Femtosecond laser settings for optimal bracket bonding to zirconia.

Bonding orthodontic brackets to ceramic materials is a challenging procedure; femtosecond (FS) laser conditioning could provide improved results, but the ideal settings for effective bracket-zirconia bonding have never been established. This study aimed to analyze the differences in surface roughness and shear bond strength (SBS) produced by different femtosecond laser settings and establish a protocol to prepare zirconia surfaces for optimal adhesion to metal orthodontic brackets. One hundred eighty zirconia samples were assigned to six groups according to surface treatment: (1) control; (2) air-particle abrasion (APA); (3) FS laser irradiation (300 mW output power, 60 µm inter-groove distance); (4) FS laser irradiation (200 mW, 100 µm); (5) FS laser irradiation (40 mW, 60 µm); and (6) FS laser irradiation (200 mW, 60 µm). Surface roughness was measured. Orthodontic brackets were bonded to the zirconia specimens, and SBS was measured. SBS in groups 3 and 6 was significantly higher than the other groups (5.92 ± 1.12 MPa and 5.68 ± 0.94 MPa). No significant differences were found between groups 1, 2, 4, and 5 (3.87 ± 0.77 MPa, 4.25 ± 0.51 MPa, 3.74 ± 0.10 MPa, and 3.91 ± 0.53 MPa). Surface roughness was significantly greater for FS laser than for control and APA groups (p = 1.28 × 10-8). FS laser at 200 mW, 60 µm can be recommended as the ideal settings for treating zirconia surfaces, producing good SBS and more economical energy use.

High sampling rate single-pixel digital holography system employing a DMD and phase-encoded patterns.

A single-pixel digital holography system with phase-encoded illumination using a digital micromirror device (DMD) as a spatial light modulator (SLM) is presented. The enhanced switching rate of DMDs, far exceeding the stringent frame-rate of liquid crystal SLMs, allows recording and reconstruction of complex amplitude distributions in just a few seconds. A single amplitude binary modulation device is used for concurrently displaying the phase-encoded sampling patterns, compensating the distortion of the wavefront, and applying phase-shifting, by means of computer generated holograms. Our detection system consists of a simple photodiode that sequentially records the irradiance fluctuations corresponding to the interference between object and reference beams. The system recovers phase and amplitude information even when a diffuser is placed in front of the photodiode.

Single-pixel digital holography with phase-encoded illumination.

We demonstrate imaging of complex amplitude objects through digital holography with phase-structured illumination and bucket detection. The object is sampled with a set of micro-structured phase patterns implemented onto a liquid-crystal spatial light modulator while a bucket detector sequentially records the irradiance fluctuations corresponding to the interference between object and reference beams. Our reconstruction algorithm retrieves the unknown phase information from the full set of photocurrent measurements. Interestingly, the sampling functions can be codified onto the reference beam, so they can be nonlocal with respect to the object. Finally, we show that the system is well-fitted for transmission of the object information through scattering media.

Generation of programmable 3D optical vortex structures through devil's vortex-lens arrays.

Different spatial distributions of optical vortices have been generated and characterized by implementing arrays of devil's vortex lenses in a reconfigurable spatial light modulator. A simple design procedure assigns the preferred position and topological charge value to each vortex in the structure, tuning the desired angular momentum. Distributions with charges and momenta of the opposite sign have been experimentally demonstrated. The angular velocity exhibited by the phase distribution around the focal plane has been visualized, showing an excellent agreement with the simulations. The practical limits of the method, with interest for applications involving particle transfer and manipulation, have been evaluated.

Ultrashort pulse fiber delivery with optimized dispersion control by reflection grisms at 800 nm.

We experimentally demonstrate a compact and efficient arrangement for fiber delivery of sub-30 fs energetic light pulses at 800 nm. Pulses coming from a broadband Ti:Sapphire oscillator are negatively pre-chirped by a grism-pair stretcher that allows for the control of second and third orders of dispersion. At the direct exit of a 2.7-m long large mode area (LMA) photonic crystal fiber 1-nJ pulses are temporally compressed to 29 fs producing close to 30 kW of peak power. The tunability of the device is studied. Comparison between LMA fibers and standard SMF fibers is also discussed.

Closed-loop adaptive optics with a single element for wavefront sensing and correction.

We propose a closed-loop adaptive optical arrangement based on a single spatial light modulator that simultaneously works as a correction unit and as the key element of a wavefront sensor. This is possible by using a liquid crystal on silicon display whose active area is divided into two halves that are respectively programmed for sensing and correction. We analyze the performance of this architecture to implement an adaptive optical system. Results showing a closed-loop operation are reported, as well as a proof of concept for dealing with aberrations comparable to those typically found in human eyes.

Single-shot color digital holography based on the fractional Talbot effect.

We present a method for recording on-axis color digital holograms in a single shot. Our system performs parallel phase-shifting interferometry by using the fractional Talbot effect for every chromatic channel simultaneously. A two-dimensional binary amplitude grating is used to generate Talbot periodic phase distributions in the reference beam. The interference patterns corresponding to the three chromatic channels are captured at once at different axial distances. In this scheme, one-shot recording and digital reconstruction allow for real-time measurement. Computer simulations and experimental results confirm the validity of our method.

Single-shot digital holography by use of the fractional Talbot effect.

We present a method for recording in-line single-shot digital holograms based on the fractional Talbot effect. In our system, an image sensor records the interference between the light field scattered by the object and a properly codified parallel reference beam. A simple binary two-dimensional periodic grating is used to codify the reference beam generating a periodic three-step phase distribution over the sensor plane by fractional Talbot effect. This provides a method to perform single-shot phase-shifting interferometry at frame rates only limited by the sensor capabilities. Our technique is well adapted for dynamic wavefront sensing applications. Images of the object are digitally reconstructed from the digital hologram. Both computer simulations and experimental results are presented.

High-visibility interference fringes with femtosecond laser radiation.

We propose and experimentally demonstrate an interferometer for femtosecond pulses with spectral bandwidth about 100 nm. The scheme is based on a Michelson interferometer with a dispersion compensating module. A diffractive lens serves the purpose of equalizing the optical-path-length difference for a wide range of frequencies. In this way, it is possible to register high-contrast interference fringes with micrometric resolution over the whole area of a commercial CCD sensor for broadband femtosecond pulses.

Synthetic aperture single-exposure on-axis digital holography.

We present a system for reconstructing single-exposure on-line (SEOL) digital holograms with improved resolution using a synthetic aperture. Several recordings are made in order to compose the synthetic aperture, shifting the camera within the hologram plane. After processing the synthetic hologram, an inverse Fresnel transformation provides an enhanced resolution reconstruction. We show that recognition capacity for high frequency details is increased. Experimental results with a test target and with a microscopic biological sample are presented. Both visualization and correlation results are reported.

Applications of short-coherence digital holography in microscopy.

We present an optical system based on short-coherence digital holography suitable for the imaging of three-dimensional microscopic objects. The short temporal coherence properties of the light source allow optical sectioning of the sample. Proper reconstruction of different layers within biological samples is possible up to a depth of a few hundred micrometers, but multiple scattering and inhomogeneities in the refractive index reduce the imaging quality for deeper layers. We have studied the possibility of numerically correcting sample-induced aberrations, and we now propose a method of improving image quality. Numerical simulations and preliminary experimental results show that compensation of these aberrations is possible to some extent.