FR2690254A1 - Method for deriving light signals from an evanescent field surrounding an optical waveguide - Google Patents

Abstract

The invention relates to a method for deriving light signals from an evanescent field surrounding an optical waveguide sheathed with a potting material. A light conductive probe (35) comprises an optical fiber having a free end which is embedded in the potting material (32) so as to elastically deform it (41) to the extent permitted by the mechanical properties of that material. . The fiber of the probe (35) is then tilted relative to the optical waveguide so that part of the light signal traveling through the waveguide is picked up in the probe (35). Field of application: Optical transmissions.

Description

The invention relates to a method for deriving light signals from

  an evanescent field surrounding an optical waveguide which is sheathed with a coating material, using a light-conducting probe which comprises an optical fiber having a free end, the derivation of the light signals being carried out without removing the coating material or without breaking the waveguide, and the derivation of said light signals being effected with minimized damping attenuation. In the fiber transmission domains

  particularly in the field of telecommunications.

  current, it is desirable to be able to derive light signals in order to be aware of the traffic state of the optical fiber. Currently, the light signals are taken from the fiber using a permanently attached bypass device. The fiber consists of a light conducting core and a sheath. To make the light signal accessible to the bypass device, either the sheath is removed at the branch point or the fiber is bent. to take the light signal from the heart through its evanescent field. U.S. Patent No. 3,982,123 discloses two methods of deriving a light signal from an optical fiber without requiring fiber breakage. The inventive concept of this patent is to examine the fiber in order to surrender account of its state of traffic, and a derivation of signals can be carried out at any point without disturbing the traffic. For this purpose, the bypass device is placed, which in this case consists of a material comprising a photodetector, on a heart

  The optical fiber consists of a core with low optical losses and a cladding whose refractive index is smaller than that of the optical fiber. heart.

  A first method described in the patent consists in removing all or substantially all of the cladding material of the fiber. The detector is then fixedly placed on the conducting core of the light, the stripped zone of which must be at least three times the length

wave in the optical fiber.

  A second method of derivation of light signals consists in bending the optical fiber without removing the cladding material. This allows the light signals to be

  extracted through the sheath and captured by a photodetector.

  The derivation is carried out permanently in both cases. U.S. Patent No. 4,784,452 discloses a method in which a bypass is effected by means of a bypass device placed on an optical fiber. The fiber consists of a light conducting core and at least one cladding material The shunt device, namely a probe, is an optical fiber of

  same type as the fiber from which the signals are derived.

  This probe has a free end which comprises a light-conducting core. To derive light signals from the fiber, it is necessary to remove the sheath in order to expose the core. The probe is used in this exposed area. the end of the probe being placed against the bare part of the fiber To obtain the best possible derivation effect, it is necessary to adapt the angle defined by the axis of the probe and the axis of the fiber. coupling connects the area of the probe and the stripped portion of the fiber and conducts light signals from the stripped portion of the fiber to the probe The coupling medium, which is a solid material and

  hard, fixes the probe relative to the fiber.

  Various tests have shown that the light-conducting core may consist of a polyimide In the article "Dependence of Precursor Chemistry and Curing Conditions on Optical Loss Characteristics of Polyimide Wageguides" from CP Chien and KK Chakravorty to Boeing Aerospace and Electronics, Seattle, USA, SPIE volume 1323, Optical Thin Films III, New developments (1990), it is described that a polyimide is a good material for the core of an optical fiber A polyimide has good stability

  and a dielectric index of 3.5, which is compatible with

  with other integrated circuit materials The material works well as a light emitter, for example in optoelectronic circuits in the GH frequency band. The advantage of a polyimide is that in the manufacture of cores These can be closely grouped. Another data concerning a polyimide is that it has a refractive index of 1.6 (1.58-1.62) and optical losses in the core of about 1 d B / cm.

  when exposed to ultraviolet light.

  Assays were performed with a silicone elastomer as an index matching medium for the light-conducting core "Index Matching Elastomers for Fiber Optics" by Robert W Filas BH Johnson and CP Wong at AT & T Bell Laboratories , NJ EUA in IEEE magazine, Proc Electron Compon Cont, 39th, 486-9, indicates that silicone elastomers are good core index matching materials Copolymer reflection

  depending on the concentration of diphenyl and the temperature

  ture is obtained by measuring the reflection force of a monomode waveguide whose core has been embedded in an elastomer It is possible to obtain the same refractive index as that of the core with a material of the silicone rubber type. silicone rubber can be used as interface between different components Another method is to use silicone rubber as a

  protection against moisture and dust, for example.

  Currently, air is used as the refractive medium with the light-conducting core of the lightwave conductor. The air has a refractive index much lower than that of a polyimide. The refractive index of the is equal to 1 while the refractive index of the polyimide is 1.6 and the index of

  refraction of the silicone rubber is 1.5.

  A disadvantage of previously known solutions is that the light signals are derived from the optical fibers by means of permanently fixed devices. This means that the light signals are derived from the fiber at a specific point on it, where the sheath has been The previous solutions are affected by a number of other disadvantages. One of these disadvantages is that the light signals can be derived only on fiber waveguides and that it is necessary to eliminate the sheath. from where the bypass must take place The bypass device must be placed firmly on the optical fiber at this point where the sheath has been removed. The bypass in permanent connections leads to losses.

excessively high.

  The object of the present invention is to eliminate the disadvantages encountered with previously known methods for deriving light signals from optical fibers. The invention relates to a method of derivation of a light signal by means of a light-conducting probe directly on an optical waveguide which is embedded in an elastic material. The method of derivation of a light signal can be implemented on two types of waveguide In one case, the method can be applied to a light waveguide which extends on a substrate, and in the other case to an optical fiber The method is executed by inserting the probe into direct contact with the conducting core of the light of a waveguide, so as to extract light signals from the evanescent field of the core without it being necessary to eliminate the material embedding or bending the waveguide to derive the light signals The probe comprises an optical fiber which is of the same type as the fiber from which the light signals are derived. The fiber of the probe has a free end. The method is as follows in a first step, the end of the fiber of the light conductive probe is lowered under pressure to the coated optical waveguide. In a second step, the end of the fiber of the probe is pressed under pressure in the coating material 32 while elastically deforming this material to a degree permitted by the elastic properties of the material or so that the residual deformation, the deflection, does not become permanent In a final step, the end of the fiber of the probe is inclined with respect to the optical waveguide 23 so that part of the light signal is captured by the probe The probe is inclined so as to derive a given light signal If the angle is modified, a other signal

bright is obtained in the probe.

  It should be noted that the end of the fiber of the light-conducting probe is of a width equal to that of the optical waveguide so that the best possible derivation is obtained. same material as that of the light conducting core or in a material having such a high refractive index or higher. The method also allows the light-conducting probe to be permanently attached to the optical waveguide, if desired.

  Deriving a light signal is completed, the probe is removed without leaving any traces of its previous presence. When deriving light signals from light waveguides, no further action is required to allow derivation to take place.

  Before performing the first step of the process on the optical fiber, the fiber must be placed on a surface of

  hard support for the derivation to take place.

  When the fiber is too flexible, it is impossible to insert a probe into the fiber without first placing the

  fiber on a hard surface of support.

  The invention has the advantage of allowing the derivation of light signals without it being necessary to fix the permanent bypass device at a given point of derivation on the optical waveguide. Another advantage is that it is possible to derive light from the light waveguide when the waveguide is firmly applied to the substrate Until now light signals were not derived from light waveguides mounted on a substrate. light signals

  can be easily derived using such a system.

  Other advantages lie in the fact that a bypass can be carried out temporarily if desired, and

  that the elastic coating protects from environmental influences.

  outside, for example dust, air and

  moisture, during a process of derivation of light.

  The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and in which: FIG. 1 is a view from above of a light waveguide mounted on a silicon disk, according to a first embodiment of the invention; FIG. 2 is an enlarged partial sectional view of the light waveguide mounted on the silicon disk, along the line AA of FIG. 1, and showing a method of derivation of light signals from the waveguide FIG. 3 is an enlarged fragmentary sectional view of the light waveguide mounted on the silicon disk, along the line BB of FIG. 1, showing a light-emitting probe with a light-conducting probe; a method of deriving light signals from the light waveguide using a light-conducting probe; and FIG. 4 is a sectional view of an optical fiber, illustrating a method of deriving a light signal from the optical fiber using a probe

  conducting light according to a second embodiment

of the invention.

  The figures in the attached drawings are not to scale and show only the parts necessary for

  understanding of the concept of the invention.

  Figures 1 to 3 represent a first form

embodiment of the invention.

  FIG. 1 illustrates an arrangement 20 which consists of a substrate 21, a connection device 25, a light waveguide 23 and an optical component 24

  that emits or receives light.

  FIG. 2 is a sectional view of the device 20 illustrated in FIG. 1, along the line AA of FIG. 1 passing through the optical component 24. A very thin layer, applied to the substrate 21, is intended to function as a medium A very narrow gap 31 appears between one end of the core 30 and the component 24 The connecting device is connected directly to the other end of the core 30 The component 24 and the core 30 are covered The discontinuous line 34 illustrates a light-conducting probe 35 which is applied under pressure against the coating material 32 and which derives a light signal 33 from the core 30. The probe captures a field The light signal 33 taken from the core 30 by the probe 35 forms a light signal 36 which corresponds to the light signal. ineux 33 but whose

  the energy is much lower.

  FIG. 3 is a sectional view on an enlarged scale of the light waveguide 23 along the line BB of FIG. 1. FIG. 3 illustrates the light waveguide 23 which consists of the light-conducting core 30 of the light waveguide 23. thin layer 40 on the substrate 21 and the material

  The refractive index of the enamel material

  The layer 40 extends over the substrate 21 and the core 30 extends over the top of the layer 40. The encapsulating material 32 covers everything on the substrate 21. The light-conducting probe 35 is lowered under pressure towards the core 30, a temporary deformation 41 appears in the coating material 32 When the probe 35 is lowered under pressure to its maximum position to achieve an optimal derivation, a very short interval narrow 42 is formed between the probe 35 and the

heart 30.

  The substrate 21 illustrated in FIG. 1 is a silicon disk of the type of which semiconductors normally consist. The substrate 21 may comprise several light waveguides 23, components 24 and connection devices 25 mounted on this substrate. consist of a circuit board material, a glass material or any type of material provided that the substrate 21 has a refractive index lower than that of the core 30 It is important that the damping in the heart 30 also be

as low as possible.

  In the embodiment illustrated in FIGS. 2 and 3, the light waveguide 23 consists of three parts. These parts are the thin layer 40 applied.

  on the substrate 21, the core 30 and the coating material 32.

  When the substrate 21 is made of silicon, the layer 40 is made of silicon dioxide It is necessary that the refractive index of the layer 40 is lower than that of the core 30 so that the layer 40 does not conduct the light coming from the In the case of the illustrations of FIGS. 2 and 3, the light-conducting core 30 is a multimode core, although it can also be produced under the

shape of a monomode heart.

  The coating material 32 is a silicone elastomer, for example silicone rubber. The coating material is designed to allow light signals to be derived from the core. Since the coating material is elastic, the sensor 35 The silicone rubber is optically conductive The coating material 32 may be applied to the substrate 21 and to the components 24 and 25 mounted on this substrate while this material may be further molded, after which it is possible to to the material to harden and at the same time to become elastic. The assembly 20 described above is used to derive light signals 33 with the light-conducting probe 35 inserted directly on the optical waveguide. By inserting the probe 35 through the elastic coating material, it is possible to bring it close enough to the heart 30 to capture the field

  evanescent surrounding the heart 30

  It is important that the probe 35 is not inserted too far into the coating material, since the deformation 41 thereof can then become permanent. Moreover, if the probe 35 is not pressed sufficiently into the coating material 32, it is unable to capture the evanescent field The distance between the probe 35 and the core 30 must be of a correct order of magnitude, less than 6 gm To obtain the same distance to

  each derivation operation, an instrument can be used

  distance measurement in order to obtain the distance

correct.

  The method comprises the steps of: lowering the end of the conductive probe of light toward the coated optical waveguide; pressing the end of the fiber of the probe 35 into the coating material 32 while elastically deforming said material to a degree permitted by the elastic properties of the material; and tilting the end of the fiber of the probe 35 relative to the optical waveguide so that part of the light signal is picked up by the probe. The probe 35 is inclined to derive a given light signal. we change the angle, we get another signal

bright in the probe.

  It should be noted that the end of the fiber of the light conductive probe 35 is as wide as the optical waveguide so that the light signal 33 is obtained as strong as possible. In addition, the probe 35 is made of the same material as that of the core 30 or in a material having such a high refractive index or higher. The probe 35 can also be made from

of a plastic fiber.

  The method also allows the light conductive probe to be permanently attached to the optical waveguide if desired. The probe 35 is removed at the end of the light signal bypass operation without Residual deformation of the material at the location where the probe was inserted When shifting light signals onto the light waveguide 23, it is not necessary to take other measures to enable the

derivation to take place.

  FIG. 4 illustrates another embodiment of the assembly. FIG. 4 shows an optical fiber 1 which consists of a light conducting core 2 and an elastic coating material 3. The refractive index of the core 2 is higher than that of the coating material 3. A probe 35 is pressed into the coating material 3 towards the core 2, revealing a deformation 41 of the coating material 3 When the probe 35 is depressed up to its maximum position in which its best derivability is obtained, a very narrow gap 42 is formed between the probe 35 and the light-conducting core. The core 2 may consist of, for example, a polyimide and is coated with the elastic material, which is advantageously a silicone rubber Since the material is elastic, the probe 35 can be inserted into the coating material 3 and a bypass can begin when the probe 35 reaches the evanescent field The bypass device can be removed, if desired. Optical fibers extend between different telephone sets, or, for example, between different computers. The distances involved can be significant and it is sometimes necessary to access the fibers and In the illustrated embodiment, the optical waveguide is not attached to a substrate, but

  is an optical fiber 1 extending freely.

  The method applied for deriving light signals from light waveguides 23 may also be used to derive a light signal from optical fibers. When signals are derived from optical fibers, the fibers must be placed on a hard surface of the fiber. support before the first step of the process so that a derivation can take place When the optical fiber is too flexible, it is not possible to insert a probe in the fiber, unless the fiber rests on a hard surface of Another advantage of using an elastic coating material is that a probe can be embedded in the material and that light signals can be derived. Another advantage is that optical waveguides according to The embodiment described above can be manufactured cheaply and simply. The method makes it possible to derive light signals from optical waveguides. tick on temporary occasions. It goes without saying that many modifications can be made to the process described and shown without

depart from the scope of the invention.

Claims (4)

  A method for deriving light signals from an evanescent field surrounding an optical waveguide (23) which is encapsulated with a sheath (32), using a light-conducting probe (35) which comprises an optical fiber having a free end, the derivation of the light signals being effected without the coating material being removed or without the optical waveguide being sensed, and with minimized attenuation impairments, the method being characterized by the steps of inserting the fiber end of the light conductive probe toward the coated optical waveguide; pressing the end of the fiber of the light-conducting probe into the coating material while elastically deforming (41) said material to the degree permitted by the mechanical properties of that material; and tilting the end of the fiber of the light conductive probe relative to the optical waveguide so that a portion of the light signal present in the waveguide is sensed in the probe.   Method according to claim 1, characterized in that the probe is as wide as the optical waveguide so that the best result of possible derivation.   3. Method according to one of claims 1 and 2,   characterized in that the probe is made of the same material as the conducting core of the light of the waveguide, or in a material having a refractive index as high or higher.   Process according to any one of the claims   1, 2 and 3, characterized in that the light-conducting probe is permanently fixed to the optical waveguide. Method according to any one of the preceding claims, in which the waveguide is an optical fiber, characterized in that it consists in placing the optical fiber on a hard support surface during a first step of the method , before the operation of derivation of the light.