Optical waveguides – With optical coupler
Reexamination Certificate
2000-11-22
2003-04-22
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
C385S003000, C385S024000, C385S036000, C385S039000
Reexamination Certificate
active
06553158
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a semiconductor optical resonator having a non-continuous waveguide and a phase shift element selectively movable into and out of an optical path defined through and by the resonator.
BACKGROUND OF THE INVENTION
Optical resonators are of great interest in the telecommunication industry because of their ability to couple specific wavelengths from a multi-wavelength optical signal (e.g., WDM, DWDM, UDWDM, etc.) and route those wavelengths to a desired destination. A typical optical cross-connect includes input and output waveguides arranged so that a portion of each of the waveguides is disposed adjacent to a resonator. A desired wavelength of a multi-wavelength optical signal propagating in an input waveguide may be coupled to a particular output waveguide by providing a resonator tuned or tunable to that desired wavelength. Other wavelengths in the optical signal not on-resonance with a particular resonator will continue to propagate through and along the input waveguide and may ultimately be output therefrom. Consequently, a resonator can serve as a wavelength-specific routing device which guides particular wavelengths of light from an input path to one of several output paths.
The resonant wavelength for a circular resonator may be determined using equation (1):
λ
=
2
π
Rn
m
(
1
)
In equation 1 R is the resonator's radius, n is the effective index of refraction of the optical signal, and m is an integer of value 1 or greater.
If the resonator is not circular, the resonant wavelength is given by the equation (2):
λ
=
Ln
m
(
2
)
In equation 2 L is the resonator's length, n is the effective index of refraction of the optical signal, and m is an integer of value 1 or greater.
Today resonators are frequently employed as part of the cross-connect architecture of optical networks, such as depicted in FIG.
14
. Resonators are especially well-suited for use in optical data and tele-communication systems such as, for example, DWDM systems. These systems efficiently transmit data by simultaneously transmitting a plurality of wavelengths of light over a single optical fiber or waveguide and then, selectively coupling a desired wavelength from the multi-wavelength signal at a desired location and routing that wavelength to a desired destination.
Cross-connect waveguide architecture is described in International Patent Appln. No. WO 00/50938, entitled “Vertically Coupled Optical Resonator Devices Over a Cross-Grid Waveguide Architecture”.
For example, and as depicted in
FIG. 14
, a M×N optical cross-connect
35
provides a plurality of input waveguides 37 (M
1
and M
2
), a plurality of resonators
41
, and a plurality of output waveguides
39
(N
1
, N
2
, and N
3
). Each resonator
41
may be tuned (or tunable) to a desired wavelength (i.e., an on-resonance wavelength) and thus couple only that wavelength from the multi-wavelength optical signal propagating in and through either input waveguide
37
. That wavelength is then coupled from the resonator
41
to the corresponding output waveguide
39
. Off-resonance wavelengths are not coupled and thus continue to propagate through the input waveguide
37
.
Each of the input waveguides
37
may receive and transmit an optical signal from/to a long-distance transmission medium (e.g., a fiber-optic cable). Similarly, each of the output waveguides
39
may connect to a long-distance transmission medium. For example, each of the output waveguides
39
may connect to a fiber-optic cable over which an optical signal having a single wavelength may propagate.
Since the different wavelengths provided in the multi-wavelength optical signal are intended for different destinations, it is necessary to separate and suitably route each of those different wavelengths as separate and distinct optical signals. Resonators
41
perform this routing function quickly and efficiently—since each resonator
41
can couple a particular wavelength of light traveling in an input waveguide
37
to an output waveguide
39
.
If the resonators used in a cross-connect can only separate out a single wavelength of light, it will be necessary to provide the cross-connect with M×N resonators. However, if the resonators can be tuned sufficiently, each of the resonators could be selectively tunable to a plurality of wavelengths, thereby eliminating the need for some resonators and simplifying the cross-connect structure.
It will be appreciated that the terms “input” and “output” are used for convenience, and that light could be transmitted in the opposite manner, that is, from the “output” waveguide to the “input” waveguide.
To be useful to the telecommunication market, resonators should meet two basic requirements: small size; and high tunability range.
Small size is desirable for two reasons. First, small resonators require less wafer real estate, which reduces costs. Second, small resonators have large free spectral range (FSR) characteristics, as is clear from equation 2:
FSR
=
λ
λ
(
2
π
Rn
)
(
3
)
where &lgr; is the wavelength of the optical signal, R is the radius of the resonator and n is the effective refractive index of the medium through which the optical signal propagates (i.e., the resonator material). Referring to
FIG. 16A
, the FSR of a resonator having a 10 &mgr;m radius is graphically depicted. Such a resonator has a FSR that accommodates approximately twenty-five 200 GHz channels in an optical signal transmitted at a wavelength of 1550 nm. In contrast, a resonator having a radius of 40 &mgr;m has a FSR that accommodates approximately six 200 GHz channels, as depicted in
FIG. 16B. A
large FSR may be preferred because it allows for a higher number of optical channels to be multiplexed in a single fiber, resulting in better utilization of the fiber optical bandwidth.
Resonator operation can be enhanced if the resonator's resonant wavelength can be varied, i.e., a tunable resonator, as that enables selective modification of the resonator's switching behavior. Since the resonant wavelength of a resonator is related to the material from which the resonator is constructed, and to its index of refraction, changing the resonator index of refraction yields a corresponding change in the resonator resonant wavelength.
Increased resonator tunability is generally desirable because it provides a network administrator with the opportunity to reconfigure the network on the fly (without interrupting service) according to usage considerations and the demands of their clients. For example, and with reference to
FIGS. 15A and 15B
, any resonator in an optical cross-connect may be selectively tuned to a desired resonant wavelength. As depicted in
FIG. 15A
, the left-most resonator (in the figure) may be tuned to wavelength &lgr;
2
, and right-most resonator to wavelength &lgr;
1
. That configuration may be selectively changed, as desired, so that the left-most resonator (in the figure) may be tuned to wavelength &lgr;
1
, and the middle resonator to wavelength &lgr;
2
, as depicted in FIG.
15
B.
Resonator tunability is important for another reasons. Using currently available semiconductor fabrication processes and techniques, resonators cannot easily be manufactured with the dimensional precision required to insure that the resonators perform as required. Resonator size is important because as explained below a resonator's radius directly affects the resonator's resonant wavelength, and, at least for telecommunication applications, resonator wavelength is strictly specified by the ITU grid, a telecommunication standard which specifies a plurality of optical channels that are typically separated by fractions of a nanometer. Dimensional variations caused by the resonator manufacturing process may cause resonators to have resonant wavelengths that do not meet the ITU grid requirements. For example, a 10 nm variation for a nominal 10 &mgr;m radius resonator (and this presses the limits of what
Doan Jennifer
Edwards & Angell LLP
Lee John D.
LNL Technologies, Inc.
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