All-silicon and in-line integration of variable optical attenuators and photodetectors based on submicrometer rib waveguides.

We demonstrate a monolithic integration of variable optical attenuators (VOAs) and photodetectors (PDs) based on submicrometer silicon (Si) rib waveguide with p-i-n diode structure for near infrared (NIR) light. To make the Si PD absorptive for NIR, we introduced lattice defects at the rib core by means of argon ion implantation. At reverse bias of 5 V, the PD exhibits dark current of ~1 nA, responsivity of ~65 mA/W at 1560-nm wavelength, and a 3-dB cut-off frequency of ~350 MHz, while the VOA shows ~100 MHz. The PD has an absorption coefficient as low as ~0.5 cm−1, which is favorable for an in in-line PD configuration, where the PD absorbs a small portion of the optical power. For DC light, the PD precisely detects the optical power attenuated by the VOA with a detection range of over 40 dB. The 3-dB cut-off frequency of synchronous operation between the VOA and PD is ~50 MHz, which is limited by RF noise originating from the VOA drive current. Putting an isolation groove between the VOA and PD is effective for avoiding performance degradation in DC and RF operation.