The output power of high power femtosecond laser breaking through the key bottleneck of average power can reach the order of 100 watts

High energy, high average power femtosecond laser due to the attosecond high order harmonic generation, precision processing and manufacturing, biomedical and national defense and other fields of extensive application needs, is the forefront of ultrafast super laser technology research in the past decade.

Especially fiber laser due to stable and reliable operation characteristics, compact structure, excellent beam quality, low cost and other advantages. It has received much attention from people and is also a popular laser product with an average output power of up to 100W. 

However, due to the harmful nonlinear effects in the fiber, the single pulse energy generated by a single fiber is difficult to break through the bottleneck of the millisjiao while ensuring the pulse quality and beam quality in the time domain, which limits many important applications that require laser intensity.

The coherent synthesis technique is a feasible method to obtain femtosecond pulses with high average power and millifocal order by combining the multi-channel amplified femtosecond pulses together. There are two kinds of coherent components: active coherent synthesis and passive coherent synthesis. 

The power and energy of active coherent synthesis can be increased with the increase of the number of synthesis paths, but complex and expensive electronic control locking system is needed. However, passive synthesis does not need an electronic phase stabilizer, and the device is relatively simple, but limited by the number of synthesis paths, the synthetic average power and single pulse energy are low.

In view of the above problems and difficulties, the L07 group of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center of Condensed Matter Physics, based on years of research on high-power ultrafast fiber lasers, proposed that Static Mode Degradation (SMD) in fibers is a key bottleneck to limit the average power of passive coherent synthesis schemes. 

Based on this, a bidirectional isolator that can effectively inhibit SMD has been invented. After achieving an average power of 100W in 2021 (Opt.Lett. 46, 3115 (2021)), recently based on a passive synthetic ytterbium-doped ultrafast fiber laser system, not only further obtained the results of a maximum average power of 200 W. At the repetition rate of 100 kHz, the single pulse energy reaches 1.07 mJ, and the synthesis efficiency of the system exceeds 85%. the results are published in the latest issue of the Journal of the Optical Society of America B. The first author of the paper is Shi Zhuo, a doctoral student supervised by Chang Guoqing Special Researcher.

Figure 1. Experimental device diagram

As shown in FIG. 1, the polarized laser pulse provided by the front end with an energy of 0.80μJ and an adjustable repetition frequency between 100kHz and 1MHz is widened, reflected by PBS1 and transmitted by PBS2. After the time splitting device consisting of PBS3 and PBS4 is divided into two small pulses with an interval of about 2ns, the beam splitting device is divided into two small pulses. Further amplification by PBS5 is divided into four pulses into the Sagnac loop. 

Two of the pulses are transmitted in a clockwise direction and the other two are transmitted in a counterclockwise direction and are circularly polarized using quarter wave plates (QWP1 and QWP2) before entering the bar fiber. A polarizing beam splitter PBS6 is inserted between the two gain fibers to polarize the pulses, and the light in both directions is transferred for a circle at PBS5, and the pair-wise synthesis is performed. Some of the depolarized light leaks out from the synthesis, forming a depolarization port.

 Most of the light is returned from the original path, and becomes a pulse through the time domain coincidence at the time division pulse device. Some of the unsynthesized light is not output at the synthesized port, and the synthesized light is output from the synthesized port. The experimental results show that the average power of the synthesized port reaches 160W at a repetition frequency of 150kHz. 

When the repetition rate is reduced to 100kHz, the single pulse energy after pulse compression is 1.07mJ, and no obvious SMD phenomenon is observed during the amplification process. Figure 2 shows the main measurement results at this energy. The display pulse width is 240fs, spectral width is 8.7nm, the corresponding RMS within 3 hours is less than 0.5%, the beam quality M2 factor is 1.11×1.27, and the longitudinal beam distortion is mainly from the grating pair.

Figure 2. Results of (a) autocorrelation curve, (b) spectral distribution, (c) power stability and (d) beam quality at 1.07 mJ

Compared with previous high-power ytterbium-doped fiber femtosecond laser sources based on single amplification or active synthesis, this study uses a passive synthesis method with simple structure, and obtains results greater than 1mJ, breaking through the bottleneck of conventional femtosecond fiber laser monopulse energy, and has an average power output capacity of up to 200W, excellent beam quality and stability. 

It is expected to play an important role in the generation of high repetition frequency attosecond high harmonics, precision machining and cutting of special materials, semiconductor chip defect detection and biomedical imaging. The devices and core devices related to this progress have applied for national invention patents.

Source: Sohu