Carlos J. Román-Moreno

Aberration effects on femtosecond pulses generated by nonideal achromatic doublets

There are three main effects that affect the femtosecond pulse focusing process near the focal plane of a refractive lens: the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper we study in detail these effects generated by nonideal achromatic doublets based on a Fourier-optical analysis and Seidel aberration theory considering lens material, wavelength range, lens surface design, and temporally and spatially uniform and Gaussian intensity distributions. We show that the residual chromatic aberration in achromatic lenses, which has been neglected so far, has a considerable effect on the focusing of pulses shorter than 20 fs in the spectral range between the UV and IR, 300 to 1100 nm, and is particularly important in the blue and UV spectral range. We present a general fitted function for an estimation of the pulse stretching parameter, which depends only on the numerical aperture and focal length of the doublet as well as the wavelength of the carrier of the pulse.

Analytical method for calculating the electric field envelope of ultrashort pulses by approximating the wavenumber up to third order

For optical pulses shorter than 20 fs duration or highly dispersive materials in the visible range of the spectrum, high-order terms in the Taylor expansion for the wave vector, around the carrier frequency, should be considered. By expanding the wave vector near the center of optical frequency ω0 in a Taylor series up to the third-order approximation, we present an analytical method for calculating the electric field envelope of a pulse after it has propagated through a medium that contributes second- and third-order group velocity dispersion. To verify the method we present some examples for both 20 and 15 fs pulses propagating through pieces of glass made of low and high dispersive material. Limitations of the method are discussed.

Achromatic doublets using group indices of refraction

One main function of short pulses is to concentrate energy in time and space [1]. The use of refractive lenses allows us to concentrate energy in a small volume of focusing around the focal point of the lens. When using refractive lenses, there are three effects that affect the concentration of energy around the focal point of the lens. These are the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper, we study lenses which are diffraction limited so that the monochromatic aberrations are negligible; the group velocity dispersion and the propagation time difference are the main effects affecting the spreading of the pulse at the focus. We will show that for 100-fs pulses the spatial spreading is larger than the temporal spreading of the pulse. It is already known that the effect of spatial spreading of the pulse due to PTD can be reduced by using achromatic optics. We use the theory proposed by A. Vaughan to analyze simple lenses and normal achromatic doublets, where normal means doublets that we can buy from catalogs. We then use the Vaughan theory to design achromatic doublets in phase and group, which produce no spatial spreading of the pulse, i.e., PTD = 0, when the doublet is designed for the carrier of the pulse. We compare these phase and group achromatic doublets with normal achromatic doublets. Finally, we show that apochromatic optics can give a much better correction of PTD than using normal achromatic doublets.

Direct inversion methods for spectral amplitude modulation of femtosecond pulses

In the present work, we applied an amplitude-spatial light modulator to shape the spectral amplitude of femtosecond pulses in a single step, without an iterative algorithm, by using an inversion method defined as the generalized retardance function. Additionally, we also present a single step method to shape the intensity profile defined as the influence matrix. Numerical and experimental results are presented for both methods.

Chlorine-Substituted Bent-Core LC-Based Sono-Gel Hybrid Materials: Synthesis and Optical Properties

We implemented the catalyst-free sonogel method to disperse and encapsulate a chlorine substituted thermotropic bent shaped liquid crystalline compound. By this means, advanced organic-inorganic hybrid materials with high optical and mechanical quality, suitable for different kind of optical applications, were obtained. Bent-core molecules have shown interesting optical properties, which have not yet been investigated in the solid-state. Therefore, we implemented the sonogel route to fabricate highly pure SiO2 porous glassy networks which allowed the inclusion of this kind of mesogens in the colloidal sol-state. In this work, we present the preparation and optical performance of these amorphous “banana”-based hybrids via absorption and fluorescence spectroscopies, electro-optical and cubic third-harmonic generation nonlinear optical measurements.

Review of applications of medical lasers

Since the invention of lasers almost thirty years ago, physicians and researchers have worked in an increasing number of applications in medicine. The physical characteristics of lasers such as monochromaticity, coherence, and directionality, identifies them as unique light sources. A review of the main applications of medical lasers is presented. We present applications with photothermal effects with gas lasers, photoablation with solid lasers, photochemical effects with tuned lasers and recently, pulsed laser in photo-biology and tomography. A summary of applications is listed and a partial list of references is given.

Webcam autofocus mechanism used as a delay line for the characterization of femtosecond pulses

In this work, we present an electromagnetic focusing mechanism (EFM), from a commercial webcam, implemented as a delay line of a femtosecond laser pulse characterization system. The characterization system consists on a second order autocorrelator based on a two-photon-absorption detection. The results presented here were performed for two different home-made femtosecond oscillators: Ti:sapph @ 820 nm and highly chirped pulses generated with an Erbium Doped Fiber @ 1550 nm. The EFM applied as a delay line represents an excellent alternative due its performance in terms of stability, resolution, and long scan range up to 3 ps. Due its low power consumption, the device can be connected through the Universal Serial Bus (USB) port. Details of components, schematics of electronic controls, and detection systems are presented.

A high resolution hand-held focused beam profiler

The shape of a beam is important in any laser application and depending on the final implementation, there exists a preferred one which is defined by the irradiance distribution.1 The energy distribution (or laser beam profile) is an important parameter in a focused beam, for instance, in laser cut industry, where the beam shape determines the quality of the cut. In terms of alignment and focusing, the energy distribution also plays an important role since the system must be configured in order to reduce the aberration effects and achieve the highest intensity. Nowadays a beam profiler is used in both industry and research laboratories with the aim to characterize laser beams used in free-space communications, focusing and welding, among other systems. The purpose of the profile analyzers is to know the main parameters of the beam, to control its characteristics as uniformity, shape and beam size as a guide to align the focusing system. In this work is presented a high resolution hand-held and compact design of a beam profiler capable to measure at the focal plane, with covered range from 400 nm to 1000 nm. The detection is reached with a CMOS sensor sized in 3673.6 μm x 2738.4 μm which acquire a snap shot of the previously attenuated focused beam to avoid the sensor damage, the result is an image of beam intensity distribution, which is digitally processed with a RaspberryTMmodule gathering significant parameters such as beam waist, centroid, uniformity and also some aberrations. The profiler resolution is 1.4 μm and was probed and validated in three different focusing systems. The spot sizes measurements were compared with the Foucault knife-edge test.

Shack-Hartmann wavefront sensor using a Raspberry Pi embedded system

In this work we present the design and manufacture of a compact Shack-Hartmann wavefront sensor using a Raspberry Pi and a microlens array. The main goal of this sensor is to recover the wavefront of a laser beam and to characterize its spatial phase using a simple and compact Raspberry Pi and the Raspberry Pi embedded camera. The recovery algorithm is based on a modified version of the Southwell method and was written in Python as well as its user interface. Experimental results and reconstructed wavefronts are presented.

Mode coupling enhancement by astigmatism compensation in a femtosecond laser cavity

In this work we present a numerical analysis of the mode coupling between the pump-beam and the laser-beam in a Ti:Sapphire crystal used as a gain medium of a femtosecond laser. Using the Matrix ABCD and propagation gaussian beam models, we obtained an optimal configuration for compensate the astigmatism in the output beam laser. Also we analysed pump-beam propagation and got the settings to fix the astigmatism in the crystal. Furthermore we apply this configuration to a homemade femtosecond laser, accomplishing an overall efficiency of laser to 20% in continuum wave (CW) and 16% in mode looking (ML) operation. The femtosecond laser have 30 nm bandwidth to FWHM at 810 nm corresponding 30fs.