Daniel Leandro

Real-Time FFT Analysis for Interferometric Sensors Multiplexing

In this paper, a theoretical and experimental study of two interferometric sensor multiplexing schemes has been carried out by means of the fast Fourier transform (FFT) analysis. This study addresses one of the main drawbacks of photonic crystal fiber (PCF) sensors, that is, its multiplexing capability. Using a commercial optical interrogator combined with a simple FFT measurement technique, the simultaneous real-time monitoring of several PCF sensors is achieved. A theoretical analysis has been performed where simulations matched with the experimental results. For the experimental verification, highly birefringent (HiBi) fiber sections that operated as sensing elements were multiplexed and tested in two configurations. Due to the FFT analysis, both multiplexing schemes can be properly interrogated by monitoring the FFT phase change at the characteristic spatial-frequency of each sensor. For this purpose, a commercial interrogator and a custom Matlab program were used for computing the FFT and for monitoring the FFT phase change in real time (1 Hz).

L-Band Multiwavelength Single-Longitudinal Mode Fiber Laser for Sensing Applications

In this work, a novel single-longitudinal-mode (SLM) four-wavelength laser configuration for sensing applications in the L-band is proposed and experimentally demonstrated. The sensor system presented here is based on ring resonators, and employs fiber Bragg gratings to select the operation wavelengths. The stable SLM operation is guaranteed when all the lasing channels present similar output powers. It is also experimentally demonstrated that when a SLM behavior is achieved, lower output power fluctuations are obtained. Characterization of the lasing structure for temperature sensing is also shown.

Remote (155 km) Fiber Bragg Grating Interrogation Technique Combining Raman, Brillouin, and Erbium Gain in a Fiber Laser

We present and demonstrate a technique for remote sensing of fiber Bragg gratings (FBGs) beyond 150 km. The system combines Raman, Brillouin, and erbium gain in a linear cavity fiber laser and deploys a novel heterodyne detection technique to avoid the signal-to-noise ratio limitations produced by Rayleigh scattering. Two FBGs located in series were interrogated at 155 km away from the processing unit using 0.6 W of Raman pump and less than 2 mW of Brillouin pump provided by a tunable wavelength-swept laser. Heterodyne detection brings forth a signal-to-noise ratio of approximately 10 dB in our measurements.

Narrow-Linewidth Multi-Wavelength Random Distributed Feedback Laser

In this paper, narrow-band emission lines are generated by means of two random distributed feedback fiber laser schemes. Spectral line-widths as narrow as 3.2 pm have been measured, which significantly improves previous reported results. The laser is analyzed with the aim of obtaining a spectral line-width as narrow as possible. Additionally a variation of this setup for multi-wavelength operation is also validated. Both schemes present a simple topology that use a combination of phase-shifted fiber Bragg gratings and regular fiber Bragg gratings as filtering elements.

Stable Dual-Wavelength Erbium Fiber Ring Laser With Optical Feedback for Remote Sensing

In this paper, a new stable dual-wavelength erbium fiber ring laser for remote temperature measurements is proposed and experimentally demonstrated. The sensing element is composed of two fiber Bragg gratings and sensor interrogation is achieved with a dual-wavelength erbium fiber ring laser. This configuration is made by creating two symmetrical laser cavities with similar optical power. This topology allows the performance of two laser emission lines in single-longitudinal mode (SLM) and with power instability lower than 0.23 dB, and an optical signal-to-noise ratio higher than 30 dB for all the emitted wavelengths. The application of this system for remote temperature measurements has been demonstrated even though the SLM regime cannot be maintained.

High-Resolution Sensor System Using a Random Distributed Feedback Fiber Laser

In this paper, high-resolution measurements using random distributed feedback fiber lasers have been attained. A phase-shifted fiber Bragg grating is used to detect temperature shifts with a resolution under 0.01 °C. Due to the longitudinal-modeless behavior and the inherent frequency stability of random fiber lasers, the resolution limit of conventional fiber lasers has been overcome. The frequency shift of the grating is measured in the electrical domain by beating an external laser source with the laser line generated. A second setup that measures axial strain without the need of temperature compensation is also proposed and validated, reaching an axial strain resolution under 0.2 με.

High resolution polarization-independent high-birefringence fiber loop mirror sensor

In this work, two all polarization-maintaining (PM) high-birefringence (Hi-Bi) fiber loop mirrors (FLM) which are immune to external polarization perturbations are validated both theoretically and experimentally. Simplified and stable versions of classical FLMs were attained using a PM-coupler and by fusing the different Hi-Bi fiber sections with an adequate rotation angle between them. Since the polarization states are fixed along the whole fiber loop, no polarization controllers are needed. This simplifies the operation and increases the stability of the systems, which were also validated as ultra-high resolution sensors, experimentally obtaining a resolution of 6.2∙10-4 °C without averaging.

Random DFB Fiber Laser for Remote (200 km) Sensor Monitoring Using Hybrid WDM/TDM

In this paper, a random distributed feedback fiber laser is proposed as a multiplexing scheme for ultralong range measurements (up to 200 km). Optical fiber sensors are time and wavelength multiplexed overcoming one of the main limitations of long-range sensing setups, which is their limited multiplexing capability. The direct modulation of the lasers cavity allows the interrogation of sensors by measuring the reflected power for different wavelengths and distances. Fiber Bragg gratings placed at different fiber locations and wavelengths have been interrogated in two different sensor networks. In addition, in order to improve the performance of the system, some features have been analyzed.

Bidirectional Dual-Wavelength Raman Fiber Ring Laser

A new stable dual-wavelength fiber ring laser based on Raman amplification, deploying a dispersion-compensating fiber (DCF) as gain medium, is introduced. The gain competition effects in the fiber Raman amplification are mitigated because the two emission lines, selected by fiber Bragg grating within the resonant ring cavities, travel in opposite directions through the gain medium. The experimental results confirm that the novel multiwavelength ring laser source is stable and it offers a high optical signal-to-noise ratio.

Fiber Bragg grating interrogation technique for remote sensing (100km) using a hybrid Brillouin-Raman fiber laser

We propose and demonstrate the feasibility of a novel Fiber Bragg Grating interrogation technique for remote sensing based on the use of a hybrid Raman-Brillouin fiber laser configuration. The laser comprises 100 km of standard singlemode fiber (SMF) in a linear cavity configuration with four Fiber Bragg Gratings (FBGs) arranged in series. The FBGs are used both for the sensing function and for the selection of the lasing wavelengths. A wavelength-swept laser pumps Brillouin gain in the fiber cavity, which is previously set just under lasing threshold by the Raman gain. Furthermore, the sensor signal is detected in the radio frequency domain instead of the optical domain so as to avoid signal to noise ratio limitations produced by Rayleigh scattering. Experimental results demonstrate that the shift of the Bragg wavelength of the FBG sensors can be precisely measured with good signal to noise ration when the FBG are used for temperature sensing.