Rodolfo Martínez Manuel

Location of a time-varying disturbance using an array of identical fiber-optic interferometers interrogated by CW DFB laser

A novel technique for distributed detection and localization of vibrational disturbances is presented. It is based on a serial array of identical low-finesse Fabry-Perot interferometers interrogated at a fixed wavelength by DFB diode laser intensity modulated at 10 KHz. Interferometers are formed directly in the single mode SMF-28 fiber by pairs of fiber Bragg gratings with reflectivity < 0.05% each. A simple signal processing based on the Fourier transform of detected signals and evaluation of phases for different signal components enables localization of a perturbed interferometer with a high accuracy. In experiment, a localization accuracy of 10 meters has been demonstrated for 5 km long fiber after 1 s of averaging. The system has the capability of using well in excess of 100 interferometers in a single fiber channel. A simple sensor configuration and the use of low-frequency components make it potentially inexpensive and suitable for applications where a continuous monitoring of long structures has to be performed for appearance of vibrations.

Vibration Disturbance Localization Using a Serial Array of Identical Low-Finesse Fiber Fabry-Perot Interferometers

An optical fiber sensor system for localization of a vibrational disturbance along the fiber is presented. The sensor system is based on a serial array of identical Fabry-Perot interferometers, formed directly in the single mode SMF-28e fiber by pairs of fiber Bragg gratings with reflectivity of 0.04% each. Interferometers were interrogated by an intensity modulated distributed feedback diode laser. We present a signal processing algorithm for localization of a disturbed interferometer and experimental results for a serial array of 14 in-fiber interferometers. Simple sensor configuration and the use of low-frequency components make it potentially inexpensive and suitable for applications where continuous monitoring for appearance of vibrations is required.

Stable multi-wavelength fiber lasers for temperature measurements using an optical loop mirror

In this work, two novel stable multi-wavelength fiber laser configurations are proposed and demonstrated by using a spool of a single-mode fiber as an optical loop mirror and one or two fiber ring cavities, respectively. The lasers are comprised of fiber Bragg grating reflectors as the oscillation wavelength selecting filters. The influence of the length of the spool of fiber on the laser stability both in terms of wavelength and laser output power was investigated. An application for temperature measurement is also shown.

Phase-sensitive correlation optical time-domain reflectometer using quantum phase noise of laser light.

We propose and experimentally demonstrate a simple approach to realize a phase-sensitive correlation optical time-domain reflectometer (OTDR) suitable for detection and localization of dynamic perturbations along a single-mode optical fiber. It is based on the quantum phase fluctuations of a coherent light emitted by a telecom DFB diode laser. Truly random probe signals are generated by an interferometer with the optical path difference exceeding the coherence length of the laser light. Speckle-like OTDR traces were obtained by calculating cross-correlation functions between the probe light and the light intensity signals returned back from the sensing fiber. Perturbations are detected and localized by monitoring time variations of correlation amplitude along the fiber length. Results of proof-of-concept experimental testing are presented using an array of ultra-low-reflectivity fiber Bragg gratings as weak reflectors.

Active Q-switching of a fiber laser using a modulated fiber Fabry–Perot filter and a fiber Bragg grating

We propose and demonstrate a simple and robust actively Q-switched erbium-doped fiber ring cavity laser. The Q-switching is based on dynamic spectral overlapping of two filters, namely a fiber Bragg grating-based filter and a fiber Fabry–Perot tunable filter. Using 3.5 m of erbium-doped fiber and a pump power of only 60 mW, Q-switched pulses with a peak power of 9.7 W and a pulse duration of 500 ns were obtained. A pulse repetition rate can be continuously varied from a single shot to a few KHz.

Wavelength and power stabilization of a three wavelength Erbium doped fiber laser using a nonlinear optical loop mirror

This paper describes the use of a Nonlinear Loop Mirror to achieve wavelength and power stabilization in a three wavelength Erbium doped fiber ring laser. The laser uses three fiber Bragg grating reflectors as the oscillation wavelength selecting filters. The influence of the length of the Nonlinear Loop Mirror (NOLM) on the laser stability both in terms of wavelength and laser output power was investigated. The laser performance was improved by changing the length of the Loop Mirror to an optimal length and three simultaneous wavelength oscillations with acceptable power and wavelength stability were achieved.

Distributed detection and localization of multiple dynamic perturbations using coherent correlation OTDR

A novel technique for distributed disturbance sensor is presented. It is based on Coherent Correlation OTDR operating in CW regime. The sensor utilizes a standard telecom CW DFB diode laser as a light source and a very simple configuration. Probe signals were generated by interferometer with OPD exceeding the coherence length of the laser light. Speckle-like OTDR traces were obtained by calculating cross-correlation function between the probe light intensity signal and the signal returned back from the sensing fiber. Perturbations are detected as time variation of correlation amplitude at disturbance locations. Preliminary experiments proved operability of the sensor.

Wide range refractive index sensor using a twin-grating interferometer for intensity reference

An optical fiber refractometer based on the Fresnel reflection in the fiber tip and twin FBG interferometer located near the fiber tip for light intensity referencing is reported. A wavelength scanning DFB diode laser was used for RI sensor interrogation. Signal processing in the Fourier domain allows intensity-independent accurate measurements for liquids and gases with practically any RI. A resolution of 5 - 10 5-5 was demonstrated experimentally using different liquids including an anti-reflection index-matching liquid. Simple construction and the use of low-cost components make it interesting for many applications.

Implementation of active Q-switching based on a modulated fiber Fabry-Perot filter in linear cavity erbium doped fiber laser

A linear cavity Q-switched fiber based laser is designed and demonstrated experimentally. The Q-switching technique is based on dynamic spectral overlap between a modulated fiber Fabry-Perot tunable filter and a fiber Bragg grating. To characterize the Q-switched fiber laser performance, a 3m length of Erbium doped fiber was used. Fiber laser parameters such as output coupling ratio, pump power and modulation frequency were optimized to obtain a laser output of 5.6W peak power with a 450ns pulse width and a 1 kHz repetition rate. Using the optimized fiber laser cavity design, the extinction ratio of the fiber laser up to 36 dB was achieved. This Q-switching technique for achieving laser pulses maintains the intrinsic robustness and simplicity of an all fiber laser system.

Cascaded Ultra-Low Reflective Fiber Points for Distributed Sensing

Distributed fiber sensors based on the frequency domain analysis of Rayleigh backscattered light are well established. They exhibit very good performance in both sensitivity and spatial resolution, but their application can be limited due to their cost and the complexity of the analysis. In this work we present a system based on coherent optical frequency domain reflectometry, used in Rayleigh distributed sensors, implemented with more readily available components and simplified analysis. A sensing fiber is prepared by printing uniformly spaced, ultra-low reflectivity fiber Bragg gratings of the same Bragg wavelength. When tuneable source light is introduced to the fiber the reflections from the gratings interfere with the reflection from the tip of the fiber. The gratings’ reflectivity varies randomly which produces a frequency domain trace that shares some of the properties of a Rayleigh spectral trace, but is significantly stronger. This removes the need for specialized detection equipment. These Bragg gratings act as reflectors and not as sensors per se. Use of a reference interferometer and signal processing algorithms make it possible to replace a high precision linearly tuneable laser with a standard tuneable laser as optical source.