Recent progress of hollow fiber HCF

Recently, there seems to be a little too much written about optical fiber. Hehe, this year is the 50th year of the invention of"Low-loss optical fiber." One is to take advantage of this big background to commemorate it, a nod to the industry's big names and top companies, and a nod to the recent OFC paper that follows this trend. G. 654e-compatible ultra-low-loss, large-effective-area optical fiber has become an essential element for large-capacity, long-distance coherent transmission, as you may be aware, in fact, the capacity of coherent optical communication is ultimately determined by the loss and non-linear two factors, or even it is not an exaggeration to say that the essence of the loss of optical fiber entirely restricts the transmission distance of optical fiber. The reason, think about it will naturally understand. The trend is that people seem tired of improving performance and capacity at a 0.5dB granularity. This can be seen from the hot degree of research on space division multiplexing optical transmission technology, such as multi-core, few-mode, etc. . Even the conservative industry, now also on C + L multi-band, and even extended band, all-band optical communication devices, equipment, the pursuit of almost obsessive. It had to be said that cutting costs was never as fast or direct as open source. After all, the new technology could double or even increase the capacity of traditional optical fiber by an order of magnitude or two.

The best way to honor the legend is to break his record. Transcendence is the greatest respect for the elders. Today I want to introduce the hollow fiber, but also to the conventional solid glass fiber beyond it. First of all, we introduce the principle and advantages of hollow-core optical fiber, research history and technical evolution, and then talk about its application progress.

1. Hollow-core optical fiber light guiding principle of the most simple direct hollow-core optical fiber light guiding principle should be the following picture, the most intuitive. Different from the total reflection principle of the conventional optical fiber waveguide, the hollow fiber core is air, so the light guide depends on the light restraint of the cladding. In figure 1, for example, light is reflected by a highly reflective silver that is confined to the air core. Although the appearance is a little rough, but the general meaning should be in place. The technique, first proposed in 1960s, involves coating the inner walls of glass capillaries with a reflective film that transmits mid-infrared light in the middle. However, because the hole is larger to film, but the hole is larger transmission mode is more, this structure is more difficult to achieve long-distance single-mode transmission. Figure 1. Concept of Hollow Core Fiber with the development of technology, special designed cladding structure, such as Hollow Core Fiber, has been put forward in the 1980s and 1990s. Its light guiding principle is photonic band gap effect. Similar to the bandgap concept in semiconductors, the cladding air-hole structure of the fiber has strict periodicity. When the periodic structure is destroyed by the introduction of the core, the defect or local state with a certain bandwidth is formed, and only light waves with a certain frequency can propagate in the defect region, light waves of other frequencies can not travel and thus form a constraint on light. With this structure, the refractive index of the core layer need not be higher than that of the cladding layer, thus more practical hollow-core fiber comes into being, its structure is various, as shown in Figure 2. Figure 2. The cross sections of hollow-core photonic crystal fibers with different structure designs have a very large initial loss, which is basically the ~ db/cm level. Until now, after more than 20 years of development, the loss of hollow-core fiber with this structure may be the best and it is very difficult to achieve below 2DB/km. In order to overcome the high loss of hollow-core fiber, a new kind of hollow-core fiber based on anti-resonant principle is proposed recently, as shown in Figure 3. It uses the coherent reflection of light back and forth between the tubular glass films in the fiber to confine the light near the air core and propagate along the axis. The effect of the glass film in the fiber is similar to that of the FP resonator, which makes the transmission line appear multi-peak, and the peaks are separated into multiple high-reflection regions, also known as anti-resonant windows. In these windows, grazing incidence from the hollow core will result in very high reflection, thus greatly reducing the leakage loss of the optical fiber. The characteristics of band-gap-guided fiber depend mainly on the special design of cladding microstructure, and the low-loss band of this kind of anti-resonant fiber can be realized only by changing the thickness of glass film, this kind of fiber can provide lower loss than conventional fiber at any wavelength. Figure 3. Low-loss anti-resonant hollow fiber structure, the light field is restricted in the middle of the hexagonal region

2. The advantages of hollow-core optical fibers such as hollow-core optical fibers are widely studied, because of its obvious advantages: a) low time delay, light mainly transmits in the core region near the air hole, the refractive index is lower than the solid glass, the transmission rate is faster, and the end-to-end optical fiber transmission delay is 31% smaller than the existing optical fiber. This is very important for current and future time-sensitive communication situations. B) ultra-low nonlinearity, the nonlinear effect of hollow-core fiber is 3 to 4 orders of magnitude lower than that of conventional fiber materials. This point for the existing optical fiber in the current non-linear bottleneck encountered, simply does not exist. C) large mode field diameter, hollow core fiber module diameter can be much larger than ordinary single mode fiber even when it can guarantee single mode transmission, up to 30um, which greatly reduces the power density in the fiber, and the damage threshold power of the fiber is greatly increased, so it is not afraid of burning fiber any more. D) low dispersion, hollow-core fiber can provide low dispersion of ~ 2ps/nm/km in the ultra-wide spectrum range of thousands of NM, which is nearly 10 times smaller than the existing fiber, and can compensate the dispersion almost differently in optical domain or even electric domain. E) ultra-wide operating frequency band, designed to provide ultra-wide transmission frequency bands from mid-infrared to 3UM with a band range of more than 1000 nm, easily supporting the O, S, E, C, L, U bands of ordinary optical fibers. F) potential ultra-low loss. Although the loss of hollow-core fiber is relatively large, in theory, the theoretical minimum limit of hollow-core fiber in communication window can be as low as 0.1 db/km, which is smaller than 0.14 db/km of normal quartz fiber. H) controllable polarization state, because the photonic crystals in the cladding are easy to form birefringence, the polarization state in the hollow core fiber is easy to be maintained, that is, it has the function of polarization preservation.

3. The history of hollow-core fiber research and technological evolution as mentioned above, although the idea of hollow-core fiber was first proposed in the 1960s, but at that time, no suitable natural material was found to fill the hollow core to provide high reflectivity, so the deep research of hollow fiber didn't really start until the 1990s. In 1991, Russel proposed the pioneering idea of filling two-dimensional photonic band-gap crystal materials into optical fibers, as shown in Figure 4(a) . He also predicted that microstructured glass capillary arrays could act as fiber cladding materials, thus, the low loss transmission of light in the air core is realized. By 1995, Briks et al developed the out-of-plane photonic band gap theory and demonstrated the existence of two-dimensional photonic band gap in the hollow core filled with a triangular array of air holes filled with massive quartz. In 1999, Cregan and Knight et al first observed the phenomenon of light conduction in hollow-core optical fibers. It is proved that it is feasible to use micro-structured artificial materials to replace the conventional total reflection light guide, that is, photonic band-gap photonic crystal fiber or hollow fiber. Figure 4. Development and cross-section of hollow-core fiber in 2002, Conning announced the realization of a photonic bandgap hollow-core fiber with a loss of 13 db km and a 7 cell, the Bath team then used a 19-cell design to reduce the loss to 1.7 db/km in 2004. Due to the high proportion of air filled, greater than 0.94, the air holes changed from a circular ring into a rounded hexagon, the geometry of the cladding changed into a regular hexagon, and the glass nodes were connected by a network of glass films, as shown in Figure 4(D) . These glass nodes can support different order of bound photon resonators and mode coupling between adjacent glass nodes to create photonic band gap effect in the hollow core. These glass films are indispensable for mechanical support, but they may also adversely affect the bandwidth and loss bandwidth of optical fibers. Since the discovery in 2010 of a core with a negative curvature radius, the loss of this hollow-core fiber has quickly dropped from DB/M to tens of DB/km, and many structural improvements have been made until 2018, the loss of 2DB/km at 1512nm is realized based on the cojoined tude fiber. By 2019, the University of Southampton will have reduced the loss of hollow-core fibers to 1.3 db km using double-glazed tube-embedded resonant-resistant node-free fiber (NANF) technology, and the C + L band loss will be reduced to 0.65 db km after structural optimization, it is shown for the first time that the loss of hollow-core fiber can approach the potential of ordinary fiber. Further, at this year's OFC, University of Southampton researchers pushed the record to 0.28 db km loss and fiber length to 1.7 km. Figure 5. Loss Reduction in University of Southampton NANF hollow fibers we focus on University of Southampton improvements to low loss hollow fibers. In 2018, the ECOC reported a loss of 1.3 db/km in hollow-core fiber, which was cut in half to 0.65 db/km in last year's ECOC PDP paper, the supported wavelength window covers the C and L communication bands. Based on loop experiment, 61-wave WDM PM-16QAM signal can transmit 125km hollow fiber, while single-wave PM-QPSK can transmit 341km hollow fiber. Some design improvements have been made to improve the micro-bending loss (reducing core diameter size) and leakage loss (reducing azimuth clearance, improving the angle direction of the pipeline, and better longitudinal consistency) , this year OFC successfully reduced the loss record of hollow-core fiber to less than half again. The loss is 0.28 ± 0.04 db/km in the range of 1510-1610 nm and slightly higher at 1640 nm, about 0.3 db/km, which is very close to the requirement of single-mode long-distance transmission system. In the past 18 months, they have reduced the loss of hollow-core fibers by a factor of 10, from 3.5 to 0.28 db/km, just twice the minimum loss of existing fibers. It also increased the maximum transmission distance by a factor of 10, from 75km at the start to 750km. The reduction of optical fiber loss mainly depends on the introduction of two technologies. One is that there is no node connection at the contact of the glass tube, which avoids the increase of the glass thickness at the node to produce the resonance effect on the fiber performance. The second is to embed small glass tubes in large glass tubes to reduce light leakage from the hollow region by a thousand times. At the same time, the thickness of the coated glass tube is reduced by half to 0.5 um. This not only reduces the loss, but also expands the low-loss window of the hollow-core fiber by 3 times to more than 120 nm of 1520 ~ 1650 nm. The increase in fiber length is due to improvements in fiber-drawing technology, from 500m to 1.2 km to 1.7 km recently, with the use of larger prefabricated rods for better consistency. Furthermore, by comparing the finite element simulation with the experimental results, they also decomposed the hollow fiber loss into light leakage loss, micro-bending loss and surface scattering loss in a certain proportion, it is an irregular scattering caused by Surface roughness, which is the ultimate limiting factor of hollow-core fiber loss, and its potential limit can be less than 0.1 db km.