Cascaded Long-period Fiber Grating Research Articles

Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP01-LP11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields.

A high sensitivity sensor based on novel CLPFG with wavelength and intensity modulation for simultaneous measurement of SRI and temperature

In this paper, a novel cascaded long-period fiber grating (CLPFG) sensor based on the modulation of both wavelength and intensity is demonstrated, which is able to simultaneously detect surrounding refractive index (SRI) and temperature. The appropriate grating periods are designed by theoretical simulations. One grating operate in single-peak resonance state and the other one operates at slight deviation of resonance (SDOR) state. When the SRI and temperature change, the wavelength of resonant peak in the single-peak resonance state shifts, and the intensity of the resonant peak at the SDOR state changes accordingly. In experiments, the CLPFG is inscribed based on the theoretical design and the responses of the sensor to SRI and temperature are investigated. The experimental results show that the SRI sensitivities are −177.6 nm/RIU and −202.7 dB/RIU, and the temperature sensitivities are 0.1175 nm/℃ and 0.0997 dB/℃. The SRI and temperature sensitivities of our proposed sensor are 30 times and 10 times higher than those of other sensors with dual-parameter modulation of wavelength and intensity. The proposed sensor with simplified structure, good linearity, and high sensitivity can be expanded to the area of multi-parameter sensing.

Development of long‐distance power supply system with high power laser over single mode fiber

AbstractThe technology of long‐distance power supply with high‐power laser (HPL) over single‐mode fiber (SMF) is of great significance in specific environment. The nonlinear effect of fiber and photovoltaic conversion (PC) efficiency are the key factors restricting the application of this technology. The wide linewidth laser and cascaded long period fiber grating (CLPFG) band rejection filter are used to suppress the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) effects, respectively, integrating sphere like, scattering light guide cone and thermoelectric power generation improve PC efficiency. A HPL over SMF remote power supply system is developed and tested. The injected fiber power is 10 W, the wavelength of 1550 nm laser is transmitted through 103 km single‐mode fiber, and then 26 mW power is converted for the sensor. This is also the highest level of the open literature. It is expected to be applied to the power supply of high voltage transmission line remote monitoring information system in remote areas.

Characteristics of novel tail ring-shaped cascaded long period fiber grating sensor

Abstract In this paper, a novel tail ring-shaped cascaded long period fiber grating (TR-CLPFG) sensor is fabricated by connecting two LPFGs with ring-shaped optical fiber. The results show that the sensor can effectively reduce full width at half maxima (FWHM) and improve refractive index sensitivity. The parallel fibers in the ring-shaped structure make the two LPFGs almost identical and improve the fabrication efficiency. The smaller the sensing area leads to fewer samples to be measured. The TR-CLPFG sensor has compact structure, which is conducive to its application in narrow space. In addition, this U-shaped structure allows the sensor to be inserted into the solution measurement, which changes the previous measurement methods of LPFG sensor and makes the sensor more flexible in application. In general, this novel TR-CLPFG sensor not only maintains the advantages of traditional linear cascaded long period fiber grating (CLPFG) sensor, but also gets many superiorities in the fabrication process and application. This TR-CLPFG sensor has great potential in future detection applications.

Design of a high-sensitivity cascaded long-period fiber grating sensor operating at PMTP

This paper presents a novel cascaded long-period fiber grating (CLPFG) sensor which is cascaded by two identical coated long-period fiber gratings (LPFGs) operating at the phase-matching turning point (PMTP). This CLPFG sensor operates at the PMTP, so it has the highest sensitivity compared with those LPFGs operating near the PMTP. In addition, the spectrum of this CLPFG sensor is composed of a series of narrow-bandwidth loss peaks, which are easy to be detected by an ordinary wavelength demodulator. In theory, the transmission spectrum of the CLPFG can be calculated by the transfer matrix method. We select two loss peaks with highest sensitivity from the transmission spectrum of the CLPFG for dual-peak sensing. The simulation results show that the wavelength spacing of the two peaks changes from 24.6 to 74.8 nm with the surrounding refractive index changing from 1.330 to 1.334. The average sensitivity is 12550 nm/RIU, which is about 6 times higher than traditional LPFG sensors where the LPFG is operating near the PMTP, so this kind of CLPFG sensor is suitable for high-sensitivity sensing.

Simulation of novel intensity modulated cascaded coated LPFG sensor based on PMTP

This paper presents a novel intensity modulated cascaded long-period fiber grating (CLPFG) sensor which is cascaded by two same coated long-period fiber gratings (LPFGs) operating at the phase-matching turning point (PMTP). The sensor combines the high sensitivity of LPFG operating at PMTP and the narrow bandwidth of interference attenuation band of CLPFG, so a higher response to small change of the surrounding refractive index (SRI) can be obtained by intensity modulation. Based on the coupled-mode theory, the grating parameters of the PMTP of a middle odd order cladding mode of a single LPFG are calculated. Then this two same LPFGs are cascaded into a CLPFG, and the optical transmission spectrum of the CLPFG is calculated by transfer matrix method. A resonant wavelength of a special interference attenuation band whose intensity has the highest response to SRI, is selected form CLPFG’s spectrum, and setting the resonant wavelength as the operating wavelength of the sensor. Furthermore, the simulation results show that the resolution of SRI of this CLPFG is available to 1.97 × 10−9 by optimizing the film optical parameters, which is about three orders of magnitude higher than coated dual-peak LPFG and cascaded LPFG sensors. It is noteworthy that the sensor is also sensitive to the refractive index of coat, so that the sensor is expected to be applied to detections of gas, PH value, humidity and so on, in the future.

High-sensitivity sensor design based on cascaded long-period fiber grating with film coating

A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented. Two long-period fiber gratings (LPFGs) are cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for ambient refractive index (RI) measurement of the CLPFG sensor are analyzed in a form of transfer matrix based on rigorous coupled-mode theory. The results indicate that it is highly sensitive to the film RI and surrounding RI, so it can be used as a gas sensor or a solution sensor. Data simulation shows that the resolution of the RI of the films is predicted to be 10 −7 . Further, a cascaded chirped LPFG is introduced to study its sensing performance. The influence of the chirp coefficient and the grating structural parameters on the transmission spectrum of the cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the RI of the films is predicted to be 10 −9 . In contrast to the conventional measurement method based on interrogating the wavelength change, intensity detection of this cascaded chirped LPFG sensor means that no optical spectrum analyzer is required in the measuring system, which is favorable for practical applications, especially for in situ environmental motoring.

Characteristics of twist and residual stress relief based cascaded long period fiber gratings

A simple three-side exposure method for releasing residual stress in cascaded long period fiber gratings (CLPFG) is proposed. Experimental results show the amplitudes of loss peaks noticeably reduce and loss peak finally vanish with residual stress gradually released by three-side high-frequency CO2 laser irradiation, which demonstrates the gratings are erased. Furthermore, experiments demonstrate that loss peaks are insensitive to twist directions owing to a symmetrical wavelength shift under different twist directions (clockwise or counterclockwise).

Dynamic strain measurement using two wavelength-matched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating

In this work, we describe a fiber Bragg grating (FBG) sensing system using two wavelength-matched FBG sensors for static and dynamic strain measurement. A cascaded long-period fiber grating (CLPG)-based demodulation technique has been used to interrogate the two wavelength-matched FBG sensors. Experimental results of static strain measurement show that the proposed system has a strain resolution of 1 με. This system has also been used for dynamic strain measurement. An eddy current displacement meter-based system has been used as a comparison measurement. Experimental results of dynamic strain measurement have proved that the FBG sensing system has a good performance in the measurement of dynamic strain. The results of static and dynamic strain measurement indicate that the sensing system using two wavelength-matched FBG sensors is superior to the single FBG sensor system.

All-optical clock extraction from 40-Gbit/s NRZ data using cascaded long-period fiber grating

All-optical clock extraction from a 40-Gbit/s NRZ input signal is demonstrated using a cascaded long-period fiber grating (CLPG) and a mode-locked fiber ring laser. The CLPG has a Mach–Zehnder configuration with two arms along the core and cladding regions. Using the difference in propagation delay between two arms, the non-return-to-zero (NRZ) signal is converted to the pseudo-return-to-zero (PRZ) signal. To obtain repetitive pulses as a clock signal from the PRZ signal, a ring laser with a semiconductor optical amplifier (SOA) is used. Subsequently, the measured carrier-to-noise ratio (CNR) of the PRZ and clock signals are enhanced up to 30 dB and 31 dB, respectively, compared to that of the original NRZ signal. Also, the clock signal centered at 40 GHz has a low timing jitter of <1.3 ps. It is expected that this method can be applied to high speed fiber-optic systems of >40 Gbit/s due to its small time delay between the core and cladding regions.

OSNR monitoring technique based on cascaded long-period fiber grating with optically tunable phase shifter

An optical signal-to-noise ratio (OSNR) monitoring technique using a cascaded long-period fiber grating with a tunable phase shifter is proposed for non return-to-zero on-off-keying (NRZ-OOK) and differential phase shift keying (DPSK) signals. This method is based on two key points: first, the cascaded long period fiber grating (CLPG) acts as an optical delay interferometer (ODI) by Mach-Zehnder configuration consisting of the core and cladding modes inside the CLPG. Secondly, an optically tunable phase shifter using an Yb(3+) doped optical fiber (YDF) is inserted between the CLPG. The phase of the modulated signal propagating along the core mode of YDF is controlled by adjusting pumping power of 976-nm laser diode (LD). By combining the CLPG with the optically tunable phase shifter, the OSNR monitoring could be successfully executed.

A new method of measuring the waveguide dispersion factor and the thermo-optic coefficient of long-period fiber gratings

Based on the Mach–Zehnder effect between the core mode and the cladding modes, the interference fringes are formed by a pair of cascaded long-period fiber gratings (CLPFGs). Theoretical analyses show that the spectral spacing and the wavelength of these fringes are functions of the waveguide dispersion factor γ, which is a characterizing parameter to LPFG and with theoretical and applicational significance. By measuring the characteristics of the transmission spectra of CLPFGs, the absolute value of γ can be obtained. At the same time, the thermo-optic coefficient of effective refractive index difference between core and cladding modes, μ, can also be obtained by measured the temperature sensitivity of these fringes. In the experiments, ∣γ∣ and μ were measured by this method to be 0.874 and 4.08×10−5 °C−1, respectively, for LPFGs with period of 450 μm and with a HE14 resonant peak at 1554 nm.

All-optical signal gating in cascaded long-period fiber gratings

An all-optical signal gating has been demonstrated in cascaded long-period fiber gratings using cross-phase modulation by means of Q-switched pump wave packets. The obtained cascaded twin long-period fiber gratings showed an extremely narrow band of rejection of 0.5-nm bandwidth. Using this narrowband rejection filter, all-optical switching efficiency in long-period fiber grating devices could be enhanced so that they can be operated with relatively low pump intensities. Approximately a 20% modulation depth of a signal wave at 1565 nm was obtained with a pump intensity of 1 GW/cm/sup 2/ at 1064 nm. The experimental results are in good agreement with numerical expectations by the discretized coupled-mode theory, which was previously proposed for the nonlinear long-period fiber grating analysis.