Pair Of Long-period Fiber Gratings Research Articles

Spectral characteristics and space division multiplexing scheme of long period fiber grating pair

Due to the transmission mode and multiple loss peak of a long period fiber grating pair (LPGP), there are some difficulties in multiplexing and demodulating it. To realize the multi-point detection of LPGP, a new distributed demodulation system based on space division multiplexing is proposed. The relationship between the spectra of the LPGP and the grating parameters (grating length, grating period, separation of the long period fiber grating) were investigated, and a series of rules for optimizing the spectrum characteristics of the LPGP were obtained. By using optical switches, the traditional spectrometer demodulation is combined with computer technology to design a multiplex demodulation system of distributed LPGPs. Spectrum reduction, resonant wavelength, and temperature demodulation of the LPGP were realized. The experimental results show that the demodulation speed of the system is 1.1 s and the wavelength accuracy is 0.01 nm. The designed system has the advantages of high precision, strong compatibility and friendly operation interface. This multiplexing method is appealing to construct distributed LPGP sensors since it is easy to extend multiplexing channels.

Interferometric Fiber Optic Sensors

Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair.

Tunable filter based on a pair of special long-period fiber gratings and its application in fiber ring laser

A filter based on a Mach-Zehnder interferometer (MZI) formed by cascading a pair of longperiod fiber gratings (LPFGs) written by CO2 laser in a twisted single-mode fiber is proposed. The transmission spectrum of the filter is tuned by modifying the bending curvature of the MZI, and thus wavelength tuning is achieved. And a tunable erbium-doped fiber (EDF) ring laser employing the filter as a wavelengthselection component is experimentally demonstrated. The laser can be tuned continuously from 1573.05 to 1595.75 nm with a side-mode suppression ration of ∼50 dB over the total tuning range.

Torsion sensing with a fiber ring laser incorporating a pair of rotary long-period fiber gratings

We experimentally demonstrate a fiber ring laser for high-resolution torsion measurement, where the laser cavity consists of a Mach-Zehnder interferometer formed with a pair of long-period fiber gratings written in a twisted single-mode fiber by a CO 2 laser. The emitting wavelength of the laser provides a measure of the rate of the torsion applied to the grating pair, while the direction of the wavelength shift indicates the sense of the applied torsion. The narrow linewidth and the large side-mode suppression ratio of the laser can provide a much more precise measurement of torsion, compared with passive fiber-optic torsion sensors. The torsion sensitivity achieved is 0.084 nm/(rad/m) in the torsion range ± 100 rad/m, which corresponds to a torsion resolution of 0.12 rad/m, assuming a wavelength resolution of 10 pm for a typical optical spectrum analyzer. The ultimate resolution of the sensor is limited by the linewidth of the laser and could be an order of magnitude higher.

A study on a long period fiber grating pair's spectral characteristics

This paper demonstrates the spectrums of a long period fiber grating pair (LPGP) by changing the following physical parameters: the separation of the LPG, the length of single LPG and the number of single LPG .The study of the response of the LPGP to different parameters enables a rational design of the sensor element. These results produce an important foundation of our further study of LPGP spectral characteristics. On the basis of the above conclusion, a LPGP is produced by the amplitude mask method, in which the germanium-doped fiber is exposed by the UV. The LPGP's resonance wavelengths are shifted as the refractive index of medium surrounding the cladding of the long-period grating. The sugar solution was experimentally measured, and results show that, this fiber-based device can rely on LPGP's the wavelength drift to differentiate solution's concentration based on their refractive index.

一种通道数可变的光纤光栅梳状滤波器

An in-fiber Mach-Zehnder interferometer filter based on a pair of long-period fiber gratings (LPGs) had been operated in the reflection mode by incorporating a broadband chirped-fiber Bragg grating (CFBG) or an optical fiber mirror reflector.As a result,the extinction ratio has been nearly doubled and,by tuning chirp rate,i.e.reflection bandwidth of the CFBG using a beam-bending method,channel number of the formed optical comb filter can be easily changed.In our experiment,two comb filters with channel numbers being changed among odd ones from 1 to 5 and 3 to 9 have been achieved,respectively.Even channel filtering can also be achieved if the center wavelength of the chirped fiber grating is changable.

Intensity-based optical fiber strain sensor using long-period fiber gratings and a core mode blocker

We report an intensity-based fiber strain sensor using a pair of long-period fiber gratings (LPFGs) and an in-line core mode blocker. By inserting a core mode blocker fabricated by the arc discharge method between two LPFGs, a band-pass filter can be formed. We used an LED and an optical power meter to measure the transmittance near the resonance wavelength of the LPFGs and we obtained a relation between the axial strain applied on one of the LPFGs and the transmitted power. The measurement results indicate that the sensitivity of the power meter output voltage to the applied strain is 6.37 pV με−1.

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.

Hydrogen Sensor Based on A Palladium-Coated Long-Period Fiber Grating Pair

We propose a simple hydrogen detection scheme based on a Mach-Zehnder interferometer formed with a pair of palladium-coated long-period fiber gratings (LPGs). Since an LPG pair offered a fine-structured interference fringe in its transmission spectrum, the resolution as a sensor could be appreciably enhanced compared to that of a single LPG. As the palladium layer absorbed hydrogen, the effective refractive indices of the cladding modes were increased so that the interference spectrum was blue-shifted up to 2.3 nm with a wavelength sensitivity of -0.29 nm/min for 4% of hydrogen concentration.

用非线性光纤连接的长周期光栅对的光开关特性

用非线性光纤连接的长周期光栅对的光开关特性

Tunable WDM filter with 0.8-nm channel spacing using a pair of long-period fiber gratings

A strain-tuned wavelength-division-multiplexing filter with 0.8-nm channel spacing has been demonstrated by using a pair of long-period optical fiber gratings. The response of a transmission spectrum to applied axial strain is studied theoretically and experimentally. A large operation bandwidth of over 20 nm and a linear tuning rate of -0.6 pm/μ/spl epsiv/ within a near 3-nm range have been achieved.

Broad-band tunable all-fiber bandpass filter based on hollow optical fiber and long-period grating pair

We report a tunable all-fiber bandpass filter based on a short hollow optical fiber serially concatenated between a pair of long-period fiber gratings. With novel core mode blocking in the hollow core fiber and its optimal design, the device showed a low insertion loss of 1.5 dB and broad-band passband tuning range of 84.3 nm covering both S- and C-band.

Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings.

We demonstrate implementation of an all-fiber Mach-Zehnder interferometer formed in a photonic crystal fiber (PCF). We formed the all-PCF Mach-Zehnder interferometer by mechanically inducing two identical long-period fiber gratings (LPGs) in the PCF. The spectral properties of a LPG and a LPG pair were investigated. The interference fringe formed within the stop band of the LPG pair varied with the period and the strength of the gratings, and the fringe spacing was decreased with increasing grating separation. From the fringe spacing measurement the differential effective group index of the PCF was calculated to be deltam approximately equal to 2.8 x 10(-3).

All-optical switching application based on optical nonlinearity of Yb3+ doped aluminosilicate glass fiber with a long-period fiber gratings pair

We propose a new fiber-type all-optical switching device based on the optical nonlinearity of Yb(3+) doped fiber and a long-period fiber gratings(LPG) pair. The all-optical ON-OFF switching with the continuous wave laser signal at ~1556nm in the LPG pair including the 25.5cm long Yb(3+) doped fiber was demonstrated up to ~200Hz upon pumping with the modulated square wave pulses at 976nm, where a full optical switching with the ~18dB extinction ratio was obtained at the launched pump power of ~35mW.

Novel all-fiber bandpass filter based on hollow optical fiber

We report an in-line all-fiber bandpass filter based on a hollow optical fiber (HOF) core mode blocker along with a long-period fiber grating (LPG) pair. It is theoretically and experimentally confirmed that selective core mode blocking at the HOF region located in the middle of the LPG pair could provide efficient passband channels corresponding to the resonant wavelengths of the LPGs. The filter characteristics in passbands and the nearfield patterns of guided modes in the proposed device are discussed.

Measurement of refractive-index variation with temperature by use of long-period fiber gratings

The thermo-optic coefficient of the core material of a fiber is analyzed by use of a pair of long-period fiber gratings. First the effective index difference between the core and the cladding modes is measured from the peaks of the interference fringe generated by the grating pair. The order of the cladding mode is decided by the cutoff wavelength and the numerical aperture of the fiber. The material index of the fiber core is obtained in terms of wavelength. At each wavelength the index is chosen to minimize the difference between the measured and the calculated spectra of the grating pair. Finally the thermo-optic coefficient of the fiber core is calculated by repetition of the measurement at different temperatures. With a germanosilicate-core fiber and a boron codoped germanosilicate-core fiber, the thermo-optic coefficients were 1.1x10(-5)/( degrees )C and 0.75x10(-5)/( degrees )C, respectively.

Resonant optical nonlinearity measurement of Yb^3+/Al^3+ codoped optical fibers by use of a long-period fiber grating pair

A new method of measuring optical nonlinearity for resonant nonlinear optical fibers is proposed. A long-period fiber grating (LPG) pair was used to measure the nonlinear refractive index n(2) of a Yb(3+)/Al (3+) codoped optical fiber, which was spliced between the two LPGs, as the fiber was pumped with a laser diode. The nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm depended on the pump power. As the pump power increased, the nonlinear refractive index decreased. At launched pump powers of 2-8 mW, the nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm was ~7.5x10(-15)m (2)/W .

High-Resolution Interrogation Technique for Fiber Bragg crating Sensor Using Long-Period Fiber Grating Pair and Erbium-Doped fiber

A novel interrogation scheme to detect fine Bragg wavelength shift using a long period fiber grating pair with erbium-doped fiber inserted between the two gratings is reported. The technique is shown to feature high resolution and much more immunity to temperature perturbation compared to the conventioned Mach-Zehnder interferometer demodulation system. For quasi-static strain measurement, this approach provides high wavelength resolution of 0.05 pm that corresponds to 41.7 ne in strain and $3.8 $\times$ 10^{-3}$ $^{\circ}C$ in temperature. This interrogation system is also employed in dynamic measurement to obtain the minimum detectable strain perturbation of ~ 8.76 ne/H $z^{{\frac}{1}{2}}$ at 100 Hz. Moreover, this interrogation system has prominent thermal stability. This thermal stability comes from the fact that two arms of the interferometer, the core and cladding in erbium-doped fiber, are exposed to nearly the same environment .

Dependence of fringe spacing on the grating separation in a long-period fiber grating pair

The spectral spacing of the interference fringes formed by a pair of long-period fiber gratings was investigated. The variation of the fringe spacing was measured while the separation between the gratings was changed from 22 to 500 mm. When the grating separation was much longer than the length of the individual grating, the inverse of the fringe spacing became linearly proportional to the grating separation and to the differential effective group index of the fiber. In the third stop band of the grating pair, made along a dispersion-shifted fiber centered at 1.55 microm, the differential effective group index was calculated to be approximately 6.4 x 10(-3), which is approximately twice the differential effective index of the fiber. The discrepancy between the two indices was observed to decrease with the band order, a phenomenon that is explained by the first-order dispersion of the fiber. The measured interference fringes were not regularly spaced in the frequency domain, but regular spacing is required in wavelength-division multiplexing communication systems. Analysis of the second-order dispersion of the fiber and the grating-induced nonlinear phase shift within grating regions as the factors that induce chirping on the fringe spacing is presented.