Silicon Photonics is set to revolutionize the Optical Communications industry due to its low cost, volume manufacturability and intrinsic compatibility for monolithic integration with CMOS circuits. An optical modulator is a vital component of an optical transceiver. Current commercial optical modulators are fabricated using expensive materials such as III-V compound semiconductors or Lithium Niobate (LiNbO₃). Furthermore, such modulators necessitate the use of hybrid integration techniques during manufacture, involving higher cost and complexity compared to monolithic integration.

IME researchers have developed monolithic silicon optical modulators capable of wavelength independent operation at data rates of up to 10 Gbs^-1. Excellent phase-shifting efficiency and jitter performance was also demonstrated.

IME's silicon modulators are based on a Mach-Zehnder interferometer (MZI) design and exploit the free carrier plasma dispersion effect to achieve optical modulation. In MZI modulators, light from a continuous laser source enters the modulator and is split into two beams. The two beams are re-combined at the output after each of them passes through a phase-shifter waveguide. By electrically controlling the carrier density in each phase-shifter, the refractive index of silicon can be controlled and in turn induces a phase difference in the two beams of light, resulting in constructive or destructive interference at the point of re-combination. Hence, the intensity of light at the output can be modulated by controlling the carrier density in the phase-shifters.

The modulators utilize p-n junction phase-shifters which are embedded in the silicon-on-insulator (SOI) rib waveguides. These are operated in the carrier depletion regime, resulting in excellent speed characteristics. This can be attributed to the much faster carrier transport when operating in the carrier depletion regime compared to the carrier injection regime. Fig. 1 shows the eye pattern measurements for a modulator tested at 10 Gbs^-1. An extinction ratio of 6 dB was achieved with an electrical driving signal of 5 V peak-to-peak. Excellent jitter performance of ~4 ps was also measured. The short rise and fall-times of the waveform also indicate that the modulator can be operated at even higher data rates. The phase-shifting efficiency (V_π.L_π ~2.6 V.cm) is comparable to or exceeds those of the best reported carrier depletion type modulators.

Fig. 1 Eye pattern measurements showing an extinction ratio of ~6 dB and jitter (RMS) of ~4 ps for operation at 10 Gbs^-1

The monolithic carrier depletion-type silicon optical modulator will be an attractive component for future silicon photonic transceivers due to its high performance and simplicity in fabrication. Such monolithically integrated silicon optical modulators will become enabling components for low-cost optical transceivers. It is likely that this can extend high performance optical communications capabilities even to mainstream consumer applications in the near future.