Post on 03-Aug-2020
© Finisar Corporation 1
VCSELs in Various Sensor Applications
Dr. Jim Tatum
Jim.tatum@finisar.com
First published in proceedings of
Sensors Expo 2005
AN-2140
© Finisar Corporation 2
Contents
VCSEL 101
What is a VCSEL? Where is it used? Why?
VCSEL Sensor Attributes
Advantages, Uniqueness, etc.
Types of Sensors
Reflective, transmissive, scattering, etc.
VCSEL Horizons
© Finisar Corporation 3
1993
Honeywell VCSEL Research Begins
1996
First Commercial VCSEL Product
1996
TO-46 VCSEL 1995
Technology Transfer
to Richardson
2000
10,000,000 VCSELs Shipped
2000
Honeywell VCSEL Optical Products SBE Formed
1998
1,000,000 VCSELs shipped
1999
Oxide VCSEL R&D
Red VCSELs
demonstrated
2000
LC & MTRJ OFEs
2001
4 & 12 Channel Arrays
STABILAZETM VCSEL
850nm SM VCSELs
Production oxide VCSELs
2006
1300nm VCSELs
1996
First VCSEL Reliability Study
2000
Industry Benchmark
VCSEL Reliability Study
2001
20,000,000 VCSELs shipped
1998
SC sleeve VCSEL
2002
850nm MM2 VCSEL
850nm 10GB VCSELs
780nm VCSELs
1310 & 1550nm VCSELs
demonstrated
2002
25,000,000 VCSELs shipped
Worldwide
VCSEL
Leaders
Q1 2005
35,000,000
VCSELs shipped 2004
Finisar Purchase-AOC Formed
AOC (née Honeywell)
© Finisar Corporation 4
What is a VCSEL?
Vertical Cavity Surface Emitting Laser (VCSEL) first commercialized by Honeywell in 1996
Primary application was high speed data communications on multimode optical fiber
More than 50M VCSELs shipped by multiple vendors, AOC has shipped 35M+ VCSELs
VCSEL filled market need for high reliability
© Finisar Corporation 5
Semiconductor Optical Sources
LED VCSEL EEL
All sources are grown be either MOCVD or MBE
- Incoherent - Lambertian emission from all facets
- Coherent - Symmetrical, low divergent optical beam - Mirrors formed vertically during growth
- Coherent -Elliptical, astigmatic optical emission - Mirrors formed by cleaving and coating
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Semiconductor Optical Sources
VCSEL Advantages
Planar structure with vertical emission can be tested in wafer form before sawing and building higher level assemblies
High efficiency
High degree of usable optical emission
Reliability
Attribute Symbol UnitsSM
VCSEL
MM
VCSEL
EE
LaserLED
Electrical Power Pelec mW 5 20 60 60
Optical Power Popt mW 1 5 10 1
Efficiency at Popt=1mW h % 20 10 5 2*
Wavelength l nm 760-860 670-870 630-1300 400-1300
Spectral Width Dl nm 0.01 0.5 2 50
Spectral Tuning
(Temperature)Dl/DT nm/°C 0.06 0.06 0.3 0.3
Spectral Tuning (Current) Dl/ DI nm/mA 0.25 0.09
Beam Angle (full width at
half of maximum value)Ð ° <15 ~15
15 par.
35 perp120
© Finisar Corporation 7
VCSEL Value in Sensors
High efficiency for battery powered applications
Single optical wavelength, high coherence
High radiance optical source
Flexible packaging options
Reliability
© Finisar Corporation 8
In other words, the “ilities”
Manufacturability – all-vertical construction enables the use of traditional semiconductor manufacturing equipment
Integrability –compatible with semiconductor manufacturing and wafer integration of the emitters with detectors and circuitry
Reliability –without the failure modes of traditional laser structures such as dark line defects and catastrophic optical damage; very long wearout life
Testability – complete testing and burn-in in wafer form
Arrayability – VCSELs can be easily fabricated into one or two dimensional arrays
Packageability – VCSELs allow use of traditional low cost LED packaging; chip on board technology for VCSEL-based sensors
Low power consumption (OK, not strictly an ility)– extends battery life and reduces thermal design constraints in larger equipment systems
© Finisar Corporation 9
Efficiency
Measuring efficiency with the total emitted optical power,
VCSELs are typically 20%
LEDs are typically 20%
EELs are typically 10%
0.01
0.1
1
10
100
1000
10000
10 100 1000 10000Battery Rating (mA-Hrs)
CW
Op
era
tin
g T
ime (
Hrs
)
Miniature
Batteries
AAA AA C
VCSEL
EEL
LED
0%
5%
10%
15%
20%
25%
0 5 10 15 20 25
Current (mA)
VC
SE
L E
ffic
ien
cy
(%
)
© Finisar Corporation 10
Simplicity
Power variation in a VCSEL can be made very minor over temperature and process with simple passive electrical components
Tatum, et, al, “VCSEL SPICE Model,” at http://www.adopco.com
© Finisar Corporation 11
High Peak Power from a VCSEL
Pulsing a VCSEL with short electrical pulses reduces the joule heating, and allows for much higher optical power emission. Powers more than 10x the DC limits are readily achievable
© Finisar Corporation 12
Optical Spectrum
VCSELs can be made to emit at a single wavelength. Some EELs can be made to emit a single wavelength. LEDs are broadband emitters
-70
-60
-50
-40
-30
-20
-10
0
Wavelength (nm)
Inte
ns
ity
(d
Bm
)
1nm Lasers
10nm LED Dl/DI
(nm/mA)
Dl/DT
(nm/°C)
VCSEL 0.09 0.06
EEL 0.1 0.3
FP 0.1 0.3
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850nm Single Mode VCSELs
Single longitudinal and
spatial mode
High efficiency
0.0
0.3
0.6
0.9
1.2
1.5
0 1 2 3 4 5
Current (mA)
Po
we
r (m
W)
1
1.3
1.6
1.9
2.2
2.5
Fo
rwa
rd V
olt
ag
e (
V)
-60
-50
-40
-30
-20
-10
0
845 847 849 851 853 855
Wavelength (nm)
Inte
ns
ity
(d
Bm
) 3mA
4mA
5mA
-20 -15 -10 -5 0 5 10 15 20
Divergence Angle (Degrees)
No
rma
lize
d In
ten
sit
y
3mA
4mA
5mA
© Finisar Corporation 14
VCSEL Optical Properties
Symmetrical beam
No Astigmatism
Low divergence
Simple, low cost
optics
LED VCSEL EEL
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50
Aperature Width (mm)P
ow
er
Tra
ns
mis
sio
n
VCSEL
LED
EEL
LED
VCSEL
© Finisar Corporation 15
VCSELs are fast!
A VCSEL can turn on and off again in ~100 ps, producing a pulse of light short both in time and in space
Less than 1.5 inches
© Finisar Corporation 16
VCSEL Packaging Flexibility
Can be put in any form factor of an LED and most EEL form factors
2D Arrays
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Reliability
302010521.5.2.1.05
70°C ambient
1,000
10,000
100,000
1,000,000
10,000,000
Cumulative Failures (%)
Op
era
tin
g H
ou
rs
1998
1996
1999 850 nm VCSEL
1995
1300 nm FPScreened CD
Unscreened CD
© Finisar Corporation 18
Optical Sensor Applications
Many techniques for optical sensing
Reflection
Transmission
Absorption
Scattering
Interference
Self Mixing
Remote power
Scattering Medium
Signal Wavelength
Reference Wavelength
l specific absorber
Optical Output Encoding Strip
Optical Input
Strip movement
(a)
(b)
(c)
© Finisar Corporation 19
Some Optical Sensors
Taken from www.balluff.com
© Finisar Corporation 20
VCSEL Application examples exploit:
Narrow wavelength and stability
differential absorption sensors, e.g. blood gas
WDM and fiber sensors
Narrow beam and small source
wide-gap transmissive and reflective sensors
photoelectrics
scatterometers, e.g. turbidity sensor
light guide illumination
Low current and high speed
hand-held optical ranging
air data links
Low current, size, and coherence
high-precision encoders
speckle-based sensors
Integration
© Finisar Corporation 21
Reflective Sensor
Reflective sensors can work on either
specular or scattering reflections
Most ubiquitous optical sensor
Automatic flush
Proximity sensors
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10 12
Distance (mm)
No
rmalized
IC
E
Driver and
control
electronics
R
X
T
X
Reflector Optical
system
Specular Reflection
© Finisar Corporation 22
VCSEL Photoelectric Control
With little scattered light, VCSEL is 10x more efficient than LED in similar package
Increased discrimination to background light
Improved precision on object location
Lower power dissipation
Surface mount on board, direct replacement of LED
Picture: Courtesy of Eaton
© Finisar Corporation 23
Transmission Sensor
A transmission sensor is an unfolded reflective sensor that can
provide a higher degree of discrimination
Transmission sensors can also quantitatively measure the amount of
transmission, absorption, or other losses
Electronics L
Object Electronics D
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Absorption Sensor for Oxygen
NLgPTSII 0
0 ,exp I Intensity at frequency
I0 Initial intensity at frequency
S(T,P) – Temperature, Pressure dependent linestrength G(-0) lineshape function N = Number density of absorber L = path length
Wang, et al, Applied Physics B, 2001
© Finisar Corporation 25
Scattering Sensor
Scattering sensors measure the amount of light
scattered relative to the amount of light
transmitted
Colloidal suspensions are uniform scatterers
Some suspensions preferentially scatter into
particular angles (blood, for example)
Electronics L
Elec
tro
nic
s D
Reference D
Controller
Cabuz, et al, DARPA contract MDA-972-00-C-0029
Applications
• Dishwashers
• Laundry machines
• Pool/Spa water
• Process controls
• Mixing controls
© Finisar Corporation 26
Speckle Sensor
Optical Sub-Assembly
IC
(Photo courtesy of Kanitech)
Optical Mouse
pen
• Speckle is induced by
coherent sources
• Speckle is the
summation of waves
with a definite phase
relationship
• Speckle patterns
translate with the
reflecting surface
• Image correlation can
be used to determine
direction and amount of
motion
• Speckle sensors are
statistical sensors, not
absolute measurements
White Light
LED
Laser
© Finisar Corporation 27
Interference Sensor
Uses the property of coherence to create
an interference of the laser electric field.
Typical example is a Michelson
interferometer
Variation (vibrations, movement, etc) in
one of the optical paths will translate to
fringe patterns
Practical examples
• Ellipsometry (measurement of thin optical
and semiconductor films) is a practical
example
• Surface flatness measurement
© Finisar Corporation 28
MicroE Interferometric Sensor
MicroE’s Patented Position Sensor
LED encoder uses slotted mask and detects on/off signal
Laser encoder uses diffraction grating which produces interference pattern (requires coherence of laser)
Resolutions: rotary: LED: 1M counts/revolution; MicroE:300M counts/revolution
linear: LED: up to 0.1mm; MicroE: up to 0.0006mm
VCSEL Advantages
• Compact Package
• High coherence
• Low power dissipation
• Low beam divergence
© Finisar Corporation 29
MicroE VCSEL encoder
Ultra high resolution encoder Packaging of the laser is key to the application Single mode VCSEL
Photos courtesy of MicroE Natick, MA
© Finisar Corporation 30
Atomic Clocks
Traditional Atomic Clocks
use an optical source to
interrogate an Alkali vapor
in an RF cell. The RF cell
is tuned to the hyperfine
transition of the ground
state.
• Coherent Population Trapping (CPT) eliminates the need for an RF chamber by placing the RF signal on the interrogating laser beam
Ground State HF
Single l Laser Laser Absorption
RF Excitation
Ground State HF
FM Laser Laser Absorption
RF
Ch
amb
er
© Finisar Corporation 31
Atomic Clocks
Alkali Center
Wavelength
HF
Cs 894nm 9.2GHz
852nm
87Rb 852nm 6.834GHz
795nm
85Rb 795nm 3.036GHz
VCSEL
Optics include l/4 waveplate, attenuator, and collimating lens
Heater Heater
Alkali Cell Detector
From Lutwak, et. al, Symmetricon website From Knappe, et al, Optics Express, vol. 13, pp. 1249ff, 2005
DARPA Chip Scale Atomic Clock (CSAC) Project
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Self Mixing Sensor
A weak signal is fed back into the laser from a distant reflector. The feedback causes instability in the laser. When the reflector is in motion, a Doppler signal is incorporated on the feedback which can be used to determine movement and direction of the reflector
D
c
L
c
vt
LP
044cos
Duijve, et. al SID 2003
Diode laser
cavity External cavity
Movement causes Doppler effect,
even though the size of the external cavity remains constant
Diode laser
cavity External cavity
Movement causes Doppler effect,
even though the size of the external cavity remains constant
Drive
current
Forward
movement
Backward
movement
time
© Finisar Corporation 33
Self Mixing Sensor
Self mixing can be used to measure a position and movement of an object
Self mixing can directly measure velocity using the Doppler shift in the laser
http://www2.rnw.nl/rnw/en/features/science/021216laserbeetle.html
Duijve, et. al SID 2003
© Finisar Corporation 34
Remote Power Sensor I
In some applications for sensors, it is impractical or
dangerous to have electrical wiring
Magnetic Resonance Imaging
Hazardous material processing (e.g. flammable materials)
Oil wells and pipelines
Harsh EMI environments
One common example is the optocoupler (optoisolator)
Ground Isolators DC Isolators AC coupling of signals
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Remote Power Sensor II
One way to achieve remote power is to deliver energy optically to a detector, which converts the optical energy to electrical energy.
The electrical energy can be used to charge a battery, or it can be used to directly power an electrical circuit.
Several detectors can be connected in series to gain voltage multiplication
http://www.photonicpower.com
The major drawback to remote power devices today is that the return signal must be generated in a separate optical component. This requires additional fibers, power drain, and another optical component.
© Finisar Corporation 36
VCSELs in Remote Power Solutions
The power conversion
photodiodes and the VCSEL
can me monolithically
integrated. More than 3V
and 25mA can be supplied
to an external circuit. The
VCSEL requires less than
5mW to send optical signals.
Requires only a single fiber
to operate
Can power almost any type
of sensor
Can be made very small
(less than 7Fr) for in vitro
applications
Various types of side views of one possible embodiment.
top of VCSEL
16%, so carrier
reflector
p
p+
nn+-p+ tunnel jcn
p
nn+-p+ tunnel jcn
p
n
n+ contact, plus
16% or higher for
carrier reflector
830 nm808 nm
electrical
contact 1
electrical
contact 2
optional
electrical
contact 3
US Patent publication 200030223756 – Optical Transceiver
© Finisar Corporation 37
VCSELs in Laser Printing
0
25
50
75
100
125
150
0 600 1200 1800 2400
Resolution (dpi)
Pag
es P
er
Min
ute
1 Beam
4 Beam
32 Beam
10k RPM polygon mirror
Monochromatic printing
FujiXerox DocuColor 1256 GA
VCSELs enable 2 dimensional arrays to be
used for printing
Increases speed and resolution
Allows faster color reproduction
Excellent beam quality
Competition from various SLMs
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Future VCSEL Directions
Other wavelengths
650nm to 1550nm
Higher power, better efficiency
Integration of functions on chip
Photodiodes
Lenses
Packaging with control electronics
© Finisar Corporation 39
Summary
The value of VCSELs in optical sensors has been
demonstrated with practical examples
High efficiency
Optical beam quality
Optical spectrum
Packaging flexibility
High speed