Defect centers play a major role in the radiation-induced transmission loss for silica optical fibers. We have investigated
characteristics of the best known defect centers E' in silica optical fiber material irradiated with γ ray at room temperature,
and measured by using electron spin resonance (ESR) and spectrophotometer. The results show that the defect
concentrations increase linearly with radiation doses from 1kGy to 50kGy. We have established the mechanism models
of radiation induced defect centers' formation. We have also studied the influences of thermal annealing on defect centers.
The radiation induced defect centers can be efficiently decreased by thermal annealing. Particularly, the defect
concentration is less than the initial one when the temperature of thermal annealing is over 500°C for our silica samples.
These phenomena can also be explained by the optical absorption spectra we have obtained.
Normalized frequency, normalized propagation constant and asymmetry measure are introduced to the left-handed slab
waveguides. The dispersion relations expressed by normalized parameters are then derived. All the possible waveguide
configurations with the left handed materials as core, substrate, and cover are considered and divided into four cases.
Universal dispersion curves, and dispersion properties have been obtained analytically. It is found that guided mode
properties differ dramatically for these four cases. For some cases, fundamental mode does not exist. For some cases,
double degeneracy of modes appears. In one case, the first order mode only exists in a small frequency range, while in
another case the fundamental mode exists only in a small frequency range. Both TE and TM oscillating guided modes
In this paper, we present two new photonic crystal structures, which are composed of fractal Cantor multilayer with defects embedded in its middle. Optical transmission matrix method is used to calculating the transmittance and reflectance. Compared with general Cantor multilayer, we find these new structures have wider stopbands and show super narrow bands in the middle of wider stopbands. They can be served as super narrow bandpass filters. The pass band obtained can be less than 0.6nm near the infrared 1530 nm when there is a defect embedded in the cantor multilayer. The optical transmission in the center wavelength is higher than 99%. This means a very low insert loss. If there are three detected layers, three super narrow peaks can be found in the middle of the stopband. The center wavelengths are 1232.4 nm, 1372.8nm and 1538.3 nm, respectively. It is more superior to other kind narrow band filters. These kinds of photonic crystal super narrow band optical filters may find applications in super dense wavelength division multiplexing for optical communications and precise optical measurement.
In this paper, we present a novel cross section shape DCF. Truncated from a circular DCF, the inner cladding of this DCF is bounded by two equal angular spiral curves. By using the concept of mean transverse path between successive encounters of the core, we developed a new method to calculate the absorption efficiency per unit fiber length. For a spiral shape DCF with the largest radius R=100μm , radius of doped core r0=4μm , and the angle parameter of spiral curve θ=0.04 , we calculated the mean transverse path between successive encounters of the core lr=2903μm . The corresponding lc with other truncated shape cross section having the same cross sectional area is lc=3454μm . Final results show that the absorption efficiency η increases 16% for our spiral shape DCF.
The absorption efficiency of graded-index double-clad fiber lasers and amplifiers is investigated. As ray optics are no longer valid in the case of a graded index, a mode analysis method is used. The calculated results show that, in symmetric cases, absorption efficiency is higher with graded index than that with step index. In the case of an offset core, graded index can achieve the same absorption efficiency with a much smaller offset distance. Absorption efficiency for different graded-index profiles of the inner cladding is also studied and compared.
In this paper, a novel chiral photonic crystal structure is presented. The formula of reflection coefficient of multi-layer chiral media is applied to dielectric-chiral photonic crystal structure, which is composed of thin chiral layers sandwiched by conventional media. To compare with previous literature, we consider the dielectric structure with alternate glass and GaAs layers. The power reflectance as a function of wavelength for this photonic crystal structure has been calculated. The results are in good agreement with that of Reference. However, our method is simpler. From these graphs, it is found that 100% reflectance is only in finite wavelength ranges, and reflection bandwidth is also small for conventional photonic crystal structure. For chiral photonic crystal, the results show that the chiral photonic band gap (PBG) structure gives nearly 100% reflections in the near-infrared region in addition to some parts of the visible region of the wavelengths. Therefore, it can be used as a broadband reflector and filter.
The absorption efficiency for circular or offset double-clad optical fibers is investigated with modal analysis method. First, calculate the number of all the propagating modes approximately using WKB method and derive the analytic formulas for calculating the number of propagating modes that can't be absorbed by active core. Then the absorption efficiency can be obtained analytically. Comparison of modal method with the ray optics method is made and the condition of using both methods is also discussed.
The absorption efficiency of graded-index double-clad fiber lasers or amplifiers is investigated. As ray optics method is no longer valid for graded-index case, the mode analysis method is used in this paper. Calculated results show that absorption efficiency for a graded-index is higher than that for a step-index in symmetric case. In offset core case, the graded-index can achieve the same absorption efficiency with much smaller offset distance. The absorption efficiency for different graded-index profiles of the inner cladding is also calculated and compared.
In this paper, the formula of reflection coefficient of multi-layer chiral media is derived by non-symmetric transmission-line method. Then, it is applied to 1-D chiral photonic crystal structure, which is composed of thin chiral layers sandwiched by air. The results show that it is difficult to obtain photonic band gap for general dielectric when the difference of two media refractive indices isn't large, and the reflection coefficient is small. With the increasing of the refractive index of the medium, reflection coefficient becomes gradually large, and reflection bandwidth basically keeps unchanged. These characteristics are agreed with results of theoretical analysis of photonic crystal. However, for chiral photonic crystal, although the refractive index of chiral layer is small, the wave spectrum obtained contains forbidden zones and the reflection coefficient from such a structure is found to be almost equal to 1, i.e., the wave is almost totally reflected through adjusting chiral parameter. Therefore it is easier to obtain an ideal photonic band gap.
Based on the theory of mode analysis, we present here a new method to calculate the absorption efficiency for the double-clad optical fibers. The ratio of the number of the absorbable modes to total number of guided modes is taken as absorption efficiency. Taking the radius of caustic surface as a threshold value, we can estimate whether or not an individual mode is absorbable. Results show that for a symmetric circular DCF, there is a large fraction of power which can never be absorbed and its absorption efficiency is low. Increasing the radius of RE-doped core can raise the absorption efficiency, but the single mode operation will be destroyed. For an offset core DCF, the high absorption efficiency can be achieved by increasing the offset distance. Also there exists an offset limit, beyond this the absorption efficiency reaches saturation. The result is that we can improve the absorption efficiency by increasing offset distance instead of the radius of fiber core. This conclusion is in principle consistent with precious 2-D ray optics methods.
An approximate solution has been presented for the single-input nonlinear optical directional coupler under the most commonly used condition: operating near the critical power. By introducing the approximation in previous step, a simplified integral equation has been obtained. It can be integrated as simple functions instead of elliptic functions. Numerical examples of our results have been given and compared with the exact solution and the approximate solution derived by A. T. Pham et al. Our results are more accurate than that of A. T. Pham et al.