Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-08-11
2001-11-20
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Switch
C385S018000
Reexamination Certificate
active
06320995
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to optical switching arrangements and more particularly to connectivity among switching units in a multistage optical switch.
BACKGROUND ART
While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, reliable optical switching systems have been developed. 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. Another such matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element
10
is shown in
FIG. 1
, while a 5×5 matrix
32
of switching elements is shown in FIG.
2
. The optical switch of
FIG. 1
is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch
10
includes planar waveguides defined by a lower cladding layer
14
, a core
16
, and an upper cladding layer, not shown. The core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers are formed of a material having a refractive index that is different from the refractive index of the core material, so that optical signals are guided along the waveguides.
The core material
16
is patterned to form an input waveguide
20
and an output waveguide
26
of a first optical path and to define a second input waveguide
24
and a second output waveguide
22
of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap is formed by etching a trench
28
through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the crosspoint
30
of the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide
20
to the output waveguide
22
, unless an index-matching fluid resides within the crosspoint
30
between the aligned waveguides
20
and
26
. The trench
28
is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides.
The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the switching element
10
between a transmissive state and a reflective state. The element includes at least one heater that can be used to manipulate fluid within the trench
28
. One approach is illustrated in FIG.
1
. The switching element
10
includes two microheaters
38
and
40
that control the position of a bubble within the fluid-containing trench. The fluid within the trench has a refractive index that is close to the refractive index of the core material
16
of the four waveguides
20
-
26
. Fluid fill-holes
34
and
36
may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching element, one of the heaters
38
and
40
is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In
FIG. 1
, the bubble is positioned at the crosspoint
30
of the four waveguides. Consequently, an input signal along the waveguide
20
will encounter a refractive index mismatch upon reaching the trench
28
. This places the switching element in a reflecting state, causing the optical signal along the waveguide
20
to be redirected to the output waveguide
22
. However, even in the reflecting state, the second input waveguide
24
is not in communication with the output waveguide
26
.
If the heater
38
at crosspoint
30
is deactivated and the second heater
40
is activated, the bubble will be attracted to the off-axis heater
40
. This allows index-matching fluid to fill the crosspoint
30
of the waveguides
20
-
26
. The switching element
10
is then in a transmitting state, since the input waveguide
20
is optically coupled to the collinear waveguide
26
.
In the 5×5 matrix
32
of
FIG. 2
, any of the five input waveguides
42
,
44
,
46
,
48
and
50
may be optically coupled to any one of the five output waveguides
52
,
54
,
56
,
58
and
60
. The switching matrix is sometimes referred to as a “non-blocking” matrix, since any free input fiber can be connected to any free output fiber regardless of which connections have already been made through the switching matrix. Each of the twenty-five optical switches has a trench that causes TIR in the absence of a fluid at the crosspoint of the waveguides, but two collinear waveguides of a particular waveguide path are optically coupled when the crosspoint associated with the waveguides is filled with the fluid. Trenches that are in the transmissive state are represented by fine lines that extend at an angle through the intersections of the optical waveguides in the matrix. On the other hand, trenches of switching elements in a reflecting state are represented by broad lines through points of intersection.
In
FIGS. 1 and 2
, the input waveguide
20
is in optical communication with the output waveguide
22
, as a result of TIR at the crosspoint
30
. Since all other crosspoints for allowing the input waveguide
48
to communicate with the output waveguide
54
are in a transmissive state, a signal that is generated at input waveguide
48
will be received at output waveguide
54
. In like manner, the input waveguide
42
is optically coupled to the output waveguide
60
, the input waveguide
44
is optically coupled to the output waveguide
56
, the input waveguide
46
is optically coupled to the output waveguide
52
, and the input waveguide
50
is optically coupled to the output waveguide
58
.
One concern with optical switching elements
10
of this type is that in the transmissive state, there is a small but potentially objectionable amount of reflection. If the index of refraction of the fluid is different than that of the core material
16
, reflections occur. A precise match between the indices of refraction is problematic, since there are other considerations in the selection of a fluid. For example, since the fluid is manipulated using thermal energy, the thermal properties of the liquid must be considered. Thus, there is a loss of signal strength at each transmissive crosspoint within the matrix
32
.
In
FIG. 2
, an optical signal that enters the input waveguide
48
Input Port
3
) will pass through one crosspoint in the horizontal direction and three crosspoints in the vertical direction, since the optical signal will be reflected at the switching element
10
. The total loss is 1 k+3 k=4 k, where k is the loss associated with each crosspoint. On the other hand, an optical signal entering the input waveguide
50
(Input Port
4
) will propagate through the output waveguide
58
(Output Port
3
), thereby passing through three crosspoints in the horizontal direction and four crosspoints in the vertical direction. The total loss is 3 k+4 k=7
Agilent Technologies , Inc
Font Frank G.
Lauchman Layla
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