Silicon photonics
Which correctly describes VπL ("V-pi-L"), a figure of merit for Mach-Zehnder modulators?
- AThe product of the voltage needed to change the phase by π and the phase-shifter length; smaller means more efficient modulation
- BThe product of the maximum optical power the modulator can accept and its length; larger means high-power lasers can be used directly
- CThe product of the modulator's electrical bandwidth and its length; larger means higher speed and lower power consumption
Show answer and explanation
Answer: A. The product of the voltage needed to change the phase by π and the phase-shifter length; smaller means more efficient modulation
Vπ is the voltage needed to make the phase difference between the two arms π, switching the output from the bright state to the dark state. A longer phase shifter needs less voltage, so efficiency is compared by the product of voltage and length, VπL. For silicon depletion-type modulators, typical values are roughly 1–3 V·cm, depending on doping and operating conditions. A smaller VπL allows a shorter modulator or a lower drive voltage. However, a longer modulator increases optical loss and electrode capacitance and lowers bandwidth, so there are trade-offs among length, drive voltage and bandwidth.