Nonlinear confocal positioner for micron-scale target alignment.

This paper presents a novel target positioner system that exhibits high sensitivity and accuracy. Specifically, the system is capable of precisely locating rough target surfaces within a micron-scale in the focal plane. The high sensitivity comes from the nonlinear detection scheme which uses the two-photon-absorption process in a Si-photodiode and a CMOS sensor at 1550 [nm]. The setup employs a confocal configuration that is easy to align and does not require a conjugated focal plane selective aperture (pinhole), thus demonstrating its feasibility and tilt tolerance of the target. Moreover, the system offers high accuracy up to 5 [µm], which corresponds to the step size of the focus scanning. The presented positioner system has potential applications in microfabrication with lasers and laser-driven plasma accelerators even at high repetition rates, limited by the detection bandwidth of the photodiode. Additionally, the principle can be extended to cameras if spatial information is needed and the system design can be extended to other spectral ranges with minimal changes.

High accuracy astigmatic-focusing system for laser targets.

An accurate location of the focal position with respect to a solid target is a key task for different applications, for instance, in laser driven plasma acceleration for x-ray generation where minimum required intensities are above 1014W/cm2. For such practical applications, new approaches for focus location and target delivery techniques are needed to achieve the required intensity, repeatability, and stability. There are different techniques to accomplish the focusing and target positioning task such as interferometry-, microscopy-, astigmatism-, and nonlinear-optics-based techniques, with their respective advantages and limitations. We present improvements of a focusing technique based on an astigmatic method with potential applications where maximum intensity at the target position is necessary. The presented technique demonstrates high accuracy up to 5 µm, below the Rayleigh range, and also its capability to work in rough surfaces targets and tilt tolerance of the target, with respect to the normal of the target surface.

Deep photothermal effect induced by stereotactic laser beams in highly scattering media.

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.

Merging Mie solutions and the radiative transport equation to measure optical properties of scattering particles in optical phantoms.

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 (µa and µs', 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 µa and µs' 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 SiO2 nanospheres, ns, which is estimated by fixing the fitted values of µa and µs', using the known value of the real part of ns, and finding the corresponding value of Im(ns) 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.

Wavelet-based method for spectral interferometry filtering.

In this work, we study the effects of noise present on spectral interferometry signals, for femtosecond pulse retrieval such as in the SPIDER technique (spectral phase interferometry for direct e-field reconstruction). Although previous works report SPIDER robustness, we have found that noisy signals with low signal-to-noise ratio (SNR), in the acquired spectral interferogram, could cause variations in the temporal pulse intensity retrieval. We demonstrate that even in a filtered SPIDER signal, following standard procedures, at some point the noise on the spectral interferogram could affect the spectral phase retrieval. As a novel alternative for spectral interferograms filtering, we have applied the wavelet transform and propose a target criterion to automatize the optimization algorithm. We apply this method on SPIDER signals and analyze its effectiveness on the spectral phase retrieval. We present numerical and experimental results to show the improvement in the phase retrieval and the temporal pulse reconstruction after applying this filtering method and compare the results with a standard method.

Efficiency signal conversion parameter to evaluate astigmatic femtosecond-optical parametric oscillator cavities.

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.

Rapid scanning optical delay line based on a diffraction grating pair for a low-coherence reflectometer.

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.

Comparison of spatially and temporally resolved diffuse transillumination measurement systems for extraction of optical properties of scattering media.

This paper discusses the main differences between two different methods for determining the optical properties of tissue optical phantoms by fitting the spatial and temporal intensity distribution functions to the diffusion approximation theory. The consistency in the values of the optical properties is verified by changing the width of the recipient containing the turbid medium; as the optical properties are an intrinsic value of the scattering medium, independently of the recipient width, the stability in these values for different widths implies a better measurement system for the acquisition of the optical properties. It is shown that the temporal fitting method presents higher stability than the spatial fitting method; this is probably due to the addition of the time of flight parameter into the diffusion theory.

Time-domain measurements reveal spatial aberrations in a sub-surface two-photon microscope.

We show that in a nonlinear microscopy system the effects of chromatic and spherical aberrations are revealed by a difference in the focal positions corresponding to the shortest pulse duration and the minimum lateral resolution. By interpreting experimental results from a high-numerical-aperture two-photon microscope using a previously reported spatio-temporal model, we conclude that the two-photon autocorrelation of the pulses at the focal plane can be used to minimize both the chromatic and spherical aberrations of the system. Based on these results, a possible optimization strategy is proposed whereby the objective lens is first adjusted for minimum autocorrelation duration, and then the wavefront before the objective is modified to maximize the autocorrelation intensity.

Autocorrelation z-scan technique for measuring the spatial and temporal distribution of femtosecond pulses in the focal region of lenses.

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.

Time of flight dependent linearity in diffuse imaging: how effective is it to evaluate the spatial resolution by measuring the edge response function?

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.

Temporal widening of a short polarized pulse focused with a high numerical aperture aplanatic lens.

We present a theoretical analysis of the field distribution in the focal plane of a dispersionless, high numerical aperture (NA) aplanatic lens for an x-polarized short pulse. We compare the focused pulse spatial distribution with that of a focused continuous wave (CW) field and its temporal distribution with the profile of the incident pulse. Regardless of the aberration free nature of the focusing aplanatic lens, the temporal width of the focused pulse widens considerably for incident pulses with durations on the order of a few cycles due to the frequency-dependent nature of diffraction phenomena, which imposes a temporal diffraction limit for focused short pulses. The spatial distribution of the focused pulse is also affected by this dependence and is altered with respect to the diffraction limited distribution of the CW incident field. We have analyzed pulses with flat top and Gaussian spatial irradiance profiles and found that the focused pulse temporal widening is less for the Gaussian spatial irradiance pulse, whereas the spatial distribution variation is similar in both cases. We present results of the focused pulsewidth as a function of the NA for the two spatial irradiance distributions, which show that the Gaussian irradiance pulse outperforms the flat top pulse at preserving the incident pulse duration.

Temporal spreading generated by diffraction in the focusing of ultrashort light pulses with perfectly conducting spherical mirrors.

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.

Gauss-Legendre quadrature method used to evaluate the spatio-temporal intensity of ultrashort pulses in the focal region of lenses.

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.

Effects of primary spherical aberration, coma, astigmatism and field curvature on the focusing of ultrashort pulses: homogenous illumination.

We analyze the spatiotemporal intensity of pulses with durations of 20 fs and shorter and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens. The incident pulse is well-collimated, and we use the Seidel aberration theory for thin lenses to evaluate the phase change due to the aberrations of the lens. In a set of cemented thin lenses with the stop at the lens, there is only spherical aberration, coma, astigmatism and field curvature, whereas the distortion aberration in the phase front is zero. We analyze the effect of these aberrations in the focusing of ultrashort pulses for homogenous illumination. We will show that the temporal spreading introduced by these aberrations in pulses shorter than 20 fs at 810 nm is very small but the spatial spreading is not, which reduces the intensity of the pulse considerably.

Effects of primary spherical aberration, coma, astigmatism, and field curvature on the focusing of ultrashort pulses: Gaussian illumination and experiment.

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.

Designer femtosecond pulses using adaptive optics.

We describe a femtosecond pulse shaper using a deformable membrane mirror. The pulses are measured with a real time second-harmonic-generation frequency-resolved optical gating system. Pulse shapes are modified according to a prescribed spectral phase. Accurate spectral phase design as well as pulse intensity modulation was achieved by using negative feedback mirror-surface control. Convergence to the chosen spectral phase design was typically achieved within several seconds.

High-average-power, 1-MW peak-power self-mode-locked Ti:sapphire oscillator.

We describe a novel self-mode-locked Ti:sapphire ring laser that produces 13-fs pulses with peak powers exceeding 1 MW, pulse energies of 13 nJ, and average mode-locked output powers of 1.5 W at a cavity repetition frequency of 110 MHz. A complete resonator analysis describing the optimum mode-locking configuration is presented, together with fringe-resolved autocorrelation and sonogram measurements of the output pulses.