Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-09-21
2003-07-22
Lee, Eddie (Department: 2815)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S102000, C372S020000, C385S129000, C385S130000
Reexamination Certificate
active
06597721
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical devices, and more particularly to lasers formed from photonic band gap structures and sub-wavelength grating structures.
BACKGROUND OF THE INVENTION
Light has several advantages over the electron. As used herein, “light” means not only signals in the spectrum of visible light, but also signals in the full spectrum of frequencies typically handled by optical transmission systems. The speed of light is approximately three orders of magnitude higher, compared to the speed of electrons in semiconductors. Thus, photons of light can theoretically carry information approximately 1,000 times faster than electrons in semiconductors. Moreover, photons are not as strongly interacting as electrons with their environment, which allows photonic devices to dissipate less energy, produce less heat and generate less signal noise compared to electronic devices.
In spite of the numerous advantages of photons, all optical circuits have yet to be commercially available on a large scale. Some hybrid opto-electronic circuits have produced significant improvement over the performance of electronic circuits, but the difficulties in designing a multipurpose optical component analogous to the electronic transistor has severely hindered the development of all optical systems.
It is known that as the periodicity of a medium becomes comparable with the wavelength of electromagnetic waves traveling therethrough, the medium begins to significantly inhibit the wave's propagation. A photonic band gap (PBG) structure is one type of optical structure that is currently being investigated for certain electromagnetic (EM) wave applications. PBGs are formed from photonic crystals, which are composite periodic structures made up of two different dielectric materials. Both dielectric materials should be nearly transparent to electromagnetic radiation in the frequency range of interest. However, the composite periodic structure may not be transparent to the frequency range of interest, due to electromagnetic scattering at the interfaces between the two dielectric components. Intervals of prohibited frequencies are called photonic band gaps.
Relying on the subwavelength wave inhibition effect, PBG structures are two or three-dimensional periodic array structures in which the propagation of EM waves may be described by band structure types of dispersion relationships resulting from scattering at the interfaces between the two dielectric components. Waveguide dispersion is the term used to describe the process by which an electromagnetic signal is distorted by virtue of the dependence of its phase and group velocities on the geometric properties of the waveguide. These photonic band gap structures provide electromagnetic analogs to electron-wave behavior in crystals, with electron-wave concepts such as reciprocal space, Brillouin zones, dispersion relations, Bloch wave functions, Van Hove singularities and tunneling having electromagnetic counterparts in a PBG. This has enabled the development of many new and improved types of photonic band gap devices, including devices in which optical modes, spontaneous emission, and zero-point fluctuations are substantially reduced.
PBG structures can also be formed with added local interruptions in an otherwise periodic photonic crystal, thereby generating defect or cavity modes with discrete allowed frequencies within an otherwise forbidden photonic band gap range of frequencies. Generation of an allowed defect state in an otherwise forbidden band gap enables applications such as high-Q resonators or filters.
In the absence of external currents and sources, Maxwell's equations for a photon in a dielectric waveguide may be represented in the following form:
{
∇
×
1
ε
(
r
)
∇
×
}
H
(
r
)
=
ω
2
c
2
H
(
r
)
where H(r) is the magnetic field of the photon, &ohgr; is its frequency, c is the speed of light and ∈(r) is the macroscopic dielectric function of the waveguide. The solutions H(r) for and &ohgr; are determined completely by the magnitude and symmetry properties of ∈(r). If ∈(r) is perfectly periodic, as in a photonic crystal comprising a dielectric waveguide having a periodic array of features therein, such as a series of holes etched into the waveguide, the solutions to Maxwell's equation are quantized, characterized by a wavevector k and a band index n. Thus, the periodicity of the waveguide dielectric constant removes degeneracies that would otherwise allow free photon states at the Bragg plane, forming a photonic band gap. The region of all allowed wavevectors is referred to as a Brillouin zone and the collection of all solutions to the above equation is termed a band structure. Thus, in a perfectly periodic photonic crystal, allowed photonic states are quantized, with band gaps having no allowed states between discrete allowed states.
When a periodic array of features, such as holes, is introduced into a waveguide material to form a perfectly periodic photonic crystal, the wavevector k becomes quantized and limited to Π/a, where a is the spatial period of the holes. In addition to putting a limit on wavevector values, the introduction of an array of holes in a waveguide has the effect of folding the dispersion relations (&ohgr;
n
(k)) of the strip waveguide and splitting the lowest-order mode to form two allowable guided modes. The splitting at the Brillouin zone edge is referred to as a band gap. The size of the band gap is determined by the relative dielectric constants of the waveguide material and the material filling the periodic structures, such as air in the case of holes. The larger the difference in relative dielectric constants, the wider the gap.
If a defect is included into an otherwise periodic PBG structure, an allowed photonic state can be created within the band gap. This state is analogous to a defect or impurity state in a semiconductor which introduces an energy level within the semiconductor's band gap. A defect in the otherwise periodic PBG structure is formed by incorporating a break in the periodicity of the PBG structure. PBG defects can take the form of a spacing variation using constant features, use features having a different size or shape, or use a different material. Introduction of a PBG defect may result in the creation of a resonant wavelength within the band gap.
The resonant wavelength of a PBG structure may be shifted by changing the defect. For example, a PBG structure using a defect in feature spacing can shift the resonant wavelength by altering the length of the defect in feature spacing. Increasing the defect spacing length increases the resonance wavelength to a longer value and also reduces the cavity's Q. The Q of an optical resonant cavity is its figure of merit, defined as 2Π×(average energy stored in the resonator)/(energy dissipated per cycle). The higher the reflectivity of the surfaces of an optical resonator, the higher the Q of the resonator and the less energy loss from the desired mode. An increase in defect length results in a corresponding increase in the effective refractive index felt by the resonant mode due to a reduced density of lower refractive index holes in the higher refractive index waveguide material. The increase in effective refractive index of the waveguide material results in the reduction of the frequency of the resonant mode. This reduction enhances the coupling of the resonant mode to the waveguide mode. This increases the cycle average radiation out of the cavity resulting in a lower Q. A reduction in defect spacing length is expected to produce the inverse result.
Alternatively, the hole spacing may be held constant, but a column of holes having a different size compared to the other PBG holes may be used to introduce an allowed photon state within the PBG band gap. For example, a column of holes may be placed in the PBG hole array having a radius greater or less than the nominal hole radius. As a further alternative, a ro
Hutchinson Donald P.
Richards Roger K.
Akerman & Senterfitt
Landau Matthew
UT-Battelle LLC
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