## 1 Introduction
In contrast to the tactile probes, the measurement signal in fibre probe (i.e., the position of the probing sphere) is directly detected by an optical CCD camera without the mechanical transfer of the probe styli [24]. The advantages are that the probing forces can be very low (1 \(\upmu\)N to 100 \(\upmu\)N) and stylus tips are available with much smaller diameters (down to 25 \(\upmu\)m). The 2D version fibre probe has been used for many years to measure injection nozzles, turbine blade cooling holes, and a variety of other small, tight tolerance features. An upgrade of this probe, the 3D fibre probe, has been commercially available for some time [25]. PTB collaborated with the Werth Messtechnik company are continuing the investigations on this microprobe, which will be detailed in this paper. The design idea of the fibre probe was followed by [PERSON] et al. [26] who proposed a spherical coupling fibre probe as an attempt to overcome the shadowing effect. Some other kinds of tactile-fibre probes have also been proposed, where Bragg grating strain sensors [27], fibre optical displacement sensors [28] and micro-focal-length collimation techniques [29] are applied to measure the position of the probing sphere or of the stylus. In addition, [PERSON] et al. [30] proposed an electrical tunnelling current probe for force-free probing; and [PERSON] et al. [31] put forward a probe based on the laser trapping technique. System calibration and performance verification are crucial tasks for 3D micro-coordinate metrology. To qualify, calibrate and verify these micro-CMMs, calibration standards as well as the traceable calibration of these standards are essential. However, this is still a significant challenge today due to the extremely low uncertainty demanded, as is also summarized in a review paper by [PERSON] et al. [32]. It is extremely difficult to achieve the required calibration uncertainties at calibrated test lengths, due to the limited availability of both high-quality physical standards and metrological services. For example, with the current micro-CMMs exhibiting an uncertainty of 100 nm or less, it is essential for any test length used for verification to be calibrated with an uncertainty that is five- or even ten-times lower, i.e., an uncertainty of better than 20 nm. The micro-CMMs are not covered by the current international standard documents for CMM testing or are only partly covered by them (e.g., VDI 2617 12.1 [33]). Problems also occur when trying to apply the existing acceptance tests defined in ISO 10360-2 [34] to micro-CMMs. According to the test procedures defined in ISO 10360-2, for example, it is required to measure five different calibrated test lengths which are locatedat seven orientations within the measurement volume of the CMM, four of which must be the space diagonals. However, the shaft length of a micro-CMM probe is usually kept short in order to enhance the measurement stability, which limits its measurement accessibility to all reference features of the calibration artefact. Therefore, the development of a traceable metrological capability and of physical as well as documentary standards is still an urgent task for promoting the commercialization and application of micro-coordinate measuring tools today. State-of-the-art tactile micro-coordinate metrology has recently been reviewed by [PERSON] et al. [35] and three key aspects--stage and metrology system design, probe developments, and system calibration and performance verification--have been well summarized. Therefore, this paper is focused on giving an overview of the research activities carried out at PTB.