Electromagnetic pulse train generation for testing optical...

Optics: measuring and testing – For optical fiber or waveguide inspection

Reexamination Certificate

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Reexamination Certificate

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06762828

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an emitter for emitting electromagnetic pulses, to a system for testing fiber-optic components and to a method of testing them.
BACKGROUND OF THE INVENTION
Said emitter, of the type comprising:
a generator for generating at least one electromagnetic pulse, especially a light pulse; and
at least one optical fiber capable of transmitting an electromagnetic pulse generated by said generator for the purpose of injecting it,
is able to be applied more particularly, although not exclusively, to a test system for determining characteristic parameters, especially the losses, of a fiber-optic component, in particular a fiber-based component, a fiber-based link or a fiber-optic network.
U.S. Pat. No. 5,251,002 discloses such a test system, which comprises:
such an optical emitter or source capable of emitting a light pulse;
a photoreceiver capable of measuring optical characteristics of a light pulse emitted by said optical source and transmitted by a fiber-optic component; and
data acquisition, storage and processing means which receive the measurements generated by said photoreceiver for said fiber-optic component to be tested and for a reference fiber-optic component and which determine, on the basis of these measurements, the losses of said fiber-optic component to be tested.
In a known manner, said optical source or said emitter has emission conditions, especially with regard to the emission solid angle and the illumination surface, which are fixed and uniform.
In addition, said known test system allows tests to be carried out only for a defined configuration of the light beam used, as emitted by said optical source. This known test system therefore uses a method which measures only the losses relative to a single defined pulse emitted by said optical source.
However, in practice, the configuration of the light beam generated by the optical source is not always that actually used in said optical component to be tested. Consequently, the reliability of this test system is unsatisfactory.
To increase the reliability, it is necessary to suitably sample the aforementioned illumination or emission conditions, which may vary from 0% to 100% of the area of the core in the case of the emission surface, and from 0% to 100% of the numerical aperture, in the case of the emission solid angle. In the situations usually encountered, the conditions may vary from 70% to 100%.
For this purpose, it is advised to provide a sampling increment of at most 2%. This therefore assumes that at least some fifteen different optical sources are used, each of which has appropriate illumination conditions, in order to obtain satisfactory sampling.
This solution is therefore hardly satisfactory, especially because of the high cost, the lengthy implementation time and the considerable amount of handling.
SUMMARY OF THE INVENTION
It is an object of the present invention to remedy these drawbacks, and especially those associated with the existence of fixed and uniform emission conditions of the optical source.
It relates to an emitter for emitting electromagnetic pulses, which is capable of emitting electromagnetic pulses having emission geometry characteristics (emission solid angle and emission area) which are variable.
For this purpose, according to the invention, said emitter for emitting electromagnetic pulses, comprising:
a generator for generating at least one electromagnetic pulse; and
at least one optical fiber capable of transmitting an electromagnetic pulse generated by said generator for the purpose of emitting it,
is noteworthy in that it includes in addition at least one optical cavity:
which is placed in the path of an incident electromagnetic pulse transmitted by said optical fiber; and
which has an input provided with a first partially reflecting mirror and an output provided with a second partially reflecting mirror, said first and second mirrors being arranged so as to create at the output of said optical cavity, from a single incident electromagnetic pulse, a train of emitted electromagnetic pulses which have variable geometry characteristics and are associated with said incident electromagnetic pulse.
Thus, by virtue of said optical cavity, the emitted pulse train comprises pulses subjected to variable numbers of reflections off said mirrors and therefore traveling variable distances in said cavity. However, since the aforementioned geometry characteristics vary, in a known manner, according to the distance traveled, the various pulses of said pulse train have variable geometry characteristics (emission solid angle and emission area).
Consequently, by choosing appropriate characteristics of said optical cavity, and especially the reflection/transmission ratio of the partially reflecting mirrors and the distance between them, a plurality of pulses having predetermined emission characteristics can be formed.
In particular, when said emitter is applied to a test system of the aforementioned type, it is possible to form a particular pulse train comprising the various possible configurations of the light beams likely to be transmitted by the component to be tested.
This enables the aforementioned drawbacks of the known test system, described in particular in U.S. Pat. No. 5,251,002, to be remedied since it is no longer necessary to provide a plurality of different optical sources in order to carry out an overall test.
Moreover, advantageously, at least one of said first and second mirrors is linked to two lengths of the optical fiber:
in a first embodiment, directly by opposed faces; and
in a second embodiment, via an associated optical coupling means.
In this second embodiment, said or each of said optical coupling means advantageously comprises:
in a first embodiment, two lenses optically linking said two lengths of the optical fiber, the mirror associated with said optical coupling means being placed between said lenses;
in a second embodiment, a single half-wave graded-index lens, said mirror being placed on that face of said optical coupling means which is internal to said optical cavity; and
in a third embodiment, two quarter-wave graded-index lenses optically linking said two lengths of the optical fiber, the mirror associated with said optical coupling means being placed between said lenses.
Moreover, the emitter according to the invention advantageously comprises means preventing a pulse generated by said generator from returning toward the latter. For this purpose, said means preferably include at least one light trap which cooperates with the optical coupling means associated with said first mirror.
Furthermore, said generator is advantageously capable of generating at least two pulses, of different wavelengths, thereby making it possible in particular, when applying it to a test system, to measure simultaneously the losses at several operating wavelengths of certain types of links such as, for example, video communication links operating at wavelengths of 0.85 &mgr;m and 1.3 &mgr;m.
The present invention also relates to a test system of the aforementioned type.
According to the invention, said test system is noteworthy in that the optical source of this system comprises a light-pulse emitter, as mentioned above.
In addition to the advantages indicated above, this test system according to the invention has an operating life (or lifetime) longer than that described in U.S. Pat. No. 5,251,002. This is because this known latter system, which includes electrooptic switches, only allows a relatively small number of uses and, in addition, it is sensitive to the environmental conditions, something which is not the case in the system according to the invention, which has no mechanical moving parts.
It should also be noted that the optical fiber of the pulse emitter generally has characteristics (core diameter and numerical aperture) identical to those of the fiber-optic component to be tested. This makes it possible to vary over time, in the same direction, the emission area and the emission solid angle, these latter two geometry characteris

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