Technology: Fiber Lasers Continue to Attract Users

Melinda Rose, melinda.rose@laurin.com

Fiber lasers are gaining in popularity over other types because they offer advantages in beam quality, scalability and efficiency. They also are more cost-effective than traditional laser designs (Manufacturers can weld, cut and drill with the same fiber laser) and require only an electrical source to operate.

Fiber lasers have compact diodes, generate no chemical-reactant waste and are highly efficient at converting electrons to photons to generate a high-quality beam. Their efficient operation (promoted as up to 15 times more efficient than conventional lasers) minimizes thermal loading of the glass fiber, requiring less cooling and enabling up to kilowatts of output power.

The lasers’ ever-growing output power is opening up a host of new applications for the devices, which began with an output power of tens of watts and now can reach several kilowatts. They were first used in the telecommunications industry, then moved into marking, welding and cutting applications as their output power increased.

The Raydiance Discovery system scaled to high energy levels and focused to a small spot delivers a power intensity high enough to cause the optical breakdown of air (pictured here), much the same as what occurs during lightning. For fiber-based ultrashort-pulse lasers, breakdown in air occurs at peak optical intensities around 20 terawatts (trillion watts) per square centimeter. The Raydiance laser being developed for the Navy can produce more than 10 times this level of optical intensity. Photo courtesy of Business Wire.

Fiber lasers were found to have high-quality, energy-efficient beams as well as ruggedness, small size and the potential for eye-safe operation, making them attractive for medical procedures and directed energy weapons systems that can “dazzle” combatants.

In the UK, a consortium led by ultrafast fiber laser maker Fianium is working to develop advanced white-light (supercontinuum) fiber lasers for biomedical applications.

The $3.6 million WhiteLase project is co-funded by the UK Technology Strategy Board and involves the Centre for Photonics and Photonic Materials at the University of Bath and Edinburgh Instruments Ltd., which will evaluate the lasers developed in fluorescence imaging applications.

The aim of the project is to offer ultraviolet content supercontinuum sources and ultrabright visible sources to below 300 nm and to provide spectral power densities an order of magnitude higher than those of existing commercial white-light lasers.

Fiber’s short pulse, high peak power and broad bandwidth capabilities are also being explored for medical imaging applications, such as optical coherence tomography (OCT). Thulium-doped fiber lasers also are gaining interest as surgical tools because they operate at wavelengths between 1.8 and 2.1 μm, which is the water-absorption range. They are seen as a lower-cost, high-precision alternative to the holmium and frequency-doubled solid-state lasers commonly used for these applications.

Coherent beam combining – or stringing together a bunch of lower-powered fiber lasers to generate a single high-power beam – is a power-scalable technique that overlays the wavelength-offset outputs of many laser emitters into a single, near-diffraction-limited beam.

Companies exploring such techniques include Aculight Corp., which has used spectral beam combining to demonstrate power scaling of multiple fibers into a diffraction-limited beam of more than 500 W.

In December 2007, Southampton, UK-based SPI Lasers, now a member of the Trumpf Group, anounced that it was investigating new applications for fiber lasers to challenge the perception of the capabilities of near-infrared lasers. Researchers at SPI’s applications lab in Santa Clara, Calif., reported excellent results using fiber lasers in a number of new areas.

The most surprising results were in the area of plastics welding, something previously thought unsuitable for the high brightness and beam characteristics of fiber lasers. Testing, however, showed that the laser easily produces the types of plastic welds needed in the medical and mobile phone industries, SPI said.

In April, Raydiance Inc. announced that it had demonstrated, at the eye-safe wavelength of 1552 nm, the highest pulse energy ever achieved in a fiber laser.

At SPIE Photonics West 2009 this month, Fibercore Ltd. of Southampton, UK, is launching an all-silica erbium-ytterbium, co-doped, dual-clad fiber.

“Erbium-ytterbium co-doping is frankly the only practical way to achieve high-power fiber amplification at 1550 nm,” said Dr. Chris Emslie, Fibercore’s managing director. “Making these technologies available to more engineers and scientists working on a broader range of applications can only help boost fiber laser and high-power amplifier development worldwide.”

Northrop Grumman is developing single-frequency fiber amplifiers for the new Revolution in Fiber Lasers (RIFL) program funded by DARPA. Single-frequency fiber lasers and amplifiers now typically are limited to several hundred watts by an effect known as stimulated Brillouin scattering, which reflects power backward and damages low-power components. A key goal is to demonstrate such amplifiers working at 3 kW by the end of the project’s 18-month second phase.

High-power fiber lasers are increasingly being accepted by automakers, as evidenced by IPG Photonics’ announcement in July that its subsidiary in Germany will be providing multikilowatt, continuous-wave ytterbium lasers with 63 kW of power to the BMW Group in Munich for welding auto doors.

“We are seeing particular interest in our high-power fiber lasers in automotive applications because of their proven low maintenance and smaller footprint on the factory floor,” said Dr. Valentin Gapontsev, CEO of IPG Photonics.

There are also limits to the types of lasers fiber can replace. For example, fiber lasers are limited in their ability to both store energy and generate significant amounts of peak power, so they will probably not compete with diode-pumped solid-state lasers for applications requiring pulses of 100 mJ or greater.