[Frontier] An advanced laser focusing technology-Flying Focus Technology

[Frontier] An advanced laser focusing technology - flying focus technology

In the near-diffraction limit system, the focus size and the focus longitudinal range (Rayleigh length) have always been a contradiction. If you want a small focus size, you can only get a small focus vertical range. If you want a large focus range, you can only get a large focus size.

However, many times, we want both a small focus and a large range of action, especially in the field of ultra-fast laser precision processing. This demand is almost unsolvable using traditional methods. However, Dustin H. Froula and others from the University of Rochester in the United States have solved this problem perfectly. Let us take a look at their solution.

Let’s take a picture first. Hey, isn’t it a spectrum? What's so surprising? Wrong, kids, this picture reflects the time domain, not the frequency domain. That is to say, the abscissa is not frequency, but time.

Figure 1: Chirped laser and National Day Parade

That is to say, when this pulse passes through a point, we observe this pulse at this point, and we will find that the "color" of the pulse will change, from blue to red (actually the rate of change of wavelength is very small, the color here only refers to frequency).

Imagine that during the National Day military parade, the People's Liberation Army wearing different colors of military uniforms walked majestically in front of Tiananmen Square. Everyone's steps were the same, but the passing time was in order, and the same was true for the chirp pulses.

So how is this chirped laser pulse actually generated? To explain this problem, we need to understand the second most important technology of ultrafast lasers—chirped pulse amplification technology. The most important reason restricting ultrafast laser amplification is that the laser gain medium cannot withstand ultra-high peak power. For this reason, CPA (chirped pulse amplification) came into being. CPA is divided into three processes: broadening, amplification and compression.

The broadening system generally uses dispersive elements, such as gratings or prisms, to spatially separate the frequencies of femtosecond pulses that originally have a certain spectrum width, and then uses specific optical paths to control the optical path differences of different frequency components, thereby spreading each frequency component in the time domain.

To give a practical example: In a typical petawatt laser system, a Ti: Sapphire femtosecond laser pulse of tens of fs can be broadened into a pulse with a pulse width of several hundred ps. The pulse width is broadened by as much as 10,000 times, so the peak power is also reduced. It is then amplified to saturated power through the gain medium and then compressed back to the femtosecond level pulse width to obtain a high-energy femtosecond amplified pulse with extremely high peak power.

Here, the author only uses the first third of the chirped pulse amplification technology, that is, the broadening technology. A stretcher is used to broaden the pulse width of the femtosecond pulse to the ps or even ns level, thus forming a situation where each frequency component is separated in the time domain.

Figure 2: Chirped pulse amplification technology [2]

After obtaining each frequency component separated in the time domain, Dustin H. Froula and others used color focusing technology to achieve the separation of the focus of each frequency component. So what is color focus technology? Everyone knows that there will be chromatic aberration when focusing the lens, and light of different frequencies will produce different refraction inside the lens, causing the focus to shift. People always want to compensate for such chromatic aberration, so various chromatic aberration compensation systems have been designed.

However, the authors took a different approach. They thought in reverse. By designing a diffractive lens whose line width changes with the radial distance, light of different frequencies can be focused on different focal points in space. Together with the previously obtained chirped pulses, controllable movement of the femtosecond laser focus in space can be achieved. If the chirped femtosecond pulse is controlled in time, the focus can be controlled to move in the direction of laser propagation at any speed. The article mentioned that it can even reach "super-light speed" movement.

Figure 3: Schematic diagram of color focusing diffractive lens [3]

Figure 4: Schematic diagram of flying focus technology [1]

What is the use of this flying focus? The authors stated that this technology provides a novel method to control the laser plasma, which does not require a long focal length system or a waveguide to maintain the laser intensity over a long distance, and couples the focus movement speed to the group velocity of light. For some special applications, the authors can design a nonlinear chirp signal and color focusing system to control the laser focus movement speed. For example, the flying focus technology can be used in laser accelerators to generate extreme ultraviolet light sources. In addition, flying focus technology can change the optimization method of plasma and nonlinear optical devices, such as terahertz wave generation in nonlinear crystals and laser wake field accelerators. In addition, under the condition of negative focal velocity, each frequency component is focused in front of the plasma, which fundamentally avoids the interaction of the laser plasma, thus avoiding laser filamentation and stimulated scattering. This is of great significance to building a high-efficiency laser plasma amplifier

References

References

【1】Froula, Dustin H., et al. "Spatiotemporal control of laser intensity." Nature Photonics (2018): 1.

【2】Wikipedia contributors. "Chirped pulse amplification." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 24 Apr. 2018. Web. 27 Apr. 2018.

【3】Wang, Peng, Nabil Mohammad, and Rajesh Menon. "Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing." Scientific reports 6 (2016): 21545.