[Laser Technology Sharing] How much do you know about ultrafast amplification technology?

Abstract: In the development of pulse lasers, the pursuit of faster (shorter pulse width), higher (higher average power), and stronger (stronger peak power density) has always been an eternal theme of researchers. The development of ultrafast laser pulse amplification technology is one of the effective means to obtain stronger peak power laser pulses.

CPA (Chirped Pulse Amplification):

On the road to pursuing stronger peak power density of lasers, everyone initially amplified the shortest laser pulses available at that time directly through the gain medium, fighting back and forth, and racing all the way. The ultimate peak power density that can be achieved by laser pulses rises rapidly, but soon encounters a bottleneck because the gain medium or other optical components will be damaged at too high a power density. Everyone who has done laser magnification must have experienced the situation where the crystal lens is broken and punctured.

Figure 1: Broken Titanium Gemstone

It is the emergence of CPA technology [1] that breaks this bottleneck, which is undoubtedly an important milestone in the development of laser technology. Here is a picture that has been cited countless times. The horizontal axis is time, and the vertical axis is the peak power density of the laser.

Figure 2: Development of laser peak power density over time [2]. The author is Mourou, the inventor of CPA.

If you encounter the problem of too high peak power density, then the natural idea is not to make the peak power density too high. In fact, the peak power density is not allowed to be too high for the time being, so as to protect the optical components. Let’s look at a formula:

Figure 3: Calculation of pulse laser peak power density

While maintaining the pulse energy constant, the time width of the pulse is temporarily broadened and the area is increased. In this way, the peak power density is reduced, and damage to the optical components is naturally avoided. As long as the pulse time width can be narrowed and the area is small in the end, won't the peak power rise again? From simplicity to simplicity, genius's ideas always look simple. This reminds me of when I was a kid. Every time I ate KFC, I loved the ketchup in it. I always wanted more. If I couldn’t eat it, I would have to walk around and take it home to dip it in something else. But if you go to the counter alone, the lady will only give you one or two packs. At that time, I had an idea. If I asked the whole class of 60 people to come, and each person asked for 2 bags, and finally everyone gave it to me, wouldn’t I have 120 bags at once? Even if some students in the middle may eat a few packets and lose 20 packets, it doesn’t matter. The peak ketchup density went directly from 2 to 100 (although the average ketchup density per person didn't change much)!

Figure 4: Childhood dream

So the key is, can the time width of the laser pulse be broadened? After stretching and amplifying, can it be compressed back to the pulse width before stretching? In fact, it is possible, because femtosecond laser pulses usually have a wide spectral range, and through some means, different spectral components can be made to travel different distances and then integrated together. Just like a row in the National Day military parade, it looks like there is only one person in the row, but in fact there are many people in the row. If everyone is staggered back and forth, it will look like a very long row from the side.

Figure 5: The uniform and impressive military parade

Usually this situation where different spectral components are located at different time parts of the pulse is called chirp, which is borrowed from the concept in the field of communications. The original meaning of chirp is to describe the melodious sound of a bird, with sounds of different frequencies separated in time. This is where light of different frequencies separates in time.

This technology is called chirped pulse amplification, or CPA for short. Through the previous introduction, the CPA system usually consists of three parts, the expander, the amplifier, and the compressor. As the name suggests, the stretcher broadens the time width of femtosecond laser pulses, usually to hundreds of picoseconds or even nanoseconds. Commonly used methods include Martinez-type and Ofner-type stretchers that use gratings. Chirped fiber Bragg gratings (CFBG) are commonly used in fiber lasers. When the broadening amount is not high, the dispersion of bulk materials can also be used to broaden or use chirped mirrors to broaden. Amplifiers have various structures due to different gain media and repetition frequencies, but their function is to increase the energy of the broadened pulse. We will discuss it in detail in future articles, so we will not go into details here.

The function of the compressor is to compress the pulse width of the broadened and amplified pulse back. The most commonly used is the grating pair compressor, which can be built with a transmission grating or a reflection grating. When the amount of broadening is not large, a prism pair or a chirped mirror can also be used for compression. After the emergence of CPA technology, laser amplification has developed rapidly due to the breakthrough of the bottleneck of peak power density. my country has also been at the forefront of the world in research in the field of ultrafast laser amplification. In 2011, the L07 group of the Institute of Physics, Chinese Academy of Sciences took the lead in achieving the world's highest peak power of 1.16PW (1.16x1015W). At present, the Shanghai Institute of Optics and Precision Machinery has also achieved the world's highest peak power of 10PW and is moving towards the goal of 100PW.

OPA (Optical Parametric Amplification):

The principle of the general laser amplification process can be simply summarized as follows: when a laser beam passes through a gain medium in a particle number inversion state, the original beam is amplified under the action of stimulated radiation. The energy here is first transferred from the pump source to the gain medium, causing particle number inversion in the gain medium, and then transferred from the gain medium to the amplified laser. Obviously, a lot of energy will be wasted during the transmission process.

However, in nonlinear optics, there is still such a way of energy transfer. Laser will produce nonlinear effects in the medium due to its high power density. This effect can generate lasers of new frequencies or directly exchange energy between lasers of different frequencies. As long as the appropriate conditions are matched, the energy of the pump laser can be directly coupled into the amplified laser through the nonlinear medium. The nonlinear medium here only provides a suitable nonlinear effect. The energy is directly transferred from the pump laser to the amplified laser. There is no middleman to earn the price difference, and the efficiency is naturally much higher. This process in which the medium does not participate in energy conversion is called a parametric process, and using this parametric process to amplify laser light is called optical parametric amplification, or OPA for short. We generally call the pump laser as pump light (pump), and the amplified laser is called signal light (signal). During the parameter conversion process, light of other frequencies is generally produced, called idler light (idler).

Figure 6: Schematic diagram of OPA. Among them, green is the pump light, yellow is the signal light, and red is the idler light. You can see that the yellow light changes from dim to bright and is amplified

In principle, OPA has many advantages, including high efficiency, high temporal contrast, wide spectrum amplification, and can support both high single pulse energy and high average power. However, OPA has relatively high requirements for pump light. Because there is no intermediate layer, the quality of various parameters of the pump light will seriously affect the amplification of the signal light, and requires fine angle and distance control. In addition, it is currently difficult to grow nonlinear crystals into large-sized products, which also limits the size of the beam aperture. In contrast, the requirements for ordinary non-parametric amplification are not so high. The commonly used saying is that the pump light and laser are irradiated on the crystal. And there are many laser crystals that can grow products of large sizes, which further supports the amplification of high-energy lasers. For these reasons, when amplifying super-energy femtosecond laser pulses, ordinary non-parametric amplification is still used as the main amplification of the last few stages.

OPCPA (Optical Parametric Chirped Pulse Amplification)：

The two laser amplification technologies of OPA and CPA were introduced earlier. In fact, the two are not completely unrelated. If pulse broadening is also performed before OPA (optical parametric amplification), and pulse compression is performed after amplification, then the advantages of both amplification technologies can be taken into account. This amplification technology is called optical parametric chirped pulse amplification, or OPCPA for short [3].

The laser pulses amplified by OPCPA generally have high temporal contrast (based on the mechanism of the parametric amplification process, you can think about the reasons), and are often used as the front stage of high-contrast ultra-intense laser systems. At the same time, because the OPA process has a wide spectrum and in some cases has a CEP locking effect, it can achieve an average power of tens of watts and several terawatts of kilohertz output, and has gradually become a powerful tool in current attosecond scientific research.

References

[1] Strickland, D., and G. Mourou."COMPRESSION OF AMPLIFIED CHIRPED OPTICAL PULSES." OpticsCommunications 55.3 (1985): 219-221.

[2] Mourou, G., et al."Exawatt-Zettawatt pulse generation and applications." OpticsCommunications 285.5 (2012): 720-724.

[3] Dubietis, A., G. Jonušauskas, and A.Piskarskas. "Powerful femtosecond pulse generation by chirped andstretched pulse parametric amplification in BBO crystal." OpticsCommunications 88.4-6 (1992): 437-440.

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