Silicon Ring Based Wavelength Division Multiplexing with an Ultra-wide Spectral Range of 6.4 THz

Introduction

Wavelength-division multiplexing (WDM) is a crucial technique for increasing the capacity of optical communication systems by transmitting multiple signals over a single optical fiber using different wavelengths. Silicon photonics has emerged as a promising platform for integrating WDM technology onto chips, with silicon ring resonators being a popular choice due to their compact size and excellent performance. However, the ring's free spectral range (FSR) has traditionally limited the WDM spectral range to a maximum of 3.2 THz.

In a recent breakthrough, researchers at imec have developed a novel design that extends the WDM spectral range to an impressive 6.4 THz by cascading a Mach-Zehnder (MZ) filter at the ring drop port, doubling the range of conventional ring-based WDM filters.

Design and Operation Principle

The key innovation lies in the design of the MZ filter, which utilizes half of a ring resonator as the arm length difference. This unique configuration ensures that the MZ FSR is precisely twice that of the ring FSR, aligning the MZ interference wavelengths with the ring resonance wavelengths.

The MZ filter comprises two identical directional couplers, as shown in Figure 1a, with the input and output ports arranged at a 90-degree angle. The arm length difference is determined by the half-ring bend, resulting in an MZ FSR that is double the ring FSR.

Figure 1: (a) Microscope image of the add-drop ring with a MZ filter cascaded at the drop port. (b) Measured ring and MZ individual spectra, confirming the doubling relation of the FSR between the ring and MZ.

Experimental Demonstration

To validate the design, the researchers fabricated a WDM-16×400 GHz filter by cascading eight ring-MZ blocks, each with a working wavelength spaced 400 GHz apart, as shown in Figure 2a. Doped silicon heaters with sealed under-cut (UCUT) were integrated into each ring to thermally tune the working wavelength and align it with the corresponding MZ filter (Figure 2b).

Figure 2: (a) Microscope image of the fabricated WDM-16×400 GHz filter. (b) Zoomed-in view of one add-drop ring with MZ filter and integrated doped silicon heater. (c) Measured transmission spectra of the WDM-16×400 GHz filter.

After tuning, the measured transmission spectra (Figure 2c) exhibited a 16-channel WDM filter with a channel spacing of 400 GHz, spanning an ultra-wide spectral range of 6.4 THz. The filter demonstrated a 3 dB bandwidth of 87±9 GHz, crosstalk ranging from -30.4 to -14.9 dB, and an insertion loss of 1.77 to 4.61 dB.

It's important to note that the measured insertion loss values include not only the WDM filter's intrinsic loss but also losses from the measurement setup, such as the fiber array and optical switch matrix. The actual filter insertion loss is expected to be lower than the reported values.

Significance and Future Prospects

The achievement of a 6.4 THz WDM spectral range represents a significant advancement in silicon ring-based WDM filters, surpassing the previously reported maximum of 3.2 THz. This breakthrough paves the way for the realization of advanced WDM configurations, such as the WDM-64×100 GHz filter, which can be realized by integrating the current design with an interleaver.

The researchers propose that future designs could consider reducing the straight segment length of the half-ring bend in the MZ filter to achieve precise working wavelength alignment between the ring and MZ filter, further optimizing the performance of this innovative approach.

Conclusion

The development of this silicon ring-based WDM filter with an ultra-wide spectral range of 6.4 THz represents a significant milestone in the field of integrated photonics. By cascading a novel MZ filter design at the ring drop port, the researchers have successfully doubled the WDM spectral range compared to conventional ring-based filters.

This breakthrough is expected to catalyze further advancements in silicon ring-based WDM filters, enabling the realization of high-capacity optical communication systems and advanced photonic integrated circuits. The potential for scaling to higher channel counts and tighter channel spacing makes this technology highly attractive for meeting the ever-increasing demands of data transmission and processing.

Reference

[2] Q. Deng et al., "Silicon Ring Based Wavelength Division Multiplexing With an Ultra-wide Spectral Range of 6.4 THz," presented at the IEEE Photonics Conference, Leuven, Belgium, 2024.