MEMS-Enabled Movable Cladding Phase Shifter for Silicon Nitride Photonics

Introduction

Phase shifters are critical building blocks for optical signal processing in photonic integrated circuits (PICs). In recent years, PICs have enabled advancements in optical computing for both classical and quantum applications by providing more available optical processor modes and quantum qubits. Silicon nitride waveguides are an excellent platform for this due to their low optical loss, compact circuitry, and transparency across telecom and visible wavelengths.

Traditional in-plane or out-of-plane MEMS phase shifters for silicon nitride rely on an air-cladded waveguide region to function. However, the transition from the air-cladded section to the buried waveguide leads to optical losses. Additionally, silicon nitride waveguides without an upper cladding suffer higher losses compared to those with an oxide upper cladding.

In this tutorial, we present a new MEMS phase shifter design that uses a movable silicon dioxide cladding to enable low-loss and low-power phase shifting for silicon nitride photonics.

Device Design

The key innovation is using the cladding material itself as the active region, where an air gap between the waveguide core and the movable upper cladding enables tuning of the effective refractive index.

Figure 1a shows the device structure with a 220 nm tall, 1200 nm wide silicon nitride waveguide passing through seven sections with different cladding environments.

Fig. 1. a) Scale model of the MEMS phase shifter with seven different sections, b) Change of the waveguide effective refractive index versus vertical gap changes in section 4 for two different waveguide widths, c) Cross section of the waveguide and the actuation principle

The input (1) and output (7) are standard strip waveguides. Sections 2 and 6 are interfacial regions to enable the cladding release during fabrication. 3 and 5 are short air-cladded trenches, while 4 is the core movable cladding section.

The waveguide is wider (1700 nm) under the movable cladding to reduce optical loss from the different sections to 0.9 dB.

Figure 1b shows that increasing the air gap between the core and movable cladding reduces the effective refractive index, enabling phase shifting.

The movable cladding is actuated electrostatically using a capacitive parallel plate mechanism, as depicted in Figure 1c. The cladding plate is connected via flexures and has no metal in the optical path to minimize loss.

Fabrication and Experimental Results

The devices were fabricated using a surface micromachining process on a 200 mm silicon wafer. An asymmetric Mach-Zehnder interferometer (MZI) with a 147 μm path length difference was used to characterize the phase shifter performance.

Figure 2a shows the fabricated MZI with the phase shifters. Light is coupled vertically using a fiber array.

Fig. 2. a) Fabricated device in an asymmetric MZI configuration (an enlarged view of the phase shifter is presented), b) MZI spectrum for the actuation voltage of 18 V for π phase shift

Figure 2b displays the measured MZI spectrum sweeping across the telecom C-band. A π phase shift is achieved with an 18 V actuation voltage for a 1000 nm air gap in the movable cladding section.

The phase shifter introduces only 1.1 dB of additional optical loss measured via a cut-back method. The waveguide loss of 1.45 dB/cm at 1550 nm remained unchanged after fabrication.

Conclusion

This article presented a new MEMS phase shifter design utilizing an actuated movable silicon dioxide cladding for silicon nitride photonics. The key benefits include:

1. Compact device footprint with 100 μm long phase shifter

2. Low optical loss of 1.1 dB by eliminating air-cladding regions

3. Low power π phase shift with 18 V actuation voltage

4. CMOS compatible process on 200 mm wafers

5. Potential compatibility with other waveguide platforms

The movable cladding approach enables high-performance, low-loss phase shifting critical for advanced photonic integrated circuits across applications like optical computing, communications, and quantum technology.

Reference

[1] M. Namdari, M. Blasl, T. Grasshoff, M. Wagner, and J. Grahmann, "Phase Shifter for Silicon Nitride Photonics using MEMS-Enabled Movable Cladding," Fraunhofer Institute for Photonic Microsystems IPMS, Maria-Reiche-Str. 2, 01109 Dresden, Germany, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.