Tiago A. Ortega

THz polariton laser using an intracavity Mg:LiNbO_3 crystal with protective Teflon coating

An enhancement in the performance of a THz polariton laser based on an intracavity magnesium-doped lithium niobate crystal (Mg:LiNbO3) in surface-emitted (SE) configuration is demonstrated resulting from the deposition of a protective Teflon coating on the total internal reflection surface of the crystal. In this cavity geometry the resonating fields undergo total internal reflection (TIR) inside the lithium niobate, and laser damage to that surface can be a limiting factor in performance. The protective layer prevents laser damage to the crystal surface, enabling higher pump power, yielding higher THz output power and wider frequency tuning range. With the unprotected crystal, narrow-band THz output tunable from 1.50 to 2.81 THz was produced, with maximum average output power of 20.1 µW at 1.76 THz for 4 W diode pump power (limited by laser damage to the crystal). With the Teflon coating, no laser damage to the crystal was observed, and the system produced narrow-band THz output tunable from 1.46 to 3.84 THz, with maximum average output power of 56.8 µW at 1.76 THz for 6.5 W diode pump power. This is the highest average output power and the highest diode-to-terahertz conversion efficiency ever reported for an intracavity terahertz polariton laser.

The Brazilian time and frequency atomic standards program

Cesium atomic beam clocks have been the workhorse for many demanding applications in science and technology for the past four decades. Tests of the fundamental laws of physics and the search for minute changes in fundamental constants, the synchronization of telecommunication networks, and realization of the satellite-based global positioning system would not be possible without atomic clocks. The adoption of optical cooling and trapping techniques, has produced a major advance in atomic clock precision. Cold-atom fountain and compact cold-atom clocks have also been developed. Measurement precision of a few parts in 10(15) has been demonstrated for a cold-atom fountain clock. We present here an overview of the time and frequency metrology program based on cesium atoms under development at USP São Carlos. This activity consists of construction and characterization of atomic-beam, and several variations of cold-atom clocks. We discuss the basic working principles, construction, evaluation, and important applications of atomic clocks in the Brazilian program.

Very compact and high-power CW self-Raman laser for ophthalmological applications

In this work, we present a continuous-wave yellow laser operating at 586.5nm based on self-Raman conversion in Nd:GdVO4. We report more than 4.2W CW and 5.5W instantaneous output at a 50% duty cycle regime. This is the highest CW power of a self-Raman laser to be reported so far. We also demonstrate the integration of this laser cavity into a console for applications in ophthalmology, and more specifically for retinal photocoagulation therapies.

Portable fluorescence spectroscopy platform for Huanglongbing (HLB) citrus disease in situ detection

In this work, the development of a portable fluorescence spectroscopy platform for Huanglongbing (HLB) citrus disease in situ detection is presented. The equipment consists of an excitation blue LED light source, a commercial miniature spectrometer and embedded software. Measurements of healthy, HLB-symptomatic and HLB-asymptomatic citrus leafs were performed. Leafs were excited with the blue LED and their fluorescence spectra collected. Embedded electronics and software were responsible for the spectrum processing and classification via partial least squares regression. Global success rates above 80% and 100% distinction of healthy and HLB-symptomatic leafs were obtained.

Design of high power LED-based UVA emission system and a photosensitive substance for clinical application in corneal radiation

This work presents an innovative cross-linking procedure to keratoconus treatment, a corneal disease. It includes the development of an ultraviolet controlled emission portable device based on LED source and a new formulation of a photosensitive drug called riboflavin. Thus new formulation improves drug administration by its transepithelial property. The UV reaction with riboflavin in corneal tissue leads to a modification of corneal collagen fibers, turning them more rigid and dense, and consequently restraining the advance of the disease. We present the control procedures to maintain UV output power stable up to 45mw/cm2, the optical architecture that leads to a homogeneous UV spot and the new formulation of Riboflavin.

Coaxial fundus camera for opthalmology

A Fundus Camera for ophthalmology is a high definition device which needs to meet low light illumination of the human retina, high resolution in the retina and reflection free image1. Those constraints make its optical design very sophisticated, but the most difficult to comply with is the reflection free illumination and the final alignment due to the high number of non coaxial optical components in the system. Reflection of the illumination, both in the objective and at the cornea, mask image quality, and a poor alignment make the sophisticated optical design useless. In this work we developed a totally axial optical system for a non-midriatic Fundus Camera. The illumination is performed by a LED ring, coaxial with the optical system and composed of IR of visible LEDs. The illumination ring is projected by the objective lens in the cornea. The Objective, LED illuminator, CCD lens are coaxial making the final alignment easily to perform. The CCD + capture lens module is a CCTV camera with autofocus and Zoom built in, added to a 175 mm focal length doublet corrected for infinity, making the system easily operated and very compact.

Development of the micro-scanning optical system of yellow laser applied to the ophthalmologic area

In this work, the development of a laser scanning system for ophthalmology with micrometric positioning precision is presented. It is a semi-automatic scanning system for retina photocoagulation and laser trabeculoplasty. The equipment is a solid state laser fully integrated to the slit lamp. An optical system is responsible for producing different laser spot sizes on the image plane and a pair of galvanometer mirrors generates the scanning patterns.

Strategies to run an ophthalmological CW Self-Raman laser in micro-second pulsed laser regime

Micro-second pulsed laser exposure is a new procedure for retina disease treatments. This technique consists in applying laser power sequences of 200µs pulses (1000 times less than traditional treatments). Short laser exposure duration avoids photoreceptor lesions on the retina, providing a better final vision [1]. The laser cavity in 586nm and 4W CW output power is based on Self-Raman conversion in Nd:GdVO4 [2],[3]. Initially, it was developed to continuous mode, but now it is switched to 500Hz, with minimum pulse duration of up to 100µs. This regime is called fast pulse mode. This operation mode makes the cavity PID control system velocity vital and challenging [4]. The energy delivery should be very fast for the pulse to reach the desired level before its termination, and the control feedback system has to correct any deviation of output power to keep it stable and constant. The strategy found was to build a duplicated current driver, one plugged to the laser head and the second plugged to a fake laser cavity. Each one works in synchronism to keep the drained current constant while the laser pulse in turned on and off during treatment procedure. Consequently, it eliminates the power supply time response dependency which can take around 1ms. Fig. 1 shows how the current is switched between both loads against time, fake laser current (I2) and pump diode laser current (I1), and constant drained current from power supply. The output power control is performed by a high priority software to minimize any delay in power reading. The laser head showed good performance, without any stability issues. The cavity pulse time response was of 30µs (worst case) to reach a stable mode. Fig. 2 shows the control loop effort to make output power stable by changing laser diode current. It takes 50ms to achieve the regime state. Thermal lens effects are also investigated. It causes performance differences between CW and micro-pulse mode operation, and laser cavity alignment pulse regime is responsible to favor one mode. This work resulted in a commercial ophthalmic laser.

Design of micro-second pulsed laser mode for ophthalmological CW self-raman laser

This work presents the mechanisms adopted for the design of micro-second pulsed laser mode for a CW Self-Raman laser cavity in 586nm and 4W output power. The new technique for retina disease treatment discharges laser pulses on the retina tissue, in laser sequences of 200 μs pulse duration at each 2ms. This operation mode requires the laser to discharge fast electric pulses, making the system control velocity of the electronic system cavity vital. The control procedures to keep the laser output power stable and the laser head behavior in micro-second pulse mode are presented.