Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1998-04-30
2001-06-05
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S014000, C385S027000, C385S046000
Reexamination Certificate
active
06243514
ABSTRACT:
BACKGROUND TO THE INVENTION
Optical Wavelength Division Multiplexed (WDM) systems ideally require passive optical wavelength multiplexers and demultiplexers which have isolated pass-bands which are flat-topped so as to allow a measure of tolerance in the spectral positioning of the individual signals of the WDM system within these pass-bands. One method of multiplexing or demultiplexing channels in an optical WDM system relies upon the use of multilayer dielectric interference filters. Another relies upon Bragg reflection effects created in optical fibres. A third method, the method with which the present invention is particularly concerned, relies upon diffraction grating effects.
One form that such a diffraction grating can take for wavelength multiplexing/demultiplexing is the form described in EP 0 254 453, which also refers, with particular reference to its FIG. 5, to the possibility of having a tandem arrangement of two diffraction gratings arranged to provide a combined intensity transfer function that is the product of the intensity transfer function of its component diffraction grating 40 with that of its component diffraction grating 42.
An alternative form that such a diffraction grating can take is an optical waveguide grating that includes a set of optical waveguides in side-by-side array, each extending from one end of the array to the other, and being of uniformly incrementally greater optical path length from the shortest at one side of the array to the longest at the other. Such an optical grating constitutes a component of the multiplexer described by C Dragone et al., ‘Integrated Optics N×N Multiplexer on Silicon’, IEEE Photonics Technology Letters, Vol. 3, No. 10, Oct. 1991, pages 896-9. Referring to
FIG. 1
, the basic components of a 4-port version of such a multiplexer comprise an optical waveguide grating, indicated generally at
10
, where two ends are optically coupled by radiative stars, indicated schematically at
11
and
12
, respectively with input and output sets of waveguides
13
and
14
. Monochromatic light launched into one of the waveguides of set
13
spreads out in radiative star
11
to illuminate the input ends of all the waveguides of the grating
10
. At the far end of the grating
10
the field components of the emergent light interfere coherently in the far-field to produce a single bright spot at the far side of the radiative star
12
. Scanning the wavelength of the light causes a slip in the phase relationship of these field components, with the result that the bright spot traverses the inboard ends of the output set of waveguides
14
linearly with wavelengths as depicted at
15
. If the mode size of the waveguides
14
is well matched with the size of the bright spot, then efficient coupling occurs at each of the wavelengths at which the bright spot precisely registers with one of those waveguides
14
. Either side of these specific wavelengths the power falls off in a typically Gaussian manner as depicted at
15
. While this may allow acceptable extinction to be achieved between channels, it is far from the ideal of a flat-topped response.
A tandem arrangement of this alternative form of diffraction grating can also be constructed, an example of such an arrangement being described in EP 0 591 042 with particular reference to its FIG.
3
. This tandem arrangement similarly provides a combined intensity transfer function that is the product of the intensity transfer functions of its two component diffraction gratings. The response of this tandem arrangement also provides a typically Gaussian fall off in power that is similarly far from the ideal of a flat-topped response.
SUMMARY OF THE INVENTION
The present invention is directed to the provision of an optical multiplexer/demultiplexer that achieves a response that is more nearly flat-topped without introducing excessive insertion loss. In particular, it is directed to an improvement of the type of multiplexer/demultiplexer described in the specification of PCT Application GB 97/02051 corresponding to U.S. Pat. application No. 09/194,004, now U.S. Pat. No. 6,144,783, with particular reference to its
FIG. 6
, to which specification attention is specifically directed and its teachings incorporated herein by reference.
One of the features limiting the performance of such a multiplexer/demultiplexer is departures from uniformity within the area of the wafer from which the device is constructed, particularly departures from uniformity in the thickness and composition of the layers defining the optical waveguides.
An object of the present invention is to reduce the effects of such departures upon the performance so as to enable higher performance multiplexer/demultiplexer devices to be constructed from a given standard of wafer uniformity.
According to the present invention there is provided an optical multiplexer/demultiplexer for the multiplexing/demultiplexing of optical signal channels at a substantially uniform optical frequency spacing, which multiplexer/demultiplexer includes a set of input/output ports optically coupled with an output/input port via a tandem arrangement of first and second optical waveguide diffraction gratings that provide multiple optical paths from each member of the set of input-output ports to the output/input port via different grating elements of the gratings,
wherein the difference in optical path length occasioned by paths via adjacent optical waveguide elements of the first grating is greater than that occasioned by paths via adjacent optical waveguide elements of the second grating,
wherein said difference in optical path length defines for its associated grating a frequency range, the Free Spectral Range, being the frequency range over which said optical path length difference produces a phase difference whose value ranges over 2&pgr;,
wherein the Free Spectral Range of the first diffraction grating is matched with the optical frequency spacing of the optical signal channels,
wherein the Free Spectral Range of the second diffraction grating is at least as great as the sum formed by the addition of the difference in frequency between adjacent frequency channels of the multiplexer/demultiplexer to the difference in frequency between the highest and lowest frequency channels of the multiplexer/demultiplexer,
wherein the portion of the optical coupling between the set of input/output ports and the output/input port that extends between the first and second diffraction gratings couples spatial information between the two gratings in addition to intensity information,
wherein the optical waveguide elements of each diffraction grating consist of a plurality of optical waveguides extending in side-by-side relationship in a set of arcuate optical paths and wherein the arcuate optical paths of the optical waveguides of one of said first and second diffraction gratings is configured to embrace the set of arcuate paths of the optical waveguides of the other of said first and second gratings.
Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, the drawings and the claims.
REFERENCES:
patent: 5450512 (1995-09-01), Asakura
patent: 5488680 (1996-01-01), Dragone
patent: 5745616 (1998-04-01), Zirngibl
patent: 5926587 (1999-07-01), Chen et al.
patent: 0254453A2 (1988-01-01), None
patent: 0591042A1 (1994-04-01), None
patent: 2222891A (1990-03-01), None
patent: WO98/04944 (1998-02-01), None
Ishida, et al., “Loss-Imbalance Equalization in Arrayed-Waveguide-Grating (AWG) Multiplexer Cascades,” 8217 Journal of Lightwave Technology 13 (1995) Jun., No. 6, New York, NY US, pp. 1155-1163.
Dragone et al., “Integrated Optics N×N Multiplexer on Silicon,” IEEE Photonics Technology Letters, vol. 3, No. 10, Oct. 1991, pp. 896-899.
Tsai, et al., “Multiband Wavelength-Division Demultiplexing with a Cascaded Substrate-mode Grating Structure,” Applied Optics, vol. 34, No. 25, Sep. 1, 1995, pp. 5582-5588.
Bovernick Rodney
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Stahl Michael J.
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