The main difference between hollow core fiber (HCF) and traditional optical fiber is that it guides light through a hollow area in the center, rather than traditional fiber relying on its glass texture to spread the beam. In hollow-core fiber, the light beam is confined to propagate in a hollow within tiny pores in the surrounding glass material, which means that only a very small portion of the optical power is transmitted through the solid fiber material (usually glass).
Figure 1. Cross-section of bare hollow core fiber
Hollow core fiber (HCF) has several advantages over traditional optical fiber. Its nonlinearity is reduced by several orders of magnitude, its damage threshold is higher, and there is less overlap between the transmitted beam and the surrounding glass, providing unprecedented power for fiber-based lasers. Its lower signal loss enables longer distance transmission without the need for repeaters. Because light travels faster in air than other media such as glass, hollow-core fiber offers higher transmission speeds and lower latency, as well as higher bandwidth because each fiber is physically separated by a single mode. Dispersion in optical fibers can be controlled by design, especially for photonic bandgap fibers with small mode areas. This is particularly important for the guidance of ultrashort pulses, where large amounts of dispersion and nonlinearity can lead to severe pulse distortion. Optical fibers with large hollow cores generally exhibit weak dispersion and low dependence on design details, making them ideal ultrashort pulse transmission media [1]. This could enable groundbreaking applications in areas such as laser manufacturing, laser ignition, defence, attosecond science, nonlinear endoscopy/microscopy and gas mid-infrared lasers.
Figure 2. Photonic crystal fiber
One of the core features of photonic crystal fiber (PCF) is its periodic microhole structure, which can be used to control the propagation of light waves. Photonic crystal fiber uses the Photonic Band Gap (PBG) effect to guide light waves in the hollow (air) core of the fiber. In photonic crystals, periodic micropores or microstructures form photonic band gaps, which are similar to the electronic band gaps of electrons in crystals. Within the photonic band gap, light waves in a specific frequency range cannot propagate but are reflected or absorbed. This is because at these frequencies, the periodic structure of the photonic crystal causes the scattering of light waves to interfere with each other, forming a forbidden band gap. This means that light waves at frequencies within the PBG cannot propagate through the cladding, but can only be conducted in the core of the fiber. Unlike optical fibers based on refractive index guidance, the refractive index of the core region does not need to be higher than the refractive index of the cladding [2].
Hollow-core photonic crystal fiber (HC-PCF) combines the advantages of both types of optical fibers and revolutionizes traditional optical fiber technology by guiding light into a hollow with a periodic microhole structure. This unique waveguide structure is ideal for sensing, imaging and ultrashort pulse applications. Our hollow-core photonic bandgap fibers deliver ultrashort pulses without causing nonlinear effects or material damage, and can maintain stable transmission under tight bends.
Currently, two low-loss optical fiber technologies have been developed that have light-guiding properties that are significantly different from step-index optical fibers. Both fiber types utilize a two-dimensional periodic structure of cladding surrounding a hollow core to guide light, and are therefore collectively known as hollow-core photonic crystal fibers (HC-PCF). [3,4] Although both types of optical fibers share several common characteristics, the physics behind their light guidance, and subsequent optical properties, are very different.
Microscope images of different hcf
(a) Photonic bandgap fiber, (b) Kagome fiber, (c) non-contact tubular fiber, (d) hexagonal antiresonance fiber
The first fiber was the much-anticipated experimental result of photonic bandgap (PBG) lightguiding technology, resulting in HC-PCF without cladding modes at the frequency and index of the HC guided mode [5]. Today, our understanding and engineering of PBG HC-PCFs has greatly matured with the intuitive “photon tight binding model” [6].
The other type is Kagome lattice HC-PCF, which contrasts with PBG HC-PCF. Kagome HC-PCF is characterized by providing a broadband guidance spectrum and its cladding structure does not rely on the PBG effect. It was not until 2007, with the introduction of inhibitory coupling (IC) guidance, that the guidance mechanism was elucidated. Here, the cladding no longer requires a bandgap in the core mode space, but the structure and its dimensions are carefully designed so that the cladding supports continuation modes with strong phase mismatches with the core modes, thereby inhibiting the latter from escaping the core.
In HC-PCF using an air-silica structure, the cladding modes are effectively confined within the thin silica layer of the cladding and exhibit rapid lateral vibrations (with high azimuth numbers). [7] From this perspective, PBG in fiber is defined as cladding modes that do not phase match the core modes, and these modes propagate light outward from the core. In IC (inhibited coupling) guided fibers, the surrounding cladding plays a role similar to PBG in the core mode frequency index space.
To date, the largest ultrashort pulse lasers that can be guided in optical fibers have been limited to nanojoules in silica core fibers, and a few microjoules in photonic band gap (PBG) guided hollow photonic crystal fibers (HC-PCF) [9]. The pulse energy limit of solid conventional optical fiber is determined by the inherent catastrophic material damage of silica [8]. The limitation of PBG-guided HC-PCF is mainly due to the strong optical overlap between the core-guided pattern and the silicon dioxide core surround [9]. This effect is exacerbated near the cross-resistant spectral range between surface modes and core modes [10].
In addition to material damage limitations, fiber nonlinearity and dispersion are two other major limiting factors for ultrashort pulse laser waveguides and transmission. These factors can significantly affect the temporal distribution of the pulse. For example, under conditions of normal group velocity dispersion (e.g., wavelengths below 1300 nm), pulses propagating through silica undergo rapid diffusion, further exacerbating the temporal and spectral distortion of the pulse. As the pulse energy increases to the level of a few nanojoules, the Kerr and Raman responses of the medium will make additional contributions to diffusion, which will lead to spectral broadening and greater dispersion.
Application areas
The application of C-PCF technology in existing fields such as communications, sensors, medical imaging, etc. will continue to expand. In addition, emerging application fields, such as quantum computing, deep space communications, and biomedical research, will also become the focus of HC-PCF's future development.
With the growing demand for high-performance optical fibers in the field of optics and photonics, especially in terms of increasing data transmission speeds and expanding bandwidth, hollow-core photonic crystal fibers (HC-PCF) have shown their unique advantages. HC-PCF, with its remarkable low signal loss and extended bandwidth characteristics, can effectively meet these growing technical needs. Especially in the development of high-speed communication networks, these characteristics of HC-PCF make it a key component to support the next generation of communication technology.
Laser transmission link between laser head and workpiece,
Ultrashort pulse lasers have made huge advances in power scaling and their expanding use in industrial applications, which requires flexible and powerful beam delivery systems across many meters. Until recently, the maximum USP energy levels that could be guided in optical fibers were limited to nanojoules for silica core fibers and a few microjoules for hollow-core photonic crystal fibers.
In a recent collaboration, GLO Photonics, Amplitude Systemes and the GPPMM research group at the University of Limoges jointly demonstrated a breakthrough result. They successfully transmitted millijoule-level ultrashort pulses of 600 femtoseconds in a several-meter-long Kagome-type hollow-core photonic crystal fiber (HC-PCF) in a stable single-mode transmission method. This experiment not only demonstrated the excellent performance of HC-PCF in high-intensity pulse transmission, but also achieved 50 femtosecond pulse self-compression, reaching petawatt/cm² level intensity. This highlights the great potential of HC-PCF in ultrafast beam delivery applications.
GLO hollow photonic crystal fiber's exceptional ability to handle record laser pulse energies and deliver these pulses in a flexible manner makes GLO fiber an excellent solution for all developers and end-users in emerging laser micromachining markets and applications.
Through its proprietary Kagome HC-PCF, GLO brings its customers innovative and superior solutions for laser power processing and delivery and/or fiber photonic components. In particular, GLO Kagome fibers and their functionalized forms of PMC are well suited to realize the full potential of high-power ultrafast fiber lasers in materials micromachining and microprocessing. GLO products and/or technologies can be used in the laser micromachining field in a variety of ways. It combines flexibility, low transmitted light loss, low pulse distortion and access to very small and unobtrusive processing spaces. The second example is laser pulse timing control. GLO provides customer-customized PMC to compress the laser pulse.
The man behind the revolution in laser micromachining, bioengineering and surgery
Engineered laser pulses of short duration (sub-picosecond duration range) are powerful drivers in many scientific and industrial applications. Materials can now be processed, engraved and cut with high precision without the need for heating. Another example is the extremely challenging laser-based particle accelerator, where one of the key technical challenges is a device capable of handling large pulse energies (>100 mJ) and compression below 10 fs. GLO hollow fiber and PMC technology enable unprecedented control of light pulses, allowing sub-picosecond laser pulses to be compressed by more than 10 times in a single, compact fiber photonic assembly. GLO can design and produce fiber-based pulse compressors that are tailored for the wide range of ultrashort pulse (USP) lasers that exist on the market.
Biomedical research and surgical applications
Bringing laser light to cells or patient body parts in a safe and user-friendly manner remains one of the highly sought-after commodities among surgeons and biologists. GLO Kagome fibers and their functionalized forms, PMCs, are ideally suited to provide laser delivery links to any cell or body part, with excellent flexibility, low transmission optical loss, low pulse distortion, and access to very small and unobtrusive machine spaces. GLO's current experience working with players in the biomedical field is its asset in developing innovative fiber laser tools for end users or medical machine manufacturers for surgical and biomedical applications. In addition, GLO can help develop specific lasers that are now used medically at various visible light wavelengths. For example, high-power lasers operating at specific wavelengths in the yellow part of the spectrum can be used to treat a variety of vascular conditions, such as removing unwanted leg veins and facial capillaries. It also has applications in ophthalmology, for example in the treatment of retinal detachments.
Through its unique laser beam delivery and laser frequency conversion solutions, GLO technology can address most biophotonics applications such as cytometry, imaging, DNA sequencing or forensics.
References
1)P. J. Roberts et al., “Ultimate low loss of hollow-core photonic crystal fibres”, Opt. Express 13 (1), 236 (2005);
2)Dr. Rüdiger PaschottaRP Photonics AG,Photonic Crystal Fibers,
3)F. Benabid et al.,Science298, 5592, 399–402 (2002).
4)N. Venkataraman et al.,Proc. ECOC 2002, PD1.1, Copenhagen, Denmark (2002).
5)T. Birks et al.,Electron. Lett.31, 22, 1941–1943 (1995).
6)F. Couny,Opt. Express15, 2, 325–338 (2007).
7)F. Benabid et al.,Laser Focus World44, 9, 60–64 (2008).
8)X. Liu, D. Du, and G. MourouIEEE J. Quantum Electron.33(10), 1706–1716 (1997).
9)G. Humbert, Opt. Express12(8), 1477–1484 (2004)
10)J. A. West, C. M. Smith, Opt. Express12(8), 1485–1496 (2004)