Tunable coupler device and optical filter

Details Tunable coupler device and optical filter Tunable coupler device and optical filter

2002-10-10

2004-07-20

Kim, Ellen E. (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S032000

Reexamination Certificate

active

06766083

ABSTRACT:

The present invention relates to a tunable coupler device according to claim
1
and an optical filter incorporating said tunable coupler device according to claim
11
.
More particularly, the present invention relates to a tunable coupler device designed for use in devices including but not limited to directional couplers, Mach-Zehnder interferometers, optical ring resonators, infinite impulse response filters, dispersion compensating devices, add-drop multiplexers, optical wavelength converters or optical modulators.
BACKGROUND OF THE INVENTION
An optical signal is often split from one input port to two output ports for signal distribution or monitoring. This can be accomplished passively by using a directional coupler with two separate single-mode waveguides which are brought together for some interaction region. A gaussian-shaped single-mode wave propagating in a waveguide will have most of its energy residing in the core accompanied by an evanescent field tail propagating alongside the core within the cladding region. The evanescent tail of a single-mode wave, which is propagating along the interaction region in a first waveguide of a directional coupler, will therefore partially fall into the range of the second waveguide exciting an optical wave therein. In this way power is gradually coupled from the first to the second waveguide (see Mool C. Gupta, Handbook of PHOTONICS, CRC Press, Boca Raton 1997, pages 642-646).
According to Govind P. Agrawal, Fiber Optic Communication Systems, Wiley Series in microwave and optical engineering, New York 1992, chapter 6.2.1, pages 232-234, optical signals can be modulated by means of a Mach-Zehnder interferometer comprising two arms wherein the phase of optical carrier signals is shifted according to electrical binary data. As long as the phase of the optical carrier signals, which originate from the same source, is identical, the corresponding optical fields interfere constructively. An additional phase shift of adequate size introduced in one of the arms destroys the constructive nature of the interference of the optical carrier signals which are superpositioned on an output line of the ASK-modulator. The additional phase shift in the given example is introduced through voltage-induced index changes of the electro-optic materials (e.g. LiNbO
3
) used for said arms of the Mach-Zehnder interferometer.
In C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo and L. T. Gomez, Phase Engineering Applied to Integrated Optical Filters, IEEE Lasers and Electro-Optics Society, 12
th
annual meeting, San Francisco 1999, allpass filter rings and linear delay response architectures for dispersion compensations are described. A basic ring architecture consists of a tunable optical waveguide ring which is coupled to an optical waveguide through which optical signals are transferred. The thermo-optic effect is used to shift the phase of the signals within the ring. In order to obtain a desired filter response, it is critical to accurately fabricate the desired coupling ratio. To reduce the fabrication tolerances on the couplers and simultaneously to obtain a fully tunable allpass response, the basic ring architecture is preferably enhanced with a Mach-Zehnder interferometer (see FIG.
1
). This enhanced ring structure, below called ring resonator, is briefly explained with reference to
FIGS. 1 and 2
.
FIG. 1
shows a prior art tunable balanced Mach-Zehnder interferometer with a first and a second waveguide
10
,
11
aligned in parallel, with a first and a second directional coupler
31
,
32
, through which optical signals can be exchanged between said waveguides
10
,
11
, and with one thin-film heater
21
covering a part of the first waveguide
10
lying between the directional couplers
31
,
32
. An optical signal entering the first waveguide
10
at port A will partially be coupled in the first directional coupler
32
to the second waveguide
11
. Between the directional couplers
31
,
32
the phase of the remainder of the optical signal transferred in the first waveguide
10
will be shifted according to the thermal energy applied to the first waveguide
10
by means of the thin-film heater
21
. The optical signal in the first waveguide
10
then interferes in the second directional coupler
32
with the optical signal of the second waveguide
11
. Depending on the phase relationship between the optical signals, the signal intensity in the second waveguide
11
will be increased or reduced.
In case that the second waveguide
11
is formed as a ring and enhanced with a thin-film heater
22
for phase-shifting purposes, then the architecture shown in
FIG. 1
corresponds to the tunable ring resonator shown in
FIG. 2
respectively [
3
],
FIG. 1
which may be used for dispersion compensation.
In order to obtain a desired shift of the phase of the optical signal in the first waveguide
10
relative to the phase of the optical signal in the second waveguide
11
, thermal energy provided by the thin-film heater
21
is applied to the first waveguide
10
and not to the second waveguide
11
. In the region of the thin-film heater
21
the waveguides
10
,
11
are traditionally spaced apart at a distance which is sufficient to avoid a transfer of thermal energy from the thin-film heater
21
to the second waveguide
11
.
Since the waveguides
10
,
11
of the tunable ring resonator are kept apart from each other between the directional couplers
31
,
32
over a relatively long distance, the architectures shown in
FIGS. 1 and 2
are difficult to realize in small sizes as required for high frequency applications operating for example in the range of 25 GHz to 75 GHz.
It would therefore be desirable to create an improved tunable coupler device.
It would be desirable in particular to create a tunable coupler device which in conjunction with related circuitry can be fabricated at reduced cost and in high packing density.
More particularly it would be desirable to create a tunable coupler device which can easily be fabricated in planar waveguide technology.
It would further be desirable to create a tunable coupler device designed for use in devices including but not limited to directional couplers, Mach-Zehnder interferometers, optical ring resonators, infinite impulse response filters, dispersion compensating devices, add-drop multiplexers, optical wavelength converters or optical modulators.
SUMMARY OF THE INVENTION
The above and other objects of the present invention are achieved by a device according to claim
1
and a tunable optical filter according to claim
11
.
The tunable coupler device is disposed on a substrate, comprising a first and a second waveguide for guiding optical signals, and comprising a heater element disposed adjacent the first waveguide in order to shift the phase of the optical signal in the first waveguide by means of the thermo-optic effect in response to a control voltage applied to the heater element. The heater element is disposed in an interaction region of the optical signals, such that, within the interaction region, a temperature gradient across the first and the second waveguide is generated in dependence on the applied control voltage. In order to increase the temperature gradient, the substrate may be designed to absorb thermal energy.
In order to further increase the temperature gradient, the heater element can be disposed adjacent the first waveguide and a heat sink element can be disposed adjacent the second waveguide such that thermal energy which passes from the heater element to the first and partially to the second waveguide is absorbed by the heat sink element.
In case that the second waveguide is formed as a closed loop or a ring, the heat sink element can be placed within said loop or ring, thereby reducing hindering the placement of further elements on the substrate.
Regions wherein optical signals are interacting and the region wherein the phase of the optical signals is shifted are therefore not separated in the tunable coupler device. Compared to prior art tunable couplers such as the

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