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High-power all-solid-state femtosecond lasers

High-power all-solid-state femtosecond lasers

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Project Output/Brief Introduction:
The power density of high-power femtosecond laser focusing is very large, which can easily induce molecular valence bond cleavage and atomic ionization of almost any material, and the irreversible changes in state and physical properties such as phase transformation caused by this provide a physical basis for ultrafast laser material processing. Due to its high intensity, ultrafast laser processing has a broad spectrum of materials for a wide range of materials, including diamonds, sapphires, glass, semiconductors, metals, ceramics and polymers. Since the excitation process is ultra-fast, thermal relaxation is avoided, making ultrafast excitation a quasi-adiabatic process. Traditional 3D printers use nanosecond pulsed lasers, which can process some low-temperature materials, but if you want to process higher-temperature materials, you need more laser power, and the thermal stress generated at this time will affect the printing effect. 3D printing with femtosecond lasers creates little to no thermal stress, giving it an inherent advantage in creating denser parts. At present, ultrafast laser micromachining has been applied in consumer electronics, biomedical, aerospace, information technology, new energy, new materials and other industries.

At present, the commercially available high-power femtosecond lasers mainly include titanium-sapphire lasers and fiber lasers. Titanium-sapphire lasers are very mature, but they are extremely expensive. Due to the very high nonlinear effect and long pulse width (>500 fs) at high power output, fiber lasers still have problems such as insufficient processing accuracy and slow processing speed in special material cutting and special-shaped hole processing. In contrast, all-solid-state femtosecond lasers have the characteristics of large single pulse energy, narrow pulse width (<100 fs), low cost, etc., which can not only realize the fine processing of special materials (glass, ceramics, special brittle materials), but also show unique advantages in the fields of laser medicine (dentistry, ophthalmology, stone surgery), and have great application prospects. Therefore, the development of high-power all-solid-state femtosecond lasers has important applications.

High-power ultrashort pulse lasers have emerged in industrial and scientific research fields such as precision lasers, micro-nano manufacturing, etc. Based on all-solid-state femtosecond laser technology, this project develops an all-solid-state femtosecond laser with high average power, narrow pulse width and high repetition rate for fine processing of special materials and multiphoton real-time microscopy. We have carried out in-depth research on ytterbium-doped all-solid-state femtosecond lasers pumped by laser diodes (LD), and have successfully developed a variety of high-power, narrow-pulse-width all-solid-state femtosecond laser oscillators (as shown in Figure 1) and amplifiers (as shown in Figure 2), with internationally advanced laser indicators and leading in China. Benchmarking the series products of Lithuanian Light Conversion, an internationally renowned laser brand, it is planned to carry out the industrialization research of all-solid-state femtosecond lasers based on the existing high-power narrow-pulse Kerr lens mode-locked all-solid-state femtosecond lasers. By optimizing the unit device technology of all-solid-state femtosecond laser, the engineering process design, quality control and integration research of the whole laser system, the high-power all-solid-state femtosecond laser with small structure, high efficiency, stability and reliability has been successfully developed.

Figure 1  Principle prototype and output parameters of high-power narrow pulse femtosecond oscillator\

                           Figure 2    All-solid-state femtosecond magnifier photo

High-power underpump thermal management technology

On the one hand, the high-average power femtosecond laser output requires a high-power CW laser pump, and on the other hand, the optical efficiency of ytterbium-doped all-solid-state femtosecond lasers is generally 10%-30%, so a large part of the pump light absorbed by the crystal is converted into heat. In order to alleviate the influence of heat accumulation on the beam quality and power stability of femtosecond mode-locked lasers, effective thermal management of crystals is required. We've mastered the core technology, and Figure 3 shows us using our patent-pending thermal management scheme to achieve an average power output of >20 W at 100 W pump power with a beam quality of better than 1.2 and less than 0.5% power jitter.

Figure 3  At 100W power pump (57% absorption efficiency), the femtosecond amplifier can output 20W of average power with a beam quality better than 1.2

High-Power Kerr Lens Mode-Locking Technology The main technical means for LD-pumped all-solid-state femtosecond lasers to produce narrow pulse output is Kerr lens mode-locking technology. We use Kerr lens mode-locking technology to obtain pulses as short as 33 fs from ytterbium-doped all-solid-state femtosecond lasers (Photonics Research 3(6), 335 (2015)). However, the output power of conventional Kerr lens mode-locked lasers is generally in the order of -100 mW, which limits their application in many fields. We have developed a new dual-focus resonator structure that allows both high-power laser output and Kerr lens mode-locking to be guaranteed. With LD direct pumping, we got an average power of 1.5 W and a pulse width of 68 fs in the Yb:CYA laser, an order of magnitude higher than the previous output power. The related research results were published in Photonics Research 6(2), 127 (2018() CAS Area I), and an invention patent (an LD-pumped high-power Kerr lens self-locking mode laser, ZL 2017 1 0568672.6) was authorized in 2019, as shown in Figure 4.

      Figure 4    High-power Kerr lens mode-locked laser based on dual confocal cavities

By further optimizing the cavity design, the high-power narrow pulse width Kerr lens mode-locked operation with an average power of 6.2 W and a pulse width of 59 fs and an average power of 10 W and a pulse width of 97 fs was achieved, with a maximum single pulse energy of 124 nJ and a peak power of 1.85 MW, which is the highest peak power obtained in the current block Yb all-solid-state femtosecond laser, and the related results were published in Optics Express 27, 21448 (2019) and Optics Letters 46, 1297 (2021)。

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