Unlocking visible femtosecond fiber oscillators: progress in laser science

The emergence of ultrafast laser pulses marks an important milestone in laser science, triggering astonishing progress in a wide range of disciplines such as industrial applications, energy technology, and life sciences. Among various laser platforms that have been developed, fiber optic femtosecond oscillators are highly praised for their compact design, excellent performance, and cost-effectiveness, and have become one of the mainstream technologies for femtosecond pulse generation.

However, their working wavelengths are mainly limited to the infrared region, ranging from 0.9 to 3.5 μ m. This in turn limits their applicability in many applications that require visible wavelength light sources. For a long time, expanding compact femtosecond fiber oscillators to new visible light wavelengths has been a challenging but eagerly pursued goal in laser science.

Currently, most visible light fiber lasers use rare earth doped fluoride fibers, such as Pr3+, as effective gain media. Over the years, significant progress has been made in the development of wavelength tunable, high-power, Q-switched, and mode-locked visible light fiber lasers.

However, despite significant progress in the near-infrared field, achieving femtosecond mode locking in visible light fiber lasers remains a highly challenging task. This challenge is attributed to insufficient development of ultrafast optical components for visible light wavelengths, limited availability of high-performance visible light modulators, and extremely normal dispersion encountered in visible light fiber laser cavities.

Recent attention has been focused on near-infrared femtosecond mode-locked fiber oscillators using phase biased nonlinear amplification ring mirrors. PB-NALM eliminates the need for accumulated phase shift in long cavity fibers.

This innovation not only promotes tuning flexibility and long-life operation, but also provides the opportunity to manage intracavity dispersion in a larger parameter space, from normal dispersion state to abnormal dispersion state. Therefore, it is expected to promote the breakthrough of direct femtosecond mode locking in visible light fiber lasers and push fiber femtosecond oscillators towards the visible light band.

According to reports, researchers from the Fujian Key Laboratory of Ultra Fast Laser Technology and Applications at Xiamen University have recently developed a visible light mode-locked femtosecond fiber oscillator and amplifier.

The fiber optic femtosecond oscillator emits red light at 635 nm and adopts a 9-shaped cavity configuration. It uses double clad Pr3+doped fluoride fibers as visible light gain media, adopts visible light wavelength PB-NALM for mode locking, and utilizes a pair of customized high-efficiency high channel density diffraction gratings for dispersion management. The visible self starting mode locking established by PB-NALM directly generates red laser pulses with a pulse duration of 199 fs and a repetition rate of 53.957 MHz from the oscillator.

Accurate control of the spacing between grating pairs can switch the pulse state from dissipative or stretching pulse solitons to traditional solitons. In addition, the chirped pulse amplification system built together with the oscillator greatly improves laser performance, achieving an average output power of over 1 W, a pulse energy of 19.55 nJ, and a pulse duration of 230 fs.

Professor Luo Zhengqian, Director of the Department of Electronic Engineering at Xiamen University, said, "Our research results represent a solid step towards high-power femtosecond fiber lasers that cover the visible spectrum region and may have important applications in industrial processing, biomedical research, and scientific research.".

The author expects that their new solution for generating high-performance visible light femtosecond fiber lasers will lay the foundation for applications such as precision processing of special materials, biomedical, underwater detection, and optical atomic clocks.

Source: Laser Net