Transcript Laser Diodes - Optical Tomography Group

Biomedical Imaging II
Class 6 – Optical Tomography II:
Instrumentation
03/13/06
BMI II SS06 – Class 6 “OT Instrum.” Slide 1
Measurement principle
Measure optical intensity migrating from small
irradiation spot (source, S) to detector (D)
position
“Scan” object to obtain measurements for many
S-D pairs
Light propagation is scatter-dominated 
Signals are obtained under any angle
Signal is strongest near source
(backscattered)
BMI II SS06 – Class 6 “OT Instrum.” Slide 2
DOT characteristics summary
Functional imaging method
Sensitive to hemoglobin oxygenation states (contrast mechanism)
Low spatial resolution (~mm - ~cm)
Excellent temporal resolution (~ms), capability of studying hemodynamics
DOT assesses tissue function rather than providing an accurate
image of anatomical features.
Examples: breast cancer, functional brain imaging
BMI II SS06 – Class 6 “OT Instrum.” Slide 3
Continuous Wave (C.W.) Measurements
Simplest form of OT: lowest spatial resolution, “easy” implementation,
greatest penetration
Measuring transmission of constant light intensity (DC)
Simple, least expensive technology  most S-D pairs
High “frame rates” possible
BMI II SS06 – Class 6 “OT Instrum.” Slide 4
Example: Optical brain imaging
“Partial view” or back reflection geometry
Source / Detector 1
Detector 2
2-3 cm
Detector 3
Scalp
Bone
CSF
Cortex
BMI II SS06 – Class 6 “OT Instrum.” Slide 5
Time-Resolved Measurements
Prompt or ballistic Photons (t = d/c)
I
“Snake” Photons
I
Diffuse Photons
t
t0
t
d
t0
Measuring the arrival time/temporal spread of short pulses (<ns) due to
scattering & absorption (narrowing the “banana”)
Expensive, delicate hardware (single-photon counters, fast lasers, optical
reflections, delays…)
Long acquisition times (low frame rates)
Potentially better spatial resolution than DC measurements
BMI II SS06 – Class 6 “OT Instrum.” Slide 6
Frequency-Domain Measurements
I
I
t
t0
Photon density waves
t
t0
I
I
Modulation
Phase
t
t
t0
t0
Propagation of photon density waves (PDW): PDW = 9 cm, cPDW = 0.06 c (*
Measure PDW modulation (or amplitude) and phase delay
RF equipment (100MHz-1GHz)
Wave strongly damped, challenging measurement
(*
f = 200 MHz, μa = 0.1 cm-1, μs’ = 10 cm-1 n = 1.37
BMI II SS06 – Class 6 “OT Instrum.” Slide 7
Principle components of a DOT system
DAQ
storage
Timing,
control
Light
source
Delivery
Collection
Source
scan
Detector
scan
Target
Detector
BMI II SS06 – Class 6 “OT Instrum.” Slide 8
Multi-detector implementation
Scanning of single detector (only used in lab setups):
• Safes hardware components, cost
• Long acquisition times
Parallel multi-detector acquisition
• No “time skew”
• Stable setup
• Added hardware
Mixed approach (Scanning limited number of detectors)
• Feasible for “static imaging”
• Used in TR, FD methods because of expensive detection hardware
BMI II SS06 – Class 6 “OT Instrum.” Slide 9
Multi-source position implementation
Time-division multiplexing : One source position is illuminated at one time for
the duration of the detection (~10-100 ms).
Time skew between sources
Switching mechanism necessary:
• Optical switch (challenges: isolation, stability, size)
• Electronic switching of multiple sources (multiple laser sources &
drivers – cost, complexity)
Frequency encoding: All sources are on at the same time.
Intensity modulation at different frequencies allows electronic separation
of the signals originating from different sources.
No time skew
Reduced dynamic range
BMI II SS06 – Class 6 “OT Instrum.” Slide 10
Dynamic range
Ratio of largest to smallest “useable” signal
(saturation limit  noise limit)
Typically 1:104 (80 dB) for detection electronics
Signal falls of rapidly (~ factor 10 per cm distance on surface)
Determines the maximum tissue volume that can be probed
One source
With second source
BMI II SS06 – Class 6 “OT Instrum.” Slide 11
Solution: Detector gain switching
S1
S2
100
I1
1
I
I2  1
10
I
I3  1
100
I
I4  1
1, 000
I1
10, 000
0
1
100
1
0
1
1
10,000
0
1
10,000
100
1,000
0
1
1,000
10
1
10,000
10,000
100
100
1
1,000
10
1
10,000
100
10
10,000
100
1,000
10
1,000
10
100
1
10,000
100
1,000
10
1,000
10
1
10,000
10
1
I5 
100
1,000
10
1,000
0
1
10
1
10,000
1,000
10,000
BMI II SS06 – Class 6 “OT Instrum.” Slide 12
Semiconductors I
Electron energy
Energy levels in solids have band structure :
-
Conduction band
Free electrons
Eg < 5 eV for insulator
Eg 1 eV for semiconductor
Eg = 0 eV for metals
+
Valence band
Bound electrons
 Eg 


2 3  2 kT 
ni  pi  T e
Thermal excitation creates intrinsic carriers (electron-hole pairs):
ni = np  1.51010 cm-1 (Si at room temperature, kT = 0.025 eV)
Photoelectric excitation possible for
h  Eg

  hc Eg
BMI II SS06 – Class 6 “OT Instrum.” Slide 13
Semiconductors II
Doping with impurities increases number
of free carriers (~ typ. by factor of 107)
according to Ea,d  0.045 < kT
Donor: Pentavalent impurity (e.g., P)
provides excess e-  n-type
semiconductor
Acceptor: Trivalent impurity (e.g., B)
“captures” e- creates additional holes p
-type semiconductor
Donor doped: n-type
DEd
Acceptor doped: n-type
Internal photoelectric effect:
DEa
h  Ea ,d

  hc Ea ,d
BMI II SS06 – Class 6 “OT Instrum.” Slide 14
Photodiodes (PD)
Diode junction of p-type and n-type
semiconductors:
1.
2.
Diffusion of carriers  potential across
junction (n-type is left positively charged, ptype is left negatively charged)
Recombination at junction  region of
depletion of free carriers  high resistance
 voltage drop
3.
Carriers generated within diffusion length of
the depletion region are separated by
potential slope
4.
Photoelectric current Ip produced by
photodiode (proportional to irradiation
intensity)
Negative
net charge
+
+
+
p +
+
+
+
Positive
net charge
Diffusion length
+
+
+
+
+
-
-
-
-
-
-
-
n
Ip
Anode
Cathode
BMI II SS06 – Class 6 “OT Instrum.” Slide 15
Photodiode Operation
BMI II SS06 – Class 6 “OT Instrum.” Slide 16
Photodiode (Transimpedance) Amplifier
PD
Converts photocurrent to voltage according to:
Bandwidth:
f3db
Vout  I PD  R f
1

2 RC f
Highly linear
“Photovoltaic Mode”
BMI II SS06 – Class 6 “OT Instrum.” Slide 17
PD characteristics
BMI II SS06 – Class 6 “OT Instrum.” Slide 18
Photomultiplier tubes (PMT)
External photoelectric effect converts light
intensity into current of free electron
Cascade of secondary electron emission /
multiplication
Signal amplification G =  N typ. ~106 (N:
no. of dynodes, : gain per dynode ~4)
BMI II SS06 – Class 6 “OT Instrum.” Slide 19
PMT spectral sensitivity
BMI II SS06 – Class 6 “OT Instrum.” Slide 20
Avalanche Photodiodes
Reverse biased with high voltage
(~100V)
Internal 10-1000× amplification
through avalanche effect
Gain temperature sensitive ->
requires cooling/regulation
Available in ready-to-use modules
Pricey
BMI II SS06 – Class 6 “OT Instrum.” Slide 21
Comparison PD vs. PMT
Property
PD
PMT
Sensitivity
10-12 W
10-15 - 10-16 W
Active area
~ mm2
~ cm2
MHz - GHz (small area)
~ GHz
>109
<105
Size
Small (mm)
Medium to large (~cm)
Power supply
Low voltage
High voltage (~0.1-1 kV)
Ruggedness
Very good
Limited
Cheap – moderate (~$10)
Expensive (~$100)
Speed
Dynamic range
Cost
BMI II SS06 – Class 6 “OT Instrum.” Slide 22
Light sources I
Near infrared range (600-900 nm)
Power ~1-100 mW: Signal quality vs. exposure limit (~ mW/mm2)
Laser diodes (semiconductor lasers): Most widely used
+ Small
+ Inexpensive (o.k…. ~$10 - $1000)
+ High efficiency, easy-to-operate
+ RF modulation possible
+ ps-pulsed systems available
(Poor beam quality)
Discrete wavelengths (760, 785, 800, 810, 830,.. nm)
BMI II SS06 – Class 6 “OT Instrum.” Slide 23
Semiconductor-based light sources
“Forward bias” causes reduction of
potential wall  diode in conducting
mode
Electrons and holes recombine in
depletion layer, carriers are replenished
by current source
Emitting of recombination radiation 
light emitting diode
For special diode geometries and
reflecting end faces, laser action can be
achieved  laser diode
+
+
+
p +
+
+
+
+
+
+
-
+
-
-
+
-
-
-
-
-
-
-
-
n
+
-
+
+
-
- - - - -- -
+
+ ++ +
+
+
+ +
+
+
+
+
-
-
+
LD
LED
BMI II SS06 – Class 6 “OT Instrum.” Slide 24
Types of laser diodes
“5-mm can /
9-mm can:”
Low / mid-power
(mW-100 mW)
Hi-power (~W)
“TO3”
Telco app.
“Butterfly”
“HHL”
“Fiber pigtail”
“C-mount”
Hi-power (~W)
BMI II SS06 – Class 6 “OT Instrum.” Slide 25
Laser diode drivers
Laser diodes require a controlled current source
LD are highly sensitive to ESD, short pulses, and all kinds of electromagnetic
interference
Line filters
Power on ramping
Off shorting
LD require cooling and often temperature control to stabilize the output
power  Thermoelectric cooling (TEC) elements
BMI II SS06 – Class 6 “OT Instrum.” Slide 26
Light sources II
Solid state lasers: Optically active crystals (TiSa)
+ Short pulses (< ps, time-resolved systems)
+ Good beam profile
Bulky (requires pump laser)
Expensive
Difficult to operate
Non-laser sources: light emitting diodes (LED)
Broad wavelength range
Diffuse emitter
Power ~ 10mW
BMI II SS06 – Class 6 “OT Instrum.” Slide 27
Light propagation in optical fibers I
Snells’ law:
sin a1 n0

sin a 0 n1
a0
a1
Total internal reflection for
a > ac when going from
n1 to n2 < n1:
n2
sin ac 
n1
Fiber components:
Core (n1)
Cladding (n2)
Coating (mechanical stab.)
n1 > n2  “guided modes”
BMI II SS06 – Class 6 “OT Instrum.” Slide 28
Light propagation in optical fibers II
Acceptance angle a: Maximum incoupling angle ai resulting in guided
transmission:
sin a 
1
n12  n22
n0
“Numerical Aperture” NA = sin a
Divergence angle: Maximum exiting angle ad (ai  ad  aa)
BMI II SS06 – Class 6 “OT Instrum.” Slide 29
Properties of some optical materials
Important interfaces
Various fiber materials
BMI II SS06 – Class 6 “OT Instrum.” Slide 30
Fiber modes
Different modes of optical propagation = different spatial intensity patterns
Number of possible modes depending on core radius, refractive indices
Multimode (MM) fibers
large core > 50 m
Higher efficiency
Higher power
(Cheaper)
Single-mode (SM) fibers
small core < 10 m
Better beam quality
No pulse shape
distortion (Telecom apps)
BMI II SS06 – Class 6 “OT Instrum.” Slide 31
Intensity profile for MM fibers
BMI II SS06 – Class 6 “OT Instrum.” Slide 32
Fiber transmission losses
Absorption losses
Bending losses
BMI II SS06 – Class 6 “OT Instrum.” Slide 33
Coupling light into optical fibers
Focusing optics must provide:
Focus spot size s  core
diameter
Beam convergence angle 
acceptance angle
Mechanical alignment:
Focus on fiber core front face
(x-y-z)
Beam perpendicular to front
face (-)
Fiber face cut, polished
BMI II SS06 – Class 6 “OT Instrum.” Slide 34
DYNOT system (best DOT imager around!!!)
6
2
Source fiber
bundles
Collecting fiber
bundles
Measuring
head
OTDM
4
Target
Mirror
7
MDU
Incoupling
optics
13
3
Motor
controller
5
Bifurcated
fibers
LD 2
1
Laser
diodes
Fiber
pigtails
12
Personal
computer
Beam
splitter
LD 1
Data acquisition
board
PCI bus
8
Laser controller
f1
Laser
current
9
f2
LDD +
TECD
LDD +
TECD
f1
DPS 1
Reference
reference
signals
signals
11
10
DPS 2
f2
BMI II SS06 – Class 6 “OT Instrum.” Slide 35
Fiber Optics
Deliver light to/from tissue
Bifurcated design ( co-located S/D pairs)
Probing end
Soft
jacket
Reinforced jacketing
Detector fiber bundle
Source fiber bundle
Bifurcation
BMI II SS06 – Class 6 “OT Instrum.” Slide 36
Laser Diodes
Electrical connectors
830 nm
780 nm
Laser Diodes
“Fiber pigtails” = opt. output
BMI II SS06 – Class 6 “OT Instrum.” Slide 37
Laser Controller
Commercial Newport 8000 Laser
diode and temperature controller
Thorlabs Inc. OEM laser driver
BMI II SS06 – Class 6 “OT Instrum.” Slide 38
Fast Multi-Channel Optical Switch
circular fiber
array
DC servomotor
incoupling
unit
fiber
pigtails
beamsplitter
cube
Multi-wavelength
32 fibers
~70 Hz switch speed = 2Hz frame
rate @ 30 sources
focusing
optics
rotating
mirror
source
fiber
bundles
BMI II SS06 – Class 6 “OT Instrum.” Slide 39
Commercial fiber-optic switch
BMI II SS06 – Class 6 “OT Instrum.” Slide 40
Multi-Channel Detector
Gain switching
32 Parallel detection
channels
Electronic wavelength
separation
Gain Setting of detector
determines its sensitivity
S/H
Gain (TTL)
1
1
 1000  1000
SiPD
PTIA
PTIA
Ref. f2
Lock-in
Lock-in
@ f2
f2
@
S/H
S/H
Out
@ 2
Lock-in
Lock-in
@ f1
f1
@
S/H
S/H
Out
@ 1
PGA
PGA
Ref. f1
BMI II SS06 – Class 6 “OT Instrum.” Slide 41
The DYNOT (DYnamic Near-infrared OT) Instrument
5
5
6
4
4
9
6
8
3
3
2
2
7
1
5
1
1 – power supply, 2 – motor controller, 3 – detector, 4 – laser controller, 5 – host PC w/ monitor, 6 – fiber
optics, 7 – optical switch, 8 – optics shielding cover, 9 – laser diodes
BMI II SS06 – Class 6 “OT Instrum.” Slide 42
Helmet
Helmet kit can be configured depending on
application
Probes individually spring loaded
BMI II SS06 – Class 6 “OT Instrum.” Slide 43
Measurement Geometries
1
3
4
1. Unilateral temporal arrangement (motor cortex)
2
2. Distributed Arrangement (frontal, temporal,
parietal)
3. Installing / adjusting the optical probes
4. Complete 56 arrangement 
30 sources  30 detectors = 900 data channels
BMI II SS06 – Class 6 “OT Instrum.” Slide 44
Baby Helmet
BMI II SS06 – Class 6 “OT Instrum.” Slide 45
Dual Breast Measurement Head
Patient in prone position
Simultaneous dynamic bilateral breast imaging
Fiber protrusion individually adjusted (manually; pneumatic possible)
Measuring cup positions individually adjusted
BMI II SS06 – Class 6 “OT Instrum.” Slide 46
Highly Flexible 2×-Breast Setup
BMI II SS06 – Class 6 “OT Instrum.” Slide 47
Adjusting Mechanism
BMI II SS06 – Class 6 “OT Instrum.” Slide 48
Probe Placement
BMI II SS06 – Class 6 “OT Instrum.” Slide 49
Animal Imaging Studies
Optical Fibers
BMI II SS06 – Class 6 “OT Instrum.” Slide 50