We investigate the potential of photonic crystal fiber (PCF) to realize high quality and high-power transmission. We utilize the PCF with a quasi-uniform air-hole structure, and numerically clarify that the quasi-uniform PCF can realize the effective area (Aeff) of about 500µm2 with bending loss comparable with that of a conventional single-mode fiber for telecom use by considering the quasi single-mode transmission. We then apply the quasi-uniform PCF to kW-class high-power beam delivery for the single-mode laser processing. The cross-sectional design of the PCF with the high-power delivery potential of more than 300kW·m is numerically and experimentally revealed. A 10kW single-mode beam at 1070nm is successfully delivered over a 30m-long optical fiber cable containing a fabricated PCF with single-mode class beam quality of M2 =1.7 for the first time.

A single-polarization single-mode (SPSM) photonic crystal fiber (PCF) based on double-hole unit core is proposed in this paper for application to cross-talk free polarization splitter (PS). Birefringence of the PCF is obtained by adopting double-hole unit cells into the core to destroy its symmetry. With an appropriate cladding hole size, single x- or y-polarized PCF can be achieved by arranging the double-hole unit in the core along the x- or y-axis, respectively. Moreover, our proposed SPSM PCF has the potential to be applied to consist a cross-talk free PS. The simulation result by employing a vectorial finite element beam propagation method (FE-BPM) demonstrates that an arbitrary polarized incident light can be completely separated into two orthogonal single-polarized components through the PS. The structural tolerance and wavelength dependence of the PS have also been discussed in detail.

We propose a novel single polarization photonic band gap fiber (SP-PBGF) with an anisotropic air hole lattice in the core. An SP-PBGF with an elliptical air hole lattice in the core recently proposed can easily realize SP guidance utilizing the large difference of cutoff frequency for the x- and y-polarized modes. In this paper, in order to achieve SP guidance based on the same principle of this PBGF, we utilize an anisotropic lattice of circular air holes instead of elliptical air holes to ease the fabrication difficulty. After investigating the influence of the structural parameters on SP guidance, it is numerically demonstrated that the designed SP-PBGF has 381 nm SP operating band.

Polarization-maintaining photonic crystal fiber couplers (PM-PCFCs) were fabricated using a CO2 laser irradiation technique. We could control the states of air holes in the tapered region of couplers by adjusting the laser power density in the fusion and the elongation processes. It was demonstrated that the air hole remaining PM-PCFC exhibited polarization-splitting characteristics and that the air hole collapsed PM-PCFC had polarization insensitive coupling characteristics.

We investigate numerically the applicability of photonic crystal fiber (PCF) with a uniform air hole structure as a wide-band transmission medium. We show that accumulated dispersion over the PCF can be reduced effectively by optimizing the index profile of dispersion compensating fiber (DCF). We also show that a bandwidth of more than 300 nm will be available for 40 Gbit/s NRZ transmission by using the PCF as a transmission medium instead of conventional 1.3 µm zero-dispersion single-mode fiber (SMF).

We describe the applicability of photonic crystal fiber (PCF) with an enlarged effective area Aeff to a distributed Raman amplification (DRA) transmission. We investigate the DRA transmission performance numerically over a large Aeff PCF taking account of the signal-to-noise ratio (SNR) improvement RSNR in the S, C, and L bands. We show that an RSNR of 3 dB can be expected by utilizing DRA with a maximum pump power of 500 mW when the Aeff of the PCF is 230 µm2.

For the construction of photonic crystal fiber (PCF) systems using their unique properties, a PCF coupler (PCFC) is one of the key components of the systems. The characteristics of the PCFC depend on the state of air holes in the tapered region of the PCFC because the state of air holes in the tapered region affects light propagation in the PCFC taper. In this paper, coupling characteristics of PCFCs were theoretically investigated. In PCFCs with air hole remaining tapers, we found that a smaller elongation ratio i.e. a stronger elongation is required to obtain optical coupling as an air hole pitch or a ratio of air hole diameter to pitch is larger. In PCFCs with air hole collapsed tapers, it was clarified that a dependence of extinction ratio on air hole collapsed elongation ratio is higher for smaller elongation ratio. It was also clarified that an air hole remaining PCFC has slow wavelength characteristics in extinction ratio compared to an air hole collapsed PCFC. Air hole remaining PCFCs and air hole collapsed PCFCs were fabricated using a CO2 laser irradiation technique. We could successfully control whether air holes in the PCFC taper were remaining or collapsed by adjusting the irradiated laser power in the elongation process of the PCFC fabrication. It was experimentally clarified that the air hole remaining PCFC has slow wavelength characteristics in extinction ratio compared to the air hole collapsed PCFC. The tendencies of the measured wavelength characteristics of PCFCs agree with those of numerical results.

In this paper, we propose and demonstrate a novel type of PCF that has two cladding layers with Ge rods at the center core. We numerically show that it is possible to design a single mode PCF with large effective area greater than 200 µm2 over the whole wavelength above 1.2 µm. The proposed large mode area PCF (LMA-PCF) exhibits a high negative dispersion coefficient from -186 to -158 [ps/(nm-km)] in all wavelengths ranging from 1.2 µm to 1.8 µm. Effective single mode operation of LMA-PCF is confimed for the entire band of interest.

This paper describes near-zero ultra-flattened chromatic dispersion and low confinement loss that can be achieved from a decagonal photonic crystal fiber (D-PCF). The finite difference method with anisotropic perfectly matched boundary layer (PML) is used for the numerical analysis. It is demonstrated that it is possible to design a four-ring D-PCF with ultra-flattened dispersion of 0 0.69 ps/(nm-km) in a 1.30 to 1.75 µm wavelength range and 0 0.22 ps/(nm-km) in a 1.35 to 1.65 µm wavelength range with very low confinement losses of order 0.0011 dB/km. The proposed D-PCF shows promising dispersion tolerance.

We achieved the first 10 Gb/s WDM transmission at 1064 and 1550 nm over 24 km of photonic crystal fiber (PCF). We confirmed an improvement in the bit error rate (BER) performance after the transmission, namely "negative power penalties" of -0.5 and -0.3 dB at 1064 and 1550 nm, respectively. Our experimental result and theoretical estimation revealed that the signal degradation induced by the chromatic dispersion can be effectively suppressed by employing the pre-chirp technique with a conventional Z-cut lithium niobate (LN) modulator. We also show theoretically that we can expect to realize 10 Gb/s transmission over a 24 km PCF with negligible BER degradation in the 1060 to 1600 nm wavelength range by using the pre-chirp technique.

This paper reports a novel design in Photonic Crystal Fibers (PCFs) with nearly zero ultra-flattened dispersion characteristics. We describe the chromatic dispersion controllability taking non-uniform air hole structures into consideration. Through optimizing non-uniform air hole structures, the ultra-flattened zero dispersion PCFs can be efficiently designed. We show numerically that the proposed non-uniform air cladding structures successfully archive flat dispersion characteristics as well as extremely low confinement losses. As an example, the proposed PCF with flattened dispersion of 0.27 ps/(nmkm) from 1.5 µm to 1.8 µm wavelength with confinement losses of less than 10-11 dB/m. Finally, we point out that full controllability of the chromatic dispersion and confinement losses, along with the fabrication technique, are the main advantages of the proposed PCF structure.

This study proposes a novel structure of index-guiding square photonic crystal fibers (SPCF) having simultaneously ultra-flattened chromatic dispersion characteristics and low confinement losses in a wide wavelength range. The finite difference method (FDM) with anisotropic perfectly matched layers (PMLs) is used to analyze the various properties of square PCF. The findings reveal that it is possible to design five-ring PCFs with a flattened negative chromatic dispersion of 0-1.5 ps/(nm.km) in a wavelength range of 1.27 µm to 1.7 µm and a flattened chromatic dispersion of 01.15 ps/(nm.km) in a wavelength range of 1.25 µm to 1.61 µm. Simultaneously it also exhibited that the confinement losses are less than 10-9 dB/m and 10-10 dB/m in the wavelength range of 1.25 µm to 1.7 µm.

In this paper, we describe detailed experimental demonstrations of blue/violet light generation by the injection of ultrashort optical pulses into photonic crystal fibers (PCFs). Two lightwaves appear one on each side of the injected pulses in the spectral domain. They simultaneously evolve in the PCFs, changing their center wavelengths so as to spectrally stand apart from each other. Such behaviors are explained on the basis of the theory of nonlinear optics. The final center-wavelength difference between the two lightwaves at the end of the PCFs, depending on the power density of the injected pulse, is increased up to a limit imposed by the PCFs. Owing to this increase, the shorter wavelength limit reaches approximately 400 nm, which shows that short-pulse injection in PCFs is a promising method of realizing simple blue/violet light sources.

In this paper, the confinement loss of octagonal photonic crystal fibers (PCFs) with an isosceles triangle lattice of air-holes are numerically investigated. Taking into account the confinement loss, the mode field diameter (MFD), the effective area (Aeff) and the chromatic dispersion of octagonal PCFs are calculated, compared to conventional hexagonal PCFs. It is found from confinement loss and MFD results that the octagonal PCFs can confine the field strongly than the hexagonal PCFs due to the different air filling fraction. Moreover, it is shown that the octagonal PCFs are obtained not only positive but also negative larger dispersion values and smaller Aeff values compared to the hexagonal PCFs.

Propagation characteristics of acoustic waves in photonic crystal fibers (PCFs) have been theoretically investigated in details. In order to evaluate acoustic band structures and guided modes for out-of-plane propagation in PCFs, analysis methods based on the finite element method are newly formulated. It is shown through numerical results that complete acoustic band-gaps (ABGs) exist in the cladding region of PCFs and that acoustic guided modes could be localized in the defect region of PCFs by the ABG effect. Furthermore, it is shown that acoustic guided modes could also be localized in the defect region of PCFs by the total internal reflection. These confinement mechanisms of acoustic waves propagating along the fiber length are completely different to those of lightwaves.

Press-induced long-period fiber gratings exhibiting strong core-to-cladding mode coupling were formed in photonic crystal fiber. Only one resonance peak was observed over a 600 nm spectral range and the resonant wavelength was tuned over the whole range by tilting a groove plate before pressing the fiber. The resonant wavelength decreased with increasing periodicity of the grating, which was opposite to the trend of the step-index conventional optical fiber. Meanwhile, the resonant wavelength increased with increasing the ambient refractive index, which was also opposite to that of the conventional optical fiber.

We report the empirically obtained conditions for the fusion splicing with photonic crystal fibers (PCF) having large mode areas. By controlling the arc-power and the arc-time of a conventional electric-arc fusion splicer, the splicing loss between two PCFs could be lowered down to 0.2 dB in average. For the splicing PCF with a conventional single mode fiber (SMF), the loss was increased due to the modal field mismatch, but still below 0.45 dB in average. The tensile strength was weakened by the splicing from 2.83 GPa down to 1.04 GPa for the PCF-PCF case and 0.89 GPa for the PCF-SMF one.

Photonic crystal fiber (PCF) is a promising candidate for future transmission media due to its unobtainable features in a conventional single-mode fiber. We discuss some important problems to realize a PCF for transmission purpose. We also present recent progress on the PCF as a transmission media.

Recent progress on photonic crystal fibers is reviewed aiming at their application to high performance networks. A photonic crystal fiber has an array of air holes surrounding the silica core region. Light is confined to the core by the refractive index difference between the core and the array of air holes. Photonic crystal fibers have special characteristics compared with conventional single mode fibers. One is that the dispersion characteristics can be designed. Another characteristic, that strong birefringence can be established by sizing and/or arranging the air holes, is expected to realize a polarization maintaining fiber with high birefringence of the order of 110-3. This paper will describe the characteristics of dispersion controlled PCFs and polarization maintaining PCFs that include supercontinuum generation and absolute single polarization characteristics for various types of optical devices in high performance network systems.

We describe the first highly nonlinear dispersion-flattened polarization-maintaining photonic crystal fiber designed for nonlinear optics applications in the 1.55 µm region. The nonlinear coefficient of the fiber is 19 (W-1km-1), which is ten times that of dispersion shifted fiber. The chromatic dispersion and dispersion slope of the fiber at 1.55 µm are -0.23 ps/km/nm and 0.01 ps/km/nm2, respectively. We demonstrate the generation of a supercontinuum using the photonic crystal fiber. A symmetrical supercontinuum over 40 nm is obtained by injecting 1562 nm, 2.2 ps, and 40 GHz optical pulses into the 200 m-long photonic crystal fiber.