We report on the development of an instrument for the measurement of the Encircled Angular Flux (EAF) and on establishing its metrological traceability at the required level of uncertainty. We designed and built for that purpose two independent EAF measuring instruments, both based on the analysis of the two-dimensional far field intensity profile observed at the output of an optical fibre, using either CMOS or CCD cameras. An in depth evaluation of the factors influencing the accuracy of the measurements was performed and allowed determining an uncertainty budget for EAF measurements, which was validated by a first series of inter-comparisons. Theses comparisons were performed between the two independent EAF measuring systems, using a 850 nm LED coupled into a gradient index fibre as a test object. We demonstrated a very good equivalence between the two systems, well within the absolute measurement uncertainties that were estimated at the 10-3 level. Further inter-comparisons using light sources coupled to step-index, large core and small core multimode fibres are still ongoing, with the aim to confirm the performances of the instrument under various illuminating conditions.
A stable reproducible optical standard source for measuring multimode optical fiber attenuation is required as recent
round robin measurements of such fibers at several international companies and national standards organizations showed
significant variation when using a source having only the encircled flux in the near field emerging from it defined. The
paper presents and compares the far field modal power distributions for (i) 2 km and 3 km step-index multimode Hard
Plastic Cladding Fibers, HPCF, (SI-MMF) with 200 μm silica core diameter, 0.37 numerical aperture (NA) and polymer
cladding, (ii) a 10 m silica graded-index multimode fiber (GI-MMF) with 50 μm core diameter and 0.2 NA, and (ii) a
near field Encircled Flux Mode Convertor or “modcon”. A free space method for measuring the far field using a Lightemitting
diode (LED) centered at 850 nm wavelength with 40 nm 10 dB-bandwidth and a charge-coupled device (CCD)
camera is compared with a f-theta multi-element lens based far field pattern (FFP) system. Mandrels of different
diameter and different numbers of turns of the fiber around them were used to achieve an equilibrium mode distribution
(EMD) for the GI-MMF. The paper defines encircled angular flux (EAF) as the fraction of the total optical power
radiating from a multimode optical fiber core within a certain solid angle in the far field. The paper calculates the EAF
when the solid angle increases from the far field centroid.
Measurement of the laser beam propagation factor M2 is essential in many laser applications including materials
processing, laser therapy, and lithography. In this paper we describe the characterisation of a prototype device using a
cross-distorted diffraction grating known as an Image Multiplex (IMP(R)) grating, to measure the M2 value of laser beams.
The advantage of the IMP(R) grating instrument lies in its ability to simultaneously image nine positions along the beam
path. This enables beam propagation parameters to be calculated both for pulsed lasers and lasers with rapidly changing
propagation characteristics. This is in contrast to the scanned technique recommended by the ISO, which is relatively
slow and in practice can only be easily used with cw sources. The characterisation was accomplished by comparison of
results from the IMP(R) grating device with those obtained using the accepted methodology described in the ISO 11146
series of standards through measurements conducted by the National Physical Laboratory. The scope of the work also
included provision of a traceability route to international standards, and an uncertainty budget, to allow the intended user
community to have confidence in measurements obtained when using the device, and to enable them to use it as part of
their quality framework.
In the past, the use of optical and digital three-dimensional correlation methods have been demonstrated to extract velocity data from the complex amplitude distribution of particle images in holographic particle image velocimetry (HPIV). Recently we have proposed a digital shearing method to extract three-component particle displacement data throughout a complete image field. In contrast to full three-dimensional correlation, it has been shown that all three components of particle image displacement can be retrieved using just four two-dimensional fast Fourier transform (FFT) operations and appropriate coordinate transformations. In this paper we describe three-dimensional correlation and digital shearing methods and compare their performance in terms of computational efficiency and measurement accuracy. The simulated results show that the digital shearing method has comparable accuracy to three-dimensional correlation but is significantly faster.