Javier Madrigal

Tilted fiber Bragg gratings in multicore optical fibers for optical sensing

We have inscribed a tilted fiber Bragg grating (TFBG) in selected cores of a multicore optical fiber. The presence of the TFBG permits to couple light from the incident-guided mode to the cladding modes and to the neighbor cores, and this interaction can be used for optical sensing. We measured different magnitudes: strain, curvature magnitude and direction, and external refractive index. The curvature results show a linear dependence of the maximum crosstalk with the curvature magnitude with a sensitivity of 2.5  dB/m-1 as the curvature magnitude increases and at the same time a wavelength shift of 70  pm/m-1. Changes in the external refractive index gradually vanish the cladding modes resonances and the crosstalk between the different cores, obtaining a reduction of the 90% of the optical spectra integral area for refractive indexes between 1.398 and 1.474.

Directional curvature sensor based on long period gratings in multicore optical fiber

Multicore optical fiber can be used to implement multidimensional optical fiber sensors including curvature sensors. In this paper, a selective core inscription technique is used in order to inscribe a single long period grating in each of the outer cores of the optical fiber. A set of three different long period gratings is inscribed for implementing the curvature sensor. The ability of the sensor for measuring the magnitude and the direction of curvature is demonstrated. The optical fiber sensor is characterized experimentally for curvature magnitudes from 0 m−1 to 1.77 m−1 and curvature directions from 0° to 360°. The maximum curvature sensitivity of the developed sensor is −4.85 nm/m−1.

Characterization of a FBG sensor interrogation system based on a mode-locked laser scheme

This paper is focused on the characterization of a fiber Bragg grating (FBG) sensor interrogation system based on a fiber ring laser with a semiconductor optical amplifier as the gain medium, and an in-loop electro-optical modulator. This system operates as a switchable active (pulsed) mode-locked laser. The operation principle of the system is explained theoretically and validated experimentally. The ability of the system to interrogate an array of different FBGs in wavelength and spatial domain is demonstrated. Simultaneously, the influence of several important parameters on the performance of the interrogation technique has been investigated. Specifically, the effects of the bandwidth and the reflectivity of the FBGs, the SOA gain, and the depth of the intensity modulation have been addressed.

Microwave Photonic Filtering for Interrogating FBG-Based Multicore Fiber Curvature Sensor

We propose and experimentally demonstrate an approach to perform high-resolution and temperature-insensitive interrogation of a fiber Bragg grating (FBG)-based multicore fiber (MCF) curvature sensor using a microwave photonics filtering technique. By combining two reflected sample signals from the two FBGs inscribed in MCF and a dispersion device, a two-tap notch microwave photonic filter (MPF) is formed. The notch frequency of MPF is dependent on the time delay difference between the two FBG reflected signals, which is related to wavelength spacing of the two FBGs. Due to the wavelength spacing of the two FBGs is proportional to the curvature, the curvature can be readily interrogated by monitoring the notch frequency of MPF. The proposed interrogation scheme with the sensitivity of 92 MHz/

Microwave photonics filtering interrogation technique under coherent regime for hot spot detection on cascaded FBG fiber

\text{m}^{-1}

Phase modulation to intensity modulation conversion for sensitive FBG sensor interrogation

is achieved, whilst the sensitivity can be easily adjusted. Moreover, the proposed interrogation scheme is temperature insensitive.

FBGs based multicore fiber curvature sensor interrogation using microwave photonics filtering techniques

An interrogation technique of cascaded FBGs sensors based on a Microwave Photonics filtering technique under coherent regime is presented. The sensing information of a 5m fiber coil with 500 weak FBGs with a similar central wavelength is retrieved. The principle of operation is based on the calculation of the impulsive response by recording the electrical frequency response of the system. Hot spot detection and temperature measurement are demonstrated through experimental measurements with a temperature sensitivity of 0.6dB/°C. The resolution in the measurement is related to the bandwidth of the electrical measurement and we have obtained a resolution of 20 cm for an electrical bandwidth of 1 GHz and a SNR bigger than 16dB.

Multiplexing FBG sensors combining microwave photonics and phase modulation

An interrogation technique based on phase modulation to intensity modulation conversion due to FBG filtering is presented. A 10 GHz tone is used to phase modulate an optical carrier located at the Bragg wavelength of a given FBG. The modulation index is set to a small value to keep Bessel identities close to 0 in order to avoid higher harmonics. Changes of the Bragg wavelength cause a power change in the photodetected 10 GHz tone. A remarkable linear sensitivity of 1 dB/pm for a shift up to 10 pm of the Bragg wavelength is demonstrated through experimental measurements. The range with linear sensitivity can be enlarged sweeping the source wavelength. This proves that the presented interrogation technique is able to interrogate FBGs with a resolution far below 1 pm and no need of extra post-processing.

Multicore fiber-Bragg-grating-based directional curvature sensor interrogated by a broadband source with a sinusoidal spectrum

A high-resolution and temperature-insensitive interrogation scheme for fiber Bragg grating (FBG) based multicore fiber curvature sensor using microwave photonics filtering techniques is proposed and verified by experimental demonstration. The principle of operation exploited for curvature interrogation is based on monitoring the notch frequency shift of a two-tap microwave photonics filter (MPF), which is formed by combining two reflected sample signals from the two FBGs inscribed in multicore fiber (MCF) and a dispersion device. The proposed interrogation scheme with the sensitivity of 92 MHz/m−1 is achieved, whilst it is naturally insensitive to any temperature variations.

Tilted Fiber Bragg Gratings for Selective Coupling in a Multicore Optical Fiber

We apply phase modulation techniques at different wavelengths to generate microwave tones that allow us to interrogate multiplexed sensors. The interrogation technique has a resolution far below 1 pm with a simple system and no need of post-processing. This approach relies on basic concepts of microwave photonics technique operating under low coherence regime. A remarkable linear sensitivity of 1 dB/pm for a movement up to 10 pm of the Bragg wavelength is demonstrated through experimental measurements.