M. G. Shlyagin

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.

Simple refractometer based on in-line fiber interferometers

A very simple but accurate optical fiber refractometer based on the Fresnel reflection in the fiber tip and two in-line low-reflective mirrors for light intensity referencing is reported. Each mirror was generated by connecting together 2 fiber sections with FC/PC and FC/APC connectors using the standard FC/PC mating sleeve. For the sensor interrogation, a standard DFB diode laser pumped with a sawtooth-wave current was used. A resolution of 6 x 10-4 was experimentally demonstrated using different liquids. A simple sensor construction and the use of low cost components make the reported system interesting for many applications.

Multiplexing FBG-based sensors using unmodulated continuous wave DFB diode laser

We present an inexpensive technique capable to interrogate multiplexed sensors based on ultra-low-reflective Bragg gratings written in a long standard telecom fiber. The technique is suitable for distributed detection and localization of disturbances in security and early warning systems for pipeline monitoring, in linear temperature sensors for fire detectors, etc. It is based on the correlation OTDR principle measuring cross-correlation between a noise-like probe signal and the signal reflected back from the sensing fiber. In order to simplify the sensor configuration, an unmodulated CW DFB diode laser was used as a light source. A noise-like probe signal was generated by conversion of phase noise of the DFB laser into intensity noise with a help of unbalanced Michelson interferometer. Results of the experimental verification of the proposed technique are presented.

Location of vibrational disturbance using a serial array of identical low-finesse fiber Fabry-Perot interferometers

An optical fiber sensor system for location of a vibrational disturbance along the fiber is presented. The sensor system is based on a serial array of identical F-P 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 a DFB diode laser which was intensity modulated at 10 kHz. A method for localization of a disturbed interferometer and experimental results for a serial array of 14 sensors are presented. 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.

Long range multiplexed sensors based on identical Bragg gratings with ultra-low reflectivity

We present very simple and sensitive techniques capable to interrogate ultra-weak Bragg gratings written in a long SMF-28 fiber. The techniques are suitable for distributed detection and localization of alarm conditions in early warning systems. Also, a high multiplexing capability was demonstrated in a multi-point measuring system utilizing an array of identical FBGs. This technique is based on measuring correlation between the probe and reflected signal. A DFB laser operating in a CW regime was used as a light source. We present results of experimental verification of the techniques in different sensor configurations for static strain and vibration measuring. Multipoint sensor using Bragg gratings with reflectivity of 0.01% printed in a 3-km long fiber was demonstrated.

Information Capacity Of Spatial Light Modulators

An experimental technique for estimation of information capacity of optically addressed spatial light modulators (SUB) in coherent optical systems is suggested. The technique can be used for comparison of different types and a proper choice of SLMs.