Optical waveguides – Accessories – Attenuator
Reexamination Certificate
2002-05-09
2004-11-09
Font, Frank G. (Department: 2877)
Optical waveguides
Accessories
Attenuator
C385S003000, C385S040000
Reexamination Certificate
active
06816665
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
Integration of optical components on a single chip is a desired feature. Integration reduces the number of components, reduces the device size and eliminates all the fiber interconnection. Thus the device reliability is increased, the performance is improved and the overall cost is significantly reduced.
The Planar Lightwave Circuit (PLC) SiO
2
on Si technology is a natural and mature technology for integration for the following reasons: (1) many components such as AWGs (Array Waveguide Gratings), switches, VOAs (Variable Optical Attenuator), splitters, taps, etc, have already been produced with this technology, and (2) PLC technology uses almost the same equipment and processes as used by the mature microelectronic industry. Some PLC integrated components have already been fabricated, most of them in the configuration of the basic OADM (Optical Add Drop Multiplexer) module [T. Saida, A. Kaneko, T. Goh, M. Okuno, A. Himeno, K. Takiguchi, K. Okamoto, “Athermal silica-based optical add/drop multiplexer consisting of arrayed waveguide gratings and double gate thermo-optical switches,” Elect. Lett., 36, 528-529, 2000], shown in FIG.
1
.
FIG. 1
shows an integrated PLC OADM module
10
with an AWG multiplexer (“passive” component)
12
, N “add/drop” 2×2 switches
14
(“active” components) and an AWG multiplexer
16
(“passive” component). A major problem in this integration lies in preventing the active (heat producing) components from affecting the performance of the (temperature) sensitive passive components. A change in the configuration of a thermo-optic switch matrix is usually followed by a variation in the local distribution of the operating heaters, as shown in U.S. Pat. No. 6,259,834 to Shani. There, the distribution of the operating (“on” stage) switches depends on the switch matrix configuration, and switches that change from a heated to a non-heated state induce local temperature non-uniformities on the wafer. Thus, even if the whole wafer is temperature-stabilized, changing a switch matrix configuration can affect the integrated passive components performance.
Prior art methods for removing the temperature sensitivity include: (1) fabricating active and passive components on separate chips and butt-coupling them; (2) separating between the active and passive components which are integrated on the same chip, and (3) designing the passive components to be a-thermal components [e.g. M. Ishii, Y. Hibino, F. Hanawa, H. Nakagome, K. Kato, “Packaging and environmental stability of thermally controlled AWG multiplexer module with thermoelectric device,” J. Light. Technology, vol 6, 258-264, 1998; N. Kail, H. H. Yao, C. Zawadzki, “Athermal polarization-independent AWG multiplexer using an all polymer approach,” European Conference on Integrated Optics, Paderborn, Apr. 4-6 2001, Post-deadline paper]. However, these prior art methods suffer from a number of drawbacks and disadvantages, including the use of non-mature polymer technology, and hybrid integration of half waveplates.
The prior art operation and use of thermo-optic switches is illustrated next.
FIG. 2
shows a common Mach Zehnder Interferometer (MZI) thermo-optic switch
100
. Such switches are known in the art, and a detailed description of one is provided for example in M. Kawachi, “Silica waveguides on silicon and their application to integrated-optic components,” Optical and Quantum Electronics, 417-426, 1990 (hereinbelow KAW90). Switch
100
consists of two 3 dB couplers
102
and
104
combined by two waveguide arms
106
and
108
, with an electrode (“heater”)
110
on one of the waveguide arms (in this case, arm
106
). In the “off” position or stage, electrode
110
is not activated (not turned on) and therefore introduces no phase difference, and the light passes from an input
112
on one arm (e.g. arm
106
) to an output
114
on the other arm (
108
), i.e. following the “1”>“2” path in FIG.
2
. In the “on” position electrode
110
is activated, a 180 degree phase shift due to the thermo-optic effect is introduced by the electrode on the light passing in arm
106
, and the light stays in the input waveguide (arm
106
) leading to an output
116
, i.e. following the “1”>“1” path. The action of the switch is “digital” in the sense that it operates in two positions only, one position requiring heating, the other not requiring it. The passage from a heated to a non-heated state is the main source of temperature non-uniformity on the wafer or chip.
In order to build a constant power operating switch, i.e. to have a constant heating power during both the “on” and “off” stages (and therefore remove the temperature non-uniformity), a 90 degree phase shift is added between the waveguide arms, and electrodes
110
and
120
are positioned on both MZI arms (
106
and
108
respectively), as shown in FIG.
3
. In this case, with no heating (“zero power consumption”) no additional phase shift is introduced and the light goes to both output arms (
114
and
116
), i.e. the switch functions as a 3 db splitter. This is the main use of this architecture, as described in both U.S. Pat. No. 6,259,834 to Shani and in KAW90. For a “switch” operation, path “1”>“1” is connected when heater
110
is turned “on” (and heater
120
is “off”), and path “1”>“2” is connected when heater
120
is turned “on” (and heater
110
is “off”). Thus, in this “switch” or “digital” mode, there is always one heater in an “on” position (and one heater in an “off” position), independently of whether the connection is “1”>“1” or “1”>“2”. However, if the device is used both as a splitter (both heaters “off”, zero power) and as a switch (one heater “on”, the other “off”), there is still a non-uniform and time dependent temperature distribution on the chip.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for operating such a switch that does not suffer from the disadvantages of prior art switches, and that provides a uniform temperature distribution on a PLC, thus not affecting the integrated passive components performance.
SUMMARY OF THE INVENTION
The present invention is of a method used to eliminate temperature variations caused by active components in PLCs. More specifically, the method of the present invention can be used to design an active component (e.g. switch) in such a way that its generated heat does not depend on its configuration (i.e. the same heat generation exists if the active component is at an “on” or at an “off” position).
According to the present invention there is provided a method for obtaining a constant and uniform temperature on a planar lightwave circuit, comprising: a) providing at least one active element having two connecting configurations and operative to have a phase change in a light beam passing through each of the connecting configurations, and b) constantly heating both of the connecting configurations, thereby achieving a substantially constant and uniform temperature distribution on the planar lightwave circuit.
According to one feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the step of providing at least one active element includes providing at least one thermo-optic switch.
According to another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the step of providing at least one thermo-optic switch includes providing a waveguide Mach Zehnder Interferometer switch having one input waveguide, two identical waveguide arms, and two output waveguides, wherein the connecting configurations include a first connecting configuration defined by connecting the input waveguide to one of the output waveguides, and a second connecting configuration defined by connecting the input waveguide to the other of the output waveguides.
According to yet another feature of the method of the present invention for obtaining a constant and uniform tem
Kopelovitz Ben-Zion
Shani Yosi
Font Frank G.
Friedman Mark M.
Kianni Kevin C
Lynx Photonic Networks Inc.
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