Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1997-12-22
2001-01-30
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S015000, C385S039000
Reexamination Certificate
active
06181848
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the technical field of light guides.
The present invention proposes an optical multiplexer and/or demultiplexer usable in the fields of telecommunications and of remote sensors.
The device of the invention may constitute an optical receiver for a direct-detect reception circuit, e.g. for optical-link systems. Such a device may be advantageous for separating signals of different wavelengths arriving over a common fiber so as to distribute them to different users, each receiving a specific wavelength.
The device of the invention may also be coupled (in hybrid or integrated manner) with transmission lasers or optical amplifiers.
The device of the invention is particularly advantageous in switching, cross-connection, and local loop functions. It may be also used in optical interconnections between, for example, highly integrated high-speed electronic chips, or between computers, or else within a computer.
BACKGROUND OF THE INVENTION
Various optical multiplexer and/or demultiplexer structures have been described in the literature.
For example, reference may be made to the following documents:
[1] ECOC 96, “Extremely compact 1.31 &mgr;m-1.55 &mgr;m phased array duplexer on InP with −30 dB crosstalk over 100 nm”, R. Mestric et al., which describes a duplexer having two wavelengths: 1.3 &mgr;m and 1.55 &mgr;m; and
[2] ECOC 96, “Compact low loss 8×10 GHz polarisation independent WDM receiver”, C.A.M. Steenbergen et al., which describes an integrated demultiplexer having detectors, and proposes to use two waveguide structures to solve the problem of polarisation.
The (de)multiplexers that are generally used today are of the type having an array of waveguides, as shown in accompanying FIG.
1
. Such a demultiplexer is made up of two plane optical surfaces
10
and
12
separated by an array
14
of waveguides. The difference in path length &Dgr;L between two consecutive waveguides
14
is constant, and it makes it possible to perform phase-shifting, and therefore demultiplexing. In
FIG. 1
, P
1
represents the equiphase plane for an input signal while P
2
represents the equiphase plane for a signal coming from one of the outlets.
Such known multiplexers/demultiplexers have already done good service. Unfortunately, they are not entirely satisfactory.
The main drawback with such known devices is their high degree of temperature dependency which is intrinsic to the material used.
The refractive index of the material varies as a function of temperature, and so the path-length difference between two consecutive waveguides changes with changing temperature, thereby causing the peaks to be offset relative to the output waveguides.
SiO
2
has a coefficient of variation of refractive index as a function of temperature that is small (giving rise to an offset of about 1 nm per 100° C.). Unfortunately, that material offers only limited possibilities as regards monolithic integration (integration on the same material) of devices such as lasers, optical amplifiers, or detectors.
Monolithic integration, which enables production costs to be reduced (compared with hybridizing on different materials), is possible on InP. Unfortunately, the refractive index of InP varies considerably with temperature (giving rise to an offset of about 1 nm per 10° C.). The Publication [3] “Polarisation independent 8×8 waveguide grating multiplexer on InP”, Electronics Letters, Jan. 21, 1993, vol. 29, No. 2, M. Zirngibl et al., gives a variation of 1.5 nm per 10° C. That heavy dependency requires the temperature to be controlled by means external to the device, e.g. in the form a Peltier-effect element, which increases the cost of implementing the device.
Naturally, for a wide-line demultiplexer with a small number of lines, such as the 2-line duplexer in reference [1] ECOC 96 with lines of 100 nm, temperature does not disturb demultiplexing or disturbs it only slightly. However, for a multi-wavelength application, and lines of 0.65 nm, the temperature instability must be controlled.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a device making it possible to reduce and/or to cancel out the effect of temperature on a (de)multiplexing device without it being necessary to take any action external to the device, and regardless of the material used (SiO
2
, InP, polymer, Al
2
O
3
, LiNbO
3
, etc.)
An additional object of the present invention is to provide a device organized either to reduce the effect of temperature, or to increase it to offer means of controlling the signal, by means of temperature on the desired channel. In this way, the invention makes it possible to offer greater tolerance as regards the geometrical configuration of the device.
By cancelling out the effect of temperature, it is no longer necessary to provide an external regulation device.
The invention achieves the above-mentioned objects by means of a device forming an optical multiplexer and/or demultiplexer of the type including two zones that are not guided laterally, that act as planar lenses, and that are separated by an array of waveguides having controlled differences in length, wherein each waveguide comprises at least two spans placed in series and having respective lengths and refractive indices that are suitable for controlling the positions at which various wavelengths are output from the device, which positions depend on fluctuations in temperature.
In a first embodiment of the present invention, the refractive indices of the materials of both spans of each waveguide vary as a function of temperature in the same direction, and the two spans have respective lengths that vary in opposite directions from one guide to another.
In a second embodiment of the present invention, the refractive indices of the materials of the two spans of each guide vary as a function of temperature in opposite directions, and the two spans have respective lengths that vary in the same direction from one waveguide to another.
The present invention thus proposes a (de)multiplexer that is either temperature independent, or that has temperature dependence (d&lgr;/dT) that is chosen and desired.
REFERENCES:
patent: 5341444 (1994-08-01), Henry et al.
patent: 5623571 (1997-04-01), Chou et al.
patent: 5799118 (1998-08-01), Ogusu et al.
patent: 5838844 (1998-11-01), Van Dam et al.
patent: 5982960 (1999-11-01), Akiba et al.
patent: 6049640 (2000-04-01), Doerr
patent: 0662621 (1995-07-01), None
patent: 2732777 (1996-10-01), None
Patent Abstracts of Japan; vol. 018, No. 146 (p. 1707) Mar. 10, 1994.
Rigny A et al. Taper Assisted Polarisation Compensation in Efficiently Fibre Coupled Inp Demultiplexer Metal Finishing; vol. 32, No. 20, Sep. 26, 1996.
Bissessur H at al. “Extremely Small Polarization Independent Phased Array Demultiplexers on INP” vol. 8, No. 4 Apr. 1, 1996.
Bruno Adrien
Rigny Arnaud
Blakely & Sokoloff, Taylor & Zafman
France Telecom
Lee John D.
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