Optical Channel Monitor
Liang-yu Chen
Jian Li
Arvind Narayanaswamy
Dilan Seneviratne
Zhenhai Zhu
Mentored by
Prof. George Barbastathis
Jump to first page
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Goals
Device
• Monitor optical power in fiber carrying WDM
signals
Applications
• Real-time optical performance monitoring of
DWDM networks
• Optical add/drop monitoring and diagnostics
• EDFA gain balancing
Design Requirements
• Scan 10 wavelengths (l1 … l10) in 10ms
• 5% diffraction efficiency
• Central wavelength: 1.5mm
• Operate under 50V
• +/- 10% variation between l1 and l10
Device concept
Grating operation
actuator
grating
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Grating design
t
• Type of grating - amplitude or
phase
• Pitch and spacing between
beams (nominal duty cycle)
1
0
p
x
s
Amplitude grating
f
fo
0
p
x
s
Phase grating
Grating Design
• Angle of incidence
• Thickness of beams
-1
-2
-1
0
0
+1
+1
Normal incidence
Incidence at an angle
Design challenges
• Focusing on the detector

spot size

spot overlap
detector
l3 l4
l5
l6 l7
l4
l5
l6
Design challenges
• Focusing on the detector

spot overlap
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-40
-30
ln-1
-20
-10
ln
0
10
ln+1
20
30
40
Overview of the Whole Device
Large number of grating beams needed
Natural Frequency is too low
Split into pieces
Efficiency reduction caused by comb drive and space
between two pieces
......
....
......
......
•
•
•
•
Grating
Comb
Drive
Flexure
Anchor
.........
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Grating Design


Lumped model
Resonant frequency >1khZ, for fabrication limitation
and operation limitation
F
F
F
...
F
Grating
Flexure
Anchor
Grating Design (2)




Resonant frequency
• Grating beam: 4khZ
• Whole system: 1.5khZ
Number of gratings for each piece: N=500
Displacement of the gratings: N*22.5nm=11.3um
Actuation force Needed: k*22.5nm=53uN
Actuator
• Requirements:
 Displacement: 11.3um
 Force 53mN
 Voltage 0-50V
• Options:
 Electrostatic: Ease of fabrication, Low power, high stroke
 Piezoelectric: Low strain
 Thermal: High power, long response time
Electrostatic Actuators
• Electrostatic actuators - ease of fabrication
• Different electrostatic actuators
t
t
h
t
g
d
L
h
0
d
• Comb Drive
1
t 2
F  N  0 U
2
g
• Capacitor Plate
• Zipping Actuator
2
1  0Lt U
F  
2
2
d
3
4
3
2
 h  U t
h 
 0
F  Cons tant

d
Electrostatic Actuators (2)
• Force Displacement curve
Force-Displacement
Force(mN)
5.0
4.0
Zipper
Comb Drive
Capacitor Plate
3.0
2.0
1.0
0.0
0
20
40
Displacement(um)
60
Results
• Displacement 20um
• Force 255uN @ 50V
• 4 times larger than needed
• Can be used to calibrate
the device
• Air damping: quality factor Q
~ 30
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Fabrication: overview
Two masks, SOI wafer, KOH back etch
Fabrication
SOI
strip PR
KOH etch
HF wet etch
DRIE
Silicon
Photo Resist
Silicon Oxide
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Detector
• Sensitivity – flat response at 1.5mm
1.49625 mm
to
1.50375 mm
1.5mm
Lenses
• Collimating system
 Lenses
• Transmission - > 99.5%
Lenses (2)
• Magnification system
 M = 22 = f1/f2 = 44mm / 2mm
Bi-convex
• EFL
- 44 mm
Bi-concave
• EFL
- 2 mm
f2
f1
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Drive signal
• Assembly and alignment
• Summary
Assembly and Alignment
• Requirements:
 Tolerances permissible by design
• Perpendicular to optical axis – 0.2 – 0.4 mm
• Along optical axis - 0.8mm
 Precision achievable by assembly – +/- 10nm
• Assembly outsourcing

Axsys Technologies
 Zygo -
From Axsys Technologies
Calibration
l1
ql
Detector
l10
qS
qS Total swing possible
Detector plane
ql Total swing required
Total permissible error = (qS - ql)/2
Calibration (cont.)
Outline
• Introduction
• Grating Design
• Actuator
• Fabrication
• Sub-components
• Assembly and alignment
• Drive signal
• Summary
Drive Signal
Drive Signal (Cont.)
AC/
DC
50V
110/
60
3.3V
110/
60
AC/
DC
Oscillator
+
-
+
-
Drive Signal (Cont.)
AC/
DC
50V
110/
60
+
-
Conclusion(s)
Central wavelength: 1.5mm
Scan 10 wavelengths in 10ms
5% diffraction efficiency (+/- 10%)
Operate under 50V
Device size - ~ 10cm X 10cm
Easily calibrated
Minimum alignment
In-expensive fabrication