Researchers demonstrate first femtosecond fiber laser with visible wavelength

Researchers have developed the first fiber laser capable of producing femtosecond pulses in the visible range of the electromagnetic spectrum. Fiber lasers that produce bright, ultrashort visible wavelength pulses could be useful for a variety of biomedical applications as well as other areas such as materials processing.

Visible femtosecond pulses are usually achieved using complex and inherently inefficient configurations. Although fiber lasers represent a very promising alternative due to their robustness/reliability, small footprint, efficiency, lower cost and high brightness, it has not been possible, until now, to produce visible pulses with durations in the femtosecond (10^-15 s) scope directly with such lasers.

“Our demonstration of a femtosecond fiber laser operating in the visible spectrum paves the way for a new class of reliable, efficient and compact ultrafast lasers,” said research group leader Ral Valle of Laval University in Canada.

In Optics letters, the researchers describe their new laser, which is based on a lanthanide-doped fluoride fiber. By emitting red light at 635 nm, the laser achieves compressed pulses with a duration of 168 fs, a peak power of 0.73 kW, and a repetition frequency of 137 MHz. Using a commercial blue laser diode as the optical source of energy, or pumping source, helped make the overall design robust, compact, and cost-effective.

‘Provided that higher energies and powers can be achieved in the near future, many applications could benefit from this type of laser,’ said Marie-Pier Lord, a doctoral student involved in the project. “Potential applications include high-quality, high-precision ablation of biological tissue and two-photon excitation microscopy. Femtosecond laser pulses also allow for cold ablation during material processing, a process that can perform much cleaner cuts [than longer pulses] because it does not produce thermal effects.”

Obtaining visible light from fiber lasers

In a fiber laser, an optical fiber doped with rare earth elements serves as the laser medium. Although fiber lasers are among the simplest, most robust, and most reliable of high-brightness laser systems, the use of silica fibers tends to limit them to the near-infrared spectral region. Valle’s group worked to extend the spectral range of these laser sources by using fibers made of fluoride instead of silica.

“We have previously focused on the development of mid-infrared fiber lasers, but have recently gained interest in visible fiber lasers,” Lord said. “Although the lack of compact and efficient pump sources for such lasers has hindered their development for a long time, the recent advent of semiconductor-based laser sources operating in the blue spectrum has provided a key technology for the development of efficient visible fiber lasers “.

After demonstrating fiber lasers that continuously emit visible wavelengths, the researchers wanted to extend the progress to ultrafast pulsed sources. Thanks to the refinement of the fluoride fiber manufacturing process, it is now possible to obtain lanthanide-doped fibers with properties essential for the development of efficient visible fiber lasers.

Integrating technologies into a new laser

The new pulsed fiber laser developed by Valle’s team combines a lanthanide-doped fluoride fiber with a commercially available blue diode pump laser. To create and maintain a pulsed output, the researchers also had to figure out how to carefully manage the polarization of light in the fiber.

“Developing a laser at a new wavelength, where the material properties of the optical components are different from those used previously, can sometimes be challenging,” said co-author Michel Olivier.

“However, our experiments showed that the performance of our laser was in very good agreement with our simulations. This confirmed that the system was performing well and was being understood, and that important system parameters were correctly characterized and well suited for the pulsed lasers, especially the properties of the optical fiber we used.”

Next, the researchers would like to improve the technology by making the setup completely monolithic, meaning that the individual fiber-wound optical components would all be directly connected to each other. This would reduce the optical losses of the setup, improve efficiency and make the laser more reliable, compact and robust. They are also investigating different ways to improve the laser pulse energy, pulse duration and average power.