Localized thermal tuning of ring resonators

Details Localized thermal tuning of ring resonators Localized thermal tuning of ring resonators

2000-11-22

2003-10-21

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S032000, C385S016000

Reexamination Certificate

active

06636668

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to an optical ring resonator and the like having a wavelength tuning range which can be varied with accuracy.
BACKGROUND OF THE INVENTION
Optical ring resonators are of great interest in the telecommunication industry because of their ability to provide cross-connect architectures and because they can be made very compact in size. Other technologies that have been used to provide cross-connect architectures include thin-film interference filters, fiber gratings and arrayed waveguide gratings.
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”.
One example of a cross-connect architecture using optical ring resonators is shown in
FIG. 1
, and is discussed hereafter.
An optical semiconductor resonator
7
has plurality of microcavity resonators
5
and input and output waveguides
1
,
3
formed from semiconductor materials. The input
1
and output
3
waveguides are arranged so that a portion of each of the two waveguides is disposed adjacent to the microcavity resonator
5
. Light propagating in the input waveguide
1
with a wavelength on resonance with the resonance wavelength of the microcavity resonator
5
is coupled to the microcavity resonator
5
, and from the microcavity resonator
5
the light is coupled to the output waveguide, by way of example, n
1
, for transmission therefrom. Light propagating in the input waveguide
1
with a wavelength that is off resonance with the microcavity resonator
5
is not coupled to the microcavity resonator
5
, but continues to propagate in the input waveguide
1
for 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.
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.
The resonance wavelength for a ring resonator is a direct function of the ring resonator's structure, and can be given as:
&lgr;
i=
2&pgr;Rn/i  (1)
where &lgr;
i
is the resonance wavelength, R is the resonator radius (for a circular resonator), n is the resonator's effective index of refraction, and i is any positive integer.
If the resonator is not circular, the resonant wavelength is given by the equation (2):
λ
=
L
⁢
 
⁢
n
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.
Resonator operation can be enhanced if the resonator's operating wavelength can be varied, as that allows modification of the resonator's switching behavior. For example, a user can select which wavelengths of light transmitted by a first waveguide are coupled to the resonator and to a second waveguide by changing the resonator's resonance wavelength to match the wavelength of light sought to be routed.
There are several ways to alter a resonator's index of refraction and so control the resonator's operating wavelength. In accordance with equations (1) and (2), a resonator's resonance wavelength is related to the resonator material and its index of refraction, so changing the resonator index of refraction leads to a corresponding change in the resonator resonance wavelength. Alternatively, the resonator's size (i.e., radius) will determine the resonance wavelength.
Certain materials used in ring resonators have indices of refraction which vary with temperature. A ring resonator made from such a material could be thermally tuned. Changing the ring resonator's temperature will alter the resonator's index of refraction and size, as discussed in greater detail below, and thus produce a corresponding change in the resonance wavelength.
Another way to control a ring resonator's resonance wavelength is to inject current into the resonator ring. Some of the semiconductor materials that can be used in ring resonators exhibit electro-optic behavior. A material having electro-optic properties experiences a change in its index of refraction when an electric field is applied thereto. A ring resonator constructed from an electro-optic semiconductor material can therefore be tuned through the application of a suitable electric field.
As already noted, ring resonators are frequently employed as part of the cross-connect architecture of optical networks. Ring resonators are well-suited for use as telecommunications systems switching devices in Wave Division Multiplexing (“WDM”) systems, various types of which systems will be discussed later on. These systems efficiently transmit data by simultaneously sending several different wavelengths of light over a single optical fiber or waveguide and then, at the appropriate point, separating (de-multiplexing) the combined signals into individual optical fibers or waveguides and routing those signals to their desired end-points or destinations.
FIG. 1
depicts a typical network architecture based on ring resonators, namely, an M×N optical data network cross-connect
7
. In cross-connect
7
each resonator
5
can take the signal coming from a horizontal input waveguide
1
, and couple it into a vertical output waveguide
3
, provided the wavelength of the optical signal in the input waveguide
1
is on-resonance with the resonance wavelength of the resonator
5
. On the other hand, if the wavelength of the optical signal in the input waveguide
1
and the resonance wavelength of the resonator
5
are different, the optical signal remains undisturbed, i.e., does not couple to the resonator
5
in the input waveguide
1
and thus can travel toward and encounter a second resonator
5
downstream along the optical path of that waveguide
1
.
Each of the M input waveguides
1
can be a long-distance transmission medium (i.e., fiber-optic cable or waveguide) which simultaneously carries a number of different wavelength signals between widely-separated points. The N output waveguides
3
may connect to optic fibers which carry a particular wavelength(s) of light between the long-distance transmission medium and a single device or user. Incidentally, it should be understood that while the foregoing discussion refers to optical fibers, the, input and/or output lines
1
,
3
, also could be any other suitable optical transmission devices, including by way of non-limiting example, waveguides.
Since the different wavelengths of light which are carried by each of the M input waveguides
1
are intended for different destinations, it is necessary to separate and suitably route each of those different wavelengths of light. As noted above, ring resonators
3
perform this routing function quickly and efficiently—since each ring resonator
5
can couple a particular, wavelength of light traveling in an input waveguide
1
to an output wave guide
3
, ring resonators
5
can be used to “pick off” the different wavelengths of light from a multi-wavelength optical signal, e.g., a WDM signal.
One common type of cross-connect is a multiplexer (MUX)/demultiplexer (deMUX). A MUX/deMUX is a cross-connect that links a multi-wavelength optical waveguide carrying N wavelengths of light to a total of N waveguides. Thus, in a MUX/deMUX, a single multi-wavelength waveguide will have a total of N corresponding ring resonators.
If the ring 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 ring resonators can be tuned sufficiently, each of the ring resonators could separate out multiple wavelengths of light, and so some resonators could be omitted and the cross-connect structure could be simplified.
To be useful to the telecommunication market, resonators should meet two basic r

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