Optical Frequency Comb-Based Photonic Sampling for Microwave Characterization of Wafer-Level Silicon Photonic Transceiver Chips

Introduction

Silicon photonic integrated circuits (PICs) have emerged as a promising solution for meeting the increasing bandwidth demands of high-speed communication and high-performance computing. These PICs employ optical signals instead of electrical signals, replacing electrical wires with broadband and low-power optoelectronic transceivers consisting of transmitters (e.g., semiconductor laser diodes and intensity modulators) and receivers (e.g., photodiodes). However, characterizing the optical-electrical (O-E) and electrical-optical (E-O) response parameters of these wafer-level PICs is crucial for chip fabrication and optimization, but it presents significant challenges.

Conventional Measurement Techniques

The commonly used electro-optic frequency sweep method (EOFS) employs a microwave network analyzer (MNA) and O-E or E-O transducer standards to measure the frequency response of optoelectronic chips. However, this method requires intense E-O or O-E calibration to determine the responses of the transducer standards. Additionally, in the case of wafer-level transceiver measurements, it is challenging to decouple the individual responses of the intensity modulator (IM) or the photodiode (PD) chips from the combined response contributed by both chips using the transducer standards, unless the integrated transceiver is decomposed into discrete IM or PD chips.

Proposed Optical Frequency Comb-Based Photonic Sampling Method

To address the limitations of conventional measurement techniques, researchers have proposed an optical frequency comb-based photonic sampling method for characterizing wafer-level silicon photonic transceiver chips. This method enables the simultaneous extraction of the intrinsic frequency responses of the IM and PD chips in a wafer-level E-O-E link, without the need for extra O-E or E-O calibration.

Figure 1 shows the schematic setup of the proposed on-chip measurement method and photographs of the chip under test.

Fig. 1. Schematic setup of the proposed on-chip measurement method and the photographs of the chip under test. MLL: mode-locked laser; MNA: microwave network analyzer; IM: intensity modulator; PD: photodiode; A and B: microwave adapter networks

Experimental Results

In the demonstration, a silicon photonic integrated circuit consisting of a Mach-Zehnder modulator (MZM) chip and a PD chip was used as the optoelectronic transceiver under test. The microwave stimulus signal in the IM chip of the integrated E-O-E link was sampled by the optical pulse train from a mode-locked laser. The sampled optical signal was then coupled to the PD chip of the integrated E-O-E link, and the photocurrent was collected by the built-in receiver of the MNA.

Figure 2(a) shows the measured relative frequency responses of the MZM, PD, and transceiver chips using the proposed method and the EOFS method with the help of O-E and E-O transducer standards.

Fig. 2 (a) Measured relative frequency responses of the MZM, PD and transceiver chips by EOFS method and the proposed method. (b) Measured modulation depth and half-wave voltage of the MZM chip by the OSA method and the proposed method.

The good consistency between the measured results of the proposed method and the calibrated results of the EOFS method indicates that the proposed method achieves self-reference measurement of the transceiver chip.

Furthermore, the modulation depth and half-wave voltage of the MZM chip were measured using the optical spectrum analysis (OSA) method and the proposed method, as shown in Figure 2(b), verifying the feasibility of the proposed method.

To confirm the robustness of the proposed method to impedance mismatch, the reflection coefficients of the MZM and PD chips before and after de-embedding the microwave adapter networks were extracted and plotted on a Smith chart, as shown in Figure 3.

Fig. 3 Measured reflection coefficients of the MZM and PD chips before and after de-embedding the microwave adapter networks on the Smith chart.

The results indicated that the MZM and PD chips were far from a perfect 50 Ω match, yet the proposed method could still provide accurate measurements.

Advantages and Applications

The proposed optical frequency comb-based photonic sampling method offers several advantages over conventional techniques:

1. Simultaneous measurement of the intrinsic frequency responses of the IM and PD chips in a wafer-level transceiver chip.

2. No need for extra O-E or E-O calibration.

3. Robustness to impedance mismatch.

4. Non-invasive characterization during chip fabrication.

5. Predictable outcome and yield analysis before packaging.

This method can be extensively used for characterizing various optoelectronic integration circuits, such as InP-based photonic integration circuits, in addition to silicon photonic integrated transceiver chips.

Conclusion

The optical frequency comb-based photonic sampling method provides a self-reference and non-invasive approach for microwave characterization of wafer-level silicon photonic transceiver chips. By simultaneously extracting the intrinsic frequency responses of the IM and PD chips without the need for extra calibration, and being robust to impedance mismatch, this method offers a promising solution for optimizing chip fabrication and predicting yield before packaging. Its applicability extends beyond silicon photonics to other optoelectronic integration circuits, making it a valuable tool for the advancement of high-speed communication and computing technologies.

Reference

[1] J. Zhu, X. Zou, Y. Xu, C. Jing, Y. Zhang, Z. Zhang, Y. Liu, and S. Zhang, "Optical frequency comb-based photonic sampling for microwave characterization of wafer-level silicon photonic transceiver chips," Research Center for Microwave Photonics, University of Electronic Science and Technology of China, Chengdu, China, 2024.