Active reflection and anti-reflection optical switch

Details Active reflection and anti-reflection optical switch Active reflection and anti-reflection optical switch

2002-02-11

2003-02-04

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Having particular chemical composition or structure

C359S320000

Reexamination Certificate

active

06515791

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application relates to co-pending U.S. pat. application, Ser. No. 09/884,702, filed on Jun. 19, 2001, titled “Piezoelectric Actuated Optical Switch,” which claims the priority of U.S. provisional patent application, Ser. No. 60/246,284, filed on Nov. 6, 2000, both of which applications are assigned to the same assignee as the present application, and are incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates generally to optical signal switching, and particularly to a piezoelectric actuated device for switching an optical signal. More specifically, the present invention relates to an active optical modulator that allows switching from a reflecting state to an anti-reflecting state and vice versa. The switch is based on the precise controlling of an air gap between a thin film membrane and a substrate. The thin film membrane is deformed by a miniaturized adaptive material, such as electrostrictive or piezoelectric (PZT) material. Maximum optical reflection is realized when the air gap is equal to a quarter wavelength of the optical beam, while anti-reflection is achieved when the thickness of the air gap is different from the quarter wavelength.
BACKGROUND OF THE INVENTION
With the increasing popularity of the World Wide Web (“the web”), there is a continual need to increase the available communication bandwidth. The constant traffic on the web requires an infrastructure that is dynamic to accommodate new needs as they emerge. One of the most pressing challenges is the underlying pipeline, that is the bandwidth which accommodates new users and applications. Some of these applications include as video on demand, video conferencing, and so forth.
A number of photonic solutions have been proposed to increase the available network bandwidth. These solutions range from point to point connections to wavelength division multiplexed passive optical network systems. The latter solution is effective in principle, however the cost associated with photonic devices in these systems has been an impediment to their acceptance and rapid deployment.
Optical data transmission offers many advantages over electrical and broadcast transmission. However, switching optical data from one channel to another has proven to be problematic. Fundamentally, a beam of light is unaffected by passage through an electric or magnetic gradient, thus the usual solid-state methods for switching electric signals are not effective to switch optical signals. Accordingly, various mechanical techniques relying typically on reflection or refraction have been developed to divert optical signals.
FIG. 1
is a schematic diagram of a conventional optical switching array
10
. The switching array
10
includes a plurality of input ports, i.e.,
12
,
16
, and output ports
14
arranged in columns and rows. To switch an optical signal from a first input port
16
to the output port
14
, a diverter
18
located at a point of intersection between the axes of the two ports
16
and
14
, diverts the beam from the input port
16
to the output port
14
. The diverter
18
can be a mirror, a light pipe, a refractive medium, or the like. Most diverters
18
require a form of actuation to move them into or out of the path of a light beam.
FIG. 2
shows a cross-section of a MEMS diverter
18
. The diverter
18
is comprised of a base
32
suspended within a frame
34
. The base
32
includes a reflective coating
36
. Between the frame
34
and the bottom of the base
32
is an interdigitated electrostatic actuator
37
comprising interdigitated fingers
38
and
39
of the base
32
and frame
34
, respectively. The interdigitated electrostatic actuator
37
is actuated by applying electric charges to surfaces of fingers
38
and
39
to cause them to attract each other. The electric charges can be applied to specific fingers
38
and
39
, or to sets of fingers
38
and
39
, to modify how much force is applied, and in what direction, to control the induced tilting of base
32
.
Conventional MEMS diverters, however, suffer from some drawbacks. In addition to being expensive to produce, they are also sensitive to electrostatic discharges (ESD) and microcontamination. It will be readily appreciated that ESD can destroy the interdigitated electrostatic actuator
37
by melting or fusing fingers
38
and
39
. Similarly, microcontamination in the form of fine particles or surface films, for example, can mechanically jam the interdigitated electrostatic actuator
37
and prevent it from actuating. Microcontamination can also create an electrical short between fingers
38
and
39
, thereby preventing actuation.
A low-cost silicon optical modulator based on micro electro mechanical systems principles (MEMS) has been proposed, offering a low-cost, high production volume modulator. This device has been designated MARS, which is an acronym for Moving Anti-Reflection Switch. In one form, this device has a multi-layer film stack of polysilicon/silicon nitride/polysilicon, wherein the polysilicon is doped and forms the electrode material. A precisely controlled air gap between the film stack and the substrate allows switching from a reflecting state to an anti-reflecting state.
The operating principle of a conventional MARS device
100
is illustrated in
FIGS. 3
,
4
, and
5
, and is based upon the change in an air gap
105
between a suspended membrane
110
, e.g., a silicon nitride film, and an underlying substrate
120
. The membrane
110
has a refractive index equal to the square root of the refractive index of the substrate, and a thickness equal to ¼ the wavelength (&lgr;/4) of an incident light beam.
If the membrane
110
is suspended above the substrate
120
such that when the air gap
105
equals &lgr;/4, a high reflection state is achieved, otherwise, including when the air gap
105
is close to zero, an anti-reflection state is achieved. These states also hold true for any value of m&lgr;/4, wherein an even number m represents an anti-reflecting state (or mode), and an odd number m represents a reflecting state. An exemplary MARS structure that is referred to as a double-poly MARS device, is described in U.S. Pat. No. 5,654,819.
To activate this MARS device, two electrodes are provided and positioned on top of the membrane
110
and the substrate
120
, with a voltage selectively applied therebetween. The applied voltage creates an electrostatic force that pulls the membrane
110
physically closer to the substrate
120
. When thickness (depth) of the air gap
105
between the membrane
110
and the substrate
120
is reduced to substantially &lgr;/2, an anti-reflective device exhibiting substantially zero reflectivity is produced.
While this MARS device
100
provides certain advantages over other prior conventional devices, it has a potential catastrophic failure mode due to the lower polysilicon metallization. This failure mode is illustrated in
FIG. 5
, where in certain adverse conditions, such as large changes in the dielectric properties of the air gap
105
, or with unusual voltage surges (i.e., electrostatic discharge or ESD) in the switching signal the membrane
110
undergoes excessive deflection, and shorts to the substrate
120
, resulting in a device (
100
) failure.
Accordingly, it would be desirable to have an optical switching device that can redirect a beam of light that is less susceptible to microcontamination and ESD failures, and that is readily fabricated according to developed microfabrication technologies.
SUMMARY OF THE INVENTION
The present invention addresses and resolves the foregoing concerns that could lead to potential failure of the MEMS-based devices, namely (i) spurious voltage spikes and (ii) large changes in the dielectric properties of the air in the air gap.
The active optical switch of the present invention includes a thin film membrane, that is suspended over a substrate, and that is mechanically deformed by a miniaturized motor, to perform the reflection and anti-reflection switching. In a preferre

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