Temperature-Insensitive Mach-Zehnder Interferometer Filters Using Silicon Nitride Waveguides

Introduction

Silicon photonics has been extensively researched and employed in various devices such as filters, modulators, and sensors. However, one major challenge with silicon photonic devices is their sensitivity to temperature variations due to silicon's high thermo-optic coefficient (TOC) of 1.86×10-4/°C. This can cause issues in practical implementation.

Common methods to mitigate thermal sensitivity include using heaters for temperature stabilization, selecting cladding materials with negative TOCs to counteract silicon's positive TOC, and implementing precise temperature control techniques. An alternative approach is to use silicon nitride (SiN) waveguides instead of silicon.

Silicon nitride has emerged as an attractive waveguide core material due to its CMOS compatibility, moderate core/cladding refractive index contrast, and wide transparency window from visible to mid-infrared wavelengths. Crucially, it has a low TOC of just 2.45×10-5/°C, about 75 times lower than silicon. This enables SiN waveguide devices to maintain stable performance over a wide temperature range without being affected by temperature fluctuations.

In this article, we examine the thermo-optic properties of SiN waveguides and present a temperature-insensitive Mach-Zehnder interferometer (MZI) filter design utilizing optimized SiN waveguide widths.

Device Design and Fabrication

The designed MZI structure consists of a silicon substrate, 2 μm SiO2 undercladding, and a 280 nm SiN waveguide layer deposited by LPCVD, as shown in Figure 1(a). It comprises two directional couplers - the first splits input light equally into two arms with different widths and lengths, while the second recombines the light into the output waveguide.

The arm waveguide widths W1, W2, and W3 were optimized to 1 μm, 2 μm, and 0.6 μm respectively based on simulations in Figure 1(b) showing the variation of TOC with waveguide width. This width engineering exploits the difference in TOCs to minimize the device's overall thermal sensitivity.

The arm lengths L1 and L2 were designed as 90 μm and 300 μm to achieve a free spectral range (FSR) of 20 nm in the C-band.

Fig. 1 (a) The structure of the thermal insensitive MZI. (b) Simulated thermal-optic coefficients for various silicon nitride widths with a consistent thickness of 280 nm.

Simulation and Measurement Results

Simulations comparing the conventional and temperature-insensitive MZI designs were carried out, shown in Figures 2(a) and 2(b) respectively. For the conventional MZI, a clear redshift in the filter wavelength from 1557 nm to 1558.15 nm is seen as temperature increases from 25°C to 55°C, corresponding to a sensitivity of ~17 pm/°C. In contrast, the temperature-insensitive MZI exhibits negligible wavelength shift over the same temperature range due to the compensating effects of the different arm widths. Fitting reveals an ultra-low sensitivity of only 0.05 pm/°C.

Fig. 2 Simulated transmission spectra of (a) conventional MZI and (b) temperature-insensitive MZI at different temperatures.

To experimentally validate the design, both conventional and temperature-insensitive MZI filters were fabricated. Figure 3 shows an optical image of the fabricated temperature-insensitive device along with SEM images of the two waveguide arms with engineered widths.

Fig. 3 Top-view microscope image of temperature-insensitive MZI and SEM images of the two arms.

The measured transmission spectrum from 1500-1560 nm of the temperature-insensitive MZI at different temperatures from 25°C to 55°C is plotted in Figure 4(a). An FSR of 20.3 nm is observed with no discernible wavelength shift, even with the 30°C temperature change.

This is clearly seen in the zoomed-in view of a single filter peak in Figure 4(c) for the temperature-insensitive MZI, which remains stable in wavelength. In contrast, the peak from the conventional MZI in Figure 4(b) redshifts with increasing temperature.

Quantifying the wavelength shifts in Figure 4(d) reveals temperature sensitivities of 16.2 pm/°C and only 0.5 pm/°C for the conventional and temperature-insensitive MZIs respectively. This confirms the simulated results, with the optimized SiN waveguide dimensions reducing thermal sensitivity by over 30 times compared to the conventional design.

Furthermore, the fabricated temperature-insensitive filter exhibits a low insertion loss of 1.61 dB along with an extinction ratio of 18 dB, highlighting its practicality.

Fig. 4 (a) Transmission spectra of temperature-insensitive MZI at different temperatures. Spectra of a single filtering peak of both the (b) conventional and (c) temperature-insensitive MZI at various temperatures. (d) Temperature-dependent wavelength shift of two MZI.

Conclusion

The authors have demonstrated a Mach-Zehnder interferometer filter using silicon nitride waveguides with a 20 nm free spectral range in the C-band. By optimizing the widths of the two interferometer arms to 2 μm and 0.6 μm, the device's temperature sensitivity was dramatically reduced from 16.2 pm/°C to just 0.5 pm/°C. This 32-fold improvement makes the SiN MZI filter essentially athermal and robust to temperature fluctuations, a highly desirable feature for practical applications. The low insertion loss and high extinction ratio further highlight the performance benefits of this temperature-insensitive silicon nitride photonic integrated filter design.

Reference

[1] L.-Y. Chen, Y.-J. Chen, W.-H. Huang, C.-T. Wang, "Temperature-insensitive Mach-Zehnder Interferometer Filters Based on Silicon Nitride Waveguides," Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan; Taiwan Semiconductor Research Institute, TSRI, Hsinchu 30078, Taiwan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.