Optical waveguides – With optical coupler
Reexamination Certificate
2001-04-20
2004-09-14
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
C385S016000, C385S017000, C385S039000
Reexamination Certificate
active
06792172
ABSTRACT:
BACKGROUND OF INVENTION
Multimode interference (MMI) based devices have become important components within integrated optical circuits. The operation of optical MMI devices is based on the self-imaging principle of multimode waveguides wherein an input field profile is reproduced in multiple “images” at periodic intervals along the propagation axis of the waveguide such that the MMI device can function, for example as a power splitter or other device. The basic well known structure of an MMI coupler requires a relatively wide multimode waveguide region designed to support a large number of modes. In order for light to enter and exit from the relatively wide multimode waveguide region, a number of relatively narrow access waveguides are placed at the beginning and end of the MMI region. Devices configured as such are generally referred to as N×M MMI couplers, where N and M are the respective number of input and output waveguides (collectively known as access waveguides). A review of MMI devices may be found in
Soldano and Pennings, Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications
, Journal of Lightwave Technology, Vol. 13, No. 4, April 1995.
Integrated optical circuits which currently require N×M power splitters include ring lasers, Mach-Zehnder interferometers and optical switches. See, P. A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit,
Optical Bandwidth and Fabrication Tolerances of Multimode Interference Couplers
, Journal of Lightwave Technology, Vol. 12, pp. 1004-1009, 1994; L. B. Soldano and E. C. M. Pennings,
Optical Multi-mode Interference Devices Based On Self-imaging: Principles and Applications
, Journal of Lightwave Technology, Vol. 13, pp. 615-627, 1995; and L. H. Spiekman, Y. S. Oei, E. G. Metaal, F. H. Groen, I. Moerman, and M. K. Smit,
Extremely Small Multimode Interference Couplers and Ultrashort Bends on InP By Deep Etching
, IEEE Photon. Technol. Lett., Vol. 6, pp. 1008-1010, 1994. Perhaps the most important current MMI structure is the 2×2 coupler due to its applicability to all of these devices, the most common being the “3-dB” power splitter which will separate an input signal into two output signals of approximately equal power. Other MMI based devices include mode width expanders or spot size converters which are used to magnify or diminish the intensity distribution of the periodic images within a multimode region. See, e.g., Simpson et al.,
Mode
-
Width Expanders Based on the Self Imaging Effect in Tapered Multimode GaAs/AlGaAs Waveguides
, Proc. 7th Eur. Conf. on Int. Opt., 1995 at 29. Applications may be expected to multiply, for example, due to improvements in telecommunications networks which require flexibility, increased capacity and reconfigurability, all of which can be enhanced by the use of photonic integrated circuits for optical communications. To date, known MMI based devices that maintain approximately even power distribution across the output images have been configured using straight sidewalls to confine the MMI region.
To better use these MMI devices for large scale photonic integrated circuit production, there is a desire to make these devices with smaller dimensions and improved fabrication tolerances. Currently, for example, “straight” 3-dB MMI couplers have decreased to the “extremely small” regime with typical limits in length of a few hundred microns. Length scaling of MMI devices is most readily done via control of the width of the MMI region, W
MMI
. A straight 3-dB coupler has a device length, L
MMI
, proportional to the square of the width of the MMI device, i.e., L
MMI
∝W
2
MMI
.
The width, W
MMI
, for a straight 2×2 coupler must be greater than or approximately equal to the combined width of the two input waveguides plus the width of the MMI region between the access waveguides. This total width has practical limitations that constrain further reducing the device size/length. At least the following limitations exist.
First, the width of the region between sets of access waveguides, which in turn restricts the lower limit of W
MMI
, cannot be smaller than manufacturing tolerances allow. Manufacturing defects such as lithographic gap fill-in can occur when the devices are patterned on a semiconductor wafer. These defects can be caused by optical (or electronic) proximity effects encountered during the photolithographic (or electron-beam lithographic) patterning, poor image contrast of the optical system (i.e., poor focusing, or aerial image contrast), or inefficient chemical process development. These defects limit the width of the region between access waveguides for practical applications. See, e.g.,
Microlithography Process Technology for IC Fabrication
, D. Elliott, New York:McGraw-Hill Book Co., 1986;
VLS
1
Fabrication Principles,
2nd ed., S. Ghandi, New York:John Wiley & Sons, Inc., 1994.
Second, if access waveguides are too closely spaced there may occur unwanted power transfer between waveguides. This parasitic optical phenomena is due to the effect known as optical coupling, and further serves to limit the reduction of space between waveguides, and thus W
MMI
, in practical applications. Optical or directional coupling is examined in A. Yariv,
Optical Electronics,
4th ed., New York:Holt, Reinhard, and Winston, 1991.
Third, while it is possible to reduce W
MMI
by shrinking the width of the access waveguides while maintaining the distance between waveguides constant, this may cause unacceptable signal loss. For example, losses due to etch induced roughness of the walls of waveguides causes proportionately greater loss in thinner as opposed to thicker waveguides. Thus, if the access waveguides are made too thin, unacceptable transmission losses may occur. See, R. Deri, E. Kapon, and L. Schiavone,
Sattering in Low-Loss GaAs/AlGaAs Rib Waveguides
, Appl. Phys. Lett., Vol. 51, pp. 789-91, 1987; M. Stern, P. Kendall, R. Hewson-Browne, P. Robson,
Scattering Loss From Rough Sidewalls in Semiconductor Rib Waveguides
, Electronics Letters, Vol. 25, pp. 1231-2, 1989.
Fourth, because the manufacturing “width tolerance” of the MMI region, &Dgr;W, is proportional to the square of the width of the access waveguides, W
WG
, if the access waveguides are too thin, the required manufacturing “width tolerance” will be impractical or unachievable by commercial manufacturing processes. The “width tolerance”, &Dgr;W, is the variation in the designed width of the MMI region, caused by manufacturing errors, that can be tolerated and still preserve the imaging functionality of the MMI device. Because &Dgr;W∝W
2
WG
, it is desirable to have wider access waveguides to allow for larger manufacturing tolerances. See, P. Besse, M. Bachmann, H. Melchior, L. Soldano, and M. Smit,
Optical Bandwidth and Fabrication Tolerances of Multimode Interference Couplers
, Journal of Lightwave Technology, Vol. 12, pp. 1004-9, 1994. However, in conventional “straight” MMI couplers, wider access waveguides can cause the entire device width to increase, thereby increasing the device size/length, in that L
MMI
∝W
2
MMI
.
Accordingly, it is an object of the present invention to provide for N×M MMI couplers that approximately maintain the power splitting functionality across output images of MMI devices with straight line sidewalls that overcome the limitations of the prior art (1) by allowing for the manufacture of smaller MMI devices by causing the average W
MMI
to decrease; (2) by allowing for access waveguides of MMI devices to remain sufficiently spaced to avoid lithographic gap fill-in or optical coupling while reducing the average W
MMI
and thus the overall size/length of the coupler, and (3) by allowing for the use of wider access waveguides without increasing the average W
MMI
and the corresponding length, L
MMI
, thereby decreasing transmission losses and increasing manufacturing width tolerances, &Dgr;W.
Additional uses and advantages of the present invention will be apparent to those skilled in the art upon review of the detailed desc
Levy David S.
Osgood, Jr. Richard M.
Scarmozzino Robert
Baker & Botts LLP
Font Frank G.
Kianni Kevin C.
The Trustees of Columbia University of the City of New York
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