Dual-Mode Silicon Ring Resonator Based on Subwavelength Grating Directional Couplers

We propose and experimentally demonstrate a dual-mode silicon ring resonator which could process TE0 and TE1 modes simultaneously. By introducing subwavelength grating into the coupling region, compact footprint of ∼90 μm × 20 μm is achieved. The proposed device is fabricated on the standard silicon-on-insulator platform. The measured results indicate the mode crosstalk within C band are less than −12.2 and −11.5 dB for TE0 and TE1 modes, respectively. Furthermore, heaters for resonant wavelength tuning are fabricated and measured with good uniformity for both modes.


I. INTRODUCTION
I N THE past decade, silicon photonics is commonly regarded as a promising solution for inter/intra data-center connections due to its advantages as low-cost, small footprint and compatibility with matured Complementary Metal-Oxide Semiconductor (CMOS) technology [1], [2]. To meet the increasing capacity requirement of optical interconnection, multiplexing technologies such as the wavelength division multiplexing (WDM), polarization division multiplexing (PDM) are intensively utilized [3], [4], [5], [6]. Nowadays, mode division multiplexing (MDM) technology attracts more and more attention due to its compatibility with other multiplexing technologies and ability on further increasing the transmission capacity [7]. In order to build a chip-level MDM network, a variety of fundamental building blocks, including mode multiplexer [8], multi-mode crossing [9], coupler [10], [11], [12], [13], have been extensively investigated. Advanced circuits, such as multi-mode switch and router, are further proposed and demonstrated [14], [15].
Due to the severe mode dispersion in silicon multimode waveguides, demultiplexing-process-multiplexing architecture is commonly used for MDM signal processing, resulting in a high-loss and inefficient layout. Recently, a novel multi-mode signal processing architecture attracts more and more attention for its ability on processing MDM signals simultaneously [16], [17]. Thus, MDM signal processing circuits with high performance and small footprint could be achieved. Micro-ring resonator (MRR) as an essential element in silicon integrated circuit, is widely utilized as filter [18], modulator [19] and sensor [20] in single-mode regime. With the development of MDM technology, MRR that could process multi-mode signals is highly desired. However, it is difficult to achieve a multi-mode MRR with compact footprint, due to the severe mode dispersion in the coupling region. Reported MRR [21] used a conventional directional coupler to achieve uniform two-mode coupling with a racetrack length of 95 μm, resulting an inefficient layout. MRR based on free space geometric optics [22] could overcome the mode dispersion and support more modes. However low Q-factor limits its application in practical system.
In this paper, we propose and experimentally demonstrate a dual-mode MRR with compact footprint. By introducing subwavelength grating [23] into the coupling region, length of the racetrack is reduced to 20 μm. The proposed device is fabricated on the standard silicon-on-insulator (SOI) platform. The measured results indicate the mode crosstalk within C band are less than -12.2 and -11.5 dB for TE 0 and TE 1 modes, respectively. Furthermore, heaters for resonant wavelength tuning are measured with good uniformity for both modes.

II. DESIGN AND SIMULATION
The scheme of the proposed device is shown in Fig. 1(a). The device consists of two 180°bends and two coupling regions. TE 0 and TE 1 modes at resonance wavelengths are coupled into the MRR through the input port and coupled out at the drop port. Coupling regions are designed as the SWG waveguides, which could enhance the coupling efficiency to realize compact coupling length. Bend radius is designed as 45 μm to support TE 0 and TE 1 modes transmission with low crosstalk. Thus, uniform process on TE 0 and TE 1 modes could be achieved. Design details of the SWG waveguides and multi-mode bends are introduced in following sections. TE 0 and TE 1 modes, Λ and η need to be carefully designed to meet both the SWG working condition and fabrication rules.

A. Subwavelength Grating Directional Coupler
According to the equivalent dielectric theory [24], SWG waveguide could be regarded as a uniform equivalent dielectric material. The equivalent refractive index n eq is given by n si and n sio 2 are the refractive indices of the silicon and silica.
To meet the SWG working condition [24], the period Λ needs to be designed as λ min stands for minimum operating wavelength. To make the device work within C band, Λ and η are chosen as 280 nm and 0.5, respectively. Coupling gap is chosen as 220 nm. Furthermore, a SWG based taper with width of 600 nm and length of 1.4 μm is introduced to realize adiabatic mode coupling between channel waveguide and SWG waveguide. Fig. 2(a) shows the simulated coupling efficiencies with different coupling length at 1550 nm for TE 0 and TE 1 modes. To achieve uniform light coupling for dual-mode signal process, coupling length is chosen to be 20 μm. Insertion losses of TE 0 and TE 1 modes are simulated to be ∼0.26 dB and 0.4 dB, respectively. While mode crosstalk is simulated as ∼-18 dB for both TE 0 -TE 1 and TE 1 -TE 0 . The simulated light propagating evolution in SWG waveguides are shown in Fig. 2(b) and (c). To be noted, resonance of the mode field is caused by the Bloch-Floquet modes of the SWG structure [25], which could be reduced by selecting a smaller Λ.

B. Bend Waveguide
In order to reduce the mode crosstalk between TE 0 and TE 1 modes, arc waveguides with radius of 45 μm is usually used to form the dual-mode bends. Simulated transmission spectra of the bends are shown in Fig. 3(a), (b) and (c) illustrate the simulated light propagation for TE 0 and TE 1 modes.  In the legend "TE 0 -TE 1 ", the first "0" stands for the input TE 0 mode, while the second "1" stands for the output TE 1 mode. Results indicate insertion loss for TE 0 and TE 1 modes are∼0.01 and 0.05 dB, respectively. While the mode crosstalk between TE 0 and TE 1 modes are<−22.2 dB within the C band.

C. Dual-Mode Ring Resonator
Combining the two key elements discussed above, the dualmode MRR can be formed. In order to characterize the proposed device, the drop port transmission spectra for TE 0 and TE 1 modes are further simulated, as shown in Fig. 4. The simulation results indicate that bandpass filtering for dual-mode signals is successfully achieved. The insertion loss for TE 0 and TE 1 modes are simulated to be ∼0.67 and 1.58 dB. While the mode crosstalk are ∼-17.7 and -15.3 dB for TE 0 and TE 1 modes, respectively. Free spectrum ranges (FSRs) for TE 0 and TE 1 modes are simulated to be 1.9 and 1.7 nm. This can be increased by optimizing the bent waveguide to reduce its size.

III. FABRICATION
The proposed device is fabricated on a SOI wafer with a 220 nm-thick top-silicon layer and SiO 2 buried layer of 2 μm. Fig. 5 shows the microscope image of the fabricated device. 248 nm deep ultraviolet photolithography and inductively coupled plasma (ICP) etching are used to form the waveguide structure. To form the channel waveguide, full etching is adopted. The waveguide width is chosen to be 1 μm to support dual-mode transmission. Λ and η of the SWG waveguide are chosen as 280 nm and 0.5, respectively. To achieve uniform dual-mode processing, coupling length and gap of the SWG coupling region are set as 20 μm and 220 nm. Furthermore, an adiabatic taper with width of 600 nm and length of 1.4 μm was introduced to realize adiabatic mode coupling between channel waveguide and SWG waveguide. The bending radius of the MRR is set to be 45 μm. In order to adjust the resonance wavelength of the ring, an integrated TiN heater is fabricated on top of the ring. The thickness and the width of the heater are 200 nm and 5 μm, respectively. The grating couplers (GCs) are used to couple light into and out of the chip. The circuit is fabricated at the Advanced Micro Foundry (AMF) in Singapore. Footprint of the fabricated MRR is 90 μm × 20 μm. In order to test the proposed MRR, adiabatic mode MUX and De-MUX are adopted. The insertion

IV. MEASUREMENT
The experiment setup is shown in Fig. 6. In order to verify the performance of the fabricated device, the transmission spectra over C band of the fabricated device is measured, by introducing the light from a broadband source (BS). Assisting by the polarization beam splitter (PBS) and polarization controller (PC), linearly polarized light input can be achieved. The chip is set on the coupling test platform. With the help of Thermo Electric Cooler (TEC), temperature of the test environment is kept at 25±0.01°C. On-chip heater is used to tune the resonance wavelength for both TE 0 and TE 1 modes. A fiber based beam splitter is utilized to split 1% of the output light into power meter (PM) for power monitoring, and 99% of the output light is launched into the optical spectrum analyzer (OSA) for transmission spectra measurement. The light input from port In 1 stands for the input TE 0 mode. The transmission spectrum of TE 0 is measured at port Out1, and the crosstalk is measured at port Out2. Details of the measured results are demonstrated and discussed in the following section.

V. RESULTS AND DISCUSSION
Transmission spectra within C band for TE 0 and TE 1 modes are measured. The results are shown in Fig. 7.
The total losses of the fabricated circuit for TE 0 and TE 1 modes are measured to be 13 and 14.73 dB. Coupling loss of the grating coupler fabricated on the same chip is measured to be ∼5 dB. Thus the insertion losses of the fabricated MRR are ∼ 3 and 4.73 dB for TE 0 and TE 1 modes, respectively. The FSRs for TE 0 and TE 1 modes are measured to be 1.912 and 1.704 nm. The quality factors (Q-factors) for TE 0 and TE 1 modes are measured to be ∼3300 and 5400, respectively. The crosstalk for TE 0 and TE 1 modes are measured to be <-12.1 and -10.3 dB. To be noted, relatively low extinction ratio (∼8.3 dB) is due to the strong coupling in SWG waveguides, which could be improved by selecting a suitable coupling gap.
To make a clear comparison of the figure of merit (FOM) between simulation and experiment results. Extinction ratio (ER), FSR and crosstalk for TE 0 and TE 1 modes are collected and shown in Table I. The results indicate good consistency between simulation and measurement. Furthermore, uniformities of these FOMs indicate good performance of the proposed dual-mode signal processing. To be noted, the slight performance degradation is due to the imperfect fabrication process.
In order to adjust the resonance wavelength of the ring, an integrated TiN heater is fabricated on top of the MRR. To characterize the performance of the heater, resonant wavelength with different heating powers for TE 0 and TE 1 modes are measured respectively. The measured results are shown in Fig. 8. The tuning efficiency for TE 0 and TE 1 modes are measured to be 0.0504 and 0.0505 nm/mw, respectively.

VI. CONCLUSION
In summary, we propose and experimentally demonstrate a dual-mode silicon ring resonator which could process TE 0 and TE 1 modes simultaneously. By introducing subwavelength grating into the coupling region, compact footprint of ∼90 μm × 20 μm is achieved. Measured results show good multi-mode processing is successfully achieved. The proposed device could be utilized in advanced mode division multiplexing network.