1

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