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All issues Volume 80 / No 3 (December 2017) Eur. Phys. J. Appl. Phys., 80 3 (2017) 30501 Full HTML
Issue
Eur. Phys. J. Appl. Phys.
Volume 80, Number 3, December 2017
Article Number 30501
Number of page(s) 5
Section Photonics
DOI https://doi.org/10.1051/epjap/2017170176
Published online 29 November 2017

© EDP Sciences, 2017

1 Introduction

The asymmetrical optical directional coupler on silicon-on-insulator (SOI) is one of the key fundamental elements for application in optical switches, power splitters, polarization selectors, micro ring resonators [1–16], optical modulators, multiplexers [5,11], logic gates [17], quantum vortex states [18], optical switches [19] and optical filters [20]. SOI based slab structured directional couplers have a low propagation loss and high coupling, extinction ratios compatibility with optical fibers [21–25]. Therefore, SOI based optical directional couplers are anticipated to be exploited in various low-losses guided integrated devices for optical sensing phenomenon. Optical directional couplers on SOI platform are highly polarization dependant. So, these couplers are usually practice to design devices to operate in both polarization modes such as TE and TM modes.

In this paper, we have shown how the asymmetrical directional coupler coupling and extinction ratios could improve the polarization dependent modes with coupling parameters such as width, wavelength [26,27], waveguide spacing, and coupling length. In this article, we propose a design of asymmetrical directional coupler on SOI platform by using an R-Soft CAD tool and simulated using beam propagation method at 1550 nm wavelength.

2 Design of asymmetrical optical directional coupler

The schematic diagram of the proposed asymmetrical optical directional coupler is shown in Figure 1. It consists of two waveguides that are straight and S-bend waveguides on SOI substrate with slab structure. S-bend waveguides are separated by 20 µm at the output ends. Total length is 3 cm, input power is 1 µW, width is 3 µm, height of the device is 5 µm, the coupling length of the waveguide is 4000 µm, free space wavelength is 1.55 µm, and waveguide separation is 2.8 µm, under TE and TM polarization modes, which are basic parameters of the directional coupler.

thumbnail Fig. 1

Schematic diagram of asymmetrical optical directional coupler (g = waveguide spacing, Lc = coupling length).

3 BPM simulation results

The beam propagation method (BPM) is the most powerful method to investigate linear and nonlinear lightwave propagation phenomena in axially varying waveguides. Recently, BPM is the most widely used propagation technique for modeling integrated and fiber optic photonic devices. The BPM is essentially a particular approach for approximating the exact wave equation for monochromatic waves, and solving the resulting equations numerically. The basic approach is illustrated by formulating the problem under the restrictions of a scalar field (i.e. neglecting polarization effects) and paraxiality (i.e. propagation restricted to a narrow range of angles). The scalar field assumption allows the wave equation to be written in the form of the well-known Helmholtz equation for monochromatic waves. (1)

Here the scalar electric field has been written as E(x,  y,  z,  t) = Φ(x,  y,  z)e-iwt and the notation k(x, y, z, t) = kon(x, y, z) has been introduced for the spatially dependent wave number, with ko = 2π/λ being the wavenumber in free space. The geometry of the problem is defined entirely by the refractive index distribution n(x, y, z). Assuming that axis is predominantly along the z-direction, it is beneficial to factor the rapid phase variation out of the problem by introducing a so-called slowly varying field u along the direction z (2) is a constant number to be chosen to represent the average phase variation of the field Φ. Then, introducing the expression into the Helmholtz equation yields the following equation for the slowly varying field (3) (4)

This is the basic BPM equation in three dimensions (3D), simplification to two dimensions (2D) is obtained by omitting any dependence on y [28].

Simulation has been carried out by BPM tool by considering the propagation of an optical signal of fundamental TE and TM modes through the asymmetrical optical directional coupler. Here we depicted some of the simulation profiles in Figure 2. Figure 2(a) shows the launching field of the optical directional coupler with slice approach. Figure 2(b) shows the light propagation direction of the optical directional coupler, from this profile we can calculate the transmitted power. Figure 2(c) shows the mode profile of the optical directional coupler. Figure 2(d) shows the propagation of amplitude view throughout the optical directional coupler, from this profile we can confirm the light propagate through other port. Figure 2(e) shows the 3D view of simulation of asymmetrical optical directional coupler with 5 µm width, 1550 nm wavelength, 3 cm length, 10 000 coupling length, and 20 µm gap between the two output ports.

Simulation results enable the extinction ratio (ER) and coupling ratio (R) which is used to calculate these values by using the following formulae. (5) (6) (7) where ER is the extinction ratio (dB) of an optical directional coupler, is defined as the ratio between the transmitted powers of two polarizations at the same output port. PTM is the transmitted power at TM-mode, PTE is the transmitted power at TE-mode [29]. Simulations were carried out for different in put modes and rest of varying parameters were kept as constant for asymmetrical optical directional coupler.

The coupling parameter (w) takes part in an imperative task in asymmetrical optical directional coupler for changes the coupling ratio, extinction ratio, and insertion loss. Figure 3 shows the coupling and extinction ratios with component width of the asymmetrical directional coupler for TE polarization mode. The coupling ratio is decreased with width, which has got more than 96% at 6 µm width of the component at port-II in TE and TM modes because of the higher widths enable the good confinement of the light into the cross-section of the device due to the coupling coefficients between the waveguides is varied with core width of the directional coupler [30].

The extinction ratio is increased with width, which has got maximum that is 1.79 dB at 8 µm width of the asymmetrical directional coupler because of the coupling efficiency of the polarization decreases with width due to effective index and coupling efficiency and phase matching condition is sensitive to the width of the waveguide and the insertion loss was calculated with width of the asymmetrical optical directional coupler for two different modes. The minimum insertion loss was achieved 0.33 dB at 6 µm for both TE and TM modes at 1550 nm wavelength through port-II. Polarization changes in asymmetrical optical directional coupler depend on the component width of the waveguide, height of the waveguide and ratio of slab height to the height of cross-section of the device.

The coupling parameter (λ) takes part in an imperative role in asymmetrical optical directional coupler for changes the coupling ratio, extinction ratio, and insertion loss. Figure 4 shows the coupling and extinction ratios with wavelength of the asymmetrical optical directional coupler for TE polarization mode. The coupling ratio is decreased with wavelength, which has got highest value is 93% at 1.55 µm wavelength at port-II in TE and TM modes because of directional coupler is strongly polarization dependent with central wavelength in photonic spectrum band (1500 nm < λ < 1700 nm).

The extinction ratio is increased with wavelength, which has got maximum that is 0.7 dB at 1700 nm wavelength of the directional coupler because of equal distribution of transmitted power on the both output ports, and the interaction length is takes place important role to obtain extinction ratio due to coupling coefficients in two waveguides are slightly different and also calculated insertion loss with wavelength of the symmetrical optical directional coupler for two different modes. The minimum insertion loss achieved 0.3 dB for both TE and TM modes at 1550 nm wavelength through port-II. The insertion loss of the asymmetrical directional coupler is between 2 and 12 dB in the wavelength range 1500–1700 nm.

The coupling parameter (g) takes part in a key role in asymmetrical optical directional coupler for changes the coupling ratio, extinction ratio, and insertion loss. Figure 5 shows the coupling and extinction ratios with waveguide spacing of the directional coupler for TE polarization mode.

The coupling ratio is decreased with waveguide spacing, which has got highest value is 98% at 2 µm waveguide spacing at port-II in TE and TM modes because of its lesser distance between the separation of waveguides due to the effective index increases as the gap reduces. When separation is smaller there will be more proportional of optical field and effective index is higher because of this coupling ratio is high [31]. The extinction ratio is also decreased with waveguide spacing, which has got minimum value that is almost 0 dB at 3.6 µm waveguide spacing of the directional coupler because of equal distribution of propagation of light on the both output ports, and also the insertion loss was calculated with waveguide spacings 2, 2.8, 3.6, and 4.4 µm of the asymmetrical optical directional coupler for two different modes. The insertion losses were achieved 0.09, 0.3, 27 and 0.2 dB for TE and 0.08, 0.3, 27 and 0.2 dB TM modes through port-II.

In optical directional coupler, coupling phenomenon takes place in the coupling region in which the odd and even normal modes can propagate with propagation constants βe and βo. The coupling length depends upon the coupling coefficient and propagation constant. The coupling length of either TE or TM polarization is given by [32] (8)

The coupling parameter Lc takes part in a key role in asymmetrical optical directional coupler for changes the coupling ratio, extinction ratio, and insertion loss. Figure 6 shows the coupling and extinction ratios with coupling length of the asymmetrical directional coupler for TE polarization mode. The coupling ratio is decreased and increased with coupling length, which has got highest value is 99% at 2 mm coupling length with 2.8 µm waveguide separation at port-II in TE and TM modes because of the coupled mode theory. The extinction ratio is increased with coupling length, which has got maximum that is 6 dB at 10 mm coupling length of the directional coupler because of the coupling efficiency is increased with decreasing of coupling ratio, and also calculated the insertion losses with coupling length of the asymmetrical optical directional coupler for two different modes. The insertion losses were achieved for 2, 4, 6, 8, and 10 mm coupling lengths 0.08, 2, 16, 5 and 0.3 dB for TE mode and 0.08, 2.1, 17, 5.1, 0.31 dB for TM modes through port-II at 2.8 µm waveguide spacing.

thumbnail Fig. 2

Simulated profiles for asymmetrical optical directional coupler (a) launching profile with slice structure type, (b) propagation field view, (c) mode field view, (d) amplitude view, (e) power launching field view in 3D.
thumbnail Fig. 3

Waveguide width as a function of coupling ratio and extinction ratio for TE mode.
thumbnail Fig. 4

Wavelength as function of coupling ratio and extinction ratio for TE mode.
thumbnail Fig. 5

Waveguide spacing as function of coupling and extinction ratio for TE mode.
thumbnail Fig. 6

Coupling length as function of coupling and extinction ratio for TE mode.

4 Conclusions

In summary, we proposed an asymmetrical optical directional coupler exhibiting switching phenomena for changing waveguide spacing between two waveguides of the coupler. We found from our BPM simulation results the coupling ratio can be found with waveguide parameters such as width, wavelength, waveguide spacing, and coupling length, extinction ratio can be found with waveguide parameter such as width, C, L, U-band regions wavelength, waveguide spacing, and coupling length, and insertion loss also can be found with width, waveguide spacing, and coupling length, wavelength, The minimum insertion loss reached 0.02 dB at 8 µm component width for TE mode, 0.08 dB for TM mode, 0.35 dB at 1550 nm wavelength for TE mode polarization, 0.31 dB for TM mode, 0.09 dB at 2 µm waveguide spacing for TE mode 0.08 dB for TM mode, 0.008 dB at 2 mm coupling length for TE and TM polarization modes. The device shows high extinction ratio, high coupling ratio, and low insertion loss on SOI platform. These asymmetrical optical directional couplers can be developed as key components for integrated optical communication devices and useful for build up functional devices, optical clock distribution or arranged as I/O ports on SOI ULSI devices. This work supplies a capable solution for all optical directional couplers on a SOI chip.

Acknowledgments

This work was supported by the UGC-BSR-New Delhi India fund for Ph.D. program.

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Cite this article as: Nagaraju Pendam, Chunduru Parvatha Vardhani, Enhancement of coupling ratios in SOI based asymmetrical optical directional couplers, Eur. Phys. J. Appl. Phys. 80, 30501 (2017)

All Figures

thumbnail Fig. 1

Schematic diagram of asymmetrical optical directional coupler (g = waveguide spacing, Lc = coupling length).
In the text
thumbnail Fig. 2

Simulated profiles for asymmetrical optical directional coupler (a) launching profile with slice structure type, (b) propagation field view, (c) mode field view, (d) amplitude view, (e) power launching field view in 3D.
In the text
thumbnail Fig. 3

Waveguide width as a function of coupling ratio and extinction ratio for TE mode.
In the text
thumbnail Fig. 4

Wavelength as function of coupling ratio and extinction ratio for TE mode.
In the text
thumbnail Fig. 5

Waveguide spacing as function of coupling and extinction ratio for TE mode.
In the text
thumbnail Fig. 6

Coupling length as function of coupling and extinction ratio for TE mode.
In the text

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