Broad-band optical communications typically require high-speed electro-optical modulators (EOM) to modulate light at a desired data rate. One common type of a broad-band EOM is a Mach-Zehnder modulator (MZM) that uses a waveguide Mach-Zehnder (MZ) interferometric structure with RF-driven optical phase modulators in each arm. The waveguide arms of the MZM are typically formed in an electro-optic material, for example a suitable semiconductor or electro-optic material such as LiNbO3, where optical properties of the waveguide may be controlled by applying a voltage. Such a waveguide modulator may be implemented in an opto-electronic chip as a photonic integrated circuit (PIC). A silicon photonics (SiP) platform based on Silicon on Insulator (SOI) technology may be particularly attractive for implementing broad-band modulators as it enables a natural integration with CMOS-based high-speed electronic drivers.
One common technique to high-speed modulation of propagating light, in particular at modulation rates on the order of 10-20 Gigabit per second (Gb/s) and higher, is the travelling wave approach, when the modulating electrical RF signals are applied to properly terminated electrical transmission lines that are electro-optically coupled and velocity-matched to the optical waveguides of the EOM. FIG. 1 schematically illustrates an example broad-band EOM in the form of an MZM 10 with two waveguide arms 11, 12 coupled to two electrical transmission lines 30 of length L, each formed by an inner electrode 22 and an outer electrode 21 with a transmission line termination 25. In the SiP platform, the electrodes 21, 22 may be overlaying p/n junctions formed across the waveguide arms that may either inject carriers (forward bias) or deplete carriers (reverse bias) in the waveguide core to modulate the refractive index of the waveguide by means of the carrier plasma dispersion effect. A common approach is to have the inner electrodes 22 connected to the ground, and to use a differential RF signal to drive the outer electrodes 21, so as to effectively double the phase modulation amplitude at the output combiner for a given peak-to-peak (PP) drive voltage Vpp applied to each electrode. In one common implementation, the transmission lines 30 modulate reverse-biased PN junctions formed along the length of the line, which modulate the light that is travelling in the waveguide arms 11, 12 at the same velocity as the RF signals propagating in the transmission lines 30.
Such traveling-wave modulators however present a design trade-off: increasing the length of the transmission lines in the EOMs improves the optical eye opening at the output of the modulator, but reduce the modulator bandwidth and increases the size of the modulator. The optical eye opening at the output of the modulator may be expressed in terms of the optical modulation amplitude (OMA), which may be defined as the difference in optical power between the “ON” and “OFF” levels for the On-Off-Keyed (OOK) modulation. Thus, designing EOMs having a sufficiently high OMA at a target bandwidth remains a challenge.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for providing high-bandwidth optical waveguide modulators with a suitably low OMA loss.