Optical coherence tomography (OCT) has developed rapidly and is widely used in different fields such as biomedicine and optometry. The characterization and calibration of OCT systems is essential when testing the system and during normal use to ensure that there is no misalignment or distortion that could affect clinical decisions. Imaging distortion is a significant challenge for OCT systems when viewing through non-planar surfaces. Here we present a new multi-purpose plano-convex OCT phantom which is designed to be used for OCT characterization and calibration as well as to validate the post-processing algorithm for the imaging distortion of the OCT systems. A femtosecond laser direct writing technique is used to fabricate this phantom which consists of a landmark layer with radial lines at a 45-degree angular spacing inscribed at 50μm in apparent depth (AD) underneath the planar surface. Below that there are a further 8 layers of a spherical inscription pattern which has a 150μm (in AD) separation between each layer. The first spherical layer is located at 150μm (in AD) underneath the planar surface. Due to the laser power loss when travelling through the deeper layer, an increased power is applied to the deeper layers. The spherical pattern overcomes orientation issues seen with existing calibration phantoms. The landmark layer is applied so that it can easily tell the exact location when scanning which will also benefit the image distortion correction process.
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.
We describe a parallel multi-threaded approach for high performance modelling of wide class of phenomena in
ultrafast nonlinear optics. Specific implementation has been performed using the highly parallel capabilities of
a programmable graphics processor.
We present the results of comparative numerical study of energy deposition in single shot femtosecond laser
inscription for fundamental and second harmonic of Yb-doped fiber laser. We have found that second harmonic
is more efficient in absorbing energy which leads to lower inscription threshold. Hence this regime is more
attractive for applications in femtosecond laser microfabrication.
The optical layouts incorporating binary phase diffractive grating and a standard micro-objective were used for femtosecond microfabrication of periodical structures in fused silica. Two beams, generated in Talbot type interferometer, interfered on a surface and in the bulk of the sample. The method suggested allows better control over the transverse size of the grating pitch, and thus control the reflection strength of the waveguide or fibre grating. We present the examples of direct inscription of the sub-micrometer periodical structures using a 267 nm femtosecond laser radiation.
A series of LPGs was inscribed in photonic crystal fibre by a low repetition femtosecond laser system. When subjected
to bending they were found to be spectrally sensitive to bend orientation and displayed a strong polarisation dependence.
We present results on characterization of lasers with ultra-long cavity lengths up to 84km, the longest cavity ever reported. We have analyzed the mode structure, shape and width of the generated spectra, intensity fluctuations depending on length and intra-cavity power. The RF spectra exhibit an ultra-dense cavity mode structure (mode spacing is 1.2kHz for 84km), in which the width of the mode beating is proportional to the intra-cavity power while the optical spectra broaden with power according to the square-root law acquiring a specific shape with exponential wings. A model based on wave turbulence formalism has been developed to describe the observed effects.
A numerical model of the sensitivity of long period gratings fabricated by electric-arc in photonic crystal fibres to strain,
temperature and refractive index is proposed and evaluated by comparison to the experimental results. It is shown to be
superior to the commonly used semi-analytical method. The generalized modelling procedure is thoroughly explained in
order to facilitate its application to a wide range of long period gratings in different types of fibres.
This paper describes experimental and numerical results of the
plasma-assisted microfabrication of subwavelength structures by
means of point-by point femtosecond laser inscription. It is shown
that the spatio-temporal evolution of light and plasma patterns
critically depend on input power. Subwavelength inscription
corresponds to the supercritical propagation regimes when pulse
power is several times self-focusing threshold. Experimental and
numerical profiles show quantitative agreement.
We present recent results on experimental micro-fabrication and numerical modeling of advanced photonic devices by means of direct writing by femtosecond laser. Transverse inscription geometry was routinely used to inscribe and modify photonic devices based on waveguiding structures. Typically, standard commercially available fibers were used as a template with a pre-fabricated waveguide. Using a direct, point-by-point inscription by infrared
femtosecond laser, a range of fiber-based photonic devices was fabricated including Fiber Bragg Gratings (FBG) and Long Period Gratings (LPG). Waveguides with a core of a couple of microns, periodic structures, and couplers have been also fabricated in planar geometry using the same method.
We present an adaptive mesh approach to high performance comprehensive investigation of dynamics of light and plasma pattens during the process of direct laser inscription. The results reveal extreme variations of spatial and temporal scales and tremendous complexity of these patterns which was not feasible to study previously.
Long period gratings in two types of photonic crystal fibre have been studied. The gratings display negligible temperature sensitivity but useful sensitivity to other measurands. Theoretical modelling suggests that unusual phase matching conditions apply.
Long period gratings (LPGs) were written into a D-shaped optical fibre, which has an elliptical core with a W-shaped refractive index profile. The LPG's attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15nm between the two orthogonal polarisation states. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature, surrounding refractive index, and directional bending. These LPG devices produced blue and red wavelength shifts of the stop-bands due to bending in different directions. The measured spectral sensitivities to curvatures, dλ/dR, ranged from -3.56nm m to +6.51nm m. The results obtained with these LPGs suggest that this type of fibre may be useful as a shape/bend sensor. It was also demonstrated that the neighbouring bands could be used to discriminate between temperature and bending and that overlapping orthogonal polarisation attenuation bands can be used to minimise error associated with polarisation.
In this paper we review the sensing features of long period Bragg gratings and report on studies aimed at optimizing the sensitivities to various measurands by using different fiber geometries.
The model of the pulse laser nephelometry polarization methods of using in biotechnologies and ecology is elaborated. It is shown that informativity of the laser nephelometry measurements increases when polarization self- action effects of short pulses of the laser radiation are taken into account by multiple scattering is dispersed biological media with high concentration of weighted particles. The soliton and simulton regimes of the polarization transformation of the laser pulses have been investigated. Conditions of the modulation instability are determined of the pulse laser radiation of arbitrary polarization in nonlinear scattering biomedia.
We demonstrated the results of investigations on the creation of the new high-precision laser methods and technologies of the on-line detection and operative quantitative analysis of the small and super-small concentrations of sugar, glucose, fructose and other matters dissolved in the transparent liquids based upon the effect of the natural optical activity of such matters. The record- breaking improvement has been achieved of the sensitivity of the registration systems as well as measurement precision in determination of small rotation angles of the radiation polarization plane due to the natural optical activity of solutions.
In this paper we consider an effective variational description for the optical pulse evolution in fiber links with dispersion management (varying dispersion). Instead of solving a nonlinear partial differential equation with varying coefficients, one can gain some important information about the features (e.g. breathing solitons) by an analysis of two ordinary differential equations. This approach provides a clear physical picture of the pulse evolution in a transmission line under the combined actions of nonlinearity, varying dispersion, fiber loss, and periodic amplification. An optical pulse propagation along a transmission line must be periodically reproduced.
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