Air-core anti-resonant fibers: a new generation of high-power laser transmission technology

The article briefly introduces the development of hollow-core optical fibers, and summarizes the key technologies and research progress of high-power laser transmission based on hollow-core anti-resonant fibers. Finally, the future development direction of high-power laser transmission technology based on air-core anti-resonant fiber is prospected.

Cover interpretation

The cover shows the application of high-power laser transmission based on a hollow-core deresonant fiber against the backdrop. The hollow-core anti-resonant fiber confines the light transmission in the air medium of the fiber core through the anti-resonant reflection light waveguide effect, which greatly reduces the overlap between the mode field and the quartz material, and has excellent properties such as high damage threshold, low dispersion and low nonlinearity compared with the traditional solid-core energy transfer fiber. In the transmission band, it is almost unaffected by the intrinsic absorption of the material, and the laser transmission from ultraviolet to mid-infrared can be realized with low loss. The high-power laser transmission technology based on air-core anti-resonant fiber will become a new generation of transmission solutions, promoting the rapid development of related application fields.

1. Background

With its advantages of high conversion efficiency, stable performance, good beam quality and compact structure, high-power fiber lasers are widely used in various research fields such as industrial processing, national defense and military, biomedicine, environmental monitoring, etc., which have greatly promoted the development of human society. At present, fiber lasers have achieved 20 kW continuous laser single fiber output in the 1 μm band, and 200 kW laser output has exceeded through beam synthesis technology. In terms of output laser pulse duration, fiber lasers can achieve full coverage from quasi-continuous to femtosecond through Q-switched and mode-locked technologies, and the peak power of picosecond and femtosecond fiber lasers can reach GW. In terms of output wavelength, the output spectrum range can be from ultraviolet to mid-infrared bands by selecting different rare earth ion doped fibers and the nonlinear interaction between light and matter to meet the application requirements.

With the continuous improvement of transmission power, the traditional solid-core quartz energy transfer fiber greatly affects the further improvement of transmission power and transmission length due to the limitation of core material damage threshold and nonlinear effect. In addition, the serious material absorption of quartz materials in the mid-infrared band and the large Rayleigh scattering loss at short waves affect the transmission laser range. How to carry out efficient and stable flexible transmission of multi-band high-power laser is still an important development direction.

In order to break the limitations of traditional solid core quartz fibers, hollow-core fibers have emerged, especially air-core anti-resonant fibers (HC-ARF).A hollow-core anti-resonant fiber is a microstructured optical fiber that relies on a resonant reflected optical waveguide for light guidance, which confines the light field to an air fiber core with a low refractive index. Compared with the traditional solid-core optical fiber, the air-core anti-resonant fiber uses air as the transmission medium, which reduces the overlap between the mode field and the quartz material, and has a series of excellent properties such as high damage threshold and low nonlinearity, which is expected to play its unique advantages in the field of high-power laser flexible transmission. The use of hollow-core anti-resonant fibers to realize high-power laser transmission in special bands is also an important development direction. By adjusting the quartz wall thickness of the hollow core anti-resonant fiber to select the transmission passband, the transmittable of the laser band is broadened, and the optical fiber uses the core air to guide the light, and the overlap between the mode field and the quartz wall is low, which greatly reduces the material absorption loss and scattering loss, and can realize the low-loss transmission of laser from ultraviolet to mid-infrared band.

2. Light guiding mechanism of hollow-core anti-resonant fiber

The air-core anti-resonant fiber is based on the anti-resonant reflection optical waveguide theory (ARROW) to achieve light guidance, and its light guide model is shown in Figure 1, the light transmitted in the fiber core is determined by the thickness of the cladding quartz wall, and its resonant wavelength can be expressed as

where t is the quartz wall thickness, nglass is the quartz refractive index, nair is the air refractive index, and m is a positive integer. When the transmitted wavelength satisfies the resonance condition, the light is significantly leaked; When the resonance condition is not met, the light is restricted to transmission in the core. As a result, hollow-core anti-resonant fibers have a wider light-guiding passband than photonic bandgap fibers. At the same time, the transmission window can be selected by adjusting the thickness of the quartz wall. At present, air-core de-resonant fibers have achieved low-loss transmission in many important bands, and most of them are lower than the loss limits of traditional solid-core quartz fibers in the corresponding bands, attracting more and more scientific research institutions and universities to carry out research on high-power laser transmission based on air-core de-resonant fibers.

Fig.1 ARROW structure and its transport spectrum

3. Research progress of high-power laser transmission technology based on air-core anti-resonant fiber

1. Flexible transmission of near-infrared laser hollow-core anti-resonant fiber

Near-infrared band lasers have complete technology, mature industry, higher power, greater energy laser output, and a wide range of applications, so there has always been a huge application demand for high-power laser energy transmission. At the same time, compared with other bands, hollow-core anti-resonant fibers are relatively less difficult to fabricate, and it is easier to achieve low-loss transmission. As a result, many of the record-breaking air-core deresonant fiber laser transmissions have been done for near-infrared lasers.

In terms of high-power CW laser transmission, the advanced laser and fiber technology research team of Beijing University of Technology designed and fabricated a multi-mode nested hollow-core anti-resonant fiber with a core diameter of 65 μm, as shown in Figure 2(a). Through simulation calculation, the fiber can achieve low-loss transmission of at least 5 modules. The transmission experiment was carried out for the low-mode laser with a beam quality of 1.38, and the laser output of 2951 W was realized at a transmission length of 10 m, and the transmission efficiency was 95.2%, and the output beam quality and spectrum were well maintained. Transmission tests were then carried out with a longer fiber, and a 2850 W laser output was achieved at a transmission length of 110 m with a transmission efficiency of 92%, as shown in Figure 2(b). This work lays a foundation for the high-efficiency transmission of low-mode and multi-mode lasers, and is expected to achieve higher power transmission based on multi-mode air-core anti-resonant fibers.

Table 1 shows the research progress of high-power CW laser transmission in the near-infrared band based on air-core anti-resonant fibers at home and abroad. It can be seen that the continuous laser transmission has achieved a corresponding improvement in two aspects, on the one hand, the increase of the transmission distance, and on the other hand, the increase of the transmission power.

Table 1 Research progress of high-power CW laser transmission in the near-infrared band based on air-core anti-resonant fibers

2. Flexible transmission of mid-infrared laser hollow core anti-resonant fiber

Mid-infrared laser has become the focus of research in the field of optics due to its important application background and great demand. However, the flexible transmission of mid-infrared laser is still a problem that needs to be solved urgently. Traditional quartz fibers have strong material absorption effects after 2.4 μm, while soft glass fibers based on fluoride and chalcogenide materials have low transmission losses, but they have the disadvantages of difficult preparation, poor physicochemical properties, and low damage threshold. Quartz-based hollow-core anti-resonant fiber can not only maintain the good physical and chemical properties of quartz substrate, but also because of its core air light guide, the overlap between the mode field and the quartz wall is low, which greatly reduces the influence of material absorption on its transmission performance, and can achieve low-loss transmission in the mid-infrared band.

Our team has further broken through the high-power continuous laser transmission power in the mid-infrared band based on air-core anti-resonant fibers. A mid-infrared gas laser was prepared based on the hollow-core anti-resonant fiber acting as a gas cavity, and the core was filled with acetylene gas to achieve a 3.1 μm laser output by inverting the number of particles, with a maximum output power of 21.8 W, and the output laser had good beam quality. Subsequently, the laser transmission was carried out in the hollow-core anti-resonant fiber, with a maximum output power of 20.05 W, a transmission efficiency of 88.09%, and an output beam quality of 1.25/1.26 (X/Y).

3. Visible and ultraviolet band laser air-core anti-resonant fiber flexible transmission

Visible and ultraviolet lasers are widely used in many fields, such as laser processing, quantum information processing, precision spectroscopy, etc. Due to the intrinsic defects of quartz materials, traditional solid-core optical fibers have large Rayleigh scattering at short waves, which increases the transmission loss. In addition, in the ultraviolet band, irradiation causes the transmission medium to produce color centers, i.e., the sun effect, which causes irreversible damage to the optical fiber. Hollow-core anti-resonant fibers provide a new idea for visible and ultraviolet laser transmission.

In 2017, our team conducted a research on 532 nm green light high-power picosecond laser transmission experiments. The optical fiber used is a hollow core anti-resonant fiber with a core diameter of 26 μm and a singleturn structure with a loss of 80 dB/km@532 nm, and finally achieves an average power output of 32 W at a fiber length of 0.3 m, with a single pulse energy of 144 μJ and a peak power of 7.2 MW. In 2024, Fu et al. at the University of Southampton in the United Kingdom will use a singleturn structure air-core anti-resonant fiber to achieve 100-meter-level green high-power nanosecond laser transmission.

In terms of ultraviolet laser transmission, we prepared a single-turn hollow core anti-resonant fiber with a core diameter of 15 μm and a loss of 0.3 dB/m@355 nm. Subsequently, the ultraviolet laser with an output wavelength of 355 nm, a pulse width of 20 ps, and a repetition rate of 1 kHz was used to carry out the transmission experiment, and the laser output of a single pulse energy of 106 μJ was realized at the fiber length of 1 m, and the corresponding peak power was 5.3 MW, which was the first time to realize the transmission experiment of high-power ultrashort pulse laser in the ultraviolet band. The transmission potential of air-core anti-resonant fiber in ultraviolet high-power laser flexibility in the ultraviolet band is verified and is expected to play an important role in the field of ultraviolet precision machining.

4  summary and outlook

By limiting the light field to the core air, hollow-core de-resonant fiber has the characteristics of low nonlinearity, low dispersion, high damage threshold and controllable number of transmission modes, which makes the high-power laser transmission technology based on air-core de-resonant fiber a research hotspot. With the continuous improvement of the structural design and preparation process of air-core de-resonant fiber, the transmission loss of many important laser bands has been greatly reduced, and the research reports on high-power laser transmission based on air-core de-resonant fiber have been emerging, and the related work of high-power CW laser and pulsed laser energy transfer from ultraviolet to mid-infrared bands has been realized, but there is still a large room for improvement in this field, such as further improving the transmission power and transmission efficiency, and exploring the integrated transmission mode of full fiber under high power. With the in-depth research and solution of related technical problems, high-power laser transmission technology based on air-core de-resonant fiber will become a new generation of transmission solutions and promote the rapid development of related application fields.