Since the advent of mode-locking technology in the 1960s, narrower and stronger laser pulses have been the goal that people have been pursuing, and various high-quality ultrafast solid gain media have also been explored. Ti:sapphire mode-locked lasers and their chirped pulse amplification technology have led the rapid development and prosperity of femtosecond ultrafast lasers for a long time. To this day, if you want to achieve ultra-high peak power (such as PW lasers), titanium sapphire lasers are still unique.
Ti:sapphire crystal has excellent thermal conductivity, high damage threshold and other characteristics. Its fluorescence emission wavelength covers the range of 600~1100nm, so it can obtain a pulse width of <10fs. This is the fundamental reason why it can support ultra-high peak power (peak power = single pulse energy/pulse width).
Due to its unique advantages, Ti:sapphire has not been completely replaced by other gain media. However, it also has shortcomings that cannot be ignored. From the perspective of energy conversion, achieving laser operation requires a very complicated process. As shown in Figure 1, taking the 532nm pumped Ti:sapphire laser as an example, it needs to go through:
- The flash lamp or semiconductor laser is driven by voltage/current to generate pump light that excites the Nd:YAG crystal;
- Nd:YAG laser outputs 1064nm laser;
- The 532nm green light generated after frequency doubling of the 1064nm laser pulse is used as the pump light to excite the titanium sapphire crystal;
- 532nm green light pumped titanium sapphire crystal outputs broadband 800nm laser pulses.
Figure 1 Ti:sapphire laser energy conversion diagram
Ti:sapphire lasers go through complex energy loss and heat generation processes. Not only do they have low conversion efficiency, but they are also unable to achieve high repetition frequency output and cannot obtain high average power. This greatly limits the application of Ti:sapphire lasers in wider fields, especially in industrial scenarios.
In addition to titanium sapphire, laser crystals doped with the rare earth element ytterbium (Ytterbium, symbol Yb, atomic number 70) are also typical gain media for femtosecond lasers. Common ones include Yb:YAG, Yb:fiber, Yb:KGW, Yb:CaF2, and Yb:Glass. Take the most widely used Yb:YAG as an example. It has an absorption peak at 940nm, which happens to be matched by mature semiconductor lasers. The 940nm semiconductor laser directly pumps the Yb:YAG crystal to output a broadband 1μm laser, which greatly simplifies the intermediate conversion process. As shown in Figure 2, its absorption wavelength and emission wavelength are very close, the Stokes loss is less than 10%, and the conversion efficiency is very high. It is a perfect choice! Although the gain bandwidth of Yb-doped media is much narrower than that of titanium sapphire crystal, and it is impossible to achieve laser pulses as narrow as titanium sapphire, the pulse width on the order of hundreds of femtoseconds can already meet the needs of a very rich application scenario.
Figure 2 Yb:YAG laser energy conversion diagram
With the maturity of chirped pulse amplification technology, the growth process of Yb-doped laser gain media and the advancement of high-power semiconductor lasers, ultrafast lasers based on Yb-doped media have achieved rapid development in recent years. There are three main types: fiber ultrafast lasers, sheet ultrafast lasers and slab ultrafast lasers.
At present, there are mature high repetition frequency fiber ultrafast lasers with less than one hundred watts and several hundred femtoseconds on the market, which have been widely used in the industrial field. Fiber lasers have the characteristics of low threshold, high efficiency, and compact structure. However, the core diameter of the fiber is only on the order of micrometers. A single fiber femtosecond laser can only achieve pulse energy on the order of hundreds of microjoules, which cannot meet the dual requirements of single pulse energy and average power in some special application scenarios, such as attosecond science, terahertz, nonlinear optics, extreme materials processing and other scientific research and industrial applications. The most advanced optical fiber coherent synthesis technology can only achieve single pulse energy on the order of millijoules, and the system is complex and has not yet been maturely applied on a large scale.
The clear aperture of the laser medium of slab lasers and sheet lasers is not limited, so they can break through the limitations of pulse energy. The following will mainly introduce the end-pumped slab laser (InnoSlab), and we will focus on ultrafast lasers based on thin sheet media in the next article.
Slat laser technology is not a new technology. As early as the beginning of the 20th century, scientific researchers had successfully obtained 100kW continuous laser output through slab technology. We all know that multi-mode semiconductor lasers have very different divergence angles in the XY direction. The cross-section of its output beam is a strip shape. InnoSlab itself is also strip-shaped, which just adapts to this feature of semiconductor lasers. Therefore, the InnoSlab laser does not need to perform particularly complex shaping of the pump light. As shown in Figure 3, the InnoSlab end pump structure is very simple.
Figure 3 Pump structure of InnoSlab
In the Innoslab crystal, the temperature gradient occurs in the thickness direction of the slab, the pump light is fully absorbed in the thickness direction, and the temperatures of the heat source and the heat dissipation source are very close, which can basically avoid the thermal lens effect and thermo-optical distortion effect, and greatly increase the laser output power.
As one of the famous manufacturers of Innoslab ultrafast lasers, Germany's AMPHOS was founded in
2010. It is a company incubated by the Fraunhofer Institute of Technology and RWTH Aachen University in Germany. It now has more than 50 employees and is headquartered in the Herzogenrath Technology Park, covering an area of 1,200m2. In January 2018, AMPHOS was acquired by the TRUMPF Group and became its wholly-owned subsidiary.
AMPHOS is mainly committed to the research and development of ultra-high-power ultra-fast lasers with InnoSlab amplification technology, serving the industrial and scientific research markets! It currently occupies an important share in all major markets in the world, including Germany, Japan, South Korea and the United States. It performs extremely well in the field of accelerators. Its unique high repetition frequency and "Burst mode" technology dominates the needs of DESY in Germany and SLAC in the United States. It can now obtain an average power output of up to 20,000 watts. Its technical indicators and physical diagram are shown in Figure 4.
Figure 4 AMPHOS’s Burst Mode ultrafast laser used in the accelerator field
For standard products used in the industrial field, AMPHOS's technical route is to inject femtosecond seed laser into the InnoSlab amplifier, combined with chirped pulse amplification technology, which can output up to 1kW (20mJ@50kHz) pulse laser. The structure of the InnoSlab amplifier they designed is shown in Figure 5:
Figure 5 Structural diagram of AMPHOS InnoSlab amplifier
At present, AMPHOS has four series of standard products, whose main parameters are shown in Table 1:
Table 1 AMPHOS company standard product parameter table
As the exclusive agent of AMPHOS in China, Singapore and Malaysia, Boduo Technology has focused on ultrafast laser technology and services for many years. In the future, it will join hands with the world's top ultrafast laser brands and the most advanced laser technology to provide industrial and scientific research customers with ultrafast laser systems with both high average power and high peak power. Combined with Boduo Technology's rich independent research and development capabilities, it will also provide customers with comprehensive solutions for ultrafast laser products and applications. Customers are warmly welcome to inquire!
Preview for the next issue: The next article will introduce to you Trumpf Scientific Lasers, the thin-film ultrafast laser exclusively distributed by Wavelion Technology, so stay tuned!