Coherent light generators – Particular resonant cavity
Reexamination Certificate
1998-04-07
2001-07-10
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular resonant cavity
C372S108000, C385S146000
Reexamination Certificate
active
06259717
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to resonant optical cavities and, more particularly, to asymmetric optical cavities which provide directional light emission at a wavelength dependent upon the structure of the optical cavity.
BACKGROUND OF THE INVENTION
A new class of optical resonators is comprised of convex dielectric bodies which are substantially deformed from cylindrical or spherical symmetry. See: J. U. Nöckel, A. D. Stone and R. K. Chang, Optics Letters 19, 1693 (1994); A. Mekis, J. U. Nöckel, G. Chen, A. D. Stone and R. K. Chang, Phys. Rev. Lett. 75, 2682 (1995); J. U. Nöckel and A. D. Stone, in
Optical Processes in Microcavities,
editted by R. K. Chang and A. J. Campillo (World Scientific Publishers, 1996). Such asymmetric resonant cavities (henceforth ARCs) exhibit high-Q (quality factor) whispering gallery (WG) modes (Q>1000) at distortions as large as 50% of the undeformed radius, R, of the corresponding circular (symmetric) resonant cavity. The emission pattern from these modes is highly directional, in contrast to the isotropic emission from symmetric cavities.
WG modes of symmetric cavities (dielectric spheres and cylinders) have high Q (long lifetime) because the light trapped in such a mode (when described by ray optics) always impinges on the boundary at the same (conserved) angle of incidence, &khgr;, where sin &khgr;>=1
(n is the index of refraction of the dielectric); hence the light is almost totally internally reflected. Due to the curvature of the surface, there is an exponentially small correction to the law of total internal reflection which allows light to escape after very long times (this is called “evanescent leakage”). In ARCs on the other hand, the dominant mechanism for emission of light is not evanescent leakage but direct refractive escape via Snell's law because the angle of incidence sin &khgr; is not conserved. In the ray-optics language these ARC resonances correspond to ray trajectories which initially are in WG orbits with sin &khgr;>1
, but after a large number of reflections with the boundary eventually impinge on it with sin &khgr;<1
and are then directly emitted according to Snell's law of refraction. The high-intensity regions in the near-field correspond to the regions on the boundary of the ARC where most of the refractive escape occurs; the far-field directionality can be determined by following the refracted rays.
In
FIG. 1
, a prior art cylindrical resonator of radius R is shown. The motion of a light ray in a WG mode circulating around its cross-section is shown in
FIG. 2
a
. The motion forms a regular pattern with the angle of incidence the same at each reflection. As noted above, since this angle is initially above the angle required for refractive escape and remains so indefinitely, escape of optical energy occurs isotropically (equally in all directions) by the exponentially slow process of evanescent leakage (this escape is not shown in the figure).
Nöckel, Stone and Chang, in the paper entitled “Q spoiling and directionality in deformed ring cavities”, introduced the idea that a deformation of the cross-section might induce directional emission due to refractive escape. The deformation they considered, shown in
FIG. 2
b
, causes irregular (chaotic) ray motion and leads to refractive escape at a point
10
(
FIG. 2
b
) with the far-field high emission directions shown in
FIG. 2
c
. Note that the high emission directions are not parallel and hence intersect in the near-field leading to interference effects not describable by ray-optics. Moreover there are secondary (split) peaks at angles near those of the largest peaks. The near-field behavior of ARCs was not known at that time due to the limitations of the ray model used.
The occurrence of multiple and non-parallel emitted beams make the prior art deformed resonator unsuitable for optical devices employing ARCs. Moreover the lack of information about the near-field radiation pattern of ARCs made it impossible to design input and output couplers for this resonator.
SUMMARY OF THE INVENTION
We have now determined that the high emission intensity points typically correspond to the regions of highest curvature on the ARC surface and that an ARC based on the deformation of
FIG. 2
b
would produce non-parallel beams because the regions of highest curvature on the boundary are not 180° opposite one another. This defect is avoided in ARCs with two-fold reflection symmetry, as discussed below. It is also shown how to avoid the occurrence of multiple intersecting beams.
Accordingly, it is an object of this invention to provide an ARC wherein the light is emitted at a predetermined location and in a predetermined direction.
It is another object of this invention to provide an improved ARC wherein the emitted light comprises a single beam or parallel (non-intersecting) beams.
It is still another object of this invention to provide an ARC with predetermined Q-value and directional emission pattern so as to maximize the input-output coupling to a range of optical devices.
An asymmetric resonant optical cavity is constructed of an optically transmissive dielectric material having an index of refraction n. The cavity may be in the form of a cylinder with a cross-section deformed from circularity or a spheroid with an oblate or prolate deformation. These deformations should be larger than 1% of the undeformed radius R but it is crucial that they maintain convexity, i.e. at no point should the curvature of the relevant cross-section change its sign. In the cylindrical case the resulting cross-section should be oval in shape and we will use this descriptive term henceforth. The oval cross-section will have a major (long) axis and a minor (short) axis and the ARC should have mirror symmetry with respect to both axes. The shape and index of refraction of the ARC is chosen so as to avoid multiple beams. For signal processing in optical communications, an ARC will be coupled in the near-field region to an input optical signal at a wavelength &lgr; which is in resonance with one of the whispering gallery resonances of the ARC. Typically this signal will be transmitted (possibly in modified form) through the ARC to an output coupler also positioned in the near field. The couplers must be oriented parallel to the tangent line at the point(s) of high emission intensity, which are determined (see below) to be approximately the points of highest curvature of the boundary. ARCs may also be used in signal generation, for example as resonators in microlasers or LEDs. In signal generation ARCs will produce highly directional beams of light along directions tangent to the points of highest curvature.
REFERENCES:
patent: 5742633 (1998-04-01), Stone
Chang Richard Kounai
Noeckel Jens Uwe
Stone Alfred Douglas
Arroyo Teresa M.
Inzirillo Gioacchino
Ohlandt Greeley Ruggiero & Perle LLP
Yale University
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