fiber-optic phase-sensitive optical time domain reflectometer (Φ-OTDR) and Brillouin optical time-domain analyzer (BOTDA) are developed, respectively. For microfiber sensors, we mainly focus on the knot resonator and its application for sensing of the refractive index and acceleration, etc.

高频CO2激光脉冲写制的倾斜长周期光纤光栅光谱特性研究

高频CO₂激光脉冲写制的倾斜长周期光纤光栅光谱特性研究

Highly Sensitive Optical Refractometer Based on Edge-Written Ultra-Long-Period Fiber Grating Formed by Periodic Grooves

An ultra-long-period fiber grating with periodic groove structure (G-ULPFG) fabricated by using an edge-written method with high-frequency CO2 laser pulses is proposed in this paper. The experimental results show that the different resonant peaks of the G-ULPFG have different temperature and refractive index sensitivities and, in particular, the refractive index sensitivity is much larger than that of conventional ULPFG due to the edge-groove structure. Such a kind of G-ULPFGs could be used as a highly sensitive optical refractometer with temperature self-compensation.

Theory and experiment of refractive index change of long-period fiber grating induced by CO2 laser pulses

The mechanism of refractive index change of long period fiber gratings （LPFGs） induced by high frequency CO2 laser pulses is investigated in theory and by experiment. It is found that the release of residual stress, rapid solidification, fiber densification and fusion distortion are discovered to be the main mechanisms of formation of this kind of gratings. The above four mechanisms play different roles when different fibers, different scanning methods and the different laser power were used in the process of LPFG fabrication. The average refractive index changes of the core and cladding were measured by using M-Z interferometer formed by LPFG pair in this paper, which provides the experimental basis for fabricating all-fiber devices.

Theory and fabrication of long period fiber grating with rotary refractive index modulation induced by CO2 laser pulses

The structure of asymmetric LPFG with rotary refractive index modulation （R-LPFG） along the fiber axis is proposed and fabricated by using high frequency CO2 laser pulses. The transverse refractive index distribution of LPFG with asymmetric refractive index modulation in the cross-section is divided into multilayer circular waveguides, which are then discretized according to the magnitude of index. The mode coupling characteristics of this asymmetric LPFG is discussed by adopting coupled-mode equation sets and the revised Bragg condition. It’s observed that the transmission spectrum has different features depending on the rotation angle. For small angles, resonance peak is unique, however, when the angle becomes greater, the resonance peak will be split into two, which is different from the transmission spectrum of previous LPFGs. Moreover, the two split peaks have almost the same temperature sensitivity because they originated from the same two coupling modes. The high sensitivity torsion meter and strain meter without temperature compensation can be designed by adopting the wavelength spacing between the two split peaks.

A Highly Sensitive Fiber-Optic Refractive Index Sensor Based on an Edge-Written Long-Period Fiber Grating

A novel long-period fiber grating (LPFG) is edge-written without any destructive damage on the fiber based on the thermal shock and rapid cooling effects of the high-frequency CO2 laser pulses exposure method in this letter. The refractive index disturbance induced by high-frequency CO2 laser pulses mainly occurs in the edge region of the fiber cladding rather than in the fiber core. An edge-written LPFG with a resonant peak of ~18 dB is obtained using a standard telecommunication fiber. The experimental results show that such a novel LPFG structure has much higher refractive index sensitivity over conventional LPFGs side-written, which can be used as a refractive index sensor, or a tunable filter and modulator by controlling the refractive index sensitive film which is coated on the cladding of such an LPFG.

Multi-Edge-Written Long-Period Fibre Gratings with Low PDL by UsingHigh-Frequency CO2 Laser Pulses

A novel long-period fibre grating (LPFG) with lowpolarization-dependent loss (PDL) is fabricated by using a multi-edgeexposure method with high frequency CO2 laser pulses. The experimentalresults show that the PDL of a triple-edge-written LPFG with a peakamplitude of -16.5 dB can be as low as 0.22 dB. These novel LPFGs canfind important applications in optical communication and sensing.

Characteristics of novel ultra-long-period fiber gratings fabricated by high-frequency CO2 laser pulses

A novel ultra-long-period fiber grating with periods of up to several millimeters induced by high-frequency CO2 laser pulses is reported, for the first time to our knowledge. The refractive index change occurs mainly within the cladding area of the fiber and has an unsymmetrical distribution on the cross-section of the ULPFG. The resonant loss peaks of the ULPFG are generated from the coupling of the forward propagating core mode to the cladding modes of fundamental and harmonic orders. The characteristics of temperature, strain, torsion and refractive index are investigated by experiment, which offers greater potential for applications in measurement of multi-parameters and optical fiber communication when compared with conventional LPFGs. In addition, several new approaches for the potential applications of such an ULPFG are proposed.

A high sensitivity fiber-optic torsion sensor based on a novel ultra long-period fiber grating

A high sensitivity fiber-optic torsion sensor is described,which measures twist rate and determines twist direction simultaneously by using a novel ultra long-period fiber grating (ULPFG) with periods of up to several millimeters fabricated by high frequency CO2 laser pulses. The torsion characteristics of a ULPFG are analyzed based on the coupling theory and birefringence phenomena. The experimental results show that a higher torsion sensitivity of 0.2244nm/(rad/m) for high order resonant modes was obtained, which is four times higher than that of the long-period fiber grating written by high frequency CO2 laser pulses. Finally, an intensity-type demodulation approach for the realization of real-time torsion measurement is proposed and demonstrated based on the edge filtering effect of the ULPFG.

Transverse-load characteristics of twisted long-period fibre gratings written by high-frequency CO2 laser pulses

The transverse-load characteristics of a twisted long-period fibre grating (LPFG) written by high-frequency CO2 laser pulses are investigated for the first time to the authors' knowledge. It is observed that such a twisted LPFG can weaken the strong orientation-dependence of the transverse load and enhance the transverse-load sensitivity considerably, when compared with a torsion-free LPFG, offering the potential for use as a dynamic load sensor by means of simple intensity detection.

Torsion characteristics of long-period fiber gratings induced by high-frequency CO2 laser pulses

It is demonstrated, for the first time to our knowledge, that torsion characteristics of resonant wavelength of long-period fiber gratings (LPFGs) induced by high-frequency CO2 laser pulses depend strongly on the twist directions. That is, the resonant wavelength shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, whereas it shifts linearly toward the shorter wavelength as the LPFG is twisted anticlockwise. On the other hand, the loss peak amplitude decreases gradually whether the LPFG is twisted clockwise or anticlockwise. If the twisted fiber is much longer than the twisted LPFG, the resonant wavelength shifts wavelike toward the longer and shorter wavelength as the LPFG is twisted clockwise and anticlockwise, respectively, and the loss peak amplitude decreases wavelike with the twist rate applied. An asymmetric refractive-index distribution exists within the cross section of the LPFG fabricated by high-frequency CO2 laser pulses, which results in obvious linear birefringence in the LPFG. The right- and left-rotatory elliptical birefringence are induced when the LPFG with a linear birefringence is twisted clockwise and anticlockwise, respectively. So the twist-induced right- and left-rotatory elliptical birefringence in the LPFG show that the shift of resonant wavelength is dependent on the twist directions. The twist-induced circular birefringence in the fiber shows that the resonant wavelength and amplitude of the LPFG change wavelike.

A novel dynamic LPFG gain equalizer written in a bend-insensitive fiber

The bending characteristics of the long-period fiber gratings (LPFG) induced by high-frequency CO2 laser pulses using a bend insensitive fiber is reported, for the first time, to our knowledge. The experimental results show that the resonant peak amplitude of the LPFG written in a bend insensitive fiber is quite sensitive to bending while its resonant wavelength is insensitive to bending at some circular orientations. In addition, the resonant peak amplitude of the LPFG written in a bend insensitive fiber has a much higher bend sensitivity than that written in a standard telecommunication fiber. Based on this finding, a novel all-fiber dynamic gain equalizer is proposed to flatten the gain of EDFAs by controlling the bend of the LPFG directly. The experimental results show that such a device would be a practical and simple approach to realize gain flattening of an EDFA with a high precision for gain flatness control.

Long period fibre grating torsion sensor measuring twist rate and determining twist direction simultaneously

A novel fibre-optic torsion sensor based on a long-period fiber grating written by high-frequency CO2 laser pulses is proposed and has been demonstrated. Such a torsion sensor can not only measure the applied twist rate directly, but also determine the twist direction simultaneously.

Unique characteristics of long-period fibre gratings fabricated by high-freque ncy CO2 laser pulses

In this paper, we report novel long-period fibre gratings (LPFGs) written by focused high-frequency CO2 laser pulses. A number of unique characteri stics of such a LPFG, such as stroin, temperature, bend, torsion and transverse-load, etc., are observed by experiment, for the first time to our knowledge. These uni que features are mainly due to the asymmetrical distribution of the refractive i ndex on the cross section of the LPFG induced by high-frequency CO2 l aser puls es. Based on these unique characteristics, we have proposed a novel bend-insensi tive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurements, a novel torsion sensor that can realize absolute measure ment of twist rate and a load sensor that can achieve simultaneous measurement o f transverse load and temperature using a single LPFG element.

An All-Fibre Dynamic Gain Equalizer Based on a NovelLong-Period Fibre Grating Written by High-Frequency CO2 Laser Pulses

An all-fibre dynamic gain equalizer based on a novel long-period fibregrating (LPFG) written on a photosensitive fibre by high-frequencyCO2 laser pulses is demonstrated for the dynamic gain flattening ofEr-doped fibre amplifiers. The resonant peak amplitude of such adynamic gain equalizer can be adjusted linearly by bending the LPFG.The gain profile of the Er-doped fibre amplifier constructed has beenflattened to within ±0.7 dB over a 32 nm bandwidth using such adevice.