Temperature Stabilized
Measurements of Laser Spectra
T. Flick, Wuppertal University
Mini Opto Workshop
4.-5. March 2010
CERN
05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Overview
• Introduction
▫ Measurement purpose
▫ Measurement principle
• Setup
• Performed measurements
▫ Temperature behavior
▫ Spectra
• Status and future plans
2
05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Introduction
• In the innermost of the existing HEP detectors VCSEL need to
stand severe radiation environments
• This will get worse with future experiments
• Main damaging effects for lasers:
▫ Radiation Damages
▫ Temperature effects inside the semiconductor material (at the
junction)
• Mostly both effects come along together, but heat can be cooled
away.
• This study is investigating the possibility to quantify a measure
and prepare an improvement possibility for the cooling.
• Similar work has been investigated by Markus Axer (Jan,
Francois) for the CMS experiment and we inherit a lot from this
work.
▫ I will use several slides from him to explain the principle
3
05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Wavelength Spectrum
•
•
•
•
The wavelength spectrum emitted by a laser diode is a perfect indicator of the device’s internal
temperature – the junction temperature Tj
The wavelength spectrum is red-shifted when the device is heated by increasing the ambient
temperature or the input power
If a given cavity mode remains at
the same wavelength, the
junction temperature Tj must be
constant
The change in junction
temperature due to varying the
input power Pin to the laser can
be cancelled by a change in the
heat sink temperature, so as to
keep the selected mode fixed in
wavelength (nulling method 
Paoli method)
The thermal resistance is found
from the ratio of the change in
heat sink temperature to the
change in input power.
Optical Output Power [dBm]
•
-20
Typical wavelength
spectrum of a
Fabry-Perot type
laser measured
with an Optical
Spectrum Analyzer
-30
-40
-50
-60
1292
1296
Wavelength [nm]
1300
4
T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
L-I Characteristic
Light-Current (L-I) characteristic of a
non-irradiated laser at Tamb=20°C
Thermal
rollover
Ith
DL
DI
 Threshold current Ith
laser starts to emit coherent light
 Efficiency Eff
slope of L-I curve in linear part
 Thermal rollover
non-linear part of L-I curve where non-radiative
recombination mechanisms (Auger) become
dominant due to internal temperature
Eff=DL/DI
5
T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
6
Spectral Behavior during Irradiation
Peak Mode Wavelength [nm]
1298.4
1298.2
1298.0
1297.8
1297.6
The behavior of certain mode
peaks is unique for all LDs:
55mA
XP439D03, C
D/DFluence = 0.24656
D/DFluence = 0.090094
D/DFluence = 0.061354
D/DFluence = 0.049894
D/DFluence = 0.042622
45mA
• “Slight”  red-shift with
increasing fluence at the
same input current level
25mA
1297.4
• “Large”  red-shift when
increasing the input current
1297.2
10mA
1297.0
0
1
2
3
4
14
5
2
20 MeV Neutron Fluence [10 n/cm ]
Rth =
T j  Tamb
Pdiss
=
T j  Tamb
Pin  Popt
=
T j  Tamb
I 2 Rs  I thV j  Popt
  
•Rs is constant during
irradiation
•Ith increases during
irradiation
 term is mainly affected
by I
 term is mainly affected
by irradiation
 Popt is affected by I and
by irradiation
T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
Paoli Method
7
Rth ( Pin,1  Pin, 0 )
-20
T0,DC0
-30
-40
-50
-60
Optical Output Power [dBm]
Optical Output Power [dBm]
= (T j ,1  T1 )  (T j , 0  T0 )
T0,DC0
-20
= T0  T1
T0,DC1
-30
with
-40
-50
-60
1295.2
1295.4
1295.6
1295.8
1295.2
Specific Mode Peak Wavelength [nm]
1295.4
1295.6
1295.8
Specific Mode Peak Wavelength [nm]
T0,DC0
T0
Optical Output Power [dBm]
external cooling
Ambient Temperature
internal heating
TX,DC1
-30
-40
-50
-60
1295.2
1295.4
1295.6
1295.8
Optical Output Power [dBm]
Specific Mode Peak Wavelength [nm]
T1,DC1 = T0,DC0
T1
T0,DC0
-20
T0,DC0
-20
T1,DC1
-30
-40
-50
-60
1295.2
1295.4
1295.6
Specific Mode Peak Wavelength [nm]
Input Pulse Duration
1%
100%
1295.8
Pin, x = I x  Vx  DC x , x = 0,1
T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
8
The Paoli Method
Rth =
Tj,0  T0
Pin,0  Popt,0

Tj,0  T0
Pin,0
Rth 

T j ,1  T1
Pin,1
, T j ,1 = T j , 0
Rth ( Pin,1  Pin, 0 ) = (T j ,1  T1 )  (T j , 0  T0 ) = T0  T1
with
Pin, x = I x  Vx  DC x , x = 0,1
05.03.2010
T.
Flick, Temperature
Stabilized Measurements
of Laser Spectra
9
The Paoli Method Step by Step
Extraction of
spectrum properties
Temp=30.04(°C)
Current=-10(mA)
Total Power=-10.42(dBm)
-20
Optical Power [dBm]
•
-30
-40
-50
-60
1298
1300
1302
1304
1306
1308
1310
Wavelength [nm]
-20
Temp=30.04°C
Mean=1304.3nm
Width=2.7274nm
'Peak Maximum'
'Fit'
Optical Power [dBm]
Gain Envelope Optical Power [dBm]
-20
-25
-30
-35
'Peak Maximum'
'Fit'
Temp=30.04(°C)
-30
-40
-50
-60
1304.2
1304.4
Wavelength [nm]
-40
1298
1300
1302
1304
Wavelength [nm]
1306
1308
1310
Gain
1304.6
Cavity Mode
Thermal Effects during Irradiation
A parameter that describes the device’s efficiency to release heat generated
inside the laser is called Thermal Resistance Rth
D/DTamb measured in an oven:
Ambient Temp Tamb [°C]
Optical Output Power [dBm]
•
D / DPin
DT T j  Tamb
0.0153 C
C
=
= 170
=

0.09 mW
W
Pdiss Pin  Popt D / DTamb
•
T=24.2°C
T=24.5°C
T=24.9°C
-20
-30
-40
-50
-60
1295.3
1295.4
1295.5
1295.6
1295.7
24.8
D / DTamb = 0.09nm / C
24.6
24.4
D/DPin monitored during irradiation:
Optical Output Power [dBm]
Rth =
Input Power [mW]
•
-35
-40
Pin=52.8mW
Pin=55.0mW
Pin=56.9mW
-45
-50
-55
-60
1310.0
1310.1
1310.2
57
56
D / DPin
55
= 0.0153nm / mW
54
53
24.2
1295.3
1295.4
1295.5
Wavelength [nm]
T. Flick, Temperature
1295.6
1295.7
1310.0
1310.1
Wavelength [nm]
1310.2
05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Measurement Setup
• In Wuppertal a similar setup as used by Markus
has been realized:
▫ DUT is kept in a thermally isolated box
▫ Cooling and heating capabilities are realized using a
Peltier element and a temperature control /
regulation circuit
▫ Optical fibres connected to an OSA
(Yokogawa A6319)
▫ Laser driving using external pulser / waveform
generator and current source.
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T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
Setup Schematic
Data Stream
Waveform
Generator
PC
LabView
Control
Program
Thermal Enclosure
Peltier
Current
Source
DUT
Spectra Mesaurement
Cooling
Temperature
Regulation
OSA
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T. Flick, Temperature Stabilized Measurements of Laser Spectra
05.03.2010
Setup Pictures
Waveform
Generator
Thermal
Enclosure
Current
Source for
Laser
Spectrum
Analyser
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Temperature Studies
• Different regulation
algorithms have been
studied
• PID algorithm has been
chosen to control the
Peltier element
• Temperature regulation
is very fast
▫ O(few mins)
• Temperature remains
very stable
▫ < ±0.05 °C
22 min
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Optical Measurements
• OSA measurement time is
depending on the resulotion
and span:
▫ 1.5 - 25 s per measurement
(10 pm resolution)
• Different analysis possible,
directly in the OSA or
offline on the raw data
• Scan of temperature
dependent spectra shows
the wished behavior
• Red shift of the spectrum
while warming the laser
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
▫ 0.0780 nm/K
▫ 0.0779 nm/K
▫ 0.0786 nm/K
• Zooming into the range of 1618°C measured in 0.1°C steps
shows a jump
• It is not yet fully understood and
needs further investigation
Temperature [°C]
Wavelength [nm]
• Monitoring 3 peaks from the
spectrum under temperature
change
• Temperature range 10-30°C in
1°C steps
• Spectrum peaks change by
Wavelength [nm]
Red Shift vs Temperature
Temperature [°C]
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
• Peak does not shift, but
more transforms into
another
• Polarization effect?
Intensity [dBm]
Interesting Topic to Look at
Wavelength [nm]
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Duty Cycle Dependency
• First DC measurements
performed
• Increase of wavelength
with introduced power
• Error is RMS of the peak
• Careful handling of the
peak error needed
• Inclusion of this
measurement into the
Paoli Method to be done
• This measurement
shows the working
principle only
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05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
19
Status of the Setup and Further Plans
• The measurement itself (Paoli Method) is automized
• Temperature depending spectra and duty cycle (power) depending
spectra are taken automatically
• Analysis tools are under investigation:
▫
▫
▫
▫
▫
Evaluate peaks
Fit the Gaussian
Extract the l shift and the gain curve
Conclude for thermal resistance
…
• Different types of optical components (simple diode, transmitter board,
…) need to be implemented, but this is prepared already.
• Planned:
▫ Laser package optimization studies
▫ Test several different laser diodes (different materials, speed, wavelength …
compare properties)
▫ Package optimization studies (heat coupling)
▫ Irradiation
05.03.2010
T. Flick, Temperature Stabilized Measurements of Laser Spectra
Summary and Outlook
• The setup used at CERN for CMS studies has been
reproduced in Wuppertal
• First measurements have been taken
• Spectra measured in dependence on temperature and
power have been performed
• Measurement can be run automatically
• Analysis software is under way
• More devices will be tested and the setup will be
qualified further
• Will be used to qualify lasers afterwards
20