Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2000-03-13
2003-10-28
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C385S012000
Reexamination Certificate
active
06639681
ABSTRACT:
TECHNICAL FIELD
The present invention relate to a device for reading spectral lines contained in an optical spectrum.
It finds applications notably in the field of optical communications.
The invention applies particularly to networks of sensors with optical fibres.
These comprise notably networks of deformation sensors with optical fibres and very often photo-inscribed Bragg gratings constituting the components transducing deformation (or even pressure or temperature).
One of the first architectures of networks to have been published uses an optical source with a spectral width greater than the spectral band containing the spectra of the Bragg gratings and sequentially analyses the wavelengths reflected by the different sensors (demultiplexing in terms of wavelength and then spectral analysis of the different signals).
In this regard, reference should be made to documents (1) to (4) which, like the other documents cited below are set out at the end of the present description.
Such networks of sensors can be used for monitoring structures in the following fields: building, civil engineering, transportation, aeronautics and aerospace.
PRIOR ART
Four techniques are known for effecting integrated optics demultiplexing: a first technique using an engraved grating, a second technique using Mach-Zehnder interferometers, a third technique using an array of microguides or PHASAR (standing for PHASe-ARray), and a fourth technique using balanced Mach-Zehnder interferometers or 100% couplers with a Bragg grating photo-inscribed identically on the two arms (“ADD-DROP multiplexer”).
The first technique uses the diffraction of light by a concave grating (with a circular or plane exit field) engraved and blazed to a high degree.
Vertical engraving is possible in the case of silica on silicon guides and can attain a depth of 25 &mgr;m.
In this regard, document (6) should be consulted.
The demultiplexing component then consists of an input fibre connected to a planar guide sending the light in the direction of an engraved diffraction grating.
In the case of a grating with a circular exit field, the incident light and the diffracted light, refocused at different angular incidences, are located on the Rowland circle.
In the case of a plane field grating (see document (6)), the stigmatic points dispersed in terms of wavelength are aligned on a straight line orthogonal to the reflected field.
As the grating functions by reflection, it is metallised.
The engraving profile of the grating can consist of a set of ellipses, as taught by document (7).
The diffracted beam is refocused on monomode guides having for example a mode diameter of 9 &mgr;m and a spacing of 16 &mgr;m, as taught by document (6), or on photodiodes forming an array as taught by document (5).
The network preferably functions at a high degree of diffraction (ranging from 4 in document (6) to 50 in document (5)) with the intention of effecting a high-density demultiplexing (for telecommunications).
The second technique is based on putting several interferometers of the Mach-Zehnder type in series, which are all unbalanced with regard to their optical paths, with a characteristic imbalance value.
In this regard, document (8) should be consulted.
For a demultiplexer with four channels, two interferometers are for example used, whose imbalances are equal respectively to &Dgr;L
1
and &Dgr;L
2
=&Dgr;L
1
+&lgr;/
4N
, and a third interferometer whose imbalance &Dgr;L
3
is equal to 2.&Dgr;L
1
(typically around 50 &mgr;m to 100 &mgr;m) in order to obtain an inter-channel separation of 7.5 nm to 1,550 nm, N being effective index of the mode.
The third technique uses an optical phase-array which consists of a set of parallel monomode dephasing guides connecting two plane input and output guides by means of circular interfaces.
In this regard, document (9) should be consulted.
Input guides and output guides are connected to the other circular interfaces of the plane guides.
The light injected by any one of the input guides lies in the input guide plane and covers all the dephasing guides situated at the interface.
From one dephasing guide to another, there is a constant difference in length so that the light beams emerging from the output guide plane interfere as if they were reflected by an inclined concave diffraction grating.
The offset in the optical path caused by the dephasing guides produces the same effect as an inclination of the wave front with respect to the interface.
The PHASAR, which functions by transmission, must behave like a grating with a concave diffraction of a very high degree (approximately 50 to 100) and with a high multiplexing capacity.
In this regard, document (10) should be consulted.
The greater the number of dephasing guides, the better the spectral resolution.
For example, in document (11), 60 dephasing guides are used.
In order to cancel out the polarisation dependency of this circuit, one possible solution is to insert a half-wave plate in the middle of the optical circuit formed by the dephasing guides.
The fourth technique uses balanced Mach-Zehnder interferometers or 100% couplers with a Bragg grating photo-inscribed in an identical fashion on the two arms. The light is injected at the port
1
and emitted at the port
3
(100% coupling) for all the distinct wavelengths of the Bragg wavelength; the light at the Bragg wavelength is reflected selectively at the port
2
. In this regard, document (29) should be consulted, from which all the references in the description of the fourth technique, given in the present section, are derived.
Three kinds of material are used for producing the components used in the above four techniques:
glass, silica on silicon and semiconductors of the InP type.
In particular, engraved gratings and PHASARs have been produced in integrated optics on silicon whilst demultiplexers with interferometers have been produced in integrated optics on silicon and on glass.
The components by means of which these four known techniques are implemented are only demultiplexers which serve merely to separate different spectral contributions.
These components do not make it possible to determine the Bragg wavelengths directly with the required precision.
In addition, these techniques require a compromise between cross-talk and spectral space occupied.
Cross-talk, that is to say the light coupling between the outputs, must be minimised since it contributes to falsifying the wavelength measurements.
Typically, a cross-talk of −25 dB to −30 dB is sought and the spectral occupation is derived accordingly.
In the case of a diffraction grating in integrated optics on silicon, the light coupling between the outputs is caused by the diffusion in the guide (because of engraving imperfections) and by the coupling between the output guides when these are two close together.
Between the centres of two adjacent spectral channels, the cross-talk is typically around −20 dB to −35 dB whilst it is no more than −10 dB to −15 dB at the intersection of the transfer functions corresponding to these channels (at half the spectral period).
In this case, a spectral space unoccupied by a transducer is therefore necessary so as to guarantee the minimum of cross-talk necessary.
Typically, this cross-talk is achieved with a spectral occupation of around 0.8 nm on 2 nm of period (see documents (5) and (6)).
The characteristics of cross-talk and occupied space of the PHASAR and engraved grating are equivalent.
Typically, a cross-talk better than −30 dB is achieved in the case of document (11), for a spectral occupation of 0.8 nm and a period of 2 nm, with 60 dephasing guides and a degree of diffraction equal to 60.
In the case of Mach-Zehnder interferometers, the cross-talk depends on the accuracy of adjustment of the separation couplers (3 dB couplers).
By way of example, in document (8) a demultiplexer is described which consists of three interferometers formed from 3.1 dB couplers (instead of 3 dB) and which is characterised by a cross-talk of approximately −20 dB.
I
Ferdinand Pierre
Grand Gilles
Magne Sylvain
Commissariat a l'Energie Atomique
Font Frank G.
Harness & Dickey & Pierce P.L.C.
Natividad Phil
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