Silicon waveguide based splitter

H. Zhang, J. Zhang, S. Chen, L. Ding, T.-Y. Liow, M. Yu, and G.-Q. Lo Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research)

11 Science Park Road, Singapore Science Park II, Singapore 117685.

Abstract- A silicon waveguide based splitter is a key device for polarization diversity circuit. A bilayer waveguide structure was proposed by MIT to split the input light into its orthogonal components. It is challenging to fabricate a bilayer silicon waveguide by the conventional two-step-etching process due to precise alignment and accurate etch thickness required. In addition, the partial etched waveguide would introduce additional insertion loss due to its non-perfect surface. Here we propose a novel method to fabricate splitters with various dimensions by using selective silicon epitaxial growth to achieve bilayer structure. The performance of the splitter fabricated is evaluated through Finite-difference-time-domain simulations. The insertion loss and efficiency of the splitters obtained is experimentally investigated. The proposed approach is potentially useful for integrated photonic circuit consisting of passive waveguides and active photodetector devices.

Figure 1 Schematic of the polarization splitter: a) the device design in 3D configuration. The left waveguide is the input waveguide, which adopts a horizontal rectangular cross section. b) Top view of the device. c) Cross section view at dotted lines aa, bb and cc in a), respectively.

I. INTRODUCTION

Polarization control and manipulation are crucial to the design and the operation of optical devices. In general, the polarization sensitivity could cause a polarization-dependent wavelength shift and phase shift in optical devices, such as spectrometers, Mach–Zehnder interferometers, and ring resonators, which are unfavorable and hence is preferred to be excluded. Great effort has been spent on reducing the polarization sensitivity and eventually achieves polarization transparent optical circuit.[1]-[3] A polarization splitter hence is a key element in the polarization diversity circuit, which aims to separate the two orthogonally polarized TE and TM components and process them individually. Different configurations of polarization splitters such as the grating coupler [4], slotted waveguides [5], directional coupler [6][7],and arrayed waveguide grating (AWG) [8] have been reported. An MIT group demonstrated a polarization splitter and rotator with an asymmetric core cross section for SiN waveguides [2][9].

Here we report the fabrication and experimental results for a passive polarization splitter based on the configuration proposed by MIT group [9] using a silicon-on-insulator (SOI) platform. More importantly, we propose a non-destructive approach by selective silicon epitaxial growth to realize the bi-height waveguide. The method eliminates the process of partial etch on waveguide, which may introduce additional process complexity and insertion loss due to etched top surface.

II. DESIGN OF SILICON-WAVEGUIDE-BASED SPITTER

A. Design of the polarization splitter The Si-based polarization splitter is designed based on

the configuration proposed by Watts et al. [9]. The schematic of the device is illustrated in Fig. 1, which consists of two waveguides: one horizontal and one vertical waveguide, having rectangular cross sections of width (w) by height (h) and w/2 by 2h, respectively. As shown in Fig. 1a, the horizontal waveguide is the input waveguide. The vertical waveguide, transitioned from a finite tip size (tip) to a width of w/2, is separated with the input waveguide by a gap (distance, g). The coupling length between the horizontal and vertical waveguides is denoted as L. It is expected that the fundamental TE mode of the combined structure remains in the horizontal waveguide while the fundamental TM mode transitions from the input guide to the vertical waveguide.

B. Simulation result The performance of the polarization splitter using

silicon waveguide under the two different launching conditions was studied using three-dimensional finite-difference-time-domain (FDTD) simulation. The simulation was performed with the center wavelength at 1550 nm. The grid size was 20 nm in x-axis and y-axis, and 40 nm in z-axis. The structural dimensions for single mode conditions were chosen as follows: h = 200 nm, g = 150 or 200 nm, tip= 50 nm, L = 10 �m and w = 400 nm. The refractive index of Si waveguide and the cladding material SiO2 are 3.5 and 1.45, respectively.

To simulate the polarization effect, both TM and TE modes were launched at the input plane z = 0 �m and outputs at the end of the two waveguides were monitored. Fig. 2 shows the TM and TE waves along a 10 �m polarization splitter. Ex is the electrical field in x-axis and Ey is the

Figure 3 schematic of process flow to fabricate the polarization splitter.

electrical field in y-axis. In Fig. 2a, only Ey corresponding to TM mode was launched at the input. It is observed that the wave is quickly coupled into the vertical waveguide in less than 5 �m transition region. In the contrast, when TE mode was launched at the input, the wave propagates along the horizontal waveguides only. Hence, the polarization at the input is completely splitted.

III. FABRICATION AND EXPERIMENTAL RESULTS

Conventionally, to achieve a configuration having waveguides with two height dimension in a SOI platform, a two-step etch is used. [3] Zhang et al have fabricated a Si-based rotator by partial etching of Si waveguide to obtain the bi-height waveguide configuration. The etching process may introduce defects into Si waveguide due to plasma striking on Si surface, potentially leading to additional insertion loss.

Here we propose a novel method to achieve the designed configuration as the process flow illustrated in Fig. 3. We started with a SOI wafer with active layer thickness of 200 nm (Fig. 3a). Silicon oxide layer of 200 nm thickness was deposited by plasma-enhanced chemical vapor deposition system (PECVD) on the wafer and patterned to open the active window for later Si growth (Fig. 3b). The oxide layer was firstly partially etched (~180 nm) by reactive ion etching (RIE) and then wet etched (remaining 20 nm) by diluted hydrofluoric acid (DHF) dipping to ensure the silicon surface untouched by plasma. The perfect silicon surface is critical for silicon epitaxial growth. Selective epitaxial growth of Si was performed in an ultrahigh vacuum chemical vapor deposition (UHCVD) epitaxy reactor (Fig. 3c). It should be noted that there is a 54.7 o angle between the

sidewall and the bottom surface due to Si (111) facet. After stripping of the oxide layer in DHF, another layer of silicon oxide having 80 nm thickness was deposited as hard mask by PECVD (Fig. 3d). The splitter was then patterned and etched by RIE (Fig. 3e-f).

Figure 2 Simulation results of the polarization splitter: a) light with TM polarization is coupled to the vertical waveguide at the right; b) light with TE polarization propagates along the horizontal waveguide at the left and is not coupled to the vertical waveguide.

Preliminary results are shown in Fig. 4. Fig. 4a is the scanning electron microscopy (SEM) image of the polarization splitter in top view. The window containing the vertical waveguide was the opening for Si epitaxial growth. Fig. 4b is the side view of the device to illustrate the height profiles of the horizontal and vertical waveguides.

The devices fabricated were tested on a standard integrated optics setup with a broadband amplified spontaneous emission (ASE) source with wavelength range from 1530 to 1570 nm. In order to improve light coupling to the waveguides, a tapered and lensed fiber with beam spot diameter of 2 �m was used. TE and TM measurements were done separately. The input light polarization state is controlled by a fiber polarization controller. The output power after the polarizer was about -1dBm. At the output, another polarizer was connected in the setup to confirm the light polarization status. At first, light in TE mode was coupled to Port In as indicated in Fig. 2. The output at Port 1 and 2 were measured to be -3.5 dB and -22.7 dB,

Figure 4 SEM images of the polarization splitter. a) Top view. b) Side view.

respectively. The result shows that majority of TE propagated along the horizontal waveguide with a weak cross talk about -19 dB. TM mode was then launched at Port In and the output at Port 1 and 2 were measured to be -10.6 and -10.1 dB, respectively. Part of TM input was coupled into the vertical waveguide; however, a noticeable portion still remained in the horizontal waveguide. The non-ideal performance may result from the weak coupling due to the gap dimension of the splitter. The designed gap between the horizontal and vertical waveguide is 200 nm. The fabrication variation may lead to dimension non-ideality and hence degrade the device function. Another possible reason is the much larger tip size than designed 50 nm tip at the vertical waveguide, which may significantly affect the splitter performance. Further optimization of the device design and fabrication process is currently ongoing.

IV. CONCLUSION

In conclusion, we have presented the Si-based waveguide splitter. The simulation results have been obtained to demonstrate the device function. Importantly, we have proposed a novel method to fabricate the device by selective silicon epitaxial growth. Preliminary results have been obtained and shown in the device fabrication and characterization. More efforts will be spent on process optimization to improve the device performance.

REFERENCES

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