High power sources are increasingly finding important industrial applications such as micromachining, material processing and biomedical imaging. In particular, femtosecond sources can emit optical pulses at durations below one picosecond. High-power femtosecond sources can be obtained with different types of lasers, including scientific and industrial lasers. With the increasing industrial application of femtosecond sources, many studies are also being carried out to improve their performance and expand their application scope.
The application of high-power femtosecond sources can be achieved in numerous ways. One popular scheme uses Yb-doped gain media, which continue to attract significant research attention owing to their low quantum defect rate, long fluorescence lifetimes and broad gain bandwidth. Among the known Yb-doped gain media, ytterbium doped Yttrium Aluminium Garnet (Yb:YAG) is a promising candidate for power amplification applications, especially in low average output power sources. This can be attributed to their relatively large emission cross-section and high thermal conductivity.
Heat dissipation in the gain medium is one of the main parameters when applying a high-power laser diode to Yb-doped gain media. To this end, increasing surface-to-volume ratio has been the research focus on gain structures. Although various femtosecond amplifiers have been developed, their efficiency and suitability for high-power source amplification remain poor. For example, thin-disk and InnoSlab amplifiers require complex pump and amplifier configurations to attain adequate gain levels, while all fiber amplifiers are limited by nonlinear optical effects.
Previous findings established that a seed beam with a center wavelength of 1030 nm is suitable for achieving power amplification by Yb:YAG. All polarization-maintained (PM) fiber oscillators are examples of femtosecond oscillators with a center wavelength of 1030 nm. These oscillators have been investigated for various operation regimes, especially the soliton regime. Although a 1030 nm oscillator can be constructed in the dissipative soliton regime using a nonlinear optical loop mirror, the associated pulse generation mechanism is generally complex. Interestingly, these limitations can be addressed using a semiconductor saturable absorber mirror (SESAM), whose feasibility is still underexplored.
Herein, Dr. Jun Wan Kim, Ms. Seolwon Park, Dr. Guang-Hoon Kim, Dr. Vladimir Yashin and Dr. Juhee Yang from Korea Electrotechnology Research Institute engineered an all-polarization-maintained SESAM mode-locked fiber oscillator and demonstrated its amplification by Yb:YAG thin rod and Yb-doped fiber. The SESAM configuration was created with commercially available components. The operation of all-PM dissipative soliton fiber oscillators in regimes near 1030 nm was explored. Their work is currently published in the journal, Laser Physics.
The authors showed that the dissipative soliton fiber oscillator exhibited stable and self-starting mode-locking operation near a wavelength of 1030 nm. The incorporation of three-stage fiber amplifiers pumped by single-mode fiber-coupled laser diodes and single-stage Yb:YAG thin-rod amplifiers played a vital role in boosting the performance of the oscillator. For instance, its output power was boosted to 11.3 W at a center wavelength of 1030 nm and 495 kHz repetition rate. These repetition rates were successfully controlled by inserting an acousto-optic modulator.
The main advantage of this approach was that the system complexity and volume could be minimized by using fiber amplifiers pumped by SMF-coupled laser diodes and thin rod amplifier to amplify the output power. After pulse compression via diffraction grating, amplified pulses exhibited a pulse duration of 758 fs achieved at an output power of 9 W, peak power level of 21.1 MW and pulse energy of 18.2 µJ. Notably, the combination of Yb:YAG thin-rod amplifier and all-PM SESAM mode-locked oscillator proved to be an efficient scheme for amplifying micro-Joule-level pulse energy and generating femtosecond pulse.
In summary, the feasibility of fiber and a Yb:YAG thin rod in amplifying 1030 nm all-PM SESAM mode-locked dissipative soliton fiber oscillator was demonstrated. Overall, the results showed a simple and efficient amplification of pulses to high energy levels. In a statement to Advances in Engineering, the researchers stated that their findings contribute to developing robust, compact and cost-effective femtosecond laser sources for a wide range of industrial applications.
Kim, J. W., Park, S., Kim, G.-H., Yashin, V. E., & Yang, J. (2022). A 1030 nm all-PM SESAM mode-locked dissipative soliton fiber oscillator and its amplification with Yb-doped fiber and a Yb:YAG thin rod. Laser Physics, 32(10), 105102.