FR2683052A1 - Fibre-optic polariser, its method of manufacture and application to a sensor - Google Patents

Abstract

Fibre polariser comprising a hollow fibre (2) in which an internal housing (3, 4) located close to the core of the fibre (1) is at least fitted with one metal element (5, 6) over part of the length of the fibre. Applications and advantages: such a structure integrates a polariser into the optical fibre and makes it possible to avoid any coupling, such as that currently entailed between a polariser and a transmission fibre.

Description

FIBER OPTIC POLARIZER, ITS METHOD
MANUFACTURING AND APPLICATION TO A SENSOR
The invention relates to an optical fiber polarizer, its method of manufacture and its application to a sensor. It applies in particular to a polarizer integrated in an optical fiber having, throughout the fiber, at least one recess (hollow fiber).

 It is well known that polarization conservation properties can not be obtained perfectly in an optical fiber structure. Although extinction rates greater than 20 decibels are obtained between crossed polarizers over fiber lengths of several hundred meters, this value can be strongly altered by the conditioning of the fiber. It is therefore necessary to perfect the extinction coefficient (and therefore the maintenance of the polarization) by the use of an additional polarizer element at the output of the fiber.

 In known systems, independent polarizers are provided which are optically coupled to the other elements of the system.

 However, free propagation in any part of a transmission "network" or sensors is a significant practical limitation (need to design near-perfect alignments in quality and stability). In addition some applications do not allow this hybrid approach (pollution of the dioptres ...).

 Optical fiber polarizers based on differential losses obtained by curvatures are available on the market, but have a large size (coils several centimeters in diameter). In addition, the radius of curvature must be adjusted very precisely according to the wavelength of the source used.

Polarizers made in the form of optical fiber portions are also known in the art. Such a polarizer, as described in the document "Optical Fiber
Sensors "by John DAKIN and Brian CUSHAW, pages 262 to 264, consists of a portion of optical fiber having a metal element along its length, according to which the polarizer described is an individual polarizer and is not integrated. to an optical assembly, it must be assembled (optically coupled) with other optical elements of a system.

 The use of a separate optical fiber polarizer requires at least one additional splice in the link, which constitutes a significant source of attenuation: geometric (different fiber diameter) and optogeometric (different mode diameters) mismatch .

 To avoid these drawbacks, the invention proposes to integrate the polarizer with the optical fiber to avoid the use of an additional optical element which causes a discontinuity of the wave guided by free propagation at the polarizer.

 The invention therefore relates to an optical fiber polarizer characterized in that it comprises an optical fiber comprising, all along the fiber, at least one recess located near the core of the fiber, said recess comprising at least one metal element on only part of the length of the fiber.

 The invention also relates to a method for manufacturing an optical fiber polarizer, characterized in that, starting from an optical fiber comprising, all along the fiber, at least one recess, a portion of molten metal is inserted into the and then let the metal solidify in the recess.

 The polarizer function is also obtained by introducing metal into an optical fiber element which has two recesses located on either side of the guide core, to produce a polarizer by metallic absorption.

The advantage of the invention lies in the fact of filling with metal the recesses at the ends of the optical fiber used for the connection or the sensor.

 The invention applies to any optical fiber system for which the polarization of the light wave is of interest. By way of non-limiting example, mention may be made of coherent bonds, polarization multiplexing, polarizer realization at the level of the primer fiber of a semiconductor light source, etc.

The various objects and features of the invention will appear more clearly in the description which follows and in the appended figures which represent
FIG. 1, an exemplary embodiment of the polarizer according to the invention;
FIG. 2, an alternative embodiment of the polarizer according to the invention;
FIGS. 3 and 4, embodiments of the invention of the polarizers of FIGS. 1 and 2;
FIGS. 5 to 7, optical fiber sensors of known type
FIG. 8, an optical fiber sensor with independent polarizer;
FIG. 9, an optical fiber sensor according to the invention;
FIGS. 10a and 10b, optical fiber sensors according to the invention operating in series
- Figure 11, fiber sensors according to the invention operating in parallel.

 Referring to Figure 1, we will describe an exemplary embodiment of a polarizer according to the invention. This polarizer comprises a fiber having a guide core 1 and an optical cladding 2. The sheath 2 has at least one cavity 3 made parallel to the core. The cavity is made along the entire length of the fiber. According to the embodiment of Figure 1, the fiber has two cavities 3 and 4. In a limited portion of the length of the fiber cavities 3 and 4 contain metal elements 5 and 6. These metal elements will act as absorbent on iine polarization of the light guided in the heart of the fiber.

 According to Figure 1, the elements 5 and 6 are located at one end of the fiber.

 According to Figure 2, they are located in an intermediate zone of the fiber.

 It is also possible, although it is not shown to provide metal elements in different localized areas of the fiber.

 Each metal element has a length of the order of a few centimeters for example.

 When the fiber has several cavities as shown in Figures 1 and 2> the metal elements of the same area have the same length and are located at the same place in the fiber.

 Figures 3 and 4 show methods of making the fibers of Figures 1 and 2.

A hollow fiber Fi is firstly produced according to a known technique such as that described in the Application for
French Patent No. 89 15872. One end of the fiber F1 is then dipped in a molten metal M contained in a sealed enclosure C and maintained at a predetermined temperature so that the metal remains in the molten state. Pump urn P has a tubing penetrating into the container C and allows to inject a gas or air into the chamber. The molten metal M is under the effect of the pressure pushed back into the cavities of the fiber. If the metal elements must be located at the end of the fiber, it stops the pressure created by the pump P when the metal elements are of appropriate dimensions (by opening the valve V for example).

If the metallic elements are to be located in an intermediate zone of the fiber, when the metallic elements are of appropriate dimensions, the end of the fiber is removed from the molten metal M. As a result, the pressure introduced by the pump P into the enclosure C has the effect of pushing the metal elements in the fiber Fl. When the metal elements have arrived in the appropriate zone, the pressure in the chamber is cut off by opening the valve
V, or if we want to introduce other metal elements, simply re-dip the end of the fiber in the molten metal and proceed again as before.

 FIG. 4 represents another embodiment of the invention. One end of the fiber Eil is soaked in the molten metal contained in the enclosure M (the enclosure M is not sealed in this process). The other end of the fiber F1 is connected to a suction pump which draws in the necessary amount of molten metal. When it is desired to move the metal elements in the fiber, the end of the fiber is drawn out of the molten metal and the pump continues to be sucked up.

 Referring to Figures 5 to 11, will now be described an application of the optical fiber polarizer according to the invention to the realization of sensors.

 Polarimetric (interferometric) type optical fiber sensors use the birefringence properties of the fiber subjected to the variations of the parameter to be measured (for example temperature, hydrostatic pressure, stresses, etc.).

 In the case of so-called intrinsic sensors, the intrinsic birefringence of the fiber is modified as a function of the quantity to be measured, which results in a variation of the differential propagation of the two modes of polarization.

At the output of the sensor fiber, the measurement of the variations of the quantity to be measured is obtained by measuring the induced phase shift. An optical fiber sensor is an optical fiber element of length L. Subject to variations in the quantity to be measured, the total phase shift used is given by a simple linear relation: the product of the sensitivity (to the value to be measured) per unit of length of fiber by the length of the fiber (and the variation of the value to be measured). The measurement is therefore the integral of the variations of the quantity to be measured over the length of the sensor fiber.

 This implies that the value of the quantity to be measured is assumed to be constant along the fiber. It therefore appears necessary to limit (limit) the sensor fiber element to differentiate it from the connecting fibers (light source-sensor and sensor-detector) and furthermore it is imperative that the link fibers do not disturb the information useful for measurement (phase shift induced in particular). In Figure 5, there is a sensor fiber F10 coupled outwardly by connecting fibers Fil and F12.

 A known solution consists in using the same optical fiber, this time considering not the differential propagation but the propagation according to a mode of polarization.

 This approach is possible provided that a coupling point is achieved between the axes of propagation of the fiber according to a particular mode of orientation that is recalled below.

 During the differential propagation, the two eigen modes of the fiber are excited. In Figure G, it can be seen that the two modes entering the fiber propagate at different speeds. An element of the same optical fiber oriented at 450 and spliced at the previous one (FIG. 7) behaves like an analyzer, the energy contained in each of the propagation modes of the sensor fiber F10 is projected onto each of the modes of the fiber. F13 analysis. The length of the sensor fiber is chosen so that the waves propagating in the two polarization modes are decorrelated (the minimum length is determined from the temporal coherence of the source and depending on the birefringence of the source). If this condition is realized the components of the waves projected on each of the axes of the analysis fiber F13 can not interfere, the phase shift information induced on the sensor fiber F10 can be deported to the detector system without disturbances.

It appears that the useful information is projected on the two axes of propagation of the analysis fiber, whereas a single projection is useful for the measurement. One method used to cancel one of the components is to place a properly oriented polarizer element at the output of the fiber
F13 analysis (and offset) before the reading system (Figure 8).

 Can be used for this purpose, mass polarisers of conventional optics or optical fiber polarizers.

 Depending on the architecture of the network this solution is not enough. Indeed, it may be necessary to include the polarizer function in the network itself between the different sensors to minimize or cancel spurious effects from second order polarization couplings (and higher orders).

 An "in-line" polarizer solution is possible thanks to the optical polarizer of the invention. For this, as shown in FIG. 9, an optical fiber with a hollow structure is used as a sensor element over a length L1. At point P1 is achieved polarization coupling by controlled rotation of the axes (splicing or rotation-fusion). The coupling point induces a coupling at 3 decibels (coupling at 450) A From the coupling point P1, the fiber length L2 is used as analysis and offset fiber. To obtain the polarizer function, metal is introduced into the recesses of the fiber over a sufficient length 2P to obtain an extinction rate adapted to the application.

 For example, extinction rates of the order of 30 to 40 dB can be achieved after a few centimeters of propagation.

 On the fiber length after the metal-containing part only the information contained in the clean mode not attenuated by the metal propagates. On this part of the optical fiber denoted L2D the phase shift information of the sensor is offset without differential phase disturbances.

 In this way, the fiber section L2D can be directly connected to the reading system without adding additional polarizing elements.

 The invention is particularly suitable for interferopolarimetric sensor networks (serial or parallel modes).

 The realization of polarizing elements located in a series network of coherently addressed polarimetric sensors makes it possible to spatially separate the different sensors of the network. Figures 10a and 10b show the improvement provided by the invention in a serial network.

 In the configuration of FIG. 10a, the different sensors Ci> C2,......> Cn are adjacent two by two. Two sensors are separated only by a bias coupling point PI, P2, ..., Pn.

 In the configuration of FIG. 10b, the polarizing elements LlP1,..., LnPn make it possible to separate two consecutive sensors C1,..., Cn by a long fiber length LlD1J ... LnDn (up to several tens of meters) rendered insensitive to the environment by suppressing the differential propagation between polarization modes.

 In the case of a parallel architecture} represented in FIG. 11> the polarizing elements LP1J ...> LPn placed at the output of the sensors C1>..., Cn make it possible to use a single return bus B2 for the set sensors. According to this configuration, the sensors are addressed in time and read sequentially in coherence. The polarizers are made in the sensor fibers.

 The invention is also applicable to the production of a polarization-maintaining transmission line. A strictly monomode transmission line is indeed difficult to achieve because of technical imperfections.

According to the invention, there is thus provided a fiber with a hollow structure comprising, in the recess or recesses, metallic elements acting as polarizers.

 The recesses can also be filled with metal throughout the fiber.

 It is obvious that the foregoing description has been made only by way of example. Other variants may be envisaged without departing from the scope of the invention. In particular, the shape of the fibers, the shape of the various elements of the fiber and the nature of the materials are only indicated to illustrate the description.

Claims (10)

 i. An optical fiber polarizer characterized in that it comprises an optical fiber comprising, all along the fiber, at least one recess located near the core of the fiber said recess comprising at least one metal element on only part of the length fiber.  2. Fiber polarizer according to claim 1, characterized in that the metal element is located at one end of the fiber.  3. Optical fiber polarizer according to claim i> characterized in that the metal element is located in an intermediate portion of the fiber.  4. polarizer optical fiber according to claim 1. characterized in that it comprises a plurality of metal elements distributed in different areas of the fiber.  5. Fiber optic polarizer according to claim 1J characterized in that the fiber has two recesses located on either side of the core of the fiber, each recess comprising at least one metal element, these two metal elements being located in the same fiber area.  6. A method of manufacturing an optical fiber polarizer characterized in that, starting from an optical fiber having throughout the fiber at least one recess is inserted a portion of molten metal into the recess and that then let the metal solidify in the recess.  7. Method according to claim 6, characterized in that the insertion of a portion of molten metal in the recess is made by contacting one end of the fiber with a molten metal portion and by suction by the other end of the fiber.  8. Method according to claim 6, characterized in that the insertion of a molten metal portion into the recess is by injection pressure of a molten metal portion.  9. Method according to claim 6, characterized in that the fiber has at least two recesses and that molten metal portions of the same volume are inserted into the recesses, two metal portions of the two recesses being placed at one and the same location. the fiber.  10. An optical fiber sensor applying the polarizer according to any one of claims t to 5 comprising a hollow optical fiber, characterized in that it comprises at least one metal element located near the core of the fiber.  he. Polarization maintaining transmission line applying the polarizer according to any one of wool of claims i to 5, characterized in that it comprises throughout the fiber at least one element containing at least one metal element.