a 1550nm telemeter for outdoor application based on off-the-shelf components

Transcription

1 a 155nm telemeter for outdoor application based on off-the-shelf components Joffray Guillory, Jean-Pierre Wallerand, Jorge Garcia Marquez, Daniel Truong (mechanical engineering), Christophe Alexandre (digital electronics). LNE - Cnam, France 1 st Workshop: Metrology for Long Distance Surveying Lisboa, Portugal November 21, 214

2 An absolute distance meter in 4 points 1. A telemeter able to measure distances up to 1km with a resolution better than 1µm. 2. An user-friendly system. 3. An affordable solution with off-the-shelf components already available today on the market. 4. A telemeter that compensates the fluctuation of the air refractive index with temperature. 3

3 Outline 1. Presentation of the telemeter principle and of our aims 2. Experimental setup 3. Measurement accuracy over 5m 4. Measurement precision up to 1km 5. Conclusion and Next steps 2

4 Principle of the Telemeter Phasemeter RF in Laser RF out Φmeas. Modulator RF out Φref. Beam Splitter Photo detector 2 Photo detector 1 1 L ( 2 1 ( rad) L ( ref meas) ( ref meas) position1 k 2 ) 2 n air position 2 c f 2 RF Corner cube AIM: distances up to 1km with a relative resolution of 1-7, i.e. 1µm over 1km. Issue: to achieve a resolution of 1µm over 1km, the temperature has to be known at ±.1 C along the optical path. Solution: developing a two-wavelength system (78 and 155 nm) and measuring the distance at each wavelength for a refractive index equal to 1. The error can then be calculated using the temperature dispersion relation (method valid for dry air), and therefore the measured distance can be corrected [K.B. Earnshaw et al., Appl. Opt., Vol. 11, Issue 4, 1972]. We are currently developping the telemeter for the first wavelength, at 155nm. 3

5 Phasemeter Experimental setup at 155nm RF amplifier bias voltage S = RF splitter Φ measure bias PD PD tee LO mixer lens S Φ reference LO optical power meter DFB = distributed feedback laser diode = photodetector = local oscillator EDFA = erbium doped fiber amplifier SMF = single mode fiber Att. = variable optical attenuator EAM = electro-absorption modulator off-axis parabolic mirror corner cube Target corner cube RF S SMF not used Distance single chip DFB EAM EDFA isolator optical splitter I BIAS + I Noise Down-conversion into IF: we perform RF amplification at IF to be less sensitive to the AM/PM conversions occurring in the electronic stages (as suggested in [D.H. Phung, Ph.D. Thesis, UNS, France, 213]). 4

6 Phasemeter Experimental setup at 155nm RF amplifier Φ measure mixer S Φ reference bias voltage bias tee LO PD lens optical power meter S = RF splitter DFB = distributed feedback laser diode PD = photodetector LO = local oscillator EDFA = erbium doped fiber amplifier SMF = single mode fiber Att. = variable optical attenuator EAM = electro-absorption modulator off-axis parabolic mirror corner cube Target corner cube RF S SMF not used Distance single chip DFB EAM EDFA isolator optical splitter optical switch Att. Reference path fibered mirror I BIAS + I Noise 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. 4

7 Reference distance (mm) Experimental setup at 155nm Measure distance (mm) 1 Measure path Time (min) RF carrier at 1.3GHz The number of modulo 2πis undetermined and fixed to. 5

8 Reference distance (mm) Reference distance (mm) Experimental setup at 155nm Measure distance (mm) Measure path Reference path Time (min) RF carrier at 1.3GHz The number of modulo 2πis undetermined and fixed to. 5

9 Measure distance - Reference distance (mm) Experimental setup at 155nm Measure path Reference path Only real distance variations, no drift from the system Time (min) RF carrier at 1.3GHz The number of modulo 2πis undetermined and fixed to. 5

10 PLAN 1. Presentation of the telemeter principle and of our aims 2. Experimental setup 3. Measurement accuracy over 5m 4. Measurement precision up to 1km 5. Conclusion and Next steps 8

11 Measurement Accuracy up to 5m Comparison of the measured distances with an interferometric bench up to 5m (VSL) VSL interferometer Position 1 Position 2 Distance off-axis parabolic mirror LCM telemeter Currently, we do not measure yet the number of synthetic wavelength (c / f RF 5cm). L ( 2 1 ( rad) k 2 ) 2 c n f RF 23

12 Measurement Accuracy up to 5m 12

13 error (µm) Measurement Accuracy up to 5m Probably due to operator error. This should be removed when the different paths (reference and measure) will have the same RF levels. To this end, automation under progress distance (m) 24

14 PLAN 1. Presentation of the telemeter principle and of our aims 2. Experimental setup 3. Measurement accuracy over 5m 4. Measurement precision up to 1km 5. Conclusion and Next steps 22

15 Forêt de Montmorency Distances up to 1km

16 Measurement Precision 2m Forêt de Montmorency 1

17 Humidity = 82% Temperature = 1 C Wind = south southeast 14 km/h Measurement Precision 2m 1

18 Distance (mm) RF level (dbm) Measurement Precision 2m Main issue = Amplitude Modulation to Phase Modulation conversions occuring in the reception stages. The received light is subject to intensity noise due to beam bending and scintillation phenomenon..4 Standard deviation = 12µm measures in 53 secondes time (s) 1

19 Distance (mm) RF level (dbm) Measurement Precision 2m The solution consists in selecting the phase shift values / the distances having a same amplitude..4 Standard deviation = 7µm RF level = -2.4 dbm ±.5dB time (s) 12

20 Distance (mm) RF level (dbm) Measurement Precision 2m Standard deviation = 7µm time (s) RF level = -2.4 dbm ±.5dB Data selection = 76.4% 13

21 Distance (mm) RF level (dbm) Measurement Precision 1m Standard deviation = 11µm time (s) RF level = dbm ±.5dB Data selection = 24.1% 14

22 Distance (mm) RF level (dbm) Measurement Precision 2m Standard deviation = 15µm time (s) RF level = -4. dbm ±.5dB Data selection = 21.8% 15

23 Measurement Precision 2m 11

24 Distance (mm) RF level (dbm) Measurement Precision 41m Standard deviation = 12µm time (s) RF level = -3.5 dbm ±.5dB Data selection = 12.8% 16

25 Measurement Precision 97m 11

26 Distance (mm) RF level (dbm) Measurement Precision 6m Standard deviation = 15µm time (s) RF level = dbm ±.5dB Data selection = 8.5% 17

27 Distance (mm) RF level (dbm) Measurement Precision 8m Standard deviation = 19µm time (s) RF level = dbm ±.5dB Data selection = 4.2% 18

28 Distance (mm) RF level (dbm) Measurement Precision 97m Standard deviation = 36µm time (s) RF level = dbm ±.5dB Data selection = 3.6% 19

29 Distance (mm) RF level (dbm) Measurement Precision 97m Standard deviation without drift = 12µm time (s) The drift of 122µm can be explained by an average temperature change of.13 C. This should be removed when the two-wavelength system will be achieved. 2

30 Measurement Precision 97m 11

31 Standard Deviation (µm) Measurement Precision To sum up: Untreated with data selection (±.5dB) with data selection + drift removed Distance (m) 21

32 Conclusion & Next steps Conclusion: We have developed a first version of a distance meter at 155nm based on a well-known technique, the latter has been improved using: - All-digital data processing including FPGA-based phasemeter - optoelectronic components coming from the telecommunication industry affordable system. - an internal free-space corner cube to perform quick distance measurements and remove the drifts of the system. To meet the wanted performances, it has been necessary to overcome: - the AM/PM conversions select the phase shift values of same amplitude. Next Steps: - Determining the number of modulo 2π of the phase in order to know the absolute distance. - Duplicating the setup at the second wavelength, at 78nm, to compensate the fluctuation of the air temperature. Acknowledgements: This work is funded within the European Metrology Research Programme (EMRP) as JRPs SIB6 Surveying ( joffray.guillory@cnam.fr 25

33 Thank you for your attention 26