Investigation of dispersion and nonlinear characteristics of liquid core optical fiber filled with olive oil

*Corresponding author: aldergazly@gmail.com © The Author 2020. Published by ARDA. Abstract This paper introduces a prospective material for photonic laser applications. Different high purity samples of olive oil were subjected to spectrophotometer to determine the absorption spectra. The transmission behavior of olive oil is about (44.28) % at wavelength of 532 nm, while it is (62.94) % at 1064 nm. Then, the nonlinear optical properties represented by nonlinear refractive index and nonlinear absorption coefficient are determined using a highly sensitive method known as Zscan technique. Z-Scan experiment was performed using a 1064 nm CW Nd:YAG and 532 nm SHG Nd:YVO4 lasers. n2 = 3.99×10 W/cm and β = -0.0017 m/W for 1064nm wavelength and n2 = 2.45×10 W/cm and β =-7.26×10 m/W for 532nm wavelength. This paper gives simulation results for a liquid core optical fiber (LCOF) filled with olive oil. The nonlinear propagation constant and group velocity dispersion (GVD) properties are estimated. The simulation of generation of LCOF is also obtained. The calculations show that LOCF can provide huge nonlinear parameter and large span of slow varying GVD characteristics in the visible and infrared region, which have potential applications in optical communications and nonlinear optics. The material is olive oil which is classified as organic compound, having a good nonlinear optical properties making it a potential candidate to be for photonic applications.


Introduction
Organic materials have been used in nonresonant photonic applications due to their high nonlinear coefficients γ, high damage threshold and good transparency. Olive oil behaves as a nonlinear material where it has a ''blue shift'' in some excited wavelengths [1]. Therefore, the nonlinear optical properties of olive oil which are of prominent importance for photonics applications are investigated in this work. Lyotropic liquid crystals (LLC) are mixtures of amphiphilic molecules and a solvent self-assembled in aggregates at certain condition of temperature and relative concentrations [2]. Besides, LOCF has advantage of easy fabrication and robustness compared with silica nano-wire. The olive oil represents the one class of lyotropic liquid crystal in nematic phase [3]. When applied optical field to the Liquid crystals the behavior of the crystals appear as nonlinear optic crystal materials. The polarized light beam from laser source can induce an alignment or ordering in isotropic phase.
These result in a change in refractive index. In isotropic phase the change in refractive index is due to the density change following a rise in temperature. In the nematic phase the change in the refractive index depend directly on the temperature. The reorientation of molecules in liquid crystals depends on the phase of liquid crystals. For the isotropic phase the liquid crystal molecules are randomly oriented owing to thermal motion. So, an intense laser field will force the anisotropic molecules to align themselves in the direction of optical field through the dipolar interaction.

Fiber losses
One of the important fibers parameters is the measure of power loss during transmission of optical signals inside the fiber. Several factors contribute to the losses of conventional fibers, with material absorption and Rayleigh scattering as a dominating contribution. In photonic crystal fiber (PCFs), there is another source of losses which is called confinement losses. The origin behind this type of losses is that the core material is made from (silica) and therefore it is difficult to confine the light in the core without increasing the number of hole rings in the cladding layer [4]. Consider the simplified case of propagating a monochromatic optical wave in a fiber extended along the z axis. When fiber dispersion, optical nonlinearity, and boundary conditions are neglected, the general form of the electric field for the optical wave in the fiber can be written as [5]: where, Re[ ] stands for the real part of the argument, Ao= Field amplitude at z = 0, α = Fiber optical loss coefficient, t = Time, ω = angular frequency of the optical wave, β is Propagation constant (wave number). The propagation constant β = 2n/, where λ is the optical wavelength. Then Eq. (1) can be written as: where, n is the complex refractive index Thus,  is related to the imaginary part of the complex refractive index The optical power is proportional to the square of the electric field amplitude E.

Fiber chromatic dispersion
Chromatic dispersion is the phenomenon wherein different spectral components of the transmitted laser signal travel at different velocities in the fiber, arriving at different times at the receiver. Depending on system length and bit rate per wavelength, the amount of chromatic dispersion in a system must be carefully managed with dispersion compensators to achieve the right amount of residual dispersion [6].
Chromatic dispersion plays a critical role in propagation of short optical pulses in single-mode fibers because different spectral components associated with the pulse travel at different speeds given by c/n (ω) where c is the speed of light in vacuum. Thus, the origin behind chromatic dispersion is the frequency dependence of the refractive index n(ω). The mode-propagation constant becomes frequency dependent according to [7,8].
Expanding the mode-propagation constant in a Taylor series about the frequency o at which the pulse spectrum is centered [9]: where, The parameters 1 and 2 are related to the refractive index n and its derivatives through the relations … (7) where, ng is the group index and g is the group velocity. Note that the envelope of the optical pulse moves at the group velocity while the parameter 2 represents dispersion of the group velocity and is responsible for pulse broadening.
The group-velocity dispersion (GVD) parameter 2 is related to the dispersion parameter D that is commonly used in the optical communication literature. This quantity is usually specified with units of ps/(nm km) (picoseconds per nanometer wavelength change and kilometer propagation distance) [7,8].

… (8)
The forgoing discussion highlights the concepts behind material dispersion which is due to the frequency dependence of the refractive index of the material of the fiber core. However, because of dielectric wave guiding, the effective mode index is slightly lower than the material index n of the core, the reduction itself being frequency dependent. This results in a waveguide contribution that must be added to the material contribution to obtain the total dispersion. In standard single-mode fibers, the total second-order dispersion D vanishes at λ = 1300nm [6]. Pulse propagation near this wavelength requires inclusion of the third-order dispersion (TOD) parameter 3 or equivalently inclusion of the slope of GVD, … (9) When short optical pulses propagate inside a fiber, both dispersive and nonlinear effects influence their shape and spectrum. The pulse propagation is described by the nonlinear Schrodinger (NLS) Equation [9,10] … (10) where A(t, z) is the slowly varying envelope of the electric field and  is the nonlinear parameter … (11)  is the center frequency of the pulse spectrum, and Aeff is the effective core area.

Olive oil spectrum and refractive index measurements
The refractive index of silica and olive oil vs. wavelength were calculated using Sellmeier Equation [11] as shown in Figure 1.  Figure 1 shows that the refractive index of the olive oil is larger than the refractive index of silica so it is achieved with total internal reflection ncore>ncladding The experimental work is based on testing the olive oil transmission spectrum using UV-VIS spectrophotometer. Figure 2 shows the transmission spectrum of olive oil.

Dispersion measurements
The parameters 2,  and  were determined using Eq. (7) for olive oil related to the refractive index n as show in Figures 3,4 and 5. This results in a waveguide contribution that must be added to the material contribution to obtain the total dispersion.

Nonlinear parameter of olive oil
Using Z-scan techniques [12] to determine the nonlinear refractive index and nonlinear absorption coefficient of the olive oil. The nonlinear refractive index of olive oil n2 versus input powers and wavelengths is determined which are equal n2=3.99×10 -6 W/cm 2 for 1064nm wavelength and n2=2.45×10 -7 W/cm 2 for 532nm wavelength.
Olive oil has a saturation effect for both wavelengths 532nm and 1064nm in CW regime. The nonlinear absorption coefficients are β = -0.0017 m/W for 1064 nm and β = -7.26×10 -4 m/W for 532nm wavelength.
The nonlinear parameter of olive oil will be determined at 532 nm and 1064 nm wavelength for different core diameter using Eq. (11) as shown in Figure 8 and Figure 9.

Conclusions
• In this paper we have obtained the dispersion parameter of olive oil 2,  ,  these parameters can be used for many applications like Four Wave Mixing.
• D and S for olive oil will be comparing with the other materials olive oil show lower dispersion that approaches to zero.