EMRP ENV59: ATMOZ

„The FTS instrument developed for outdoor sun

measurements“ D3.1.1. Entrance optics for Fourier Transform Spectroradiometer (FTS)

developed and combined instrument characterized

Ingo Kröger

Slide 2 of 18

“Determination a high resolution extraterrestrial solar spectrum using a

Fourier-transform spectrometer and medium resolution array

spectroradiometers with an absolute uncertainty of ±2 % over the

wavelength range 310 nm to 350 nm.”

• PTB will develop the entrance optics for the Fourier Spectroradiometer (FTS) and characterize the combined

instrument. (PTB) (D3.1.1 – September 2015)

• PTB will develop the transportable housing for the FTS so that it can be used for outdoor operation. (PTB) (D3.1.2 –

March 2016)

• PTB will then produce the characterised Fourier Spectroradiometer (FTS) for outdoor measurement of spectral solar

direct irradiance in the range 300 nm to 350 nm (to be used in the Task 2.3 Izaña field campaign) and report written. (PTB)

(D3.1.3 – August 2016)

• PTB will use the FTS from D3.1.3 to measure the direct solar UV spectrum with a resolution exceeding 0.05

nm between 300 nm - 350 nm performed at the Izaña field campaign. One spectrum < 30s (PTB) (D3.1.4 – October

2016)

• SFI Davos will combine PTB’s measurements from the FTS in D3.1.4 with REG(ULL)’s medium resolution

direct solar UV irradiance measurements from array spectroradiometers in D2.3.4 to determine an absolute high

resolution ETS. This will be done by applying the Langley-Plot analysis to the direct solar UV irradiance measurements. The

requirements are for a resolution of 0.05 nm in the range 300 nm to 350 nm and an uncertainty of ±1 % in the range 310 nm

to 350 nm, and ±10 % between 300 nm and 310 nm. (SFI Davos, REG(ULL), PTB) (D3.1.5)

Task 3.1: Technical objective

Slide 3 of 18

Development of entrance optics of FTS

Slide 4 of 18

Task 3.1: The Fourier Transform Spectroradiometer

Bruker VERTEX 80 FT-IR Spectrometer • GaP-Diode 50.000cm-1 – 25.000cm-1 : 200nm – 400nm • Resolution: >0.06 cm-1

300 310 320 330 340 350WL / nm

0,00

0,02

0,04

0,06

d /

nm

: dk = 5 cm-1

: dk = 4 cm-1

: dk = 3 cm-1

: dk = 2 cm-1

: dk = 1 cm-1

: dk = 0,5 cm-1

: dk = 0,2 cm-1

Slide 5 of 18

Task 3.1: The Fourier Transform Spectroradiometer at PTB

External detector chamber

Dual source

entrance

InGaAs

Si

GaP

Slide 6 of 18

200 300 400 500 600

/ nm

: GaP spectral response

: UG11 filter Transmission

: GaP+UG11 filter

Task 3.1: FTS – Modifications

Detector modifications • GaP detector at position of shortest beam path (4 detector

positions available)

• Combination of GaP-diode and UG11 filter 1

• Spectral responsivity: 250 nm – 400 nm

• Cutting off solar irradiance > 400 nm higher

amplification for UV measurements possible

©Thorlabs

GaP-diode

UG11

1 P. Meindl

TAR

GET

Slide 7 of 18

Task 3.1: FTS – Entrance optics

Aperture

The aperture limits the size of a light source coupled into the FTS interferometer

• Point source at FTS entrance is imaged on the aperture by

focussing mirrors

• Maximum size of aperture: 8mm

• Solar tracker compatible entrance optics needs to be based

on optical fibre, i.e. a fibre bundle Approach: Fibre bundle with core diameter ≈ 8mm, length 3m

Aperture wheel

Slide 8 of 18

Task 3.1: FTS – Entrance optics

Fibre coupling into FTS

• 2 Fibres: IR and UV on x-y-z- translation stages

• Fibres adjusted for optimal imaging of fibre exit plane on aperture of FTS

• x-y-z-stage firmly arrested after adjustment

• Closed housing + bending protection of fibres

IR fibre

UV fibre

xyz-translation stage (can be firmly arrested to adjusted position)

bending protection

Slide 9 of 18

Task 3.1: FTS – Entrance optics for direct irradiation

Irradiation coupling into fibre

We need as much light as possible coupled into the 7mm fibre • Using 2“ lens, f = 200mm, dlens = 50 mm dfibre = 7 mm : 50x higher signal

• Fibre positions well outsite focal distance of lens quasi

homogeneous illumination of fibre entrance, no imaging of light source

• Chromatic abberation of lens will „calibrated into the instrument“, since entrance optics is keeped fixed

2“ Lens, f = 200mm

2“ tube

bending protection

Fibre entrance

dLens-fibre = 150 mm < fLens

Aperture = 25 mm FOV = ±3.5°

lens

fibre

Slide 10 of 18

Task 3.1: That is, how it looks like right now

FTS

Transfer mechanics (total weight FTS ≈ 150kg)

3 meter fibre bundles

Entrance optics

Now…characterization…

Slide 11 of 18

Task 3.1: FTS – radiometric calibration

300 310 320 330 340 350 / nm

10-3

0,005

0,01

0,05

0,1

0,5

E /

W/m

² nm

: standard lamp (halogen 1000W)

: AM1.5 (IEC 60940)

Radiometric calibration against standard lamp

We need a radiometric correction of the measured spectra! We encounter 2 major problems…

• Low spectral irradiance of standard lamp in UV range

• Standard lamp is a point source with divergent light,

entrance optics is made for parallel light (diffractive elements: lens, numerical aperture of fibre)

First try standard radiometric calibration

Slide 12 of 18

Task 3.1: FTS – radiometric calibration

Radiometric calibration against standard lamp

• A radiometric correction function could be derived (averaging 90 minutes of measurement)

• Major impact of background

• Is that strange feature at 338 nm an background related artefact?

Periodic features might originate from a hum/noise/vibration

of the interferometer, that leads to a periodic signal during the interferometer scanning process.

Investigate impact of scanner frequency

260 280 300 320 340 360 380 400 / nm

-510-5

0

510-5

1,010-4

1,510-4

2,010-4

2,510-4

S(

) /

a.u

.

0

5

10

15

20

25

30

rel.

un

cert

ain

ty (

k=2

) /

%

Signal background

?

Scanner frequency f = 5 kHz

Slide 13 of 18

Task 3.1: FTS – Background features dependent on scanner frequency

• Periodic features in the background (dark measurement)

• Highly reproducable

• Structure dependent on scanner frequency of interferometer

background must be subtracted for measurements with low SNR (i.e. radiometric calibration with SL), frequency should be set to lowest value: 2.5kHz!

300 320 340 360 380 / nm

300 320 340 360 380 / nm

f = 5 kHz

f = 7.5 kHz

300 320 340 360 380 / nm

f = 2.5 kHz

Slide 14 of 18

Task 3.1: FTS – radiometric calibration: f = 2.5 kHz vs f = 5 kHz

280 300 320 340 360 380 / nm

rel.

un

its

FTS UV+GaP+UG11, f = 2.5kHz

: measured data

: Background

: background corrected data

: spectral irradiance standard lamp

: radiometric correction function

280 300 320 340 360 380 / nm

rel.

un

its

FTS UV+GaP+UG11, f = 5kHz

: measured data

: Background

: background corrected data

: spectral irradiance standard lamp

: radiometric correction function

Radiometric calibration against standard lamp

Compare 2.5 kHz (lowest frequency) to 5 kHz

• Less pronounced features

• Much better/reasonable result

• Retrieve radiometric correction function

Measure some UV-spectra and apply this correction

Slide 15 of 18

300 310 320 330 340 350 / nm

0,000

0,002

0,004

0,006

0,008

0,010

0,012

S(

) /

a.u

.

Task 3.1: Test of entrance optics with natural sunlight

Test with natural sunlight

• Manual tracking, no clear sky no stable irradiance • Aperture: 8mm, Res.: 4cm-1, t = 25s, Amplifier: 16xB

• 12.10.2015, 14:27 – 14:47

• Compare 47 spectra

326,0 326,5 327,0 327,5 328,0 / nm

1,510-3

2,010-3

2,510-3

3,010-3

3,510-3

4,010-3

4,510-3

5,010-3

S(

) /

a.u

.

y-A

chse

Raw data 47 spectra

Wavelength scale stable

Slide 16 of 18

Task 3.1: Test of entrance optics with natural sunlight

300 310 320 330 340 350 / nm

0,00

0,02

0,04

0,06

S(

) /

a.u

.

: mean measured spectrum

: corrected spectrum

: radiometric correction function

Slide 17 of 18

344 345 346 347 348 349 350 / nm

0,2

0,4

0,6

0,8

1,0

S(

) /

a.u

.

: corrected spectrum

: Extraterrestrial spectrum

Task 3.1: Test of entrance optics with natural sunlight

300 310 320 330 340 350 / nm

0,0

0,2

0,4

0,6

0,8

1,0

S(

) /

a.u

.

: corrected spectrum

: Extraterrestrial spectrum

First results:

• Spectral range: ok

• Spectral resolution ≈ 0.05nm • Agreement with Extraterrestrial spectrum

• SNR: ok

• Time per spectrum: 25s

Slide 18 of 18

Task 3.1: Outlook + next steps

• Perform a radiometric calibration against the spectral responsivity of a photodiode using a tunable light source (DSR-facility)

• more parallel light • Higher irradiance in UV than standard lamp • Compare against radiometric calibration with standard lamp

• Investigate stability/reproducability of combined instrument (!!) • Investigate „special features“ of FTS dependent on FTS parameter settings

• Ghost peaks • „mathematical“ noise resulting from Fourier Transformation • …or even more surprises…

• Development of transportable housing D3.1.2 (March 2016)

Slide 19 of 18

300 320 340 360 380 / nm

0

210-5

410-5

610-5

810-5

1,010-4

1,210-4

1,410-4

1,610-4

S(

) /

a.u

.

: Hg Pencil Style lamp

Task 3.1: Measure Hg-Pencil style lamp

312,4 312,6 312,8 313,0 313,2 313,4 / nm

210-5

410-5

610-5

810-5

1,010-4

1,210-4

1,410-4

1,610-4

S(

) /

a.u

.

: Hg Pencil Style lamp

312.58 nm

313.19 nm

Literature values: 312.57 nm 313.18 nm 365.02 nm

362 363 364 365 366 367 368 / nm

0

210-5

410-5

610-5

810-5

S(

) /

a.u

.

: Hg Pencil Style lamp

365.03 nm

Δλ1/2 = 0.06 nm

Δλ1/2 = 0.07 nm Δλ1/2 = 0.09 nm

Slide 20 of 18

Task 3.1: FTS parameters

FTS Parameter used for solar irradiance measurements

GaP diode + UG11 (Bruker) Si diode (Bruker) Si diode (Bruker) InGaAs diode (Bruker)

direct fibre (UV/VIS) direct fibre (UV/VIS) direct fibre (VIS/IR) direct fibre (VIS/IR)

xpm-file 150820 UVFaseroptik+GaP+UG11.xpm 150819 UVFaseroptik+Si.xpm 150819 IRFaseroptik+Si.xpm 150817 IRFaseroptik+InGaAs.xpm

wavelength range 250 nm - 400 nm 350 nm - 1100 nm 500 nm - 1100 nm 1000 nm - 2500 nm

wavenumber range 25000 cm^-1 - 40000 cm^-1 9000 cm^-1 - 29000 cm^-1 9000 cm^-1 - 20000 cm^-1 4000 cm^-1 - 10000 cm^-1

Instrument parameter

Aperture 8 mm 2 mm 8 mm 8 mm

Scanner Frequency 2.5 kHz 5 kHz 5 kHz 5 kHz

Low pass filter frequency Automatic Automatic Automatic Automatic

Gain 16xB 1xA 1xB 1xB

Aquisition parameter

Frequency limits 0 - 34000 cm^-1 0 - 31600 cm^-1 0 - 31600 cm^-1 0 - 15800 cm^-1

High pass filter On On On On

Acquisition mode Single Sided, Forward-Backward Single Sided, Forward-Backward Single Sided, Forward-Backward Single Sided, Forward-Backward

No. Of scans 30 20 20 20

Scan time 8 scans = 25 s 25 s 25 s 25 s

Spectral resolution 4 cm^-1 5 cm^-1 5 cm^-1 5 cm^-1

Phase resolution 8 10 10 10

Fourier Transformation

Phase correction mode Mertz Mertz Mertz Mertz

Apodization function Blackman-Harris 3-Term Blackman-Harris 3-Term Blackman-Harris 3-Term Blackman-Harris 3-Term

Zero filling factor 2 2 2 2

Laser frequency: 15802.38cm^-1 (air, 632.82 nm)