Fabrication and sensing characteristics of intrinsic Fabry–Perot interferometers in fiber tapers

This Letter presents intrinsic Fabry–Perot interferometers in the fiber tapers fabricated by the femtosecond laser micromachining technique. The sensing of temperatures as high as 1000°C based on the fiber device is characterized, with a sensitivity of 15.28 pm∕°C. A nearly linear refractive index sensing is also obtained by using the fringe visibility to characterize, with a sensitivity of 73.05 dB∕RIU. These intrinsic Fabry–Perot interferometers in fiber tapers may be useful in applications of high-temperature and linear refractive index sensing.

Polymer diaphragm based sensitive fiber optic Fabry-Perot acoustic sensor

A sensitive polymer diaphragm based fiber Fabry-Perot （F-P） sensor for aeroacoustic wave measurement is presented. A novel polymer material poly （phthalazinone ether sulfone ketone） （PPESK） diaphragm is used as the acoustic sensing element. The effective dimensions of the diaphragm are 4 mm in diameter and 6 μm in thickness. Owing to the good mechanical and optical features of the diaphragm and application of the interferometric/intensity demodulation, a system sensitivity of 31 mV/Pa is achieved in the frequency range of 0.1-12.7 kHz, and a signal-to-noise ratio （SNR） of 29 dB at 1 kHz is obtained. The linear response of the sensor is from 0.35 to 2.82 Pa, corresponding to 85 103 dB sound pressure level （SPL） （re： 20 μPa）. The sensor has the potential to be used as a universal and low-cost optical microphone.

Theoretical and experimental investigation of the mode-spacing of fiber Bragg grating Fabry-Perot cavity

The mode-spacing of the fiber Bragg grating Fabry-Perot （FBG F-P） cavity is calculated by using the effective cavity length which contains the effective length of the FBG. The expression of the effective length, defined by using the phase-time delay, is obtained and simplified as a function of the peak reflectivity at the Bragg wavelength, the band edges, and the first zero-reflectivity wavelength. The effective length is discussed from the energy penetration depth point of view. Three FBG F-P cavities are fabricated in order to validate the effective length approach. The experimental data fits well with the theoretical predictions. The limitation of this method is also pointed out and the improved approach is proposed.

Multichannel fiber optic Fabry-Perot nonscanning correlation demodulator

A multichannel fiber-optic Fabry-Perot （F-P） demodulator based on nonscanning correlation demodula- tion is proposed. The demodulator principle is analyzed, and the prototype of nonscanning correlation demodulation fiber-optic F-P demodulator is made and tested. The measurement range of the prototype is from 10 to 40 μm, the demodulation resolution is 8 nm, and its stability is 7 nm. This method provides a feasible solution, which guarantees the practicability of the fiber-optic F-P sensor network.

Generation of tunable multi-wavelength optical short pulses using self-seeded Fabry-Perot laser diode and tilted multimode fiber Bragg grating

We experimentally demonstrate the simultaneous generation of tunable multi-wavelength picosecond laser pulses using a self-seeding configuration that consists of a gain-switched Fabry-Perot laser diode （FPLD） with an external cavity formed by a tilted multimode fiber Bragg grating. Dual- and triple-wavelength pulses are obtained and tuned in a flexible manner by changing the temperature of the FPLD. The side mode suppression ratio larger than 25 dB is achieved at different dual- and triple-wavelengths and the typical pulsewidth of the output pulses is ,-70 ps. In the experiment, the wavelength separation can be narrowed to 0.57 nm.

Temperature-independent refractive index measurement based on Fabry-Perot fiber tip sensor modulated by Fresnel reflection

A simple fiber tip sensor for refractive index （RI） measurement based on Fabry-Perot （FP） interference modulated by Fresnel reflection is proposed and demonstrated. The sensor head consists of an etching- induced micro air gap near the tip of a single-mode fiber. The microgap and the fiber tip function as two reflectors to form a FP cavity. The external RI can be unambiguously measured by monitoring the fringe contrast of the interference pattern from the reflection spectrum. The experimental results show that the proposed sensor achieves temperature-independent RI measurement with good linear response. The proposed sensor achieves a high RI resolution of up to 3.4× 10^-5 and has advantages of low cost and easy fabrication.

Wavelength conversion of chaotic message through gain modulation by injection-locked Fabry-Perot laser diode

We propose a wavelength conversion scheme for chaotic optical communications（COC） based on a Fabry-Perot （FP） laser diode.The FP laser,as a wavelength converter,is injection-locked at one of longitudinal modes by an external continuous-wave（CW） light.The simulation results demonstrated that the chaos masked signal at wavelengthλ_1,which corresponds to the other longitudinal mode of FP laser,can be converted to the injection-locked mode（wavelengthλ_2） based on cross-gain modulation in a closed-loop COC link.A 1.2-GHz chaos masked sinusoidal signal is successfully decoded with signal-to-noise ratio （SNR） beyond 8 dB in 15-nm wavelength conversion range,and the effects of SNR on the signal frequency and conversion span are also investigated.

In-line Fabry-Perot refractive index sensor based on microcavity

An in-line Fabry-Perot （FP） refractive index （RI） sensor based on an intrinsic FP cavity and fabricated by the etching and fusion splicing method is proposed. The experimental results demonstrate that the sensor possesses a high resolution of 1508 nm/RIU for the measurement of acetylene gas RI. The temperature- response measurement shows that the sensor is insensitive to room temperature variations. The FP RI sensor is suitable for applications in biosensing and environmental monitoring because of its high sensitivity and structural simplicity, thereby making it suitable for low-cost mass production.

Temperature-insensitive refractive index sensor based on an optical fiber extrinsic Fabry–Perot interferometer processed by a femtosecond laser

An optical fiber extrinsic Fabry-Perot interferometer （EFPI） is designed and fabricated for refractive index （RI） sensing. To test the RI of liquid, the following two different methods are adopted： the wavelength tracking method and the Fourier-transform white-light interferometry （FTWLI）. The sensitivities of sensors with cavity lengths of 288.1 and 358.5 μm are 702.312 nm/RIU and 396.362 μm/RIU, respectively, by the two methods. Our work provides a new kind of RI sensor with the advantages of high sensitivity, mechanical robustness, and low cross sensitivity to temperature. Also, we provide a new method to deal with gold film with a femtosecond laser.

Cost-effective tunable fiber laser based on self-seeded Fabry-Perot laser diode using a Sagnac loop reflector

We propose and demonstrate a simple configuration of wavelength-tunable fiber laser made up of a tunable band-pass filter, a Sagnac loop refiector, and a Fabry-Perot laser diode. Based on the self-seeded operation, the proposed fiber laser can obtain a single-longitudinal-mode output in the wavelength tuning range of 1544.69–1563.39 nm with tuning step of 1.34 nm. The performances of output power （-9 dBm）, optical side-mode suppression ratio （ 65.5 dB）, and power and wavelength stabilities are well exhibited.

A novel smart steel strand based on optical-electrical co-sensing for full-process and full-scale monitoring of prestressing concrete structures

As the main load bearing component,the steel strand has a significant impact on the safety of civil infrastructure.Real-time monitoring of steel strand stress distribution throughout the damage process is an impor-tant aspect of civil infrastructure health assessment.Hence,this study proposes an optical-electrical co-sensing(OECS)smart steel strand with the DOFS and CCFPI embedded in.It can simultaneously measure small strains in the initial damage phase with high accuracy and obtain information in the large deformation phase with relatively low precision.Several experiments were carried out to test its sensing performance.It shows both DOFS and CCFPI have good linearity,repeatability and hysteresis.In comparison to DOFS,CCFPI has a relatively lower accuracy and resolution,but a large enough measurement range to tolerate the large strain in the event of a steel strand failure.To verify the reliability of the proposed smart steel strand in real structures,the strand strain distribution in the full damage process of bonded prestressed beams under four-point bending loading was monitored using the smart steel strand as a prestressing tendon.The strain measured by the OECS steel strand is shown to reflect the deformation and stiffness variation of prestressed beams under different load.

EFPI-FBG hybrid sensor for simultaneous measurement of high temperature and large strain

We report an extrinsic Fabry Perot interferometer-fiber Bragg grating （EFPI-FBG） hybrid sensor in this letter. The interferometric cavity of the proposed hybrid sensor is composed of a glass capillary tube, a section of single-mode fiber, and a section of single-mode metal fiber with one FBG. The FBG processed by high-temperature annealing is used to measure temperature, whereas the fiber EFPI is adopted for strain measurement. One of the two aligned fibers is free along the axial direction, which is different from the traditional structure that both the fibers are fixed to glass capillary tube. Experimental results show that the sensor can measure high temperature and large strain simultaneously. The measuring range of temperature and strain for the hybrid sensor is up to 500℃ and 10,000μs, respectively. Effective temperature compensation of the hybrid sensor is realized, too.

InGaN/GaN laser diode characterization and quantum well number effect

The effect of quantum well number on the quantum efficiency and temperature characteristics of In- GaN/GaN laser diodes （LDs） is determined and investigated. The 3-nm-thick In0.13Ca0.87N wells and two 6-am-thick GaN barriers are selected as an active region for Fabry-Perot （FP） cavity waveguide edge emitting LD. The internal quantum efficiency and internal optical loss coefficient are extracted through the simulation software for single, double, and triple InGaN/GaN quantum wells. The effects of device temperature on the laser threshold current, external differential quantum efficiency （DQE）, and output wavelength are also investigated. The external quantum efficiency and characteristic temperature are improved significantly when the quantum well number is two. It is indicated that the laser structures with many quantum wells will suffer from the inhomogeneity of the carrier density within the quantum well itself which affects the LD performance.

Optical fiber acoustic sensing multiplexing system based on TDM/SFDM

We propose a system of time-division multiplexing （TDM） and spatial frequency-division multiplexing （SFDM）. Extrinsic Fabry Perot interferometric sensors are applied to detect weak acoustic signals. The broadband source is employed, the light from it is modulated by a pulse signal sequence and is efficiently amplified by semiconductor optical amplifiers. Experimental results show that the equivalent noise pressure spectrum level is -97.2 dB re 1 rad/√Hz below 1250 Hz, and the cross talk between two sensors in one TDM channel is -32.7 dB with a cavity length difference of 60 μm. The number of sensors in this multiplexing system can theoretically reach 160.

Reflection-type infrared biosensor based on surface plasmonics in graphene ribbon arrays

We propose a reflection-type infrared biosensor by exploiting localized surface plasmons in graphene ribbon arrays. By enhancing the coupling between the incident light and the resonant system, an asymmetric Fabry-Pcrot cavity formed by the ribbons and reflective layer is employed to reshape the reflection spectra. Simulation results demonstrate that the reflection spectra can be modified to improve the figure of merit （FOM） significantly by adjusting the electron relaxation time of graphene, the length of the Fabry-Perot cavity, and the Fermi energy level. The FOM of such a biosensor can achieve a high value of up to 36/refractive index unit （36/RIU）, which is -4 times larger than that of the traditional transmission-type one. Our study offers a feasible approach to develop biosensing devices based on graphene plasmonics with high precision.

High-power double-clad large-mode-area photonic crystal fiber laser

A high power double-clad ytterbium-doped large-mode-area photonic crystal fiber（LMA PCF）laser was demonstrated using a unique Fabry-Perot （F-P） continnous wave （CW） output power of 50 W at～L04μm with a slope efficiency of 76.3% is obtained, Single transverse mode operation is achieved without any thermal-optical problems, This laser has the potential for scaling to much higher output power.

Frequency-locked tunable LD laser with a broad bandwidth

Using lock-in amplifer and proportional, integral, and derivative (PID) electric circuit, the frequency of diode laser is stabilized on a highly mechanical stable Fabry-Perot (FP) cavity transmission peak. When the frequency locking system is on, the frequency tunable range of the laser is about 4 GHz around the D1 transition of Rb. The laser frequency tuning is implemented by scanning the FP cavity length. The fluctuation of frequency of the output laser is less than 1 MHz, and the drift of the center frequency is less than 1.5 MHz in 1.5 min. This system has great potential of the application in the experimental investigation of the interaction between light and atoms, especially, for the case of far off the atomic resonance.

Novel approach to reduce the pattern effect in 10-Gb/s clock recovery

A Fabry-Perot (F-P) etalon and a semiconductor optical amplifier (SOA) were combined to preprocess the data signals before clock recovery. With this technology in the 10-Gb/s clock recovery utilizing injection mode-locked laser (IMLL) based on SOA, the amplitude fluctuation and timing jitters caused by the pattern effect in recovered clock pulses were greatly reduced, experimentally. It also demonstrated that clock could be recovered from the very degraded signals.