In this paper, the influences of fiber link and laser source on performances of Radio over Fiber (RoF) including error vector magnitude (EVM), constellation and eye diagram are investigated by simulation using Opti-system12 (trial version). The investigated RoF network is built on IEEE 802.16a WiMAX, with 16 QAM and a Mach-Zehnder modulator for intensity modulation. The mechanism of that the dispersion in fiber link makes the constellation rotate is investigated. The relationship between the rotation angle of the constellation and dispersion is analyzed, where we first put forward a fitting formula to describe this approximate quantitative relation. In the analysis of the influence of the laser source on the network, where the dispersion compensates fiber (DCF) is applied to compensate the rotation in constellation caused by fiber link, the threshold in the relationship curve of the linewidth and EVM is obtained. It is found that if the laser linewidth exceeds this threshold, the EVM will increase rapidly, then, the performance decreases.
A compact and high sensitivity curvature sensor based on twin core fiber (TCF) coupler is proposed and demonstrated
experimentally in the curvature range from 0 to 9.30 m-<sup>1</sup>. The TCF coupler is formed by splicing a section of 86.85 mm
TCF between two single mode fibers (SMFs). A nonlinear blue-shift of the wavelength was observed when increasing
the curvature. In the range from 3 m<sup>-1</sup> to 9.30 m-<sup>1</sup>, the minimum and maximum sensitivity are - 2.5 nm/m<sup>-1</sup> and - 14.7
nm/m<sup>-1</sup>, respectively. The dynamic range can be tailored by proper selection of the TCF length and the resonance dips.
A novel side-leakage PCF with high birefringence is designed and fabricated by introducing a linear side-leakage region
into both sides of the elliptical Ge-doped core. A curvature sensor using the homemade side-leakage PCF based Sagnac
interferometer is proposed and demonstrated experimentally. Experimental result shows that a high bending sensitivity
of 10.798nm/m<sup>-1</sup> can be achieved when the linear side-leakage region is in the vertical with the direction of curvature.
With the measuring matrix obtained from sensor's sensitivity matrix by measuring the wavelength shifts of two dip in
the transmission spectrum, this sensor can also realize simultaneous measurement of curvature and temperature.
A simple modal interferometer is proposed, to be produced by splicing a section of a novel ring-core photonic crystal fiber (RPCF) and two segments of single-mode fibers. Owing to the effects of the collapsed region, the ring modes in the RPCF can be effectively activated. This is the first demonstration of an interferometer based on the interference between ring modes, as opposed to previously reported interferometer based on the interference of core modes or cladding modes. The effects of the length of RPCF and the wavelength on the temperature and strain characteristics of the interferometer are investigated. The strain sensitivity increases with RPCF length and with wavelength. Based on the interferometer, a strain sensor with low temperature sensitivity is proposed, which has strain sensitivity of 1.777 pm/(µm/m).
We demonstrate the fabrication of a kind of asymmetrical twin core fiber, which is easy spliced with standard single
mode fiber. This fiber is designed to be used for passive mode-locking in fiber lasers.
We propose a novel structure of single-polarization single-mode(SPSM) holey fiber designed based on resonant coupling
effect. The proposed fiber is able to achieve SPSM operation over an ultra-broad bandwidth as large as 920nm ranging
from 1.48μm to 2.4μm, within which only one polarization state of the fundamental mode can be effectively guided.
With the aid of six smaller holes around the central core, the effective mode area of the fiber is enlarged and the
chromatic dispersion curve is flattened. The numerical results indicate that the proposed fiber achieves nearly zero
ultra-flattened dispersion over a wide spectral range and the effective mode area is approaching ~78μm2 at 1550nm.
Moreover, the structure shows superiority in output beam quality owing to the symmetry of the central core region, and
also exhibits great modal compatibility with SMF.
Switchable dual-wavelength fiber laser with photonic crystal fiber (PCF) Sagnac loop and broadband fiber Bragg grating
(BFBG) at room temperature is demonstrated. By adjusting the polarization controller (PC) appropriately, the laser can
be switched between the stable single- and dual-wavelength lasing operations by exploiting polarization hole burning
(PHB) and spectral hole burning effects (SHB).
A highly negative dispersion photonic crystal fibre with less germanium doped core and central index dip is numerically
investigated by fully vectorial finite element method. By adjusting the pitch, the depths and radius of the central index
dip and the diameter of six small air holes in the third ring, Highly negative dispersion value (-1325.5ps/nm.km) and
large mode effective area (71.4um<sup>2</sup>) DCPCF around the wavelength of 1.55μm is obtained.
A fully vectorial effective index method and multiple-cladding method (FVEIM&MCM) is developed for modeling
photonic crystal fibers. Large negative dispersion photonic crystal fibers can be designed by changing the diameters of
the first, second and third ring of air-holes.
The very high dispersive dual-core photonic crystal fiber is analyzed by the full-vector finite element method. The
dependence of the phase-matching wavelength (PMW) and the full width at half-maximum (FWHM) on the refractive
index of the doped inner core, the diameter of air holes and the hole pitch is demonstrated. The dispersion value is as
large as -1427 ps/(nm km) and the effective mode area of the fundamental mode, 82.06 um<sup>2</sup> are obtained.
Through introducing two sizes of hole in the cladding of PCF, a kind of highly birefringent two-mode photonic crystal
fiber (PCF) is proposed. Modal properties are analyzed numerically by the plane wave expansion method. Numerical
results demonstrate that only two-mode, i.e. LP<sub>01</sub> and LP<sub>11</sub><sup>even</sup> can propagate in the wavelength range from 500nm to
1890nm in the two-mode PCF which is nine times as wide as that in elliptical core fibers. The theory of highly
birefringence two-mode PCF for constituting fiber sensor is also discussed. According to the structure proposed, we
successfully fabricate the highly birefringent two-mode PCF with stack-drawing technique and gas-inert pressurization
control at furnace temperature 1900°C and gas-inert pressure 900pa. They could extend significantly the application
range of these two-mode devices and open up new possibilities.
Highly nonlinear fibers with nearly zero flattened dispersion over a wide band range has very wide applications in future
high-capacity, all-optical networks. Because of its flexibility of structure design and much larger index contrast between
the core and effective cladding than the conventional fibers, photonic crystal fibers (PCFs) are becoming to be an
attractive candidate to form this kind of highly nonlinear fibers. Based on quantitative analysis on the effect of the
difference of hole-sizes between the first ring and the other's, and the effect of Ge-doped concentration in the core region
on the PCF's dispersion curve, a new way to design PCF with high nonlinear coefficient and nearly zero flattened
dispersion is proposed. Based on this, a PCF is designed, which has dispersion values between ±0.8ps/nm/km over
S+C+L wavelength bands, and the dispersion slope of 0.005ps/nm<sup>2</sup>/km, nonlinear coefficient of 46.6W<sup>-1</sup>km<sup>-1</sup> at 1.55μm.
Some PCF fabrication techniques are discussed. Effects of draw conditions on the capillaries and the geometry of the final photonic crystal fiber (PCF) are investigated experimentally. The cross-sectional hole structure can be adjusted to a certain extent by controlling the parameters such as the temperature, the feed and draw speed or combinations of these. Since the improvement is limited and hardly can a satisfactory fiber be obtained, the inert gas pressurization method is introduced. It is testified feasible and effective in tuning the geometry of the final PCF and PCFs of good uniformity are fabricated experimentally.
Recently a significant advancement in THz source and detector technology have arisen a number of potential application such as space-based communication, THz sensing and imaging for military and security, biology and so on. However, THz spectroscopic techniques uses mainly free space propagation and guided wave still remains a challenge in this intermediate spectral region. In this paper, a square lattice microstructure terahertz waveguide is proposed. Combining super-lattice full-vector method with beam propagation method, its optical properties, especially dispersion, confinement loss and single mode property, are analyzed. Numerical results demonstrate that this waveguide can operate at single mode with low dispersion and low confinement loss.
A method for judging mode cutoff in photonic crystal fibers (PCFs) with non-uniform holes
by analyzing the break phenomena of the mode field radius during the increasing of
wavelength is brought forward. And three kinds of PCFs with different structure are analyzed
and discussed in detail using this new method.
In this presentation, the universal structure of one-dimensional photonic crystal (1-D PC) is constructed, and its optical transmission properties are analyzed by transfer matrix method (TMM). A case that there are two kinds of medium as a period is studied in detail. It is concluded that the reflectivity in photonic band-gap (PBG) increases with the increasing of periodical number, and the bandwidth of PBG has direct relation with the difference between two kinds of dielectric constant, three methods for extending PBG are discussed. When defect layer is inserted, a defect mode appears in the PBG. The concept of optimal periodical number is presented, and it is found that this optimal periodical number is only relative to the ratio of dielectric constant (K). Using multi-objective optimization method, we educe the curve and equation relation between optimal periodical number and K for the first time. In addition, the change in the number of defect mode with the variation of the defect layer's thickness is analyzed, and it is explained by the theory of F-P cavity.
Photonic crystal fiber (PCF) has aroused growing interest over the past few years becauce PCFs exhibit many unusual properties, especially an endlessly single mode and highly tunable dispersion. Polyethylene have distinguish advantages such as the real part of dielectric constant keep constant in the frequency range from 0.1THz to 2THz that the material dispersion contribute little to total dispersion, and the imaginary part of the dielectric constant is close to zero in THz frequencies that the material loss is low. This offer a new possibility of constituting polyethylene PCF for low loss and low dispersion THz waveguide. In this paper, propagation properties of polyethylene PCF in THz frequencies are analyzed by full-vector model. Numerical results demonstrate that effective index of fundamental mode decreases with frequency decreasing and near zero-dispersion can be obtained over a wide frequency region.
The splice losses between PCF and SMF and between two PCFs with different structure are analyzed based on mode field radius of the fibres, respectively. And the effect of each structure parameter on the splice loss was discussed.
The modal characteristics of dual-core photonic crystal fibers are analyzed by a full vector supercell method. The fundamental and second order modes of dual-core PCF consist of a pair of even and odd modes with different polarization, the parity properties of the modal electric field are illuminated. Based on the analysis of parity of modal electric fields, we investigated the vector modal interference in dual-core PCF. The power transfer induced by interference of different mode pair is investigated. It is the interference between two same polarized modes that contribute to the inter-core coupling of power. It is shown that the optical power will oscillate from one core to the other. The dependence of the coupling coefficients on wavelength and structural parameter for different polarization is discussed.
In this paper, we proposed a novel SPSM PCF and analyzed its optical properties with a full vector model. Considering the opposite parity of each guided mode in the fiber with symmetric structure, we improved the full vector model we developed previously. The numerical result demonstrated that the algorithm proposed is very efficient for analyzing the PCFs with symmetric structure. It only needs relatively few terms to obtain good results that the computation time can be reduced greatly. It is also confirmed that the fiber structure proposed is very efficient and can operate at SPSM region from the wavelength 1.37mm to 1.70mm.
The optical properties of near elliptical core polarization maintaining photonic crystal fiber are analyzed by using a full vector model. We classify guided modes in the near elliptical core photonic crystal fibers (PCFs) according to the minimum waveguide sectors and its appropriate boundary conditions. Because the field patterns of the near elliptical core PCF is similar to that of rectangular waveguide, the guided modes are labeled in this PCF in the same way as in rectangular waveguide. The numerical results exhibit that the modal birefringence of elliptical core PCF is at least one order of magnitude higher than the conventional elliptical polarization-maintaining fibers (PMF). Zero walkoff point occur at the longer wavelength than that of convention elliptical PMF. This can restrain the first order polarization mode dispersion. This fiber has a number of potential applications in polarization control and management.
Combining perfectly matched layer (PML) for the boundary treatment, we present an efficient compact 2-dimensional finite-difference time-domain (2D FDTD) method for modeling photonic crystal fibers. For photonic crystal fibers, if we assume that the propagation constant along the propagation direction is fixed, three-dimensional hybrid guided modes can be calculated by using only a two-dimension mesh. Because of using the real variable method, the computation time, i.e., it is of order N. Comparing with the plane wave expansion method, FDTD make the computation time and computer memory are significantly reduced. The numerical results for a triangular lattice
photonic crystal fiber are in very good agreement with the results from the local basis function method. This method can easily be used for any complicated inclusions.
A full vector method based on supercell lattice method is applied for modeling the microstructured optical fibers (MOF). With this new method, the class and degeneracy of modes in MOF are discussed based on symmetry analysis. We classify the modes of MOF into nondegenerate or degenerate pairs according to the minimum waveguide sectors
and its appropriate boundary conditions. It is shown that the modes of MOF can be labeled by its step index fiber analogs, except the modes with the same symmetry as MOF. The doublet of the degenerate pairs in which both have the same symmetry as MOF will be split into two nondegenerate modes by the reduced symmetry of the fiber.
A novel supercell overlapping method is developed to analyze the PCFs. The dielectric constant of the PCF is considered as the sum of two different periodic dielectric structures which can be expanded in cosine functions.