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Calibrated light

Researchers from USP invent a methodology for measuring laser beams

A group of researchers from the Chemistry Institute (IQ) of the University of São Paulo (USP) has created an unprecedented methodology for calibrating laser beams. The novelty will serve not only the industrial concerns that make the laser emitting devices, but also the professionals who use these instruments, for example, in eye surgery and need, from time to time, to verify the manufacturer’s specifications.

The new technique will be used in characterizing continuous lasers in which the distribution of energy in the beam shows a pattern of intensity of light that is weaker at the edges and stronger in the center. Lasers with this profile, called Gaussian lasers, are the ones most commonly found in the market, employed in a vast field of applications, such as optical measures, soldering, metrology and medical use (surgeries, photodynamic therapy, and odontology, amongst others).

The method developed at the IQ will make it possible to determine the diameter or radius of the laser beam at any position along its axis of propagation. This information is fundamental for the various applications of lasers in medicine, in industry and in academic research to have the expected performance. “Our methodology will make it possible to obtain the parameters of the beam quickly and accurately”, explains chemical engineer and postdoctoral student Marcos Gugliotti, who invented the device under the supervision of Professors Mario José Politi and Maurício Baptista, both from the IQ’s Biochemistry Department.

The conception of the equipment took into account the optical phenomenon of a thermic lens, which is observed when a laser beam passes through a substance in which the light is absorbed and transformed into heat. This transformation induces a temperature variation (gradient), which in turn causes a variation in density. This variation causes a change in the substance’s level of refraction, which starts acting like a lens, putting the beam into or out of focus. “It is this lens effect produced by the generation of heat in a given medium, in this case a liquid, that is called a thermal lens”, Gugliotti explains.

In the methodology proposed by Gugliotti, the beam must hit a liquid sample containing a suitable dye, proper for inducing the thermal lens effect. The necessary parameters for characterizing the laser beam are determined by measuring the focusing and defocusing effect of the beam that passes through the sample. This makes it possible to determine the radius of the beam in any position, besides other characteristic measurements. “Theoretically, there are no limits to the size of the beam. In principle, the method could be applied both to beams with a radius of 2 centimeters and to beams with a radius in the order of tenths of micrometers (millionth parts of a meter), or even smaller”, Gugliotti explains.

By chance
Although the effects of the thermal lens have been known since 1965, they have never been used for characterizing laser beams. The researchers from the IQ were the first to glimpse the possibility of using it for this purpose. “It happened almost by chance”, says Gugliotti. “In the course of my doctorate in studies of surfaces, without any connection with characterizing lasers, I worked with photothermic techniques, which are ultrasensitive techniques based on the transformation of light into heat and proper for determining very small quantities (traces) and the thermo-optical properties of materials. This made me realize that I could measure the diameter and other properties of laser beams with a thermal lens”, he explains. “I discovered something that I wasn’t looking for. I stumbled across it”.

By virtue of the unprecedented nature of this methodology, the group of researchers filed a patent request with the National Institute of Industrial Property (INPI) in July this year. They had the support of FAPESP, through the Intellectual Property Support Program (PAPI) and the Nucleus for Licensing and Patenting Technologies (Nuplitec). In the patent, measurements were presented comparing the new technology with the conventional technique for characterizing laser beans, known as the knife-edge scan. “I arrived at the same results using the two methods. They show the same precision in measuring the radius of the beam, in the order of micrometers”, explains the researcher. “The commercial devices have a precision of tenths of micrometers, but I did the measurements manually. When using more sophisticated instruments for measuring, like a step motor (an electric motor controlled by digital circuits), it is reasonable to imagine that this precision will increase”.

Advantages in price
According to its inventors, the thermal lens technique shows a series of advantages over knife-edge scanning. The main one of them is the low cost of the future equipment. The devices based on the knife-edge scan method cost about US$7,000 at the moment; and the apparatus from the IQ researchers, when it is ready, is likely to be sold at a price from 10% to 20% lower than this amount. “This will be possible because the devices used in it are very simple and cheap”, Politi says. But there are other benefits as well. Besides the instrument being easy to handle, the new technique makes it possible to determine several parameters for the beam with a lower number of measurements, that will be less laborious and will be able to be done in less time.

According to Gugliotti, in spite of there not being a prototype of the instrument, some companies are already interested in its manufacture. “We are negotiating with large American companies that make equipment for diagnosing laser beams”, explains the researcher, who prefers not to name them. “Of the six companies contacted since July, two have shown interest, and we are in the early stage of negotiations”, Gugliotti says. The first prototype can be gotten ready in a few weeks after a contract is signed with one of the interested parties. “The equipment will be very compact, smaller than a shoe box, and will weigh between 1 and 2 kilos”, he says. The greatest demand for the device will come from abroad, in particular from the United States, because the Brazilian laser market is still in its infancy. Optimists, the researchers are now hoping to put into effect a production line for the new equipment.

The project
Determining the Radius of a Gaussian Laser Beam by the Thermic Lens Technique (nº 01/08143-8); Modality Intellectual Property Support Program (PAPI); Coordinator Mário José Politi – Chemistry Institute at USP; Investment R$ 6,000.00