Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-09-28
2001-02-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S004000, C385S016000, C385S018000
Reexamination Certificate
active
06195478
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to optical switching elements and more particularly to methods and mechanisms for manipulating optical signals within a switch.
BACKGROUND ART
While signal exchanges within telecommunications networks and data communications networks have traditionally been accomplished by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. A single switching element
10
is shown in
FIGS. 1 and 2
. Waveguides are fabricated by depositing a lower cladding layer, a core, and an upper cladding layer on a substrate
12
. The switching element is shown as including first and second input waveguides
14
and
16
and first and second output waveguides
18
and
20
. The core material is primarily silicon dioxide, but with other materials that affect the refractive index of the core. The cladding layers are formed of a material having a refractive index that is substantially lower than that of the core material, so that optical signals are guided along the core material.
A trench
22
is etched through the core material to the silicon substrate in which the cladding layers and core material are formed. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench is filled with a vapor or gas. One wall of the trench
22
intersects the crosspoints of the waveguides
14
-
20
. Thus, TIR diverts light from the first input waveguide
14
to the second output waveguide
20
, unless an index-matching fluid is located within the gap between the first input waveguide
14
and the first output waveguide
18
. The fluid within the trench has a refractive index that substantially matches the refractive index of the core material. An acceptable liquid is a combination of isopropyl alcohol and glycerol. Another acceptable liquid is M-pyrol.
In the embodiment of
FIGS. 1 and 2
, two microheaters
24
and
26
control the position of a bubble
28
within the fluid-containing trench
22
. In the operation of the switching element
10
, one of the microheaters is brought to a temperature sufficiently high to form the gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the microheater. In
FIG. 1
, the bubble is located at the intersection of the core waveguides
14
-
20
. Consequently, an input signal along the first input waveguide
14
will encounter a refractive index mismatch upon reaching the wall of the trench
22
. TIR causes the input signals to be diverted to the second output waveguide
20
. Thus, the switching element is shown in a reflecting state in FIG.
1
. The activation of the microheater
24
pins the bubble at the intersection, so that the reflecting state is maintained as long as the microheater is activated.
In
FIG. 2
, the microheater
24
at the intersection of the waveguides
14
-
20
has been deactivated and the second microheater
26
has been activated. The bubble
28
is strongly attracted to the activated microheater. This allows index-matching fluid to fill the gap at the intersection of the waveguides. The switching element is in a transmitting state, since the first input waveguide
14
is optically coupled to the collinear first output waveguide
18
. Moreover, the second input waveguide
16
is optically coupled to the collinear second output waveguide
20
.
FIGS. 1 and 2
represent only one available approach to manipulating fluid within a trench of a switching element. Other approaches are described in the Fouquet et al. patent. For example, a single heater may be used to vaporize index-matching fluid at the intersections of waveguides in order to toggle a switching element from a reflective state to a transmissive state.
The testing of a switching matrix which utilizes bubble manipulation to control signal paths has yielded very positive results. However, testing for long-term reliability (e.g., 25-year operation) has not been completed, particularly for large scale switching matrices. Consequently, there are still some concerns regarding the bubble-manipulation approach for directing signals in a telecommunications or data communications network. Other types of optical switches are commercially available, but suffer from one or more of cost efficiency, unwieldy size, poor performance, or a known lack of long-term reliability.
What is needed is an optical switching element and a method for fabricating switching matrices that enable optical switching with low insertion loss, low crosstalk, and high scalability, with long-term reliability.
SUMMARY OF THE INVENTION
A switching element for a planar lightwave circuit includes a waveguide substrate in which at least two light-transmitting waveguides are formed of a core layer to extend along the substrate to a trench, so that optical coupling between the waveguides is dependent upon optical characteristics exhibited at the trench. A displaceable member is moved relative to the trench, such that a selected pair of waveguides is optically coupled when the displaceable member is in the first position and the same pair of waveguides is optically isolated when the displaceable member is in the second position. In the preferred embodiment, the displaceable member is the micromirror that is manipulated between the second position (i.e., a non-reflecting position) in which a first input waveguide is optically coupled to a first output waveguide and the first position (i.e., a reflecting position) in which the first input waveguide is optically coupled to a second output waveguide.
In some embodiments of the switching element, there is no liquid within the trench. It follows that the angle of incidence of a waveguide at the wall of the trench must be less than the critical angle required for total internal reflection (TIR). Consequently, an optical signal can enter the trench to be reflected by the micromirror when the micromirror is in the reflecting state. As a result of the ray bending (i.e., refraction) that will occur at the interfaces of the air in the trench and the input and output waveguides, the waveguides that are on opposite sides of the trench should be parallel, with an offset that is calculated using Snell's Law. Unidirectional reflection can be reduced by depositing an anti-reflection coating on the sidewalls of the air-filled trench, so that the resulting switching element is less susceptible to loss and crosstalk. If using two cross-connect switching arrays, one for each polarization component, then the angle of incidence of the waveguides to the trench can be chosen to be Brewster's angle, which in this case would be approximately 34°. This substantially eliminates reflections from the waveguide/air interfaces at trenches which do not contain liquid for one of the two polarizations.
In other embodiments, the trenches are normally filled with liquid. In these embodiments, the angle of incidence of the waveguides to the trenches can assume a large range of values. However, in practice, it is desirable to avoid very large angles of incidence, because large angles tend to lead to long path lengths in the trench. Since an optical signal is not guided as it propagates through the trench, long path lengths lead to higher insertion loss from divergence.
Movement of the mirror between the reflecting and non-reflecting positions may be in the direction perpendicular to the major surfaces of the waveguide substrate. For example, a modified multi-pin dot matrix printer engine may be used. Alternatively, recently available micro-electromechanical system (MEMS) actuators are displaceable in a direction perpendicular to the substrate surfaces and may be used to
Agilent Technologie,s Inc.
Palmer Phan T. H.
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