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We present a Silicon-based Charge-Coupled Device (Si-CCD) sensor applied as a cost-effective spectrometer for femtosecond pulse characterization in the Near Infrared region in two different configurations: two-Fourier and Czerny–Turner setups. To test the spectrometer’s performance, a femtosecond Optical Parametric Oscillator with a tuning range between 1100 and 1700 nm and a femtosecond Erbium-Doped Fiber Amplifier at 1582 nm were employed. The nonlinear spectrometer operation is based on the Two-Photon Absorption effect generated in the Si-CCD sensor. The achieved spectrometer resolution was 0.6 ± 0.1 nm with a threshold peak intensity of 2×106Wcm2. An analysis of the nonlinear response as a function of the wavelength, the response saturation, and the criteria to prevent it are also presented.

The transmission properties of a photonic crystal immersed in several different oils have been characterized using terahertz time domain spectroscopy in the spectral range of 0.3–1.5 THz. As in previous works, oil samples can be distinguished using terahertz transmission measurements. When the same oils are introduced into a photonic crystal, we find that the effective refractive index of the photonic crystal is sensitive to the properties of the oils and shows differences not seen in bulk measurements. These effects are described in detail and have potential applications in both the sensing of very small volumes of oils and in the fine control of the refractive indices of photonic crystals.

When a low-power, monochromatic Gaussian beam is focused by a thin lens in air and the waist of the beam is in the plane of the lens, there is a shift of the focus position if the waist of the beam is much smaller than the size of the lens. The point of maximum intensity relative to the geometrical focal point shifts closer to the lens. We show that for ultra-intense light beams, when the Kerr effect is unavoidable, there is a nonlinear focal shift. The nonlinear focus position shifts closer to the lens for laser powers below the critical power. To avoid the nonlinear focal shift below the critical power, the correct combination of Gaussian beam waist and focal system has to be used in the experimental setup. It will be shown that as the Fresnel number N w associated with the Gaussian beam radius increases, the nonlinear focal shift first increases and then begins to decrease.

In this work we theoretically confirm that the constant term of the measured signal of the Fringe-Resolved Autocorrelation technique using a two-photon absorption photodiode as sensor provides the same information as the intensity autocorrelation amplitude obtained from this technique. We achieve this result by computing the temporal intensity pulse distributions of focused femtosecond pulses around the focal region of an aberrated system and calculating both the intensity autocorrelation and the quadratic interferometric autocorrelation equation. Pulses with initial Full-Width Half-Maximum pulse widths of 50.0 and 20.0 [fs] were focused with a BK7 double convex lens.

Plasmonic photothermal therapy (PPTT), as an increasingly studied treatment alternative, has been widely regarded mostly as a surface tissue treatment choice. Although some techniques have been implemented for interstitial tumors, these involve some grade of invasiveness, as the outer skin is usually broken to introduce light-delivering optical fibers or even catheters. In this work, we present a potential non-invasive strategy using the stereotactic approach, long employed in radiosurgery, by converging multiple near infrared laser beams for PPTT in tissue-equivalent optical phantoms that enclose small gel spheres and simulate interstitial tissue impregnated with plasmonic nanoparticles. The real-time in-depth monitoring of temperature increase is realized by an infrared camera face-on mounted over the phantom. Our results show that a significant reduction in the surface heating can be achieved with this configuration while remarkably increasing the interstitial reach of PPTT, assuring a ∼ 6 ∘ C temperature increase for the simulated tumors at 10 mm depth and ∼ 4 ∘ C at 15 mm depth and opening up new possibilities for future clinical applications.

We have developed a continuous wave sub-wavelength terahertz (THz) imaging system that combines two prominent classical optical techniques: solid immersion microscopy and interferometric detection. This combination allows for resolution beyond the diffraction limit at 703 GHz. We experimentally demonstrate sub-wavelength spatial resolution working with a relatively low-cost pyroelectric detector and with both high and low contrast samples.

We present a new method to calculate the complex refractive index of spherical scatterers in a novel optical phantom developed by using homemade monodisperse silica nanospheres embedded into a polyester resin matrix and an ethanol–water mixture for applications in diffuse imaging. The spherical geometry of these nanoparticles makes them suitable for direct comparison between the values of the absorption and reduced scattering coefficients ( μ a and μ s ′ , respectively) obtained by the diffusion approximation solution to the transport equation from scattering measurements and those obtained by the Mie solution to Maxwell’s equations. The values of the optical properties can be obtained by measuring, using an ultrafast detector, the time-resolved intensity distribution profiles of diffuse light transmitted through a thick slab of the silica nanosphere phantom, and by fitting them to the time-dependent diffusion approximation solution to the transport equation. These values can also be obtained by Mie solutions for spherical particles when their physical properties and size are known. By using scanning electron microscopy, we measured the size of these nanospheres, and the numerical results of μ a and μ s ′ can then be inferred by calculating the absorption and scattering efficiencies. Then we propose a numerical interval for the imaginary part of the complex refractive index of S i O 2 nanospheres, n s , which is estimated by fixing the fitted values of μ a and μ s ′ , using the known value of the real part of n s , and finding the corresponding value of I m ( n s ) that matches the optical parameters obtained by both methods finding values close to those reported for silica glass. This opens the possibility of producing optical phantoms with scattering and absorption properties that can be predicted and designed from precise knowledge of the physical characteristics of their constituents from a microscopic point of view.

In this work, we present a CMOS sensor implemented for 2D-temporal characterization of femtosecond pulses, based on the Two Photon Absorption (TPA) process. We demonstrate the CMOS camera TPA capability to measure the Fringe Resolved Autocorrelation (FRAC).

In this work, the DC two-photon absorption signal offset ( D C T P A ) and the amplitude of the autocorrelation ( A A C ) are measured simultaneously around the focal point of an apochromatic microscope objective using the z-scan autocorrelation technique. The A A C is obtained from the nonlinear sensor response given by the two-photon-absorption, generated in a GaAsP photodiode, for femtosecond laser pulses. We verify that the change in the D C T P A signal along z is coincident with the amplitude of the intensity autocorrelation, and that the highest amplitude of the AC is reached at the same position as the highest amplitude of the D C T P A signal. The D C T P A signal is typically used as a reference for the alignment in a collinear intensity autocorrelator, and we show that it can also be used as a practical procedure to estimate the depth of focus. The D C T P A signal measurement allows us to locate the optimum spatial-temporal coupling given by the highest amplitude of the intensity autocorrelation. Additionally, we find a variation in the pulse duration within the same region due to the radially varying group delay dispersion.

Optical-coherence tomography (OCT) is a technique that employs light in order to measure the internal structure of semitransparent, e.g. biological, samples. It is based on the interference pattern of low-coherence light. Quantum-OCT (QOCT), instead, employs the correlation properties of entangled photon pairs, for example, generated by the process of spontaneous parametric downconversion (SPDC). The usual QOCT scheme uses photon pairs characterised by a joint-spectral amplitude with strict spectral anti-correlations. It has been shown that, in contrast with its classical counterpart, QOCT provides resolution enhancement and dispersion cancellation. In this paper, we revisit the theory of QOCT and extend the theoretical model so as to include photon pairs with arbitrary spectral correlations. We present experimental results that complement the theory and explain the physical underpinnings appearing in the interference pattern. In our experiment, we utilize a pump for the SPDC process ranging from continuous wave to pulsed in the femtosecond regime, and show that cross-correlation interference effects appearing for each pair of layers may be directly suppressed for a sufficiently large pump bandwidth. Our results provide insights and strategies that could guide practical implementations of QOCT.

We describe an implementation of continuous wave microscopy in the millimeter - terahertz wave region in with phase imaging is realized using a simple low cost detection scheme. Samples are illuminated using a Backward Wave Oscillator system and a detection scheme is presented in which soft or semitransparent samples are imaged in reflection or transmission using an interferometer. The main advantage of this approach is that simple pyroelectric detectors can be used and can in principle be extended to use in near field measurements.

In this paper, the temporal and spatial intensity pulse distributions are calculated around the focal region of an optical system using a combination of ray tracing and a wave propagation method. We analyze how to measure the width of the intensity pulse distributions to estimate pulse duration and spot size in order to study the impact of the variation of spherical aberration with frequency in a pulse on the intensity distributions. Two experimental techniques used in the laboratory are also modeled: the knife-edge test to measure spatial distribution and the intensity autocorrelation technique to measure the temporal distribution. We use two measuring criteria, the full-width half-maximum (FWHM) and standard deviation ( σ ), to compare the spatial and temporal intensity distributions of the calculated diffraction patterns and those obtained from the simulated experimental techniques. We show that the FWHM is not a good criterion, since it gives different results in the measured intensity distributions in time and space when they are measured directly from the theoretical modeling and when they are measured from the modeled experimental techniques used in the laboratory. The standard deviation, however, is a consistent criterion, giving the same results for the calculated intensity distributions and the modeled experiments.

We have developed an algorithm to filter the noise in the spectral intensity of ultrashort laser pulses. The filtering procedure consists of smoothing the noise by using the Savitzky–Golay filter, removing the offset, and using a super-Gaussian window to truncate the frequencies of the spectrum. We have modeled bandwidth-limited ultrashort pulses with Gaussian modulated frequencies to show the estimation of the carrier wavelength, reconstruction of the intensity pulse profile, and pulse duration after applying the algorithm. Theoretical results are presented for pulse durations between 5 fs and 100 fs with a carrier wavelength of 825 nm and three different amounts of signal-to-noise ratio (SNR): 30 dB, 20 dB, and 15 dB, normally found in experiments. The algorithm is also applied to an experimental spectral intensity from a homemade Ti:sapphire laser that produces pulses of about 20 fs at 825 nm at 100 MHz. We will show that using only a low-pass Fourier filter and removing offset is not enough to recover the spectral intensity when a large SNR is present, which may be the case when the ultrashort laser beam has been manipulated to compensate for the group velocity dispersion of an external optical system. In cases like this, the use of the Savitzky–Golay filter prior to the super-Gaussian filter improves the recovery of the carrier wavelength and the spectral intensity. We will also show that the algorithm presented in this paper is suitable for experimental analysis and requires limited user intervention.

Robust coupling between a pulsed laser beam and a photoelectric circuit is an important issue in the development of miniaturized, integrated, and embedded terahertz instrumentation. Here, we present a study of the effect of varying the focus and alignment parameters of an excitation laser pulse on the emission characteristics of a standard Hertzian- dipole type terahertz photoelectric source. The objective is to quantify the tolerance of a terahertz time-domain spectroscopy system, and we study the variation of peak amplitude, waveform, phase, and energy distribution as a function of excitation position and defocus. We find that a terahertz source can be made relatively tolerant to variations in focus, alignment, and details of the geometry of the photoelectric system, providing a window for a more robust field operation.

The effect of Seidel aberrations on the focusing of ultrashort optical pulses by a perfectly conducting spherical mirror, which is a dispersionless system, is presented in this paper showing that aberrations change the pulse duration.

In this work we present an Autocorrelation z-scan technique to measure, simultaneously, the spatial and temporal distribution of femtosecond pulses near the focal region of lenses. A second-order collinear autocorrelator is implemented before the lens under test to estimate the pulse width. Signals are obtained by translating a Two Photon Absorption (TPA) sensor along the optical axis and by measuring the second-order autocorrelation trace at each position z. The DC signal, which is typically not considered important, is taken into account since we have found that this signal provides relevant information. Experimental results are presented for different lenses and input wavefronts.

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.

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.

In this work, we define the efficiency signal conversion numerical parameter, Veff, useful to evaluate the operation efficiency of femtosecond-Optical Parametric Oscillator (fs-OPO) cavities considering the astigmatism effect. For the validation of the Veff, we have performed experimental measurements. We present different high efficiency home-made singly resonant fs-OPO cavities, with signal tuneability from 1.1 µm to 1.6 µm based on a 0.5 mm Periodically Poled Lithium Niobate doped with MgO (MgO:PPLN) crystal. We have also defined the pump energy threshold per crystal unit length, ζp,th. Pump threshold, achieved by following the Veff, was 142 mW at 810 nm, and ζp,th = 2.10 nJ/mm, the lowest value, in comparison with other studies. The Veff is based on an ABCD matrix Gaussian beam propagation method, which calculates the mode coupling between the pump and signal beams along the crystal under different cavity configurations taking into account the astigmatism. The model was compared and tested with 3 different experimental singly resonant fs-OPO ring cavity configurations that we have defined as single-folded, two-folded, and direct-pump cavity.

We present the construction of a femtosecond laser, based on a titanium sapphire crystal. This laser produce pulses of 20 fs. We also present theoretical results for the electric field distribution near the focal plane of a lens for gaussian illumination under the influence of primary aberrations: spherical aberrations, coma, astigmatism and field curvature, for an achromatic doublet. The theoretical results are compared with results obtained with the laser system constructed.

In this work we analyze the spatio-temporal intensity of sub-20 fs pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achro­matic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). For sub-20 fs pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect.

We calculate the electric field of few-optical-cycle pulses at the focus of a perfectly conducting mirror by adding coherently the Airy diffraction patterns for each pulse frequency. We will show that the pulse suffers temporal spreading generated by a change in the spectrum of the pulse as a function of position in the focal plane, which introduces spatial chirp to the pulse. A double pulse appears near to the diffraction minima of the carrier frequency due to the variations in the spectra.

In this work we present a method used to study the spherical and chromatic aberrations contribution near the focal point of a refractive optical system. The actual focal position is measured by scanning a pinhole attached on the front of a power detector, which are scanned along the optical axis using a motorized stage with 1 μm resolution. Spherical aberration contribution was analyzed by changing the pupil aperture, by modifying the size of the input iris diaphragm and for each case, measuring the actual laser power vs the detector position. Chromatic aberration was analyzed by performing the same procedure but in this case we used an ultra-broad-band femtosecond laser. The results between ML and CW operation were compare. Experimental results are presented.

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.

We propose the design of a new technique for measuring the spectral resolution of a Czerny-Turner Spectrometer based on spectral interferometry of ultrashort laser pulses. It is well known that ultrashort pulse measurement like SPIDER and TADPOLE techniques requires a precise and well characterized spectrum, especially in fringe resolution. We developed a new technique, to our knowledge, in which by measuring the nominal fringe spacing of a spectral interferogram one can characterize the spectral resolution in a Czerny-Turner spectrometer using Ryleigh’s criteria. This technique was tested in a commercial Czerny-Turner spectrometer. The results demonstrate a consistent spectral resolution between what was reported by the manufacturer. The actual calibration technique was applied in a homemade broadband astigmatism-free Czerny-Turner spectrometer. Theory and experimental results are presented.

Three numerical methods for the design of Kerr Lens Mode-Locking (KLML) ultrashort pulse cavities that use a solid state Brewster-cut nonlinear gain medium are compared. The nonlinear medium is modeled first deploying a matrix approximation that considers non-coupled (tangential analysis is independent of sagittal analysis) Kerr and thermal self-focusing; and second with a differential equation that relates the real and imaginary parts of the inverse of the complex Gaussian beam parameter. The third comparison is against a matrix analysis method that considers the coupling between the sagittal and tangential modes inside the nonlinear medium in order to determine the impact of this effect. The three methods search the self-consistency condition for the complex beam parameter and the results are compared.

We present a simple low-coherence time-domain interferometric reflectometer with a rapidly scanning optical delay line (RSODL) based on a non-parallel diffraction grating (DG) pair. The novelty of the solution is that the lightwave in the reference channel is focused on a galvo-mirror in a sub-mm static spot, which allows implementation of fast microelectromechanical systems scan optics. It is shown that the DG pair can be operated as a non-dispersive element that provides dynamic group delay of a reference lightwave. The DG pair system is also capable of tuning the RSODL dispersion from negative to positive values. The experimental depth range in air was obtained as large as 2.5 mm for axial resolution of 20 μm.

The general equation for third-order group velocity dispersion is derived in this paper, which contains all the terms of equation (11) in Arissian and Diels [1], as well as one extra term, which we have verified is not negligible. To verify our equation we have modeled the sum of Gaussian modulated frequencies. We show the electric field of bandwidth limited incident ultrashort pulses with a time duration of: 5 fs, 10 fs, 15 fs, 20 fs, 30 fs and 50 fs after propagating through a pair of identical isosceles prisms for a central frequency of 0.8 µm and 0.620 µm. The model is applied by using (a) the finite phase, (b) the phase approximated up to the second-order dispersion and (c) up to the third-order dispersion for each frequency component of the pulse. Graphs are presented for prisms made with fused silica and SF14.

Ultrafast electron dynamics in 5 nm thick gold films have been investigated as a function of film morphology. The transient reflectivity data reveal a sub-100 fs rise time and a complex relaxation profile on the picosecond timescale. These data are discussed in terms of a modified two-temperature model of the electron dynamics accounting for the initial relaxation of the non-thermal population. The electron–phonon coupling may be sensitive to film morphology. The possible origin of this behaviour is discussed and results are placed in the context of enhanced photochemistry on thin metal films.

The effect of both nonthermal and thermal hot electron populations on the ultrafast transient reflectivity of thin and ultrathin gold films has been considered. The hot electron dynamics are calculated based on the recent model of Carpene [E. Carpene, Physical Review B 74 (2006) 024301], modified for film thickness phenomena. The effect of laser fluence, film thickness, probe wavelength and metal band structure are all considered. The consequences of the model for calculating rates of hot electron attachment to adsorbates are discussed. Finally the predictions are compared with experimental data.

A detailed stability analysis of a Kerr Lens Mode Locked cavity-dumped Ti:Sapphire laser based on a non symmetrical two-folded cavity design is presented. Pulse intensity stability was measured both for the oscillator alone and cavity dumped sources. Stability conditions were compared with a numerical model based on an ABCD matrix Gaussian beam propagation method. The continuous wave and pulsed operation regions were well predicted as was the hard aperture position for mode locking.

We describe an improved instrument for measuring at video rate (30 frames per second) the second-harmonic frequency-resolved optical gating trace of femtosecond pulses from a mode-locked laser oscillator. The system comprises separate scanning acquisition and pulse retrieval elements which together enable the exact pulse profile to be viewed in real time with a typical refresh rate of 1 Hz. Details are given of the optical system used, the electronic synchronization circuits and the acquisition and retrieval software employed.

We show experimentally and theoretically that the cavity design plays an important role in achieving stable bi-directional operation in a mode locked ring laser. We developed a numerical model for a bi-directional Kerr Lens Mode Locked (KLML) Ti:Sapphire ring laser. We verified the model by comparing its results with three ring laser cavities, which we constructed in the laboratory. The three ring lasers were a standard x-cavity with two curved mirrors, a cavity with four curved mirrors, and a cavity with four curved mirrors that included an additional nonlinear element. We show that this model can predict conditions for the bi-directional operation. Our model predicts, as our experiments show that the presence of an extra fold in the four-mirror ring cavity permits a more stable bi-directional mode locked operation. Furthermore, our model helps us to understand why addition of an extra fold in the cavity could improve the stability of mode locked operation. In general, the repetition rate of the laser in the two opposite directions was different, and depended on the position of the laser rod. We further experimentally show that, this difference can be eliminated if another Kerr element is added to the cavity. It was also confirmed experimentally and numerically, that the additional Kerr element reduces power threshold for mode locking.

We present a new technique that we have defined as the z-scan confocal method to determine the location and size of the focal spot in a tightly focused ultrashort laser pulse. The method permits accurate positioning of a target in the focal spot with a fast response. The technique is designed to help to automatize the location of an overdense target in focus in a laser plasma experiment and suitable to be implemented in a closed loop configuration setup. The method allows for a fast localization of the focal position and the relative motion of the target with respect to it. As an example of the potential of the technique we have measured the defocusing of a fast rotating disc in several radii to reconstruct the motion of the disc at focus.

Solid-immersion lens (SIL) microscopy requires the dimensions of the SIL to be precisely fabricated, otherwise spherical aberrations will be encountered [1]. Uncorrected spatial aberrations not only degrade the lateral and axial resolutions of a sub-surface microscopy system, but also couple into the time domain to stretch the pulses beyond their minimum transform-limited durations [2]. Chromatic aberration remaps the arrival times at the focal plane of the different colors in the pulse, while spherical aberration changes the position of the best focal plane for each frequency of the pulse. Therefore, time-domain measurements can be applied to diagnose the presence of spatial aberrations, and even to inform an experimental strategy of minimizing their impact. Here we describe an investigation of this approach in the context of sub-surface SIL microscopy of integrated circuits.

We present three methods for calculating the spatio-temporal electric field of ultra-short pulses near the focal point of an optical system. The first method is the Riemann’s sum-based direct-integration method, which is used as a comparison for the other two methods. The other methods are a triple fast Fourier transform (FFT) algorithm and a combined FFT/quadrature algorithm. The proposed triple FFT method reduces dramatically the numerical integration time compared to the other two methods. In the FFT, the pixel size in the focal plane depends on the wavelength, so the pixel size changes for each frequency of the pulse. In this paper, we propose a way to overcome this problem giving a reliable, fast focus-field calculation. As an example, the electric field of the pulse at the focal region of a low numerical aperture mirror is evaluated by using the scalar diffraction theory, and results of the focusing pulse with the three methods are presented for a one-optical-cycle unchirped pulse beam, 2.7 fs at 810 nm, incident on the mirror. However, the FFT algorithm also works for the focusing of more complex optical systems such as high numerical aperture optical systems with aberrations and dispersion.

To pump a solid-state femtosecond laser cavity, a beam from a CW laser is focused by a single lens into the laser crystal. To increase the output power of the laser, the overlap of the laser mode with the pump mode should be maximized. This is particularly important in the so-called mode coupling and the Kerr-lens mode locking (KLM) operation, where the change in beam waist at the position of the gain medium is exploited to enhance the mode overlap with the pump laser in the crystal. In this paper, the astigmatism in the pump beam is reduced by tilting the pump lens. A Gaussian beam is propagated through the complete focusing system-pump lens, tilted spherical mirror, and crystal cut at Brewster's angle-to show the astigmatism inside the crystal as a function of the tilt of the pump lens. A genetic algorithm is presented to optimize the mode coupling between the pump and laser beam inside the crystal by tilting the pump lens. Experimental results are presented to verify the design, showing an increase in the output power of the laser cavity of about 20%.

We study femtosecond pulses at the focal plane of a perfectly conducting spherical mirror which is a dispersionless system, that is, it introduces no group velocity dispersion and no propagation time difference to the pulses after reflection. By using the scalar diffraction theory we will show that the neglected terms in the diffraction integral, when using the approximation of the bandwidth being smaller than the frequency of the carrier, have a significant influence on imaging if a laser pulse of a few femtoseconds is used in time-resolved imaging. The neglected terms introduce temporal spreading to extremely short pulses of a few optical cycles incident on the mirror, which avoids a fully compensated pulse, i.e., a one optical cycle pulse, at the focus of the mirror. The study in this paper also applies to refracting optical systems such as microscope objectives or lenses.

We have developed a numerical model which can be used to understand the operation conditions of a unidirectional or bi-directional Kerr lens mode locked ring laser. The model is based on a time independent ABCD gaussian beam propagation and includes thermal and Kerr effects in the gain mediumm in addition to astigmatism in the cavity. Other important effects that were not included in previous work are also considered. We have verified the validity of the model by comparing the results of our calculations with a laser constructed in our laboratory. In this work we report the results of the comparison of our model with our laser under, unidirectional self-Kerr lens mode locked operation. We found that under certain conditions of alignment and feedback we can control the operation direction of the laser. This model can predict the direction of the operation and can be used to understand the conditions of the bi-directional operation of a Kerr lens mode locked ring laser.

We describe the behavior of linearity in diffuse imaging by evaluating the differences between time-resolved images produced by photons arriving at the detector at different times. Two approaches are considered: Monte Carlo simulations and experimental results. The images of two complete opaque bars embedded in a transparent or in a turbid medium with a slab geometry are analyzed; the optical properties of the turbid medium sample are close to those of breast tissue. A simple linearity test was designed involving a direct comparison between the intensity profile produced by two bars scanned at the same time and the intensity profile obtained by adding two profiles of each bar scanned one at a time. It is shown that the linearity improves substantially when short time of flight photons are used in the imaging process, but even then the nonlinear behavior prevails. As the edge response function (ERF) has been used widely for testing the spatial resolution in imaging systems, the main implication of a time dependent linearity is the weakness of the linearity assumption when evaluating the spatial resolution through the ERF in diffuse imaging systems, and the need to evaluate the spatial resolution by other methods.

We analyze the spatio-temporal intensity of sub-20 femtosecond pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achromatic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. The numerical errors in this method are negligible by taking 96 nodes and the computational time is reduced by 95% compared to the integration method by rectangles. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). This last effect, however, is also significant. For sub-20 femtosecond pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect. The design of an apochromatic doublet is presented in this paper and the spatio-temporal intensity of the pulse at the focal region of this doublet is compared to that given by the achromatic doublet.

We analyze the spatiotemporal intensity of Gaussian temporal envelope pulses with initial durations of 200 fs and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens for a well-collimated incoming pulse beam by using the Seidel aberration theory for thin lenses with the stop at the lens. We analyze the effect of these aberrations in the focusing of ultrashort pulses for Gaussian illumination and present experimental results for 200 fs pulses focused by a near-IR achromatic doublet.