一种新型高非线性色散平坦光子晶体光纤结构

提出了一种新的高非线性色散平坦光子晶体光纤结构,引入了一个衡量非线性和色散平坦的品质因子δ。采用平面波展开法,研究了气孔尺寸对光子晶体光纤色散特性和非线性的影响。新结构在第一圈空气孔的中间插入六个附加小孔,使得光子晶体光纤有更小的有效模场面积,提高了光纤的非线性。通过控制第一圈和第三圈空气孔以及附加小孔的直径,使得该光子晶体光纤在大约330 nm的波长范围内,光纤的色散系数介于±0.5 ps/(km.nm)之间,在大约230nm的波长范围内,光纤的色散系数介于±0.1 ps/(km.nm)之间,在大约200 nm的波长范围内,光纤的色散系数D的值介于±0.05 ps/(km.nm)之间。光纤的有效模场面积为2.26μm2。衡量非线性和色散平坦的品质因子δ=11.8 ps.W/μm2。     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

Holey optical fibres: Fundamental properties and device applications

The presence of wavelength-scale holes in the transverse profile of a holey fibre can lead to novel optical properties that cannot be achieved in more conventional forms of optical fibre. Examples of such properties include broadband single-mode guidance, the extremes of fibre nonlinearity, from fibres providing tight mode confinement to those offering large mode areas, and a range of remarkable dispersive properties, including broadband flattened dispersion, anomalous dispersion below 1.3 μm, and large normal dispersion values at 1.55 μm. Fundamentals and recent progress are reviewed, ranging from design and fabrication through to applications and devices based on this emerging fibre type. To cite this article: T.M. Monro, D.J. Richardson, C. R. Physique 4 (2003).     

Nonlinearity of refractive index in glasses based on heavy metal oxides with different lead and tellurium contents

We have studied the nonlinearity of the refractive index at a wavelength of 1.08 μm for optical lead silicate and lead phosphate tellurite glasses of different compositions. We have shown that the nonlinear refractive index n₂ increases as the lead content increases in lead silicate glasses and as the tellurium content increases in lead phosphate tellurite glasses, where the latter are typically have higher values of n₂, as high as 24·10⁻¹³ cgs units (47·10⁻¹⁶ cm²/W), and lower Rayleigh losses. We have established a correlation between the nonlinear refractive index and the microstructure of the studied glasses. The highly nonlinear glasses are distinguished by extensive fluctuations in the heavy metal oxide concentration which are “frozen” during cooling of the glass melt. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 780–784, November–December, 2007.     

Study on pulse compression in tapered holey fibres

This paper proposes three kinds of tapered holey fibres with a multi-layer of holes whose pitch of air holes at the end of untapered and tapered are 5.8 μm and 1.8 μm. The central wavelength which locates in the anomalous dispersion region is 1.55 μm. An adaptive split-step Fourier method is numerically used to study the pulse propagation in tapered holey fibres. For the considered convex tapered holey fibre, at a wavelength of 1.55 μm, a compression factor of 136.7 can be achieved by initial width of 800 fs propagation through a length of 0.8 m. It demonstrates that in anomalous dispersion region, pulse can be compressed with the increase of nonlinearity coefficient and the decrease of dispersion coefficient.     

A broadband ultra flattened chromatic dispersion microstructured fiber for optical communications

A double-cladding microstructured fiber (MF) is proposed in this paper. The inner cladding of this optical fiber is composed of elliptical air holes and silica. The dependence of dispersion on the diameter of the air holes, the pitch, and the axes of the elliptical holes is investigated numerically. The proposed fiber possesses an ultra flattened dispersion curve over a wide wavelength range, and its dispersion value is small. The effective mode area is approaching to 60 μm², and the confinement loss is as low as <0.025 dB/km at 1550 nm. While choosing suitable structure parameters, an ultra dispersion-flattened MF within a broadband from1000 nm to 1900 nm can be achieved. The dispersion fluctuation is 0.6-1.0 ps/(nm·km) in all S, C and L band.     

双零色散光子晶体光纤结构参数的变化对其性能的影响

应用多极法,研究了正六边形结构光子晶体光纤的结构参数改变时,波长范围在0.8—1.8μm之间的双零色散光子晶体光纤的色散特性和非线系数随波长的变化规律.对具有相同结构参数的正六边形结构和正八边形结构进行比较,得到正六边形结构的双零色散光子晶体光纤的色散更加平坦,非线性系数有明显增大的结果.因此,正六边形结构更容易获得色散平坦的高非线性双零色散光子晶体光纤.最终设计了在800nm附近具有平坦色散和高非线性的正六边形双零色散光子晶体光纤.     

Holey optical fiber with circularly distributed holes analyzed by the radial effective-index method

We analyze a holey fiber that consists of a circular distribution of air holes by the radial effective-index method. By this method, we show that the holey fiber is a leaky structure and its extended single-mode operation is governed by the differential leakage loss between the first two modes of the fiber. The effects of the hole size and the hole separation on the leakage losses of the first two modes are calculated. The leakage loss of the fundamental mode of the fiber is found to be comparable to that of a conventional holey fiber that has a hexagonal distribution of air holes.     

Nonlinearity in holey optical fibers: measurement and future opportunities

Holey fibers combine two-dimensional microstructuring with one-dimensional longitudinal propagation, resulting in fibers with tailorable dispersive and nonlinear properties. We measure the effective nonlinearity of a typical holey fiber. The small effective area that is possible in this type of fiber significantly enhances its effective nonlinearity relative to standard fiber.     

Theoretical realization of holey fiber with flat chromatic dispersion and large mode area: an intriguing defected approach

We present a novel design approach for realizing holey fibers (HFs) with flat dispersion characteristics and large mode area based on the existence of an artificially defected air-hole ring in the cladding, and on the inclusion of additional defected air holes in the core of the fiber. This unique type of HF can be used for achieving remarkable flat dispersion characteristics as well as a large mode area, which are particularly useful for high-speed data transmission. The validation of the proposed design is done by adopting an efficient full-vectorial finite element method for optical characterization of HFs. The proposed fiber can be employed in reconfigurable optical transmission systems for performing wavelength division multiplexing operation. Typical characteristics of the proposed HF are a flattened dispersion of 6.3 +/- 0.5 ps/km/nm from 1.45 to 1.65 microm and an effective mode area as large as 100 microm2 in the same frequency range.     

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.     

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.     

Broadband nearly-zero ultra-flattened dispersion single mode index guiding holey fiber

This paper proposes and demonstrates a novel type of silica index guiding holey fibers (IGHFs) that has two cladding layers at the defective innermost structures. The proposed IGHFs exhibit remarkable chromatic dispersion properties such as nearly-zero and flattened dispersion over a wide spectral range and single mode guidance along with very low confinement loss. The numerical results indicate that 5 air-hole rings of nearly zero ultra-flattened dispersion single mode IGHFs can be designed with desire flattened dispersion of over a 340 nm bandwidth including the entire band of interest with low confinement loss of less than 10⁻⁶ dB/m.     

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.     

Dispersion-flattened polarization-maintaining photonic crystal fiber for nonlinear applications

In this paper, we propose a novel photonic crystal fiber design to achieve high birefringence, high nonlinearity with a flattened dispersion profile. The air holes which are embedded in silica are arranged in triangular lattice configuration. In the core region, an air hole is introduced as the central defect to flatten the dispersion curve. The shape of the defect air hole is deliberately designed elliptical so that high birefringence in the PCF is obtained for polarization-maintaining purpose. The structural parameters are carefully selected to optimize the optical properties.     

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.     

Multiplex frequency conversion of femtosecond laser pulses in trefoil and quatrefoil channels of a holey fiber

We report that trefoil and quatrefoil silica channels with sizes ranging from 0.6 μm to as large as 4–5 μm in a holey fiber can strongly confine the light field and act as highly nonlinear waveguides. Supercontinuum emissions, efficient dispersive-wave generation and parametric four-wave-mixing processes were achieved in these channels with sub-nanojoule femtosecond Ti:sapphire laser pulses. Two dispersion curves corresponding to two orthogonal modes in each channel were simulated and used to explain the experimental results. PACS 42.81.Gs; 42.81.Qb     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

Large optical second-order nonlinearity of poled WO3-TeO2 glass

Second-harmonic generation, one of the second-order nonlinear optical properties of thermally and electrically poled WO>(3)-TeO>(2) glasses, has been examined. We poled glass samples with two thicknesses (0.60 and 0.86 mm) at various temperatures to explore the effects of external electric field strength and poling temperature on second-order nonlinearity. The dependence of second-harmonic intensity on the poling temperature is maximum at a specific poling temperature. A second-order nonlinear susceptibility of 2.1 pm/V was attained for the 0.60-mm-thick glass poled at 250 degrees C. This value is fairly large compared with those for poled silica and tellurite glasses reported thus far. We speculate that the large third-order nonlinear susceptibility of WO>(3)- TeO>(2) glasses gives rise to the large second-order nonlinearity by means of a X((2)) = 3X((3)) E(dc) process.     

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.