Analysis of Micro and Nanostructured Photonic Crystal Fibers Dihan Md. Nuruddin Hasan, Md. Nazmul Hossain and K. M. Mohsin Supervisor: Dr. Md. Shah Alam Department of Electrical and Electronic Engineering Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh.

1. Introduction With the advent of the photonic crystal a new concept in fiber optics called photonic crystal fiber (PCF) has come to forefront in fiber research. Usually PCF are all pure silica fibers with a regular array of air-holes running along the length of the fiber acting as the cladding. A “defect” in the periodical structure acts as a core. PCFs with a microstructure of air holes around a solid central core, guide light through modified total internal reflection. Because of their new structures, PCFs have several properties that conventional fibers cannot provide, for example: endless single-mode, ultra-flattened dispersion, and super-continuum generation. Moreover, tailoring the air hole arrangement can yield easy manipulation of the dispersion curve, very high nonlinearity with utmost confinement, tailoring the ZDW towards visible region and many other exotic auspicious properties.

2. Background In this work we have investigated various interesting linear and nonlinear features of several variants of PCFs. In this work, a modified design of Equiangular Spiral PCF (ES-PCF) [1-2] is proposed to tailor the ZDW in the visible region with very low air filling fraction (AFF) helpful for fabrication. Besides, a new parameter, suspension factor, is introduced to facilitate the study of high ESnonlinearity of the emerging Suspended SCF PCF Core PCF (SCF). Moreover, variant NW characteristics of nanowire has also been studied. Several applications like THESIS supecontinuum generation, LC tuning of group velocity dispersion and optical sensing have been considered with utmost preference for justifying the significance of the outcomes of the work.

ESPCF Silica+As2Se3 Visible+MidIR

Modified ESPCF Silica Visible

SCF As2Se3 Mid-IR

Fig. 2. Cross-section of SCF (a) cross-section with maximum suspended condition. (b) at different suspended condition.

4. Results and Discussion 3. Designing of the Devices The proposed fiber is composed of eight spiral arms of air holes in a silica background. Each spiral arm contains three air holes and hence three circular rings each containing eight air holes are formed. The number of arms determines the number of air holes in each circular ring whereas the number of holes in each arm attributes the number of ring. Fig. 1 shows the cross sectional view of the proposed ES-PCF designed in this work.

r2 2rh

Silica Air

It can be seen that the maximum and minimum values of GVD curve shifts toward lower wavelengths as SF increases.

5. Conclusion The proposed ES-PCF shows improvement over the HPCF by tailoring the both ZDWs in the visible region with a very low AFF (43% only). Again, the simulation for the structure shows ultrahigh nonlinearity (1441W−1km−1), and very low confinement loss (<3%). Besides, a geometrical design parameter, SF, has been proposed to control the suspension of the core of the SCF. Tailoring of various parameters of the SCF including dispersion, nonlinearity etc can be achieved with this geometrical parameter towards different applications based on SCF.

Future Prospects:

r1

Fig. 3 Dispersion and nonlinearity versus wavelength as a function of core radius of the proposed ES-PCF.

r0 r0=rc+rh r1=r0+0.8(2rh) r2=r1+0.8(2rh)

a)

A full-vector FEM has been used to characterize the PCF design. Fig. 3 shows the GVD and nonlinearity versus wavelength as a function of the radius of the core of the ES-PCF and compares the feasibility of the proposed ES-PCF over conventional hexagonal PCF (HPCF) considering the AFF.

Fig. 4. GVD versus wavelen gth as a function of suspens ion factor.

b)

Fig. 1 a) Cross section of the ES-PCF (AFF=43%), b) Comparable HPCF (AFF=97%).

The proposed SCF with 8 air holes is shown in Fig. 2. In the analysis, each hole in the structure is formed with the combination of a circular hole and a second degree Bézier curve. Different suspended condition for the core can be obtained by manipulating the control points of Bézier curve. A new parameter Suspension Factor (SF) has been introduced in this regard [3].

In HPCF, tailoring both first and second ZDWs around the visible region will only be possible for high value of AFF (above 90%), whereas the proposed ES-PCF requires only 43% of AFF to do that.

HPCF

1. Supercontinuum Generation using ES-PCF. 2. Application Specific Sprial PCF Design. 3. Using SCF as a Gas Sensor.

ES-PCF

Fig. 4 shows how the proposed SF parameter controls the desired profile of the GVD along with controlling the ZDWs of the designed SCF for super continuum generation.

6. References [1] A. Agrawal, N. Kejalakshmy, J. Chen, B. M. A. Rahman, and K. T. V. Grattan, “Soft glass equiangular spiral photonic crystal fiber for supercontinuum generation,” IEEE Photon. Technol. Lett., vol. 21, no. 22, pp. 1722–1724, Nov. 2009. [2] M. N. Hossain, M. S. Alam, Dihan M. N. Hasan, and K. M. Mohsin, “A Highly Nonlinear Spiral Photonic Crystal Fiber for Tailoring two Zero Dispersion Wavelengths in the Visible Region”, Photonics Letter of Poland (PLP), vol. 2, No. 3, pp. 143-145, Sept. 2010. [3] K. M. Mohsin, D. M. N. Hasan, M. N. Hossain, and M. S. Alam, “Suspended Core Fiber Characterization with a New Geometrical Parameter,” Applied Optics, OSA, submitted for publication.