Optical waveguides – Having nonlinear property
Reexamination Certificate
2001-07-23
2003-10-28
Healy, Brian (Department: 2874)
Optical waveguides
Having nonlinear property
C385S024000, C385S015000, C385S129000, C385S130000, C385S037000, C385S141000, C438S031000, C438S032000, C359S199200
Reexamination Certificate
active
06640034
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to method of controlling the propagation characteristics of radiation in waveguides by means of photonic band gaps, to optical devices and, in particular, to optical devices which influence the transmission of radiation by means of photonic band gaps. Such devices may be formed by etching a substance which supports propagation of radiation at a wavelength of interest. Although the embodiments described herein are concerned with visible radiation, the principles involved are equally applicable to techniques for controlling the propagation of other forms of electromagnetic radiation such as ultra-violet, infra-red, terahertz and microwave radiation. In this specification, the term “optical” includes such other forms of radiation.
For some periodic dielectric structures, the propagation of electromagnetic radiation can become forbidden in certain lattice directions. These structures are known as photonic band gap structures. Structures based upon a cubic or triangular lattice of deep air rods in a background dielectric material can exhibit a photonic band gap (PBG). The size and position of the band gap is dependent upon the wave polarisation state, direction of wave propagation, dimensions of the photonic crystal, and the dielectric contrast. The frequency extent of the band gap is of the order of the lattice spacing. Semiconductor materials are ideal for the fabrication of PBGs because of their large dielectric constant. It has also been shown that two-dimensional photonic lattices can have a three dimensional band gap, that is to say, the band gap remains open even when there is a large out of plane wave component.
Photonic band structures with band gaps at optical frequencies have several interesting applications. An important property of photonic band gaps is the ability to enhance or inhibit spontaneous emission within the band gap energy range. This has important implications for direct band gap optoelectronic devices such as semiconductor lasers and light-emitting diodes (LEDs).
Photonic band gap structures can also be fabricated in fluorescent (including laser) materials. The PBG can make these active materials useful as sensors, or to make one transition (or group of transitions) more likely to occur than others.
As a sensor, the PBG may be fabricated to fluoresce at a specified wavelength when the air holes in the structure are filled with air. If however the air holes fill with a different gas, such as pure carbon dioxide, or carbon monoxide, the different refractive index of the gas (compared to ordinary air) could be made to tune the PBG off the fluorescent line which would be easily detected. The PBG structure may be used in a similar way for liquid sensing.
Some laser glasses emit at several different wavelengths (example neodymium-doped GLS glass). Frequently, it is desirable to choose preferentially to amplify just one line. This line may be the weakest transition of the group of lines. A PBG structure in the glass may be employed to prevent the fluorescence of the unwanted lines and promote the transmission of the required wavelength.
A particularly important application would be to make a high energy laser transition in a glass favourable by preventing direct transitions from lower lying radiative levels. In a typical laser system, the lower lying transitions are stronger and more likely to occur. However, there may be useful higher energy levels (for example in the blue region of the spectrum) that could be used, but that are unusable because the lower energy transitions are taking all the energy. A suitably engineered PBG in such a laser system could prevent the lower energy transitions from occurring, thus allowing lasing at the higher energy level.
PCT patent application No. WO 94/16345 (Massachusetts Institute of Technology) discloses low-loss optical and optoelectronic integrated circuits with light guides fabricated in a structure having a photonic band gap. This publication does not disclose methods for the determination of the transmission characteristics of such waveguides, other than the centre frequency of the band gap. Furthermore, it describes embodiments which would not operate in the manner described therein, due to adverse interaction between a photonic band gap and a dielectric waveguide. Another disclosed embodiment would not produce the promised advantage due to the influence of back reflection in a tapered dielectric waveguide.
An etched silicon structure is disclosed by V. Lehmann in
J. Electrochem. Soc
. Vol. 140, No. 10, page 2836, October 1993. However, use of the etched silicon structure as an optical device is not discussed. The etched silicon structure disclosed is formed by etching an homogeneous slab of bulk silicon by placing it in an acid bath. Etching is achieved by establishing an electric field across two opposite, substantially planar, faces of the silicon slab, and illuminating the rear surface. The resultant structure has an array of substantially equally spaced holes or pores formed therein. These holes or pores are referred to as macro pores and occur as a result of an electro chemical reaction in conjunction with the phenomenon of a self adjusting charge distribution at a tip of a macro pore.
Krauss T.F. et al., in Nature 1996 Oct. 24, 1996) Vol. 383 at pages 699-702, describe a photonic bandgap (PBG) device. The device is a two-dimensional lattice in the form of an homogeneous array of holes formed in a semiconductor waveguide of high refractive index silicon. Krauss notes that radiation from a tunable source, incident on the structure at certain angles, is detected as it emerges from a waveguide positioned on a substantially opposite side to where the radiation is incident.
SUMMARY OF THE INVENTION
According to the present invention there is provided an optical device including a waveguide formed in a first region of a first optically-transmissive material bounded by a second region or regions having an array of sub-regions arranged therein to create a photonic bandgap at least partially non-transmissive to radiation of a predetermined frequency or frequencies wherein the frequency transmission characteristics of said waveguide are at least partially determined by the transmission characteristics of said second region or regions.
There is also provided an optical transfer device having a first plurality of input ports and a second plurality of output ports coupled by a waveguide at least partially bounded by a photonic band gap wherein at least one of said first plurality of ports is adapted to pass an optical signal having a first range of frequencies and at least one of said second plurality of ports is adapted to pas an optical signal having a second range of frequencies, said first and second range of frequencies being defined by said photonic band gap.
There is also provided an active optical device having a waveguide comprising a region of optically-transmissive material bounded by a photonic band gap and containing a dopant adapted to induce quasi-stable energy levels in said material.
The invention further provides a hybrid optoelectronic signal translation device having a first region adapted to transfer a signal by means of movement of electrical charge carriers and a second region adapted to transfer a corresponding signal by means of electromagnetic radiation and electro-optic transducer means disposed between said first and second regions to convert said signal from or to said corresponding signal wherein said second region includes a third region at least partially bounded by a photonic band gap.
The invention further provides a coupler to a waveguide defined by a photonic band gap having an input or output port for the transfer of radiation to or from said waveguide wherein said input or output port includes a region having a graded refractive index to enhance the transfer of radiation to or from said waveguide.
There is also provided a method of fabricating an optical device comprising the steps of forming a waveguide in a first region of a first opt
Charlton Martin David Brian
Parker Gregory Jason
BTG International Limited
Healy Brian
Nixon & Vanderhye P.C.
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