Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-12-10
2001-11-13
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S015000, C385S016000, C385S017000, C385S043000, C385S014000, C385S129000, C385S130000, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06317536
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a phased array of optical waveguides for wavelength division multiplexing and demultiplexing of optical signals. In particular, the novel optical waveguide multiplexer/demultiplexer includes an array of waveguides which are configured to efficiently collect light from the phased array, at a spatial location relatively close to the phased array, while minimizing coupling among the waveguides of the output array.
Wavelength division multiplexing and demultiplexing (WDMD) have gained in importance as system data rate and system capacity requirements have increased. Usually, the two types of signal processing, i.e., multiplexing and demultiplexing are discussed together because many embodiments of WDMD devices are reciprocal. That is, multiplexed signals which are transmitted through a WDMD device in a first direction are separated into component wavelengths, each propagating in a pre-selected one waveguide. Alternatively, signals of different wavelengths propagating in separate waveguides are multiplexed when transmitted through the device in a direction opposite the first direction. This reciprocity of function is found in the devices disclosed and described herein. Thus it will be understood throughout this document that a discussion of the multiplexing function includes the reciprocal demultiplexing function. Choosing one or the other function for discussion removes the need for discussion of its obvious reciprocal, thus simplifying the description without sacrificing completeness.
The parts or segments of a WDMD device typically include a phase shifting array of waveguides which can sort signals by wavelength, one or more waveguides for transmitting or receiving multiplexed signals, and a waveguide array for transmitting or receiving a plurality of demultiplexed signals. The transmitting or receiving segments must be properly coupled to the phase shifting array. In particular, the one or more waveguides which launch multiplexed signals into the phase shifting array must be so coupled to the phase shifting array as to uniformly distribute signal intensity onto the ends of the waveguides of the array. It is understood that the intensity of the light in a plane does have some variation. In this context, the term uniform means that the shape the intensity distribution of light may have is not such that unacceptable crosstalk appears in the multiplexed or demultiplexed signals. In addition, proper demultiplexing of signals occurs when light intensity from each of the waveguides of the phase shifting array is distributed uniformly onto each waveguide of the array which propagates a demultiplexed signal. The multiplexing and demultiplexing functions may be described using a different conceptual model. For example, one may speak of the wavefront exiting the phase shifting array as being tilted at an angle which provides for constructive interference of the phase shifted light. A different tilt angle corresponds to different light wavelengths. The results, i.e., the structure of a WDMD is, of course, not effected by the choice of descriptive model. The alternative description is mentioned here to avoid confusion between this application and other publications or previously filed applications.
The characteristics of a satisfactory WDMD device are:
minimum signal attenuation within the device;
separation of signals by wavelength sufficient to meet a pre-selected bit error rate ceiling;
small size; and,
ease of manufacture which can translate into lower cost. A particular one of these characteristics may gain in importance depending upon the requirements of the system in which the device is used.
There is a strong need within the telecommunications industry for WDMD designs which provide adequate signal separation and low signal attenuation in a device which is relatively small in size and simple to manufacture. U.S. patent application Ser. No. 08/586,134, filed Jan. 11, 1996, discloses a WDMD device which simplifies the spatial configuration of the phase shifting array. The phase shifting function is performed by an array of waveguides each of which has a unique propagation constant, thus eliminating the need for a process in which the waveguides are arranged in a pattern which allows the waveguides to have unique lengths. Because adjacent waveguides of the phase shifting array have different propagation constants, cross talk among the waveguides of the array is small and thus a source of error is removed from the WDMD. Stated differently, because neighboring waveguides have different propagation constants, there is no need to adjust the geometry of the waveguides at their input or output ends to avoid cross talk error. In particular, the spacing between neighboring waveguides in the phase shifting array may be brought closer together, reducing the dimensions of the WDMD without effecting performance.
The novel work disclosed herein is an additional step toward decreasing WDMD component size without negatively effecting either waveguide cross talk or signal attenuation in the device.
DEFINITIONS
Waveguide—An elongated structure for containing and transmitting light signals, the structure made of materials which are substantially transparent to light signals. To contain and transmit light a generally central portion of the structure has a maximum index of refraction greater than the maximum index of refraction of a layer which surrounds and is in contact with at least a part of the central portion.
Refractive index profile—The central portion of the waveguide and sometimes the surrounding layer may be characterized by the index of refraction which occurs at points along a reference line in the waveguide. The reference is typically a diameter in the case of a circular waveguide or a side to side dimension for waveguides of general geometric shape.
Refractive index &Dgr;%—A waveguide region having a maximum refractive index n
1
has a &Dgr;% relative to another region of the waveguide having a maximum refractive index n
c
defined by the relation &Dgr;%=(n
1
2
−n
c
2
)/2n
1
2
. The region having maximum index n
c
is usually located in the surropunding or clad layer of the waveguide.
Propagation Constant—This is a constant found from a scalar wave equation descriptive of the electromagnetic fields which are supported by a particular waveguide geometry. An example of a scalar wave equation is found in “Optical and Quantum Electronics”, J. P. Meunier et al., 15, (1983), pp. 77-85.
SUMMARY OF THE INVENTION
The novel optical component of this application meets the need for small, low cross talk, high performance WDMD devices by decreasing the distance between input or output waveguides and the phase shifting waveguide array. As described in U.S. patent application Ser. No. 081586,134 cited above, the WDMD device comprises at least one input waveguide which transmits a multiplexed light signal, a phase shifting array of N waveguides which provides a distinct optical path length for light signals of different wavelengths, and an output array of M waveguides each of which transmits a single signal having a pre-selected wavelength. The number, N, of waveguides in the phase shifting array is equal to or greater than the number of wavelengths present in the multiplexed signal. That is, the integer N is in general much larger than the integer M. The greater the number of distinct paths in the phase shifting array, the better, in general, is the signal to noise ratio or crosstalk among the multiplexed or demultiplexed signals. The at least one input waveguide and the output array of waveguides are optically coupled to the respective first and second ends of the N waveguides of the phase shifting array. Although the terms input and output have been applied to the waveguides coupled to the phase shifting array, it will be understood that these waveguides may either transmit signals to or receive signals from the phase shifting array depending upon whether the WDMD is being used as a multiplexer (the phase shifting array receives a p
Bhagavatula Venekata A.
Nolan Daniel A.
Chervenak William J.
Corning Incorporated
Healy Brian
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