A new design of allnormal and near zero flattened dispersion based on allsilica photonic crystal fibers (PCFs) using selectively liquid infiltration technique has been proposed to realize smooth broadband supercontinuum generation (SCG). The investigation gives the details of the effect of different geometrical parameters along with the infiltrating liquids on the dispersion characteristics of the fiber. Numerical investigations establish a dispersion value of −0.48 ps/nm/km around the wavelength of 1.55
Broadband smooth flattened supercontinuum generation (SCG) has been the target for the researchers for its enormous applications in the field of metrology, optical sensing, optical coherence tomography, wavelength conversion, and so forth [
Conventional PCFs have cladding structures formed by airholes with the
The fabrication of such a fiber is simplified due to the uniformity of the airholes in the cladding. To manufacture these PCFs, one must first selectively block specified airholes and infuse the liquid into the unblocked holes. One possible way is to employ the fusion splicing technique [
There are certain issues related to the infiltration of liquid to the airholes, such as whether the fluid wets glass and how viscous it is. If the liquid does not wet glass, then surface tension will oppose entry of the liquid into the airhole, thereby making the process difficult to realize. The required pressure to push such a liquid into the airholes can be worked out with the value of its surface tension and contact angle. With an airhole diameter of the order of 0.60
If the fluid does wet glass, then the airhole should be filled but the fillingup speed will depend upon viscosity. It can be worked out how quickly the airholes will be filled using the expressions for Poiseuille flow in a pipe [
The selective hole filling technique provides a couple of advantages. Firstly, all the airholes are of the same diameter, which is easier to be fabricated compared to fibers with multiple different submicron airhole sizes. Secondly, a regular PCF with selective infiltration with a liquid provides huge flexibility for tremendous applications. This is why the paper pursues selective airhole filling approach towards the target of allnormal near zero dispersion in a microstructured optical fiber. Notably, the technique comes out in designing fibers for various other applications [
As an initial attempt, we consider a regular PCF with four rings of airholes with
Cross section of the proposed photonic crystal fiber. The shaded regions represent airholes infiltrated with liquid with refractive indices
Modal fields as well as dispersion parameters are calculated by using commercially available CUDOS software along with MATLAB for numerically calculating the dispersion relation. The total dispersion (
Here
The SCG with the liquid filled near zero allnormal dispersion silica based PCF for smooth flattened broadband spectra has been studied by solving the nonlinear Schrödinger equation (NLSE) [
The nonlinear effects in a PCF strongly depend on the dispersive characteristic and the length of the employed optical fiber [
Computed dispersion of the PCF as a function of pitch (
Dispersion behaviour as calculated for varying
Variation of dispersion as a function of airhole diameter (
Then, we started the second stage of the optimization procedure as the following. For this step we tried to keep
Allnormal near zero dispersion with
Having obtained a preliminary design using an artificial, dispersionless liquid, we continue with the third optimization stage. We select oil (calling it as oil number 1) whose RI is close to the above optimized value in the target communication wavelength band and is given by Cauchy (
Cauchy equation for oil number 1 is known to be [
With this liquid an allnormal near zero dispersion PCF with
The optimized design of the allnormal near zero dispersion with
The effect of liquid infiltration in the first airhole ring upon dispersion. Not only the dispersion values but also the slope of the graph has been drastically altered.
The contribution of the material dispersion of the liquid and background silica material dispersion towards the total dispersion.
NLSE, as mentioned in (
The values of
Higher order betas  Beta values 















Evolution of the spectrum as the pulse propagates along the fiber. The input pulse and the output spectrum after each 25 cm of propagation.
Propagation of the spectrum as it travels through the fiber.
Optical spectrum with the optimized fiber after propagation of 1.2 meter. A FWHM of 375 nm is obtained with the optimized fiber.
It is well known that the dispersion can be tailored in a PCF according to the requirement by changing the geometrical parameters. However, once a design is fixed and fabricated the dispersion of that particular fiber is also fixed. So, to have a different or optimized design we need another fiber to be fabricated. Now here comes the advantage of liquidfilled PCF. Fluidfilled PCFs have created novel opportunities for tunable dispersion control, with applications in tunable delay lines, nonlinear optics, and long period grating designs [
Dispersion curves for the liquidfilled PCF operated at various temperatures.
(a) Peak dispersion values of the liquidfilled PCF operated at variant temperatures. (b) Peak dispersion wavelength of the liquidfilled PCF operated at variant temperatures.
We have demonstrated a new design of allnormal near zero dispersion PCF based on selectively liquid infiltration and explored its properties for flattened broadband SC generation in near IR wavelength regions. We have studied the effect of individual PCF parameters, namely, Λ and
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Dr. Boris Kuhlmey, University of Sydney, Australia, for providing valuable suggestions in understanding the software for designing and studying the properties of different structures. Thanks are also due to Dr. Alessandro Tonello of Xlim Institute, UMR CNRS Limoges Cedex, France, for providing valuable suggestion in developing and understanding the NLSE codes. The authors acknowledge sincerely the Defence Research and Development Organization, Govt. of India and CRF of IIT Kharagpur for the financial support to carry out this research.