Absolute frequency measurement of the HeNe/I 2 standard at 633 nm J.E. Bernard * , A.A. Madej, K.J. Siemsen, L. Marmet Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Ont., Canada K1A 0R6 Received 13 July 2000; received in revised form 26 October 2000; accepted 1 November 2000 Abstract The absolute frequency of a HeNe laser locked to a hyper®ne component in 127 I 2 is measured using a frequency standard based on a single, trapped and laser-cooled 88 Sr  ion. A CO 2 laser, whose frequency is measured with a Cs- based frequency chain, is used to span the frequency interval between the ion standard at 445 THz and the HeNe/I 2 laser at 474 THz. The frequency of the f component of the 11-5 R(127) transition of our HeNe/I 2 standard laser is found to be 473 612 353 600 180  720 Hz at the exact operating conditions speci®ed for the realization of the recommended value of this radiation. Crown Copyright Ó 2001 Published by Elsevier Science B.V. All rights reserved. PACS: 42.72.g; 06.30.Ft; 42.62.Fi; 42.62.Eh; 42.50.Lc; 42.55.Lt Keywords: Optical frequency standard; Single ion; Frequency chain 1. Introduction The present de®nition of the metre as the dis- tance light travels in vacuum in 1/(299 792 458) s was adopted by the 17th Conf erence G en erale des Poids et Mesures in 1983 [1]. At the same time, the Comit e International des Poids et Mesures (CIPM) made recommendations for the practical realiza- tion of the metre, referred to as the mise en pratique of the de®nition. Various sources of radiation have been recommended as standards of wavelength and have been updated by the CIPM over time [2]. One of the most important recommended radiations in the ®eld of length metrology and worldwide pre- cision measurements is that at 474 THz (633 nm) from a HeNe laser, stabilized on an absorbing hyper®ne component in 127 I 2 . This laser is used in many laboratories around the world as the practi- cal means of realizing the SI metre and is com- monly used for calibrating the frequency of lasers employed in gravitometers, length measurement, and precision measurement in atomic physics. The ®rst observation of hyper®ne structure in saturated absorption of I 2 inside a HeNe laser cavity was by Hanes and Dahlstrom in 1969 [3]. Soon afterwards, Hanes and Baird [4] used an in- terferometric technique with a 86 Kr standard to measure the wavelength of the i component of the 11-5 R(127) transition to a relative uncertainty of 3  10 8 . By the mid 1970s, the application of the third-harmonic locking technique [5,6] resulted in wavelength (frequency) reproducibilities for 1 January 2001 Optics Communications 187 (2001) 211±218 www.elsevier.com/locate/optcom * Corresponding author. Fax: +1-613-952-1394. E-mail address: john.bernard@nrc.ca (J.E. Bernard). 0030-4018/01/$ - see front matter Crown Copyright Ó 2001 Published by Elsevier Science B.V. All rights reserved. PII:S0030-4018(00)01085-3