1. Department ArtemisObservatoire de la Cote d'Azur1 A Sagnac interferometer with frequency modulation for sensitive saturated absorption (and applications for LISA!) Glenn de Vine, Matthieu Vangeleyn, Alain Brillet, C. Nary Man David McClelland, Malcolm Gray Observatoire de la Côte d'Azur Département ARTEMIS NICEglenn.devine@obs-nice.fr

2. Department ArtemisObservatoire de la Cote d'Azur2 Talk Outline: 1. LISA - lasers and frequency noise 2. Sagnac interferometer basics 3. Saturation spectroscopy basics 4. Sagnac interferometer for noise-rejection 5. Details of the technique 6. Theoretical modeling 7. Experimental results 8. The Future…

3. Department ArtemisObservatoire de la Cote d'Azur3 The LISA Interferometer  Arm lengths = 5 million km  Arm length difference ≈ 50,000 km (1%)  Frequency noise now couples in due to unequal arm length  Equal arm length Michelson freq noise is common and freq noise is common and not a concern white light interferometer white light interferometer

4. Department ArtemisObservatoire de la Cote d'Azur4 Frequency Noise Coupling

5. Department ArtemisObservatoire de la Cote d'Azur5 Measurement Sensitivity  In order to measure a relative arm length difference, dx = 2 pm/  Hz, using: we require a detector (laser) frequency sensitivity (stability), d, of we require a detector (laser) frequency sensitivity (stability), d, of 6x10 -6 Hz/  Hz

6. Department ArtemisObservatoire de la Cote d'Azur6 LISA Lasers  LISA will employ the most stable CW lasers currently available: Nd:YAG lasers at 1064 nm Nd:YAG lasers at 1064 nm Intensity noise requirements should be met with noise-eaters Intensity noise requirements should be met with noise-eaters Laser frequency noise needs to be overcome: Laser frequency noise needs to be overcome: Typical free running laser frequency noise: 10 4 /f Hz/  Hz LISA detection band is 100  Hz to 1 Hz At 100  Hz we require a stability improvement of over 13 orders of magnitude

7. Department ArtemisObservatoire de la Cote d'Azur7 Frequency Stabilisation Methods  Arm locking - stable reference, well established in ground-based GWD’s  Time-delay interferometry - new technique, currently being tested  Mechanical reference (cavity) - ULE, ZeroDur, etc  Atomic or molecular reference  No method alone will achieve the 13 orders of magnitude improvement required  Solution will be a combination

8. Department ArtemisObservatoire de la Cote d'Azur8 Atomic vs Mechanical (Cavity)  Atomic - for: for: absolute reference, best long term stabilityabsolute reference, best long term stabilityagainst: not space-rated, absorptions typically very weak at 1064 nmnot space-rated, absorptions typically very weak at 1064 nm  Cavity - for: for: simple, space-rated, best short term stabilitysimple, space-rated, best short term stabilityagainst: not absolute, aging, long term stability is susceptible to thermal variationsnot absolute, aging, long term stability is susceptible to thermal variations

9. Department ArtemisObservatoire de la Cote d'Azur9 Iodine Spectroscopy for LISA Laser Frequency Stabilisation  develop high performance frequency stability by locking a laser using Doppler- free saturated absorption spectroscopy of iodine at 532 nm for 1064 nm absolute stability  achieve LISA laser frequency stability requirement of < 1 Hz/√Hz from 100  Hz to 1 Hz

10. Department ArtemisObservatoire de la Cote d'Azur10 Iodine  Sufficient absorption from hyperfine resonances at 532 nm (the harmonic of 1064 nm - weak absorptions: Cs 2,CO 2,C 2 H 2 )  Commercially available lasers with doubled (532 nm) and fundamental (1064 nm) outputs  The spectroscopy (and thus, frequency stability) can benefit from improved techniques to enhance the signal and/or reduce the noise

11. Department ArtemisObservatoire de la Cote d'Azur11 Sagnac Interferometry

12. Department ArtemisObservatoire de la Cote d'Azur12 Saturation Spectroscopy  Energy levels of I 2 : 1. electronic 2. vibrational 3. rotational

13. Department ArtemisObservatoire de la Cote d'Azur13

14. Department ArtemisObservatoire de la Cote d'Azur14 Saturation Spectroscopy  Energy levels of I 2 : 1. electronic 2. vibrational (1 GHz) 3. rotational (1 MHz)

15. Department ArtemisObservatoire de la Cote d'Azur15 Saturation Spectroscopy  Energy levels of I 2 : 1. electronic 2. vibrational (1 GHz) 3. rotational (1 MHz)  Boltzmann thermal distribution - Doppler shifts transition frequencies relative to laser frequency  Doppler shifting is greater than hyperfine linewidth  Counter-propagating pump and probe fields - both interact only with molecules of zero longitudinal velocity (to first order)

16. Department ArtemisObservatoire de la Cote d'Azur16 Saturation Spectroscopy  Pump saturates vibrational transition, allows probe to interact with hyperfine (rotational) transitions  When pump and probe frequency are coincident with hyperfine transition, the transparency from the hole burnt by the pump produces the inverted Lamb dip

17. Department ArtemisObservatoire de la Cote d'Azur17 A new spectroscopy technique

18. Department ArtemisObservatoire de la Cote d'Azur18

19. Department ArtemisObservatoire de la Cote d'Azur19

20. Department ArtemisObservatoire de la Cote d'Azur20 3rd Harmonic Sagnac Spectroscopy

21. Department ArtemisObservatoire de la Cote d'Azur21

22. Department ArtemisObservatoire de la Cote d'Azur22 Experimental Results

23. Department ArtemisObservatoire de la Cote d'Azur23

24. Department ArtemisObservatoire de la Cote d'Azur24 Applications for LISA 1.Laser frequency stabilisation 2.Initial phase- locking of LISA lasers 3.Could use Cs 2 at 1064 nm

25. Department ArtemisObservatoire de la Cote d'Azur25 Further Work   Optimise error signal: fringe visibility, show 1st harmonic. Then stabilise laser   Complete 2nd identical system   Independent long-term laser frequency stability measurement against LISA requirements   Compare with modulation transfer results   Simple, yet powerful (potentially shot- noise-limited) technique can be used for any spectroscopic application