Heterodyne Many-Wavelength Interferometry
Jutta Mildner1,*, Karl Meiners-Hagen1 and Florian Pollinger1
1
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
* Corresponding author: [email protected]
Outline
• Introduction to
– Heterodyne many-wavelength interferometry in
general
– Comb-based heterodyne interferometry in particular
• Preliminary studies on
– Generating cavity enhanced electro-optic comb
sources
– Application to absolute distance measurements
• Frequency comb-based interferometer
– Principle of cavity mode filtering
– Simulation & experimental setup
– First results
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
2 / 19
Introduction
Heterodyne Many-Wavelength Interferometry
PBS
λ/4
PBS
Laser 1: Signal
HWP
Meas. Beamline
Laser 2: Local
Polarizer
Measurement
Reference
SR
SM
Sampling &
Processing
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
Absolute Distance
3 / 19
Introduction
Fixed Synthetic Wavelength Interferometry
λ1 λ2
Measurement principle:
• Combination of phase information of
more than one wavelength
☺ Range of non-ambiguity increases to half
the synthetic wavelength
Λ jk =
ΛS /2
λ j λk
λ j − λk
Uncertainties are scaled up quickly
u (l jk ) ∝
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
Λ jk
λj
4 / 19
Introduction
Ideal Multi-Wavelength Interferometry
l
ϕ j − ϕk
l ≈ l jk = N
+
Λ jk
2
4π
Λ jk
Λ1
Λ2
Λ3
The ideal implementation:
• As many lasers as possible
• Varying wavelength differences, both small and big
• Complete chain of scales (synthetic wavelengths)
from m to nm
…
…
Λi
For broader use:
• Relatively simple source
• Simple and robust implementation
• Affordable price
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
5 / 19
Comb-Based Heterodyne Interferometry
fSr
fSr = fLr
fLr
Lay et al., Opt. Lett. 28, 890 (2003)
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
Yang et al., Opt. Lett. 39, 5834 (2014)
6 / 19
Outline
• Introduction to
– Heterodyne many-wavelength interferometry in
general
– Comb-based heterodyne interferometry in particular
• Preliminary studies on
– Generating cavity enhanced electro-optic comb
sources
– Application to absolute distance measurements
• Frequency comb-based interferometer
– Principle of cavity mode filtering
– Simulation & experimental setup
– First results
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
7 / 19
Cavity-Enhanced Frequency Comb Generator
CEFCG
~ fm
SM Laser
EOM
~ fm
SM Laser
EOM
Kourogi et al. IEEE Journal of Quantum Electronics 29, 2693 (1993)
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
8 / 19
Generating Two CEFCGs
with Synchronized PDH-Stabilization
v0
v0
PBS
f0S
frS
EOM1
AOM1
ECDL
Signal comb
PZT
λ/2
f0S
~
frS
~
Synchronous
f0L
~
AOM2
frL
Stabilization
f0L
~
EOM2
• Commercially available RF EOMs fr = 9.2 GHz
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
PZT
frL
Local comb
Range of non-ambiguity 1.6 cm
9 / 19
Results: The Generated Comb(s)
• “Bandwidth”
300 GHz
• Mode spacing fSr = 9.2 GHz
• Optically down-converted
• Mode spacing fr = 5 kHz
• Digitalization with standard
AD card possible
• Phase retrieval by digital lock-in
Yang et al., Opt. Lett. 39, 5834 (2014)
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
10 / 19
Results: Exploiting Redundant Information
L ≅ 10 m, tmeas = 50 min , n = -7 … +7
m=1
m=4
νi
µm


uemp = ± 10µ m + 0.25
× lm 
m


1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
more bandwidth = lower uncertainty
11 / 19
Outline
• Introduction to
– Heterodyne many-wavelength interferometry in
general
– Comb-based heterodyne interferometry in particular
• Preliminary studies on
– Generating cavity enhanced electro-optic comb
sources
– Application to absolute distance measurements
• Frequency comb-based interferometer
– Principle of cavity mode filtering
– Simulation & experimental setup
– First results
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
12 / 19
Alternative Approach
Filtering a Femtosecond Frequency Comb
L
fr
Frequency comb:
250 MHz
Filtering Cavity
FSR = c 2 L = mf r
~10 GHz
• Curved cavity mirrors (R ~ 99%)
• Piezo-actuator: Piezomechanik GmbH
20 mm diameter, 13.5 mm height
12 µm travel range
• Home designed Macor-Spacer
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
13 / 19
Filter Cavity
Design and Transmission Simulations
Simulations:
•
•
•
Spacing between modes ~300 µm
Lateral width of resonance ~10 µm
Substructure on the order of 3 nm
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
mth Mode
FSR / GHz
Spacing L / mm
38
9.50
15.778550
39
9.75
15.373972
14 / 19
Filter Cavity
Experimental Setup
Spectrum
Analyzer
9.75 GHz
PLL
PD
12 GHz
~
PD
LPF
+50 mm
Fs-Laser
Piezo
Driver
ECDL2
ECDL1
PD
12 GHz
PD
1 GHz
Y-Coupler
+100 mm
Cavity
+300 mm
+300 mm
FPI
1 GHz
λ/2
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
λ/2
15 / 19
First Results
Transmission of Diode Laser
Cavity Finesse
Linewidth
F = 312
δν = 35 MHz
1,2
ECDL
Amplitude / V
1,0
0,8
0,6
0,4
0,2
0,0
0,0
0,5
1,0
1,5
2,0
2,5
Frequency / GHz
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
16 / 19
First Results
Signal Change of fs-Comb
normalized
Signal
NormalizedBeat
Signal
/1 /1
Signal rise:
Mode suppression of 10 dB
Change
9.25
GHz
Comb
Change of
of9.75
9.75GHz
GHzvs.
Beat
Node
vs.
9.25
with
a width
of Mode
2GHz
nm
10
8
6
4
2
0
1,65
1,66
1,67
1,68
1,69
1,70
1,71
1,72
1,73
Cavity Length Change / µm
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
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Summary & Outlook
Preliminary studies with CEFCGs
realization of two combs heterodyne interferometer
EOMs used both for comb generation & PDH-stabilization
achieved uncertainty of σ = ±10 µm
Setup of prototype cavity with PLL-stabilization
finesse of 312 and linewidth of 35 MHz
filtering efficiency of -10 dB
transmission peak width of 2 nm confirming simulations
Outlook
Amplification of laser power
Improvement of experimental stability
Implementation of PDH-stabilization
1st Workshop in Metrology for Long Distance Surveying – IPQ, Caparica – Jutta Mildner
18 / 19
Acknowledgements:
Physikalisch-Technische Bundesanstalt
Braunschweig und Berlin
Bundesallee 100
D-38116 Braunschweig
Germany
Dipl.-Phys. Jutta Mildner
5.42 Multiwavelength Interferometry for Geodetic Lengths
Tel.:
+49 (0)531/592-5422
E-Mail: [email protected]
www.ptb.de