IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, zyxwvutsrqpon VOL. 40. NO. 2, APRIL zyxwvutsr 1991 181 Recent Activities at BIPM in the Field of Stabilized Lasers-Radiations Recommended for the Definition of the Meter Jean-Marie Chartier, Lennart Robertsson, and Susanne Fredin-Picard Abstract-The results of international comparisons of iodine stabi- lized He-Ne lasers at zyxwvutsrqponm X = 633 nm between the Bureau International des Poids et Mesures (BIPM) and eleven national laboratories are pre- sented. The measured frequency differences are all inside *3 parts in 10". New investigations concerning the He-Ne lasers at X = 612 nm using internal or external iodine cells are discussed. The frequency re- producibility is limited by, among other things, the quality of the io- dine cells. Therefore the contamination of iodine cells has been studied using laser-induced fluorescence. First results obtained on the green line of He-Ne at X = 543 nm stabilized on iodine are also presented. The frequency difference between two such systems was found to be 14 kHz with a repeatability of zyxwvutsrqpo 5 kHz. INTRODUCTION IVE radiations were recommended by the ComitC Interna- F tional des Poids et Mesures (CIPM) in 1983, for the prac- tical realization of the metre. The BIPM is at present equipped to realize four of these: 633 nm, 612 nm, 515 nm, and 3.39 pm. Concerning the international comparisons of stabilized la- sers, the main activity is still with the helium-neon lasers at X = 633 nm. New studies at X = 612 nm have been started to improve the understanding of the behaviour of such lasers using either an internal or an external iodine cell. Preliminary results are also presented of work on a He-Ne laser stabilized on an iodine transition at X = 543 nm, a possible candidate for even- tual inclusion in the list of recommended radiations. LASERS AT = 633 nm Several international comparisons of He-Ne lasers stabilized by saturated absorption of iodine at zyxwvutsrqpo h = 633 nm, have already been carried out, [1]-[3]. After the adoption of the new defi- nition of the metre, a new series of such comparisons was be- gun. For this wavelength, the BIPM has been involved in five international comparisons since 1986 which have included eleven different national laboratories. These laboratories con- cerned are: JILA (USA), May 1986 zyxwvutsr [4]; NRLM (Japan), June 1988 [5]; ASMW (GDR), CSMU (Bratislava and Prague, Czechoslovakia), CSAV (Brno, Czechoslovakia), OMH (Hun- gary), and VNIIM (USSR), August 1988 [6]; ETCA (France), December 1988 [7]; INM (France), February 1989 [7]; and SP (Sweden), June 1989 [8]. The results are presented in Fig. 1. Each comparison consists of the determination, by the beat fre- quency method, of the frequency difference between the labo- ratory laser and the BIPM laser which is treated, arbitrarily, as Manuscript received June 12, 1990; revised October 5, 1990. The authors are with the Bureau Inemational des Poids et Mesures IEEE Log Number 904 1627. (BIPM), 92312 Sevres Cedex, France. , 20 I 15 +r,,:, lor,, , zyxwvut , I -20 8.1988 Laboratories Fig. 1. International comparisons of iodine stabilized He-Ne lasers (X = 633 nrn) by beat frequency methods. zyxwv Af = (frequency of the National Lab- oratory laser)-(frequency of the BIPM laser). JILA (Joint Institute Lab- oratory for Astrophysics, Boulder, USA), NRLM (National Research Laboratory of Metrology, Tsukuba, Japan), ASMW (Amt fur Standardi- sierung, Messwesen und Warenpriifung, Berlin, German Democratic Rep.), CSMU.B and CSMU.P (Czechoslovak Institute of Metrology, Bratislava and Prague, Czechoslovakia), CSAV (Czechoslovak Academy of Sci- ences, Brno, Czechoslovakia), OMH (National Office of Measures, Bu- dapest, Hungaria), VNIIM (D.I. Mendeleev All Union Scientific Research Metrological Institute, Leningrad, USSR), ETCA (Etablissement Tech- nique Central de l'Armement, Arcueil, France), INM (Institut National de MCtrologie, Paris, France), SP (Swedish National Testing Institute, Boras, Sweden). the reference. It is worth noting that the mean value of the fre- quency differences measured between each laser and the BIPM lasers is less than 0.5 kHz, and has an estimated standard de- viation of 8 kHz for one frequency determination on one laser. Among the principal reasons for carrying out all these com- parisons are; first, to check the uniformity of the dissemination of the metre which is the most important; second, comparisons between national laboratories are necessary to reveal systematic errors such as contamination of the iodine cells. When all the parameters which disturb the frequencies of the lasers, such as modulation amplitude, iodine pressure, laser power, and dc offsets of the electronic servo are carefully ad- justed and controlled, a relative frequency spread of +3 parts in 10" can be achieved. It has been confirmed that the fre- quency dependence on modulation amplitude of some lasers is strongly related to the laser cavity configuration. At the BIPM, the frequency of the radiation of such lasers has been kept by a group of five lasers for more than fifteen years with a long term frequency stability of + 3 parts in 10". The stability has also been confirmed for two of these lasers at each international comparison. LASERS AT X = 612 nm The situation for the orange line at h = 612 nm differs from that for the X = 633-nm lasers. Far fewer iodine stabilized laser 0018-9456/91/0400-0181$01.00 zyxwvut 0 1991 IEEE