Interactions of shaped laser beams with highly scattering media

When light propagates in a medium, different effects can emerge depending on properties of light and medium. One of these effects is light scattering and in some applications the scattering can play critical role. For instance, if scattering and beam seperation is reduced controlledly, new methods can be developed to investigate deeper in turbid media. Shaping of intensity profile of light beam can give better results for certain applications. Laser cavities produce beams with Gaussian profiles. A Gaussian beam propagating in media diffracts. The beam size almost does not change along the certain distance called as Rayleigh range. Rayleigh range varies inversely proportional to the square of the beam size. Therefore, when beam is focused to smaller size, it will diffract much faster and diffraction angle will be larger. In the last decades, there have been various developments to obtain "non-diffraction (or diffraction-free) beams" by virtue of spatially shaping transverse profiles of laser beams. Among the most commonly used nondiffraction beams is the Bessel beams. While propagating, their spot size do not change over long distances in comparison to Gaussian beams with the same spot sizes. Another intriguing nondiffraction beam type is Airy beam. The main lobe of Airy beam moves along a parabolic path instead of a straight line. Due to this exotic behaviour, Airy beams are usually called as "accelerating beams". In addition to, Bessel and Airy beams exhibit "self-healing" (self-reconstruction) characteristic. When the beams encounter an obstacle, they reproduce their original intensity profiles after brief propagation. We considered that these properties of shaped laser beams can reveal differences according to Gaussian beam when propagating in highly scattering media. Therefore, in the second chapter of the thesis, we experimentally investigated behaviour of shaped laser beams, propagating in scattering media. We generated Bessel and Airy beams by using appropriate optical elements. Liquid phantoms with variable density intralipid solutions were prepared as scattering media. Change in maximum intensities of the beams and self-healing of the profiles were compared. We observed that maximum intensities of Bessel and one dimensional Airy beams decrease slower with increasing scatterer density. Due to self-healing property, an energy flow occurs from rings to beam center for Bessel beam and from fringes to main lobe for Airy beam. However, in highly scattering media, the beam profiles of shaped beams deteriorate more than the profile of Gaussian beam. The highest deterioration was observed for Bessel beam. As occupied volume of the beam in scattering media increases, the beam will encounter more scatterers and the beam profile will deteriorate faster. Consequently, shaped beam can be used to obtain intense light deeper into highly scattering media. However, all properties of the beam in turbid media have to be considered according to purpose of application. Laser beam can be shaped by using spatial light modulator (SLM) as well as various optical elements. SLM based on liquid crystal technology controlling by applying a voltage. When a direct voltage is applied to molecule in liquid crystal, molecule changes orientation and then the index of refraction of this region of the crystal changes. Thereby, the phase of part of the beam passing through that region retards. In this chapter, laser beam shaping experiments were performed by transmissive SLM. To optimize the modulator, phase and intensity modulations were measured for chosen wavelength. Human tissue is a scattering medium for the light and both properties of tissue and light determine process of laser-tissue interaction. In the third chapter, laser-tissue inetraction mechanisms were summarized. In laser-tissue applications, appropriate laser parameters such as wavelength, power, pulse duration have to be used according to desired result. The fourth chapter of the thesis includes several biomedical applications about interaction between soft tissue and femtosecond pulsed laser. Ultrashort laser pulses can generate microbubbles in soft tissues. This interaction type is called as photodisruption. These microbubbles split tissue without the need for mechanical contact. In this study, firstly, microbubble patterns were formed in agar gel of solid phantom. This application used in ophthalmology field was also repeated for ovine cornea. After femtosecond laser exposure, a thin tissue layer could be removed from the surface of cornea. Furthermore, laser ablation experiment in agar gel was done by using Bessel beam. Since Bessel beams exhibit much longer compared to Rayleigh distances of focused Gaussian beams, surface of agar gel was ablated. Bessel beam starts from the end of axicon. According to results of restriction of Bessel zones experiments presented in fisrt chapter, arising of Bessel beam can be retarded by placing blocks in coming beam. Thus, laser ablation can occur inside the phantom not surface. Another studied topic in ophtalmology field is about ablation of intraocular lenses. If eye's natural lens is damaged by cataract or any reason, the natural lens is removed and an artificial lens called as intraocular lens is placed inside eye surgically. In general, intraocular lenses are produced from flexible, foldable and transparent polymer material. Accommodation ability of natural lens provides to view both distant and near objects. In other words, natural lens can change its shape and thus it can change optical power. However, when intraocular lens is implanted in patient's eye, patient needs glasses or contact lens for near vision. For this reason, multifocal intraocular lenses are developed to overcome this problem. Our purpose in the project is to convert a monofocal intraocular lens to multifocal through femtosecond laser ablation patterning in the form of Fresnel zone plates. A Fresnel zone plates is a circular diffraction grating. Consecutive transparent and opaque rings comprise Fresnel zone plates. In the first experiments, laser beam was scanned shaped like ring on the surface of contact lens by using scanning Galvo system. When light of wavelength 633 nm was passed through this patterned contact lens, we observed that the lens focused light at 47 mm as designed. However, this method could not be used for intraocular lens efficiently because of self-focusing effect. Therefore, in the next experiments, sample was moved in perpendicular plane to the incoming beam. The sample was placed on two dimensional stage controlled by computer. Thus, Fresnel zone plates were formed on the surface of intraocular lens in more controlable way. Furthermore, bovine teeth were etched via femtosecond laser as an example of hard tissue ablation and experimental results were also reported in the fifth chapter. Laser etching offers significant advantages as compared to conventional acid etching method. However, excessive increase in tissue temperature during laser irradiation can cause undesirable side effects such as microcracks. In these laser etching experiments, we used femtosecond laser system to minimize probability of heat diffusion to dental tissue. Therefore, intrapulpal temperatures were measured during laser treatments and measured the temperature increases were below the threshold for possible histological changes. In consequence of microscopic investigations, thermal damage or microcracks were not be observed on enamel surfaces of samples. Afterwards, brackets widely used in orthodontic treatments were bonded to the etched samples. Bond strength od each tooth sample was measured during debonding. Debonding force is the most significant parameter to evaluate achievement of etching process. For laser etching experiments using appropirate laser powers and ablation patterns, measured debonding forces were similar to acid etching method. Besides, after debonding process, it is prefered that no resin remain on enamel surface. In this study, quite clean surfaces were obtained after bracket debonding. The possible depth of etched enamel is ranging from 10 to 50 um according to literature. Average ablation depths measured in our study were 11 um and obtained maximum depth was 14.5 um. Therefore, this amount of enamel loss is definitely acceptable. In conclusion, femtosecond laser etching method is very suitable for enamel roughening and this method is fast, highly controllable and chemical or water-free dissimilar to classical acid etching method. In addition to, we tried to etch tooth surface using Bessel beam without laser beam scanning. We aimed to obtain ablation pattern composed of rings on the surface. We observed that the central spot of Bessel beam caused ablation but rings did not. Because energy of Bessel beam decreases from center to outer rings, thus the energies of rings are below ablation threshold of dental tissue. In the sixth chapter, we studied fs laser cleaning of historical papers as part of art conservation. Laser cleaning has certain advantages as compared to conventional cleaning methods. For instance, laser method is applied without contact and using chemical. Besides, it is more easier to control the process. Laser light can clean fragile paper artworks without damaging to their orginalities as long as proper laser parameters are used. In experiments, laser beam was shaped to bring to line focus. Therefore, sample surfaces were scanned in the shape of ablation lines. To determine appropriate scan speed, scan pitch between lines and laser power, we used artificially contaminated and aged paper models before cleaning original historical papers. Four paper types were soiled with two different contaminants and aged under same conditions. Since papers with sizing were used especially in Islamic handwritten documents, we also prepared papers with sizing. These prepared papers and historical papers were cleaned using various laser powers. Afterwards, color of all papers were measured. We observed that brightness of all the samples were enhanced as increasing laser power. On the other hand, other colors usually changed slightly for low laser powers. Minimal variations in colors are very important in respect to retain originality of artifact. Additionally, fiber structure of the paper sample should be undamaged after laser cleaning. Microscopic investigations demonstrated that fiber chains were not broken even when high laser powers were used. Also, we measured mechanical strengths of paper samples. In all cases, laser cleaning diminished mechanical strength inevitably. But this is acceptabable unless fibers do not be damaged. Consequently, fs laser cleaning is very effective technique in order to clean historical papers. The seventh chapter involves applications on metal machining. In the first part, we tried to decrease light scattering effect during metal drilling. Plasma formation and roughness on hole walls cause scattering of incoming light during laser ablation. Therefore, scattering leads to decrease intensity of light in hole and limits achievable hole depth. In this study, we used various methods in order to decrease scattering effect and obtain deep holes on copper metal during femtosecond laser ablation. Firstly, to decrease roughness of hole wall, copper sample was moved during laser machining. However, we observed that there was no difference between depths of moving and fixed samples from profile measurements. Secondly, holes were drilled by means of circularly polarized laser light. These holes were lesser deeper than the holes fabricated by linearly polarized laser light. Thirdly, two fs laser beams that have different wavelengths were used together during ablation process. But, deeper holes were not obtained. Finally, incoming laser beam was divided into two beams by Michelson interferometer and these beams were focused on the metal surface. Time difference between two pulse trains was changed from 200 fs to 66.7 ps. We observed hole depths depends on separation of pulses. In the second part of the seventh chapter, micro and nanoscale circular structures were formed on gold thin films using Bessel-vortex laser pulses. Since Bessel beams have long focal distances, micromachining with these beams is easier than Gaussian beams due to convenience in sample alignment. We showed that structure size depends on laser power. Also, ablation profiles depend on laser power. In other words, single or multiple rings can be obtained via adjusting laser power.