Optical coherence tomography (OCT) is increasingly used in areas such as ophthalmology and contact lens metrology. However, in such cases, image distortion can occur due to the non-planar nature of the measured sample. Postprocessing algorithms can be implemented to correct this distortion. Here we present an OCT phantom designed to confirm the validity of post-processing algorithms used for measuring curved surfaces. A multi-purpose OCT phantom has been created within a fused silica plano-convex lens using the direct femtosecond laser writing technique. This phantom can be used to calibrate and quantitatively assess the performance (e.g. resolution, sensitivity and distortion) of OCT systems and associated post-processing algorithms for curved structures such as lenses. This novel OCT phantom has been characterized using an optical microscope and OCT systems.
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.
Recently, we have extended fibre grating devices in to mid-IR range. Fibre Bragg gratings (FBGs) and long-period
gratings (LPGs) with spectral responses from near-IR (800nm) to mid-IR ( ~ 2μm) have been demonstrated with
transmission loss as strong as 10-20dB. 2μm FBG and LPG showed temperature and refractive index (RI) sensitivities
of ~ 91pm/°C and 357nm/RIU respectively. Finally, we have performed a bio sensing experiment by monitoring the
degradation of foetal bovine serum at room temperature. The results encouragingly show that the mid-IR LPGs can be an
ideal biosensor platform as they have high RI sensitivity and can be used to detect concentration change of bio- samples.
Measurement of the laser beam propagation factor M<sup>2</sup> 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<sup>(R)</sup>) grating, to measure the M<sup>2</sup> value of laser beams.
The advantage of the IMP<sup>(R)</sup> 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<sup>(R)</sup> 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.