Laser ablation offers cleaning method for historical and art works effectively. Difference between ablation threshold of contamination layer and the surface yields to remove contaminants from surface without detriment to historical material. In particular, lasers with ultrashort pulse duration are very convenient for brittle historical papers, fibers of which should be intact after cleaning treatment. Since duration of laser irradiation and material interaction is very short, the possibility of damage to the paper is very low. One of the other crucial issues after paper cleaning treatment is color variation on the surface. Authentic color of the historical paper has to be preserved after the procedure. In this paper, we present results of paper cleaning via femtosecond (fs) laser running at a wavelength of 1030 nm. In the first stage of this experimental study, we determined optimum laser parameters on artificially soiled and aged paper samples, then cleaned a handwritten manuscript with ‘sizing’. In calligraphy, ornamentation and miniature arts, sizing is applied on paper as a protective layer which increases strength of paper and renders it more useful. Papers with sizing have been prevalently used in Islamic or Ottoman culture. We observed that after laser irradiation, artificial soling layer is effectively removed, while original color of the subs-layers did not alter. We used scanning electron microscopy (SEM) to examine fiber integrity, and determined that the sizing layer was not removed when optimized parameters were used, thus the fibers were not damaged.
We present a new method to form liquid-core optofluidic waveguides inside hydrophobic silica aerogels. Due to their
unique material properties, aerogels are very attractive for a wide variety of applications; however, it is very challenging
to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore,
there is a need to develop alternative processes for formation of complex structures within the aerogels without
damaging the material. In our study, we used focused femtosecond laser pulses for high-precision ablation of
hydrophobic silica aerogels. During the ablation, we directed the laser beam with a galvo-mirror system and,
subsequently, focused the beam through a scanning lens on the surface of bulk aerogel which was placed on a three-axis
translation stage. We succeeded in obtaining high-quality linear microchannels inside aerogel monoliths by
synchronizing the motion of the galvo-mirror scanner and the translation stage. Upon ablation, we created multimode
liquid-core optical waveguides by filling the empty channels inside low-refractive index aerogel blocks with highrefractive
index ethylene glycol. In order to demonstrate light guiding and measure optical attenuation of these
waveguides, we coupled light into the waveguides with an optical fiber and measured the intensity of transmitted light as
a function of the propagation distance inside the channel. The measured propagation losses of 9.9 dB/cm demonstrate the
potential of aerogel-based waveguides for efficient routing of light in optofluidic lightwave circuits.
The aim is to investigate femtosecond laser ablation as an alternative method for enamel etching used before bonding orthodontic brackets. A focused laser beam is scanned over enamel within the area of bonding in a saw tooth pattern with a varying number of lines. After patterning, ceramic brackets are bonded and bonding quality of the proposed technique is measured by a universal testing machine. The results are compared to the conventional acid etching method. Results show that bonding strength is a function of laser average power and the density of the ablated lines. Intrapulpal temperature changes are also recorded and observed minimal effects are observed. Enamel surface of the samples is investigated microscopically and no signs of damage or cracking are observed. In conclusion, femtosecond laser exposure on enamel surface yields controllable patterns that provide efficient bonding strength with less removal of dental tissue than conventional acid-etching technique.
We show that axicon-focusing of ultrashort laser pulses facilitates the generation of filaments with specific spatial
profiles in the form of nonlinear Bessel beams. For conditions ensuring that the energy arriving from each Bessel ring is
in quasi equilibrium with that absorbed by multiphoton processes, these Bessel filaments generate meter long plasma
channels whereas the same pulses focused by a standard lenses generate plasma channels which do not exceed a few
centimeters long. Measurements with different lasers show that the length and the homogeneity of the plasma channels
are enhanced by the use of large beams and sharp-tip (ideal) axicons whereas blunt-tip axicons induce a lens effect
leading to oscillations of the plasma density along the propagation axis.
We describe two techniques for measuring the complete spatio-temporal intensity and phase, E(x,y,z,t), of an ultrashort
pulse. The first technique is an experimentally simple and high-spectral resolution version of spectral interferometry,
which uses fiber optics to introduce the pulse that is to be characterized into the device. By scanning the fiber around the
focus, this device can be used to measure the spatio-temporal field of a focusing ultrashort pulse. We illustrate this technique by measuring the spatio-temporal filed for several different focused pulses. The other technique measures the complete spatio-temporal field of a pulse using a very simple experimental setup. While this technique will not work at the focus, it is single shot and requires only a single camera frame to reconstruct the complete filed versus space and time. This technique involves measuring multiple holograms, each at a different wavelength, and all in a single camera frame. To test this technique we show that it can accurately measure the spectral phase. We also illustrate this technique by measuring, E(x,y,t) of a single laser pulse.
We review the state of the art of ultrashort laser pulse characterization techniques. Two main methods will be mentioned: frequency-resolved optical gating (FROG) and spectral-phase interferometry for direct electric-field reconstruction (SPIDER). Basics of the techniques are introduced, and a comparison will be made on the pros and cons of both methods. We will then present some recent developments in the field of pulse characterization, including the development of an ultracompact and robust pulse characterization device-GRENOUILLE, the extension of pulse-measurement techniques into both time and space, and the measurement of extremely complex and extremely weak pulses.
We describe two novel, practical ultrashort laser pulse measurement devices, which are also experimentally very simple. The first one is an "ultra-broadband" pulse characterization device that is based on FROG, but uses transient grating (TG) process. TG FROG involves forming an induced grating in a piece of glass by crossing two pulses in space and time and then diffracting a third pulse off it to create a fourth diffracted pulse. The TG process is inherently very broadband and automatically phasematched. We have implemented an ultrasimple TG FROG device, which can also operate single-shot. First, three beams are created using a simple mask. Then, a cylindrical beams line-focuses the beams horizontally, where the induced grating is generated. The variation of the relative delay is achieved by crossing the two grating-creation beams at an angle using a Fresnel biprism. Then, by detecting the diffracted pulse with spatial resolution, the TG FROG trace is captured. The second device that we present aims to measure ultrashort pulses with complex spectral and temporal structure. Spectral interferometry (SI) works perfectly for this purpose. SI simply involves measuring the spectrum of the sum of the unknown (shaped) and known (reference) light waves. Unfortunately, SI is very difficult to align and maintain aligned, as it requires that the two beams be nearly perfectly collinear. We solved this problem by utilizing optical fibers. Spectral resolution is also significantly improved by using spatial fringes, avoiding time-domain filtering.
Ultrashort laser pulses are usually expressed in terms of the temporal and spectral dependences of their electric field. This approach disregards any couplings between the spatial coordinates and time and/or frequency. This assumption, however, often fails, as the generation and manipulation of ultrashort pulses require the introduction of spatio-temporal couplings. Furthermore, disregarding these couplings in ultrashort pulses also greatly limits the potential applications that could only be possible by exploiting the spatio-temporal behaviors. For these reasons, spatio-temporal couplings are receiving increased attention from researchers in recent years. Most of the work presented to date, however, focuses on a few particular couplings, lacking a general and rigorous analysis. We present a rigorous and mathematically elegant theory of first-order spatio-temporal distortions of Gaussian pulses and beams. We write pulses in four possible domains, xt, xω, kω, and kt, including the couplings. We identify couplings in intensity profiles as: pulse-front tilt, spatial dispersion, angular dispersion, and time vs. angle. We identify four new couplings that occur in phase: "wave-front rotation," "wave-front-tilt dispersion," "angular temporal chirp," and "angular frequency chirp." In addition, we provide normalized, dimensionless definitions for them, which range from -1 to 1. Finally, we show that for such parameters as pulse length, bandwidth, beam spot size and divergence angle, two separate definitions are required as "local" and "global" quantities, in presence of the couplings. Our approach completely determines the explicit relations between various spatio-temporal couplings in Gaussian pulses and beams. It can be generalized to arbitrary profiles by using computational analysis.
The couplings between the spatial coordinates and time and/or frequency are very common in ultrashort laser pulses. We previously showed that, the ultrashort pulse intensity and phase measurement devices, single-shot FROG and GRENOUILLE also measure some of the very common spatio-temporal distortions. Specifically, GRENOUILLE yields a sheared trace in frequency if the input pulse has spatial chirp. It also yields a trace shifted in delay, if the input pulse has pulse-front tilt. The shear and shift can also be used to measure the distortions. While this approach holds valid for relatively simple pulse, as the pulse gets more complicated, so does the effect of the spatio-temporal distortions. Therefore, we develop methods to extract the spatio-temporal distortions from GRENOUILLE traces, even for fairly complex pulses and distortions. First, we have developed a general model of GRENOUILLE for arbitrary spatio-temporal input beams. We then develop two algorithms to be run on distorted GRENOUILLE traces. The first perturbative algorithm is approximate, but is adequate for most cases where the spatio-temporal distortions are relatively small. The advantage of this perturbative approach is that it requires little modification to the existing FROG program, which is fast, reliable and robust. The second rigorous algorithm is numerically more complicated but is capable of accurately measuring the pulse intensity and phase and the spatio-temporal distortion parameters in more general cases. We tested this algorithm with several pulses that have various complexities and showed that this new algorithm retrieves the intensity and phase and the spatio-temporal distortions very accurately.
We show that GRENOUILLE, the experimentally simple version of frequency-resolved optical gating (FROG) can measure the two spatio-temporal distortions, namely spatial chirp and pulse-front tilt, in addition to the pulse intensity and phase. This is done without a single alteration in the experimental setup. Specifically, pulse-front tilt yields a displacement of the otherwise centered trace along the delay axis while spatial chirp causes a shear to the otherwise symmetrical GRENOUILLE trace. We develop a more general FROG pulse-retrieval algorithm based on the Levenberg-Marquardt algorithm, which can retrieve not only the pulse intensity and phase but also both the spatial chirp and pulse-front tilt from GRENOUILLE traces. Lastly, we also show that, by employing the exotic nonlinear crystal Proustite, GRENOUILLE can be extended to measure fiber-laser pulses with wavelengths near 1.5μm. The high nonlinearity of Proustite compensates for the lower output power of fiber lasers. Also Proustite has so high dispersion that it can spectrally resolve these relatively narrowband pulses. We experimentally test all of these innovations and obtain perfect agreement with the expected results.