We have theoretically investigated the dispersion characteristics of dual-core PCF, based on square-lattice geometry by varying different parameters. The fiber exhibits a very large negative dispersion because of rapid slope change of the refractive indices at the coupling wavelength between the inner core and outer core. The dependence of different geometrical parameters, namely, hole-to-hole spacing (

Photonic crystal fibers (PCFs) [

The principle behind having a very large negative dispersion in these Dispersion Compensating Fibers (DCFs) being the coupling between two spatially separated asymmetric concentric cores which support two leaky modes: inner mode and outer mode. By proper design, mode matching can take place between these two modes at the desired wavelength. A few analyses have been performed to realize high negative dispersion with triangular lattice PCF [

In recent times, a square-lattice PCF preform has been realized with a standard fabrication process, stack and draw, in order to study the localization and control of high frequency sound by introducing two solid defects in the periodic distribution of air-holes [

Cross-sectional geometry of the proposed/studied fiber has been shown in Figure

Cross-section of the proposed/studied fiber. The air-hole diameter of the third air-hole ring is reduced to create the outer core, thereby creating the dual-core structure.

We solve the guided modes of the present fiber by the CUDOS MOF Utilities [

The confinement loss for the structures has been calculated through

We started our dispersion analysis with

Variation of real part of the effective indices for both the cores (inner core-black line and outer core red-line) of PCFs with

Im(

Dispersion curve for PCFs with

The dependence of dispersion upon the geometrical parameters (

Variation of dispersion for

Variation of

Variation of

In this section, we will study the fabrication tolerance of the dual core S-PCF for designing ultra-low dispersion at the required wavelength. For this purpose, we have considered the S-PCF as demonstrated in Figure

Variation of peak dispersion (a) and peak dispersion wavelength (b) for percentage change of hole-to-hole distance (

Variation of peak dispersion (a) and peak dispersion wavelength (b) for percentage change of bigger air-hole diameter (_{1}).

Variation of peak dispersion (a) and peak dispersion wavelength (b) for percentage change of smaller air-hole diameter (

Based upon the above studies we changed the available parameters and we could achieve a very high negative dispersion of −47,500 ps/nm/km around 1550 nm with ^{−2} km^{−1} and is given by [

Optimized dispersion value of −47,500 ps/nm/km achieved with

Effective area variation of the optimized PCF.

In this paper we have theoretically investigated chromatic dispersion compensation property exhibited by square-lattice geometry of the PCFs based on pure silica. We have extensively studied the effect of different geometrical parameters upon dispersion towards achieving ultra-negative dispersion. We have shown that with an increase of bigger air-hole diameter, the peak dispersion is red-shifted with higher negative dispersion at the cost of narrower FWHM while an increase of smaller air-hole diameter in the outer core again red-shifted the coupling wavelength but with smaller values of negative dispersion. Changing hole-to-hole distance has the effect of red-shifting the coupling wavelength with smaller values of negative dispersion. Based upon the above findings we could achieve an ultra-negative dispersion of −47,000 ps/nm/km around 1550 nm of wavelength by properly changing the parameters. Our designed fiber will be very useful for dispersion compensation in long-haul data transmission some thousand times more than the available DCFs. The basic principle of power transform from inner core to the outer core after the coupling can be applied for suppressing spontaneous emission after a particular wavelength.

The authors would like to declare that there is no direct financial relation with any commercial identity mentioned in their work that might lead to a conflict of interests.

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. The authors acknowledge sincerely the Defence Research and Development Organization, Government of India, and CRF of IIT Kharagpur for the financial support to carry out this research.