Over 67 GHz Bandwidth and 1.5 V Vπ InP-Based Optical IQ Modulator With n-i-p-n Heterostructure

We report novel high-bandwidth InP-based Mach-Zehnder modulator and in-phase/quadrature (IQ) modulators that we realized by combining an n-i-p-n heterostructure and a capacitively loaded traveling wave electrode. The extremely low electrical and optical loss structure enhances the 3-dB electro-optic bandwidth of over 67 GHz without degrading other properties such as driving voltage and optical loss. The modulator also exhibits a static extinction ratio of over 24 dB with a Vπ of less than 1.5 V for the entire C-band. Furthermore, we demonstrate the first 120-Gbaud rate IQ modulation without optical pre equalization, and 100-Gb/s non-return-to-zero on-off keying modulation with a dynamic extinction ratio of over 10 dB.

I. Introduction

The optical transport network requires further increases in channel capacity and modulation format flexibility [1], [2]. Recently, projects for realizing a polarization-division multiplexing in-phase and quadrature modulator (PDM-IQM) with a high bandwidth (BW), and flexible modulations for 400G/λ were established by the Optical Internetworking Forum (OIF), which considers LiNbO₃ (LN) modulators with a baud rate of up to 64-Gbaud (3-dB BW: ∼35 GHz). However, in terms of the maximum transmission reach, a higher baud rate is preferred to higher level modulations e.g. in an ideal model, the transmission distance of a 128-Gbaud/QPSK signal is theoretically more than twice that of a 64-Gbaud/16 QAM signal. Thus, a higher BW modulator is becoming a key component. Although the LN modulator used in 100/200 G digital coherent systems is well developed, further BW extension is limited by its material properties such as its dielectric constant. On the other hand, InP-based Mach-Zehnder modulators (MZM) and IQMs are becoming key components for realizing not only high-density pluggable modules but also high baud rate modulation (3-dB BW: ∼40 GHz). Several high-speed InP IQMs, which utilize a low-loss electrical line such as a capacitive-loaded traveling-wave electrode (CL-TWE), have already been reported [3]– [8]. Nevertheless, their performance is insufficient in modulations exceeding a rate of 100 Gbaud. A BW of over 60 GHz is ideally needed for a rate of beyond 100 Gbaud. In most cases, there is a trade-off relationship between electro-optic bandwidth (EO BW) and half-wavelength voltage () because the EO interaction depends strongly on electrode length, which also affects the EO BW. Thus, previously reported ultra-high speed modulators tend to shorten the electrode length, and an increase in is inevitable [7], [8]. The main obstacle to higher speed modulation with an InP-based MZM is the high series resistance of the semiconductor. In particular, a p-doped InP layer has contact and bulk resistances that are about one order of magnitude higher than those of an n-doped InP layer. Therefore, the resistance of the p-doped cladding layer must be lower if we are to extend the BW without degrading other properties such as the and the optical propagation loss.