FR2721114A1 - Fluorescent plastic optical fibre - Google Patents

Abstract

A fluorescent plastic optical fibre has a core (2) of plastic material such as polystyrene with a dia. of 0.5-1.5 mm, an optical sheath layer (11) e.g. of polymethylmethacrylate (PMMA) 25-150 microns thick, a sheath layer (12) doped with ZnS 100-200 microns thick, and an outer covering layer (13) of black pant 100-200 microns thick. Pref. the layer (12) doped with ZnS is made up of several layers of ZnS powder in an adhesive with high fluidity, partic. 'superglue'. Pref. the ZnS has a particle size of less than 10 microns.

Description

OPTICAL FIBER AND ITS MANUFACTURING METHOD
The present invention relates to a fluorescent polymer optical fiber which can be used for X-ray detection. The invention finds a particular application for monitoring the state of the vacuum in the vacuum bulbs of medium-voltage circuit breakers.

 In French patent no. 2 640 386 filed on December 9, 1988, a device has been described for the detection of visual aromas occurring in a closed enclosure, in particular inside the envelope of a high voltage installation. armored type. This device comprises a fluorescent optical fiber placed inside the enclosure and at least one end of which opens out to the outside, is connected to a photodetector.

 In French patent No. 91 14396, a vacuum circuit breaker has been described provided with self-diagnosis means of the vacuum state of the bulbs; these means comprise one or more scintillating and / or fluorescent optical fibers arranged around the outer surface of the vacuum interrupter.

 In French patent No. 92 07500, an optical fiber has been described which can be used for the detection of visual aromas, and which consists of a transparent polymer sheath containing at least one dopant derived from xanthenic acid.

The communication by Philippe PIERRE "Device using an optical fiber in fluorescent polymer for the detection of X-rays", given to FIRELEC 2nd JCGE on April 6, 7 and 8, 1994 in Grenoble (France), shows that the best fiber for the detection of X-rays, emitted in particular by a vacuum bulb, consists of a plastic core such as polystyrene and a double cladding; the sheath which adjoins the heart, called the optical sheath, is conventionally produced for example from polymethyl methacrylate or
PMMA; the second sheath which covers the assembly constituted by the core and the optical sheath, is doped with zinc sulfide
ZnS. It is this type of dopant which has the best sensitivity to the light emitted by a ray generator.
X, with a peak centered in the vicinity of a wavelength of 450 nm.

 A first object of the invention is to determine the optimal diameters of the core and of the sheaths.

 Another object of the invention is to define a process for producing the sheath doped with zinc sulfide.

 Another object of the invention is to define an electronic circuit for detecting the light signal supplied by the fiber of the invention.

 All of these objects are achieved by the invention which relates to a fluorescent plastic optical fiber comprising a plastic core such as polystyrene, a polymer optical sheath such as polymethyl methacrylate and a sheath doped with zinc sulfide covered with a layer of black paint, characterized in that its dimensions verify the following relationships: - the core of the fiber has a diameter of between 0.5 and 1.5 mm, - the optical sheath has a thickness of between 25 and 150 microns , - the sheath doped with zinc sulfide has a thickness between 100 and 200 microns - the black paint layer has a thickness between 100 and 200 microns.

 Advantageously, the zinc sulfide doped layer consists of several layers formed of zinc sulfide powder held in very fluid adhesive, for example of superglue type.

 Preferably, the particle size of zinc sulfide powder is less than 10 microns.

 The invention also relates to a process for manufacturing an optical fiber characterized in that it comprises the following operations: 1. a very fluid adhesive is coated, by dipping, a fiber comprising a plastic fiber core and a polymer optical layer, 2. this coated fiber is placed horizontally above the bottom of a box placed on a vibrating pot, said bottom being covered with a layer of zinc sulfide powder, 3. the vibrating pot, 4. repeat operations 1. to 3. until the desired thickness is obtained, 5. cover the fiber with a layer of black paint.

 The invention also relates to a device for checking the state of the vacuum of a vacuum bulb comprising a fiber of the type described above surrounding the vacuum bulb and a high voltage generator connected to the electrodes of the vacuum bulb, the light signal produced in said fiber being routed to a photodiode, the signal of which is amplified by an electronic circuit.

The invention is now explained in detail using the attached drawing in which:
FIG. 1 is a partial view of an optical fiber according to the invention,
FIG. 2 is a diagram explaining the process for manufacturing the sheath doped with zinc sulfide,
FIG. 3 is a diagram illustrating an installation for checking the vacuum of a vacuum interrupter using a fiber according to the invention,
- Figure 4 is a diagram of the electronic circuit of the installation of Figure 3.

 In Figure 1, we see a fiber generally designated by 1 and comprising a core 2 of polystyrene surrounded by a first sheath 11, called optical sheath, preferably made of polymethyl methacrylate (PMMA).

The optical sheath 11 is surrounded by a sheath 12 doped with zinc sulfide. The sheath 12 is covered with a layer of black paint 13.

It has been observed that the optical fiber is optimal when the following dimensions are respected:
- diameter of the core 2 between 0.5 and 1.5 mm,
- thickness of the optical sheath 11 between 25 and 150 microns,
- thickness of the ZnS 12 doped sheath between 100 and 200 microns.

 The production of the optical core and sheath is conventional and reference may be made, for example, to the work "Plastic optical fibers: implementation and application", published by Masson in 1994.

According to the invention, the production of the zinc sulfide doped sheath is carried out as follows, with reference to FIG. 2:
a very fluid adhesive, for example of superglue type, is coated, the surface of the optical sheath 11 of a fiber 10 comprising a core 2 and an optical layer 11, the coating being carried out by soaking followed by draining,
- The optical fiber 1, thus coated, is placed horizontally a few centimeters above the bottom 21 of a box 20 placed on a vibrating pot 22; maintaining the fiber is easily achieved by passing it through two holes 23 and 24 made in two opposite edges 25 and 26 respectively of the box; powdered zinc sulfide 27 was placed on the bottom 21; the vibrating pot is started. Under the action of the vibrations generated by the vibrating jar, whose frequency and amplitude can be adjusted, the ZnS powder is suspended in the box and is deposited homogeneously on the entire fiber. When the thickness is reached, the vibrating pot is stopped. If the thickness is not reached after refusal of the powder to stick to the fiber, the fiber is again coated with glue and the operation is repeated.

 The particle size of the zinc sulfide powder is preferably less than 10 microns.

 After constitution of the ZnS doped sheath, the fiber is coated with black paint (layer 13 of FIG. 1) by soaking followed by draining, until a thickness of 100 to 200 microns is obtained. The fiber is thus made insensitive to ambient light and can therefore be used.

 The optical fiber is connected to a connector possibly provided with a clear fiber to ensure the transmission of the light signal to the electronic operating circuit.

 This circuit allows for example to control the quality of the vacuum of a vacuum interrupter. For this, as shown in Figure 3, the vacuum interrupter 28 is surrounded by a fiber 1 of the type which has just been described. The electrodes 28A and 28B of the vacuum interrupter are kept at a distance such that there is an emission of X-rays when a sinusoidal alternating voltage is applied, using a generator 29 for example. order of 50 kV at 50Hz, for a gap between the electrodes of a few millimeters. In FIG. 3, the reference 30 designates the connector mentioned above and 31 the clear optical fiber which carries the light signal. This is addressed to a transducer 32 making it possible to transform the light signal into an electrical signal which is processed by an electronic circuit 33.

The transducer is preferably a photodiode of good sensitivity and of low noise for the wavelength delivered by the fiber; we choose for example a photodiode
GaAsP.

 An exemplary embodiment of the electronic circuit is illustrated in FIG. 4.

The circuit has three stages:
a first amplification stage receiving the signal I emitted by the photodiode 32 which receives the light signal, this first stage preferably consists of an operational amplifier 34 (for example of the OPA 111 type from Burr-Brown) with a resistance Rf of 10 G allowing good signal quality with a high signal / noise ratio; such a stage can provide a gain of 109 V / A
a second amplification stage 35 providing for example a gain of 100 between 35 Hz and 4800 Hz,
- a selective circuit, of unity gain, centered around the frequency of 100 Hz.

The electronic circuit therefore has a gain
G = 109.100.1 = 1011 V / A
However, at the wavelength considered, the sensitivity S of the diode, defined by the ratio of the intensity supplied measured in amperes to the optical energy Wopt received by the photodiode is given by:
S = 0.3 A / Wopt
We therefore obtain the "gain" g between the output voltage and the light power on the photodiode:
g = 30.109 V / Wopt
The application of the invention is not limited to the example described; it is possible to envisage equipping vacuum bulbs with a fiber according to the invention as soon as they are manufactured to allow their connection to a circuit for self-diagnosis of the quality of the vacuum.

Claims (4)

1 / Fluorescent plastic optical fiber (1) comprising a plastic core (2) such as polystyrene, an optical sheath (11) made of a polymer such as polymethyl methacrylate and a sheath doped with zinc sulfide covered with a layer of black paint, characterized in that its dimensions verify the following relationships: - the fiber core has a diameter between 0.5 and 1.5 mm, - the optical sheath has a thickness between 25 and 150 microns, - the sheath doped with zinc sulfide has a thickness between 100 and 200 microns - the black paint layer has a thickness between 100 and 200 microns. 2 / Optical fiber according to claim 1, characterized in that the zinc sulfide doped layer consists of several layers formed of zinc sulfide powder held in very fluid adhesive, for example of superglue type. 3 / optical fiber according to claim 2, characterized in that the particle size of zinc sulphide powder is less than 10 microns 4 / method of manufacturing an optical fiber characterized in that it comprises the following operations: 1. on coated with very fluid glue, by dipping, a fiber (10) comprising a plastic fiber core and a polymer optical layer, 2. this coated fiber is placed horizontally above the bottom (21) of a box (20 ) placed on a vibrating pot (22), said bottom being covered with a layer (27) of zinc sulphide powder, 3. the vibrating pot (22) is started up, 4. operations 1 are repeated. 3. until the desired thickness is obtained. 5 / Device for checking the vacuum state of a vacuum interrupter (28) comprising a fiber (1) of the type described in claims 1 to 3 surrounding the vacuum interrupter and a high voltage generator (29 ) connected to the electrodes (28A, 28B) of the vacuum interrupter, the light signal produced in said fiber (1) being routed to a photodiode (32) whose signal is amplified by an electronic circuit (33).