Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-11-29
2002-07-23
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000, C359S292000, C359S298000
Reexamination Certificate
active
06424450
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to optical modulators. More particularly, the present invention relates to micro electromechanical systems (MEMS)-based optical modulators that rely on optical interference as a principle of operation.
BACKGROUND OF THE INVENTION
Some optical modulators are capable of varying the intensity of an optical signal. This intensity variation can be achieved using optical interference principles. Modulators relying on this operating principle typically incorporate an optical cavity that is defined by two spaced surfaces having appropriate indices of refraction. Varying the size of the gap between the two surfaces alters the reflectivity of the optical cavity.
Optical modulators that operate in this fashion have been built using MEMS technology. For example,
FIGS. 1-3
depict a MEMS-based optical modulator 100 that is disclosed in U.S. Pat. No. 5,751,469.
Referring now to
FIG. 1
(cross-sectional view) and
FIG. 2
(plan view), modulator
100
includes a membrane
104
that is suspended above substrate
102
by support layer
106
. Membrane
104
and substrate
102
are parallel to one another and separated by gap
108
. In modulator
100
, membrane
104
completely overlaps optical cavity
110
which is defined laterally by the perimeter of a circular opening in support layer
106
and vertically by membrane
104
on top and substrate
102
on the bottom. Membrane
104
overlaps the optical cavity in the same manner as a drum-head overlaps the body of a drum.
As a consequence of the circular shape of optical cavity
110
, the unsupported portion of membrane
104
(i.e., the portion of the optical cavity) is, of course, circular. As described in U.S. Pat. No. 5,751,469, this configuration advantageously significantly reduces stress that would otherwise concentrate in the narrow membrane support arms that are typically used to support the membrane in other prior art designs (see, e.g., U.S. Pat. No. 5,500,761).
Membrane
104
advantageously has a plurality of holes
112
. In the embodiment depicted in
FIG. 2
, holes
112
are radially arranged, although other configurations can suitably be used. Holes
112
damp membrane vibration and are also used during fabrication to deliver etchant, typically HF acid beneath membrane
104
to create optical cavity
110
. Holes
112
are located in membrane
104
outside of a centrally located “optical window”
114
that receives optical signal
120
from an optical waveguide, such as an optical fiber (not shown).
In operation, membrane
104
moves toward substrate
102
(see
FIG. 3
) under the action of an actuating force. And, as it does so, the size of gap
108
decreases, altering optical properties of optical cavity
110
. In particular, the reflectivity of the device changes. For a membrane having a thickness equal to one quarter of a wavelength of the incident optical signal, as measured in the membrane (hereinafter “quarter-wave” layer or membrane or “&lgr;/4”), a relative maxima in reflectivity occurs when gap
108
is equal to odd integer multiples of one-quarter of the operating wavelength (“high reflectivity state”). That is, relative maxima occur at:
R
Max
=m&lgr;/4 [1]
where: &lgr; is the operating wavelength of the modulator; and
m equals 1, 3, 5 . . .
Similarly, relative minima in reflectivity occur when gap
108
is equal to zero or an even integer multiple of one-quarter of the operating wavelength (“low reflectivity state”):
R
Min
=m&lgr;/4 [2]
where: &lgr; is the operating wavelength of the modulator; and
m equals
0
,
2
,
4
. . .
The maximum contrast (see below; contrast=R
Max
/R
Min
) is obtained when, in one state of the modulator, the size of gap
108
results in a reflectivity maxima and, in the other state, the size of gap
108
results in a reflectivity minima. Consequently, in a “quiescent” or “non-actuated” state, as those terms are used herein, membrane
104
has a first position wherein the size of the gap is such that either an R
Max
or R
Min
condition is met. In an “actuated state,” as that term is used herein, membrane
104
moves to a second position nearer substrate
102
. Again, for maximum contrast, membrane
104
moves through a distance &lgr;/4 when actuated.
In practice, the difference in size of gap
104
in the non-actuated and actuated states is often less than &lgr;/4 since the membrane “snaps down” to the substrate if membrane deflections greater than about thirty to thirty-five percent (relative to the size of the gap in the unbiased state) occur. Consequently, rather than specifying, for example, that the membrane moves between a non-actuated position of 3&lgr;/4 to an actuated position of 2&lgr;/4, a more conservative design will specify that the membrane moves between a non-actuated position of about 0.7&lgr;, to an actuated position of 2&lgr;/4. In the former design, snap down will probably occur since the membrane deflects an amount equal to: (3&lgr;/4−2&lgr;/4)/(3&lgr;/4) or 33 percent. In the conservative design, snap down is avoided since the membrane deflects less than about thirty percent: (0.7&lgr;−2&lgr;/4)/(0.7&lgr;)=28.6 percent.
In some embodiments, the actuating force for moving the membrane is an electrostatic force that is generated by creating a potential difference across substrate
102
and membrane
104
. To that end, membrane
104
and substrate
102
are suitably conductive, or otherwise include a region of metallization or doping to provide such conductivity. In modulator
100
depicted in
FIGS. 1-3
, the electrostatic actuating system includes contact
116
, which is in electrical contact with membrane
104
and controlled voltage source
222
, and contact
118
, which is in electrical contact with substrate
102
and controlled voltage source
222
.
The performance of modulator
100
can be gauged using several parameters. Once such parameter is “contrast,” which, as that term is used herein, is the ratio of maximum reflectance to minimum reflectance for the modulator. Another important performance parameter is the theoretical “insertion loss,” which, as used herein, is one hundred minus the maximum reflectance of the modulator. A third performance parameter is “bandwidth,” which for the purposes of the present Specification, means the range of wavelengths over which an acceptable amount of contrast is obtained. These performance parameters of optical modulator
100
are dependent upon certain physical characteristics of the modulator such as the refractive indices of membrane
104
and substrate
102
, the thickness of the membrane
104
and the size of gap
108
.
A modulator possessing high contrast, low insertion loss and a wide bandwidth is desirable. But neither modulator
100
, nor other prior art MEMS-based optical modulators, possess the full measure of all of these characteristics. It is known, however, that these characteristics can be traded-off, as desired. The trade-off among performance parameters is accomplished by manipulating the aforedescribed physical characteristics. TABLE I provides a summary of the manner in which modulator physical characteristics have been manipulated in the prior art to achieve a desired modulator performance. Abbreviations used in the table include: n
m
for the refractive index of the membrane, n, for the refractive index of the substrate, L
1
, L
2
, etc., indicates a first layer of the membrane, second layer of the membrane, etc.
TABLE 1
Reflectivity
Membrane
of
Gap at
Thickness
Refractive
Membrane
R
max
and
Relative
and No. of
Index of
vs.
Gap at
Insertion
Relative
Relative
Source
Layers
Layers
Substrate
R
min
Loss
Bandwidth
Contrast
U. S. Pat.
Typically 1
L1 n
m
= n
s
0.5
Equal in
m(&lgr;/4):
High
Wide
High
No.
or 2. each
L2 n
m
= n
s
magnitude
for R
max
5,500,761
of which
& opposite
m = odd
are &lgr;/4
in phase
for R
min
m = even
U. S. Pat.
At lease 3,
L1 n
m
= n
s
Equal in
m(&lgr;/4)
High
Very Wide
High
No.
L1 = &lgr;/4
L2 n
m
= n
Aralight, Inc.
Dang Hung Xuan
DeMont & Breyer LLC
Tra Tuyen
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