Ultra-low noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock

0.26 MB·English

by  J. Millo

Ultra-low noise microwave generation with fiber-based optical frequency comb and
application to atomic fountain clock
∗
J.Millo,M.Abgrall,M.Lours,E.M.L.English,H.Jiang,J.Gu´ena,A.Clairon,S.Bize,Y.LeCoq andG.Santarelli
LNE-SYRTE, Observatoire de Paris, CNRS, UPMC,
61 Avenue de l’Observatoire, 75014 Paris, France
M.E. Tobar
School of Physics, University of Western Australia, Crawley 6009, Australia
(Dated: January 23, 2009)
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We demonstrate the use of a fiber-based femtosecond laser locked onto an ultra-stable optical
0
cavity to generate a low-noise microwave reference signal. Comparison with both a liquid Helium
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cryogenicsapphireoscillator(CSO)andaTi:Sapphire-basedopticalfrequencycombsystemexhibit
n astabilityabout3×10−15between1sand10s. Themicrowavesignalfromthefibersystemisusedto
a performRamseyspectroscopyinastate-of-the-artCesiumfountainclock. Theresultingclocksystem
J is compared to the CSO and exhibits a stability of 3.5×10−14τ−1/2. Our continuously operated

3 fiber-based system therefore demonstrates its potential to replace the CSO for atomic clocks with
2 highstability in boththeoptical andmicrowave domain,most particularly foroperational primary
frequency standards.

]
h
p
Atomic fountain frequency standards based on cold for long term and reliable operation, fiber-based optical
-
m atoms are the most widely used high accuracy atomic frequency combs (FOFC) are more desirable. Lipphardt
o clocks [1]. About ten fountains currently participate to etal. recentlydemonstratedmicrowavegenerationatthe
t the definition of the SI second at a level of 10−15 or bet- level of instability of 1.2×10−14 at 1s [18] with such a
a
ter [2, 3, 4, 5]. Besides classical metrological and time- system.
.
s
c keepingtasks,accurateandstableatomicfountainclocks Herewepresentadifferenttechniquetogeneratealow
i canalsoperformhighprecisionfundamentalphysicstests noisemicrowavefromaFOFCanddemonstrateaninsta-
s
y [6, 7, 8]. bility in the low 10−15 range at 1s by comparisons with
h State-of-the-art microwave atomic fountain clocks [5] bothaCSOandaTSOFCgeneratedmicrowave. Thelow
p
exhibitquantumprojectionnoise(QPN)[9]limitedshort noisemicrowavesignalat11.932GHzfromtheFOFCwas
[ term stability well below 10−13 at 1s integration time. usedasareplacementoftheCSOsignalinourfrequency

1 However, the intrinsic phase noise of the microwave sig- synthesis system [19] to perform Ramsey spectroscopy
v
nalusedasinterrogationoscillatorfortheseatomicstan- in a Cesium fountain and was locked to the atomic sig-
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dardsdegradesperformancesfromthefundamentalQPN nal. The resulting clock was measured against the CSO
5
6 limit, via the Dick effect [10, 11]. Therefore, the real- and demonstrated an instability of 3.5×10−14τ−1/2 for
3 isation of extremely low noise microwave oscillators is measurementtimeτ, identicaltotheoneobtainedunder
. of prime importance for frequency standards to reach identicaloperatingconditionsusingthe CSOasfountain
1
0 high stability [9]. Other applications of low noise mi- interrogationoscillator.
9 crowavesourcesincluderadar,deepspacenavigation[12] Essential to this work was our designing and imple-
0 and ultra-highresolution very-long-baselineinterferome- menting very low vibration sensitivity optical cavities
:
v try (VLBI) [13]. [20]. The measured frequency instability of the beat-
i
X The interrogation oscillator for the LNE-SYRTE note signal between two CW fiber lasers at 1542nm sta-
atomicfountainclocksiscurrentlyaliquidHeliumCryo- bilized on two independent ultra-stable cavities is below
r
a genic Sapphire Oscillator (CSO) at 11.932GHz [14]. A 2×10−15 at 1s [21]. A commercial Erbium doped fiber
CSO was until recently the only available technology al- femtosecondlaser [25] of repetition rate f ≃250MHz,
rep
lowing QPN limited stability of fountain clocks at a few withinbuiltf-2finterferometerformeasuringthecarrier-
10−14 at 1s. The cost of operation and maintenance as- envelop offset frequency f [22], is locked onto one of
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sociated with cryogenic cooling make it desirable to find these reference lasers. The lock technique is as follows:
an alternative technique. Optical ultra-stable reference a 30mW output port (100nm spectral bandwith) from
cavities[15,16],onthe otherhand,offerreliableandlow the oscillator is sent through a fibered Bragg grating at
maintenance high purity source although in the optical 1542 nm whose reflected light (1 nm spectral bandwith)
frequency range. Transfer of the stability of such optical is directed through a circulator to a fibered 50/50power
reference(typicallyinthelower10−15rangeat1s)tothe combinerwhereitismixedwiththereferenceultra-stable
microwavedomain by use of a Ti:Sapphire based optical reference light of optical frequency ν . The resulting
cw
frequency comb (TSOFC) has been demonstrated with beatnotesignalf =ν −N×f −f (withNalargein-
b cw rep 0
a residual instability of 6.5×10−16 at 1s [17]. However, teger)isdetectedonaphotodiodeandmixedwithf . Af-
0
2
FIG.2: Phasenoisepowerspectraldensity(PSD)at9.2GHz
of the beatnote between the microwaves generated by the
fiber-based and the Ti:Sapphire-based systems. Dashed line:
phasenoisePSDoftwoultra-stablelaserslockedonto1542nm
(i.e. 194THz) independent reference cavities (scaled to
9.2GHz).
FIG. 1: Schematic of the experimental system. CSO: liquid
Helium cooled Cryogenic Sapphire Oscillator. DDS: Direct a pigtailed C-band Telecom diode laser. This amplitude
DigitalSynthesizer. FFT:FastFourierTransformeranalyzer.
modulated optical signal is transmitted through optical
uW: microwave signal.
fibreto distantlaboratories. Tocancelthe residualnoise
of the fiber transmission line, we implemented a round-
trip active compensation scheme which is a simplified
ter filtering, the relevant sideband produces a frequency
versionoftheonedescribedin[23]. Theperformancesof
ν −N×f independentoff . Thissignaliscleanedby
cw rep 0 this optical link was measured to add an instability well
atrackingoscillatorfilter(2MHzbandwidth),dividedby below3×10−15τ−1forafewhundredmetersopticalpath.
128andmixedwithareferencefrequencysynthesizedby
aDirectDigitalSynthesizer(DDS)referredasDDS1(see
fig 1) to produce a phase error signal. This signal acts
on the pump-power controller of the femtosecond laser

through an optimized phase-lock loop filter. The servo
-14
bandwith is 120kHz, which, combined with the division 10
factor of 128, allows robust and reliable servo-locking. )
(
The high gainof the loopbetween 1Hz and1kHz allows y
the noise to be limited, in principle, to that of the ref- n
o
erence CW laser. Once locked onto the optical reference ati
ν , the repetition rate f (and all its harmonics) re- vi
cw rep e
d
produces the ultra-high stability of the optical reference n
transferred in the microwave domain. Our optical refer- lla
ence laser exhibits a long-term drift of a few 10−15s−1 A
whichis removedbyaconstantfeed-forwardlinearramp
on DDS1. -15
10 0 1 2 3
Togeneratemicrowavesignals,thetransmittedoutput 10 10 [s] 10 10
oftheBragggrating(9mW),containingallthespectrum
(not useful for generating the beatnote signal f ) is sent
b FIG. 3: Circles : Fractionnal frequency instability vs inte-
to a fast InGaAs pigtailed photodiode (Discovery model gration time (characterized bythe Allan standard deviation)
DSC40S). The output signal of the detector contains all of the microwave signal generated by the fiber-based system
the harmonics of the repetition rate, up to 20GHz. In against the CSO at 11.932 GHz. Squares: Fiber-based fem-
order to characterize and use the microwave from this tosecondsystemlockedontotheFO2atomicsignal,compared
FOFCindistantlaboratories,the harmonicofinterestis against theCSO(quadraticdrift removed). Thelatter insta-
bility scales as 3.5×10−14τ−1/2 (dashed line).
then filtered, amplified and used to amplitude modulate
3
Inafirstexperiment,theharmonicoftheFOFC’srep- in figure 2 is below 5×10−15τ−1/2 for the FO2 fountain
etition rate near 9.2GHz was sent to a nearby labora- current operational parameters.
tory about 30 meters away. There, it was comparedto a
9.2GHzmicrowavesignalgeneratedbyaTSOFC(repeti- Continuously operated fiber femtosecond optical fre-
tion rate ≃770MHz) lockedonto a separate ultra-stable quencycombstabilizedontoanultra-stablelaserwillre-
laser operating at 1062.5nm [20]. The TSOFC uses a placeCSOasaflywheelinthe nearfuture, removingthe
similar locking technique as the FOFC, although with a use of cryogenics and providing an ultra stable reference
higher bandwidth of about 400kHz. Figure 2 shows the in both optical and microwave domains. Furthermore,
resultofaphasenoisemeasurementoftheFOFC-TSOFC cross comparisons between fiber-based frequency comb,
beatnote signal. The Allan deviation was also measured Ti:Sapphire-based frequency comb, and CSO microwave
with a bandwidth of 400Hz (but not represented on fig- generation will allow full characterization and optimiza-
ure 3 for clarity). tion of the three systems and will pave the way to ex-
In a second experiment, the FOFC’s repetition rate treme low-noise microwave systems for applications in
harmonicnear11.932GHz[26]wassenttotheCSO/FO2 radar, deep space navigation and VLBI.
fountain laboratory 300 meters away, and there com-
Note: Upon completion of this letter, we have become
pared to the 11.932GHz CSO signal. In this case, the
aware of comparable results from S. Weyers et al. with
Allandeviationofthebeatnotesignalwasmeasuredwith
a fountain instability of 7.4×10−14 at 1s [24].
10Hz bandwidth (see figure 3). Both comparisons give
a phase noise power spectral density of approximatively
10−9/f2 [rad2/Hz](at9.2GHz)forFourierfrequenciesf
in the 0.1Hz-10Hz range and an Allan deviation about
3×10−15 at 1s integration time (see figures and 2 3). ∗
Electronic address: [email protected]
These performances, among the very best for microwave [1] R. Wynandsand S. Weyers Metrologia 42, 64 (2005).
sources, along with the reliability and robustness of the [2] F. Leviet al., Metrologia 43, 545 (2006).
fiber-based system qualifies it as an excellent microwave [3] K. Szymaniec et al.,Metrologia 42, 49 (2005).
sourceforlong-termoperationofatomicfountainclocks. [4] T.P. Heavner et al., Metrologia 42, 411 (2005).
[5] S. Bize et al., J. Phys. B: At. Mol. Opt. Phys. 38, S449
In a third experiment, the low phase noise signal at
(2005).
11.932GHz generated by our FOFC was used to re- [6] H. Marion et al.,Phys. Rev. Lett. 90, 150801 (2003).
place the CSO microwave as interrogation oscillator for [7] T.M. Fortier et al.,Phys. Rev. Lett. 98, 070801 (2007).
the FO2 atomic fountain, as shown in figure 1. From [8] N. Ashbyet al., Phys. Rev. Lett. 98, 070802 (2007). See
11.932GHz,ourfrequencysynthesis[19]generatesatun- also Wolf et al. Phys. Rev. Lett. 96, 060801 (2006).
able microwave signal shifted to 11.980GHz via a com- [9] G. Santarelli et al., Phys. Rev. Lett. 82, 4619 (1999).
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Interval Meeting, pp.133147 (1988).
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[11] Santarelli et al. IEEE Trans. UFFC 45, 887 (1998).
crowave signal for Ramsey spectroscopy of the cold Ce-
[12] J.D. Prestage et al.,Proc. of theSPIE 667306 (2007)
sium atoms. The sequential operation of the fountain
[13] S.S. Doeleman et al.,to appear in Proc. of the 7th Sym-
producesfrequency correctionsevery1.5s,whichareap- posiumonFrequencyStandardMetrology(2008).Seealso
plied to DDS2. The 11.980GHz signal is thereby locked S.S. Doelman et al. Nature 455, 78 (2008).
with a bandwidth of ≃0.2Hz onto the atomic frequency [14] J.G.Hartnettet al.,App. Phys. Lett. 89,203513 (2006).
reference. Theresultingprimarystandardreferencedsig- SeealsoC.R.Lockeetal.,Rev. Sci.Instrum.79,051301
(2008).
nal at 11.980GHz was compared to the CSO signal at
[15] A.D. Ludlow et al.,Opt. Lett. 32, 641 (2007).
11.932GHz. The 48MHz difference was bridged by a [16] S.A. Webster et al., Phys. Rev. A 77, 033847 (2008).
lownoisesynthesizer. Thecomparisonyieldsafountain’s [17] A. Bartels et al., Opt. Lett. 30, 667 (2005).
instability of 3.5×10−14τ−1/2 for integrationtime τ be- [18] B. Lipphardt et al.,arXiv:0809.2150v1 (2008).
tween 10s and 100s (see figure 3, 10Hz measurement [19] D. Chambon et al.,Rev Sci. Inst. 76, 094704 (2005).
bandwidth). For integration time longer than 100s, the [20] J. Millo et al., Proc. of the 2008 IEEE Int. Freq. Cont.
instability is limited by the flicker floor of 1-2×10−15 of Symp., pp 110-114 (2008).
[21] H. Jiang et al.,JOSA B 25, 2029 (2008)
the CSO. The short-term instability is identical to the
[22] B.R.Washburnetal.,Opt. Lett.29,Vol3,p250(2004).
one obtained when using the CSO as local oscillator for [23] O. Lopez et al., Eur. Phys. J. D 48, 3541 (2008).
the FO2 fountain under the same operating conditions.
[24] S. Weyerset al., arXiv:0901.2788v1.
ThefountaininstabilityislimitedbyQPNwiththenum- [25] Menlo SystemsGmbH, M-Comb + P250 + XPS1500
berofatomsof≃1×106 pershot(1.5scycletime). The [26] A motorized translation stage allows coarse adjustment
Dickeffectcalculated[11]basedonthephasenoiseshown of therepetition rate to the desired value.