Optical power variation only

Amplitude to phase conversion issue in telemetry

CONTEXT/OBJECTIVES: This work was realized within JRPs « SURVEYING »(SIB-60) and « LUMINAR » (IND-53). In both projects a compensation of air temperature and pressure variation is targeted using two different wavelengths (1550 nm and 780 nm) and air index dispersion. A first step is the realization of a robust and simple one wavelength telemeter at 1550 nm, with a resolution below 5 µm indoor. It turns out that the amplitude to phase conversion in photodetectors and electronic stage is a critical issue that has to be considered

PRINCIPLE OF THE TELEMETERINDOOR MEASUREMENTS

OUTDOOR MEASUREMENTS

0 100 200 300 400 500 600 700 800 900 1000

-27

-26.8

-26.6

-26.4

dis

tance (m

m)

0 100 200 300 400 500 600 700 800 900 1000-40

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-10

0

am

plitude (dB

m)

measurement number

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-26.8

-26.6

-26.4

dis

tance (m

m)

0 100 200 300 400 500 600 700 800 900 1000-40

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0

am

plitude (dB

m)

measurement number

in red = amplitudes equal to 7.9dB ± 2 dB

Std over 50 pointsbetween 11 & 24 µm

Standard deviation = 38 µm

Experimental setup at 1550nm

Optical switch: every second we compare the measured distance to a reference distance that do not vary during the measurement process. Thus, every variations observed on the reference path are interpreted as drifts from the system (for instance temperature evolution in amplifiers) and are so removed from the measured distance.

Joffray Guillory1, Jorge Garcia-Marquez1, Anne-Françoise Obaton1, Christophe Alexandre2, Daniel Truong1 and Jean-Pierre Wallerand1*.1 Laboratoire Commun de Métrologie LNE-CNAM (LCM), 1 rue Gaston Boissier, 75015 PARIS

2Centre d’études et de recherche en informatique et communications (CEDRIC), CNAM, 292 rue Saint-Martin, 75003 PARIS

* Corresponding author: jean-pierre.wallerand@cnam.fr

AMPLITUDE TO PHASE CONVERSION

RF

LO

S

S

mixer

Ph

ase

mete

r

IFamplifier

reference

single chip

DFB EAM

PD

AOM

powermeter

95% 5%

EDFA

biastee

bias voltage

AM generation

measure

Optical and RF power variation

RF

LO

S

S

mixer

Ph

ase

me

ter

IFamplifier

DFB EAM

PD

powermeter

95% 5%

EDFA

biastee

bias voltage

AOMDFB

50% 50%

laser 1

laser 2

AM generation

reference

measure

Metal semi conductor metal photdetector (Hamamatsu G7096-03)

InGaAs avalanche photodiode (Hamamatsu G8931-04)

Amplitude to phase variation conversion due to photodetector and electronic stage

Metal semi conductor metal photodetector InGaAs avalanche photodiode

sunny day, 35°C, along an asphalt road.

A selection of data within an amplitude interval of 4 dB greatly improves the robustness of the system.

Avalanche photodetector is less sensitive to amplitude to phase conversion. In that case the global amplitude to phase conversion is essentially due to electronic stage (mixer+amplifiers)

6 7 8 9 10 11 12 13 14 15-1000

-800

-600

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-200

0

200

400

Photodiode bias voltage (V)

d

ista

nce

/

opt

ical

pow

er (

µm

/dB

)

18.5 µW45.1 µW

93.3 µW

143.1 µW

196.1 µW251.2 µW

25 30 35 40-100

-50

0

50

100

150

200

250

300

350

400

450

Photodiode bias voltage (V)

d

ista

nce

/

opt

ical

pow

er (

µm

/dB

)

11.7 µW

28.9 µW

61.2 µW

6 7 8 9 10 11 12 13 14 15-1000

-500

0

500

Photodiode bias voltage (V)

d

ista

nce

/

opt

ical

pow

er (

µm

/dB

)

245.9 µW

25 30 35 40-50

0

50

100

150

200

250

Photodiode bias voltage (V)

d

ista

nce

/

opt

ical

pow

er (

µm

/dB

)

60.5 µW

CONCLUSION/PERSPECTIVES: A robust and compact telemeter was realized at 1550 nm using off-the-shelf components from telecommunication industry. A resolution of 3 µm was obtained indoor using a modulation frequency of 1,3 GHz. Amplitude to phase conversion was minimized by a selection of measurement data comprised in a given amplitude interval. A measurement of up to 600 m outdoor was realized (limited by available distance). Next steps will be to increase the modulation frequency and to add a second wavelength (773 nm) to the system in order to compensate air index fluctuation due to pressure and temperature variation.

lens

RF

LOS

S

mixer

Ph

ase

mete

r

10MHz.amplifier

Reference path

singlechip EAMDFB

PDbiastee

Biasvoltage

Att.

RF = radio frequency synthesizerLO = local oscillatorS = RF splitterDFB = distributed feedback laser diodeEAM = electro-absorption modulatorEDFA = erbium doped fiber amplifierSMF = single mode fiberAtt. = variable optical attenuatorPD = free-space photodiode

optical splitter(used as a circulator)

off-axisparabolic

mirror

distance D

Target

fiber connector(FC/APC)

cornercube

Measure path

optical switch

Φ

sine wave that hasbeen propagated

in free-space

electricalsine wave

10MHz

1310MHz

1300MHz

mirror

EDFA

SMFisolator

RFfn

ckD

2

2

ΦCnfRFk

the measured phase shiftthe speed of light in vacuumthe group refractive indexthe frequency modulation (1310MHz)an integer number of 2π modulowithin the distance to be measured.

RFfMeasurement during 90 mn over 2 m indoor. The modulation frequency was 1,3 GHz. The resolution (short term standard deviation) was approximately 3µm. Residual variations are due to uncompensated drifts in the optoelectronic system.

In black: electronic signals. In orange: fibered system. In red: free space light.

Over 400 m under favourable conditions:

Over 100 m under unfavourable conditions:

Selection of data

JRP IND 53

This project is performed within the joint research projects SIB60 “Surveying” and IND53 “Luminar” of the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.