Bifanoa,b, Steven Cornelissenb, Jason Stewartb, Zvi Bleierc
aBoston University Photonics Center, 8
Saint Mary's St. Boston, MA 02215;
bBoston Micromachines Corp. 30 Spinelli
Place, Cambridge, MA 02138;
40 West Jefryn Blvd. Deer Park, NY 11729;
An optical communication
system suitable for voice communication, data retrieval from remote sensors and
identification had been designed, built and tested. The system design allows
operation at ranges of several hundred meters. The heart of the system is a
modulated MEMS mirror that is electrostatically actuated and changes between a
flat reflective state and a corrugated diffractive state. A process for mass
producing these mirrors at low cost was developed and implemented. The mirror
was incorporated as a facet in a hollow retro-reflector, allowing temporal
modulation of an interrogating beam and the return of the modulated beam to the
interrogator. This modulator unit thus consists of a low power, small and light
communication node with large (about 60o) angular extent. The
system's range and pointing are determined by the interrogator /detector /
demodulator unit (the transceiver), whereas the communicating node remains
small, low power and low cost. This transceiver is comprised of a magnified
optical channel to establish line of sight communication, an interrogating
laser at 1550nm, an avalanche photo diode to detect the return signal and
electronics to drive the laser and demodulate the returned signal and convert
it to an audio signal. Voice communication in free space was demonstrated at
ranges larger than 200 meters. A new retro-reflector design, incorporating more
modulated mirrors had been constructed. This configuration was built and
tested. Its performance and advantages as compared to the single mirror
retro-reflector are discussed. An alternative system design that allows higher
bandwidth data transmission is described.
have been used in optical systems for some time in applications requiring a
precise return of a parallel beam of light to its source. A common application
uses a retro-reflector as a passive component to return incident laser pulses
to the location of a pulsed laser source, allowing round trip time of flight measurement
and thus precise range measurement. Active retro-reflectors are a more recent
invention, and are generally retro-reflectors that can modulate an incident laser beam before returning it to the source.
These so-called modulated retro-reflectors (MRRs) offer an intriguing option
for laser communication, since they automatically return remote information
(without the need for beam steering) to an interrogating source, and do not
require transmission source at the MRR. This allows low-power, covert
communication. The concept was first patented in an embodiment comprised of a
retro-reflector and a liquid crystal shutter . Subsequently, researchers at
the Naval Research Laboratory have made considerable progress in developing
MRRs using multiple quantum well (MQW) shutters [2-5]. These devices offer
bandwidths of tens or hundreds of MHz, and can be quite compact. The MQW
approach works over a narrow range of interrogation laser wavelengths, and is
relatively expensive to manufacture. Therefore, it is not suitable for low-cost,
flexible, remote-sensing applications. A number of groups have developed very
short range MRRs using microelectromechanical systems (MEMS) using both MEMS
corner cubes [6-8] and micro-scale cat's eye retro-reflectors [9-10]. These
promise low cost, but are range limited to several tens of meters.
The present work
is intended to fill a gap in these approaches: MEMS-based MRRs suitable for
low-cost, compact communication over hundreds of meters. A compact, low-power
device has been developed to detect and modulate an incoming laser beam, and
then return the modulated signal back to the location of the sender. MRR at the
heart of this device consists of a MEMS based electromechanical modulator
combined with a passive retro-reflector (e.g. a hollow retro-reflector). The
modulator serves as one (or more) of the three reflective facets of the retro-reflector.
The MEMS modulator can be made to act as a plane mirror, maximizing the amount
of light that is retro-reflected, or it can be made to act as a non-plane
(corrugated) mirror, reducing the amount of light that is retro reflected. One
of these states requires no power to maintain, and the other uses only
microwatts. The modulation frequency can go up to at least a megahertz with
conventionally processed MEMS fabrication tools and materials. Data to be sent
from the MRR could include voice communication, remotely sensed environmental
information, or identification information. A key attribute of this system is
that the transmission power and pointing control required for long-distance
communication are delivered by the interrogating beam. That is, the strength of the interrogating beam determines the strength
of the returned beam (and thus the range). Furthermore, the noise-limitations
for the system are contained in the receiver co-located with the sending beam.
Almost no power is required at the MRR.
In this paper,
design, fabrication, and testing of the MEMS modulator is detailed, and
integration of the modulator with a hollow retro-reflector is described. A
prototype laser communication system built with the MRR is introduced. In that
prototype system, analog audio data from a microphone is frequency modulated
and used to drive the MEMS optical modulator that is embedded in a wearable
hollow retro-reflector. The MRR unit is interrogated by a compact laser
transceiver system that is comprised of a 1.55µm wavelength 10mW laser and
driver, a magnified optical visible channel and receiver, an avalanche
photodiode, a frequency to voltage demodulator, and an amplifier and speaker.
With this system, secure, one-way audio communication over several
hundred-meter range has been demonstrated.
1.1 Principle of
The MEMS modulator alternates
between two states: A flat state, returning most of the incident power to the
source, and a corrugated state, that acts as a diffraction grating, diverting a
significant amount of the light to higher orders, thus depleting the zero order
return. The blinking MEMS mirror modulates an interrogating CW laser beam,
returning optically encoded information to the source location, where it is
The work described in this
paper is motivated by the need to fill a gap in available MRR technology by
developing and demonstrating an inexpensive MRR communication system with
extended range operating at relatively low power. There is a need for systems
with bandwidth greater than 100kHz, size less than 100cm3, and range
up to 1 km. The basic concept of the system is illustrated in Fig. 1: An
electrically modulated deformable MEMS mirror is embedded in a hollow
retro-reflector, optically modulates an interrogating laser beam and returns
the modulated signal to the interrogating source, where it is decoded.
1: Principle of operation
system is comprised of two main parts: The Retro-Communicator and Transceiver.
The Retro-Communicator uses analog audio data from a microphone to frequency modulate
the drive signal to the deformable MEMS mirror. The modulator is embedded in a
hollow retro-reflector and thus sends the modulated signal back to the
interrogator (Transceiver), where it is received, demodulated and converted
back to an audio signal.
The Retro-Communicator is
conceptually comprised of two parts: The Optics Module and the Electronics
Module. These two parts are combined in one small box, shown in Fig. 2.
2: Retro-Communicator Module (left) and block diagram – optics and
The Optics Module contains
the deformable mirror (DM) embedded in a hollow retro-reflector as also seen in
Fig. 2. The Electronics Module contains the modulating electronics to drive the
DM, as well as batteries, high voltage DC power generation, and a microphone
The Interrogator/Transceiver is
comprised of 1.55 mm, 10mW, CW laser, a magnified visual aiming channel
and receiver, an avalanche photodiode detector, a frequency to voltage demodulator,
and an amplifier and speaker. The Transceiver and its block diagram are shown
in Fig. 3.
3: Transceiver and block diagram
In this section, we describe
the main component of the system and their characteristics.
3.1 Modulator design
To modulate the intensity of the interrogating laser beam, a
reflective MEMS mirror was designed that is optically flat in its "off" state
and a diffraction grating in its "on" state, thereby deflecting most of the incident
beam energy out of the reflection path. The architecture for the modulator,
illustrated in Fig. 4, uses electrostatic actuation to deform rows of edge-supported
narrow plates to form a diffraction grating. By maintaining the reflective gold
mirror surface, supported by a tensile silicon nitride layer, at a ground
potential and applying a voltage to the conductive substrate, an electrostatic
force is generated between those two surfaces resulting in deflection of the
flexible mirror surface.
Fig. 4: Illustration of MEMS Modulator architecture
electrostatic actuation to modulate the interrogating beam has the benefits
that the actuators consume almost no power, exhibit no hysteresis, and the
deformable mirrors are relatively easy to fabricate in standard semiconductor
fabrication processes. An illustration of the fabrication process flow is shown
in Fig. 5.
Fig. 5. Schematic cross
section of modulator fabrication process. The area in blue is
the shape of gold film and defines the active aperture of the device.
The process uses an optically-flat,
electrically-conductive silicon substrate onto which thin films of silicon
dioxide and silicon nitride are deposited (step 1). A 90-nm thick gold film is
then deposited using an e-beam evaporator (step 2). The gold film is
lithographically patterned and etched to form small, 4-mm
holes. The patterned gold film is then used as a mask to etch the small holes
into the silicon nitride using reactive ion etching (step 3) that are used to
define the shape of the actuators during release. The layout of the reflective
gold modulator, shown in Fig. 5 (right), is defined by a second photoresist
pattern which removes the gold from non-active regions (step 4). Lastly, the
silicon dioxide layer is partly etched using hydrofluoric acid, leaving behind
small actuator anchors (step 5) and forming the electrostatic actuators. The finished modulator
has an active diameter of 9 mm.
3.2 Modulator optical
and electromechanical characterizatio
and electromechanical characterization of the MEMS modulators was performed
using both a Wyko surface mapping interference microscope as well as an optical
setup that simulated retro-reflector performance in the field. In operation,
light returned from a modulator with a 9mm diameter separated at a distance on
the order of a few hundred meters from the interrogating source can be
described using Fraunhofer diffraction theory. When the modulator is in the
"on" state, the retro-reflected beam returned to the interrogator receiver
aperture corresponds to the 0th order of the diffracted wavefront.
To simulate device performance in the lab, the far-field diffraction pattern is
simulated using a lens located a focal length from the modulator. By placing a
high speed photo detector at the Fourier plane of the lens (back focal plane),
intensity changes in the 0th diffraction order can be monitored as
the modulator transitions between the on and off states, providing both static
and dynamic evaluation of modulator behavior.
optical setup for the modulator characterization system can be seen in Fig. 6.
The primary components consist of: A 635nm wavelength laser diode (LD) for
illumination, lenses (L1 and L2) to expand the illumination beam to fill the full
modulating retro-reflector mirror (MRM) aperture (at normal incidence), and
lenses (L3 and L4) to project a representative far-field diffraction pattern
onto a fast 2.1MHz bandwidth photo detector (PD). The photo detector uses an
adjustable iris to block everything but the 0th diffraction order.
Fig. 6: Modulator optical
The mirror flatness was measured using the Wyko
surface interference microscope. The results are shown in Fig. 7. Typical mirror surface
flatness for the full mirror aperture is about 10nm RMS. The local surface
quality is also on the order of 10nm RMS, primarily a result of the etch-access
holes and actuator anchor pattern.
7: Surface figure measurements of the unpowered modulator: full aperture (left)
and high magnification (right).
0th order of the "far-field" diffraction pattern is projected onto
the photo detector pupil, as shown in Fig 8, such that only this order's
intensity can be recorded for different modulator deflections. As previously mentioned,
the detected 0th order intensity is analogous to what an
interrogating illumination system would receive from the modulator in the field
if it were mounted in a retro-reflector assembly. As the modulator deflects,
the intensity of the 0th diffraction order decreases while off-axis
higher diffraction orders increase in intensity. The output voltage of the
photo detector (PDV) is recorded using an oscilloscope.
Fig. 8: "Far-field" modulator diffraction pattern
(left) and optical characterization system (right). Photo detector pupil
located at 0th diffraction order.
opto-electromechanical behavior for two modulators with a grating pitch of a
150mm, and spans of 90mm and 125mm, can be seen in Fig 9. This data was obtained using both
the optical characterization system discussed above, as well as the Wyko
surface mapping interference microscope. The combination of these datasets
provides an empirical relationship between 0th order diffraction
intensity and modulator deflection. Apparent in these characterization results
is that a reduction in modulator span requires an increase in voltage to
achieve identical modulator deflections. This is due to an increase in
modulator membrane stiffness with a decrease in span. Stiffness is inversely
proportional to span .
Fig. 9: DC characterization of modulators with 125mm
and 90mm spans (both with grating pitch of 150mm)
(Left). With increased stiffness, the applied voltage-deflection relationship
for the shorter span device shifts to the right. Deflected modulator surface
measurements shown at 80V and 105V for the 125mm
and 90mm designs respectively.
mounted in the hollow corner cube retro-reflector configuration, light is
incident on the modulator surface at a 45 degree angle, and the plane of
incidence, which is orthogonal to the plane of the grating, is rotated by 45
degrees with respect to the grating profile. This influences the phase matching
conditions that create the individual diffraction orders, which reduces the
modulation contrast achievable for a given modulator deflection value. In other
words, the relationship between modulator deflection and 0th order
diffraction intensity experiences a shift towards higher deflection, as shown
in Fig 10 for the 90mm span modulator design.
In order to achieve the same modulation contrast, the modulator must deflect
further. Also shown in Fig 10 is that modulation contrast is uninfluenced by
rotation of the grating profile with respect to the plane of incidence. At a
45 degree angle of incidence, modulation contrast remains the same when the
grating orientation is rotated by 90 degrees relative to the plane of
10: Modulation contrast for the 90um span
device at 0 degree and 45 degree incidence. A 90 degree rotation of the
modulator around its surface normal does not change modulation contrast
performance. The results are scaled by the 0V contrast value.
addition to the DC characterization of the 90mm and 125mm
span modulators, the optical characterization setup was also used to perform an
intensity-based measurement of modulator dynamics at standard pressure. A
sinusoidal voltage was applied to these devices with variable frequency and
amplitude. The modulation contrast of the detected signal at normal incidence
was measured using the photo detector, and calculated as (Vmax – Vmin) / (Vmax + Vmin). These results
can be seen in Fig. 11, where an increase in the primary resonance of about
100kHz is apparent for the stiffer 90mm
span device. Modulation contrast was examined at additional voltages, although
only a few are displayed in this figure.
Figure 11: Modulation contrast response to
sinusoidal drive voltage at variable frequencies and amplitudes.
evaluate the influence of air damping on the dynamic performance of the
modulator, the step response of the modulator to a 0 to 60V square wave (at
40kHz) was also evaluated using the optical characterization setup. The results
of these measurements for the 125mm
span modulator at 760 Torr and 250 Torr can be seen in Fig. 12. Here it is
apparent that the prototype device behaves as an under-damped harmonic
oscillator with atmospheric resonance of approximately 690kHz, and a settling
time of approximately 10µs. Faster settling time could be achieved by
increasing device damping through pressurized packaging, as well as changing
the mechanical resonance of the MRM. Also apparent in the ringing behavior of
Fig. 12 (right) is overshoot of the modulator membrane as it returns to the
off-state, seen as the double peaks in the recorded data.
Fig. 12: Dynamic response of the 125mm
span modulator to a 0 to 60V, 40kHz square wave, as seen by the photo detector
in the optical characterization system; 760 Torr (left) and 250 Torr (right).
Atmospheric resonance was approximately 690kHz, with a 10µs settling time. The
peak detector intensity corresponds to the modulator in its off (flat) state.
previously described, the characterization of the modulator was performed using
visible 635nm light, where in the retro-reflector application IR radiation with
a wavelength of 1550nm is used. To understand the change in modulator
performance as a function of wavelength, a model was used to determine the
influence of modulator shape on diffraction efficiency. In this simulation, it
was assumed that the modulator imparts a sinusoidal varying phase on the
reflected beam. These results can be seen in Fig. 13 for 532nm, 635nm and
1550nm, that are the three wavelengths we used with the system, where the
normalized peak intensity of the diffracted 0th order is plotted for
variable modulator deflection depths. In this comparison of modulation
contrast for a sinusoidal grating, similar to the modulator presented here, it
can be seen that about 200nm of deflection is necessary to achieve maximum
contrast in the green, about 240nm for red and about 600nm of deflection is
necessary at 1550nm. The 635nm results for this model agree closely with the
experimental behavior presented above in Fig. 9. Therefore to achieve optimum
modulation contrast in the infrared, a larger device gap will be necessary in
future devices. The present device is only capable of deflections on the order
of 400nm before experiencing the unstable electrostatic "pull-in" behavior.
Fig. 13: Comparison of theoretical modulation
contrast for on-axis diffraction intensity in visible and infrared wavelengths for
a 150um pitch grating with sinusoidal cross-section. Optimum performance is
achieved at a deflection of 200nm at 532nm, 240nm at 635nm and 600nm at 1550nm.
also simulated the diffraction efficiency of the modulator at zero degrees of
incidence as compared to 45 degrees. The modulator was simulated as above at
635nm; a sinusoidal grating with a pitch of 150mm. The results of the simulation are shown in fig. 14. and
agree with the test results shown in Fig. 10.
Fig. 14: Comparison of theoretical modulation
contrast for diffraction intensity with on-axis and 45 degrees of incidence for
a 150um pitch grating with sinusoidal cross-section. Optimum performance is
achieved at a larger deflection when the illumination is at 45 degrees.
4.1 Hollow retro-reflector made from silicon.
retro-reflectors are used in many applications where a parallel beam of light
needs to be returned with great precision to its source. The most significant
property of the hollow retro-reflector is that it works as a self compensating
flat mirror that will preserve its original polarization. Silicon wafer based mirror
is highly resistant to deflective distortions, having a specific stiffness (E/p - Young's Modulus divided by density) on the order of
1.5-2.5 times better than BK-7, Zerodur, Aluminum and other similar precision
mirror substrate materials. The high stiffness of the silicon enables an aspect
ratio of approximately 150 to achieve a six (6) waves @633nm flat polished
surface, with 80 angstroms surface quality, over a 6" round wafer.
silicon mirrors used in our retro-reflector are squares having the dimensions
of 10mm x 10mm x 0.7mm. The mirrors were cut from a silicon wafer and are gold
coated. Initial flatness was measured without any stress and shows 0.4 waves
P.V @ 633nm. The mirror was diced from a 6" polished silicon wafer. It is
evident from the flatness measured that distortion resulting from the dicing
operation was kept to minimum.
of hollow retro-reflector from 3 silicon wafer mirrors presents a unique
challenge. The mirrors are extremely thin, 0.7mm, they are gold coated, and the
edges are not perfectly flat since they were not ground and polished. after
successfully building such retro-reflectors, prototype retro-reflectors
comprising of one Deformable Mirror (DM) modulator and two (2) silicon wafer
mirrors were assembled. Assembly and handling the DM is even more difficult
because any contact with the DM surface can deem it un-usable. Special tools were
designed and built to avoid all front surface contact and handling and
positioning stress. The resulting wavefront was about 0.2 waves RMS, comparable
to the retro-reflector comprised of three silicon mirrors.
retro-reflectors assembled suing three DMs. The advantages of these
retro-reflectors are the improved modulation depth as well as the ability to
drive the mirrors with a 1/3 cycle delay between any two, and thus increase the
bandwidth by a factor of three.
Measurements of the performance of the modulated
retro-reflector were performed. The set-up was similar to the set-up shown in
Fig.8. The only difference was incorporation of a beam splitter after the
retro-reflector, and building the detection optics beyond the beam splitter.
The set-up is shown in Fig. 15.
The results are, as expected, very similar to the results
measured with the raw modulator, as can be seen in Fig. 16. The modulation
contrast of the mirror when incorporated in the retro-reflector at a certain
applied voltage is lower than the modulation contrast measured at the same
voltage with the mirror when not in the retro-reflector. This can be explained
when looking at Figs. 10 and 14. As can be seen in Fig. 10, the modulation
contrast at 45o at the same voltage is lower than at normal
incidence. Since the mirror is viewed at 45o when part of the
retro-reflector, these results are consistent. The same high modulation
contrast seen in the original measurements can be achieved at a higher applied
15: Modulated retro-reflector (MRR) characterization set-up
Fig. 16: Modulation
contrast response to square wave drive voltage at variable frequencies and
Using a power
meter we measured the modulation depth (Pmin / Pmax ) when
each mirror was activated separately. Then we measured all possible
combinations of two or all three mirrors activated together. Fig. 17
illustrates the measured modulation depth of all possible combinations plotted
against the calculated modulation depth for the combinations. As can be seen,
the modulation depth is indeed multiplicative, as we would expect.
Fig. 17: Measured
modulation plotted against calculated modulation for all mirror combinations in
a 3 DM retro-reflector
5.1 System performance
The system was tested at
Devens at ranges to over 200m and performed well. The audio was clear at any
range between 25 and 220m. We also performed signal to noise measurements on
the system, simulating range by optical attenuation. The measurements were done
with input at 1V rms, 1kHz, using a spectrum analyzer and an arbitrary waveform
generator. The electronic design of the system includes an Automatic Gain
Control (AGC) at the output of the demodulator. The effect of the AGC is that with
a reduction in signal level, the gain increases. When the gain gets high enough
for the noise to start increasing visibly, the AGC can still maintain the
signal level. At the limit of the gain, the signal level starts to decline. The
signal to noise measurement results are shown in Fig. 18 and meet the design goal.
The system's audio
performance as a function of angle of incidence of the interrogating beam was also
evaluated. The audio did not start deteriorating until about 30o off
axis in each direction. Signal to noise measurements as a function of angle of
incidence were performed, using the same equipment as described above. The
results are shown in Fig. 18. The signal to noise measurements agree well with
our qualitative audio evaluation. We saw the signal to noise deteriorate
rapidly at the edge of the range/angular range. Measurements show that the audio
quality deteriorates when the signal to noise ratio goes below about 25.
Fig. 18: Measurement of signal
to noise ratio of the communication system as a function of the interrogating laser
power (left) and signal to noise ratio of the communication system as a
function of angle between the transceiver and the modulator(right)
An optical communication system using a modulated MEMS
mirror embedded in a hollow retro-reflector was constructed and evaluated. The
system, using a low laser power of 10mW, demonstrated audio communication to
over 200m. This system is small, low cost, low power, and could easily be
extended to ranges of about 1 km. The system fills the need of a low cost,
small modulated retro-reflection system with extended range. The system is particularly
useful for applications when the interrogator is remote (e.g. on an aerial
vehicle) and the communication needs to be covert (e.g. getting data from
concealed unattended ground sensors, identification etc.).
The described measurements and analysis demonstrate that
some design parameters will need to change for the next generation modulator.
As seen, the mirror will need a larger range of deflection to maximize the
performance at 1.55mm. Nevertheless,
the system performed well with a non-ideal contrast.
Improving the deflection range of the mirror, as well as
increasing the laser power, can increase the system's range to about 1 km.
Work was supported at Boston
University by Army Research Laboratory Grant # W911NF-06-2-0040, and at Boston
Micromachines Corporation by Army Research Office, grant # W911NF-08-C-0006.
Dr. Bifano acknowledges a financial interest in Boston Micromachines
Corporation. The authors are grateful to Chad Demers for technical help in
demonstrating the system, Michael Datta and Mark Horenstein for helpful
suggestions regarding electronics, Michael Ciholas of Ciholas Technologies for
design and development of system electronics, and Insight Technologies for help
in design, integration, and testing.
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