Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2001-03-14
2002-12-31
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S022000, C385S017000, C385S020000
Reexamination Certificate
active
06501869
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention generally relates to the field of optical switching. More specifically, the present invention relates to optical systems comprising hollow optical waveguides and micro-mechanical deflectable mirrors.
Optical switching is a fundamental operation that is useful in optical communications, optical memory, and optical displays. In its most basic form, optical switching involves spatial redirection of information-carrying optical waves.
FIG. 1
(prior art) shows a simple and generic optical switch
100
. This one-input—two-output optical switch directs input optical signal
101
to one or both of the two optical output ports, output
1
102
and output
2
103
, under the command of a control signal
104
.
This switch may be generalized to N-output ports.
FIG. 2
(prior art) illustrates how a I×N switch
200
may be constructed from multitude of 1×2 optical switches
100
. In this particular illustration, N equals 6. In the I×N configuration, a switch may be used in a signal distribution network. Alternatively, by sequentially switching the optical signal to the N output ports that are arranged in a 2-D spatial array, the switch may be used in a display device where the input optical signal contains image information. The same switch may also be used to access different spatial locations in an optical memory.
A basic switch
300
may also be generalized to contain several input ports as illustrated in
FIG. 3
(prior art). Again, a simple switch may contain two input ports, input
1
301
and input
2
302
, as well as two output ports, output
1
303
and output
2
304
. This may be a generalization of switch
100
shown in FIG.
1
. Under the direction of an external control signal
305
, each of the input signals,
301
and
302
may be directed to one or both of the output ports,
303
and
304
respectively. When a single input is distributed to both output ports, the operation is called “broadcast or fan-out.” When both input signals are directed to a single output port, the operation is termed “concentration or fan-in.” When each output port receives one or the other of the input signals, the operation is termed “permutation”.
This switch may be generalized when the number of input signals and output ports exceed two. In general, the number of input signals, say M, and the number of output ports, say N, do not have to be equal. Such an M×N optical switch forms the heart of an optical switching network in a communication system or the interconnection fabric inside a high performance computing system. Again, one skilled in the art can easily see how such an M×N optical switch may be constructed from M optical,switches, each of which is a I×N switch of the type shown in FIG.
2
. Corresponding output lines from each of the M such switches may be combined to generate one of N output lines.
Performance parameters associated with such M×N optical switches includes but is not limited to: the size of the switch (values of M and N); the bandwidth of the optical signals the switch can carry; the rate at which the signals can be redirected or the switch can be reconfigured; the optical efficiency of the switch (loss); the isolation between ports (cross talk); the ability to perform a broadcast operation; the ability to handle optical signals of different formats (analog, digital, AM, FM, PM, multiple wavelengths); the complexity of the control signal interface; the ease of interfacing to optical fibers; the overall size; the weight; the power consumption; the mechanical stability; and the ease of manufacturing. Each performance parameter is not equally important to different applications. In fact, some of them may not be relevant to certain applications at all. For example, the ease of interfacing to optical fibers may not be relevant to display applications. To the extent that some of these parameters may not be optimized simultaneously, the trade-off between them may be governed by the specific applications for which they are being designed.
Given the widespread applications of optical switches, it is not surprising that over the years a number of different designs have been proposed and implemented. These designs primarily fall in two categories: designs where light propagates in guided wave structures inside the switch and designs where light propagates in free space inside the switch.
Guided Wave Optical Switches
FIG.
4
A and
FIG. 4B
(prior art) show schematic diagrams of a simple 1×2 optical switch involving mechanical movements and using optical fibers. As illustrated, the type of switch may have an input fiber
402
and two output fibers
404
and
406
, switched by mechanical movements
410
and
412
.
FIG. 4A
shows input fiber
402
switched to output fiber
404
and
FIG. 4B
shows input fiber
402
switched to output fiber
406
. This type of switch may be extremely simple to construct, have low loss, have very high isolation between ports, and carry full bandwidth available to optical fibers regardless of the format. However, this switch may be slow to reconfigure due the difficulty of moving a large mass, may not allow broadcast and may not be easily extendable to an M×N switch. Still, such switches, due to their simplicity have found uses in fiber optic networks.
A second type of guided wave switches based on optical propagation in planar (integrated optic) waveguides and dynamic directional couplers is illustrated in
FIG. 5
(prior art). This schematic diagram of a 1×2 switch is based on a directional coupler. Light from a single mode waveguide
520
may be coupled into another single mode waveguide
522
that is brought in close proximity, allowing interaction between each waveguide,
520
and
522
, via evanescent field coupling. By controlling the phase of the evanescent wave in the intervening region using a control mechanism
510
, the coupling efficiency may be modulated between 0 and 100% (for ideal devices). If the inputs
502
and
506
to both waveguides contain optical signals, the same switch may be made to behave like a 2×2-permutation switch, where the outputs are illustrated as
504
and
508
. The phase of the evanescent wave may be controlled at the control mechanism
510
via electrooptic effect or thermooptic effect. Several such 2×2 switches may be combined in a tree-like structure to realize an M×N permutation switch. Such integrated optic crossbar switches have been commercially available from a number of companies including Ericsson of Stockholm, Sweden, Lucent of Murray Hill, N.J., and Worldwide Telecommunications Corporation (NTT) of Tokyo, Japan. These switches may be compact, switched extremely fast (if electrically controlled), have good isolation, and carry high bandwidth optical signals. However, these switches may also be difficult to couple to fibers, be environmentally sensitive, not achieve broadcast functionality, and not extend easily to large sizes without consuming a large integrated optic chip area.
FIG. 6
(prior art) illustrates an experimental approach to integrated optical switches utilizing electrooptically-activated Bragg reflectors embedded into waveguide junctions. Two one-dimensional arrays including a first input optical waveguide
610
, a second input optical waveguide
612
, a first output optical waveguide
614
, and a second output optical waveguide
616
may be arranged orthogonally oriented to each other in a single plane as shown in FIG.
6
. With no electric field activating a grating (
620
,
622
,
624
, or
626
) at a junction, light injected in a waveguide may continue propagating along it. When the grating is activated, light may be scattered into the output waveguide that crosses the input waveguide.
FIG. 6
shows this switch structure schematically. The figure shows a 2×2 switch where input signal
1
602
is coupled to output port B
608
and input signal
2
604
is coupled to output port A
606
. This is achieved by activating gratings
624
and
622
, while leaving gratings
6
George Mason University
Grossman David G.
Ngo Hung N.
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