FR2576423A1 - OPTICAL DETECTOR AND OPTICAL DETECTION METHOD - Google Patents

Abstract

OPTICAL DETECTOR CHARACTERIZED IN THAT IT INCLUDES A RADIANT OPTICAL FIBER AND AT LEAST ONE PHOTOSENSITIVE SENSOR PROVIDED TO CAPTURE THE LIGHT EMITTED BY THE FIBER OVER ITS LENGTH DURING THE USE OF THE DETECTOR, THE OUTPUT OF THE PHOTOSENSITIVE SENSOR BEING DIFFERENT 'INTERPOSITION OF AN OBJECT, WHOSE PRESENCE WE DESIRE TO DETECT, BETWEEN THE FIBER AND THIS PHOTOSENSITIVE SENSOR.

Description

The present invention relates to optical detectors and aims at

  especially detectors using optical fibers.

  According to a first aspect of the present invention, an optical detector is proposed comprising a radiating optical fiber and at least one photosensitive sensor arranged so as to capture the light emitted by the

  fiber along its length during the use of the detector, the cap output

  photosensitive tor being modified in the event of the interposition of an object, of which

  we want to detect the presence between the fiber and this photosensitive sensor.

  According to another aspect of the present invention, there is provided a method for detecting the presence of an object, comprising emitting light along the length of a radiating optical fiber, and picking up the

  light emitted, the amplitude of the light received being reduced by the pre-

  sence of the object between the fiber and the means making it possible to capture the light. Optical fibers are, in general, designed to avoid, or reduce, the emission of energy by their souls. Conventional optical detectors using such fibers operate in different ways

  ways and use the following properties of the fiber: the transpa-

  rence, the emission by its end, the finite propagation speed, the

  sensitivity to external parameters (e.g. pressure, temperature,

  temperature, magnetic field) in relation to phase changes, amplitude variations, polarization variations, frangibility

and flexibility.

  An optical fiber that emits energy by its length, rather

  than transmitting it, can also be used in an op-

  tick. For this purpose, we use a radiating or leaking fiber which emits light in its environment and sensors distant from the fiber.

  and distributed along the latter to receive the transmitted signal.

  In the case where such a detector is used as a smoke detector, any smoke interposed on the transmission path between the fiber

  and a sensor produces a drop in the output current of the photosensitive sensor

  which is used to trigger an alarm. To this end, the cap-

  They are electrically connected to an alarm circuit.

  In the case where such an optical detector is used to detect

  ter an intruder, the latter will similarly block the trans-

  mission between the fiber and the sensors and will trigger an alarm. In such alarm devices, fiber and sensors are

  appropriately arranged with respect to each other and account

  kept in the environment so as to ensure that the alarm is effectively triggered by the possible presence of the appropriate blocking agent and that, in the case of an intruder alarm device, it it is not possible to determine a "safe" path through the sensor network.

  The sensors can be constituted by photo diodes.

  electric or by diode networks that their small size allows

easily conceal.

  Note that these alarm devices are not likely to be triggered by misleading electrical signals and that a break

  fiber would cause the alarm to start rather than stop.

  Since different gases absorb optical radiation at various wavelengths, the basic principle of using a fiber

  radiant optics can be extended to detect atmospheric pollutants

  specific. To do this, a light energy source must be used,

  which is sent in the fiber, of wavelengths-selected, and of the cap-

  corresponding photosensitive agents. Such an application can be extended to produce a spectromxtron. For this purpose, a radiating optical fiber is arranged so as to pass through an analysis chamber into which an unknown gas can be introduced. Photosensitive sensors are arranged to pick up the signal emitted by the fiber and their respective outputs are connected to circuits which provide an indication of the drop in photoelectric current when gases are introduced and thus a detection of

  their presence. In order to be able to detect the presence of several different gases

  rents with the same device, sensors sensitive to different wavelengths are necessary and the optical fiber must be capable of transmitting and transmitting the different wavelengths, or else be made up

  of several optical fibers each emitting a specific wavelength

  than. The radiating optical fiber can be of different types. It can be a single mode fiber operating at a value V less than 1.5 and in this case 50 Z of the energy passes through the sheath. Fiber can

  also be designed to have - radius mode properties -

  determined, which is for example the case of fibers

applatates.

  It is also possible to use the RAMAN effect in the fi

  brs, in which case the radiation emitted differs from the transmitted wavelength from which it is offset by the first STOKES offset. In this case, the fiber must be able to transmit by its core a signal of wavelength greater than the cut-off wavelength but below the wavelength of

  cutting of the material of the core for the wavelength of RAMAN shifted.

  For example, for a single-mode index-hopping fiber having a core of 5.1 μm in diameter (An = 5 × 10) operating at 1.3 μm, a V value of 1.5 and with a separation of 60 nm of the wavelength of the signal and of the pumping signal, the variation of the emitted energy (excluding diffusion, absorption, etc.), for the first shift of STOKES, will be

  by 5%, this energy being emitted in free space.

Claims (6)

  1. Optical detector characterized in that it comprises a radiating optical fiber and at least one photosensitive sensor arranged so as to capture the light emitted by the fiber over its length during the use of the detector, the output of the photosensitive sensor being modified by case of interposition of an object, whose presence we wish to detect, between   fiber and this photosensitive sensor.   2. Optical detector according to claim 1 characterized in that   that it is used to detect smoke.   3. Detector according to claim 1 characterized in that it   is used to detect the presence of intruders.   4. Detector according to any one of the preceding claims.   your characterized in that it includes alarm means which are triggered   if the output current of a photosensitive sensor drops.   5. Optical detector according to claim 1 characterized in that it is designed so that light of different lengths   waves can be emitted and this in order to allow the detection of different   gas rents which absorb optical radiation in different wavelengths. 6. Method for detecting the presence of an object, consisting in emitting light by the length of a radiating optical fiber, and in capturing the emitted light, the amplitude of the captured light being reduced   by the presence of the object between the fiber and the means making it possible to cap- ter the light.