Introduction
The ever-increasing demand for higher data transmission capacities in optical communication networks has driven the development of advanced modulation formats and components capable of operating at ultra-high baud rates. At the heart of these high-speed coherent optical transceivers are the coherent driver modulators (CDMs), which play a crucial role in enabling the realization of transmission speeds beyond 160 Gbaud.
In this tutorial, we will delve into the design and implementation of a high-bandwidth coherent driver modulator (HB-CDM) that can reliably operate at such high baud rates without the detrimental effects of electro-optic (EO) oscillations. We will discuss the key considerations in packaging the driver and modulator components together to achieve superior performance and the strategies employed to suppress the oscillations that can arise due to the tight integration of these components [1]
The HB-CDM Architecture
The HB-CDM is a well-established form factor that combines a high-speed modulator chip, typically based on III-V semiconductors or thin-film lithium niobate (TFLN), with a high-speed driver die in a single integrated package. This integration of the modulator and driver in close proximity enables the realization of ultra-high-bandwidth performance, surpassing the capabilities of standalone modulator components.
Figure 1 shows a photograph and schematic diagram of the fabricated HB-CDM. The package body size is 12 × 30 × 5.3 mm³, and it includes an InP-based n-i-p-n heterostructure twin-IQ modulator chip with a differential capacitively-loaded travelling-wave electrode (CL-TWE) and a 4-channel linear SiGe BiCMOS driver die with an open-collector configuration. The package and modulator chip used were the same as those previously reported, with an electrical (EE) 3-dB bandwidth of 80 GHz and a roll-off frequency of over 100 GHz, and a 3-dB EO bandwidth of over 72 GHz with a V_π of 2 V, respectively [2].
The HB-CDM utilizes a flexible printed circuit (FPC) as the RF interface, which enables excellent impedance matching and minimized RF losses, enabling operation at baud rates up to 144 Gbaud [2]. The package is soldered to a printed circuit board (PCB) that has a 100-GHz compatible sub-miniature push-on (SMPS) connector as the RF interface and a United States Military Standard (MIL)-based connector as the DC power interface. The use of a small 4-channel SMPS connector with a channel pitch of 3 mm is crucial in shortening the length of the RF transmission line and reducing RF losses on the PCB, as shown in Figure 3, where the 3-dB EE bandwidth of the evaluation board exceeds 95 GHz, and the roll-off frequency surpasses 110 GHz.
Driver Characteristics and Potential for Oscillation
The performance of the HB-CDM is heavily influenced by the characteristics of the driver die, which must be carefully designed and integrated to ensure stable operation and suppress any undesired oscillations.
Figure 2 shows the measured differential gain (Sdd21) and the differential μ-factor of the driver. The driver alone exhibits a 3-dB EE bandwidth of 104 GHz, which reduces to 96 GHz when accounting for the 120-pH wire inductance between the driver and the package or modulator. The magnitude of the peaking at 80 GHz exceeds 10 dB, which is more than the sum of the RF losses of the modulator chip and the package, indicating the potential for excessive peaking in the EO response of the HB-CDM.
More importantly, the μ-factor of both the input and output sides with 120 pH remains around or below 1 in the 60–100 GHz range, indicating that the driver is conditionally stable in this frequency range and that there is a risk of the driver oscillating, depending on the assembly conditions in the HB-CDM.
Suppressing Electro-Optic Oscillations
The tight integration of the driver and modulator in the HB-CDM can lead to the emergence of undesired EO oscillations, which can degrade the transmission characteristics, such as bit error rates. The cause of these EO oscillations is the interaction between the driver's conditional stability (μ-factor < 1) and the cavity resonance frequency within the HB-CDM package.
The cavity resonance frequency is determined by the spatial vertical distance between the driver and the lid inside the package, as illustrated in Figure 3. If this distance is such that the cavity resonance frequency matches the frequency range where the driver's μ-factor is below 1, the driver becomes unstable, leading to oscillations in the EO response of the HB-CDM.
To suppress the EO oscillations, the authors investigated the effect of varying the distance between the driver and the lid. By thickening the lid above the driver and reducing the vertical distance, as shown in Figure 4(b), the cavity resonance frequency can be shifted to a higher frequency, outside the range where the driver's μ-factor is below 1.
Figure 3 shows the small-signal EO responses of the HB-CDM with different distances between the driver and the lid. When the distance is 1.75 mm, a large oscillation is observed around 85 GHz, as shown in Figure 3(a). As the distance is decreased to 1.6 mm and 1.35 mm, the oscillation frequency shifts to higher frequencies of 90.3 GHz and above 100 GHz, respectively, as shown in Figures 3(b) and 3(c). By setting the distance to 1.35 mm, corresponding to a cavity resonance frequency above 100 GHz, the EO oscillations are successfully suppressed, and the 3-dB EO bandwidth of the HB-CDM exceeds 90 GHz, which is the highest reported to date.
This approach of adjusting the cavity resonance frequency by controlling the distance between the driver and the lid is a effective method to ensure stable operation of the HB-CDM and suppress the EO oscillations, even when the driver itself has frequencies where the μ-factor is below 1.
High-Speed IQ Modulation Performance
To evaluate the performance of the HB-CDM without EO oscillations, the authors conducted back-to-back IQ modulation experiments, as shown in the setup in Figure 5.
Figure 6 presents the BER characteristics and constellation diagrams for 160-Gbaud dual-polarization (DP) 16-QAM and 168-Gbaud DP-16QAM modulations. The HB-CDM without EO oscillation was used in this experiment, and the insertion loss per polarization and the static extinction ratio were less than 8 dB and over 28 dB, respectively.
The measured BERs at the best OSNR of 41.5 dB for the 160-Gbaud DP-16QAM and 41.1 dB for the 168-Gbaud DP-16QAM were 2.3 × 10^-3 and 6.9 × 10^-3, respectively. These results confirm the capability of the HB-CDM to support operation in the 160 Gbaud class and beyond, thanks to the successful suppression of the EO oscillations.
Conclusion
In this tutorial, we have described the design and implementation of a high-bandwidth coherent driver modulator (HB-CDM) that can operate at baud rates exceeding 160 Gbaud without the detrimental effects of electro-optic (EO) oscillations.
The key aspects covered include:
The HB-CDM architecture, which integrates a high-speed modulator chip and a driver die in a single package to enable ultra-high-bandwidth performance.
The importance of the driver characteristics, particularly the μ-factor, and its impact on the potential for oscillations in the HB-CDM.
The strategy of suppressing EO oscillations by adjusting the vertical spatial distance between the driver and the lid inside the package, which determines the cavity resonance frequency.
The demonstration of high-speed IQ modulation performance, including 160-Gbaud DP-16QAM and 168-Gbaud DP-16QAM, using the HB-CDM with the oscillation-suppressed EO response.
The successful development of the HB-CDM with a 3-dB EO bandwidth exceeding 90 GHz, without any EO oscillations, represents a significant milestone in the pursuit of ultra-high-speed optical transmission systems. The insights and strategies presented in this tutorial can guide the further advancement of coherent driver modulators and contribute to the realization of next-generation high-capacity optical networks.
Reference
[3] J. Ozaki, Y. Ogiso, H. Yamazaki, Y. Hashizume, M. Ishikawa, and N. Nunoya, "Oscillation Suppression of EO Response in Coherent Driver Modulator for Over 160 Gbaud Operation," in IEEE Photonics Technology Letters, vol. 36, no. 2, pp. 119-122, 15 Jan. 2024.
[4] J. Ozaki et al., "Over-85-GHz-Bandwidth InP-Based Coherent Driver Modulator Capable of 1-Tb/s/λ-Class Operation," in Journal of Lightwave Technology, vol. 41, no. 11, pp. 3290-3296, 1 June1, 2023, doi: 10.1109/JLT.2023.3236962.
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