Physicist Produces High-Efficiency Compact Laser in Brazil

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It follows an article published on day (08/19) in the english website of the Agência FAPESP noting that Physicist produces High-Efficiency Compact Laser in Brazil.

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Physicist Produces

High-Efficiency Compact Laser

By José Tadeu Arantes

August 19, 2015

(Photo: IPEN)

The power conversion efficiency is the highest ever recorded for this

type of equipment and was achieved without additional expensive

components or complex procedures.

Agência FAPESP – An innovative design has enabled Niklaus Ursus Wetter, a physicist at Brazil’s Energy & Nuclear Research Institute (IPEN), to create a laser with 60% efficiency, a world record for this type of equipment.

The result was achieved under the aegis of the research project “Development of compact and efficient diode-pumped, solid-state lasers for use in portable LIDAR and satellites”, supported by FAPESP, and reported in the article “Influence of pump bandwidth on the efficiency of side-pumped, double-beam mode-controlled lasers: establishing a new record for Nd:YLiF4 lasers using VBG”, published in the journal Optics Express.

Without adding complex or expensive components to the original equipment, Wetter achieved this result by reconfiguring the geometry of a Nd:YLF (neodymium-doped yttrium lithium fluoride) laser.

The laser is highly compact, robust and light. All three characteristics are essential for applications in satellites and other mobile devices, such as those that use LIDAR (light detection and ranging) technology.

“The 60% efficiency we achieved is the best ever reported for this type of crystal,” Wetter told Agência FAPESP. “That means more than half the power used to operate the device is converted into laser light, producing a beam of extremely high quality.”

Wetter earned his bachelor’s degree in physics at ETHZ, the Federal Institute of Technology in Zurich, Switzerland, and completed his PhD at IPEN. Since 2013, he has run IPEN’s Center for Lasers & Applications (CLA).

Wetter recalled that the lasers that were used until the early 1990s were large and inefficient. In the case of gas lasers, which emitted visible light, less than 1% of the power input was converted into laser light and more than 99% into heat.

“This required a huge cooling system that had to be installed in a building adjoining the facility that housed the laser. To generate 10 watts of light, it was necessary to remove thousands of watts of heat,” Wetter said.

Many improvements were made over the years, and solid-state lasers doped with neodymium became the best option for combining high power with high quality, but for a long time, the efficiency of these devices never surpassed 10%. Further improvements eventually increased the efficiency to 50%, especially with the advent of the high-power diode laser. The inefficient traditional pump lamp left the scene and was replaced by the diode.

“Our device is a small, robust laser that can be operated anywhere without the need for a temperature-controlled environment or vacuum. There are more efficient lasers, but these require special materials that cost a lot of money. The best laser in existence at present, which uses ytterbium, can reach approximately 80% efficiency but has to be kept at very low temperatures: 78 Kelvin, or about minus 195 degrees Celsius. That’s not very practical, obviously,” Wetter said.

He stressed that because his reconfigured design is based on the needs of the Brazilian market, it avoids any dependence on costly inputs, complex pumping systems or special care to ensure thermal insulation from the environment.

Instead of operating continuously, the laser emits very intense, short pulses lasting 7-8 nanoseconds and delivers more than 1 millijoule of power at intervals of 1 millisecond. “The high intensity makes a number of effects possible, such as second-harmonic generation, for example. This means that the laser can operate in the visible light spectrum, in the green color range, as well as in the usual near-infrared region,” Wetter said.

Green lasers are well known for their use by dermatologists to remove tattoos. Their uses are actually far more varied, however, ranging from environmental research involving pollution tracking based on the emission of pulses into the atmosphere and the subsequent collection of the reflected light to industrial engraving, cutting and marking.

Monochromaticity, Coherence and Collimation

The term ‘laser’ is an acronym for ‘light amplification by stimulated emission of radiation.’ The process consists of the production of electromagnetic radiation that is monochromatic (all waves have a single wavelength), coherent (all waves are phase-matched) and collimated (all waves are emitted practically in parallel). The virtues of a laser derive entirely from the combination of these three characteristics.

Laser light is generated when a certain material, called the active or gain medium, is pumped by an external power source such as a lamp or diode. The energy delivered by the power source excites the atoms in the material, and their electrons migrate to higher-energy orbits.

Each electron tends to spontaneously return to the minimum-energy ground state, emitting surplus energy in the form of a photon, or a quantum of light; however, rather than allowing this decay to occur randomly, the device induces it by means of another photon at the same energy level.

Each photon released by an electron stimulates the next electron to emit another photon of the same wavelength, triggering a cascade effect. Another component, the resonator, causes the produced photons to return to the gain medium, generating more stimulated emission.

The result is high-intensity emission with the three characteristics mentioned above: monochromaticity, coherence and collimation.

Geometric Reconfiguration

“The intensity of a laser beam varies radially as a Gaussian distribution, i.e., the intensity is highest along the centerline and lowest at the edge of the beam. We used geometric reconfiguration to boost the centerline intensity,” Wetter said.

The key innovation consists of polishing the crystal not only on the faces where the beam enters and exits but also on one of its sides and then pointing the beam at the polished side. Total internal reflection then exposes the core of the beam, which is pumped by the diode.

“It’s as if we opened up the laser beam with a scalpel and delivered our energy burst right in the middle, where the intensity is maximal,” Wetter said.

Although this artifice makes the laser highly efficient overall, it does not guarantee beam quality. To obtain a beam of excellent quality, Wetter devised an additional procedure whereby the beam is again fired at the pumping surface at a carefully calculated distance from the initial point. The proximity of the two lines prevents the beam from widening and losing quality.

“These two components, which are part of the same laser beam, fight for pumping energy. Because they’re very close together, their diameter can’t expand unless they steal energy from each other. As a result, the beam’s cross section remains as small as possible,” Wetter said.

Source: English WebSite of the Agência FAPESP