Comparison of cleaning performances of nanosecond and femtosecond lasers on historical papers

Properties of lasers are evolved with developing science and technology and this improved properties made usage areas of lasers wider. Interdisciplinary use of lasers is expanded by rapid and important development in optics. Ultrafast and fast lasers are used in nonlinear optics, biomedical imaging, surface ablation because of some properties of these lasers such as non-thermal effect and short repetition rate. These pulsed lasers are also used for conservation science due to properties they have. Pulsed lasers do not work as continuous wave lasers, they generate the light in pulsed form. Pulsed lasers are different then continues lasers because pulsed lasers have some characteristic features like pulse duration, pulse repetition rate frequency and peak power. These lasers are used for cutting, drilling, marking, surface restoration in industry. Heat transfer is minimized so pulsed lasers do not create nonrecoverable problems to processing material therefore, this feature made pulsed lasers more preferable. Nanosecond lasers produce pulses in nanosecond regime (1 ns = 10⁻9 s). These lasers have a special technique to generate pulses. This method is formed by putting some device which decreases the gain in laser resonator. The device works to reduce q factor. High q factor means low resonator loses in each rounds and the device which makes this procedure called 'Q - Switched' and the lasers are called 'Q - Switched laser'. There are few methods to generate of pulsed lasers with q-switching. Here are the three significant methods: mechanical shutters, electro-optic shutters and passive shutters. Femtosecond lasers generate pulses in femtosecond regime (1 fs = 10⁻15 s). There are some distributions of electromagnetic waves that have different frequencies in laser cavity. These distributions are named as 'mode'. There is also a special technique to make the mode in same phase is entitled as 'mode - locking'. The phases of all electromagnetic waves in the cavity are kept stable with mode - locking. Inside the laser cavities generally Hermite-Gauss or Laguerre-Gauss mode beams are generated. The fundamental mode of the two beams are called as TEM₀₀ and this mode is the most used mode in the applications. If an electromagnetic wave's transverse electric field and intensity distribution are in the form of Gauss, then this electromagnetic wave is called as Gauss beam Helmholtz equation solution with paraxial approximation gives Gauss beams. The region, which is efficient by higher than first order of power of the force of Electrical and magnetic field intensities, is called as non-linear optical region. The most fundamental part of the non-linear optics is generation of the second harmonic. Besides that, sum - frequency generation and difference - frequency generation, phase matching are some of the important topics. Femtosecond and nanosecond lasers are better than other options for surface ablation. It is hard to ablate a material surface and keep the process clean and proper. In the ablation process, a level of heat transmission is acceptable but determining this acceptable level is an exhausting job and this job requires preliminary work. A proper pulsed laser should be chosen according to physical, chemical properties and environmental conditions of the surface for best ablation. High energy capacity and short duration of interaction with the surface is the answer why pulsed lasers are preferred most for ablation. Existence of conservation science depends on preserving cultural heritage objects such as paper, painting, sculpture and lately, conservation provides benefit from science and technology. People who work in this job investigate reason of decomposition, decomposition mechanism of the material end new conservative methods. Using laser in this area is a consequence of searching a new method for cleaning of artifacts. Choosing a laser which is proper for removing extraneous substances gives amazing results compared to traditional methods, especially for paper samples have fragile structure. Preparing the samples is the first step of the experiments. Aim of the first step is to simulate Ottoman papers with sizing physically and chemically. Three differences sizing types are used; alum- egg sizing, corn starch sizing, wheat starch sizing. Every sized sample is glass burnished. Samples without sizing are also prepared. Two different contamination types are used for all pieces; graphite and graphite + kaolin mixture. This mixture contains graphite and kaolin with 1:1 ratio by mass. Glass burnish is used to improve the adhesion of the surface contamination. There is also a sample that is not glass burnished after contaminated. ASTM D 4714 ve TAPPI 544 cm −08 standard is preferred for ageing process. Two different experimental setups are used during this study. A Yb: Glass femtosecond laser has 1030 nm wavelength is used for a part of experiment. A x-y-z stage is used for both setup to move samples during the experiment. A cylindrical lens of focal length 25 mm is used in fs setup and the samples are moved by a computer to get 15 × 15 mm² clean areas. Pulsed energy is changed from 100 μJ to 200 μJ by 20 at a time. A Q switched system Nd:YAG laser has 1064 nm wavelength is used for the other setup. This laser has four different harmonics and two different lenses are used according to this harmonics. A diverging lens of focal length 1000 mm is used for fundamental and second harmonics. A converging lens of focal length 200 mm is used for third and fourth harmonic. Four different fluences are recorded for four different harmonics. These fluences are applied to the surfaces as 1 and 10 shots. Usage of a sizing type causes some thickening on sample surface so some new samples are prepared for determining this thickness approximately. Same papers are used for preparing for new samples. Investigate to own surface roughness of sample is the first step before specifying to affect of sizing about thickening. Burnishing process is applied to the surface and this makes sample surface more smooth than the original. Aim of burnishing implementation is demonstrate the sample roughness even if the surface exposed to burnishing. There is approximate ∼6μm differ between red and green line. Second step is determining of thickness which is created by sizing. Sizing is applied to a half of a the sample and other part of sample is kept clean. Profilometer (Veeco DEKTAK 150 Profilometer) measurements show ∼14μm difference between red and green line bordering on clean part and sizing part. According to graphic of this measurement there is some accumulation on border. Possible reason of the accumulation is applying sizing by a brush. ∼14μm thickness may the answer of what thickness of a sizing is. ∼14μm is not on the negligible level for effecting laser cleaning performance. Absorbance and transmittance values of sizing types are studied as a part of the experiment. A regular UV spectrometer (Agilent 8453) is used for determining how much of an incident light is absorbed or transmitted by a sizing type. The following result is well known: the structure of the paper with low light transmission starch will be get less damage. When the graphics of the results are examined, corn starch transmission is lower then the other starch kinds. Beside that, when the corn starch has the lowest transmission, transmission of alum egg starch is very high. Also color measurements support these results. With color measurement results, corn starched samples are cleaned best when compared with other samples. A simple spectrophotometer is applied to all cleaned surfaces and color changes measured by CIE L∗a∗b∗ color space. This color space is preferred for recording L∗, a∗ and b∗ values. A matlab code is written for comparing results with testimonial ones. 20x microscope images (Nikon, Eclipse, LV150L) of all samples which are cleared by femtosecond and nanosecond lasers with using different fluences, have been taken. The aim of this process is to investigate the fibre structure of the samples to see how much they are cleaned by the lasers. Decreasing clearing performance is obtained apparently with decreasing fluence for femtosecond laser. It is also same for the fundamental and second harmonic of the nanosecond laser. For the third and fourth harmonics of the nanosecond laser, the results looks like the same with fundamental and second harmonic. On the other hand, fibre structure of the samples are damaged, even if they are cleaned, because of the high energy. The deformation can be seen on surface of the samples caused by the broken fibre structure. Because of that, third and forth harmonics are not suitable to work with the samples which are used during the experiment. Beside these, the number of applying pulses are also important for nanosecond lasers. For each fluence, applying 10 shots gives better cleaning results than applying 1 shot. Consequently, femtosecond and nanosecond lasers are the best choice for the cleaning of artworks according to experiment results. These non-thermal pulsed lasers have to choose carefully considering various properties of paper. Sizing type, using burnishing or not, contamination type are all deterministic parameters for cleaning process. Using pulsed lasers for conservation is one of the important method to transfer paper artworks from generation to generation.