Alliance unit Aochuang Photon's new progress in self similar amplification technology in the 1030 nm band

Femtosecond laser has the characteristics of narrow pulse width, wide spectrum, and high peak power, making it an ideal tool for scientific research and industrial processing. Fiber optic femtosecond laser systems have rapidly developed with advantages such as high output power, good beam quality, and simple structure. However, due to the inherent limitations of small fiber core area and limited gain bandwidth, it is difficult to directly output high-quality, narrow pulse width high-energy pulses.

Although chirped pulse amplification technology can obtain higher single pulse energy, due to the absence of spectral broadening during the amplification process and the strong gain narrowing effect, the output pulse width can also be widened, often in the order of hundreds of femtoseconds. Self similar amplification technology accumulates linear chirp by controlling the nonlinear effects during the amplification process, which can achieve pulse spectrum broadening while amplifying pulse power, supporting high pulse energy, and the output result is independent of the shape of the incident pulse. In addition, the linear chirp generated during the pulse amplification process can be compressed through grating pairs, providing conditions for obtaining high-power and high-energy femtosecond pulse laser output.

Unlike chirped pulse amplification technology, which strives to reduce nonlinear effects in optical fibers to ensure that the pulse shape and spectral shape are not distorted by self phase modulation, self similar amplification technology aims to utilize the characteristics of self phase modulation to broaden the spectrum. According to the time bandwidth product theory, the wider the spectrum, the narrower the limit pulse. However, if any pulse is allowed to self phase modulate and broaden the spectrum, the final result must be pulse distortion, skirt generation, and reduced signal-to-noise ratio.

Research on the theory of self similar pulse amplification shows that, without considering fiber absorption loss and higher-order dispersion, when pulses are transmitted and amplified in positive dispersion gain fibers without length constraints, the nonlinear Schrodinger equation can obtain a self similar asymptotic solution with linear chirp, and its envelope shape will eventually evolve into a parabolic shape. Due to the fact that the nonlinear phase accumulated by self phase modulation is a function of the same shape as the pulse, for parabolic pulses, regardless of how self phase modulation is accumulated, the corresponding chirp is constant, which transforms the nonlinear chirp into linear chirp. Therefore, the accumulated nonlinear phase can also be compressed using a pulse compressor with linear chirp like linear chirp.

This undoubtedly provides a new way for the compression process of fiber amplification systems, and provides new possibilities for obtaining high-power, chirp free ultra short pulse lasers. Unlike chirped pulse amplification, in a self similar pulse amplification system, the spectrum is broadened together with the time-domain pulse under the action of self phase modulation. Based on the time bandwidth product, it is easy to achieve a narrower pulse width after pulse compression compared to the seed pulse width. When the parameters of the amplifier are fixed, self similar amplification is only related to the initial energy of the incident pulse and is not affected by parameters such as the shape and width of the incident pulse.

Figure 1 (a) Waveform of Pulse during Time Domain Transmission

(b) Waveform of pulse transmission in frequency domain

As Aochuang Photonics continues to overcome difficulties, it has successfully mastered the 1030 nm band self similarity amplification technology. Using the SESAM mode-locked fiber laser oscillator made by Aochuang Photonics as the seed source, a self similar amplifier with an all fiber structure was achieved through a combination of gain control and dispersion management. The pulse autocorrelation curve after dispersion compensation is shown in Figure 2, where the pulse width approaches the transformation limit. A narrower pulse width is beneficial for obtaining higher peak power during the gain process, thereby better utilizing the self phase modulation effect to achieve spectral broadening. The pulse after dispersion compensation enters the self similar amplification stage, and the envelope of the amplified pulse spectrum shows a significant parabolic shape. The central wavelength of the spectrum is located near 1030 nm, without significant deviation. The spectral bandwidth is greater than 20 nm, and it is self controllable (as shown in Figure 3). The corresponding autocorrelation curve indicates that the time-domain pulse has also evolved into a parabolic shape (as shown in Figure 4). At this point, the pulse can be easily compressed to less than 100 fs.

Figure 2 Autocorrelation image after dispersion compensation

Figure 3 Spectral shape after self similarity amplification

Figure 4 Autocorrelation image after self similarity amplification

The self similar amplification technology mastered by Aochuang Photonics has many application advantages. Firstly, provide a stable, high linear chirp, and high-power seed source for the next stage of power amplification and energy amplification. At present, Aochuang Photon has achieved a high-energy femtosecond pulse laser with a single pulse energy greater than 100 uJ and a pulse width less than 170 fs by combining self similarity amplification technology with chirped pulse amplification technology (as shown in Figure 5). Secondly, providing high-energy parabolic pulses for nonlinear pulse compression technology, improving the linear chirp of the broadened spectrum, brings new opportunities for outputting high-quality single cycle femtosecond laser pulses. Finally, the linearly chirped parabolic pulse generated by self similar amplification has important application value in fields such as high coherence supercontinuum spectrum generation, arbitrary pulse time-domain shape synthesis, and optical information processing.

Figure 5 Autocorrelation image corresponding to single pulse energy greater than 100 uJ

Since its establishment in 2018, Aochuang Photonics has applied for more than 140 patents and has mastered key core technologies such as high-energy and high-power femtosecond pulse amplification technology, chirped volume Bragg grating dispersion compensation technology, wavelength conversion, etc. In combination with self-designed and manufactured ultra-fast seed sources, temperature tunable chirped fiber Bragg gratings, and other core devices, Aochuang Photonics has successfully launched a series of femtosecond laser products, and has taken the lead in achieving mass shipment in the industrial field in China, Breaking the long-standing monopoly of foreign products in this field. At present, Aochuang Photonics continues to cater to the development pace of high-end precision industries such as aerospace, new energy lithium-ion batteries, and electronic consumption in the current market, and strengthens itself, continuously laying a solid foundation for the transformation and upgrading of advanced manufacturing industries and promoting development.

Source: Alliance unit Aochuang Photon