International comparison of He-Ne lasers stabilized with ¹²⁷I₂ at λ = 633 nm: Comparison of the fifth- and third-harmonic locking techniques

An international comparison of ¹²⁷I₂-stabilized 633 nm He-Ne lasers locked either by the fifth- or the third-harmonic technique was carried out at the Finnish Centre for Metrology and Accreditation (MIKES) in September 1998. The participating laboratories were the MIKES; the Metrology Research Institute, Helsinki University of Technology (HUT); the Chinese National Institute of Metrology (NIM); the Czech Metrological Institute (CMI); the D. I. Mendeleyev Institute for Metrology (VNIIM); and the Bureau International des Poids et Mesures (BIPM). Raw measured frequency differences between lasers were in the following ranges: −5.2 kHz to +11.9 kHz and −5.8 kHz to +4.9 kHz for the third- and fifth-harmonic locking techniques, respectively. The frequency offsets between the third- and fifth-harmonic techniques for different components (d to g) were in the range 26 kHz to 35 kHz. The relative frequency stability of lasers given by the Allan standard deviations reached 2.6 × 10⁻¹³ for two lasers using the third harmonic and only 1 × 10⁻¹² for the same lasers using the fifth harmonic for a 2000 s sampling time. This comparison is part of the ongoing laser comparison carried out under the auspices of the Consultative Committee for Length (CCL, formerly CCDM) of the Comité International des Poids et Mesures (CIPM); it is also a EUROMET project (No. 462).