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 Comite Consultatif pour la Definition du Metre (CCM; 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 Comite 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 I-127(2) at lambda approximate to 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 sigma) 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.

The wavelength standards at 633 nm and 657 nm in the visible part of the spectrum are characterized by most accurate frequency values obtained by frequency chain measurements. A cross check has been performed by an interferometric wavelength comparison and agreement was found within 3.8/spl middot/10/sup -11/ which demonstrates the applicability of both methods for a consistent wavelength scale.

A EUROMET comparison of two long gauge blocks was organized by the Physikalisch-Technische Bundesanstalt. Nine EUROMET laboratories had agreed to participate in this comparison. One of them withdrew its participation during the course of calibrations. The measurements started in autumn 1993 and ended in January 1995. The gauge blocks had nominal lengths of 600 mm and 1000 mm. The latter gauge block was damaged shortly before the end of the project and was replaced by one of 900 mm nominal length. The measurement items were the central length, the variation in length, and - if possible - the determination of the coefficient of thermal expansion. The measurements of the central length agree roughly within +/- 0, 05 mu m and the estimated values of the uncertainty overlap. The measured values of the variation in length fluctuate by a few hundred nanometres. The agreement of the coefficient of thermal expansion is satisfactory.

A detailed description of the He-Ne/127I2 laser wavelength standards developed at the National Metrology Institute of Turkey (UME), using high-frequency modulation and featuring a long-term relative instability of a few parts in 1014, is presented. The frequency dependence of the lasers on the iodine gas pressure, modulation width and saturating field intensity have been studied with a very low uncertainty level, and the common effect of these parameters on the output frequency has been demonstrated. A new method of accurately measuring modulation width has been realized, making an important contribution to the improvement of reproducibility of this type of wavelength standard. An international comparison between the laser standards of the Physikalisch-Technische Bundesanstalt (PTB) and the UME has been performed. Several iodine cells studied in a laser with an accurate servo-system, reveal a correlation between the spacing of the molecular resonances, the gas contamination in the iodine cell, and the output frequencies of the laser when locked to different iodine components.

The calibration of long gauge blocks of up to 1 m using the vacuum wavelength comparator according to Kösters, and three laser wavelength standards recommended by the CIPM, is described. The relative uncertainty of this calibration is shown to be as low as 1,3 × 10-8 for suitable gauge blocks of 1 m length. In addition, a discontinuity of the length calibration of 5 × 10-8 was introduced when the international temperature scales were changed.

An international comparison of 127I2 He-Ne stabilized lasers, used as length standards, has been made. Five laboratories from Sweden, Denmark (two laboratories), Finland and Germany were represented. In addition, two lasers from the Bureau International des Poids et Mesures took part in order to relate the results to earlier comparisons. The reproducibility of the frequency mean of the four components d, e, f and g in the transition R(127)11-5 in iodine 127 for the different lasers was found to be 15 kHz, taken as the standard deviation of the distribution of frequency among the group of ten lasers. Using laser BIPM4 as reference the frequency average of the group of lasers was +3,6 kHz, making this a good reference laser for the current ensemble. Measurements of frequency dependence on modulation amplitude, iodine pressure and, for some lasers, intracavity power are reported. The relative frequency partition of the four components d, e, f and g of each laser was also determined and shown to exhibit characteristics which are new for this type of stabilized laser.

The frequencies of six iodine-stabilized He-Ne-lasers developed at the UPT have been compared with one of the PTB lasers. The lasers were operated at λ = 633 nm. Their frequencies were stabilized to the line centres of hyperfine structure (hfs) components of the R(127) 11-5 absorption line of 127I2 using the third-harmonic detection technique. The stabilized frequencies of the UPT lasers averaged for each laser over the four hfs components d, e, f, and g are distributed in two groups. The frequencies of one group of three lasers are close to the frequency of the PTB laser with maximum deviations of +4 kHz to -9 kHz. The frequencies of the other three lasers are offset by -38 kHz with deviations of +8 kHz to -4 kHz. The maximum frequency deviations between all six lasers are approximately one order of magnitude smaller than the uncertainties of the recommended wavelength values [1] for this laser system, demonstrating its high potential for the realization of the metre and for high-precision spectroscopy.

"Zerodur", a glass ceramic, has been developed in order to have a material with an extremely small thermal expansion coefficient. The known physical properties are completed here by results on dimensional stability compiled over a number of years. Three samples of the material with different thermal histories show different shrinkage during a period of observation over 10 years. The description of the behaviour by mathematical formulas, logarithmic, hyperbolic or exponential, is in all cases nearly satisfactory. The relative alteration of length is between - 0.69 × 10-6 and - 0.03 × 10-6 per year.