Technological advances have laid the foundation for reliable, compact ultrafast lasers in areas such as biomedical applications. Researchers recently developed the first fiber laser capable of generating femtosecond pulses in the visible range of the electromagnetic spectrum. This advancement brings potential for a variety of biomedical and materials processing applications. What makes these lasers unique is their ability to produce ultrashort, bright pulses of visible wavelengths, a major advance in laser technology.
Overcoming challenges in fiber laser development
Historically, achieving visible femtosecond pulses required complex and inherently inefficient setups. Although fiber lasers are a very promising alternative due to their robustness/reliability, small footprint, high efficiency, low cost and high brightness, so far it has not been possible to generate femtosecond duration (10-15s) range measurements directly using such lasers.
Réal Vallée, leader of the research team at Université Laval in Canada, said: "The femtosecond fiber laser we demonstrated in the visible spectrum paves the way for new types of reliable, efficient and compact ultrafast lasers."
Technical details of new fiber laser
The researchers describe their new laser, which is based on rare earth-doped fluoride fiber, in Optica Publishing Group's journal Optics Letters. The laser emits red light with a wavelength of 635nm and can achieve a compressed pulse with a duration of 168fs, a peak power of 0.73kW, and a repetition rate of 137MHz. Using commercially available blue laser diodes as the light or pump source helps make the overall design robust, compact, and cost-effective.
Marie-Pier Lord, a PhD student involved in the project, said: "If higher energies and powers are achieved in the near future, many applications could benefit from this type of laser. Potential applications include high-precision, high-quality biological tissue ablation and two-photon excitation microscopy. Femtosecond laser pulses also allow cold ablation during material processing, which allows for cleaner cuts [than longer pulses] because it does not produce thermal effects."
Extending the spectral range of fiber lasers
In fiber lasers, optical fibers doped with rare earth elements act as the laser medium. Although fiber lasers are among the simplest, rugged, and reliable high-brightness laser systems, the use of silica fibers tends to limit them to the near-infrared spectral region. Vallée's team has been working to expand the spectral range of these laser sources by using optical fibers made of fluoride instead of silica.
"We previously focused on developing mid-infrared fiber lasers, but have recently become interested in visible fiber lasers," Lord said. "While the lack of compact and efficient pump sources for such lasers has long hindered their development, the recent emergence of blue spectrum semiconductor laser sources provides key technology for the development of efficient visible fiber lasers."
After demonstrating a fiber laser that continuously emits visible wavelengths, the researchers hope to extend this progress to ultrafast pulsed sources. Thanks to improvements in fluoride fiber manufacturing processes, lanthanide-doped fibers are now available, whose properties are critical for the development of efficient visible fiber lasers.
Innovation and future directions
The new pulsed fiber laser developed by Vallée's team combines a rare earth-doped fluoride fiber with a commercial blue diode pump laser. To generate and sustain pulsed output, researchers also had to figure out how to carefully manage the polarization of light in the fiber.
"Developing lasers at new wavelengths, where the material properties of the optical elements are different from those used previously, can sometimes be tricky," said co-author Michel Olivier. "However, our experiments show that the performance of our laser agrees very well with our simulations. This confirms that the system performs well and is easy to understand, and that the important parameters of the system are correctly characterized and well suited to the properties of the pulsed laser, especially the fiber we used."
Next, the researchers hope to improve the technology by making the device fully integrated, meaning the individual fiber-pigtail optical elements will be bonded directly to each other. This will reduce optical losses in the device, increase efficiency, and make the laser more reliable, compact and robust. They are also investigating different ways to increase laser pulse energy, pulse duration and average power.