1. Introduction

In the integrated photonic system, optical power splitters are important components for splitting and combining the optical signals, which have been attracted much attention throughout the world. There is a strongly increasing demand for these components in the field of integrated optics. The characteristics with low loss, low cost, compactness, wavelength and polarization independence are highly desired. Up to now, there are various reports about optical power splitters, which are generally based on Y-branch [1,2], multi-mode interference [3], photonic crystal [4,5], and subwavelength plasmonic waveguide [6]. In the latter three schemes, however, the optical power splitters commonly suffer from the problem of strong dependence on wavelength and polarization. In addition, the fabrication for the devices in the latter two schemes is a challenging task, because the optical branching ratio is highly sensitive to its structure parameters.

The optical power splitter by using conventional Y-branch usually suffers from excess loss, due to the wave-front mismatch between the input and output branches, which requires that the branching angle should be less than 1° [2,7]. Whereas, this will lead to rapid increase in length of the device, which is seriously limited in the high-density optical integrated circuits. To compensate for such a mismatch, several approaches have been proposed, by using wave-front accelerator [8], or by modifying geometry of the Y-branch structure [1]. However, these will result in higher fabrication cost, and also make fabrication much complicated. Recently, P. Nordin et. el. proposed the air-trench based splitter with asymmetric optical power output, but its optical loss is relatively high, and the vertical trench with high aspect ratio and high precision is very difficult to be achieved in the fabrication process [9,10]. As a result, it is important to design and fabricate novel 1 × N optical power splitter having characteristics such as low loss, low cost, compactness, wavelength and polarization independence.

To address these problems mentioned above, the novel optical power splitter has been proposed in this paper, in which asymmetric Y-branch waveguides based on total internal reflection have been introduced. We believe that the novel component has important potential application in the integrated photonic system. The organization of this work is as follows. The structure of 1 × N optical power splitter is designed in the section 2. The simulation and discussion is given in the section 3. The 1 × N optical power splitter is fabricated and measured in the section 4.

2. Design for the structure

Generally speaking, the waveguide with low index contrast between its core and cladding has a larger guiding width in the single-mode operation, while compared with high-index-contrast one. It has advantages such as ease for fabrication, low fiber coupling loss, and low propagation loss [10]. In this paper, the optical power splitter is assumed to be operated in the single-mode state, in which low-index-contrast waveguide materials are chosen.

The proposed 1 × N optical power splitter is composed of N-1 asymmetric Y-branch waveguide structures, which has one input port and N output ports. For simplicity, Fig. 1 shows the schematic of the 1 × 4 optical power splitter as an example, which is used for illustration. While laser source is launched into the input port through a single-mode optical fiber, the optical power can be distributed to its respective output ports, to achieve the desired optical power splitting, which is determined by specific optical branching ratio of N-1 asymmetric Y-branch waveguides. According to its desired optical power splitting, each branching ratio of asymmetric Y-branch waveguide can be easily calculated, in which the optical loss at branching point of asymmetric Y-branch waveguide should be taken into account. As shown in Fig. 1, we assume the normalized transmitted power are $T_1$, $T_2$ and $T_3$, respectively, for the first, second and third Y-branch. The desired normalized optical power splitting are ${\eta }_1$, ${\eta }_2$, ${\eta }_3$ and ${\eta }_4$, respectively, for the output ports 1, 2, 3 and 4, where ${\eta }_1+{\eta }_2+{\eta }_3+{\eta }_4=1$. The branching ratio between left and right branch arm in the first, second and third Y-branch can be easily calculated, respectively, as written below,

 

Fig. 1 Proposed structure of optical power splitter.

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Afterward, we can design structure parameters of each asymmetric Y-branch, according to its corresponding specific branching ratio presented above.

The structure of asymmetric Y-branch waveguide is depicted in Fig. 2(a) , and Fig. 2(b) shows the cross-section view of the waveguide, which is a rib-type structure. The refractive index are $n_1$, $n_2$ and $n_3$, respectively, for the upper cladding, the core and the lower cladding. The slab height, rib height and width of the core are d, h and w, respectively. For rib-type waveguide, however, design for asymmetric Y-branch waveguide with specific branching ratio is complicated, and numerical simulation is time-consuming. For this reason, the effective index method is introduced to convert the 3-D waveguide into the 2-D one. Its effective refractive index in the core region is $N_2$, and effective refractive index in the cladding region are $N_1$ for left side and $N_3$ for right side, respectively, as shown in Fig. 2(c), whose value is determined by slab height and rib height. It is noted that $N_1$ is equal to $N_3$ in our proposed structure. The theory of effective index method will not be given here, and please refer to the related literature [11].

 

Fig. 2 (a) structure of asymmetric Y-branch waveguide, (b) cross-section view of waveguide structure, (c) their corresponding effective refractive index in different region.

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In Fig. 2 (a), the asymmetric Y-branch waveguide can be simply divided into four regions. The waveguide AB and DE act as input and output part of the device, respectively. The waveguide BC is a tapered waveguide, which acts as a pre-splitter, transforming the single-mode pattern into a super mode one. For waveguide CD, the total internal reflection occurs at the inner interface between its core and cladding. In this structure, relatively large branching angle can be achieved, and its branching ratio can be adjusted by changing the offset distance $\Delta x$ of the branching point. The offset distance $\Delta x$ is equal to horizontal distance between the branch point and the center axial line of the tapered waveguide, as depicted in Fig. 2 (a). The branching angle θ can be expressed as follows,

3. Numerical simulation

Nowadays, the organic materials have been widely used to fabricate optical waveguide devices. Obviously, the polymer materials have many advantages such as their low cost, ease of fabrication, compatibility with silicon fabrication technology and unique optical characteristics, while compared with inorganic materials [12]. In this section, the polymer materials chosen as the core and cladding of waveguide are SU-8 and UV-15, respectively, whose refractive index are equal to 1.57 and 1.50 at operation wavelength of 1.55μm, respectively. The parameters of waveguide structure are as follows. The slab height, rib height and width of the core are 1.0 μm, 0.8 μm and 5 μm, respectively. The length of the tapered waveguides BC and CD are 200 μm and 286 μm, respectively, and its branching angle is 1°. Its mode field diameters at the 1/e points of amplitude value are about 5.3 μm along x axis, and about 2.8 μm along y axis. By using effective index method, the 3-D waveguide structure be converted into 2-D waveguide structure, and the effective refractive index in the core and cladding region are 1.5475 and 1.5302, respectively. Through numerical computation, this waveguide structure is operated in single-mode state.

Here, the optical performance of the asymmetric Y-branch waveguide is simulated, by using the finite-difference beam propagation method (FD-BPM) [13]. FD-BPM is an effective and widely used technique for the study of optical waveguide devices, which is based on a numerical solution of the scalar Helmholtz equation. The incident optical field is assumed as TM polarization state. The normalized output of optical power varies with horizontal offset distance $\Delta x$, as shown in Fig. 3 . The curve with sign ■ and ▲ denote the normalized optical power output of the right and left branch, respectively, which is equal to the ratio between optical power at output port and at input port. As seen from the Fig. 3, the normalized optical power output of right branch is monotonically decreasing, while the branch point is shifted from the left to the right. For the left branch, however, its normalized optical power output is monotonically increasing. As a result, this change tendency is very useful for design and fabrication of the desired branching ratio of optical power splitter. It should be noted, however, that optical power output of right branch is slightly higher than that of left branch in asymmetric Y-branch waveguide, while offset distance $\Delta x$ is equal to 0, which results from that the mode field mismatch at the interface between input and two output branches is different. In addition, while the incident optical field is in TE polarization state, their normalized optical power output at the different horizontal offset distance is approximately same as ones given in Fig. 3, which indicates that the proposed Y-branch waveguide has low polarization dependence, due to that its designed structure is based on the total internal reflection. The characteristics are useful for its application in integrated photonic system.

 

Fig. 3 Normalized optical power output of branch waveguide with different $\Delta x$.

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For the simulation results shown above, the incident optical wave is assumed as monochromatic light, but in fact the laser source always has spectrum width about several to several decade nanometer. Here, the simulated results under different incident wavelength condition are given in Fig. 4 , in which the offset distance $\Delta x$ is equal to 1.2μm and the other parameters are same with those in Fig. 3. It shows that there is little change for the optical power output of asymmetric Y-branch waveguide at large spectrum scope of 100nm. Seen from the Fig. 4, the novel device has low dependence of light wavelength, which is strongly desired in integrated photonic device. In addition, it need to be pointed out that the asymmetric Y-branch waveguide structure is simple, which has relatively large fabrication process tolerance, and therefore is easy to be fabricated. For fabrication error tolerance, its detail analysis will not be given here, and please refer to the related literature [14].

 

Fig. 4 Normalized optical power output of branch waveguide, under different operation wavelength condition.

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As a example, the simulated optical performance of 1 × 4 optical power splitter with equivalent optical power output is given here. It consists of three asymmetric Y-branch waveguides, which also has same advantages with asymmetric Y-branch waveguide. The normalized optical power splitting at each output port should be equal to 25% in 1 × 4 optical power splitter, i.e. ${\eta }_1={\eta }_2={\eta }_3={\eta }_4=25\%$. According to simulated results of asymmetric Y-branch waveguide, the normalized transmitted power is about 98%, and the optical radiation loss is very low, which is about 2%, mainly resulting from mode field mismatch at the branching point. Through numerical computation, optical branching ratio between left and right branch arm are 0.32:1, 0.49:1 and 1:1, respectively, for the first, second and third asymmetric Y-branch. The offset distance in each asymmetric Y-branch can be easily obtained, according to the simulated results in Fig. 3. The optical field propagation in the 1 × 4 optical power splitter is given in Fig. 5 (a) , under TM polarization condition, which shows that the equivalent optical power splitting can be well achieved. While the incident light wave is in TE polarization state, the optical power output is basically invariant, and its optical field propagation is shown in Fig. 5(b), which indicates that its optical performance is not sensitive to polarization state. Similarly, this proposed scheme can be used for realizing other 1 × N optical power splitters with equal or unequal optical power output (for instance, N = 2, 3, 5, 6, ^…). Here, the corresponding simulation results will not be given in this paper.

 

Fig. 5 The simulated results for optical field propagation in the 1 × 4 optical power splitter, (a) TM polarization state, (a) TE polarization state.

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4. Fabrication and measurement

In order to carry out the principle-proof experiment for the 1 × 4 optical power splitter, we have fabricated the optical power splitter on the 2-inch silicon substrate, by using photolithography process. It fabrication process is as follows. The polymer UV-15 as the lower cladding was first spin coated at the spinning speed of 4500 rpm for 50 s, and then cured by the ultraviolet source and baked in oven at 80°C overnight. Afterward, the SU-8 as the core was spin coated at the spinning speed of 5000 rpm for 30 s, and baked at 95°C for 30 min, and then was patterned, by using optical mask fabricated by the laser direct-writing method. After development and post baking, the core waveguide was obtained. Then, the polymer UV-15 as the upper cladding was next spin coated at the spinning speed of 4500 rpm for 50 s, and then cured by the ultraviolet source and baked in oven at 80°C overnight. Finally, the component has been fabricated, as shown in Fig. 6(a) , in which there are eight 1 × 4 optical power splitters, and total size is 25mm × 23mm. Each 1 × 4 optical power splitter has size of 1mm × 23mm. The Fig. 6(b) shows the close-up of waveguide structure at the branching point, which indicates that its fabrication quality is satisfactory.

 

Fig. 6 (a) fabricated 1 × 4 optical power splitter, (b) the close-up view of waveguide at the branching point.

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In measurement process, the laser source with optical power of 1.30 dBm, whose operation wavelength is 1550nm, was connected with the single-mode tapered fiber, which was used to launch optical light into the input port of optical power splitter. The optical splitter under testing was placed on a high-precision 3-axis automatic stage (From Newport Corporation) for alignment between fiber and waveguides. The optical power outputs are detected, by using the optical power meter, whose measurement range is from –70dBm to 30 dBm. The measured results are given in Table 1 . It can be observed from the Table 1 that the splitter has good uniform power output, in which fluctuation of measured optical power at different output ports is less than 0.7 dB. The experimental results show that the fabricated component has exhibited good performance, which is consistent with the simulation prediction. According to Table 1, the insertion loss of 1 × 4 optical power splitter at output ports 1~4 are 27.42dB, 28.03dB, 28.11dB, 27.62dB, respectively, which includes optical splitting of 6 dB. For the straight waveguide, the insertion loss is about 20.60 dB in our measurement. Hence, the excess loss of 1 × 4 optical power splitter at output ports 1~4 can be obtained, which are 0.82 dB, 1.43 dB, 1.51 dB and 1.02 dB, respectively. Although the optical loss is slightly high in our principle-proof experiment, which mainly results from coupling loss at input and output ports, we hold that the loss could be effectively reduced by improving coupling efficiency between fiber and waveguide at input and output ports.

Table 1. Measured optical power at each output port.

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5. Summary

We have proposed and analyzed the novel 1 × N optical power splitter, in which the asymmetric Y-branch waveguides based on total internal reflection are adopted. Through numerical simulation, the specific desired optical splitting could be obtained. The 1 × 4 optical splitter has been fabricated, by using polymer materials SU-8 and UV-15. The principle-proof experiment has been carried out, in which the measured results of optical power splitting agrees with the simulation prediction. The device has many advantages, which is useful to application in integrated optical system.