Hole-assisted Fiber Research Articles

Dispersion, birefringence, and amplification characteristics of newly designed dispersion compensating hole-assisted fibers

We propose a new design of hole-assisted fiber (HAF) that can compensate for the accumulated dispersion in single-mode fiber link along with dispersion slope, thus providing broadband dispersion compensation over C-band as well as can amplify the signal channels by utilizing the stimulated Raman scattering phenomena. The proposed dispersion-compensating HAF (DCHAF) exhibits the lowest dispersion coefficient of -550 ps/nm/km at 1550 nm with an effective mode area of 15.6 microm(2). A 2.52 km long module of DCHAF amplifies incoming signals by rendering a gain of 4.2 dB with +/-0.8 dB gain flatness over whole C-band. To obtain accurate modal properties of DCHAF, a full-vector finite element method (FEM) solver is employed. The macro-bend loss characteristics of the proposed DCHAF are evaluated using FEM solver in cylindrical coordinate systems of a curved DCHAF, and low bending losses (<10(-2) dB/m for 1 cm bending radius) are obtained for improved DCHAF design while keeping intact its dispersion compensation and Raman amplification properties. We have further investigated the birefringence characteristics that can give significant information on the polarization mode dispersion of DCHAF by assuming a certain deformation (eccentricity e = 7%) either in air-holes or in the doped core or in both at a same time. It is noticed that the distortion in air-holes induces a birefringence of 10(-5), which is larger by a factor of 10 than the birefringence caused due to the core ellipticity. A PMD of 11.3 ps/ radicalkm is obtained at 1550 nm for distorted air-holes DCHAF structure.

Recent Progress on Optical Loss Reduction in Photonic Crystal Fiber

New types of optical fiber with a silica-air microstructure, such as photonic crystal fiber (PCF), hole-assisted fiber (HAF) and photonic bandgap fiber (PBGF), have received increasing attention because of their novel guiding properties. In particular, PCF is very attractive as a transmission medium for future networks, because it can be endlessly single-mode, and its chromatic dispersion can be tailored. This paper mainly focuses on recent progress in loss reduction in photonic crystal fiber.

光カールコードを用いた住宅内光配線方法に関する検討

We have developed a new wiring system for home networks without the need to use a jointing tool or to deal with excess fiber length. Optical fiber curl cord is the key component of the system. This cord has a spiral structure, which makes it possible for the cord to stretch or contract without any loss increase. This is because it is made of a hole-assisted fiber with a low bending loss. We can employ this characteristic to realize a flexible wiring system. Optical fiber curl cord can be stretched considerably. For example, a cord with a curl length of 1 m and a curl diameter of 16 mm can be stretched to nearly 20 m. This means that we can use an optical fiber curl cord with a fixed length (here 1 m) to wire an optical home network of any length (here <20 m). To prove the feasibility of this idea, we undertook experiments where we installed optical fiber curl cord in ducts in various situations and created a calculation model to simulate wiring it through duct. This model worked well and we confirmed that it is possible to pull a 16 mm diameter curl cord with an SC connector through a 22 mm diameter pipe in any situation. And if we use this method to construct an optical home network, the workability will be greatly improved. This may be an effective way to reduce construction costs.

Bending-insensitive single-mode hole-assisted fibers with reduced splice loss

We propose and demonstrate a novel type of bending-insensitive single-mode hole-assisted fiber that has a doped core and two layers of holes with two different airhole diameters. The fiber has a 9.3 microm mode field diameter, a bending loss of 0.011 dB/turn at 1.55 microm for a bending diameter of 10 mm, and a cutoff wavelength below 1.1 microm. The fiber can be fusion spliced to a conventional single-mode fiber with low loss.

Refinement of Er3+-doped hole-assisted optical fiber amplifier

This paper deals with design and refinement criteria of erbium doped hole-assisted optical fiber amplifiers for applications in the third band of fiber optical communication. The amplifier performance is simulated via a model which takes into account the ion population rate equations and the optical power propagation. The electromagnetic field profile of the propagating modes is carried out by a finite element method solver. The effects of the number of cladding air holes on the amplifier performance are investigated. To this aim, four different erbium doped hole-assisted lightguide fiber amplifiers having a different number of cladding air holes are designed and compared. The simulated optimal gain, optimal length, and optimal noise fig. are discussed. The numerical results highlight that, by increasing the number of air holes, the gain can be improved, thus obtaining a shorter amplifier length. For the erbium concentration NEr=1.8x1024 ions/m3, the optimal gain G(Lopt) increases up to ~2dB by increasing the number of the air holes from M=4 to M=10.

Erbium-doped hole-assisted optical fiber amplifier: design and optimization

An erbium-doped hole-assisted optical fiber amplifier, to be employed in the third band of the optical communications, is designed and optimized via a tailor made computer code. The finite element method is used for the electromagnetic investigation of the microstructured fiber section. The simulation model takes into account all the rare earth physical phenomena, i.e., the pump and signal propagation, the amplified spontaneous emission, the secondary transitions pertaining to the ion-ion interactions, and so on. The device feasibility is tested via a number of simulations, realistically performed by taking into account the actual parameters pertaining to the dispersion of the germania/silica glass, the erbium emission and absorption cross sections, the propagation losses. By simulation, in the small signal operation, a gain close to 42.8 dB is demonstrated for a fiber 13-m long, using a pump power of 50 mW at the signal wavelength /spl lambda//sub s/=1536 nm, the pump and the signal being copropagating.

Design of solid and microstructure fibers for suppression of higher-order modes

Hole-assisted fibers have been proposed for a number of applications, including low-bend-loss access transmission. Suppression of higher-order modes is essential in these designs, and is explained here as the result of index-matched coupling between core and cladding modes. This physical principle is shown to explain previous empirically optimized designs, and enables intuitive generalizations. The improved tradeoff between bend loss and suppression of higher-order modes in these designs is discussed. Novel solid and microstructure fiber designs with suppressed higher-order modes illustrate these principles.

Hole-Assisted Multiring Fiber With Low Dispersion Around 1550 nm

Due to large index contrast between air and silica, many properties of optical fiber unachievable through doping can be realized easily by utilizing air holes. We present a nonzero dispersion-shifted fiber design based on multiring index profile with the assistance of air holes. The proposed fiber exhibits small dispersion values over nearly 100-nm-wide wavelength range with very low dispersion slope. It can have applications in future long-haul optical communication network.

Hole-assisted fiber design for small bending and splice losses

This letter describes the bending and splice loss characteristics of hole-assisted fibers and reveals that the distance between the hole and the fiber core is a key parameter regarding these losses. We found that the normalized hole distance against the core radius must be two or greater for practical deployment.

Multipole analysis of hole-assisted optical fibers

Hole-assisted optical fibers are microstructured fibers which guide light using a conventional high-index core while using a small number of air holes in the cladding to achieve desirable dispersive and/or polarization properties of the fiber. In this paper, we analyze the characteristics of the hole-assisted fibers using the multipole method of White et al. [Opt. Lett. 26 (2001) 1660], which is a rigorous boundary value method. Such properties as dispersion and birefringence are numerically investigated for fibers with different air hole patterns and sizes. Hole-assisted optical fibers exhibit low transmission loss and easily tailorable dispersion and birefringence, and therefore, would be valuable for dispersion management applications both in linear (WDM) and nonlinear (soliton) fiber telecommunications.

Hole-Assisted Lightguide Fiber - A Practical Derivative of Photonic Crystal Fiber

AbstractUsage of air holes in optical fibers has become a hot subject in fiber optics because of the possibilities for novel transmission properties. Although photonic crystal fibers based on photonic bandgap guidance are the most drastic innovation in this subject, optical fibers containing air holes but not having photonic crystal structures are also being intensively studied. Such air-silica microstructured fibers are more practical than the photonic bandgap fibers because the lack of photonic crystal structure makes the fabrication far easier. Even without the photonic bandgap, the microstructured fibers can exhibit valuable properties in terms of group velocity dispersion and nonlinearity, because the index contrast between air and silica is 10 or more times as large as that of the conventional optical fibers based on doped silica glasses. However, one of the major challenges for practical applications of the air-silica microstructured fibers has been their high transmission losses, which have been several tens to hundreds times higher than those of the conventional fibers. As a solution to this problem, we have proposed a more practical structure called hole-assisted lightguide fiber (HALF). In addition to the air holes for realizing novel optical properties, this structure has a material index profile for waveguiding, and hence is closer to the conventional fibers than the other microstructured fibers are. As a result, novel optical properties can be realized without severe degradation in transmission loss. In experiments, an anomalous group velocity dispersion as large as +35 ps/nm/km at 1550 nm wavelength, which would be unattainable in the conventional fibers, has been realized with a loss of 0.41 dB/km, which is comparable to those of the conventional fibers. Analyses of the losses of the fabricated HALFs suggest that the loss should be lowered by mitigating the effect of the drawing tension and minimizing the power fraction in the holes. It is also shown that the full-vector finite element method realizes accurate modeling of the properties such as dispersion and macrobend loss.

Introduction

In the 1970's researchers demonstrated air-glass fiber profiles as a possible approach to low-loss optical fiber. A few years ago, interest in air-glass fiber profiles was rekindled by the idea of creating two-dimensional photonic band gaps in optical fibers. Typically, these optical fibers have a collection of air holes that run along the length of the fiber. The discovery of a number of interesting physical effects, some not necessarily associated with photonic band gaps, has blossomed into the field of Photonic Crystal Fiber. Along with that, a potentially confusing series of monikers has developed to distinguish between the various embodiments. Fortunately, the physics describing the fibers conveniently separates them into two distinct classes, those employing photonic band gaps for guidance and those that use a type of total internal reflection for guidance. The terms holey fiber, hole-assisted fiber, microstructured fiber, and effective-index fiber refer to fibers that employ total internal reflection as the guidance mechanism. Photonic band-gap fiber, Bragg fiber, and omnidirectional waveguide refer to fibers that use photonic band gaps as the guidance mechanism. We have used the term photonic crystal fiber (PCF), the highlight of this Focus Issue, to represent all these types of fibers. Though not all of the fiber profiles contain extended periodic structures normally associated with crystal structures, most all do possess a degree of regularity in the fiber profile that imparts mechanical and optical advantage.