An interferometer on grand tour – Part 2

The traceability of measurement results of distances from a few hundred meter to a few kilometers with relative uncertainties much better than u = 1 ∙ 10-6 (one millimeter per kilometer) is a major challenge. Compensating for the influence of the air refractive index on the propagation velocity of light limits the achievable accuracy in length measurements by interferometry. According to the classical approach, temperature, air pressure and relative humidity are measured along the entire measurement distance with appropriate sensors and the air refractive index is determined from this using empirical formulas. This comes along with high measurement effort, especially for larger distances. Alternatively, methods are available with which the influence of the air refractive index can be compensated by means of light-based measurements parallel to the distance measurement in situ over the entire distance. PTB pursues the two-color method using two widely separated light wavelengths.

This principle has already been successfully demonstrated by PTB in the past. Within the framework of the European research project "Large-scale dimensional measurements for geodesy" (18SIB01, GeoMetre), PTB is currently developing a system with a range of up to five kilometers under outdoor conditions: the so-called TeleYAG-II. Such longer distances are of great interest in the geodesy especially for the verification of GPS-based distance measurement methods.

The TeleYAG-II represents a multi-wavelength interferometer consisting of three subsystems: laser source, optomechanical assembly and the phase evaluation electronics. Each subsystem was conceptually built up from the scratch to comply with the increasing demands under harsh environmental condition

• the light source is now fully-fiber coupled using 8 acousto-optical modulators and two Nd:YAG lasers emitting 532-nm and 1064-nm wavelengths,
• the optomechanical setup is based on ULE material and costum-made optics and
• the phase evaluation electronics is operating with state-the-art ADC and FPGA units.

The TeleYAG-II system was built in July 2021 and was then put into operation and tested at PTB's internal reference measuring sections (50-m comparator and 600-m baseline). After the measurement campaign in Finland last year – please see News of last year - two further campaigns were carried out in Wettzell (Germany) and Warsaw (Poland) in 2022. The two campaigns differed significantly in terms of the measurement task to be fulfilled. While the campaign in Wettzell tackled to reduce the uncertainty to determine local tie vectors, the campaign in Warsaw was about verifying innovative temperature sensing techniques. For the latter one, five different metrology and research institutes were carrying out comparison measurements at the newly built reference baseline WUT200. The PTB prepared these campaigns in advance by extensive measurements on PTB’s 600 m reference baseline and numerous adjustments. During these campaigns and their preparations, the high-precision instrument could demonstrate a high degree of long-term mechanical stability as well as its resistance to harsh environmental conditions.

In the long term, measurements against the PTB nanometer comparator are planned to push the limits of performance and further improve the optomechanical system.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

  
Left: The multi-wavelength interferometer TeleYAG-II (colour: blue), developed for measurements under outdoor conditions, in use on the reference measurement baseline WUT200 in Warsaw.
Right: Group photo with participants of the involved institutes during the measurements in Warsaw within the EMPIR research project "GeoMetre".