First step in the industry-based development of an ultra-stable optical cavity for space applications

Transcription

1 First step in the industry-based development of an ultra-stable optical cavity for space applications B. Argence, E. Prevost, T. Levêque, R. Le Goff, S. Bize, P. Lemonde and G. Santarelli LNE-SYRTE,Observatoire de Paris, CNRS, UPMC,Paris, France SODERN, Limeil-Brevannes, France CNES, Toulouse, France

2 Context Ultra-stable laser is a key element modern physics: Optical frequency standards (search for fundamental constants drift) Generation of low phase noise microwave signals. Transfer of optical stable frequencies by fiber networks Fundamental physics Tests of relativity (Michelson Morley, Lorentz Invariance) Gravitational wave detection (VIRGO, LIGO and LISA). Transportable cavities (laboratory level elegant breadboard at NIST, PTB, NPL) with low level of vibration sensitivity Scope of this work: develop a full industrial engineering model of an ultra-stable Fabry Perot cavity for space applications: Extensive thermo-mechanical modeling => low thermal and vibration sensitivities transportable and acceleration tolerant => 1st step toward space qualified system Transfer of knowhow from lab. to industry

3 Fabry-Perot Cavity Design 1.54µm => telecom wavelength Vertical optical axis cavity Cavity length: 100mm => FSR = c/2l = 1.5GHz Plano-concave mirror configuration ULE spacer, Fused Silica mirrors substrate thermal noise limit 4x10-16 (Numata 2004)

4 Cavity design Extensive mechanical simulation (FEM) to reduce thermal and vib. sensitivities Cylindrical shape with mid-plane ring Symmetrical holding

5 Thermo-mechanical design 3 gold coated aluminum shields stainless steel vacuum chamber Design to filter thermal fluctuations by a factor of 10 6 up to 1000s 2L/s Ion pump => mbar (dominated H 2 of stainless steel can)

6 Thermo mechanical interface Rigidly fix FP cavity (2.2kg) Thermal and mechanical decoupling cavity/shields Sophisticated Invar interface with cantilevers First mechanical resonance ~ 300Hz Optics Express paper soon published Von Mises stress under T = 1 C

7 Transportability 20 (6) g acc. along axial (transverse) axis T = 20 ± 53 C Total weight < 40kg, (stainless steel can ~ 20kg) Cavity assembled at SODERN company Tested at SYRTE laboratory SYRTE 25km SODERN

8 Cavity figure of merit Optimization of the coupling by offset phase locking a slave laser onto an ultra-stable laser Ultra-Stable Laser Slave Laser FP Cavity f beatnote ~ f beatnote + f m f m from 1 to 200kHz Servo KI Linewidth: 3.9kHz => Finesse = Coupling (max-min)/max > 80%

9 Optical/Electronic Locking Scheme - PDH ~ 61 MHz Oscillator Mirror at 45 Avalanche Photodiode Low Pass Filter φ shifter Cavity Mirror λ/4 PBS Servo K Bandwidth 600kHz Output Beam Diode Laser MEMS Low noise ~ 15 mw Variable Optical Attenuator 10/90 λ/2 PBS Optical Isolator Polarizer EOM

10 Optical/Electronic Locking Scheme - PDH ~ 61 MHz Oscillator Mirror at 45 Avalanche Photodiode Low Pass Filter φ shifter Cavity Mirror λ/4 PBS Servo K Bandwidth 600kHz Output Beam Diode Laser MEMS Low noise ~ 15 mw Variable Optical Attenuator 10/90 λ/2 PBS Optical Isolator Polarizer EOM

11 Optical Power Stability Strong power-to-frequency coefficient ~ 200 Hz/µW trans Injected Power = 10µW Optical power control w. fiber MEMS based voltage variable optical attenuator (VOA) (servo loop) frequency fluctuations < => RIN <-90dB Measured out-of-loop RIN < -100dB Power stability floor eq. to δν/ν at 10-16

12 Vibration Sensitivity: Measurement Set Up Active platform for the acceleration sensitivity measurements & vibration isolation PDH Loop Vector Signal Analyzer LASER Seismometer Active Platform Modulation Box Ultra stable reference laser Frequency to Voltage Converter

13 Vibration Sensitivity: Results Vibration sensitivity: Γ = 1 ν 0 S ν S a Vertical sensitivity: Horizontal sensitivity: /ms -2 Difficult to evaluate: strong coupling horizontal/vertical axes of the platform Horizontal ~ (6 ± 3) /ms -2

14 Cavity Set-up Thermal Sensitivity Measured coefficient 20 MHz/K ~ 10-7 /K ULE ring on F.S mirrors (Legero 2010) Thermal shielding: Expected 2nd order low pass filter Time const. ~ 0.9 and 2.5 days Meas. ~ 1.3 and 2.2 days Fair agreement Frequency stability target ν/ν < s to 10s T <0.1mK (active control)

15 Frequency Stability Frequency comparison: 1.54 µm vs 1.06 µm using femtosecond fiber comb Laser 1.062µm Frequency Counter Femtosecond Laser (f rep, f 0 ) Stability σ y (τ) < from 0.1 to 30 seconds Laser 1.542µm W. Zang, Z. Xu, Y. LeCoq, S.Bize Servo loop => J. Mac Ferran talk σ σ y (1s) = y (1s) = Linear drift ~ 2Hz/s removed Thermal Noise Limitation ~ (Numata)

16 Conclusions First ultra-stable FP cavity system developed by space industry: Extensive thermo-mechanical modeling Transportable, robust set-up Vibration sensitivity 4x10-11 /ms -2 (10-11 /ms -2 ) axial (transvers.) Fairly adapted to satellite environment Thermal sensitivity ~ 10-7 /K (Fused Silica mirrors) Thermal shielding 2 nd order low pass filter (time const. ~ 2 days) Very Low fractional frequency stability < 1x10-15 from 0.1s to few seconds ~ 5-6x10-16 at 1s

17 Prospects Compact, autonomous, transportable system French Space Agency (CNES) facilities in Toulouse Movable system to several French locations for experiments New version for the Strontium lattice clock (698nm), FP7/SOC2 project Future targets: Understanding vibration sensitivity (factor 3 reduction) Mass optimization (titanium enclosure, total weight < 25kg) Rescaling cavity length down to 70/50 mm (very compact and light system) ULE ring on F.S. mirrors (Legero s technique) 10-7 /K => below 10-8 /K