Researchers demonstrate the first visible wavelength femtosecond fiber laser

WASHINGTON — Researchers have developed the first fiber laser that can generate femtosecond pulses in the visible range of the electromagnetic spectrum. Fiber lasers that produce bright, very short wavelength pulses can be useful for a variety of biomedical applications as well as other areas such as materials processing.

The visible femtosecond waves are usually obtained using complex and inefficient settings. Although fiber lasers are a very promising alternative due to their ruggedness/reliability, small footprint, efficiency, lower cost and high brightness, until recently it has not been possible to generate visible pulses with durations in the femtosecond (10^-15 s) direct range with the laser.

“Our demonstration of a femtosecond fiber laser operating in the visible spectrum paves the way for a new class of ultrafast lasers that are reliable, efficient and compact,” said research team leader Réal Vallée of Université Laval in Canada.

In the journal Optica Publishing Group optical Letter, the researchers describe their new laser, which is based on lanthanide-doped fluoride fibers. Emits a red light at 635 nm, the laser achieves a compressed pulse with a duration of 168 fs, a peak power of 0.73 kW and a repetition rate of 137 MHz. Using a commercial blue laser diode as the optical energy source, or pump source, helps make the overall design robust, compact, and cost effective.

“Provided 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-precision, high-quality ablation of biological tissue and two-photon excitation microscopy. Femtosecond laser pulses also enable cold ablation during material processing, a process that can produce much cleaner cuts (than longer pulses) because it produces no thermal effects.”

Getting visible light from a fiber laser

In fiber lasers, optical fibers doped with rare earth elements act as a lasing medium. Although fiber lasers are one of the simplest, most robust and reliable high-brightness laser systems, the use of silica fibers tends to limit them to the near-infrared spectral region. Vallée’s group has been working to extend the spectral range of this laser source by using fibers made of fluoride instead of silica.

“We previously focused on developing mid-infrared fiber lasers, but more recently we have become interested in visible fiber lasers,” said Lord. “Although the lack of a compact and efficient pump source for such lasers hindered their development for a long time, the recent emergence 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 a fiber laser that emits continuous visible wavelengths, the researchers wanted to extend their progress to ultrafast pulse sources. Thanks to improvements in the fluoride fiber manufacturing process, it is now possible to obtain lanthanide doped fibers with properties important for the development of efficient visible fiber lasers.

Integrating technology into new lasers

The new pulsed fiber laser developed by the Vallée team combines a lanthanide-doped fluoride fiber with a commercially available blue diode pump laser. To create and maintain a pulsed output, researchers also have to think about how to carefully manage the polarization of light in the fiber.

“Developing lasers at new wavelengths, where the material properties of optical components differ from those used previously, can sometimes be tricky,” said co-author Michel Olivier. “However, our experiments show that the performance of our laser closely matches that of our simulations. This confirms that the system is well behaved and understood, and that the important parameters of the system are correctly characterized and well adapted for pulsed lasers, especially the properties of the optical fiber we are using.”

Next, the researchers want to improve the technology by making the arrangement completely monolithic, meaning that the individual fiber-tipped optical components will all bond directly to one another. This will reduce the optical loss of the setup, improve efficiency, and make the laser more reliable, compact, and powerful. They also investigated different ways to increase laser pulse energy, pulse duration and average power.

Paper: M. -P. Lord, M. Olivier, M. Bernier, R. Vallée, “Visible femtosecond fiber laser,” Choose. Lett.48, 14 (2023).
DOI: https://doi.org/10.1364/OL.492671.

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