Polarimetric measurement of non-depolarizing optical systems.

The use of polarization measurements has become more common in recent years, as it gives more information than pure intensity measurements. Polarimetric components such as fixed or variable retarders and polarizers must be included in optical systems to obtain the polarization parameters required, and in many cases the optical system also includes other components such as relay and/or imaging optical systems. In this work we present a simple and robust method for the polarimetric characterization of non-depolarizing polarization components and other optical elements in the system, which does not require a full polarimeter.

Two-photon absorption spectrometers for near infrared.

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.

Nonlinear focal shift due to the Kerr effect for a Gaussian beam focused by a lens.

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.

Characterization of retardance spatial variations over the aperture of liquid-crystal variable retarders.

We present a comparison of two experimental methods to measure retardance as a function of applied voltage and as a function of position over the aperture of liquid-crystal variable retarders. These measurements are required for many applications, particularly in polarimetry. One method involves the scan of an unexpanded laser beam over the aperture, and the other uses an expanded beam from a LED and a CCD camera to measure the full aperture with a single measurement.

Rough surface scattering using a source able to produce an incident beam with controlled polarization and coherence.

We present a comparison of the first numerical and experimental results for the scattering of light from rough surfaces using a recently developed variable coherence polarimetry source that permits obtaining information on the object without having to scan over incidence or scatter angle. We present, for the first time, we believe, the application of this source to a 1D rough surface and show how to analyze the scattered field to retrieve useful information about the surface. This source uses a liquid-crystal phase modulator to control the polarization as well as the coherence of the beam illuminating the rough surface.

Impact of frequency-dependent spherical aberration in the focusing of ultrashort pulses.

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.

Algorithm to filter the noise in the spectral intensity of ultrashort laser pulses.

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.

Simultaneous measurement of DC two-photon absorption signal offset and amplitude of the intensity autocorrelation in the focusing of femtosecond pulses.

In this work, the DC two-photon absorption signal offset (${{\rm DC}_{\rm TPA}}$DC) and the amplitude of the autocorrelation (${{\rm A}_{\rm AC}}$A) are measured simultaneously around the focal point of an apochromatic microscope objective using the z-scan autocorrelation technique. The ${{\rm A}_{\rm AC}}$A 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 ${{\rm DC}_{\rm TPA}}$DC signal along $z$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 ${{\rm DC}_{\rm TPA}}$DC signal.

Liquid refractive index measured through a refractometer based on diffraction gratings.

We developed two versions of refractometers to measure the refractive index of liquids. One refractometer comprises a glass cell with a surface relief grating on the inner face of one of its walls, while the other one is a microfluidic channel in the form of serpentine that behaves as a grating. Measurements of the liquid refractive index were performed by sensing the first order intensity.

Interference effects in quantum-optical coherence tomography using spectrally engineered photon pairs.

Optical-coherence tomography (OCT) is a technique that employs light in order to measure the internal structure of semitransparent, e.g. biological, samples.

Low-energy/pulse response and high-resolution-CMOS camera for spatiotemporal femtosecond laser pulses characterization @ 1.55 μm.

In this work, we present a commercial CMOS (Complementary Metal Oxide Semiconductor) Raspberry Pi camera implemented as a Near-Infrared detector for both spatial and temporal characterization of femtosecond pulses delivered from a femtosecond Erbium Doped Fiber laser (fs-EDFL) @ 1.55 µm, based on the Two Photon Absorption (TPA) process. The capacity of the device was assessed by measuring the spatial beam profile of the fs-EDFL and comparing the experimental results with the theoretical Fresnel diffraction pattern.

Liquid Temperature Measurements Using Two Different Tunable Hollow Prisms.

This paper describes the design, fabrication, and testing of two hollow prisms. One is a prism with a grating glued to its hypotenuse. This ensemble, prism + grating, is called a grism.

Mode-coupling enhancement by pump astigmatism correction in a Ti:Sapphire femtosecond laser.

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.

Reflection formulae for ray tracing in uniaxial anisotropic media using Huygens's principle.

Ray tracing in uniaxial anisotropic materials is important because they are widely used for instrumentation, liquid-crystal displays, laser cavities, and quantum experiments. There are previous works regarding ray tracing refraction and reflection formulae using the common electromagnetic theory approach, but only the refraction formulae have been deduced using Huygens's principle. In this paper we obtain the reflection expressions using this unconventional approach with a specific coordinate system in which both refraction and reflection formulae are simplified as well as their deduction.

Deviation from orthogonal polarization for ordinary and extraordinary rays in uniaxial crystals.

We have derived a closed-form expression for the angle between polarizations of ordinary and extraordinary rays in uniaxial crystals for, first, any two rays propagating in the material, and, second, for rays coming from refraction. We show the cases in which orthogonality holds and that, in general, the deviation from orthogonality is rather small, for it depends on the difference of the optical indices. Specific examples for calcite and quartz are given.

Diffraction grating-based sensing optofluidic device for measuring the refractive index of liquids.

We describe a simple and versatile optical sensing device for measuring refractive index of liquids. The sensor consists of a sinusoidal relief grating in a glass cell. Device calibration is done by pouring in the cell different liquids of known refractive indices.

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.

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.

Spectrometer and scanner with optofluidic configuration.

We present a spectrometer and scanner based on optofluidic configurations. The main optical component of the spectrometer is a compound optical element consisting of an optofluidic lens and standard blazed diffraction grating. The spectrum size can be changed by filling the lens cavity with different liquids.

Refractive index measurement through image analysis with an optofluidic device.

We present a novel optical method, to our knowledge, to measure the refractive index of liquids by means of the images produced by an optofluidic lens. In addition we propose a new method to make optofluidic lenses.

Optofluidic compound microlenses made by emulsion techniques.

Here we present a new method to make liquid lenses. It is based on the microfluidics method and involves the preparation of emulsions one drop at a time. Tests of lenses by image formation are presented.

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.

Optofluidic variable focus lenses.

Here we propose optofluidic spherical microlenses that can change their focal distance by varying the refractive index of the liquid that composes them. These lenses are fabricated in the bulk of a polymeric mixture. Results of a characterization study of the profile of the lenses, the image forming capability, and the behavior of the focal distance as a function of the refractive index are presented.

Pressure sensor with optofluidic configuration.

We present a new kind of compact, simple, and low cost optical pressure sensor. The physical principle on which the sensor is based, components, layout of the system, and characterization are described. The range of pressures in which the sensor works is from about 0.

Capillary refractometer integrated in a microfluidic configuration.

We propose a microfluidic method to measure the refractive index of liquids. This method is based on the dynamic focusing by a capillary when liquids with different refractive indexes are inserted into it. Fabrication of such a refractometer has been done by encapsulating two fibers and a capillary.