Optical transport networks require further increases in channel capacity and modulation format flexibility. Recently, digital coherent signals with a higher baud rate and a simpler modulation format are the key to maximizing the transmission distance for a given capacity per lambda. Realizing a higher baud rate requires higher speed coherent optics such as a transmitter and a receiver as well as the management of RF loss. Regarding the optical transmitter, a high-speed modulator photonics integrated circuit (PIC) has to be assembled very close to a high-speed driver IC in one package to reduce RF loss. This configuration, namely a high-bandwidth (HB) coherent driver modulator (CDM), is being discussed in the Optical Internetworking Forum (OIF) to support rates of approximately 64 GBd, and it will probably be used even in the next generation of 100-GBd-class transmitters. In modulator PICs for beyond 100 GBd, the half-wave voltage (Vπ) and optical insertion loss, as well as the electro-optic (EO) bandwidth, become more important for compensating for degradations caused by other RF components, such as decreases in the output voltage of the high-speed driver IC due to the miniaturization of transistors. InP-based modulators are promising for beyond 100 GBd because of their superior and stable material properties. However, their EO bandwidths are limited to less than 40 GHz by the high series resistance of materials and are still not sufficient for beyond 100-GBd modulations.

In this paper, we report an ultra-high speed InP-based modulator PIC for beyond 100-GBd transmitters. By replacing the p-i-n heterostructure with a new n-i-p-n heterostructure, we reduced the series resistance of the semiconductor and extended the bandwidth without degrading other properties such as Vπ and optical insertion loss. The IQ modulator exhibits an EO bandwidth of 80 GHz in a 1.5-V Vπ design, which is the best modulation performance reported so far. Furthermore, we fabricated a driver modulator sub-assembly for beyond 100-GBd CDM. The sub-assembly exhibits an ultra-high bandwidth exceeding 67 GHz, which is sufficient for practical implementation. By utilizing the sub-assembly, we also demonstrated up to 128-GBd IQ modulations without optical pre-equalization. We believe our low loss modulator to be suitable for realizing higher speed IQ modulations in future optical networks.