Optical waveguides – Integrated optical circuit
Reexamination Certificate
2003-04-16
2004-06-01
Healy, Brian M. (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S037000, C385S028000, C385S031000, C385S038000, C385S088000, C359S346000, C359S337200, C398S082000, C398S085000
Reexamination Certificate
active
06744941
ABSTRACT:
The present invention relates to a wavelength tunable optical filter for, rapidly and over a broad spectral band. The use of such optical filters is particularly advantageous in the field of optical telecommunications which use wavelength multiplexing, normally referred to as “WDM” (the acronym for the expression “Wavelength Division Multiplexing”) or “optical frequency division multiplexing”.
Devices for selecting one or more optical frequencies from an input WDM signal are already known. However, none of the conventional tunable filter technologies based for example on the variation of a cavity length by moving a mechanical piece makes it possible to obtain switching times of less than a millisecond or at best a microsecond.
In fact, in the context of rapid frequency selection for switching optical packets requiring switching speeds situated in the nanosecond range, the only solution existing at the present time is based on frequency selection devices which use optical amplifiers utilized as open or closed optical gates.
A particular architecture of such a frequency selection device is described in U.S. patent application Ser. No. 2003/0002102, published on Jan. 2, 2003. In this document a description is in particular given of a device for selecting frequency division multiplex channels, an example of functioning of which, for a number of channels equal to N, respectively f
1
to f
N
, is shown in FIG.
1
.
Thus the channel selector according to D
1
comprises a first 1×n cyclic demultiplexer Demux for demultiplexing the input multiplex as n interleaved frequency combs each consisting of m channels. The demultiplexer used is a 1 to n de-interleaving multiplexer based for example on filters of the Mach-Zehnder type, on etched gratings or on waveguide gratings of the AWG type (standing for “Array Waveguide Grating”). A second m×m cyclic demultiplexer Demux′ used as a router and consisting for example of an etched grating or a waveguide grating is provided for separating the channels of the interleaved combs. This second cyclic demultiplexer Demux′ comprises n input ports connected respectively to the output ports of the first demultiplexer Demux by means of a first array composed of n optical switches Ii, with i between 1 and n. Each of the optical switches Ii composing the first array inserted between the two demultiplexers Demux and Demux′ is advantageously formed by an optical amplifier. The frequency selector according to D
1
also comprises a first m×m cyclic multiplexer Mux′ used as a router, the m input ports of which are connected respectively to the m output ports of the second m×m cyclic demultiplexer Demux′ by means of a second array composed of m optical switches (formed by optical amplifiers) Ij, with j between 1 and m. The m×m cyclic multiplexer Mux′ is finally cascaded with a second n×1 cyclic multiplexer Mux so as to recover the selected channel or channels on a single output port OF of the device.
In fact, depending on whether the optical switches composing the arrays of optical switches inserted respectively between the 1×n multiplexer Demux and the m×m demultiplexer Demux′ and between the m×m demultiplexer Demux′ and the m×m multiplexer Mux′ are switched on or off, it is possible to select and route the required frequency fi (1≦i≦N) of the input multiplex intended for the single output port OF of the device.
In order to obtain the required functioning of the device, the numbers n and m must be prime with each other (and N=n×m, with n<m) and the components Demux, Demux′, Mux and Mux′ must be designed to function with the same spectral spacing between channels &Dgr;f.
The configuration of the frequency selector illustrated in
FIG. 1
therefore makes it possible to implement the selection of the required frequency amongst N channels by means of the activation of a first optical amplifier amongst the n making up the first array and a second optical amplifier amongst the m making up the second array.
Thus, in the frequency selection device of
FIG. 1
, the values of the frequencies processed are predetermined by the multiplexers and demultiplexers used, which ensures stability in terms of frequency, no tuning having to be carried out.
In addition, the device of
FIG. 1
ensures the rapid selection of frequency. This is because the switching elements used, typically optical amplifiers, are elements which are intrinsically rapid since switching times of around a nanosecond can be achieved by such elements.
However, this solution has drawbacks. This is because, even if the architecture of the frequency selector according to
FIG. 1
makes it possible to dispense with the need for using as many active elements, in this case optical amplifiers, as there are channels to be processed, it nevertheless involves using a consequent number of active elements. The device in
FIG. 1
consequently occupies a large amount of space (for example a surface area of around 4.5×4.5 mm
2
for a conventional 16-channel frequency selector).
Equally it is necessary to provide as many electronic control elements as there are active elements. This is because, the activated optical amplifier not always being the same according to the frequency which it is wished to select, it is necessary to integrate in the device an electronic control chip for each of the optical amplifiers used. This involves in addition providing the fitting of many electrical connections and the necessary power supplies. Thus the use of the component in
FIG. 1
will require a complex electronic card of large size.
Consequently one object of the present invention is to mitigate the above-mentioned drawbacks by proposing a rapidly tunable optical filter, that is to say one with very short tuning times, over a wide range of optical frequencies, in order to precisely obtain any one of the frequencies in the ITU (International Telecommunication Union) grid with a small spacing between consecutive frequencies of 50 or 100 GHz, and this by acting on only one control quantity, thus affording simplified control electronics.
The invention aims in particular to propose an optical filter combining the advantages set out above, whilst being very compact.
To this end, the invention makes provision for using first of all a Fabry-Perot cavity, that is to say a region delimited by two opposite reflective elements which are not selective in terms of wavelength and whose resonant modes are adjustable by electro-optical effect. Typically, an electrical field is applied to a PIN waveguide junction whose effective index is modified, according to the value of the electrical field, by virtue of a Franz-Keldish electro-optical effect or a quantum confined Stark electro-optical effect. The optical length of the Fabry-Perot cavity can thus be adjusted so as to be able to obtain the optical frequency values corresponding to the resonant modes of the Fabry-Perot cavity over the entire adjustment range &Dgr;f
T
required.
The Fabry-Perot cavity is optically coupled to an external reflector having a reflectivity which is selective in terms of frequency. This reflector consists for example of a sampled Bragg grating waveguide (“SGW”, standing for “Sampled Grating Waveguide”). The sampled Bragg grating waveguide can be photo-written in a fiber (“SFBG”, standing for “Sampled Fiber Bragg Grating”), but any other waveguide can be used, in particular silica planar circuits or devices based on polymers. The sampled grating is designed so as to have N transmission peaks over the entire adjustment range mentioned above.
Such a device therefore comprises a first Fabry-Perot cavity with an operating mode adjustable by an electro-optical effect which can be controlled, coupled to a second external cavity.
Its operating principle is as follows. The optical frequency able to be transmitted by this set of two cavities coupled to each other is the frequency of the resonant mode of the Fabry-Perot cavity which be
Jacquet Joel
Khalfallah Sabry
Avanex Corporation
Healy Brian M.
Moser, Patterson & Sheridan L.L.P.
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