Download MEMS for micro optics: from fiber optic communication to display

Invited Paper
MEMS for micro optics
--- from fiber optic communication to display --H. Toshiyoshi
Inst. of Industrial Science, the Univ. of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, JAPAN 153-8505
Kanagawa Academy of Science & Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki, JAPAN 213-0012
ABSTRACT
This paper gives a comprehensive and up-to-date review of our research activity on MEMS (micro electro mechanical
systems) for all-optical fiber optic communications. Micromechanical approach for handling optical signal has
advantages over solid-state devices in terms of high contrast intensity modulation by relatively large change of refractive
index, large spatial scan angle by optomechanical reflection or refraction, and optical transparency in wide range of
wavelength. MEMS also has industrial impact in a sense that it could potentially eliminate or cut down the cost of optical
assembly thanks to the optical pre-aligning or self-aligning capability. In particular, we take an example of MEMS
variable optical attenuator (VOA) that we have commercially released in cooperation with an industrial partner. Besides
fiber optic MEMS, we also mention other applications such as projection displays using a high-speed MEMS optical
scanner for scanning modulated laser beam to create images. Furthermore, we also introduce a MEMS color pixel
based upon the mechanically tunable Fabry-Perot interferometer made of two plastic films; the color pixel can be
assembled in an array format to create a transparent-type over-sized electronic poster which is mechanically flexible and
electrically rewritable.
Keywords: MEMS, optical MEMS, switch, VOA, display, scanner
1. INTRODUCTION
The history of Optical MEMS (Micro Electro Mechanical Systems) can be dated back to the early 1970's, when an array
of micro mirrors was used to create bitmap images [1]. After almost a quarter century, micro optics has become large
portion of the MEMS applications. Besides the projection-type display [2,3], a plenty of room has been discovered in the
field of fiber-optic networks to demonstrate the potential of optical MEMS technology. In the wavelength division
multiplexing (WDM) systems, for instance, MEMS approaches have been already acknowledged to be a fundamental
technology to develop various types of active and passive optical components, namely, wavelength tunable laser diodes
[4], optical alignment, variable optical attenuators [5, 6], optical crossconnect [7], and add/drop modules [8].
Advantages of using the MEMS approaches in micro optics can be found in the excellent optical performance such as
small insertion loss, small crosstalk, and large switching contrast. More importantly, optical MEMS devices enjoy full
bandwidth of the optical fiber network thank to the "all-optical" capability thanks to its being free from opto-electro (OE)
or electro-opto (EO) conversion. In the last few years, MEMS technology has become matured and has delivered
drastic improvement of technical potential to micro optics. MEMS has also gained commercial strength to justify the
cost-performance of their products, thanks to the improvement of production yield and device reliability in terms of
external mechanical shock, temperature change, and moisture. Reliability issues in MEMS have been improved by the
great efforts of engineers in this field, and now they are systematically understood.
In this paper, we give a comprehensive overview of the MEMS research activities in particular for the fiber-optic
application and image display devices.
Refraction Interruption
Attenuation
Screening
Lens
Interference
Polarizing
Transmitters
Interferometer
Optical Coupling
Bypass
Monitor
Attenuator
Grating
Evanescent Coupling and Spoiling
Tx λ1
WL
MOD VOA
Tx λ2
Tx λ3
WL
WL
MOD VOA
Tx λ4
WL
MOD VOA
Tx λn
WL
MOD VOA
Wavelength Tuning
Polarizer
Receivers
Optical Coupling
MOD VOA
Modulator
Rerouting
Fiber Amp
VOA
Through
Wavelength
Equalizer
Dispersion
Compensation
Figure 1: Principle of (spatial) light modulation by means of
mechanical movement of optical components.
OADM
or
OXC
Wavelength
Equalizer
Coupler
VOA
VOA
Wavelength
Equalizer
Tx
Add / Drop
Switch
DEMUX
Mirror
Diffraction
MUX
Reflection
FM
VOA
Rx λ1
FM
VOA
FM
VOA
Rx λ2
Rx λ3
FM
VOA
Rx λ4
FM
VOA
Wavelength
Filter
Rx λn
Wavelength
Tracking
Wavelength
Equalizer
Rx
Figure 2: MEMS devices used in the optical fiber network.
Optomechatronic Micro/Nano Devices and Components,
edited by Yoshitada Katagiri, Proc. of SPIE Vol. 6050
605007, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.652527
Proc. of SPIE Vol. 6050 605007-1
2. MEMS FOR FIBER OPTIC APPLICATIONS
2.1 Mechanical solution for spatial light modulation
Figure 1 illustrates various types of spatial light modulation that could be implemented with mechanical motion of the
optical components such as mirrors or prisms. Most optical MEMS devices today use reflection-type modulators or
scanning mirrors mainly because of the simplicity of MEMS fabrication processes as well as the negligible wavelength
dependence of reflection. Nevertheless it should be addressed that the reflection is not the only mechanism of spatial
light modulation but there are other possibilities such as refraction, diffraction, interference, polarization, and evanescent
coupling; it implies that more chance in MEMS optical devices are left unexplored.
DRIE
Photoresist
30-um-SOI
Mirror
2.0-um-BOX
1
005g.orecson
500-um-Si Substrate
100-nm-Al
<1 deg
Release Holes
A
//
1
h—
Photoresist
Gap-Closing
EHctrostatic
2
DRIE
/I/v,
3
/J1/ Driving Substrate
J/ Voltage
to Mirror
50-nm-Au /
30-nm-Cr
Mirror
GND
Frame
Frame
Figure 3: MEMS structure of electrostatic VOA mirror.
Chip
Figure 4: Fabrication process of electrostatic VOA with siliconon-insulator wafer
Suspension
Mirror
φ 600µ m
Electrostatic
Actuator
Suspension
Electrostatic
Actuator
l5kU 1<50
500o 021206
Figure 5: SEM image of developed MEMS VOA mirror.
2.2 Fiber optic VOA
For the first example of industrial application, we directed our technology to develop fiber optic variable optical
attenuators (VOA) in collaboration with Santec Corp., Japan [5]. Figure 2 schematically shows the fiber optic network
connecting the transmitters (left) and receivers (right). The light signals of different wavelengths are generated by the
laser diodes and modulated before combined into a single mode optical fiber; VOA is used each channel to equalize the
optical intensity before the multiplexer. VOAs are also used on the receiver side to attenuate the intensity of the
incoming light according to the full dynamic range of the receiver photodiode. Furthermore, VOA is used in the fiber
amplifier to tune the incoming intensity to match with the dynamic range and linearity of the amplifier. In other words,
VOA is the most attractive application target for the MEMS businesses because of large quantity of unit sales and that
smaller devices are preferable for the compatibility to WDM integration.
Figure 3 schematically illustrates the MEMS structure of the VOA mirror scanner. We used the two optical fibers
coupled with a mirror through a collimator lens, and control the coupling rate by electrostatically tilting the angle of the
mirror. The circular mirror was made monolithically with the electrostatic actuator and the torsion hinges by using the
active layer of a silicon-on-insulator (SOI) wafer. On the other hand, the bottom part was made of the handle wafer of
the SOI. The handle wafer was patterned into a shaped hole such that the actuator plate experienced unidirectional
electrostatic torque under applied voltage. For high cost-performance fabrication and for the ease of technologytransferability to foundry services, we tried to make the MEMS fabrication process as simple as possible by using only
two photomasks.
Figure 4 shows the fabrication chart of the VOA (cross-section). In step-1, the SOI layer (30 microns) is patterned into
the shape of the scanner by using the silicon high-aspect ratio etching technology as known as Deep RIE (reactive ion
Proc. of SPIE Vol. 6050 605007-2
etching), and then protected with the passivation photoresist. In step-2, the backside of the wafer (500 microns) is
etched in the same manner to the buried oxide (BOX, 2 microns thick). Finally in step-3, the movable structure is
released by selectively removing the silicon oxide in the hydrofluoric acid. As a result, MEMS scanner was made in a 2
mm by 3 mm chip as shown in Figure 5. The mirror is 600 microns in diameter, and the actuator plate on the side is 80
microns by 400 microns each. The supporting hinges were designed to be short (100 microns long) and thin (1.7
microns wide) with the 30-micron-thick SOI layer to attain mechanical robustness against external vibration as well as
low-voltage operation capability.
2.3 MEMS for Optical Computing
Different from the most optical MEMS devices that are electrically actuated to modulate light beams, we have recently
proposed a new device principle for the opposite effect in which a micromechanism is controlled by the impinging light.
Figure 6 shows the device crosssection, where a silicon PN-junction photodiode is integrated with an electrostatic
microactuator (mirror). The photodiode and the actuator are connected in series with an additional resistor
(implemented by the internal resistance of the SOI layer) such that the electrostatic mirror is actuated only when the light
comes into the photodiode. When no light is present, on the other hand, the actuator is not energized but the voltage
from the power source is blocked by the reverse bias effect of the photodiode. Therefore, the mirror responds to the
input light signals under constant bias voltage. This driving principle is useful when actuators are arranged in a high
density array and individual control is impossible with electrical interconnection but by parallel optical addressing using
the free space. Note that only one driving voltage is needed no matter how many actuators are to be operated in the
system.
Dark
Drive
Voltage
Photodiode
Silicon
PD
Electrostatic
Actuator
PN - Junction
R
C
Silicon Oxide
C
GND
Silicon
Light
Control
Light
PD
Actuation
R
C
Figure 6 Device configuration of optically controlled electrostatic micro actuator. A photodiode and an electrostatic actuators are
monolithically integrated in an SOI wafer.
ID
C
PD
Drive
Voltage
V
R
C
GND
1 mm
(b)
(a)
Figure 7 (a) SEM image of optically controlled micro mirror and (b) application to a logic NOR circuit for optical computing
Figure 7(a) shows an example of such optically controlled micro mirror. The photodiode was located at the tip of the
bar, and the SOI layer was patterned to provide internal electrical interconnection to the actuator part. We used the
mirror to demonstrate logical NOR circuit function as shown in Figure 7(b). Since the mirror was set to tilt in a digital
manner (until it made contact to the substrate), the reflection could be read as an output from a NOR circuit, which gave
output “1” when either one of the input light signals were present. The mirror could be operated at voltage lower than
10 V at a speed faster than 1 kHz. The performance could be further improved to demonstrate an application of optical
computing that could be used in place of the conventional system using the liquid crystal displays [9].
Proc. of SPIE Vol. 6050 605007-3
3. MEMS IMAGE DISPLAYS
3.1 MEMS Projection Display
Similar fabrication technique of the silicon micromachining (DRIE) has been used to develop projection-type image
displays. Figure 8 compares two different optical systems for creating images by mirror projection. The first scheme
in Fig. 8 (a) uses one fast scanner for the horizontal axis with another slow scanner for the vertical axis [10]. The optical
intensity from a laser is modulated synchronized with the scanner's motion. Fast scanner is usually needed to create
high-resolution images, and hence, MEMS scanner is expected to replace the conventional galvano scanners. On the
other hand, the slow axis can be controlled by the conventional method such as a polygon mirror or a galvano scanner.
The other scheme shown in Fig. 8 (b) is used to make a compact optical system of the equivalent capability by using
only one scanner of two degrees of freedom as known as 2D scanner [11].
(a)
(b)
Figure 8 Two different types of image projection system using MEMS scanners. (a) With a 2D scanner and (b) with a fast MEMS
scanner and a slow galvano scanner.
Vac -
V ac
+
Piezo actuator @
V
+
-
H-Scan
Mirror
Mirror
V-Scan
Piezo actuator H
-
Vac
+
-
+
Vac
(a)
(b)
Figure 9 Optical 2D scanner with piezoelectric uni-morph actuators formed into a double-gimbal structure
In collaboration with Stanley Electric Co., Japan, we have developed a 2D optical scanner using piezoelectric unimorph
actuators. Piezoelectric PZT material was used to generate large torque as well as fast response. Careful simulation
has shown that the scanner's performance in general depends upon the areal force (torque) density of the actuator.
Therefore, we used piezoelectric PZT film deposited on an SOI structure to for the actuator shown in Figure 9 (a). By
applying differential bias voltages, one of the cantilever bends down while the other one bends up to rotate the bar
connected in the middle. The rotation mechanism was used in the two axes for the horizontal- and the vertical-scan in
the device shown in Figure 9 (b). The scanner structure was designed to have high contrast of resonant frequencies
between the fast axis in the horizontal direction (optical 27 degrees at 16.4 kHz) and slow axis in the vertical direction
(optical 32 degrees at 77 Hz) to have high resolution.
The scanner was set under the test bench with a control PC and I/O board, as shown in Figure 10 (a), and was used to
scan a laser beam of intensity modulation. Figure 10 (b) is the results of static bitmap display of 211 by 62. Currently
the image resolution is limited by the computation speed of the I/O board, and it could be improved to maximum 1056
spots in the horizontal direction, calculated from the maximum scan angle and the mirror's number of resolvable spots.
Proc. of SPIE Vol. 6050 605007-4
77.4Hz
PC
24V
V
A
16.4kHz
I/O
24V
H
A
Base
Clock
LD
(b)
(a)
Figure 10 Fabrication process of electrostatic VOA with silicon-on-insulator wafer
3.2 Flexible Electronic Color Pixel
Unlike the projection type display, we are also developing an e-Paper like displays by using a flexible plastic films
formed into an interference color pixel [12]. Figure 11 (a) shows the idea of such color pixel that could be used like a
sheet of paper. A pair of PEN (polyethylene naphthalate) films with a semi-transparent aluminum reflector (10 nm
thick) is put together with a color-making layer of silicon dioxide (240 nm to 370 nm) and an air cavity. The air cavity
is defined by the patterned photoresist, which also works as a glue layer.
By applying dc voltage to the films, one of the PEN films is brought into contact with the counter film, where the air gap
disappears and an interferometric color is shown. In case of a green pixel, we set the silicon oxide thickness to be 310
nm. For blue and red colors, the thickness is tuned to be 240 nm and 370 nm, respectively. Preliminary experimental
results showed that the pixel of 600 um wide was operated to show the ON/OFF blinking at voltages of 120 V or higher
as shown in Figure 11 (b). Details are to be reported elsewhere [13].
/ç—Upp fi I(PEN:P&ythyI Nphth&t)
,r—LlAAtAdAllA (Al
/
AAA1tY APAAA (PhAtAAAiAt(
O4
\\\ \\\\
\ '—Eltdft@ (Al
40 60
"—SptftI sp (Si I
80 100
Voltage (V)
(a)
(b)
Figure 11 (a) MEMS structure of electronic color pixel based upon the Fabry-Perot interferomenter and (b) results of electrostatic
operation.
4. CONCLUSIONS
After almost thirty years of research and development, the optical MEMS has become an inspiring toolbox for creating
advanced optical devices and systems. Micromechanically controlled optical elements can be used various kinds of
fiber optic devices such as optical switches, gain equalizers, variable optical attenuators, and fiber dispersion
compensators. Besides fiber optics, they have found new fields of applications such as image displays, free-space optical
computing, pick-up head mechanism for optical data storages, and medical instruments. Micromechanically controlled
optical elements in optical systems occupy the place equivalent to what transistors in VLSI (very large scale integration)
system, which would be indispensable element in the future advanced electronics.
ACKNOWLEDGEMENTS
The authors would like to thank their co-workers from industry for their co-develping optical MEMS devices: K.
Isamoto, A. Morosawa, M. Tei with Santec Corp. for fiber optic VOA, H. Obi and T. Yamanoi with Koshi-Kogaku
Kogyo Co., Ltd. for fiber optic MEMS, M. Tani, M. Akamatsu, and Y. Yasuda with Stanley Electric Co., Ltd. for PZT
optical scanners, M. Yoda, H. Ito, and A. Murata with Seiko Epson Co. for resonant type optical scanners. The authors
also would like to thank the members in the research group at IIS, the University of Tokyo.
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Proc. of SPIE Vol. 6050 605007-5
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