Exploring High-Q Silicon Nitride Microring Resonators for Microwave Photonics

Introduction

Microwave photonics (MWP) is an emerging field that combines the advantages of microwave and photonic technologies, leading to reduced system complexity, low signal loss, low electromagnetic interference, high bandwidth, and reconfigurability. Microring resonators (MRRs) are widely used devices in the MWP domain due to their compact size, low optical losses, and ability to operate over a wide range of optical wavelengths. However, achieving high quality (Q) factors and wide free spectral ranges (FSRs) with a smaller device footprint is crucial for better selectivity and broad frequency tunability in MWP applications.

In this article, we will explore the design, fabrication, and application of a compact and high-Q silicon nitride (SiN) MRR for microwave photonic applications such as tunable narrow-bandpass microwave photonic filters and optoelectronic oscillators.

Device Design and Fabrication

The proposed SiN-MRR is composed of a multimode rib waveguide with a slab height of 220 nm, fabricated on a commercially acquired SiN wafer from WaferPro. To ensure propagation of only the fundamental TE mode, the width of the single-mode waveguide (W = 1.1 μm) is adiabatically tapered over a length of 200 μm to a multimode waveguide (W = 2.5 μm). Figure 1 shows the schematic top view of the proposed MRR, including important topological design parameters.

Figure 1. Schematic top view of the proposed SiN microring resonator along with important topological design parameters.

The microscopic image of the fabricated device, along with SEM images of the zoomed taper and directional coupler (DC) sections, is shown in Figure 2.

Figure 2. Microscopic image of the fabricated device with SEM images of zoomed taper and directional coupler (DC) sections.

Characterization and Results

The transmission spectrum of the MRR was recorded using a high-resolution optical complex spectrum analyzer (Apex AP2683A), with a resolution bandwidth of 0.8 pm. The extracted waveguide loss, Q-value, and FSR from the measured MRR response are 0.2 dB/cm, 1.7 million, and 1 nm, respectively, as shown in Figures 3(a) and 3(b).

Figure 3. (a) Transmission characteristics of the MRR (normalized with reference waveguide), inset: resonance at 1582 nm (FWHM ∼ 112 MHz); (b) transmission characteristics of the MRR indicating the FSR ∼ 1 nm.

Microwave Photonic Applications

To validate the practicality of the fabricated device for microwave photonic applications, a sub-GHz microwave photonic bandpass filter (MPBF) and an optoelectronic oscillator (OEO) have been demonstrated. Figure 4 shows the experimental setup for both demonstrations.

Figure 4. Experimental setup to demonstrate microwave photonic bandpass filter and optoelectronic oscillator.

Demonstration of Microwave Photonic Bandpass Filter:

The filter response (normalized to the input RF power) at 10 GHz, with a 3-dB bandwidth of 340 MHz and 25 dB rejection, is shown in Figure 5(a). The filter response was tuned from 10 - 20 GHz by detuning the laser wavelength, thus altering the spacing between the carrier and the MRR resonance, and the results are shown in Figure 5(b).

Figure 5. (a) Measured microwave photonic bandpass filter response at 10 GHz with 3-dB bandwidth of 340 MHz and rejection ratio of 25 dB; (b) Tuning of central frequency of MPBF from 10-20 GHz.

Demonstration of Optoelectronic Oscillator:

Using the MPBF in the OEO loop, tunable OEO frequency generation from 8-12 GHz has been demonstrated, as shown in Figure 6(a). The results are measured with a resolution bandwidth (RBW) of 100 kHz. Further, the spectrum was resolved with an RBW of 50 Hz to measure the sidemode suppression ratio (SMSR) and FSR of the longitudinal modes of the OEO cavity, which are 42 dB and 4.5 MHz, respectively, as shown in Figure 6(b).

Figure 6. Electrical spectrum of OEO: (a) Tuning of generated OEO frequency (limited by RF coupler bandwidth in our experiment); (b) longitudinal modes resolved to estimate SMSR and FSR of the cavity.

Conclusion

In this tutorial, we have explored the design, fabrication, and application of a compact and high-Q SiN-MRR with a Q-value of 1.7 million and an FSR of 1 nm. This device has been used to demonstrate a tunable MPBF (10 - 20 GHz) with a narrow bandwidth of 360 MHz and the generation of RF signals based on an OEO loop with a tunable frequency (8 - 12 GHz) and an SMSR of 42 dB.

These results can be further improved by reducing waveguide loss and increasing waveguide mode confinement, leading to a higher Q-value. Additionally, the proposed device has the potential to be used in various other microwave photonic applications, such as delay lines, frequency measurement, spectral shaping, and arbitrary waveform generation.

Reference

[2] Tiwari, A. Velamuri, A. Goswami, D. Venkitesh, E. Bhattacharya, and B. K. Das, "A Compact and High-Q Value SiN Microring Resonator for Microwave Photonic Applications," presented at the Centre for Programmable Photonic Integrated Circuits and Systems, Dept. of Electrical Engineering, IIT Madras, Chennai, India, 2024.