Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-05-25
2001-07-17
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000, C359S199200
Reexamination Certificate
active
06263134
ABSTRACT:
The present invention relates to a fiber-optic wavelength multiplexer-demultiplexer or router which can be used as a component in fiber-optic communications applications.
Such multiplexer-demultiplexers are already known and have been described, then progressively developed, in particular in French patents FR-2,543,768, FR-2,519,148, FR-2,479,981, FR-2,496,260 and in European patent EP-0,196,963 which are marketed under the brand name “STIMAX”.
The latter document more particularly concerns a multiplexer-demultiplexer in which the entrance fibers and exit fibers are positioned in the immediate vicinity of the focus of a concave mirror. In this way, the divergent light flux received by the entrance fibers is transformed by the concave mirror into parallel light beams which are addressed on a planar diffraction grating, which in return directs the parallel beams to the concave mirror which focuses them on the ends of exit fibers. The grating thus operates according to the Littrow configuration.
In such a device, the following relationship is established to an approximate degree:
2a sin &bgr;=k&lgr;
where a is the pitch of the grating, &bgr; is the Littrow angle, i.e. the angle formed by the light beam (incident and refracted) with respect to the normal of the grating plane, k is the order of refraction, &lgr; the wavelength and n the refractive index of the medium in contact with the grating.
As is known, the angular dispersion d&bgr;/d&lgr; in the region of the Littrow conditions is all the greater as &bgr; is high.
The spectral dispersion dx/d&lgr; is equal to f(d&bgr;/d&lgr;), where f is the focal distance of the reflecting system and x is the direction of dispersion in its focal plane. A high angular dispersion d&bgr;/d&lgr; thus allows to reduce the focal distance f for a constant spectral dispersion dx/d&lgr;.
For a given lateral distance between the fibers, the wavelength separation of the different channels is also conserved for a reduced focal plane f.
However, it is observed that increasing the Littrow angle calls for the grating to be inclined and makes it difficult to position the ends of the optical fibers.
The term fiber-optic multiplexer-demultiplexer is understood here to encompass devices in which the incoming and exiting light waves are propagated by optical fibers.
The invention also concerns fiber-optic routers in which a variable number of incoming optical signals each having a particular wavelength and propagated by spatially distinct optical fibers are addressed on exit optical fibers which may be different in number.
To simplify the description, the inventive device shall be termed multiplexer-demultiplexer, it being clearly understood that it can be a multiplexer, a demultiplexer or a router.
Known devices are globally satisfactory and have made it possible to construct multiplexed links bringing together a large number of paths with low crosstalk.
However, as these devices are being used more extensively, efforts are made to increase their performance and in particular to reduce the spectral gap between two paths (two consecutive wavelengths), while at the same time maintaining devices with reduced dimensions, i.e. with a limited focal length.
The object of the invention is thus to propose a compact multiplexer-demultiplexer with equivalent or improved properties in comparison with those of prior art devices and limited crosstalk.
To this end, the invention concerns a fiber-optic wavelength multiplexer-demultiplexer having an optical axis and comprising a diffraction grating having a dispersion plane, at least one entrance fiber having an exit face; at least one exit fiber having an entrance face, means for optically combining the exit face of the entrance fiber with the input face of the exit fiber ensuring the operation of the grating in a Littrow configuration.
According to the invention, the optical combining means comprise reflecting means allowing a high Littrow angle and ensuring a fold-back of the optical axis.
The combining means are preferably conjugating means.
In the different embodiments each featuring their respective advantages and susceptible of being associated according to many technically feasible combinations:
the optical combining means have an entrance and exit axis in the vicinity of which are located the exit face of the entrance fiber and the entrance face of the exit fiber and the reflecting means are formed by a mirror placed on said axis comprising a central opening allowing the passage of luminous flux directed on that axis;
the mirror is located at the level of the exit face of the entrance fiber;
the dispersive system is a planar diffraction grating, whose plane is parallel to the entrance and exit axis of the optical combining means;
the multiplexer-demultiplexer is composed of a set of associated optical elements, the diffraction grating being fixed on a lateral face of an element having an axis parallel to entrance and exit axis of the optical combining means;
the optical elements are adhered, and preferably glued;
the diffraction grating is fixed on said elements by resilient mounts;
the multiplexer-demultiplexer is composed of a set of associated optical elements and the diffraction grating is formed directly on one of the lateral faces of one of these optical elements;
the diffraction grating is carried on a support which can be expanded such as to cause the pitch of the grating to vary;
the support for the grating is dielectric.
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Arent Fox Kintner Plotkin & Kahn PLC
Connelly-Cushwa Michelle R.
Highwave Optical Technologies
Lee John D.
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