Characteristics of Photonic Crystal Fibers & Applications in Optical Fiber Communication

Photonic Crystal Fiber (PCF), also known as Microstructure Optical Fiber (MOF), has many unique and novel physical properties, such as: controllable nonlinearity, endless single-mode properties, adjustable singular dispersion, Features such as low bending loss and large mode field, which are difficult or impossible to achieve with conventional silica single-mode fibers.

Photonic Crystal Fiber (PCF), also known as Microstructure Optical Fiber (MOF), has many unique and novel physical properties, such as: controllable nonlinearity, endless single-mode properties, adjustable singular dispersion, Features such as low bending loss and large mode field, which are difficult or impossible to achieve with conventional silica single-mode fibers.

Therefore, microstructured optical fibers have attracted the attention of foreign scientific circles. With the advancement of microstructured optical fiber manufacturing technology, breakthroughs have been made in various indicators of microstructured optical fibers, and various new microstructured optical fiber products have emerged as the times require. It is not only used in the field of conventional optical communication technology, but also widely used in the field of optical devices, such as: high-power fiber lasers, fiber amplifiers, supercontinuum spectroscopy, dispersion compensation, optical switches, optical frequency doubling, filters, wavelength converters, Soliton generators, mode converters, fiber polarizers, medical, biological sensing and other fields.

Photonic crystal fiber, also known as microstructured fiber, has attracted widespread attention in recent years. Its cross section has a relatively complex refractive index distribution and usually contains pores in different arrangements. The size of these pores is roughly the same order of magnitude as the wavelength of light. And throughout the entire length of the device, light waves can be confined to the fiber core region. Photonic crystal fibers have many peculiar properties.

For example, only one mode transmission can be supported in a wide bandwidth range; the arrangement of air holes in the cladding region can greatly affect the mode properties; asymmetric arrangement of air holes can also produce a large birefringence effect, which provides a high level of birefringence for our design. The performance of polarizers offers the possibility.

The concept of photonic crystals first appeared in 1987, when it was proposed that the Electronic band gap of semiconductors has a periodic dielectric structure similar to that of optics. One of the most promising areas is the application of photonic crystals in optical fiber technology. The main topic it addresses is the periodic microstructure of high-refractive-index fibers (they usually consist of air holes with silica as a background material).

The fibers in question are commonly referred to as photonic crystal fibers (PCFs), and the new optical waveguides can be conveniently divided into two distinct groups. The first fiber has a high-refractive-index core (usually solid silicon) surrounded by a two-dimensional photonic crystal cladding. These fibers have properties similar to conventional fibers and work on the principle of waveguide formation by total internal reflection (TIR); the effective refractive index of the photonic crystal cladding allows a higher refractive index for the core than conventional refractive index conduction . Therefore, it is important to note that these so-called total internal reflection photonic crystal fibers (TIR-PCFs) are actually completely independent of the photonic band gap (PBG) effect.

Different from TIR-PCFs, another fiber, the photonic crystal cladding shows the photonic band gap effect, which uses this effect to control the beam within the core. These fibers (PBG-PCFs) exhibit impressive properties, the most important of which is the ability to control and guide beam propagation within a core with a lower refractive index than the cladding. In contrast, total internal reflection photonic crystal fibers (TIR-PCFs) were first fabricated, while true photonic bandgap conducting fibers (PBG-PCFs) have only recently been experimentally demonstrated.

In 1991, Russell et al. first proposed the concept of photonic crystal fiber (PCF) based on the principle of photonic crystal light transmission.

In 1996, the University of Southampton, UK, J. c. Knight and others developed the world’s first PCF, and later in the field of optical fiber communication and optical research, PCF has attracted worldwide interest.

The structure of photonic crystal fiber and its light guiding principle

In terms of structure, PCF can be divided into solid fiber and hollow fiber. The solid optical fiber is an optical fiber in which the quartz glass capillaries are arranged around the quartz glass rod in a periodic pattern. Hollow core optical fiber is an optical fiber in which quartz glass capillaries are arranged around the quartz glass tube in a periodic pattern.

The light guiding mechanism of PCF can be divided into two categories: the refractive index light guiding mechanism and the photon energy gap light guiding mechanism.

Refractive index light guiding mechanism: There is a certain difference between the refractive index of the core of the periodic defect (quartz glass) and the refractive index of the periodic cladding (air), so that light can propagate in the core, the PCF of this structure The light guiding mechanism is still total internal reflection, but with the conventional G. 652 fiber is different, because the cladding contains air, so this mechanism is called modified total internal reflection, because the hole size in the hollow core PCF is smaller than the wavelength of the transmitted light.

Photonic energy gap light guiding mechanism: In theory, the conduction conditions of solid and hollow PCFs can be derived by solving the eigen equations of electromagnetic waves (light waves) in photonic crystals, and the result is the photonic energy gap light guiding theory.

The center is a hollow core. Although the refractive index of the hollow core is lower than that of the cladding quartz glass, it can still ensure that the light is not refracted out. This is because the small hole lattice in the cladding constitutes a photonic crystal. When the distance between the small holes and the diameter of the small holes meet certain conditions, the corresponding light can be prevented from propagating within the range of the photon energy gap, and the light is confined in the central hollow core.

Recent studies have shown that more than 99% of the light energy can be transmitted in this HF, and the spatial light attenuation is extremely low, so the fiber attenuation may only be 1/2 to 1/4 of the standard fiber. But not all PCFs are photonic gap guides.

The specific explanation of the photon energy gap light transmission mechanism of hollow-core PCF is: the periodic defect in hollow-core PCF is air, and the light transmission mechanism is to use the cladding to form a photon energy gap for light of a certain wavelength, and light waves can only be transmitted in air. Core formation defects exist and propagate. Although total internal reflection cannot occur in hollow-core PCF, the small hole lattice structure in the cladding acts as a mirror, so that light is reflected multiple times at the air and quartz glass interface of many small holes.

Characteristics of PCF

PCF has the following characteristics: the structural design is very flexible, with a variety of small hole structures; the refractive index difference between the core and the cladding can be very large; the core can be made into a variety of; the “cladding refractive index” is strongly dependent on As a function of wavelength, the cladding properties can be reflected on the wavelength scale. Because of the above characteristics, PCF has the following many strange properties:

(1) Endlessly Single Mode

Transmission ordinary single-mode fiber becomes multi-mode fiber as the core size increases. And for PCF, as long as the ratio of air pore size to pore spacing is less than 0. 2. No matter what wavelength can be transmitted in single mode, there seems to be no cut-off wavelength. This is the no-cut-off single-mode transmission characteristic. This fiber can transmit single-mode in light waves from blue to 2µm.

What is even more peculiar is that this characteristic is independent of the absolute size of the fiber, so the mode field area can be adjusted by changing the air hole spacing. At 1 550 nm, it can reach 1-800 μm2, and a large mode field PCF of 680 μm2 has actually been made, which is about 10 times that of conventional fibers. A small mode field favors nonlinearity, and a large mode field prevents nonlinearity. This is of great significance for improving or reducing optical nonlinearity. Such fibers have many potential applications, such as lasers and amplifiers (using high nonlinear fibers), low nonlinear communication fibers, and high optical power transmission.

(2) Unusual chromatic dispersion

Material dispersion in vacuum is zero, and material dispersion in air is also very small. This makes the dispersion of air core PCF very special. Because the fiber design is very flexible, as long as the ratio of the aperture to the hole spacing is changed, a large waveguide dispersion can be achieved, and the total chromatic dispersion of the fiber can also reach the desired distribution state. For example, the zero-dispersion wavelength can be shifted to short wavelengths, resulting in optical arc transmission at 1 300 nm; dispersion-flattened fibers with excellent properties (the bandwidth range of hundreds of nm is close to zero-dispersion); various nonlinear devices and dispersion-compensating fibers ( up to 2 000 ps/nm・km) came into being.

(3) Excellent nonlinear effect birefringence effect

g. The nonlinear effect in 652 fiber is a phenomenon that seriously damages the transmission quality of the system due to the excessive light intensity transmitted per unit area of ​​the fiber. However, in the photonic energy gap light-guiding PCF, we can reduce the light intensity per unit effective area by increasing the diameter of the air hole in the PCF core (ie, the effective area of ​​the PCF), so as to greatly reduce the nonlinear effect. This feature of the photonic energy gap guiding light lays the technical foundation for the fabrication of PCFs with large effective areas.

(4) Excellent birefringence effect

For polarization-maintaining fibers, the stronger the birefringence effect and the shorter the wavelength, the better the polarization state of the transmitted light is maintained. In PCF, it is only necessary to destroy the circular symmetry of the PCF section to form a two-dimensional structure to form a strong birefringence. By reducing the number of air holes or changing the diameter of the air holes, high-birefringence PCF polarization-maintaining fibers can be fabricated several orders of magnitude higher than the commonly used Panda brand polarization-maintaining fibers.

Application of Photonic Crystal Fiber in Optical Fiber Communication

The potential applications of PCF in optical fiber communication systems are mainly in two aspects: transmission fibers and optical devices. The research point of PCF as a transmission fiber is to improve the manufacturing process and reduce the fiber loss. The research point of PCF as an optical device is to realize the required performance of PCF device by adjusting the structure size of PCF.

As we all know, as an optical signal transmission medium, whether G. Either 652 fiber or PCF should satisfy low loss, small dispersion and low nonlinear effect. Same as G.652 loss mechanism, PCF loss mainly comes from absorption and scattering. In addition, due to the particularity of the PCF structure, it also naturally brings some special loss sources, such as mode leakage loss and structural defect loss. Table 1 shows the loss sources of PCF.

A series of measures have been taken to reduce the loss of PCF, mainly including (1) improving the purity of the core/cladding material; (2) adopting a process to reduce contamination of the cladding material tube; (3) by rationally designing the air filling ratio/air number of holes to reduce leakage patterns.

PCF has the characteristics of low loss, small dispersion and low nonlinear effect, which makes its application in the field of optical fiber communication very promising, especially for long-distance communication systems. With the continuous improvement of PCF design methods and manufacturing processes, PCF performance is becoming more and more perfect. Especially K. Through reasonable design of structural parameters, such as air hole diameter d and air hole spacing r, and d/r ratio, Tajima et al. not only reduce the attenuation of PCF, but also improve the dispersion and dispersion efficiency of PCF. Now, PCF has entered the experimental research stage of optical fiber communication system transmission in the laboratory.

At the World Optical Fiber Communication Conference (OFC) at the beginning of 2003, K. K. of the Access Network Service System Laboratory of Nippon Telegraph and Telephone Corporation (NTT). Tajima et al. reported that they developed an ultra-low attenuation, long-length PCF with an attenuation of 0.37 dB/km. PCF has complete single-mode characteristics, the available working wavelength range is 0.458-1.7μm.

The research group of C.Peucheret et al. used a 5.6 km PCF line to carry out a transmission experiment of 40Gbit/s with a working wavelength of 1550 nm. The PCF used in this experimental system has an effective area of ​​72 square µm, an attenuation of 1.7 dB/km, and a dispersion coefficient of 32 ps/(km. nm). Experiments show that when PCF is used as an optical signal transmission medium, the performance of the system is not degraded. Compared with G.652 fiber, the biggest advantage of PCF is the dispersion coefficient, effective area and nonlinearity under the premise of ensuring a small polarization mode dispersion coefficient. The coefficients can be flexibly designed.

As mentioned above, PCF itself is a good dispersion compensation fiber. By flexibly designing the three characteristic structural parameters of PCF: core diameter, cladding air hole diameter and cladding air hole spacing, we can obtain a large positive dispersion, or a large negative dispersion, or a very wide band of flatness Dispersive PCF. In particular, the flexible dispersion and dispersion efficiency compensation bandwidth management energy of PCF is several times larger than that of G.652 fiber. Therefore, PCF has excellent dispersion compensation performance and is expected to replace ordinary dispersion compensation fiber and become a new generation of dispersion compensation fiber.

Because the core/cladding refractive index difference of common dispersion compensation fiber is small (1.45/1.3), its dispersion compensation capability is poor. However, the core/cladding difference of PCF is large (1.45/1), so PCF has strong dispersion compensation capability. Researchers from Tsinghua University theoretically calculated the dispersion value of PCF. The PCF structure parameters selected in the calculation are: the air hole spacing is 0.8 μm; the ratio of air hole diameter to air hole spacing is 0.835.

It is calculated that the dispersion value of PCF at 1.55μm can reach -2050 ps/(km. nm), which can compensate 120 times the length of G.652 fiber (17 ps/(km. nm)), and can compensate 240 times the length The G.655 fiber (8.2 ps/(km. nm)), thus greatly shortening the length of the dispersion compensation fiber. Therefore, the dispersion compensation effect of PCF will have great application value in high-speed, large-capacity, long-distance WDM systems.

PCF can form fiber lasers and fiber amplifiers. The reason is that by adjusting the diameter and spacing of cladding air holes, a PCF with a mode field area range of 1-1000 μ; m2 can be flexibly designed, which makes PCF more suitable for the development of fiber lasers and optical amplifiers. G.652 fiber has more advantages.

The PCF and optical fiber communication related applications that have made research progress include: optical wavelength conversion, Raman amplifiers, soliton lasers, fiber gratings, and continuum generators.

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