Optical Communication System

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Optical MEMS Devices

• 2007. 5. 7
• ? ? ?
• Optical Communication System

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Scope

• Optical MEMS Devices
• MEMS Optical Switch
• Active Optical Fiber Alignment Technique
• Lensed Fiber
• Variable Optical Attenuator
• MEMS Optical Tunable Filter

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MEMS Optical Switches

• InP-based Optical Waveguide MEMS Switches
• InP is suitable as a substrate material for
  active optical devices
• made of InGaAsP operating at the ?1550 nm
  wavelength
• InP-based optical switches can monolithically
  integrate laser or
• SOAs
• Losses can be compensated on-chip without the
  need for
• separate optical amplifiers ? cost saving

Ref. Marcel W. Pruessner, InP-based Optical
Waveguide MEMS Switches with Evanescent Coupling
Mechanism, J. of Microelectromechanica
l Systems, Vol. 14, No. 5(2005)
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MEMS Optical Switches

• Switching(or Coupling) Mechanism

• Evanescent coupling

• Coupled-mode equations

movable waveguides

• - control the waveguide gap electromechanically
• - 12 ? gap ? no optical coupling
• 100 nm gap ? coupling
• large ON/OFF contrast and low crosstalk
• low actuation voltages

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MEMS Optical Switches

• Design and Fabrication

• Optical Design

• Layer structure and waveguide
• geometry

Core In0.96Ga0.04As0.08P0.92 Clad
In0.99Ga0.01As0.01P0.99 ? grown by molecular beam
epitaxy (MBE)
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MEMS Optical Switches

• Design and Fabrication

• Mechanical Design

• Fabrication

• material property
• Youngs modulus 90 GPa
• intrinsic stress 45 MPa
• calculated the pull-in voltage as a function of
• waveguide length and gap

• use SiO2 as RIE etching mask
• waveguide sidewall roughness lt 50 nm
• Ni-Au-Ge-Ni-Au metal pads
• sacrificial layer In0.53Ga0.47As

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MEMS Optical Switches

• Experimental Results

• Optical Switching

• Electrostatic Actuation

• short devices (L 500 ?)
• reliable operation at the pull-in voltage
• longer devices (L 1000 ?)
• stiction after initial or repeated pull-in at
  low
• frequency actuation (f 100 Hz)
• no stiction for high frequency (f 1 kHz)

• In the OFF state (V0), 1.2 CROSS
• coupled power ? -19.2 dB channel isolation
• At 8 Vp-p, 66 CROSS coupled power and
• 25 uncoupled BAR power
• Switching loss is less than 10
• Switching speed up to 25 kHz possible

lines simulation symbol measured
stiction
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Optical Fiber Alignment Technique

• Automated Two-Axes Optical Fiber Alignment
• Opposing comb-drive actuators with integrated
  3-D wedges create a dynamic
• v-groove for positioning an optical fiber
• Grayscale technology is used to fabricate 3-D
  wedges
• Actuation of a cleaved fiber tip gt 30 ? in
  each direction
• Alignment tolerance lt 1.25 ?, Alignment time
  lt 10 sec

Ref. Brian Morgan, Jonathan McGee, Reza
Ghodssi, Automated Two-Axes Optical Fiber
Alignment Using Grayscle
Technology, J. of Microelectromechanical
Systems, Vol. 16, No. 1(2007)
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Optical Fiber Alignment Technique

• Design

Spring constant of optical fiber cantilever
E Youngs modulus of the fiber cantilever
(silica 70 GPa) r fiber radius (62.5 ?) l
length of the fiber cantilever (10 mm)
Force generated by a comb-drive actuator
N number of comb fingers (100) h comb-finger
height (100 ?) d gap between fingers (10 ?) V
applied voltage (100 V)
Expected Deflection gt 35 ? (from F kx) !!!
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Optical Fiber Alignment Technique

• Coupling Loss

Axial
Angular
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Optical Fiber Alignment Technique

• Fabrication

Pull-in
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Optical Fiber Alignment Technique

• Testing

Optical test setup for autoalignment of
MEMS-actuated fiber to other fiber of InP
waveguides
Measured fiber location for extreme actuation
voltages
Primary axes of movements
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Optical Fiber Alignment Technique

• Automated Alignment-Speed

Raster scan
Time to align within 90 peak power to a InP
waveguide
Spiral search algorithm
Estimated alignment accuracy histogram
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Lensed Fiber Optical Switch

• Needs for Lensed Fibers in the MEMS Optical
  Devices
• Packaging is one of the most significant
  challenges in commercializing MEMS technology
• - Small chip size, lower cost due to batch
  silicon wafer fabrication in MEMS devices
• MEMS optical attenuators, 1X2 and 2X2 optical
  switches require the integration of fiber optic
  lenses
• with a silicon MEMS chip
• Conventional GRIN or ball lenses ? assembly
  complexity and high product cost!!!
• ? mismatch in dimensions of the lenses and
  miniature scale of the MEMS components
• ? coupling efficiency is relatively poor
  (insertion loss of -1.1dB)
• Passive alignment
• ? optical fiber and lenses are placed in
  pre-determined positions in the silicon chip
• ? reducing the assembly time and manual labor
  cost
• ? increasing manufacturing throughput and
  reducing overall product cost
• - To enable small MEMS mirror design

Ref. G. Wu et al., Design and Use of Compact
Lensed Fibers for Low Cost Packaging of Optical
MEMS Components, J. of Micromechanics and
Microengineering, Vol. 14(2004)
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Lensed Fiber Optical Switch
TEC Fiber

• needs to be angle polished at 8 for large
  return loss- requiring a curved mirror to focus
  the beam onto the output fiber

Lensed Fiber

• thermally formed plano-convex lens fused to a
  SMF-28 fiber
• a flat MEMS mirror can be used
• high coupling efficiency by the well-controlled
  shape of the lenses
• high return loss (34 dB without AR coating, gt 50
  dB with AR coating)

OptiFocusTM lensed fiber and schematic diagram of
collimating lens system
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Lensed Fiber Optical Switch

• MEMS design and fabrication

• 2X2 optical MEMS switch designed- electrostatic
  actuation by comb-drive actuator
• ? very low power consumption
• ? fast switching speed
• MEMS mirror and optical fiber fixing channels
  (or grooves)
• fabricated

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Lensed Fiber Optical Switch

• MEMS design by ANSYS

• movable stroke of MEMS Mirror 50 ? _at_ 68 V
• resonant frequency 700 Hz
• switching ON time lt 1 ms

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Lensed Fiber Optical Switch

• Lensed fiber design and tolerancing

• Optical properties of a plano-convex lens are
  defined by thickness of lens(T) and radius of
• curvature (Rc)
• - Lensed fibers with a wide range of MFDw and DW
  can be designed

Manufacturing tolerances and measured insertion
losses for actively aligned lensed fibers
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Lensed Fiber Optical Switch

• Packaging and results

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Lensed Fiber Optical Switch

• Packaging and results

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Variable Optical Attenuator

• A variable optical attenuator (VOA) integrated
  with metal-defined optical waveguide
• (MDOW) was introduced for the first time
• Metal film stressor
• ? produces the refractive index change within
  the core layer
• ? acts as a thin-film heater for thermal tuning
  of the optical power within a MDOW
• The refractive index decreases with the increase
  of temperature due to a negative thermooptic
  coefficient in the electrooptic polymer
• ? lose optical confinement ? power loss (or
  attenuation)

- Lower cladding UV15LV polymer spin
coating UV curing - Core EO chromophore
(DH6-APC) spin coating - Upper cladding
UFC-170A polymer spin coating UV curing
Proposed VOA Design
Ref. S. K. Kim et al., Metal-Defined Polymeric
Variable Optical Attenuator, IEEE Photonics
Technology Letters, Vol. 18, No. 9(2006)
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Variable Optical Attenuator

• Fabrication and Results

Near-Field Observation
Attenuation Transfer Function
P 0 mW
Dynamic attenuation range 25 dB
P gt 0 mW
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Variable Optical Attenuator

• Fabrication and Results

Time-dependent switching characteristics

• Rising time 800 ?
• Fall time 700 ?c

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MEMS Tunable Optical Filter

• Angular rotation of an interference filter ?
  passband tuning
• MOEMS tunable filter
• ? DWDM filter MEMS actuated platform

• 200 GHz standard DWDM filter (Koncent Comm.)
• Center wavelength 1560.61 nm with a -3 dB
  spectral FWHM of 1.6 nm
• dimension 1.4 x 1.4 x 1.4 mm (6 mg)

Thermal microactuator injecting current through
narrow beams generates heat ? elongation of
the beams conversion of the linear expansion of
the beams to the angular rotation
MEMS platform
Ref. Anartz Unamuno and Deepak Uttamchandani,
Hybrid MOEMS Tunable Filter for Interrogation of
Fiber Bragg Grating Sensors, IEEE Photonics
Technology Letters, Vol. 17, No. 1(2005)
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MEMS Tunable Optical Filter

• Fabrication and Results

Tuning range 870 pm
Ring laser setup used to measure the wavelength
shift
Wavelength shift due to rotated angle
Insertion loss 3.7 dB (two collimating lens)
1.7 dB (with filter cube)
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MEMS Tunable Optical Filter
Spectral measurement

• - Wavelength resolution
• 18 pm when the 2 increase of drive current
• Stability test
• 20 pm peak deviation of the wavelength
• (monitoring over a 3-h period while keeping
  the MEMS platform at a fixed position)
• Repeatability test
• 11 pm change in wavelength
• (applying the same voltage and current values
  and monitoring the peak transmission
• wavelength)

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MEMS Tunable Optical Filter

• Small insertion loss variation and negligible
  dispersion within the passband are the
• desired properties of optical bandpass filters
• FBG(Fiber Bragg Grating) and TFF(Thin Film
  Filter) have been widely used because
• of their excellent characteristics in the
  amplitude domain
• ? nonlinear group delay near the edge of the
  pass band
• ? simultaneous and independent control of the
  center wavelength and optical
• bandwidth while maintaining low chromatic
  dispersion and flat insertion loss
• profile is difficult
• - Most MEMS tunable optical filters are based on
  a monochromator configuration that
• uses a dispersive optical component (such as a
  free-space diffraction grating)
• ? designing a specific chromatic dispersion
  profile across the optical passband is
• possible
• ? capable of controlling the center wavelength
  and bandwidth

Ref. Kyoungsik Yu, Daesung Lee, Namkyoo Park,
Olav Solgaard, Tunable Optical Bandpass Filter
With Variable-Aperture MEMS Reflector, J. of
Lightwave Technology, Vol. 24, No. 12(2006)
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MEMS Tunable Optical Filter
Schematic diagram of the MEMS variable-bandwidth
optical filter
Analog control of the aperture size using MEMS
actuators ? independent control of optical
bandwidth and center wavelength
?0
?0
?0
?0
?0
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MEMS Tunable Optical Filter
Beam profile of a monochromatic optical beam with
wavelength and unit optical power
D? angular dispersion from the grating f
effective focal length of the collimating/focusing
lens
Coupling efficiency
overlap integral(?)
Independent on the input wavelength
h reflectors height
Determine the spectral shape of the passband
x1, x2 aperture dimension
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MEMS Tunable Optical Filter

• Fabrication

• - mirror thickness 100 ?
• mirror roughness 20 nm
• folded beam spring structure

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MEMS Tunable Optical Filter

• Fabrication Results

• actuator displacement

Vi input voltage, N number of comb, C0
capacitance of the unit comb, kx spring constant
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MEMS Tunable Optical Filter

• Test Results

original position
Insertion loss profile for three different MEMS
reflector configurations
Transmission characteristics as a function of the
input wavelength for different incident angles on
the diffraction grating

• large and coarse tuning of the center wavelength
  
• is done by rotating the diffraction grating
• - transmission ripple 0.5 dB
• - polarization dependent loss 1 dB

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MEMS Tunable Optical Filter

• Test Results

- The group delay is measured by the phase-shift
method compares the phase difference of the
modulated optical signal before and after
passing the filter - Nearly zero chromatic
dispersion across the full passband
peak-to-peak group delay ripple lt 5 ps
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Summary

• Various Optical MEMS Devices are presented and
  discussed
• MEMS Optical switch
• Fiber alignment technique
• Lensed fiber its application
• Optical attenuator
• Optical tunable filters