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S. Nevas1, M. Schuster1, P. Sperfeld1, J. Gröbner2, L. Egli2, A. Sperling1, P. Meindl1, S. Pendsa1
1 Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
2 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos, Switzerland
Calibration of the QASUME reference spectroradiometer for the terrestrial solar
UV irradiance measurements
Outline
2
Motivation
Traceability chain for QASUME calibration
New developments within the project:
Tuneable laser source for calibrations in the UV
Beam preparation
Reference trap detectors
First results of the detector-based QASUME calibration
Conclusions and outlook
Primary Irradiance Standard
Black Body (PTB)
Transfer Standard Reference Spectroradiometer
QASUME (PMOD WRC)
End-User Devices Calibrated UV Network
Motivation
Traceable solar UV irradiance measurements with an uncertainty of less than 2% needed to understand decadal changes in solar UV radiation
Quality assurance at the European UV monitoring sites: Q.A.S.U.M.E
Shorter traceability chain => lower uncertainties
Cryogenic radiometer
(Aim of the project)
Transfer standards
cw-Laser sources
Si-trap detector + aperture
Filter Radiometer
Blackbody + aperture
Tunable lasers
Detector
Source
Spectroradiometer
Spectrally tuneable source
QASUME QASUME Note: calibration is stored
in monitoring sources
Traceability chain
Inclusion of q-CW lasers in the TULIP (Tunable Lasers in Photometry) facility
Main unit: mode locked Ti:Sa laser (Chameleon, Coherent Inc.)
680 nm – 1080 nm
80 MHz rep. rate, 140 fs pulse duration
Frequency doubler (SHG) and tripler (THG) unit with autotracker
Tuneable laser source
700 800 900 1000 1100Wavelength / nm
1000
2000
3000
4000
Ou
tpu
t p
ow
er /
mW
Output power of the Ti:Sa q-CW laser
Bandpass limitation Tuneable laser source
140 fs - 200 fs pulses
=> 0.5 nm -1 nm bandpass in
solar UV spectral range
Need: bandpass < 0.1 nm
Solution: Laser MC in
Czerny-Turner configuration
Ruled diffraction grating +
variable slit
High-resolution motorised
rotation stage
Spectral monitoring by an
Echelle spectrograph
(FWHM = 2 pm, uwl < 10 pm)
398 399 400
Wavelength / nm
0.0
0.2
0.4
0.6
0.8
1.0
Sig
na
l /
a.u
.
FWHM = 0.07nm
FWHM = 1.2nm
Laser Power Stabilisation
Active stabilisation needed
Low-voltage KD*P Pockels cell
Rochon polariser as analyser
Feedback signal from beam conditioning unit
KD*P
Pockels cell
Pola-
riser
Laser
MC
Beam condi-
tioning unit
Controller Feedback
photodiode
Voltage
driver
Fiber Laser
beam
Tuneable laser source
0 5 10 15 20 25 30Time / min
0.9998
0.9999
1.0000
1.0001
1.0002
Norm
alis
ed s
ignal
Laser Power Stabilisation
Temporal stability of the radiant field at the plane of measurement:
5*10-5 to 2*10-4
Tuneable laser source
The purpose of the beam conditioning unit is to generate spatially
uniform and depolarised field for the irradiance responsivity calibration
The use of integrating spheres is not possible due to the fluorescence
problems in the UV and low throughput
Implemented design:
Tapered multimode fibre (4mm/1mm) for efficient coupling of the
beam after the laser monochromator
Specially designed fibre-bundle made of different-length quartz
fibres acting as a pulse-to-cw converter
Microlens-array homogeniser for spatial homogenisation of the
field
1o holographic diffuser used as a beam splitter that also
additionally improves the uniformity of the radiant field
Beam conditioning
Schematic representation of the beam conditioning unit:
Tapered-multimode fiber (TMF) (4 mm/1 mm)
Pulse-to-cw converter (fiber bundle of varying length fibers)
Microlens array (MA) beam homogeniser
Monitor phd.
Echelle spectrograph
Lens MA
Beam
splitter
TMF Pulse-to-cw converter
(fiber bundle)
MC slit Lens
Wide-range array
spectrometer
Trap detector
with aperture
Diffuser head
(spectroradiometer
input optics)
Feedback phd.
Beam conditioning
Spectral irradiance uniformity at the plane of measurements
(z = 550 mm) determined by a detector with 2 mm aperture
280 nm
-10 -5 0 5 10x / mm
-10
-5
0
5
10
y /
mm
Normalised signal
1.000 0.998 0.996 0.994 0.992 0.990 0.988 0.986 0.984 0.982 0.982< 0.98
450 nm
-10 -5 0 5 10x / mm
-10
-5
0
5
10
y /
mm
Normalised signal
0.984
0.986
0.988
0.99
0.992
0.994
0.996
0.998
1
400 nm
-10 -5 0 5 10x / mm
-10
-5
0
5
10
y /
mm
Normalised signal
0.984
0.986
0.988
0.99
0.992
0.994
0.996
0.998
1
350 nm
-10 -5 0 5 10x / mm
-10
-5
0
5
10
y /
mm
Normalised signal
0.984
0.986
0.988
0.99
0.992
0.994
0.996
0.998
1
Uncertainty
contrb.: 0.03%
Uncertainty
contrb.: 0.02%
Uncertainty
contrb.: 0.03%
Uncertainty
contrb.: 0.10%
Beam conditioning
Spectral irradiance at the plane of measurements
(spectral bandpass < 0.1nm)
300 350 400 450 500
Wavelength / nm
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
W m
-2 n
m-1
Beam conditioning
Polarisation Reference trap detectors
Two trap detectors of S1227 photodiodes built, characterised and calibrated against an absolute cryogenic radiometer
Shunt resistance
Spatial uniformity
Pre-aged under UV irradiation
Calibration in 2013 and 2014
Spatial uniformity of spectral responsivity at 350 nm
Polarisation Reference trap detectors
Spectral responsivity and relative standard uncertainty
Polarisation QASUME calibration
First of the two planned calibration campaings carried out in March 2013
Aim: compare detector vs. source based calibrations having shortest chain of traceability
QASUME calibration was first validated directly against the primary standard of spectral irradiance at PTB, the blackbody radiator (source based calibration)
The calibration has been stored by a set of halogen lamps (monitor sources)
Calibration against the reference trap detectors was then performed using the tuneable laser source (detector based calibration)
376 378 380 382Wavelength / nm
0,0
0,2
0,4
0,6
0,8
1,0
E(
)
376 378 380 382Wavelength / nm
0,0
0,2
0,4
0,6
0,8
1,0Y
Q (,
0 )
/ a
.u.
Polarisation QASUME calibration
Spectral irradiance responsivity of the spectroradiometer is a quotient of its signal and the spectral irradiance
The spectroradiometer signal at 0 is obtained by spectral integration
Laser irradiance measured by trap
d,d,)( 0
1
00
1
00
Q
n
j
jQ
n
j
jQ YaEsaY
E
Ys
Q
Q
,,
0
0
n
j
j
Trap
TrapbA
s
YE
1
0
0
0
0 )(
Spectral irradiance of the laser source
(Echelle spectrograph) QASUME signal
±3 nm, 0.05 nm step
0
0
0
E
Ys
Q
Q
Polarisation QASUME calibration
Spectral irradiance responsivity of the QASUME spectroradiometer:
250 300 350 400 450 500
Wavelength / nm
0,5
1,0
1,5
2,0
Res
po
nsi
vit
y /
nA
*W
-1 m
2
: Detector calib.
: Source calib.
Polarisation QASUME calibration
Difference between the spectral irradiance responsivity obtained by the calibration against the trap detector and a source
Reproducibility of the QASUME values within ±0.5% (error bars)
300 350 400 450 500
Wavelength / nm
-0.02
-0.01
0.00
0.01
0.02
s det
/sso
urc
e-1
378.5 379.0 379.5Wavelength / nm
0.0
0.2
0.4
0.6
0.8
1.0
Sig
na
l /
a.u
.
: 378.945 nm
: 378.947 nm
376 378 380 382Wavelength / nm
0.0
0.2
0.4
0.6
0.8
1.0
Sig
na
l /
a.u
.
: 378.971 nm
: 378.95 nm
Polarisation QASUME calibration
Reproducibility of two consecutive QASUME scans:
Integral value:
Y1/Y2-1=0.6%
Trap: E1/E2-1=0.005%
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0 / nm
0.02
0.04
0.06
0.08
0.10
0.12
0.14S
ign
al
/ a
.u.
: 0=378.971 nm
: 0=378.95 nm
QASUME signal QASUME signal
Echelle spectrograph
Polarisation QASUME calibration
QASUME calibrations relative to its irradiance scale showing a good reproducibility of the source based calibrations
±1.2%
Polarisation QASUME calibration
Uncertainty budget for the QASUME calibration at 320 nm laser wavelengh
Source of uncertainty Standard uncertainty
Reference trap detector 0.08 %
Aperture area 0.04%
Trap positioning 0.04%
Spatial uniformity 0.10%
Stability of laser irradiance 0.02%
Laser wavelength 0.01%
Spatial stray light 0.02%
Photocurrent measurement 0.01%
QASUME diff. reference plane 0.18%
QASUME scan 0.38%
QASUME non-linearity 0.25%
QASUME stability 0.20%
Combined uncertainty 0.55%
Expanded uncertainty (k=2) 1.10%
Polarisation Conclusions and outlook
Tuneable laser facility for the detector based calibration of the QASUME in solar UV has been set up and characterised
First calibration of the QASUME spectroradiometer at a set of wavelengths has been carried out
Variance of the QASUME integral irradiance values for the laser radiation was the dominant uncertainty component
Stability of the QASUME spectral irradiance scale stored in lamps is excellent
Although the agreement between detector- and source-based calibrations of the QASUME is within the comparison uncertainty,
there seems to be a systematic difference in the spectral range of 350 nm to 400 nm
A second calibration campaign is planned for spring 2014