FR2512552A1 - FIBER OPTIC IMPURITY DETECTOR - Google Patents

Abstract

FIBER OPTIC IMPURITY DETECTOR. THE DIRT DETECTOR ACCORDING TO THE PRESENT INVENTION INCLUDES A DETECTOR 16 WITH AN OPTICAL WAVE GUIDE PROVIDED IN AN AMBIENT ENVIRONMENT AND COUPLED TO THIS ENVIRONMENT. THE WAVE GUIDE DETECTOR INCLUDES THE CORE OF A FIBER OPTIC CABLE FROM WHICH THE COATING HAS BEEN REMOVED. IMPURITY CAUSES A VARIATION IN THE REFRACTION INDEX OF THE MEDIUM SURROUNDING THE WAVE GUIDE AND CAUSES IN THE LIGHT TRANSMISSION CHARACTERISTIC OF THE WAVE GUIDE A VARIATION THAT IS DETECTED AND WHICH RESULTS IN THE APPEARANCE OF A EXIT SIGNAL INDICATING THE PRESENCE OF IMPURITY.

Description

12552

  The present invention relates to a system for detecting impurities such as gases, moisture and particles, and relates, more particularly, to a system of this kind suitable for a qualitative identification of the presence. This system has the advantage of allowing a detector to be placed in the ambient environment being studied, the detector being arranged remotely while being connected to it.

by an optical fiber cable.

  It is well known in the art of optical fibers that the relative index of refraction of a material forming the soul

  and a material forming the coating determines the

  light transmission path along the fiber optic cable core The critical angle is defined for the interface between the core and the coating refractive indices so that for angles greater than Critical angle c, the light is reflected entirely inwardly along the soul For angles greater than the critical angle, the

  light is lost for the coating according to this angle.

  As the refractive index of the core approaches that of the coating, the distribution of energy in the waveguide varies with the result that energy is lost from the

core towards the coating.

  There are many reasons for remotely detecting the presence of impurities, these reasons ranging from access difficulties for verification to the presence of hazardous chemicals It is desirable that the detector is as discrete as possible, that is, non-reactive, not subject to electromagnetic interference, etc. Impurity detection systems can be used to determine the need for periodic maintenance, for example in the case of impurity formation, as well as for

  detect failure conditions, such as a fault

  Fluid channel lance The ambient environment considered may be difficult due to electrical potentials

  high or explosive constituents.

12552

  An object of the present invention is to provide a new optical fiber impurity detector for detecting

  the presence of impurities in an environment.

  The present invention resides in an optical fiber impurity detector for determining the presence of a predetermined impurity in an ambient environment, which detector comprises a long optical waveguide disposed within the ambient environment, said guide wave generator comprising a core having a predetermined refractive index in the absence of its coating; optical fiber input and output light transmission means optically coupled to each end of the optical waveguide; a light source coupled to the optical waveguide via the optical fiber input light transmitting means; a

  light detector coupled to the optical waveguide through

  the medium of the fiber output light transmission means

  optical, said light detector generating a signal

  sensing the light transmitted through the waveguide and received by the light detector; and means for comparing the normal output signal of the light detector generated by the light transmitted through the waveguide disposed in an impurity-free ambient environment with the output signal of the light detector generated when an impurity is present in the ambient environment and is coupled

  optically around the waveguide, this impurity

  trailing an alteration of the refractive index of the medium around the waveguide and causing in the transmission of light through the waveguide a variation resulting in an output signal, the light detector which indicates the

  presence of impurity in the surrounding environment.

  The single figure of the accompanying drawing is a schematic representation of an exemplary embodiment of the present invention having a waveguide disposed within a

sampling chamber.

  The present invention can be better understood by

  Referring to this figure o the system 10 for detecting impu-

  It comprises a sampling chamber 12 in which the ambient environment under study can be admitted through an opening or sampling port 14 A long optical waveguide 16 is disposed within the sampling chamber 12 and includes the core of a conventional fiber-optic cable whose coating has been removed In this embodiment, the waveguide 16 is a generally C-shaped member, but the general configuration of the waveguide is not critical and the waveguide could be a rectilinear element, a coil or have any other configuration which ensures inside the sampling chamber the presence of a sufficient surface of the core to allow a measurement of the

  variation occurring in the transmission of light.

  The element 16 forming a waveguide, that is to say the medium

  constituting the core, has a refractive index of N 1.

  The ambient environment in the room of exchange

  12 and which can be a gas coupled to the waveguide,

  has a refractive index of N 2.

  Means 18 disposed at a distance and comprising a light source and a light detector is connected to an optical fiber cable 20 which connects the portion of the means 18 forming the light source to one end 21 of the waveguide 16 through the wall of the sampling chamber A second optical fiber cable 22 connects the other end 23 of the waveguide 16 to the part of the means 18 which forms the light detector. The part of the means 18 which forms the light detector is also connected to a means 24 for controlling and processing information, whereby a light detector output signal is sent to the means 24 to be compared with a normalized signal or reference signal obtained from a known ambient environment

  present in the sample chamber.

  During operation, the chamber of exchange is

  12 in a remote environment and 6 u hostile, for example inside a steam turbine or an electric generator housing or a steam power system of nuclear power plant L Sampling orifice 14 allows a periodic variation of the atmosphere inside the sampling chamber, for example by diffusion, so that the atmosphere of the sampling chamber

  represents the ambient environment at a given moment.

  Light from the portion of the means 18 which forms the light source is transmitted via the cable

  fiber optic waveguide 16 The atmosphere at the in-

  inside the sample chamber 12 is in direct contact

  with the waveguide or fiber optic cable core, that is

  That is, it is optically coupled with this waveguide, by various mechanisms which depend on the impurity. In the case of a vaporized impurity, this could be deposited on the core by evaporation. Other coupling mechanisms include static charge attrition and particle drop in the case of single particles The impurity of the ambient environment provides a coating refractive index N 2 which

  causes loss of light at the core-coating interface.

  This transmitted light comes out of the waveguide 16 and is transferred

  by the cable 22 to the part of the means 18 forming the detection

  The part of the means 18 which forms a conventional light detector may be a photomultiplier tube which generates an output signal depending on the light reaching the detector. This output signal of the detector is sent to the control and processing means. information can be compared with a reference signal or setpoint signal which is a function of a known ambient environment representing a state free of impurities. Then, means 24 can be used to generate a control signal which is used to stop or modify the operation of the equipment or

  system within which the sample chamber is located.

  The means 24 can also be a measuring instrument which informs an operator of the presence of the impurity and the

  need for operator action.

  The portion of the means 18 forming the light source may be a conventional high intensity light source whose

  light is directed on the cable 20 and introduced into this cable.

  It is also possible to use a laser light source, this monochromatic source simplifying the detection requirements.

12552

  light and can be easily coupled to the cable 20.

  By way of example, the detector or waveguide 16 is a 20 micrometers fused silica cable core having a refractive index of 1.46. The normal coating of the silicone rubber cable is removed from this core which forms the detection part or waveguide 16 while the cable extends in the form of the strands 20 and 22 of the cable between the waveguide portion 16 and the light source means 18 of the detector When a contaminant fluid that may be present in the form of a mist or vapor is in the sampling chamber 12, droplets of this fluid are formed on the surface of the waveguide and the intensity of the light transmitted through the waveguide decreases The output signal of the light detector is sent to the signal processing means 24 and is compared

  with a reference or set point signal that

  corresponds to the absence of contaminating fluid in the chamber 12.

  The output signal of the processing means 24 can then be used to fulfill a hardware control function, by

  example the control of a valve or a mechanical control.

Claims (2)

  An optical fiber impurity detector for detecting the presence of a predetermined impurity in an ambient environment, which detector comprises a long optical waveguide (16) disposed within an ambient environment and characterized by the said waveguide comprises a core having a predetermined refractive index, its coating having been removed; optical fiber input and output light transmitting means (20,22) optically coupled to each end of the optical waveguide; a   light source coupled to the optical waveguide through   mediator of the optical fiber input light transmitting means; a light detector coupled to the optical waveguide via the optical fiber input light transmitting means, said light detector generating a signal representing light transmitted through the waveguide and received by the detector from light; and means (24) for comparing the normal light sensor output signal generated by the light transmitted through the waveguide disposed in an impurity-free ambient with the light detector output signal generated when an impurity is present in the ambient environment and is coupled optically around the waveguide, said impurity causing an alteration of the refractive index of the medium around the waveguide and causing in the transmission of light through the waveguide a variation resulting in a light detector output signal that indicates the   presence of impurity in the surrounding environment.   2 fiber optic impurity detector according to claim 1, characterized in that the long guide   optical wave is arranged inside an exchange chamber.   <12) substantially light-tight and having a sampling port (14) for admitting the ambient environment to be sampled for the purpose of lowering the presence of impurities.