Waveguide-saturable absorber fabricated by femtosecond pulses in YAG:Cr4+ crystal for Q-switched operation of Yb-fiber laser

A waveguide-saturable absorber with low propagation loss is fabricated by femtosecond pulses in YAG:Cr4+ crystal. Q-switch operation of a Yb fiber laser with the new saturable absorber having absorption saturation parameters similar to the bulk YAG:Cr4+ crystal is demonstrated.

Waveguide inscription in YAG:Cr4+ crystals by femtosecond laser irradiation

We have observed a positive change or refractive index and formation of waveguides in YAG:Cr4+ crystals, exposed to a high-intensity femtosecond laser beam. The technique is potentially suitable for fabrication of waveguide lasers in crystal materials.

The effect of heat treatment on mechanical properties of pulsed Nd:YAG welded thin Ti6Al4V

Pulsed Nd:YAG has been adopted successfully in welding process of thin (0.7 mm) Ti6Al4V. Laser welding of such thin sheet requires a small focal spot, good laser beam quality and fast travel speed, since too much heat generation can cause distortion for thin sheet weld. The microstructures of Ti6Al4V were complex and strongly affected the mechanical properties. These structures include: a´ martensite, metastable ß, Widmanstätten, bimodal, lamellar and equiaxed microstructure. Bimodal and Widmanstätten structures exhibit a good-balance between strength and ductility. The microstructure of pulsed Nd:YAG welded Ti6Al4V was primarily a´ martensite, which showed the lowest ductility but not significantly high strength. A heat treatment at 950 followed by furnace cooling can transform the microstructure in the weld from a´ martensite structure into Widmanstätten structure.

Low loss depressed cladding waveguide inscribed in YAG:Nd single crystal by femtosecond laser pulses

A depressed cladding waveguide with record low loss of 0.12 dB/cm is inscribed in YAG:Nd(0.3at.%) crystal by femtosecond laser pulses with an elliptical beam waist. The waveguide is formed by a set of parallel tracks which constitute the depressed cladding. It is a key element for compact and efficient CW waveguide laser operating at 1064 nm and pumped by a multimode laser diode. Special attention is paid to mechanical stress resulting from the inscription process. Numerical calculation of mode distribution and propagation loss with the elasto-optical effect taken into account leads to the conclusion that the depressed cladding is a dominating factor in waveguide mode formation, while the mechanical stress only slightly distorts waveguide modes.

Low loss depressed cladding waveguide inscribed in YAG:Nd single crystal by femtosecond pulses

A depressed cladding waveguide with record low loss of 0.12 dB/cm is inscribed in YAG:Nd(0.3at.%). It is shown that depressed cladding is a dominating factor in waveguide formation, and mechanical stress has a minor contribution. © 2012 OSA.

Electronic and thermal lensing in diode end-pumped Yb:YAG laser rods and discs

The lensing effects in diode end-pumped Yb:YAG laser rods and discs are studied. Two mechanisms of refractive-index changes are taken into account, thermal and electronic (due to the difference between the excited- and ground-state Yb polarisabilities), as well as pump-induced deformation of the laser crystal. Under pulsed pumping, the electronic lensing effect prevails over the thermal one in both rods and discs. In rods pumped by a highly focused cw beam, the dioptric power of the electronic lens exceeds that of the thermal lens, whereas in discs steady-state lensing is predominantly due to the thermal mechanism. © 2009 Kvantovaya Elektronika and Turpion Ltd.

Population lensing vs thermal lensing in Yb:YAG rods and disks

The induced lenses in the Yb:YAG rods and disks end-pumped by a Gaussian beam were analyzed both analytically and numerically. The thermally assisted mechanisms of the lens formation were considered to include: the conventional volume thermal index changes ("dn/dT"), the bulging of end faces, the photoelastic effect, and the bending (for a disk). The heat conduction equations (with an axial heat flux for a disk and a radial heat flux for a rod), and quasi-static thermoelastic equations (in the plane-stress approximation with free boundary conditions) were solved to find the thermal lens power. The population rate equation with saturation (by amplified spontaneous emission or an external wave) was examined to find the electronic lens power in the active elements.

Mechanisms of a change in the refractive index of an intensely pumped Yb:YAG crystal

Mechanisms of a change in the refractive index appearing in an intensely diode-pumped Yb:YAG-laser disk element are studied with the help of polarisation interferometry and dynamic grating testing. It is found that changes in the electronic component of the refractive index arising upon changing the populations of electronic levels of Yb ions (the ground F state and the upper F level of the laser transition) and caused by the difference in the polarisability of these levels are an order of magnitude greater than thermal changes in the refractive index. It is shown that the difference Δp in the polarisability at the probe wavelength of 633 nm is 1.9 × 10 cm and at the laser transition wavelength of 1029 nm is 1.6 × 10 cm. ©2006 Kvantovaya Elektronika and Turpion Ltd.

Pain response in patients undergoing panretinal photocoagulation by continuos wave Nd-YAG laser

Purpose: Current panretinal laser photocoagulative parameters are based on the Diabetic Retinopathy Study, which used exposures of 0.1 - 0.5 second to achieve moderate intensity retinal burns. Unfortunately, many patients find these settings painful. We wanted to investigate whether reducing exposure time and increasing power to give the same endpoint, is more comfortable and effective. Methods: 20 patients having panretinal photocoagulation for the first time underwent random allocation to two forms of laser treatment: half of the retinal area scheduled for treatment was treated with Green Yag laser with conventional parameters {exposure time 0.1 second (treatment A), power density sufficient to produce a visible grey - white burns}. The other half treated with shorter exposure 0.02 second (treatment B). All patient were asked to evaluate severity of pain on a visual analogue scale ranging from 0 - 10 (0 = no pain, 10 = most severe pain). All patients were masked as to the type of treatment. The order of carrying out the treatment on each patient was randomised. Fundus photographs were taken of each hemifundus to confirm treatment. Results: Of the 20 patients, 17 had proliferative diabetic retinopathy, 2 had ischaemic central retinal vein occlusion and one had ocular ischaemic syndrome. The average pain response to treatment A was 5.11 on a visual analogue scale with a mean power of 0.178 Watt; the average pain response to treatment B was 1.40 with a mean power of 0.489 Watt. Short exposure laser burns were significantly less painful (P < 0.001). Conclusion: Shortening exposure time with increased power is more comfortable for patients undergoing panretinal photocoagulation than conventional parameters.

Generation of 49-fs pulses directly from distributed Kerr-lens mode-locked Yb:YAG thin-disk oscillator

The concept of distributed Kerr-lens mode-locking and a thin-disk Yb:YAG oscillator based on this concept are presented. The described oscillator directly generates pulses with a duration of 49 fs and spectral width of 33 nm

Fabrication and application of short and novel structure in-fibre Bragg gratings

This thesis presents details on progress made in the fabrication and application of short and novel structure fibre Bragg gratings. The basic theoretical concepts of in-fibre Bragg gratings and photosensitive mechanisms are introduced together with an overview of fabrication methods and applications presented to date. The fabrication of fibre Bragg gratings using a quadrupled Nd:YAG laser is presented and some of the issues of grating fabrication using a fabrication using a phasemask are investigated, including the variation of the separation of the fibre and phasemask, and other alignment issues. A new apodisation technique is presented, enabling the production of gratings with a wide range of spectral profiles. The technique is used to investigate the design and fabrication of length limited fibre Bragg gratings for use in telecommunication systems as filters. Application to devices designed for use in WDM systems is presented. The use of fibre Bragg gratings as high spatial resolution distributed sensors is investigated. Grating sensing arrays comprising very short apodised gratings are demonstrated and Chirped Moiré gratings are implemented as distributed sensors achieving high spatial resolution with miniature point sensing sub-elements. A novel grating sensing element designed to imitate an interferometer is also presented. Finally, the behaviour of gratings fabricated in Boron-Germania-co-doped fibre is investigated, revealing atypical behaviour of the Bragg wavelength during ageing.

Cascaded nonlinear absorption of femtosecond laser pulses in dielectrics

We investigated the energy deposition process leading to the waveguide inscription in transparent dielectrics both experimentally and theoretically. Parameters of multiphoton absorption process and inscription thresholds were measured in a range of materials including YAG, ZnSe, RbPb2Cl5 crystals, and in fused silica and BK7 glasses.

The Synthesis and properties of functionalised polymers

The aim of this project was to synthesise fluorinated polymers that might act as hot material in a guest-host system for use in non-linear optical applications. These polymers would be expected to have the advantage over materials such as poly(vinylidene fluoride) which is known to be incompatible with many nlo active materials. A series of bicyclic fluorinated monomers was prepared by the reaction of fluorinated dienophiles with cyclopentadiene in a series of Diels-Alder reactions. The monomers were purified and then used in ring opening metathesis polymerisation. The materials were then characterised by gel permeation chromatography and nuclear magnetic resonance spectroscopy and cast as films for determination of their activities as nlo materials using a Nd/YAG laser system. The second harmonic intensity of each was measured relative to quartz. However no materials of significant activity were produced. In an attempt to produce polymers that might subsequently be functionalised the polymerisation of 1,2-methylenedioxybenzene and 1,4-benzodioxane was investigated.

Fabrication of high quality optical coherence tomography (OCT) calibration artefacts using femtosecond inscription

Optical coherence tomography (OCT) is a non-invasive three-dimensional imaging system that is capable of producing high resolution in-vivo images. OCT is approved for use in clinical trials in Japan, USA and Europe. For OCT to be used effectively in a clinical diagnosis, a method of standardisation is required to assess the performance across different systems. This standardisation can be implemented using highly accurate and reproducible artefacts for calibration at both installation and throughout the lifetime of a system. Femtosecond lasers can write highly reproducible and highly localised micro-structured calibration artefacts within a transparent media. We report on the fabrication of high quality OCT calibration artefacts in fused silica using a femtosecond laser. The calibration artefacts were written in fused silica due to its high purity and ability to withstand high energy femtosecond pulses. An Amplitude Systemes s-Pulse Yb:YAG femtosecond laser with an operating wavelength of 1026 nm was used to inscribe three dimensional patterns within the highly optically transmissive substrate. Four unique artefacts have been designed to measure a wide variety of parameters, including the points spread function (PSF), modulation transfer function (MTF), sensitivity, distortion and resolution - key parameters which define the performance of the OCT. The calibration artefacts have been characterised using an optical microscope and tested on a swept source OCT. The results demonstrate that the femtosecond laser inscribed artefacts have the potential of quantitatively and qualitatively validating the performance of any OCT system.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing devices applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.