Surface cleaning and glass processing studies with cylindrical focused laser beam

Today, optics and photonics constitute the principle structure of technology. Rapid and great growths in optical industry has led to the extensive use of lasers. In this respect, femtosecond (fs) lasers have been gaining importance in laser technology since many scientific and technological applications requires a laser system running at a particular wavelength and producing short pulses. Precision metrological applications, measurement of photochemical processes, biological display, nonlinear optics, cleaning processes and micromachining are between the uses of fs lasers. In particular, surface cleaning and glass processing studies with fs laser beams have high potential to develop in optical issues. Femtosecond lasers have ultrashort laser pulses which reduces used laser energy, thus, thermal and mechanical damages in peripheral structures are minimized. Another advantage of the ultrafast laser beams is that it can be used on various types of materials from transparent substances to reflective ones. Besides, when the infrared beams are focused into a line by a cylindrical lens, it is possible to write many points at a time. Therefore, in this study cylindrical focused fs laser beams will be used in order to ablate contamination on historical papers precisely and to write Bragg gratings in the pure glass. The main concern of conservation of the historical arts is the cleaning process. The written sources that have historical, artistic and religious significance shows the way to the unknown past. Paper is the basic material of the written sources. In order to save the breakable organic materials like the old paper arts without changing the colour and the mechanical stability, the contamination should be cleaned efficiently. Ultrafast lasers have been quite attractive for cleaning historical arts since it is contactless and clean process. In this process, there is no need to use any solvent and chemical, it can work with micro sensitivity and it is possible to observe the cleaning process simultaneously. Controlling the parameters like the wavelength and the power of the laser light as well as the spatial beam profile determines the efficiency of the conservation. In the first part of the study, both historical and artificial paper samples were cleaned by using femtosecond pulsed laser. The laser source was Yb: Glass femtosecond laser at central wavelength of 1030 nm. The samples were placed on the micrometer-precision stage which can move in three dimensions. Gaussian laser beams were focused on the surface of sample by using cylindrical lens. For effective cleaning without changing the original colour of sample and damaging the surface, optimum parameters were determined by changing the laser power, speed of stage, step distance of stage and focal length of the cylindrical lens. In recent years, the fabrication of optical waveguides as well as buried reflective gratings in transparent materials and generating three dimensional photonic devices are required for many applications in optics, also. By writing fiber Bragg gratings in the glass core of the fiber with firmly focused laser beams, new optical devices can be generated successfully. Many investigations of generating Bragg Gratings in glasses by UV beams have been handled before. However, laser damage by fs laser beams has received little attention because of its low photon energy. The development of high energy density ultrashort pulse lasers, however, has encouraged the researches of the potential for inducing multiphoton reactions and optical devices in glass. In the second part of the study, the technique of Bragg grating inscription in glass and optical fibers with ultrafast infrared radiation and cylindrical lenses has proven to be as simple as the traditional UV laser grating writing techniques. The physics behind the FBGs mechanism is that if a glass sample were heated by intense laser beams, the absorption coefficient would increase due to multiphoton interactions and ionization. Therefore optical properties such as refractive index of the glass structure will change. The laser-induced refractive-index change is the base for FBG fabrication since most of the optical fibers have glass cores. Fabricated transparent damage lines inside the glass fiber core will reflect a specific wavelength: Bragg wavelength. In addition, the period of the grating structures will shift when any change occurs in the temperature or the strain. By this way, Bragg gratings inscribed in the glass fibers can be used to measure optical properties and constants by analysing the Bragg wavelength. Therefore, FBGs' optical, thermal and mechanical properties makes them effective for sensing, storing and nonlinear applications. At present, the UV technologies have been well established to fabricate refractive ındex modulations in samples, however, their application is restricted to the photosensitivity of glass. In order to increase the UV photosensitivity of the sample extra processes like germania dopant or hydrogen photosensitization may be required. Another point is that in thermal applications high temperatures may cause any erasure in the UV-induced refractive index modulation. However, in recent years, a technique based on ultrafast infrared laser pulses has been demonstrated. The fs inscription method for FBGs goes beyond the limitation of the UV photosensitivity. The interaction between ultrashort pulses and transparent materials results in a refractive index change without the need of any impurities in the optical host. When femtosecond laser pulses are tightly focused into a bulk transparent material, nonlinear absorption in the focal volume will take place, leading to optical breakdown and microplasma formation. This infrared radiation induced visible, transparent structures are also stable at high temperatures. Thus, it can be said that fs Bragg grating inscription methods in optical fibers are more versatile than excimer laser ones. This new situation has made it necessary to deeply investigate and properly characterize the devices made using the infrared laser beam fabrication technology. With the goal of being able to create optical devices for both optical and photonic innovations it is appropriate to explore the effects of femtosecond laser radiation in the glass and fiber optics. Consequently, a 1030 nm Yb: Glass laser that emits 550-fs, 1-kHz pulses and delivers an average power of 75 mW was used for the glass processing study. The beam was focused through 25mm and 75 mm cylindrical lenses and injected into 1 mm thickness glass and 60μm diameter fiber core. The average energy of the laser beam at the sample location was 75 μJ. With the help of a micro precision XYZ stage, the samples were moved in three dimensions. The exposure time was controlled with a PC connected shutter and it was 100 ms for every gratings. By focusing the laser beam through cylindrical lenses, we successfully wrote transparent and periodic Bragg gratings inside glasses and optical fiber cores. In order to characterize the structures on the glass Mach Zehnder interferometer was used. And Ava Spec 3648 fiber optic spectrometer was set up for the characterization measurements of fiber Bragg gratings. Spectrometric analysis results of the structures both in the glass and the fiber core were compared with the theoretical expectations. Our research has demonstrated that surface cleaning of historical papers by astigmatic focused femtosecond laser beams gives good results. Furthermore, writing stable Bragg gratings by creating interference in the fiber core with ultrashort pulses can be applied easily compared with the standard UV laser.