PTB‘s refractive index compensated absolute 3D laser meter

Jennifer Bautsch, Matthias Franke, Karl Meiners-Hagen, Tobias Meyer, Florian Pollinger, Günther Prellinger, Kerstin Rost, Martin Wedde, Klaus Wendt (PTB)

Denis Dontsov, Wolfgang Pöschel (SIOS)

Overview:

- Introduction

- Refractive index compensation

- 3D-Lasermeter (IFM mode)

- Results in air conditioned and harsh environment

- ADM mode of 3D-Lasermeter

- Results of ADM mode

- Conclusion

- 3D metrology based on speed of light (laser tracker, LaserTracer)

is affected by air refractive index

- In harsh environments several µm/m uncertainty is possible

- Within LUMINAR a device was proposed with:

- 3D capability like a LaserTracer

- intrinsic compensation of air refractive index

- fringe counting mode (IFM) like LaserTracer

- absolute distance measurement (ADM) mode like several laser trackers

- uncertainty 10-7 l, even in harsh conditions

- Result: 3D-Lasermeter

Introduction

Refractive index compensation by two colour interferometry

Standard Interferometry:

- Measurement of optical path l with frequency stabilised laser ⇒ l0 = n l

- Measurement of temperature, air pressure, humidity, (CO2) ⇒ refractive index n

- Distance: l = l0 /n

- Uncertainties from measurement of air parameters:

Large Volume Metrology Workshop, 18-19 May 2016, NPL TeddingtonKarl Meiners-Hagen

Parameter Refractive index

change

Temperature ∆t = +1 °C -1 x 10-6

Pressure ∆p = +1 hPa +2.7 x 10-7

Relative humidity ∆RH = +1 % -1 x 10-8

Refractive index compensation by two colour interferometry

- 3) From l1 = n1l and l refractive index n1 can be calculated: n1 = l1 / l

From n1, pressure p, and pw the temperature can be derived

- 2) Measurement of partial pressure of water vapour ⇒ pw

Distance l = f(l1 , l2 , pw) (independent on temperature and pressure)

Refractive index compensated interferometry:

- Measurement of optical path with two wavelengths ⇒ l1, l2

- 1) l = l1 - A(l2 - l1), A constant (dry air)

A ≈ 65 for 532 nm + 1064 nm

uncertainties in (l2 - l1) are scaled by 65!

Karl Meiners-Hagen

600 800 1000

2,68

2,70

2,72

2,74

2,76

n2-1

λ1 λ

2

10

4 x

(n

-1)

Wavelength

n1-1

Refractive index compensation by two colour interferometry

Influence of changes in the air parameters:

Parameter Standard Compensated

Temperature ∆t = +1 °C -1 x 10-6 ---

Pressure ∆p = +1 hPa +2.7 x 10-7 ---

Relative humidity ∆RH = +1 % -1 x 10-8 -2.4 x 10-8

⇒ Theoretically independent on temperature and pressure,

but more sensitive to relative humidity

⇒ Uncertainty enhanced by factor A: l = l1 - A(l2 - l1)

(≈ 65 for “optical“ wavelengths, ≈ 21 for “synthetic wavelengths“

at 532 nm/1064 nm)

3D-Lasermeter

- Similar design like LaserTracer

- IFM mode: frequency doubled Nd:YAG laser (1064 nm + 532 nm)

- Frequency stabilised on Iodine absorption line

- Heterodyne interferometer

Karl Meiners-Hagen

Comparison with PTB 50 m comparator (geodetic base)

Comparison with HeNe interferometer with folded beam path.

Tracking is switched off.

Karl Meiners-Hagen

3D Lasermeter

reference laserZygo 7712

Fibre to detector board

Comparison with PTB 50 m comparator (geodetic base)

- Systematic deviations around 2 m ??

- Otherwise within ±2 µm up to 20 m

Karl Meiners-Hagen

May 2015

Tracking off

0 5 10 15 20

-2

-1

0

1

2

3

4

3D

-Laserm

ete

r -

Refe

rence / µ

m

Length / m

Comparison with PTB 50 m comparator (geodetic base)

After optimisations:

- Systematic deviations around 2 m ??

- Scatter well below 1 µm on short path, otherwise within ±2 µm up to 20 m

Karl Meiners-Hagen

Jan 2016

Tracking off

0 5 10 15 20-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

3D

-La

serm

ete

r -

Re

fere

nce

/ µ

m

Length / m

Comparison with PTB 50 m comparator (geodetic base)

- Seems to be a curve in our rail

- Deviations not yet understood (no problems with interferometer calibrations)

- � = (1μm)+(10 ��) for 1D, but why the effort?

Karl Meiners-Hagen

Jan 2016

Tracking on

0 2 4-2

-1

0

1

3D

-La

serm

ete

r -

Refe

rence / µ

m

Length / m

Harsh environment at GUM (Warsaw) 50 m comparator

Two housings with heating fans installed for simulating harsh environment

Karl Meiners-Hagen

12:15 12:20 12:25 12:30 12:35-50

-40

-30

-20

-10

0

10

Le

ngth

ch

ang

e /

µm

Time

L532 / µm

L1064 / µm

Compensated / µm

Harsh environment at GUM (Warsaw) 50 m comparator

Raw optical path length changes with temperaure

Compensated result remains constant, but with larger scatter during

turbulences

Karl Meiners-Hagen

Length ≈19 m

Harsh environment at GUM (Warsaw) 50 m comparator

Scatter during turbulences probably due to signal amplitudes.

Karl Meiners-Hagen

Length ≈19 m

12:15 12:20 12:25 12:30 12:35-200

-150

-100

-50

0

Le

ngth

chan

ge / µ

m

Time

0

20

40

60

80

100

120

140

Norm

alis

ed

sig

nal am

plit

ud

e / %

12:15 12:20 12:25 12:30 12:35

18,5

19,0

19,5

20,0

20,5

21,0

21,5

Te

mp

era

ture

/ °

C

Time

from interferometer

sensor data

Harsh environment at GUM (Warsaw) 50 m comparator

Temperature from sensors 1.5 °C off: we believe in interferometer (constant length)

Karl Meiners-Hagen

Length ≈19 m

Measurements at Airbus in Filton, March 2016

- Measurements in building Q7 on a 44.7 m path

- No interferometric reference, only sensor data for temperature, pressure, humidity

- Evaluation of data:

- length uncorrected and refractivity compensated

- air index change (no absolute values)

- temperature change (no absolute values)

(for absolute values of air index and temperature the absolute optical path lengths

must be known)

Karl Meiners-Hagen

Measurements at Airbus in Filton, March 2016

Moderate turbulences: corrected length follows a straight line, uncorrected does not.

Karl Meiners-Hagen

07:25 07:26 07:27 07:28

0

20

40

60

Le

ng

th c

ha

ng

e /

µm

Time

L532 / µm

L1064 / µm

L2Color 532 / µm

2016-03-10

Measurements at Airbus in Filton, March 2016

Refractive index change and measured temperature:

Sensors can‘t follow the refractive index (measurement cycle and thermal delay)

Karl Meiners-Hagen

07:25 07:26 07:27 07:28

-3x10-7

-2x10-7

-1x10-7

0

2016-03-10

∆ n

Time

Sensors

Interferometer

19,70

19,75

19,80

Tem

pera

ture

/ °

C

Measurements at Airbus in Filton, March 2016

Heating of building turned on at 09:00

(Windy morning, up to 1 mm movement of the structure)

Karl Meiners-Hagen

08:30 08:45 09:00 09:15

-1000

-500

0

500

Le

ng

th c

ha

ng

e /

µm

Time

L532 / µm

L1064 / µm

L2Color 532 / µm

2016-03-09

Measurements at Airbus in Filton, March 2016

Refractive index difference within 5x10-7 during heating

Possible reason: temperature distribution (lowest 5 m path had no sensors)

Karl Meiners-Hagen

08:30 08:45 09:00 09:15-3x10

-6

-2x10-6

-1x10-6

0

2016-03-09

∆ n

Time

Sensors

Interferometer

10

11

12

13

Tem

pera

ture

/ °

C

☺

ADM mode

- Two frequency doubled Nd:YAG lasers (1064 nm + 532 nm)

- Phase locked wih 20 GHz (1064 nm) / 40 GHz (532 nm) offset

- Generation of additional frequencies with frequency shifters (AOM)

Karl Meiners-Hagen

40 GHz

(7.5 mm)

20 GHz

(15 mm)

Nd:YAG 532 nm

1064 nm

Nd:YAG 532 nm

1064 nm

∆ν = 20 GHz (40 GHz)

-7 MHz

+93 MHz

+186 MHz

-5 MHz

+95 MHz

+190 MHz

+97.5 MHz

+96.75 MHz

-2.5 MHz

-3.25 MHz

+195 MHz

+193.5 MHz

ADM mode

- Two frequency doubled Nd:YAG lasers (1064 nm + 532 nm)

- Phase locked wih 20 GHz (1064 nm) / 40 GHz (532 nm) offset

- Generation of additional frequencies with frequency shifters (AOM)

Karl Meiners-Hagen

197.5 MHz

(1.5190 m)

196.75 MHz

(1.5248 m)

- Coarse distance by frequency scanning with AOM

- Uncertainty scaling bad: 7.5 mm/532 nm x 21.5 = 300 000

Nd:YAG 532 nm

1064 nm

Nd:YAG 532 nm

1064 nm

∆ν = 20 GHz (40 GHz)

-7 MHz

+93 MHz

+186 MHz

-5 MHz

+95 MHz

+190 MHz

+97.5 MHz

+96.75 MHz

-2.5 MHz

-3.25 MHz

+195 MHz

+193.5 MHz

ADM mode

Karl Meiners-Hagen

Light source, now on a trolley with laser shielding

90 cm x 60 cm

+ 19‘‘ rack

ADM mode

- Fixed 40 m path at 50 m comparator in PTB

- 10 seconds averaging: 60 µm standard deviation ⇒ 0.2 nm between four

optical wavelength results

Karl Meiners-Hagen

14:00 16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

-0,5

0,0

0,5

1,0

1,5

2,0 2016-02-23/24

Length

change / m

m

Time

ADM mode

- At Airbus with 2 m path length: >1 mm variation ⇒ 3.3 nm between four

optical wavelength results

- Problems with polarisation in the interferometer

- In JRP „Surveying“ the ADM mode works up to 864 m

(<50 µm length independent standard deviation, 1x10-7 l)

Karl Meiners-Hagen

11:23 11:25 11:27 11:29 11:31 11:33 11:35-0,02

-0,01

0,00

0,01

0,02

0,03

0,04

Length

change / m

m

Time

532 nm

1064 nm

l ≈ 2 m

2016-03-03

11:22 11:27 11:32

-1,0

-0,5

0,0

0,5

l ≈ 2 m

2016-03-03

Length

change / m

m

Time

Raw data Refractive index compensated result

�

Conclusions

- IFM measurements works with 1D uncertainty � = (1μm)+(10 ��)

- Temperature range 9 °C to 22 °C verified, with gradients up to 10 °C

- 3D measurements at a CMM, 3D uncertainty around 3.5 µm

- ADM measurements suffer from problems with polarisation which were

solved in the JRP “Surveying” project

- Outlook: investigation of temperature dependence of non polarising beam

splitters

Physikalisch-Technische Bundesanstalt

Braunschweig und Berlin

Bundesallee 10038116 Braunschweig

Karl Meiners-HagenArbeitsgruppe Mehrwellenlängeninterferometrie für geodätische Längen

Telefon: 0531 592-5230E-Mail: Karl.Meiners-Hagen@ptb.dewww.ptb.de

Stand: 05/2016