Optical waveguides – Planar optical waveguide
Reexamination Certificate
2001-04-05
2003-09-09
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S014000, C385S015000, C385S045000, C385S130000, C385S131000, C359S199200, C359S199200
Reexamination Certificate
active
06618535
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to photonic band gap devices having waveguides, to optical wavelength demultiplexers, optical filters, and optical switches, particularly for use in optical communications. It also relates to integrated optical circuits, nodes for an optical network, methods of transmitting data using integrated optical circuits, and to software arranged to control the integrated optical circuits, again particularly for use in optical communications.
BACKGROUND TO THE INVENTION
Optical or microwave components making use of the concept of a photonic bandgap (PBG) are known. The definition of a photonic bandgap material is commonly accepted as being a material having a property that electromagnetic radiation, (such as light) of a range of wavelengths, is not permitted to exist when the light is incident on a given part of the material at a range of angles. This can be seen as a dip or gap in the wavelength response, hence the name “bandgap”. It is caused by interference effects arising from periodicity in the structure of the material. Any lattice structure in the material, at the molecular level or higher, can give rise to such bandgaps. Typically the periodic structure is made by sub-micron size patterns created by etching. PBG devices are also called photonic crystals, “crystal” and “lattice” being defined broadly as any material with a repeating structure, whether a molecular crystal lattice or a manufactured repeating structure, or other repeating structure.
In principle, bandgap effects can be seen in one dimensional, two dimensional and three dimensional forms. An example of a one dimensional form is a series of layers of different refractive index, such as dielectric film-based multiplexer or demultiplexer devices, or fibre bragg grating devices. Both are well known. Two dimensional devices have been proposed, in the form of waveguides created in the surface of a crystalline structure. Three dimensional devices can be seen as an extension of the two dimensional devices by making waveguides in any direction of the bulk of such a crystalline structure. The remainder of this document will be concerned with two and three dimensional devices.
Some examples of the range of applications will now be described briefly. It has been shown that perfect photonic crystals have application as reflectors for a wide range of applications ranging from antenna systems to their already current usage as reflective optical coatings. In general these applications assume that the crystal is being used as a complementary device in their application and as such is not an integral part of the device.
As photonic crystals have rejection bands which specifically forbid propagation, they also forbid spontaneous emission. By controlling spontaneous emission, or suppressing it completely, the opportunity to control and enhance the efficiency of optical devices such as light emitting diodes (LEDs), and lasers is enormous. Defects introduced into a photonic crystal have very particular properties and their frequency dependence and quality factor, (Q), amongst other properties, can be engineered to suit their intended application. Within LEDs, defects can be used as emitters with the surrounding PBG lattice suppressing propagation and enhancing the emission characteristics. Defects may be substitution, lacunar, or interstitial types. Substitution may involve changing the optical index, the size, or the shape of an element of the crystal lattice. The lacunar type involves removing an element.
Another application is in waveguides. Lines of contiguous defects in the crystal may form waveguides. They work on the principle that the defects allow a small band of wavelengths to be supported, and transmitted, within the wider band defined by the band gap of the photonic crystal. An advantage of such structures is that the waveguides can have a very small turn radius of the order of several wavelengths of the optical signal which compares favourably with a typical turn radius of the order of several millimetres, or even centimeters which would be required for traditional core-cladding waveguides described above, which rely upon total internal reflection. A second significant difference compared to conventional waveguides is that the range of wavelengths passed can be determined by the defects making up the waveguide, whereas conventionally, separate filters would be required. The compactness and greater potential for integration, arising from both differences, could be commercially significant, particularly for WDM (Wavelength Division Multiplexed) systems having tens or hundreds of wavelengths.
An example of the application of particular photonic crystals as waveguides, by introducing defects to give a band of transmission within the photonic bandgap is shown in Joannopolous, J. D., Meade, R. D., Winn, J. N., Photonic Crystals Molding the Flow of Light, Princeton University Press ISBN 0-691-03744-2, 1995, particularly chapter 5. A photonic crystal is sandwiched between parallel slabs of material having lower refractive index to contain the optical signal by internal reflection. The crystal is formed by providing a lattice in a dielectric material. The lattice is formed by lattice sites at which the dielectric properties of the medium are varied relative to the bulk properties of the dielectric material. The resulting latticed region is essentially opaque to the optical signal. A waveguide can then be formed by discontinuities in the periodic lattice, for example by omitting a contiguous set of lattice sites. This is termed a lacunar type defect. The lattice sites have been made from cylinders of dielectric material, separated by air gaps. Hence omitting a contiguous line of cylinders leaves a waveguide made from air. Bends of 90° have been introduced into such waveguides, but still suffer some consequential insertion loss due to reflection, as shown in Mekis, A., Chen, J. C., Kurland, I., Fan, S., Villeneuve, P. R., Joannopoulos, J. D., “High transmission through sharp bends in photonic crystal waveguides.” Phys. Rev. Lett. 77, 3787 1996, and Temelkuran, B., Ozbay, E., “Experimental demonstration of photonic crystal based waveguides” Appl. Phys. Lett. 74: ,4, 486-488 Jan. 25 1999.
Such devices also have light containment problems in the third or vertical direction and serious device integration, coupling and fragility problems. If a hexagonal lattice is employed rather than a square lattice then reflection at the bend still occurs and once again parasitic loss mechanisms are introduced into the system. By employing the inverse lattice, such that air holes are introduced into a dielectric material, then a similar waveguide can be formed by in-filling a chain of holes or by separating two pieces of similar crystal.
Such devices guide within the dielectric channel, with the added benefit that guiding is maintained within the periodic plane by total internal reflection, unlike the guide made from air. Compatibility with other semiconductor devices in terms of integration and coupling issues, is also improved. However these dielectric guiding devices also suffer from reflections at bends introduced into the waveguide. Applications for such devices include multiplexers, demultiplexers, and equalization devices.
U.S. Pat. No. 5,651,818, Milstein et al, discusses in the introduction a number of available techniques of manufacturing photonic band gap materials. U.S. Pat. No. 5,784,400, Joannopoulous et al, proposes to utilise two-dimensional photonic band gap materials in an optical device in the form of a resonant cavity.
It is known from U.S. Pat. No. 5,389,943, Brommer et al, to utilise the frequency selective transmission properties of such two-dimensional photonic band gap materials in a filter in which transmitted light is modified in frequency response by the optical transmission characteristics of the bulk properties of the material. Further disclosed is the active control of material forming the lattice sites, such as by the application of an external field, in order to modify the ref
Healy Brian
Nortel Networks Limited
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