Mario A. Stefani

Two-mirror telescope design with third-order coma insensitive to decenter misalignment

Misalignments always occur in real optical systems. These misalignments do not generate new aberration forms, but they change the aberration field dependence. Two-mirror telescopes have been used in several applications. We analyze a two-mirror telescope configuration that has negligible sensitivity to decenter misalignments. By applying the wave aberration theory for plane-symmetric optical systems it is shown that the asphericity in the secondary mirror, if properly chosen, can compensate for any decenter perturbation allowing third-order coma unchanged across the field of view. For any two-mirror system it is possible to find a configuration in which decenter misalignments do not generate field-uniform coma.

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.

Optical analysis of electro-optical systems by MTF calculus

One of the widely used methods for performance analysis of an optical system is the determination of the Modulation Transfer Function (MTF). The MTF represents a quantitative and direct measure of image quality, and, besides being an objective test, it can be used on concatenated optical system. This paper presents the application of software called SMTF (software modulation transfer function), built in C++ and Open CV platforms for MTF calculation on electro-optical system. Through this technique, it is possible to develop specific method to measure the real time performance of a digital fundus camera, an infrared sensor and an ophthalmological surgery microscope. Each optical instrument mentioned has a particular device to measure the MTF response, which is being developed. Then the MTF information assists the analysis of the optical system alignment, and also defines its resolution limit by the MTF graphic. The result obtained from the implemented software is compared with the theoretical MTF curve from the analyzed systems.

The Brazilian wide field imaging camera (WFI) for the China/Brazil earth resources satellite: CBERS 3 and 4

The purpose of this paper is to present the optical system developed for the Wide Field imaging Camera - WFI that will be integrated to the CBERS 3 and 4 satellites (China Brazil Earth resources Satellite). This camera will be used for remote sensing of the Earth and it is aimed to work at an altitude of 778 km. The optical system is designed for four spectral bands covering the range of wavelengths from blue to near infrared and its field of view is ±28.63°, which covers 866 km, with a ground resolution of 64 m at nadir. WFI has been developed through a consortium formed by Opto Electrônica S. A. and Equatorial Sistemas. In particular, we will present the optical analysis based on the Modulation Transfer Function (MTF) obtained during the Engineering Model phase (EM) and the optical tests performed to evaluate the requirements. Measurements of the optical system MTF have been performed using an interferometer at the wavelength of 632.8nm and global MTF tests (including the CCD and signal processing electronic) have been performed by using a collimator with a slit target. The obtained results showed that the performance of the optical system meets the requirements of project.

MTF measurements on real time for performance analysis of electro-optical systems

The need of methods and tools that assist in determining the performance of optical systems is actually increasing. One of the most used methods to perform analysis of optical systems is to measure the Modulation Transfer Function (MTF). The MTF represents a direct and quantitative verification of the image quality. This paper presents the implementation of the software, in order to calculate the MTF of electro-optical systems. The software was used for calculating the MTF of Digital Fundus Camera, Thermal Imager and Ophthalmologic Surgery Microscope. The MTF information aids the analysis of alignment and measurement of optical quality, and also defines the limit resolution of optical systems. The results obtained with the Fundus Camera and Thermal Imager was compared with the theoretical values. For the Microscope, the results were compared with MTF measured of Microscope Zeiss model, which is the quality standard of ophthalmological microscope.

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.

UVA system for human cornea irradiation

According to recent studies, an increase in corneal stiffness is a promising alternative for avoiding ectasias and for stagnating keratoconus of grades 1 and 2. The clinical treatment consists essentially of instilling Riboflavin (vitamin B2), in the cornea and then irradiating the corneal tissue, with UVA (365nm) radiation at 3mW/cm2 for 30min. This procedure provides collagen cross-linking in the corneal surface, increasing its stiffness. This work presents a system for UVA irradiation of the corneas at a peak wavelength of 365nm with adjustable power up to 5mW. The system has closed loop electronics to control the emitted power with 20% precision from the sated power output. The system is a prototype for performing corneal cross-linking and has been clinically tested. The closed loop electronics is a differential from the equipments available on the market.

Procurement and Importing in New Product Projects of Brazilian Aerospace Program

New product development is a business process with many functional interactions in a company. The concurrency of these interactions must be managed in order to meet the preestablished schedule, budget and scope. The issue of procurement is central to a succesful project. When a new project belongs to an aerospace program this issue is even more crucial. And when the aerospace program belongs to a developing country such as Brazil, the core issue involves its budget and schedule planning. This article addresses the question of procurement in a small company designing a new satellite camera for the Brazilian Government. The procurement process was mapped, a monitoring structure was created and performance indicators were developed. The performance indicators are discussed to understand the leverage of each kind of purchased item and each process step on costs and schedule.