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A new class of sensor has recently appeared: nanometre sensors. These sensors are characterized by nanometre or sub-nanometre resolution and an uncertainty of a few nanometres over a range of at least several micrometres. Instruments such as capacitive or inductive sensors, laser interferometers, holographic scales, and scanning probe microscopes belong to the class of nanometre sensors. Linearity errors and drift in the mechanical and electronic system limit the accuracy of all these sensors. In order to determine these errors in a traceable way, the instrumentation described in this paper was developed. The heart of the system consists of a Fabry-Pérot cavity. One mirror of this cavity generates the required displacement. A so-called slave laser is stabilized to the cavity length. The frequency of this slave laser is compared with the frequency of a primary length standard. In this way the displacement is measured with a resolution of a few picometres, a range of 300 µm and an uncertainty of about 1 nm. Experiments confirm the performance of this instrument and show typical deviations of the probe systems investigated.

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This second Northern European comparison between seven lasers exhibits closer results than the ® rst comparison. A standard uncertainty of 5 kHz was obtained against 15 kHz in 1990, while the frequency average of the group of lasers was about the same with 3,9 kHz, using the BIPM4 laser as reference. A large frequency difference on a laser observed during the ® rst com parison was again observed; partial explanations of the origin of this differenc e were found. This differenc e has been excluded from the calculation of the average. Typical frequency stabiliti es using Allan standard deviations of about 1,6 10 ±11 and 1,7 10 ±12 were observed on 1 s and 100 s sampling tim es, respectively. * are the characteristics of the M 1 m irror located on th e iodin e cell sid e of the lasers. ** are th e characteristics of th e M 2 m irror located on the gain tube sid e of the lasers. * T he m odulatio n w idth is alw ays give n in M H z peak-to -peak. ** E xterna l powe r of the laser.

This paper reports the fourth set of results of a series of grouped laser comparisons from national laboratories undertaken by the Bureau International des Poids et Mesures (BIPM) at the request of the Comité Consultatif pour la Définition du Mètre (CCDM; now the Consultative Committee for Length, CCL) during the period July 1993 to September 1995. The results of this comparison, involving eleven lasers from eight countries and the BIPM, again meet the goals set by the CCDM in 1992 and adopted by the Comité International des Poids et Mesures (CIPM) the same year. The standard uncertainty of the frequency of the He-Ne laser stabilized on the saturated absorption of 127 I 2 at λ 633 nm is reduced to a level of 12 kHz (2.5 parts in 10 11 ) when the lasers compared meet the recommended values of the parameters.The lasers were first compared with the BIPM4 laser with the parameters set to the values normally used in each laboratory; using the BIPM4 laser as a reference the results ranged from -15.4 kHz to 36.8 kHz. After checking and readjusting the values of all the parameters, the range was reduced to -8.6 kHz to 14.0 kHz. Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was 2.7 kHz with a standard uncertainty (1 ) of 8.1 kHz. The best relative frequency stabilities, with Allan standard deviations of about 5.5 parts in 10 12 and 4.6 parts in 10 13 , were observed with sampling times of 1 s and 100 s, respectively.