Fiber bragg grating strain sensor demodulation with quadrature sampling of a mach-zehnder interferometer

An Optical Fiber Sensor for Axial Strain, Curvature, and Temperature Measurement Based on Single-Core Six-Hole Optical Fiber

In this paper, the field distribution and effective refractive index of transmission modes in single-core six-hole optical fiber were researched by modeling and simulation experiments. Based on the simulation results, a new type of sensor for axial strain, curvature, and temperature applications measurement was designed and fabricated. The experimental results showed that the axial strain sensitivities at different dips were −0.97 pm/με and −1.05 pm/με in the range from 0 to 2000 με, and the temperature sensitivities were 35.17 pm/°C and 47.27 pm/°C in the range from 25 to 75 °C. In addition, the proposed sensor also detected the curvature change with sensitivities of 7.36 dB/m⁻¹ and 20.08 dB/m⁻¹ from −2.582 m⁻¹ to −1.826 m⁻¹, respectively. Finally, through theoretical analysis, it can be deduced that this has potential application in the field of simultaneous measurement of strain and temperature.

Multichannel Approach for Arrayed Waveguide Grating-Based FBG Interrogation Systems

In this manuscript, an optically passive fiber Bragg grating (FBG) interrogation system able to perform high-frequency measurement is proposed. The idea is mainly based on the use of an arrayed waveguide grating (AWG) device which is used to discriminate the fiber optic sensor (FOS) wavelength encoded response under test in function of its output channels. As made clear by the theoretical model studied in the proposed manuscript, the Bragg wavelength shift can be detected as in linear dependence with the proposed interrogation function which changes with the voltage produced by two (or more) adjacent AWG output channels. To prove the feasibility of the system, some experimental analyses are conducted with a custom electrical module characterized by high-speed and low-noise operational amplifiers. As static measurements, three FBGs with different full width at half maximum (FWHM) have been monitored under wide-range wavelength variation; while, as dynamic measurement, one FBG, glued onto a metal plate, in order to sense the vibration at low and high frequency, was detected. The output signals have been processed by a digital acquisition (DAQ) board and a graphical user interface (GUI). The presented work highlights the characteristics of the proposed idea as competitor among the entire class of interrogation systems currently used. This is because here, the main device, that is the AWG, is passive and reliable, without the need to use modulation signals, or moving parts, that affect the speed of the system. In addition, the innovative multi-channel detection algorithm allows the use of any type of FOS without the need to have a perfectly match of spectra. Moreover, it is also characterized by a high dynamic range without loss of sensitivity.

Dual-frequency optoelectronic oscillator incorporating a single cavity and multiband microwave photonic filter

Dual-frequency optoelectronic oscillators (OEOs) have potential applications in dual-band wireless networking and dual-parameter sensing systems. We propose a dual-frequency OEO incorporating a multiband microwave photonic filter (MPF). In particular, the two microwave signals are generated simultaneously in a single OEO cavity. By simply varying the parameters of optical spectral slicing and sampling (e.g., with a programmable optical filter) used to implement the MPF, we can readily achieve simultaneous tuning of the dual-frequency output, as well as alternate switching between single-frequency and dual-frequency output. The multi-passband nature of the MPF, enabled via optical spectral slicing, opens a path to multi-frequency OEO operation by scaling our scheme in the future. Such a structure provides a flexible way to generate simultaneously tunable and reconfigurable multi-frequency microwave signals.

Distortion-corrected phase demodulation using phase-generated carrier with multitone mixing

We present a novel phase generated carrier (PGC) demodulation technique for homodyne interferometers which is robust to modulation depth variations and source intensity fluctuations. By digitally mixing the waveform with a multitone synthetic function (a linear combination of harmonics of the modulating signal), distortion can become negligible even in presence of large variations of the modulation depth. The technique only requires two mixers and can also provide the DC component of the phase in real time, without needing any previously recorded data or ellipse-fitting algorithms. We validate the technique with simulated waveforms and with experimental data from a wavelength metering experiment using an integrated unbalanced interferometer on-chip, showing that the technique corrects distortion without increasing the noise with respect to the standard PGC technique.

Multi-wavelength output based on gold nanoparticles in erbium-doped fiber lasers

A multi-wavelength fiber laser can be used as an ideal light source device for optical communication of wavelength-division multiplexing. A type of combined filter composed of a gold nanoparticle saturated absorber and three-wave polarization controller was constructed. Its multi-wavelength laser output is realized in an erbium-doped fiber ring laser. We studied the degradation of single-wavelength, dual-wavelength, and triple-wavelength output in the range of 1555-1565 nm, the tunability of three wavelengths, and the spectral periodicity of 1 min 57 s. The interesting phenomena of subregional transmission of gold nanoparticles were discovered. We have a clearer understanding of the filtering process of gold nanosaturable absorbers and the special state between pulsed and non-pulsed when using them to achieve ultra-short pulsed lasers.

Overlap Spectrum Fiber Bragg Grating Sensor Based on Light Power Demodulation

Demodulation is a bottleneck for applications involving fiber Bragg gratings (FBGs). An overlap spectrum FBG sensor based on a light power demodulation method is presented in this paper. The demodulation method uses two chirp FBGs (cFBGs) of which the reflection spectra partially overlap each other. The light power variation of the overlap spectrum can be linked to changes in the measurand, and the sensor function can be realized via this relationship. A temperature experiment showed that the relationship between the overlap power spectrum of the FBG sensor and temperature had good linearity and agreed with the theoretical analysis.

High spatial resolution distributed fiber strain sensor based on phase-OFDR

A novel method to realize high spatial resolution distributed strain measurement is proposed based on phase demodulation scheme of optical frequency domain reflectometry (OFDR). Strain information can be demodulated directly by analyzing the phase change of Rayleigh backscattered light. Strain location can be obtained with high spatial resolution by cross-correlation method using a wide scanning range of tunable laser source. Based on the above scheme, breakpoint detection with 0.1 mm spatial resolution has been demonstrated, static and dynamic strain up to 100 Hz could be distributedly measured with 10 cm spatial resolution over 200 m sensing fiber, and the minimum measurable strain is about 1 με.

Fiber-MZI-based FBG sensor interrogation: comparative study with a CCD spectrometer

We present an experimental comparative study of the two most commonly used fiber Bragg grating (FBG) sensor interrogation techniques: a charge-coupled device (CCD) spectrometer and a fiber Mach-Zehnder interferometer (F-MZI). Although the interferometric interrogation technique is historically known to offer the highest sensitivity measurements, very little information exists regarding how it compares with the current commercially available spectral-characteristics-based interrogation systems. It is experimentally established here that the performance of a modern-day CCD spectrometer interrogator is very close to a F-MZI interrogator with the capability of measuring Bragg wavelength shifts with sub-picometer-level accuracy. The results presented in this research study can further be used as a guideline for choosing between the two FBG sensor interrogator types for small-amplitude dynamic perturbation measurements down to nano-level strain.

Interrogation of a linearly chirped fiber Bragg grating sensor with high resolution using a linearly chirped optical waveform

An approach to the interrogation of a linearly chirped fiber Bragg grating (LCFBG) sensor using a linearly frequency-modulated (or chirped) optical waveform (LFMOW) with a high resolution is proposed and experimentally demonstrated. An LFMOW is generated at a laser diode through linear frequency modulation. The generated LFMOW is then launched into an LCFBG pair consisting of two identical LCFBGs, with one serving as a sensing LCFBG and the other as a reference LCFBG. The reflection of the LFMOW from the two LCFBGs would lead to two time delayed LFMOWs. By beating the LFMOWs at a photodetector, a microwave signal with a beat frequency that is proportional to the time delay difference between the two reflected LFMOWs is generated. By measuring the frequency change of the beat signal, the strain applied to the sensing LCFBG is estimated. The proposed approach is experimentally evaluated. An LCFBG sensor with a resolution of 0.25 με is experimentally demonstrated.

Microring-based ratio-metric wavelength monitor on silicon

An integrated dual-ring ratio-metric wavelength monitor (DR-RMWM) with ultra-high wavelength resolution and compact size is demonstrated. Two microrings are used as the edge filters and designed to achieve an "X-type" spectral response in a particular wavelength range. It is fabricated with the CMOS-compatible fabrication process on the silicon-on-insulator. The measured wavelength resolution is 5 pm in a 1.2 nm wide wavelength range. By tuning the resonant wavelength thermally, the functional wavelength range can be shifted. The DR-RMWM can find applications in wavelength monitoring systems, especially the on-chip systems.

Uniform spacing interrogation of a Fourier domain mode-locked fiber Bragg grating sensor system using a polarization-maintaining fiber Sagnac interferometer

A novel linearized interrogation method is presented for a Fourier domain mode-locked (FDML) fiber Bragg grating (FBG) sensor system. In a high speed regime over several tens of kHz modulations, a sinusoidal wave is available to scan the center wavelength of an FDML wavelength-swept laser, instead of a conventional triangular wave. However, sinusoidal wave modulation suffers from an exaggerated non-uniform wavelength-spacing response in demodulating the time-encoded parameter to the absolute wavelength. In this work, the calibration signal from a polarization-maintaining fiber Sagnac interferometer shares the FDML wavelength-swept laser for FBG sensors to convert the time-encoded FBG signal to the wavelength-encoded uniform-spacing signal.

Pointwise fiber Bragg grating displacement sensor system for dynamic measurements

A method for setting up a fiber Bragg grating (FBG) sensor which can measure the pointwise, out-of-plane or in-plane dynamic displacement is proposed. The proposed FBG sensor is reusable. A multiplexing demodulation system based on a single long-period fiber grating is used in this study. The experimental results of the steady-state motion for a multilayer piezoelectric actuator and the dynamic response of a cantilever beam subjected to impact loadings are presented. These results indicate that the proposed displacement sensor has the ability to measure the out-of-plane dynamic displacement with high sensitivity. Measurements for a piezoceramic plate excited by high frequency show that the proposed displacement sensor also has the ability to provide the in-plane dynamic displacement up to 20 kHz.