Design of broadband nearly-zero flattened dispersion highly nonlinear photonic crystal fiber

We propose a new structure of broadband nearly-zero flattened dispersion highly nonlinear photonic crystal fiber （PCF）. Through optimizing the diameters of the first two inner rings of air-holes and the GeO2 doping concentration of the core, the nonlinear coefficient is up to 47 W^-1.km^-1 at the wavelength of 1.55 um and nearly-zero flattened dispersion of ±0.5 ps/（nm·km） is achieved in the telecommunication window （1460 - 1625 nm）. Due to the use of GeO2-doped core, this innovative structure can offer not only a large nonlinear coefficient and broadband nearly-zero flattened dispersion but also low leakage losses.     

Modified design of photonic crystal fibers with flattened dispersion

We present a modified method to design photonic crystal fibers with flattened dispersion characteristics. By replacing the circular air-holes of the first central ring with elliptic air-holes, we observe a more flattened dispersion curve. Plane-wave expansion (PWE) method is used to analyze the dispersion property in a high-index core PCF. The simulation results are presented, and ultra-low and ultra-flattened dispersion curves over wide wavelength range are demonstrated.     

Design and analysis of novel highly nonlinear photonic crystal fibers with ultra-flattened chromatic dispersion

The present article describes novel highly nonlinear photonic crystal fibers (HN-PCFs) with flattened chromatic dispersion and low confinement losses. The proposed design has been simulated based on the finite-difference method with anisotropic perfectly matched layers absorbing boundary condition. It is proved that the design novel HN-PCFs is obtained a nonlinear coefficient greater than 45 W⁻¹ km⁻¹ and low dispersion slope −0.009 ps/(nm².km) at 1.55 μm wavelength. In addition, results from numerical simulation show that the ultra-flattened dispersion of 0 ± 0.65 ps/(nm.km) can be obtained in a 1.36-1.62 μm wavelength range with confinement losses lower than 10⁻⁷ dB/m in the same wavelength range. Another advantage of the proposed HN-PCFs is that it possessed modest number of design parameters.     

Multiple defect-core hexagonal photonic crystal fiber with flattened dispersion and polarization maintaining properties

In this paper, we present a dispersion controlling technique with a multiple defect-core hexagonal photonic crystal fiber (MD-HPCF). By omitting air holes in the core region of the conventional HPCF and adjusting the size of air holes around the newly formed core, we can successfully design low flattened dispersion PCF with low confinement loss, as well as high birefringence. The low flattened dispersion feature, as well as the low confinement losses and high birefringence are the main advantages of the proposed PCF structure, making it suitable as chromatic dispersion controller, dispersion compensator, and/or polarization maintaining fiber.     

Design of highly nonlinear dispersion-flattened square photonic crystal fiber for medical applications

In this paper, we report the design of a highly nonlinear dispersion flattened high-index-core square photonic crystal fiber (PCF) for applications in optical coherence tomography (OCT). The finite-difference method with an anisotropic perfectly matched boundary layer is used as a numerical simulation tool. A set of optimized design parameters numerically resulted in a nonlinear coefficient of 79.9W⁻¹ km⁻¹ and a dispersion of −0:186 ps/(nm·km) at a wavelength of approximately 1.06 μm. Owing to its high nonlinear coefficient and flattened dispersion, the PCF is expected to be suitable for broadband supercontinuum generation, which is considered very important in OCT medical applications.     

New nonlinear and dispersion flattened photonic crystal fiber with low confinement loss

A new nonlinear dispersion flattened photonic crystal fiber with low confinement loss is proposed. This fiber has threefold symmetry core. The doped region in the core and the big air-holes in the 1st ring can make high nonlinearity in the PCF. And the small air-holes in the 1st ring and the radial increasing diameters air-holes rings in cladding can be used to achieve the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCFs structure parameters. A PCF with flattened dispersion is obtained. The dispersion is less than 0.8 ps/(nm km) and is larger than −0.7 ps/(nm km) from 1.515 μm to 1.622 μm. The nonlinear coefficient is about 12.6456 W⁻¹ km⁻¹, the fundamental mode area is about 10.2579 μm². The confinement loss is 0.30641 dB/km. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal fibers with high nonlinearities.     

Highly birefringent photonic crystal fibers with flattened dispersion and low effective mode area

A highly birefringent index-guiding photonic crystal fibers with flattened dispersion and low effective area is proposed by introducing elliptical air holes in the cladding and small holes both in the core area and in the cladding. With the plane wave expansion (PWE) method, the birefringent, dispersion and effective area of the fundamental modes in such photonic crystal fibers are analyzed in detail. The simulation result shows that high birefringence with a magnitude of the order of 10⁻³, flattened chromatic dispersion from 1100 nm to 1800 nm and low effective area (which mean high nonlinearity) are obtained. Furthermore, the influences on the birefringence and dispersion by geometrical parameters have also been discussed and a modest number of design parameters are given.     

Design procedure for photonic crystal fibers with ultra-flattened chromatic dispersion

A simple design procedure is used to generate photonic crystal fibers (PCFs) with ultra-flattened chromatic dispersion. Only four parameters are required, which not only considerably saves the computing time, but also distinctly reduces the air-hole quantity. The influence of the air-hole diameters of each ring of hexagonal PCFs (H-PCF, including 1-hole-missing and 7-hole-missing H-PCFs), circular PCFs (C-PCF), square PCFs (S-PCF), and octagonal PCFs (O-PCF) is investigated through simulations. Results show that regardless of the cross section structures of the PCFs, the 1st ring air-hole diameter has the greatest influence on the dispersion curve followed by that of the 2nd ring. The 3rd ring diameter only affects the dispersion curve within longer wavelengths, whereas the 4th and 5th rings have almost no influence on the dispersion curve. The hole-to-hole pitch between rings changes the dispersion curve as a whole. Based on the simulation results, a procedure is proposed to design PCFs with ultra-flattened dispersion. Through the adjustment of air-hole diameters of the inner three rings and hole-to-hole pitch, a flattened dispersion of 0±0.5 ps/(nm·km) within a wavelength range of 1.239 – 2.083 μm for 5-ring 1-hole-missing H-PCF, 1.248 – 1.992 μm for 5-ring C-PCF, 1.237 – 2.21 μm for 5-ring S-PCF, 1.149 – 1.926 μm for 5-ring O-PCF, and 1.294 – 1.663 μm for 7-hole-missing H-PCF is achieved.     

Photonic crystal fibers with high and flattened dispersion

We investigate the dispersion property of space filling mode of photonic crystal structures and find a new type of dispersion—structure induced dispersion. By incorporating this new source of dispersion we designed PCF with large normal dispersion ~ 350 ps²/km. Our simulation indicates the dispersion of such fiber changes less than 3% in 1.4-1.7 μm wavelength range and we also show that our design is insensitive to the structure changes.     

Group-velocity dispersion in photonic crystal fibers

The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mum when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mum.     

Supercontinuum generation in photonic crystal fibers made from highly nonlinear glasses

The nonlinear propagation of femtosecond pulses and supercontinuum generation in photonic crystal fibers made from glasses with high nonlinear refractive indices are theoretically investigated without the use of the slowly varying envelope approximation. For specifically designed photonic crystal fibers with two zero-dispersion wavelengths, we predict supercontinuum generation due to nonsolitonic radiation emitted by the solitons on the shorter- and on the longer-wavelength sides. The physical mechanism and the peculiarities of such radiation on both spectral sides are studied and explained by the specific phase-matching relations between the solitons and their associated radiation.     

Blind dispersion compensation for optical coherence tomography

We present a numerical method for compensating dispersion effects in optical coherence tomography that does not require a priori knowledge of dispersive properties of the sample. The method yields results equivalent to recently demonstrated quantum-optical coherence tomography, but without exploiting non-classical states of optical radiation. Dispersion compensation is accomplished by processing phase information present in standard interferograms to calculate the generalized autoconvolution function. The operation of the method can be conveniently visualized using the Wigner distribution function formalism.     

Control of dispersion in photonic crystal fibers

Photonic crystal fibers (PCFs) exploit the large index difference between air and glass to achieve modal properties unattainable by conventional fiber techniques.     

Photosensitivity-enabled dispersion controllability for quasi-phase-matching in photonic crystal fibers

A novel photonic crystal fiber featured by concentric cores is proposed to induce dispersion controllability by photosensitivity. Chromatic dispersion can be changed from -1827 to 72 ps/nm/km with refractive index modulation of 4 x 10(-4) produced in Ge-doped regions in the fiber. Effective mode area of inner mode is as small as 6.4 mum(2). The proposed fiber enables to achieve quasi-phase-matched nonlinear parametric interaction in a single piece of photonic crystal fiber, by periodically alternating dispersion and compensating for phase mismatching caused by the dispersion.     

Endlessly single-mode operation of highly nonlinear photonic crystal fibers by controlled hole collapse

We report a higher-order mode filter in highly nonlinear photonic crystal fibers (HNPCFs) made by a conventional fiber taper rig based on controlled hole collapse. The air holes of the HNPCFs will become smaller due to the surface stress when they are heated on the taper rig incorporating a simple burner configuration. So the HNPCFs' configuration parameters can be easily changed by controlling the heating time. As is well known, if the relative hole size d/Λ of PCF is less than or equal to 0.4, it will become an endlessly single mode fiber. So we can control the heating time to satisfy the criterion of endlessly single mode operation, the heated section will be a higher-mode filter in HNPCFs. The optical loss of the higher-order mode filter is very low for the fundamental mode, typically less than 0.1 dB.     

Design of double cladding dispersion flattened photonic crystal fiber with deformation insensitive outer cladding air-holes

A design of double cladding dispersion flattened photonic crystal fiber (DF-PCF) is proposed. To employ traditional stack and draw technology, the cladding of the DF-PCF is consisted of triangular periodic air-holes with the same hole to hole pitch. Simulation results show that the small air-holes in the inner cladding are mainly for dispersion management. The large air-holes in the outer cladding are mainly used for light confinement and have little impact on the dispersion tailoring. Thus, the dispersion profile of the double cladding DF-PCFs is insensitive to the deformation of air-holes in the outer cladding. Considering that the larger air-holes are apt to deform in the drawing procedure, the characteristics mentioned above make the realization of DF-PCFs relative easy by employing conventional stack and draw technology and modest air-hole rings (less than 10 rings) in the cladding.     

Tailoring the dispersion properties of photonic crystal fibers

Photonic crystal fibers (PCFs) have had a substantial impact on nonlinear fiber optics and shortpulsed fiber laser systems due to their novel dispersion properties. The large normal or anomalous waveguide dispersion available in such fibers opens up a number of new opportunities not accessible with standard fiber technology. In this contribution, the fundamentals of PCF dispersion are briefly reviewed along with earlier results. In addition, some of our recent work on dispersion tailoring to facilitate nonlinear processes, and dispersion control in lasers will be presented.     

Ultrahigh-resolution optical coherence tomography by broadband continuum generation from a photonic crystal fiber

We have developed an ultrahigh-resolution optical coherence tomographic system in which broadband continuum generation from a photonic crystal fiber is used to produce high longitudinal resolution. Longitudinal resolution of 1.3-microm has been achieved in a biological tissue by use of continuum light from 800 to 1400 nm. The system employed a dynamic-focusing tracking method to maintain high lateral resolution over a large imaging depth. Subcellular imaging is demonstrated.