Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2000-02-22
2001-12-04
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S018000, C385S008000
Reexamination Certificate
active
06327397
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to optical switches and more particularly to a thermally activated optical switch.
BACKGROUND ART
Continuing innovations in the field of fiber optic technology have contributed to the increasing use of optical fibers in telecommunications and data communications networks. With the increased utilization of optical fibers, there is a need for efficient peripheral devices that assist in the transmission of data through these optical fibers, such as optical switches. An optical switch operates to selectively couple an optical fiber to one of two or more alternative optical fibers such that the two coupled optical fibers are in communication with each other.
The coupling of the optical fibers performed by an optical switch can be effectuated through various techniques. One technique of interest utilizes micro-mirrors to selectively route optical signals from an input optical fiber to a selected output optical fiber. In the simplest implementation of the micro-mirror technique, the input optical fiber is aligned with one of two output optical fibers, such that when the micro-mirror is not placed in the optical path between these two aligned optical fibers, the two aligned optical fibers are in a communicating state. However, when the micro-mirror is interposed between the two aligned optical fibers, the micro-mirror steers, i.e., reflects, the optical signals from the input optical fiber to the other output optical fiber. The positioning of the micro-mirror in and out of the optical path between the two aligned optical fibers can be accomplished by using a micro-machined actuator that mechanically displaces the micro-mirror to a desired position.
Another technique of interest utilizes thermally created bubbles, instead of micro-mirrors, to selectively route optical signals from input fibers to target output optical fibers. This technique is implemented in a thermally activated optical switch that is described in U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention. A conventional thermally activated optical switch
10
is schematically illustrated in
FIGS. 1 and 2
. As shown in
FIG. 1
, the optical switch includes a waveguide chip
12
, a heater chip
14
, and a metal substrate
16
. The waveguide chip contains planar waveguides
18
, shown in
FIG. 2
, that serve as media for transmission of optical signals. These waveguides form a matrix of optical paths. Optical paths
20
,
22
,
24
and
26
facilitate lateral transmissions of optical signals, while optical paths
28
,
30
,
32
and
34
facilitate vertical transmissions of optical signals. The waveguide chip also contains a number of trenches
36
, located at intersections of optical paths. Each trench is positioned so that an incoming optical signal from one of the optical paths
20
-
26
will impinge upon the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR). When a trench is filled with a liquid having a refractive index generally matching that of the waveguides, optical signals propagating along the lateral optical path that extends across that trench will be transmitted through that trench. However, when a bubble is formed within the trench, the optical signals are reflected by the wall of the trench from the lateral optical path to a vertical optical path that intersects the lateral optical path at the location of the trench.
The heater chip
14
of the optical switch
10
includes heating elements
38
, i.e., resistors, and other electrical elements, such as transistors, to address individual resistors. For simplification, only the resistors are shown in FIG.
2
. The heater chip is aligned with the waveguide chip
12
so that each resistor of the heater chip is positioned below a trench
36
of the waveguide chip, where two optical paths intersect. The resistors provide the thermal energy to create the bubbles within the trenches. Therefore, by selectively activating the resistors, any optical signals that were originally propagating through the lateral optical paths
20
-
26
can be rerouted to the vertical optical paths
28
-
34
. The heater chip is attached to the metal substrate
16
of the optical switch, as shown in FIG.
1
. The metal substrate contains a reservoir
40
of the refractive index-matching liquid. The reservoir is connected to the trenches of the waveguide chip by vias (not shown), which extend through the heater chip.
In order to provide an optimized and consistent performance of the thermally activated optical switch
10
, the heater chip
14
needs to be maintained at nearly constant and uniform temperature. Large temperature variations at the intersections of optical paths, or cross points, where the bubbles are created to reflect optical signals, cause increased optical losses and cross talk, as well as perturbations to the bubble behavior. Environmental changes aside, accurate and precise temperature control of an NxN thermally activated optical switch, where N is significantly large, is difficult because N resistors can be activated simultaneously. Therefore, the total heat load of the heater chip can change by as much as N times the power required at each cross point, which results in large temperature variations.
An active temperature control device
42
, shown in
FIG. 1
, can be utilized to try to control the temperature fluctuations within the optical switch
10
. However, for packaging reasons, the temperature control device is located at a significant distance from the heater chip
14
, where the sudden heat load changes are generated. As shown in
FIG. 1
, the temperature control device is located below the optical switch. Thus, the temperature control device and the heater chip are separated by the metal substrate
16
. This implies that (i) a thermal gradient will exist between the heater chip and the temperature control device and (ii) any change in the local temperature of the switch will be resolved on a time scale limited by the heat conduction between the heat generating resistors and the active temperature control device. The amplitude of the temperature fluctuations depends both on the power required for each resistor and on the thermal resistance of the path between the heat generating resistors and the temperature control device. Therefore, if the materials along the heat path have a low thermal diffusivity, the temperature control device will have a slow response time. It follows that on-chip temperature control will benefit from reducing the power requirements of the resistors and from devising packaging solutions that maximize the heat transfer between regions of heat production and heat removal.
Although the above approaches will result in an improved on-chip temperature control for a thermally activated optical switch, additional improvement in temperature control is desired. Therefore, what is needed is a thermally activated optical switching device and a method for improving the temperature control of the switching device.
SUMMARY OF THE INVENTION
An optical switching device and a method of providing temperature control for the device utilize compensating thermal energy to maintain a consistent operating temperature. The optical switching device may be a thermally activated optical switch that routes optical signals using bubbles that are strategically created or manipulated along optical paths within the device.
The bubbles are created by thermal energy generated by switching heating elements. The compensating thermal energy may be generated by at least one compensating heating element or by at least one switching heating element that is not currently being used for optical switching, i.e., bubble creation. The compensating thermal energy is varied so that total thermal energy generated by the device is constant, which results in a consistent operating temperature.
The device includes a waveguide chip, a heater chip, a metal substrate and a control unit. The waveguide chip includes a number of waveguides that define i
Schiaffino Stefano
Schroeder Dale
Troll Mark
Agilent Technologie,s Inc.
Sanghavi Hemang
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