Apparatus comprising a micro-mechanical optical modulator

Details Apparatus comprising a micro-mechanical optical modulator Apparatus comprising a micro-mechanical optical modulator

2001-02-20

2003-11-04

Mai, Huy (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S291000, C359S248000, C359S260000

Reexamination Certificate

active

06643052

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to optical communications, and more particularly to a micro-mechanical optical modulator.
BACKGROUND OF THE INVENTION
FIG. 1
depicts passive optical network
100
. Network
100
includes central office or head-end terminal
102
, splitter
110
, wavelength routing device
112
and a plurality of network units
114
i
, i=1−n, interrelated as shown.
Central office
102
includes transmitter
104
and receiver
118
. Transmitter
104
incorporates active optical source
106
, such as a multi-frequency laser or light-emitting diode. Transmitter
104
generates optical signal
108
, which is a wavelength division multiplexed (“WDM”) signal. WDM signals comprise multiple independent data channels, each of which is assigned to a distinct optical wavelength. Central office
102
sends information over WDM optical signal
108
to the plurality of network units
114
1
-
114
n
, which each receive information over one of the distinct wavelengths.
Wavelength routing device
112
de-multiplexes WDM signal
108
into its constituent spectral components (optical wavelengths)
108
i
, i=1−n, such that the spectral components
108
i
are spatially separated from one another. Each spatially-separated spectral component
108
i
is then routed, over waveguides
113
i
, i=1−n, to the appropriate network unit
114
i
as a function of wavelength. In some embodiments, the waveguides are optical fibers.
With reference to
FIG. 2
, waveguide
109
i
delivers spectral component
108
i
to splitter
220
in network unit
114
i
. Splitter
220
routes a first portion
222
of the power of spectral component
108
i
to receiver
226
, and a second portion
224
to transmitter
228
.
Information is advantageously sent in packets to network unit
114
i
via spectral component
108
i
. The packets contain information (ie., television programming, incoming e-mail, etc.) for processing as well as continuous wave (“CW”) light or “optical chalkboard” which can be modulated with information. First portion
222
of spectral component
108
i
is converted to an electrical signal that is representative, in part, of the information contained in the packet. The electrical signal is then routed to processing electronics, not shown. Optical modulator
230
in transmitter
228
modulates information on the CW light that is contained in second portion
224
, generating modulated (ie., information-carrying) spectral component
116
i
. The information modulated onto the CW light can be, for example, phone message
232
or information
234
destined for the Internet.
Modulated spectral components
116
i
, i=1−n, returned from network units
114
1
-
114
n
, are multiplexed by wavelength routing device
112
into WDM signal
116
. Splitter
110
routes signal
116
to receiver
118
in central office
102
.
Optical modulator
230
that is used in network
100
can be a micro-mechanical optical modulator. This type of modulator typically uses optical interference principles to vary the signal strength of an optical signal (e.g., a carrier signal, such as the CW light of second portion
224
of spectral component
108
i
). One well-known implementation of such a modulator is depicted in FIG.
3
.
Modulator
230
depicted in
FIG. 3
incorporates a movable mirror, realized as movable layer or membrane
344
that is supported by supports
346
above fixed multi-layer mirror
342
. The fixed multi-layer mirror is disposed on substrate
340
. Membrane
344
forms a Fabry-Perot cavity, well known in the art, with underlying fixed mirror
342
. Membrane
344
and fixed mirror
342
are electrically connected to controlled voltage source
350
.
In operation, controlled voltage source
350
applies a voltage across membrane
344
and fixed mirror
342
thereby generating an electrostatic force. This force draws membrane
344
toward fixed mirror
342
along vector
452
, as depicted in FIG.
4
. When the applied voltage is withdrawn, membrane
344
returns to the quiescent or unactuated position depicted in FIG.
3
.
As membrane
344
moves toward fixed mirror
342
, the size of the Fabry-Perot cavity (i.e., the size of gap
448
between membrane
344
and fixed mirror
342
) changes. This change is accompanied by a change in the reflectivity of modulator
230
. The optical interference principle that governs this behavior is described with reference to FIG.
5
.
In a typical prior art modulator, membrane
344
has an optical thickness that is an odd integer multiple of one-quarter of the operating wavelength (“&lgr;/4”) of the modulator. Fixed multi-layer mirror
342
consists of anti-reflection layer
554
and coating layer
556
that each have an optical thickness that is an odd integer multiple of &lgr;/4. Membrane
344
and coating layer
556
have a refractive index that is equal to the refractive index of substrate
340
. Anti-reflection layer
554
has a refractive index that is about equal to the square root of the refractive index of the substrate
340
. See, Marxer et al., “MHz Opto-Mechanical Modulator,” Transducers '95—Eurosensors IX, The 8
th
International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 289-292.
In a modulator that is configured as described above, modulator reflectivity is at a high value (i.e., a relative maxima) when the size of gap
448
is an odd integer multiple of &lgr;/4. This configuration generates a constructive interference condition since the round trip distance of the optical signal from membrane
344
, across gap
448
, over coating layer
556
and back again is an integer multiple of &lgr;. That is, the optical signal is in-phase.
Conversely, modulator reflectivity is reduced to zero (i.e., a relative minima) when the size of gap
448
is zero or an even integer multiple of &lgr;/4. This configuration generates a destructive interference condition since the round trip distance of the optical signal is an integer multiple of 3&lgr;/2-180 degrees out of phase.
FIGS. 6 and 7
depict the performance of micro-mechanical modulator
230
having the layer arrangement and layer characteristics shown in FIG.
5
and that is designed for a wavelength, &lgr;, of 1570 nanometers.
FIG. 6
depicts reflectivity as a function of wavelength for modulator
230
wherein membrane
344
is silicon and a substrate
340
is silicon. Plot
658
shows the maximum reflectivity condition wherein the size of gap
448
is an odd integer multiple of &lgr;/4—in this case, 3&lgr;/4. For this particular configuration, maximum reflectivity is shown to be about 97 percent. Plot
660
shows the minimum reflectivity condition wherein the size of gap
448
is an even integer multiple of &lgr;/4—in this case, 2&lgr;/4. For this particular configuration, minimum reflectivity is zero at the design wavelength of 1570 nanometers.
FIG. 7
depicts reflectivity as a function of wavelength for modulator
230
wherein membrane
344
is silicon and substrate
340
is germanium. Plot
762
shows the maximum reflectivity condition wherein the size of gap
448
is an odd integer multiple of &lgr;/4, which, again, is 3&lgr;/4. For this configuration, maximum reflectivity is, as before, about 97 percent. Plot
764
shows the minimum reflectivity condition wherein the size of gap
448
is an even integer multiple of &lgr;/4, here, 2&lgr;/4. Minimum reflectivity is zero at the design wavelength of 1570 nanometers.
FIGS. 6 and 7
demonstrate that silicon and germanium can be used interchangeably as the substrate with substantially no impact on modulator performance.
FIGS. 6 and 7
also illustrate a shortcoming of this particular modulator arrangement. Specifically, while insertion loss is minimized for the modulator configuration described above, the operating bandwidth is relatively narrow. That is, minimum reflectivity rises relatively rapidly with deviations from the design wavelength (e.g., 1570 nanometers) so that contrast (ie., the ratio of the maximum reflect

Affiliated with

Also associated with

Rating

Apparatus comprising a micro-mechanical optical modulator does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Apparatus comprising a micro-mechanical optical modulator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus comprising a micro-mechanical optical modulator will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3124719