Microring modulator

A microring modulator is a ring resonator, typically a few to ten micrometers in radius, coupled to a bus waveguide, with a p-n junction built into the ring. Near resonance the ring draws light out of the bus, so the transmitted power has a narrow dip. Changing the carrier density in the junction changes the ring's effective index through the plasma dispersion effect and moves the dip; with the laser set on the slope of the resonance, the transmitted power follows the drive voltage. Reverse-biased (depletion) junctions are used for high speed, forward-biased (injection) ones where a larger index change at lower speed is acceptable.

The resonance does the work that length does in a Mach-Zehnder modulator. The resonance shifts by $\Delta\lambda = \lambda\,\Delta n_\text{eff}/n_g$: at 1310 nm with a group index of 4.2, an index change of $2 \times 10^{-4}$ moves it 0.062 nm, about half the 0.131 nm full width of a ring with a loaded Q of 10,000. A half-linewidth shift gives a useful modulation depth from a device a few micrometers across with a capacitance of tens of femtofarads, so the switching energy is small: $CV^2/4$ for a random NRZ pattern gives 5 fJ/bit for 20 fF and 1 V, excluding the driver.

The same resonance imposes the limits. Light stored in the ring must leave before the modulation can take effect, so the photon lifetime, $\tau = Q\lambda/(2\pi c)$, caps the bandwidth near $1/(2\pi\tau)$: 22.9 GHz for a loaded Q of 10,000 at 1310 nm and 45.8 GHz for 5,000. Designs therefore balance modulation depth, which favors high Q, against optical bandwidth, which favors low Q. The modulation also carries chirp, because moving a resonance shifts the phase as well as the amplitude of the light.

Temperature is the chief practical constraint. The resonance moves by about 0.058 nm/K at 1310 nm, from silicon's thermo-optic coefficient, roughly the full linewidth for each 2 K of change. Every ring therefore carries a heater and a feedback loop that holds it on its laser line, and the heater power and control electronics are part of the energy budget, as described in ring resonator thermal tuning and locking. The payoff is wavelength selectivity: a row of rings on one bus, each tuned to its own WDM line, modulates many channels with no separate multiplexer, which is why microring modulators are the default in optical I/O chiplets and many co-packaged optics engines. The resonance itself can be explored in the ring resonator explorer.

References: Q. Xu, B. Schmidt, S. Pradhan, M. Lipson, Nature 435, 325 (2005); W. Bogaerts et al., Laser Photonics Rev. 6, 47 (2012).