Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2002-03-14
2004-09-28
Patel, Nimeshkumar D. (Department: 2879)
Optical waveguides
With optical coupler
Switch
C385S018000, C385S014000, C385S016000
Reexamination Certificate
active
06798939
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to optical switches and more particularly to techniques for promoting stability in the geometry and the placement of a bubble within an optical switch.
BACKGROUND ART
Increasingly, signal transfers within a communications network are carried out using optical signaling, with information being exchanged as modulations of laser-produced light. The equipment for generating and modulating light for optical transmissions is readily available, as are the cables for transmitting the optical signals over extended distances. However, there are concerns with regard to the switching of the optical signals without a significant sacrifice of signal strength.
One technique for switching optical signals is described in U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention. An isolated optical switch that is based on the description in Fouquet et al. is shown in FIG.
1
. The optical switch
10
is formed of layers that are patterned on a substrate. The waveguide layers on the substrate include an optional lower cladding layer
14
, an optical core
16
, and an upper cladding layer, not shown. The optical core may be primarily silicon dioxide, with doping materials that achieve a desired index of refraction. The cladding layers are formed of a material having a refractive index that is significantly different than that of the core material, so that the optical signals are guided along the core. The effective phase index of the waveguide is determined by the refractive indices of the core material and the material of the cladding layers. The layer of core material is patterned into waveguide segments that define a pair of input waveguides
20
and
24
and a pair of output waveguides
22
and
26
. After the core material is formed on the lower cladding layer, the upper cladding layer is blanket deposited. A trench
28
is etched into the cladding layers and the core material. A liquid having a refractive index that substantially matches the effective phase index of the waveguides is supplied to the trench. When the liquid is aligned with the waveguides, signals will propagate efficiently through the trench. Thus, signals from the input waveguide
20
will exit from the aligned output waveguide
26
, while signals from the input waveguide
24
will exit via the aligned output waveguide
22
.
The first input waveguide
20
and the second output waveguide
22
have axes that intersect at or near (preferably near) a sidewall of the trench
28
at an angle of incidence that results in total internal reflection (TIR). When a bubble
30
resides at the intersection of the two axes, the refractive index mismatch creates the TIR condition in which an input signal along the input waveguide
20
is reflected into the second output waveguide
22
. However, it should be pointed out that the second input waveguide
24
is not optically coupled to either of the output waveguides
22
and
26
, since the misalignment of the optical axes of the waveguides inhibits optical coupling.
The patent to Fouquet et al. describes a number of alternative embodiments for switching the optical switch
10
between a transmissive state and a reflective state. In the transmissive state, the liquid within the trench fills the entire area aligned with the waveguides
20
,
22
,
24
and
26
. One approach to switching between the two states is to include a microheater
38
that controls the formation of a bubble
30
within the liquid-containing trench
28
. When the microheater is brought to a temperature that is sufficiently high to form the bubble in the index-matching liquid, the bubble is ideally positioned across the entirety of the interface between each waveguide and the sidewall of the trench. In this ideal situation, only a small quantity of the light leaks into the trench.
The problem with obtaining the ideal condition along the waveguide-to-trench interface is that a bubble is subject to many destabilizing influences. If the surface area covered by a bubble flattened against a trench sidewall is sufficient to fully encompass the lateral extent of the optical fields of the crossing waveguides, such as waveguides
20
and
22
in
FIG. 1
, the reflection is at a stable maximum. However, any reduction below full lateral extent of the optical fields will cause optical loss. Perhaps more importantly, any variation in the reduced area will cause the reflected optical signal to vary correspondingly. Therefore, any successful approach to confining a bubble within the trench
28
and maintaining the bubble at a sufficiently large size improves the stability of optical reflections, and so improves one important aspect of operational stability of the optical switch
10
.
As one approach to providing such operational stability, the electrical power to the microheaters of optical switches may be increased to deliver ample thermal power to create and maintain the bubbles across the entirety of the interface. However, this solution has limited appeal, since the power handling constraints of a large array of optical switches and because of the desirability of operating such an array at the lowest possible power consumption level. Another approach is to appropriately design the shape and size of the trenches holding the bubbles relative to the shapes and sizes of the microheaters which create the bubbles. In the above-identified patent to Fouquet et al., a trench is extended downwardly at opposite sides of the microheater. Thus, V-shaped cuts are etched into a microheater substrate in alignment with the trench. The downward extension of the trench is intended to increase bubble stability by promoting dynamic equilibrium, with fluid boiling at the heaters and condensing at the top of the bubbles. This approach improves stability, but alternative or additional techniques are desired.
SUMMARY OF THE INVENTION
Performance stability of an optical switch that has a reflective efficiency based upon the position of a bubble within a liquid-containing trench is enhanced by allowing the liquid to flow from the trench into an adjacent space, while controlling the movement of the bubble relative to either or both of the trench and the adjacent spacing. Surface features are intentionally altered in order to regulate the position of the bubble within the trench. The optical switch includes a transmissive state in which optical signals efficiently propagate from a waveguide into the liquid within the trench, since the liquid and the waveguide have similar refractive indices. The optical switch also has a reflective state in which the optical signals are reflected as a result of the bubble being at the interface of the waveguide with the trench. The adjacent spacing accommodates volume expansion when the bubble is created by activation of a microheater, but the intentionally altered surface features are designed to control the position of the bubble relative to the waveguide-to-trench interface.
The spacing that is adjacent to the trench may be generally perpendicular to the trench. Typically, the spacing is naturally or intentionally formed when a waveguide substrate is connected to a heater substrate. In a switching network, the heater substrate includes at least one microheater for each optical switch in an array of switches. The waveguide substrate includes a liquid-containing trench and two or more waveguides for each optical switch. Coupling of the optical waveguides for a switch depends upon the presence or absence of liquid in alignment with an input waveguide of the switch. As an alternative to forming the adjacent spacing as a result of connecting two substrates, adjacent spacing that accommodates volumetric expansion may be provided by using other techniques, such as layer etching.
In one embodiment, the intentionally altered surface features that control the position of the bubble are raised barriers that partially obstruct the movement or expansion of the bubble into the adjacent spacing. For example, the raised barriers may be partial bar
Anvar David James
Chen Datong
Haven Richard Earl
Stawitcke Frederick Austin
Troll Mark Andrew
Agilent Technologie,s Inc.
Dong Dalei
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