Glenn de Vine, Matthieu Vangeleyn, Alain Brillet, C. Nary Man David McClelland, Malcolm Gray

Department ArtemisDepartment Artemis Observatoire de la Cote d'AzurObservatoire de la Cote d'Azur 11

A Sagnac interferometer with A Sagnac interferometer with frequency modulation for frequency modulation for

sensitive saturated absorptionsensitive saturated absorption(and applications for LISA!)(and applications for LISA!)

Glenn de Vine, Matthieu Vangeleyn, Glenn de Vine, Matthieu Vangeleyn, Alain Brillet, C. Nary Man Alain Brillet, C. Nary Man

David McClelland, Malcolm GrayDavid McClelland, Malcolm Gray

Observatoire de la Côte d'AzurObservatoire de la Côte d'Azur

Département ARTEMISDépartement ARTEMIS

NICENICE

[email protected]@obs-nice.fr


Talk Outline:Talk Outline:

1.1. LISA - lasers and frequency noiseLISA - lasers and frequency noise

2.2. Sagnac interferometer basicsSagnac interferometer basics

3.3. Saturation spectroscopy basicsSaturation spectroscopy basics

4.4. Sagnac interferometer for noise-rejectionSagnac interferometer for noise-rejection

5.5. Details of the techniqueDetails of the technique

6.6. Theoretical modelingTheoretical modeling

7.7. Experimental resultsExperimental results

8.8. The Future…The Future…


The LISA InterferometerThe LISA InterferometerArm lengths = 5 million kmArm lengths = 5 million kmArm length difference ≈ 50,000 km (1%)Arm length difference ≈ 50,000 km (1%)Frequency noise now couples in due to Frequency noise now couples in due to unequal arm lengthunequal arm lengthEqual arm length Michelson Equal arm length Michelson

freq noise is common and freq noise is common and

not a concern not a concern white light interferometerwhite light interferometer


Frequency Noise CouplingFrequency Noise Coupling


Measurement SensitivityMeasurement Sensitivity

In order to measure a relative arm length In order to measure a relative arm length difference, difference, dxdx = 2= 2 pm/pm/HzHz, using:, using:

we require a detector (laser) frequency we require a detector (laser) frequency sensitivity (stability), sensitivity (stability), dd, of, of

6x106x10-6-6 Hz/Hz/HzHz

( ) ( ) fd

xfdx Δ=


LISA LasersLISA Lasers LISA will employ the most stable CW lasers LISA will employ the most stable CW lasers

currently available:currently available:• Nd:YAG lasers at 1064Nd:YAG lasers at 1064 nmnm• Intensity noise requirements should be met Intensity noise requirements should be met

with noise-eaterswith noise-eaters• Laser frequency noise needs to be overcome:Laser frequency noise needs to be overcome:Typical free running laser frequency noise:Typical free running laser frequency noise:

101044/f Hz//f Hz/HzHzLISA detection band is 100LISA detection band is 100 Hz to 1Hz to 1 HzHzAt 100At 100 Hz we require a stability improvement Hz we require a stability improvement

of over 13 orders of magnitudeof over 13 orders of magnitude


Frequency Stabilisation MethodsFrequency Stabilisation Methods Arm locking - stable reference, well established in Arm locking - stable reference, well established in

ground-based GWD’sground-based GWD’s

Time-delay interferometry - new technique, Time-delay interferometry - new technique, currently being testedcurrently being tested

Mechanical reference (cavity) - ULE, ZeroDur, etcMechanical reference (cavity) - ULE, ZeroDur, etc

Atomic or molecular referenceAtomic or molecular reference

No method alone will achieve the 13 orders of No method alone will achieve the 13 orders of magnitude improvement requiredmagnitude improvement required

Solution will be a combinationSolution will be a combination


Atomic vs Mechanical (Cavity)Atomic vs Mechanical (Cavity) Atomic - Atomic -

for:for:• absolute reference, best long term stabilityabsolute reference, best long term stability

against:against:• not space-rated, absorptions typically very weak at not space-rated, absorptions typically very weak at

10641064 nmnm Cavity - Cavity - for:for:

• simple, space-rated, best short term stabilitysimple, space-rated, best short term stabilityagainst:against:

• not absolute, aging, long term stability is not absolute, aging, long term stability is susceptible to thermal variationssusceptible to thermal variations


Iodine Spectroscopy for LISA Iodine Spectroscopy for LISA Laser Frequency StabilisationLaser Frequency Stabilisation

develop high performance frequency develop high performance frequency stability by locking a laser using Doppler-stability by locking a laser using Doppler-free saturated absorption spectroscopy of free saturated absorption spectroscopy of iodine at 532iodine at 532 nm for 1064nm for 1064 nm absolute nm absolute stabilitystability

achieve LISA laser frequency stability achieve LISA laser frequency stability requirement of < 1requirement of < 1 Hz/√Hz from 100Hz/√Hz from 100 Hz to 1Hz to 1

HzHz


IodineIodine Sufficient absorption from hyperfine Sufficient absorption from hyperfine

resonances at 532resonances at 532 nm (the harmonic of nm (the harmonic of 10641064 nm - weak absorptions:nm - weak absorptions: CsCs22,CO,CO22,C,C22HH22))

Commercially available lasers with doubled Commercially available lasers with doubled (532(532 nm) and fundamental (1064nm) and fundamental (1064 nm) nm) outputsoutputs

The spectroscopy (and thus, frequency The spectroscopy (and thus, frequency stability) can benefit from improved stability) can benefit from improved techniques to enhance the signal and/or techniques to enhance the signal and/or reduce the noisereduce the noise


Sagnac InterferometrySagnac Interferometry


Saturation SpectroscopySaturation Spectroscopy Energy levels of I2 :

1. electronic 2. vibrational 3. rotational



Saturation SpectroscopySaturation Spectroscopy Energy levels of I2 : 1. electronic 2. vibrational (1 GHz) 3. rotational (1 MHz)


Saturation SpectroscopySaturation Spectroscopy Energy levels of I2 : 1. electronic 2. vibrational (1 GHz) 3. rotational (1 MHz) Boltzmann thermal distribution - Doppler shifts transition frequencies relative to laser frequencyDoppler shifting is greater than hyperfine linewidthCounter-propagating pump and probe fields - both interact only with molecules of zero longitudinal velocity (to first order)


Saturation SpectroscopySaturation SpectroscopyPump saturates vibrational transition, allows probe to interact with hyperfine (rotational) transitionsWhen pump and probe frequency are coincident with hyperfine transition, the transparency from the hole burnt by the pump produces the inverted Lamb dip


A new spectroscopy techniqueA new spectroscopy technique




3rd Harmonic Sagnac Spectroscopy3rd Harmonic Sagnac Spectroscopy



Experimental ResultsExperimental Results



Applications for LISAApplications for LISA

1. Laser frequency stabilisation

2. Initial phase-locking of LISA lasers

3. Could use Cs2 at 1064 nm


Further WorkFurther 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

Documents

Glenn de Vine, Matthieu Vangeleyn, Alain Brillet, C. Nary Man David McClelland, Malcolm Gray