Method of making an index grating in an optical fiber, and a qua

Details Method of making an index grating in an optical fiber, and a qua Method of making an index grating in an optical fiber, and a qua

1991-10-03

1994-05-17

Ullah, Akm E.

Optical waveguides

With optical coupler

Particular coupling function

G02B 600, G02B 636

Patent

active

053135385

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method of making an index grating in a multimode optical fiber or a fiber in which only a few modes propagate, and also to a quasi-distributed array of sensors formed in said optical fiber.
To achieve coupling between two modes of light propagation in an optical fiber, one known solution consists in creating an index grating along the fiber in the guiding structure thereof with the index grating being periodic and having a spatial period equal to a multiple of 2.pi. divided by the difference between the propagation constants of the two modes of light propagation.
Until now, index gratings have been created by machining and chemically etching the fiber, with the drawback of weakening the fiber, or else by mechanically compressing the fiber locally, with the drawbacks of poor definition of the intermode coupling points and of making the fiber bulkier and weaker.
A particular aim of the invention is to avoid these drawbacks.
An object of the invention is to provide a method of making an index grating in a two-mode or a multimode optical fiber without weakening the optical fiber and making it possible to delimit the coupling points very accurately.
Another object of the invention is to provide a quasi-distributed array of sensors in an optical fiber by performing the method.


SUMMARY OF THE INVENTION

The present invention thus provides a method of making an index grating in a multimode optical fiber, the method being characterized in that it consists in exposing the optical fiber to a laser beam through a mask which has periodic transmission and which is applied against the optical fiber.
The exposure of the fiber to the laser beam causes the refractive index in the fiber core to vary, particularly when the core is doped with germanium.
Exposure through a mask having periodic transmission makes it very simple to obtain the desired index grating in the fiber.
Preferably, the laser beam is directed perpendicularly to the axis of the fiber.
In the mask, it is possible to superpose two different longitudinal transmission periods, thereby making it possible to achieve simultaneous coupling within the fiber, e.g. between two modes of polarization and two transverse modes of light guidance within the optical fiber, or between two transverse modes in two polarizations.
The optical fiber is exposed to the laser beam through the mask at regular intervals along the fiber, which intervals may be about 10 cm, for example.
The optical fiber then constitutes an array of sensors.
The invention also provides a quasi-distributed array of sensors in an optical fiber, the array being characterized in that the sensors are defined by segments of the optical fiber delimited by periodic index grating zones for intermode coupling formed in the optical fiber by performing the above method.
A first end of the optical fiber is connected via a polarizer to a light source such as a super light emitting diode which injects light into the optical fiber in a single propagation mode, and the second end of the fiber is connected to a demultiplexing system.
This system includes means for tuning to the different optical path lengths generated by the sensors, thus achieving coherence demultiplexing.
In a variant, the optical fiber may be connected to as many multiplexing systems as there are sensors, with each system being tuned to the optical path length difference that corresponds to a particular sensor.
Such sensors can be used for measuring physical magnitudes such as temperature, deformation, pressure, magnetic field, etc. . . . , acting on the segments of fiber that constitute the sensors.
In particular, such sensors can be used on structures, e.g. to measure the stresses in components of a structure and to keep variations of such stresses under surveillance.


BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other characteristics, details, and advantages thereof will appear more clearly on reading the following description given by way of example and made with reference to the accom

REFERENCES:
patent: 4096446 (1978-06-01), Haus et al.
patent: 4974930 (1990-12-01), Blyer, Jr. et al.
patent: 5104209 (1992-04-01), Hill et al.

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