[Instrument usage tips] Detailed explanation of titanium sapphire multi-pass amplifier FemtoPower

In 2015, the acquisition of the Austrian company Femtolasers (hereinafter referred to as Femto) by the American company Newport was a big news in the ultrafast laser industry. Since then, the ultrafast laser world, especially in the field of attosecond science, has been missing a strong laser team. The Ti:sapphire ultrafast laser market continues to be monopolized by two companies: Coherence and Spectral Physics. Although KMLabs in the United States, Avesta in Russia, and some domestic companies have certain technical capabilities, their market shares are extremely low. It is expected that the market distribution of titanium sapphire will not change much in the short term.

Returning to the topic, does the backbone technical strength of Femto Company still exist in the parent company? We don’t know about this, but we can only see a few Femto titanium sapphire ultrafast oscillators on the Spectral Physics website, but its once-famous titanium sapphire amplification system-FemtoPower is missing, which is a pity. This system is the most complex laser that the author has ever adjusted and disassembled. Here I will introduce the basic principles and design techniques of this system in detail.

Figure 1: Appearance of Femtopower

one. Basic principles

FemtoPower is a typical chirped pulse titanium sapphire amplification system. This system can be split into a seed source, a stretcher, an amplifier and a compressor to understand the basic principles of the entire system [1]. In addition, the system also has options such as CEP stabilization and Hollow-core fiber pulse nonlinear compression.

1. Seed source and stretcher

Figure 2: Oscillator, DAZZLER and material stretcher

The seed source of this laser system is a sub-10 femtosecond oscillator. The ultra-wide spectral range lays the foundation for subsequent narrower pulse widths, and also provides sufficient bandwidth for locking CEP using difference frequency. This oscillator is Femto's flagship product and is still in the spectrum physics product line.

Different from other companies' complex grating-based pulse stretchers, the old version of FemtoPower uses a very compact block material stretcher. The stretched pulse width is about 10ps. This pulse width is enough to reduce the nonlinear effects in mJ-level amplifiers. The new version of the amplification system adds the dispersion compensation artifact-Dazzler, which further improves the final pulse time quality. The use of Dazzler also further improves the pulse compressor, which will be explained in detail below.

2. Multi-pass amplifier

Femto's amplifier design was originally a 9-channel design, and was later upgraded to 10-channel, with the option of high-energy multi-stage amplification. This article only describes the 10-channel amplifier in detail.

Different from the typical regenerative amplifier designs of Coherent and Spectral Physics, FemtoPower adopts a confocal multi-pass amplification solution, which utilizes the basic principle of a concave mirror: the light rays parallel to the axis of the concave mirror converge at the focus. A titanium sapphire crystal is placed at the focus and the pump light focus is adjusted here to achieve spatial coupling of the pump and amplified light. Refer to Figure 3 for the specific optical path. The pulses broadened by the block material are incident into the confocal cavity, and the MHz repetition frequency is amplified without going through the menu. After four-pass amplification, the pulse obtains a gain of thousands of times, and then is down-converted to the kHz level through the Pockels cell system, and then injected into the confocal cavity for 6-pass amplification. The question here is: how to achieve 10 magnifications in such a small space. The explanation is as follows:

(1) Front four-way: The picture below is a top view. If viewed from the side, the height of the light spot on the first concave mirror, titanium sapphire and the second concave mirror is different. Since the light spots are very small, 10 light spots can be accommodated using 1.5-inch and 1-inch mirrors.

(2) 5-8 channels: The height of the light spot is exactly opposite to that of the first four channels, and the relationship is symmetrical.

(3) 9-10 channels: Adjust the light height to the symmetrical dividing line position of the front eight channels, amplify twice, and finally saturate the output. As shown in the figure below, in order to prevent the amplified light from reaching saturation state prematurely, the light spot is narrowed to obtain a larger focused light spot.

The question here is where is the Dazzler mentioned above? The initial version did not have a Dazzler. The new version (or out-of-print version) places the Dazzler behind the Pockels box to shape the down-converted kHz pulses.

Figure 3: FemtoPower amplifier design

3. Pulse compressor

Figure 4 Pulse compressor. (left) old version, (right) new version

The old version of FemtoPower's pulse compressor uses a biprism pair scheme [2]. Because the second-order and third-order dispersion of the prism pair can match the dispersion of the material, pulses with good compression quality can be obtained. However, the shortcomings are also obvious: the amount of dispersion introduced by the prism is very small, which requires a very long spatial distance, which in turn makes the entire system huge. The new version uses a transmission grating method, but the third-order dispersion of the grating pair and the third-order dispersion of the material have the same sign and cannot be matched, so Dazzler is needed to solve this problem. Use high-resolution Dazzler and load ultra-high third-order positive dispersion, and the problem is solved!

The disadvantage of the transmission grating is that it will introduce nonlinearity. This problem can be solved by placing a high-dispersion chirped mirror behind the compressor. The chirped mirror lengthens the pulse width on the grating and reduces the B integral.

two. design skills

Let’s summarize the various subtle techniques in the design of this laser:

1. Improved contrast: First, the first four channels use MHz amplification to allow dense pulses to block ASE from eroding the number of upper energy level particles. Secondly, a bulk material stretcher with almost no loss is used to increase the single pulse energy of the seed light and further suppress ASE.

2. Dispersion management: The new version abandons the prism compressor and replaces it with Dazzler and transmission grating, which greatly improves the stability of the system and the controllability of the pulse width.

3. The basic principle of concave mirror is used to achieve 10-pass amplification, and the saturation effect of amplification is suppressed through beam transformation.

4. Multi-pass amplification ensures the possibility of CEP locking. Currently, regenerative amplification cannot match the CEP advantage of multi-pass amplification. Femto has developed a unique CEP locking solution, please refer to the Nature Photonics article [3] for details.

5. Use this laser system as the driving source, and use the inflated Hollow-core fiber to compress the pulse to the cycle level.

Figure 5 Periodic level pulse generating device

The core technicians of Wave Technology have used and disassembled and assembled no less than 10 sets of Femto oscillators and amplifiers. They have rich theoretical foundation and technical practical experience in the operation, maintenance, disassembly and reorganization of FemtoPower laser systems. Therefore, as a reader, if you happen to use this laser system and encounter a fault, you are welcome to contact us to discuss solutions, for free!

References

【1】S. Sartania, Z. Cheng, M. Lenzner, G. Tempea, Ch. Spielmann, F. Krausz, and K. Ferencz, "Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate," Opt. Lett. 22, 1562-1564 (1997)

【2】Z. Cheng, F. Krausz, Ch. Spielmann, “Compression of 2 mJ kilohertz laser pulses to 17.5 fs by pairing double-prism compressor: analysis and performance, Compression of 2 mJ kilohertz laser pulses to 17.5 fs by pairing double-prism compressor: analysis and performance,”Optics Communications, Volume 201, Issues 1–3, 2002, Pages 145-155

【3】S. Koke, C. Grebing, H. Frei, A. Anderson, A. Assion, and G. Steinmeyer, “Direct frequency comb synthesis with arbitrary offset and shot-noise-limited phase noise,” Nat. Photonics 4, 462–465 (2010).