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Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications. Thilo Kraetschmer and Scott Sanders Engine Research Center Department of Mechanical Engineering University of Wisconsin 14 th Gordon Research Conference August 12, 2007. Outline. Motivation - PowerPoint PPT Presentation
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Simple Multiwavelength Time-Division Multiplexed Light Source for Sensing Applications
Thilo Kraetschmer and Scott Sanders
Engine Research Center
Department of Mechanical Engineering
University of Wisconsin
14th Gordon Research Conference
August 12, 2007
ConclusionsResultsDesignMotivation 214th GRC 2007
Outline
Motivation
How this laser works
Experimental results
Comparison to multiplexed diode lasers
ConclusionsResultsDesignMotivation 314th GRC 2007
SampleLight Source
Detector
Light Source
Detector
Sample
Time Division Multiplexing (TDM)
ConclusionsResultsDesignMotivation 414th GRC 2007
Desired optical output
1330 1340 1350 1360 1370 138010
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10-10
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10-6
Wavelength [nm]
Spe
ctra
l Pow
er [a
.u.]
1 2 3 ... ... 17 18 19
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0
0.2
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0.8
1
Time [ s]
Sig
nal [
V]
1 2 3 ... ... 17 18 19
A sequence of pulses, each at a unique wavelength
ConclusionsResultsDesignMotivation 514th GRC 2007
Understanding the laser design
BOA*
time
pow
er
pow
er
4-step upgrade from a CW fiber laser to a 2-color TDM source
*Booster optical amplifier, a form of semiconductor optical amplifier:
ConclusionsResultsDesignMotivation 614th GRC 2007
Step 1: Replace mirrors with Bragg gratings
time
pow
er
pow
er
Customization of laser wavelength, linewidth
BOAFBG FBG
ConclusionsResultsDesignMotivation 714th GRC 2007
Step 2: Pulsed operation
time
pow
er
pow
er
BOAFBG
1 period
L R L R L R
FBG
Pulsed operation
ConclusionsResultsDesignMotivation 814th GRC 2007
Step 3: Add second grating pair
time
pow
er
pow
er
BOAFBGFBG
1 period 1 period
1 period
Pulsed operation at an additional wavelength with a modified pulse pattern
L R L R L R L R L R L R
ConclusionsResultsDesignMotivation 914th GRC 2007
Step 4: Use the same gratings on both ends
time
pow
er
pow
er
BOA
FBGFBG
1 period
Still a linear cavity laser, enforced within ring arrangement by the pulse pattern
ConclusionsResultsDesignMotivation 10
14th GRC 2007
Animation of 3-color TDM sourcefo
r an
imat
ion:
rig
ht c
lick
on t
he f
igur
e, s
elec
t pl
ay
ConclusionsResultsDesignMotivation 11
14th GRC 2007
distance [m]
time
[s]
x-t diagram familiar to gasdynamicists
Color-map of density in a shock tube experiment: He-air-CO2, M = 2.5
ConclusionsResultsDesignMotivation 12
14th GRC 2007
TDM source x-t diagram
ASE only, no FBGs
ConclusionsResultsDesignMotivation 13
14th GRC 2007
Main reflections only, 3 FBGs
TDM source x-t diagram
ConclusionsResultsDesignMotivation 14
14th GRC 2007
All signals, 3 FBGs
TDM source x-t diagram
ConclusionsResultsDesignMotivation 15
14th GRC 2007
Schematic of 19-color realization
fiber roundtrip length ~ 3 km
repetition rate ~ 66 kHz
ConclusionsResultsDesignMotivation 16
14th GRC 2007
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0
0.2
0.4
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0.8
1
Time [s]
Sig
nal [
V]
1 2 3 ... ... 17 18 19
Experimental Results
Time trace of TDM output 19 pulses of ~ 200 ns duration
Optical power is ~ 10 mW during each pulse
ConclusionsResultsDesignMotivation 17
14th GRC 2007
1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 138010
-11
10-10
10-9
10-8
10-7
10-6
Wavelength [nm]
Spe
ctra
l Pow
er [a
.u.]
1 2 3 ... ... 17 18 19
Experimental Results
Spectrum of TDM output the gain of each wavelength was adjusted to obtain a flat output spectrum
the active linewidth is ~ 5 times narrower than the passive linewidth
ConclusionsResultsDesignMotivation 18
14th GRC 2007
1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 13800
0.1
0.2
0.3
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0.5
0.6
0.7
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0.9
1
Wavelength [nm]
Spe
ctra
l Pow
er [a
.u.]
Experimental Results
Spectrum of TDM output the BOA injection current pulse pattern was customized to form a ramped output spectrum
ConclusionsResultsDesignMotivation 19
14th GRC 2007
Experimental Results
High-speed detection strategy
ConclusionsResultsDesignMotivation 20
14th GRC 2007
0 1 2 3 4 5 6 7 8 9 10
0
0.2
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0.8
1
Time [s]
Sig
nal [
V]
Experimental Results
Liquid phase Methanol, I and Io
ConclusionsResultsDesignMotivation 21
14th GRC 2007
1330 1335 1340 1345 1350 1355 1360 1365 1370 1375 13800
0.1
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1
Wavelength [nm]
Abs
orba
nce
[a.u
.]
Methanol Measurement
Methanol ReferenceIsopropanol Measurement
Isopropanol Reference
Experimental Results
Measured spectra of Methanol and Isopropanol
single shot measurement
66 kHz rep. rate
standard deviation of 100
consecutive shots: ~0.0013
ConclusionsResultsDesignMotivation 22
14th GRC 2007
Laser features
no moving parts
individual tunability of each wavelength (typ: 1 nm)
narrow spectral linewidth of each channel (< 1 GHz)
small longterm spectral drift of each channel (< 1 GHz)
fiber coupled output, typical in 10 – 100 mW range
To build this laser you need only:
gain medium
(preferably with a broad gain bandwidth and fast switching times)
custom waveform generator applying modulation
(preferably to the gain medium)
matched compressor / stretcher
(preferably as part of a long laser cavity)
ConclusionsResultsDesignMotivation 23
14th GRC 2007
Comparison to Multiplexed Diode Lasers
Advantages of TDM source over multiplexed diode lasers
1. straightforward to reach high wavelength count N: 100s to 1000s
2. single gain medium (for N wavelengths that lie within the gain bandwidth of a single gain medium)
3. modulation decoupled from wavelength-selective element
4. no external couplers / multiplexers needed
5. simple and stable wavelength control
6. broad tunability
7. more options for custom-wavelength lasers
8. opportunities for high-power lasers
Advantages of multiplexed diode lasers over TDM source
1. long fiber not required
2. some diode lasers are very inexpensive
3. direct scanning by current modulation
Questions?
ConclusionsResultsDesignMotivation 24
14th GRC 2007
ConclusionsResultsDesignMotivation 25
14th GRC 2007
The original 19 wavelengths were chosen to align with H2O peaks – now we choose the
N wavelengths differently