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Towards an accurate frequency standard at λ778 nm using a laser diode stabilized on a hyperfine component of the Doppler-free two-photon transitions in rubidium
15 May 1994
OPTICS COMMUNICATIONS EI£EVIER
Optics Communications 108 (1994) 91-96
Towards an accurate frequency standard at 2 = 778 nm using a laser diode stabilized on a hyperfine component of the Doppler-free two-photon transitions in rubidium Y. Millerioux Institut National de M~trologie (INM), BNM, CNAM. CNRS ERS 068, 292 rue Saint Martin, 75141 Paris cedex 03, France
D. Touahri, L. Hilico 1, A. Clairon Laboratoire Primaire du Temps et des Frdquences (LPTF), BNM. Observatoire de Paris, 61 av. de I'Observatoire, 75014 Paris, France
R. Felder Bureau International des Poids et Mesures (BIPM), Pavilion de Breteuil. 92312 Skvres cedex. France
F. Biraben, B. de Beauvoir Laboratoire de Spectroscopie Hertzienne de FENS, CNRS URA 18, Universit~ Pierre et Marie Curie, 4 place Jussieu, Tour 12 EOI, 75252 Paris cedex 05, France
Received 3 December 1993
Ab~a, act \
Two systems are built in which a commercially available GaAIAs laser diode is stabilized on a hyperfine component of the 5S1/2-5D(3/2,5/2 ) two-photon transitions in rubidium at 2= 778 nm (~--385 THz). Some preliminary metrological results are
presented. The frequency repeatability has been found to be of 200 Hz (5.2 parts in of 3 × 10-13/v/~ up to 2000 s is currently obtained.
1. Introduction In June 1992, the absolute frequency o f a H e N e laser stabilized on i o d i n e at 2 - - 6 3 3 n m a n d belonging to the Institut National de M6trologie ( I N M ) was m e a s u r e d at the L a b o r a t o i r e P r i m a i r e du T e m p s et des Fr~quences ( L P T F ) with a relative uncertainty o f 7 parts in 10 ~2 [1]. In M a y 1993, this laser was i Present address: Universit6 d'Evry val d'Es.~nne, 8 bd. des Coquibus, 91025 Evry cedex, France.
1013) and a short-term frequency stability
used as a reference, both with a H e N e laser stabilized on m e t h a n e at 2 = 3 . 3 9 Ixm belonging to the Bureau International des Poids et Mesures ( B I P M ) , at the L a b o r a t o i r e de Speetroscopie Hertzienne de l'Ecole N o r m a l e Sup~rieure ( E N S ) for the first d e t e r m i n a tion o f one o f the optical frequencies o f the hydrogen a t o m by a purely frequency m e t h o d without any intefferometry: the frequency o f the 2 S - 8 S / 8 D twop h o t o n transition in hydrogen ( 2 = 778 nm, J,= 385 T H z ) was measured with a relative uncertainty o f 1.6 parts in 10 ~~, leading to a new value o f the Rydberg
0030-4018/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10030-4018 ( 94 ) 00094-B
92
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
constant, so far the most precise one, since known with a relative uncertainty of 2.2 parts in 101 ~ [ 2 ]. In the LPTF experiment, the uncertainty of the fiequency determination was mainly limited by the laser under test itself, while in the ENS experiment the performance of the latter contributed with a non negligible part to the global uncertainty on the determination of the Rydberg constant. Using the potential accuracy of modern synthesized frequency chains a n d / o r measuring hydrogen transitions more precisely calls for the development of a frequency standard in the near infrared with increased performance. Stabilizing laser diodes on certain saturated absorption peak in rubidium has already been carried out in some laboratories [ 3 ]. That led to a frequency source which should have performance similar to that of the HeNe laser stabilized on iodine at 2 = 6 3 3 nm, say, a short-term frequency stability of I part in 1012and a frequency reproducibility of a few parts in 1011 [4]. In May 1993, at the ENS, taking benefit of the experimental set-up for the determination of the Rydberg constant, the spectroscopy and the absolute frequency determination of the hyperfine components of the 5SI/2-5D3/2and 5S1/25D5/2 Doppler-free two-photon transitions in rubidium were carried out by using home-made titaniumsapphire lasers operating around 1 = 778 n m [ 5 ]. In this experiment, the stability and the reproducibility of the latter ones frequency locked on the Rb lines were very good, showing the promising metrological features of the two-photon transitions in rubidium. The frequency of such transitions lies in the range of powerful commercially available GaA1As laser diodes and these transitions have been recently observed with such laser diodes [ 6 ]. A first application of this new optical frequency reference was performed in Florence (Italy) at the European Laboratory for Nonlinear Spectroscopy (LENS). A low power laser diode (3 m W ) was frequency locked on the rubidium line [7 ] and used as a reference, for a frequency-doubled titanium-sapphire laser, needed for the measurement of the 2 3Si-3 3Po transition in helium [ 8 ]. In this paper we present a preliminary study of the metrological features of laser diodes frequency locked on the rubidium two-photon transitions. Two complete systems using such sources have been developed at the LPTF and we report the first results con-
ccrning the frequency stabilityand the reproducibility of this possible candidate as a frequency standard.
2. Experimental set-up
The principle of such a system is described in Fig. I. W e use a laser diode Spectra-Physics model 5412HI [9]. It is set in a classicalextended cavity (ECL) whose length ( I0 cm, i.e.a free spectral range of 1.5 O H z ) is defined by the rear facet of the diode and a 1200 grooves-per-mm diffractiongrating [ I0, l I ]. A collimating lens is carefuUy adjusted in order to optimize the optical feedback. In these conditions the threshold current decreases from 20 to 14 mA. The whole optical device is put in a metal box and kept at a constant temperature of 30°C. For acoustic isolation the assembly is protected by a second presswood box covered with leaded foamrubbcr. One gets a radiation of lessthan 50 kHz of spectralwidth deduced from the beat frequency between two similar devices. After passing through a pair of anamorphic prisms, the beam crosses three Faraday isolatorsin order to secure an optical isolationof more than 120 dB. Such an isolation is essential because of the sensitivityof the laser diode to the spurious feedback due to the full power returning beam. The linearly-polarized beam (I=30 m W , ~w2~ l m m 2) is then sent into a sealed cell (length= 80 m m ) containing natural rubidium (consistingof 73% SSRb and 27% 87Rb) with Brewstcr-window ends. This beam is retroreflcctedby ECL
Anamorphic prisms
tic
ag
to lock-inamplifier
Fig. 1. Schematicset-up of a laser diode s t a b ~ on a hyperfine component of a two-photon transition in Rb. (ECL: extended cavity laser, PM" photomultiplier).
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
a plane mirror. The cold finger of the cell is mainrained at about 90°C, corresponding to a pressure of 8 × 10-5 Torr. The Doppler-free two-photon transition is detected by monitoring the fluorescence at 2 = 4 2 0 nm due to the radiative cascade 5 D - 6 P - 5 S with a photomultiplier whose load resistor is 22 k~. Both the Rb cell and the photomultiplier are isolated from the earth magnetic field by putting them inside a box made of mumetal. The whole hyperfine structure of the 5S~/25D~3/2,5/2) two-photon transitions is attainable by adjusting the cavity length (by changing the voltage applied to the piezo-translator supporting the grating), by modifying the temperature a n d / o r the current of the diode laser or by slightly rotating the PZT grating assembly in the horizontal plane. An external lambdameter helps us to roughly adjust the ECL at the right wavelength. In order to stabilize the frequency of the laser on a hyperfine component of the Rb spectrum, the current of the laser diode is modulated at a frequency of 70 kHz. An error signal is obtained by demodulating the electrical signal coming from the photomultiplier with a lock-in amplifier operating at the modulation frequency. The servo system comprises two loops. The fast one reacts on the laser diode current while the slow one controls the PZT supporting the grating. Fig. 2 shows the measurement set-up. We send about 1 mW of each laser diode output beam on an avalanche photodiode through a beam splitter. In these conditions, a signal to noise ratio of more than 60 dB in a 100 kHz bandwidth is easily obtained, leading therefore to a safe frequency counting. A 80 MHz acousto-optic modulator placed on one path allows us to measure the beat frequency when we stabilise the two laser diodes on the same hyperfine peak. A microcomputer is used for the storage and the treatment of data. To change some operating conditions on the device we want to study we can introduce the following optical systems: (i) An attenuator made of a half-wave plate and a polarising cube, placed on the incoming beam into the cell. (ii) A cat's eye made of a plane mirror and a f = 70 m m convergent lens. Such a system allows us to modify the geometry and the position of the returning
BS
93 ~/2
pol.
d
FP cavity
) avalanche photodlode
\/ I
il
,rap
Fig. 2. Experimentalset-up for frequencymeasurements. (AOM: 80 MHz acousto-optic modulator. A-B: electronic device for servo-locking LD2 on LDI, BS: beam splitter).
beam by modifying the distance d between the mirror and the lens. (iii) A cavity (a plane mirror and a spherical one (radius of curvature 2 m ) 30 cm apart, leading to a waist of 0.42 m m ) around each Rb cell, frequency locked on the laser.
3. R e s u l t s
In a first experiment we have recorded the spectrum of the hyperfine structure of the 5S~/2-5D5/2 transitions. The first laser diode (LD 1 ) is frequency locked on the SSRb (Fs=3, F e = 5 ) hyperfine component. The second laser diode (LD2) is frequency locked on LD 1 with a controllable frequency shift. That allows one to calibrate and to scan the hyperfine spectrum. A typical record is given in Fig. 3. The 5D5/2hyperfine structure is clearly resolved, with for the most intense component (SSRb Fs= 3, Fe= 5), a very good signal-to-noise ratio. We observe a 500 kHz experimental linewidth (fwhm) compatible with Ref. [51. In a second step we have studied the frequency stability by locking both laser diode systems on the most
94
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
4,722 MHz 5 MHz ~=
~ I00 mV
=
500
V
8°
V
Fig. 3. (a) Recordof the spectrumof the 'SRb 5Si/2-5D~/a (F.= 3, F.=5 to 1) two-photon transition. (b) Expanded scale for the two most intense lines showingthe experimentallinewidths. (2,~) / v lOaZ ! T = 4000 X lOs
1013
° ~ o e e e• •
•
"'. tO-,4
.
10°
.
.
.
10 L
.
i
.
.
.
t0 z
.
.
.
, .
.
|0 3
*. ~ .
L z (s) 10"
Fig. 4. Typical square root of the relative Allan variance versus the integration time. intense hyperfme component (SSRb, F , = 3, Fe= 5, 5S~/2-5Ds/2 transition). In a set of measurements, the beat frequency between our two devices is recorded during 40000 s (4000 samples with 10 s integration time). Fig. 4 gives a typical result. The square root of the relative Allan variance (~r(2, z ) / v ) is of 3 × 10-m3/x/~ up to 2000 s. The position of the plateau is very dependent on the quality and the stability of the optical beams in the Rb cells. But in any case, the stability for integration times lower than 300 s exceeds by an order of magnitude that of the HeNe laser stabilized on iodine at A= 633 nm, a n d / o r that of a laser diode stabilized on the D2 rubidium line at ,~=780 nm by a classical saturated absorption technique [ 12 ]. We have performed some controls of the repeatability and the frequency reproducibility. We can see
on Fig. 3 that the frequency interval between adjacent components of the SSRb 5S1/2-5D5/2 ( F , = 3 , Fe= 5 to 1 ) is less than 5 MHz and that there exists an overlap which should result in an asymmetry of the hyperfme components. So far we did not observe such an effect by changing the modulation amplitude from one to three times the optimal one [ 13 ]. We have also carefully investigated the light shift. For this purpose the incoming power in the rubidium cell of LD 1 was varied by using the attenuator. We also modified the geometry of the returning beam retroreflected inside the Rb cell by means of the cat's eye. That allowed one to vary the size and the position of the returning-beam waist while keeping the incoming beam unchanged. During these measurements the LD2 system was maintained in steady operating conditions. The results are reported in Fig. 5. Curve (a) corresponds to the case where the cat's eye is equivalent to a plane mirror: the "w"-parameter of the reflected beam in the cell is approximately the same than the one of the incoming beam ( = 0.5 nun). Under these conditions the light shift is linearly dependent on the incoming light power and for 20 mW it is about 3 kHz. Curve (b) corresponds to an intermediate case where the "w"-parameter of the reflected laser beam in the cell is reduced. In the case of curve (c) the reflected beam is focused inside the cell. As a result the light shift is increased by more than six times that the one obtained in case (a). Zeropower extrapolations of curves (a), (b) and (c) are l
~
-
I
---
'
.....
I I
1 0
10
20
30
Fis. 5. Experimental light ~ for different optical arrangemeats. (a) The cat's eye is equivalent to a plane mirror, (b) The s/zeof the reflectedbeam is lowerthan that of the incomin~beam, (c) The reflectedbeam is focusedinto the ceil
Y. MlTlerioux et al. / Optics Communications 108 (1994) 91-96
in very good agreement, within 1 part in 1012. Finally we have studied the effect of the alignment of the reflected beam. We observed a frequency shift of about 5 kHz when the two-photon signal was decreased by 50% simply by changing the direction of the reflected beam. This shift is probably due to a residual first Doppler effect. In spite of this effect the frequency repeatability is 1 kHz when the systems are carefully adjusted. In order to reduce this effect, we have placed the Rb cells into two independent buildup cavities. Since the two counterpropagating beams are perfectly matched, the first Doppler effect is completely cancelled. With this improved set-up, the short-term stability is the same with and without cavity, but we observe a day to day frequency repeatability of 200 Hz, i.e. 3.7 × 10- ~3for each device. Under these conditions, taking into account the light shift, we have precisely measured a 1.7 kHz frequency difference (4.4 × 10- ~2) between the two systems. This frequency difference is probably due to a different pressure shift in the two Rb cells which were filled at a different time or to possible electronic effects.
4. Conclusion
95
diode on the very intense F s = 2 , Fe=4, 5S1/2-5D5/2 hyperfine component. This line is indeed well isolated, 14 MHz away from its neighbour, the F s = 2 , Fe = 3 component. In these conditions the frequency reproducibility would be about 1 part in 1012, allowing then to use the full potential of the LPTF frequency-multiplication chain to link these two-photon frequencies to the Cs clock. With such performance the applications of this potentially frequency standard would be very important for optical frequency metrology and the study of the spectrum of some atoms. With such a frequency reference and the HeNe laser stabilized on methane at 2=3.39 ttm (frequency J, (CH4, 3.39 ~tm)), it is possible to measure in a simple way two optical frequencies in the visible range, that of the popular HeNe laser stabilized on iodine at 2=633 nm (J, (I2, 633 n m ) ) [ 14 ] and that of the frequency-doubled YAG laser at 2 = 5 3 2 nm stabilized on iodine 0' (I2, 532 rim) ) which is developed by several groups [ 15,16 ]. Indeed, we have the frequency relations:
P(I2,633 n m ) ~ v(Rb, 778 nm) + I,(CH4, 3.39 p m ) ,
P(12,532 nm) = J,(Rb, 778 nm) We have built two similar devices in which a laser diode emitting at 2 = 778 nm is stabilized on a hyperfine component of the 5S~/2-5Do/2.5/2 ) Doppler-free two-photon transitions in natural rubidium. So far we have observed very good stability and repeatability in the emitted frequencies. Further measurements have to be carried out in order to characterise more precisely their frequency reproducibility. Henceforth this system seems to appear as a possible secondary frequency standard in the near infrared range with a frequency reproducibility better than the 5 kHz uncertainty given in Ref. [ 5 ]. Some improvements are straightforward. We can lock the laser diode on an external cavity in order to reduce the jitter at a 1 kHz level and then to improve the very short-term stability of the whole device. We can stabilize the temperature of the rubidium cell as well as the intracavity power. That would allow to control the Rb pressure and the light shift and then to obtain a better longterm frequency stability. It is also possible to use a cell containing pure S~Rb and to stabilize the laser
+2v(CH4, 3.39 p r o ) . An other application is to frequency lock a frequency-doubled laser diode at 2 = 1.556 ttm on a twophoton Rb line. That would provide an infrared frequency reference useful for the telecommunication with optical fiber. On the other hand, this frequency reference is directly linked to the natural atomic frequency scale which is given by the hydrogen atom [ 17,18] because of the frequency coincidence with the 2S-8S/8D hydrogen two-photon transitions. In this way it will be possible to deduce the frequency of the 1S2S and 2S-4S hydrogen two-photon transitions using the "hydrogen synthesized atom" proposed by H~insch [ 19]. Finally, with the optical frequency comb generator [20] or with the optical divider proposed by Hlinsch [ 21 ], it will be possible to measure the other 2S-nS/D two-photon transitions, n--6-12, in the range 820-750 nm.
96
Y. Millerioux et al. / Optics Communications 108 (199,1) 91-96
Aeknewledgments These two devices have been built at the LPTF with equipments coming from the different laboratories involved in this experiment. Nevertheless, the authors are pleased to acknowledge the whole team of the LPTF for their dynamism and the pleasant atmosphere created in the laboratory. This work is supported by the Bureau National de M~trologie (BNM) and by the Direction des Recherches et Etudes Techniques (DRET).
References [ l ] O. Acef, J.J. Zondy, M. Abed, D.G. Rovera, A.H. G6rard, A. Claimn, Ph. Laurent, Y. Millerioux and P. Jtmcar, Optics Comm. 97 (1992) 29. [2] F. Nez, M.D. Plimmer, S. Bourzeix, L. Julien, F. Biraben, R. Felder, Y. Millerioux and P. de Natale, Europhys. Left. 24 (1993) 635. [ 3 ] M. Tetu, N. Cyr, B. Villeneuve, S. Th6riault, M. Breton and P. Tremblay, IEEE Trans. Iustrum. Meas. 40 ( 1991 ) 191. [4 ] Comit~ Consultatif pour la D~finition du M~tre, 8e session BIPM (1992), to be published. [ 5 ] F. Nez, F. Biraben, R. Felder and Y. Millerioux, Optics Comm. 102 (1993) 432. [ 6 ] R.E. Ryan, LA. Westhng and HJ. Metcalf, J. Opt. Soc. Am, Bl0 (1993) 1643. [ 7 ] F. Biraben, M. Inguscio, F. Marin, F. Pavone, Doppler free detection of the hyperfme components of the 5S1/2-5D5/2 two-photon transitions in STRbusing a narrow band AIGaAs
laser. Laser Physics, special issue in honour of V.P. Chebotayev, in press. [8]F. Pavone, F. Marin, P. de Natale, M. lnguscio and F. Biraben, First pure frequency measurement of an optical transition in helium: Laub shift of the 23St metastable level, Phys. Rev. Lett., submitted. [9]Mention of a commercial product is for technical communication only. It does not imply endorsement, nor does it su~c~t that other products are necessarily less suitable for the application. [ 10] M. de Labachellerie and P. Ccrez, Optics Comm. 55 (1985) 174. [ 11 ] G. Santarelli, A. Clairon, S.N. Lea and G.M. Tino, Optics Comm. 104 (1994) 339. [ 12 ] G.P. Barwood, P. Gill and W.R.C. Rowley, J. Modern Phys. 37QE9 (1990) 749. [ 13 ] F. Bayer-Helms and J. Helmcke, PTB-Bericht, PTB-Me-I 7 (1977) 85. [ 14] F. Nez, S. Bourzeix, L. lulien, F. Biraben, R. Felder and Y. Millerioux, Proc. ELICOLS'93, AIP Conference Proceedings, Vol. 290 (1994) p. 9. [ 15 ] J.L. Hall, private communication. [16] A. Arie, S. Schiller, E.IC Gustafson and R.L. Byer, Optics Lett. 17 ( 1992 ) 1204. [17] L. Julien, F. Biraben and B. Cagnac, Bulletin du BNM 66 (1986) 31. [18] P. Zhao, W. Lichten, H.P. Layer and J.C. Bergquist, Phys. Rev. Lett. 58 (1987) 1293. [ 19 ] T.W. Hansch, in: The hydrogen atom, eds. G.F. Bassanl, M. Inguscio and T.W. Hiiusch (Springer, Berlin, 1989) p. 93. [20] M. Kourogi, K. Nakasawa and M. Ohtsu, presented at the International Quantum Electronics Conference, Vienna, Austria, 1992, paper TuM5. [ 21 ] H.R. Tene, D. Meschede and T.W. Hansch, Optics Lett. 15 (1990) 532.
OPTICS COMMUNICATIONS EI£EVIER
Optics Communications 108 (1994) 91-96
Towards an accurate frequency standard at 2 = 778 nm using a laser diode stabilized on a hyperfine component of the Doppler-free two-photon transitions in rubidium Y. Millerioux Institut National de M~trologie (INM), BNM, CNAM. CNRS ERS 068, 292 rue Saint Martin, 75141 Paris cedex 03, France
D. Touahri, L. Hilico 1, A. Clairon Laboratoire Primaire du Temps et des Frdquences (LPTF), BNM. Observatoire de Paris, 61 av. de I'Observatoire, 75014 Paris, France
R. Felder Bureau International des Poids et Mesures (BIPM), Pavilion de Breteuil. 92312 Skvres cedex. France
F. Biraben, B. de Beauvoir Laboratoire de Spectroscopie Hertzienne de FENS, CNRS URA 18, Universit~ Pierre et Marie Curie, 4 place Jussieu, Tour 12 EOI, 75252 Paris cedex 05, France
Received 3 December 1993
Ab~a, act \
Two systems are built in which a commercially available GaAIAs laser diode is stabilized on a hyperfine component of the 5S1/2-5D(3/2,5/2 ) two-photon transitions in rubidium at 2= 778 nm (~--385 THz). Some preliminary metrological results are
presented. The frequency repeatability has been found to be of 200 Hz (5.2 parts in of 3 × 10-13/v/~ up to 2000 s is currently obtained.
1. Introduction In June 1992, the absolute frequency o f a H e N e laser stabilized on i o d i n e at 2 - - 6 3 3 n m a n d belonging to the Institut National de M6trologie ( I N M ) was m e a s u r e d at the L a b o r a t o i r e P r i m a i r e du T e m p s et des Fr~quences ( L P T F ) with a relative uncertainty o f 7 parts in 10 ~2 [1]. In M a y 1993, this laser was i Present address: Universit6 d'Evry val d'Es.~nne, 8 bd. des Coquibus, 91025 Evry cedex, France.
1013) and a short-term frequency stability
used as a reference, both with a H e N e laser stabilized on m e t h a n e at 2 = 3 . 3 9 Ixm belonging to the Bureau International des Poids et Mesures ( B I P M ) , at the L a b o r a t o i r e de Speetroscopie Hertzienne de l'Ecole N o r m a l e Sup~rieure ( E N S ) for the first d e t e r m i n a tion o f one o f the optical frequencies o f the hydrogen a t o m by a purely frequency m e t h o d without any intefferometry: the frequency o f the 2 S - 8 S / 8 D twop h o t o n transition in hydrogen ( 2 = 778 nm, J,= 385 T H z ) was measured with a relative uncertainty o f 1.6 parts in 10 ~~, leading to a new value o f the Rydberg
0030-4018/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10030-4018 ( 94 ) 00094-B
92
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
constant, so far the most precise one, since known with a relative uncertainty of 2.2 parts in 101 ~ [ 2 ]. In the LPTF experiment, the uncertainty of the fiequency determination was mainly limited by the laser under test itself, while in the ENS experiment the performance of the latter contributed with a non negligible part to the global uncertainty on the determination of the Rydberg constant. Using the potential accuracy of modern synthesized frequency chains a n d / o r measuring hydrogen transitions more precisely calls for the development of a frequency standard in the near infrared with increased performance. Stabilizing laser diodes on certain saturated absorption peak in rubidium has already been carried out in some laboratories [ 3 ]. That led to a frequency source which should have performance similar to that of the HeNe laser stabilized on iodine at 2 = 6 3 3 nm, say, a short-term frequency stability of I part in 1012and a frequency reproducibility of a few parts in 1011 [4]. In May 1993, at the ENS, taking benefit of the experimental set-up for the determination of the Rydberg constant, the spectroscopy and the absolute frequency determination of the hyperfine components of the 5SI/2-5D3/2and 5S1/25D5/2 Doppler-free two-photon transitions in rubidium were carried out by using home-made titaniumsapphire lasers operating around 1 = 778 n m [ 5 ]. In this experiment, the stability and the reproducibility of the latter ones frequency locked on the Rb lines were very good, showing the promising metrological features of the two-photon transitions in rubidium. The frequency of such transitions lies in the range of powerful commercially available GaA1As laser diodes and these transitions have been recently observed with such laser diodes [ 6 ]. A first application of this new optical frequency reference was performed in Florence (Italy) at the European Laboratory for Nonlinear Spectroscopy (LENS). A low power laser diode (3 m W ) was frequency locked on the rubidium line [7 ] and used as a reference, for a frequency-doubled titanium-sapphire laser, needed for the measurement of the 2 3Si-3 3Po transition in helium [ 8 ]. In this paper we present a preliminary study of the metrological features of laser diodes frequency locked on the rubidium two-photon transitions. Two complete systems using such sources have been developed at the LPTF and we report the first results con-
ccrning the frequency stabilityand the reproducibility of this possible candidate as a frequency standard.
2. Experimental set-up
The principle of such a system is described in Fig. I. W e use a laser diode Spectra-Physics model 5412HI [9]. It is set in a classicalextended cavity (ECL) whose length ( I0 cm, i.e.a free spectral range of 1.5 O H z ) is defined by the rear facet of the diode and a 1200 grooves-per-mm diffractiongrating [ I0, l I ]. A collimating lens is carefuUy adjusted in order to optimize the optical feedback. In these conditions the threshold current decreases from 20 to 14 mA. The whole optical device is put in a metal box and kept at a constant temperature of 30°C. For acoustic isolation the assembly is protected by a second presswood box covered with leaded foamrubbcr. One gets a radiation of lessthan 50 kHz of spectralwidth deduced from the beat frequency between two similar devices. After passing through a pair of anamorphic prisms, the beam crosses three Faraday isolatorsin order to secure an optical isolationof more than 120 dB. Such an isolation is essential because of the sensitivityof the laser diode to the spurious feedback due to the full power returning beam. The linearly-polarized beam (I=30 m W , ~w2~ l m m 2) is then sent into a sealed cell (length= 80 m m ) containing natural rubidium (consistingof 73% SSRb and 27% 87Rb) with Brewstcr-window ends. This beam is retroreflcctedby ECL
Anamorphic prisms
tic
ag
to lock-inamplifier
Fig. 1. Schematicset-up of a laser diode s t a b ~ on a hyperfine component of a two-photon transition in Rb. (ECL: extended cavity laser, PM" photomultiplier).
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
a plane mirror. The cold finger of the cell is mainrained at about 90°C, corresponding to a pressure of 8 × 10-5 Torr. The Doppler-free two-photon transition is detected by monitoring the fluorescence at 2 = 4 2 0 nm due to the radiative cascade 5 D - 6 P - 5 S with a photomultiplier whose load resistor is 22 k~. Both the Rb cell and the photomultiplier are isolated from the earth magnetic field by putting them inside a box made of mumetal. The whole hyperfine structure of the 5S~/25D~3/2,5/2) two-photon transitions is attainable by adjusting the cavity length (by changing the voltage applied to the piezo-translator supporting the grating), by modifying the temperature a n d / o r the current of the diode laser or by slightly rotating the PZT grating assembly in the horizontal plane. An external lambdameter helps us to roughly adjust the ECL at the right wavelength. In order to stabilize the frequency of the laser on a hyperfine component of the Rb spectrum, the current of the laser diode is modulated at a frequency of 70 kHz. An error signal is obtained by demodulating the electrical signal coming from the photomultiplier with a lock-in amplifier operating at the modulation frequency. The servo system comprises two loops. The fast one reacts on the laser diode current while the slow one controls the PZT supporting the grating. Fig. 2 shows the measurement set-up. We send about 1 mW of each laser diode output beam on an avalanche photodiode through a beam splitter. In these conditions, a signal to noise ratio of more than 60 dB in a 100 kHz bandwidth is easily obtained, leading therefore to a safe frequency counting. A 80 MHz acousto-optic modulator placed on one path allows us to measure the beat frequency when we stabilise the two laser diodes on the same hyperfine peak. A microcomputer is used for the storage and the treatment of data. To change some operating conditions on the device we want to study we can introduce the following optical systems: (i) An attenuator made of a half-wave plate and a polarising cube, placed on the incoming beam into the cell. (ii) A cat's eye made of a plane mirror and a f = 70 m m convergent lens. Such a system allows us to modify the geometry and the position of the returning
BS
93 ~/2
pol.
d
FP cavity
) avalanche photodlode
\/ I
il
,rap
Fig. 2. Experimentalset-up for frequencymeasurements. (AOM: 80 MHz acousto-optic modulator. A-B: electronic device for servo-locking LD2 on LDI, BS: beam splitter).
beam by modifying the distance d between the mirror and the lens. (iii) A cavity (a plane mirror and a spherical one (radius of curvature 2 m ) 30 cm apart, leading to a waist of 0.42 m m ) around each Rb cell, frequency locked on the laser.
3. R e s u l t s
In a first experiment we have recorded the spectrum of the hyperfine structure of the 5S~/2-5D5/2 transitions. The first laser diode (LD 1 ) is frequency locked on the SSRb (Fs=3, F e = 5 ) hyperfine component. The second laser diode (LD2) is frequency locked on LD 1 with a controllable frequency shift. That allows one to calibrate and to scan the hyperfine spectrum. A typical record is given in Fig. 3. The 5D5/2hyperfine structure is clearly resolved, with for the most intense component (SSRb Fs= 3, Fe= 5), a very good signal-to-noise ratio. We observe a 500 kHz experimental linewidth (fwhm) compatible with Ref. [51. In a second step we have studied the frequency stability by locking both laser diode systems on the most
94
Y. Millerioux et al. / Optics Communications 108 (1994) 91-96
4,722 MHz 5 MHz ~=
~ I00 mV
=
500
V
8°
V
Fig. 3. (a) Recordof the spectrumof the 'SRb 5Si/2-5D~/a (F.= 3, F.=5 to 1) two-photon transition. (b) Expanded scale for the two most intense lines showingthe experimentallinewidths. (2,~) / v lOaZ ! T = 4000 X lOs
1013
° ~ o e e e• •
•
"'. tO-,4
.
10°
.
.
.
10 L
.
i
.
.
.
t0 z
.
.
.
, .
.
|0 3
*. ~ .
L z (s) 10"
Fig. 4. Typical square root of the relative Allan variance versus the integration time. intense hyperfme component (SSRb, F , = 3, Fe= 5, 5S~/2-5Ds/2 transition). In a set of measurements, the beat frequency between our two devices is recorded during 40000 s (4000 samples with 10 s integration time). Fig. 4 gives a typical result. The square root of the relative Allan variance (~r(2, z ) / v ) is of 3 × 10-m3/x/~ up to 2000 s. The position of the plateau is very dependent on the quality and the stability of the optical beams in the Rb cells. But in any case, the stability for integration times lower than 300 s exceeds by an order of magnitude that of the HeNe laser stabilized on iodine at A= 633 nm, a n d / o r that of a laser diode stabilized on the D2 rubidium line at ,~=780 nm by a classical saturated absorption technique [ 12 ]. We have performed some controls of the repeatability and the frequency reproducibility. We can see
on Fig. 3 that the frequency interval between adjacent components of the SSRb 5S1/2-5D5/2 ( F , = 3 , Fe= 5 to 1 ) is less than 5 MHz and that there exists an overlap which should result in an asymmetry of the hyperfme components. So far we did not observe such an effect by changing the modulation amplitude from one to three times the optimal one [ 13 ]. We have also carefully investigated the light shift. For this purpose the incoming power in the rubidium cell of LD 1 was varied by using the attenuator. We also modified the geometry of the returning beam retroreflected inside the Rb cell by means of the cat's eye. That allowed one to vary the size and the position of the returning-beam waist while keeping the incoming beam unchanged. During these measurements the LD2 system was maintained in steady operating conditions. The results are reported in Fig. 5. Curve (a) corresponds to the case where the cat's eye is equivalent to a plane mirror: the "w"-parameter of the reflected beam in the cell is approximately the same than the one of the incoming beam ( = 0.5 nun). Under these conditions the light shift is linearly dependent on the incoming light power and for 20 mW it is about 3 kHz. Curve (b) corresponds to an intermediate case where the "w"-parameter of the reflected laser beam in the cell is reduced. In the case of curve (c) the reflected beam is focused inside the cell. As a result the light shift is increased by more than six times that the one obtained in case (a). Zeropower extrapolations of curves (a), (b) and (c) are l
~
-
I
---
'
.....
I I
1 0
10
20
30
Fis. 5. Experimental light ~ for different optical arrangemeats. (a) The cat's eye is equivalent to a plane mirror, (b) The s/zeof the reflectedbeam is lowerthan that of the incomin~beam, (c) The reflectedbeam is focusedinto the ceil
Y. MlTlerioux et al. / Optics Communications 108 (1994) 91-96
in very good agreement, within 1 part in 1012. Finally we have studied the effect of the alignment of the reflected beam. We observed a frequency shift of about 5 kHz when the two-photon signal was decreased by 50% simply by changing the direction of the reflected beam. This shift is probably due to a residual first Doppler effect. In spite of this effect the frequency repeatability is 1 kHz when the systems are carefully adjusted. In order to reduce this effect, we have placed the Rb cells into two independent buildup cavities. Since the two counterpropagating beams are perfectly matched, the first Doppler effect is completely cancelled. With this improved set-up, the short-term stability is the same with and without cavity, but we observe a day to day frequency repeatability of 200 Hz, i.e. 3.7 × 10- ~3for each device. Under these conditions, taking into account the light shift, we have precisely measured a 1.7 kHz frequency difference (4.4 × 10- ~2) between the two systems. This frequency difference is probably due to a different pressure shift in the two Rb cells which were filled at a different time or to possible electronic effects.
4. Conclusion
95
diode on the very intense F s = 2 , Fe=4, 5S1/2-5D5/2 hyperfine component. This line is indeed well isolated, 14 MHz away from its neighbour, the F s = 2 , Fe = 3 component. In these conditions the frequency reproducibility would be about 1 part in 1012, allowing then to use the full potential of the LPTF frequency-multiplication chain to link these two-photon frequencies to the Cs clock. With such performance the applications of this potentially frequency standard would be very important for optical frequency metrology and the study of the spectrum of some atoms. With such a frequency reference and the HeNe laser stabilized on methane at 2=3.39 ttm (frequency J, (CH4, 3.39 ~tm)), it is possible to measure in a simple way two optical frequencies in the visible range, that of the popular HeNe laser stabilized on iodine at 2=633 nm (J, (I2, 633 n m ) ) [ 14 ] and that of the frequency-doubled YAG laser at 2 = 5 3 2 nm stabilized on iodine 0' (I2, 532 rim) ) which is developed by several groups [ 15,16 ]. Indeed, we have the frequency relations:
P(I2,633 n m ) ~ v(Rb, 778 nm) + I,(CH4, 3.39 p m ) ,
P(12,532 nm) = J,(Rb, 778 nm) We have built two similar devices in which a laser diode emitting at 2 = 778 nm is stabilized on a hyperfine component of the 5S~/2-5Do/2.5/2 ) Doppler-free two-photon transitions in natural rubidium. So far we have observed very good stability and repeatability in the emitted frequencies. Further measurements have to be carried out in order to characterise more precisely their frequency reproducibility. Henceforth this system seems to appear as a possible secondary frequency standard in the near infrared range with a frequency reproducibility better than the 5 kHz uncertainty given in Ref. [ 5 ]. Some improvements are straightforward. We can lock the laser diode on an external cavity in order to reduce the jitter at a 1 kHz level and then to improve the very short-term stability of the whole device. We can stabilize the temperature of the rubidium cell as well as the intracavity power. That would allow to control the Rb pressure and the light shift and then to obtain a better longterm frequency stability. It is also possible to use a cell containing pure S~Rb and to stabilize the laser
+2v(CH4, 3.39 p r o ) . An other application is to frequency lock a frequency-doubled laser diode at 2 = 1.556 ttm on a twophoton Rb line. That would provide an infrared frequency reference useful for the telecommunication with optical fiber. On the other hand, this frequency reference is directly linked to the natural atomic frequency scale which is given by the hydrogen atom [ 17,18] because of the frequency coincidence with the 2S-8S/8D hydrogen two-photon transitions. In this way it will be possible to deduce the frequency of the 1S2S and 2S-4S hydrogen two-photon transitions using the "hydrogen synthesized atom" proposed by H~insch [ 19]. Finally, with the optical frequency comb generator [20] or with the optical divider proposed by Hlinsch [ 21 ], it will be possible to measure the other 2S-nS/D two-photon transitions, n--6-12, in the range 820-750 nm.
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Y. Millerioux et al. / Optics Communications 108 (199,1) 91-96
Aeknewledgments These two devices have been built at the LPTF with equipments coming from the different laboratories involved in this experiment. Nevertheless, the authors are pleased to acknowledge the whole team of the LPTF for their dynamism and the pleasant atmosphere created in the laboratory. This work is supported by the Bureau National de M~trologie (BNM) and by the Direction des Recherches et Etudes Techniques (DRET).
References [ l ] O. Acef, J.J. Zondy, M. Abed, D.G. Rovera, A.H. G6rard, A. Claimn, Ph. Laurent, Y. Millerioux and P. Jtmcar, Optics Comm. 97 (1992) 29. [2] F. Nez, M.D. Plimmer, S. Bourzeix, L. Julien, F. Biraben, R. Felder, Y. Millerioux and P. de Natale, Europhys. Left. 24 (1993) 635. [ 3 ] M. Tetu, N. Cyr, B. Villeneuve, S. Th6riault, M. Breton and P. Tremblay, IEEE Trans. Iustrum. Meas. 40 ( 1991 ) 191. [4 ] Comit~ Consultatif pour la D~finition du M~tre, 8e session BIPM (1992), to be published. [ 5 ] F. Nez, F. Biraben, R. Felder and Y. Millerioux, Optics Comm. 102 (1993) 432. [ 6 ] R.E. Ryan, LA. Westhng and HJ. Metcalf, J. Opt. Soc. Am, Bl0 (1993) 1643. [ 7 ] F. Biraben, M. Inguscio, F. Marin, F. Pavone, Doppler free detection of the hyperfme components of the 5S1/2-5D5/2 two-photon transitions in STRbusing a narrow band AIGaAs
laser. Laser Physics, special issue in honour of V.P. Chebotayev, in press. [8]F. Pavone, F. Marin, P. de Natale, M. lnguscio and F. Biraben, First pure frequency measurement of an optical transition in helium: Laub shift of the 23St metastable level, Phys. Rev. Lett., submitted. [9]Mention of a commercial product is for technical communication only. It does not imply endorsement, nor does it su~c~t that other products are necessarily less suitable for the application. [ 10] M. de Labachellerie and P. Ccrez, Optics Comm. 55 (1985) 174. [ 11 ] G. Santarelli, A. Clairon, S.N. Lea and G.M. Tino, Optics Comm. 104 (1994) 339. [ 12 ] G.P. Barwood, P. Gill and W.R.C. Rowley, J. Modern Phys. 37QE9 (1990) 749. [ 13 ] F. Bayer-Helms and J. Helmcke, PTB-Bericht, PTB-Me-I 7 (1977) 85. [ 14] F. Nez, S. Bourzeix, L. lulien, F. Biraben, R. Felder and Y. Millerioux, Proc. ELICOLS'93, AIP Conference Proceedings, Vol. 290 (1994) p. 9. [ 15 ] J.L. Hall, private communication. [16] A. Arie, S. Schiller, E.IC Gustafson and R.L. Byer, Optics Lett. 17 ( 1992 ) 1204. [17] L. Julien, F. Biraben and B. Cagnac, Bulletin du BNM 66 (1986) 31. [18] P. Zhao, W. Lichten, H.P. Layer and J.C. Bergquist, Phys. Rev. Lett. 58 (1987) 1293. [ 19 ] T.W. Hansch, in: The hydrogen atom, eds. G.F. Bassanl, M. Inguscio and T.W. Hiiusch (Springer, Berlin, 1989) p. 93. [20] M. Kourogi, K. Nakasawa and M. Ohtsu, presented at the International Quantum Electronics Conference, Vienna, Austria, 1992, paper TuM5. [ 21 ] H.R. Tene, D. Meschede and T.W. Hansch, Optics Lett. 15 (1990) 532.