Work in Progress
Refractive devices modulate by interfering a beam with itself in some way, either in a single-pass two-beam interference as in a Mach–Zehnder interferometer structure or in some device with multiple interference, such as a resonator. Changing the relative phase of the interfering beams by changing the refractive index changes the output power.
A basic difficulty with refractive modulators is that we have no high-speed mechanism that can usefully give us refractive index changes much larger than 10⁻³. Larger index changes can be induced, e.g., in semiconductors very near to their optical absorption edge, but then such large changes only occur in the presence of substantial absorption.
Even with Δn = 10⁻³, to get a half-wavelength path-length change at 1.5 μm wavelength would require a device length L ~ 750 microns.
There has been considerable interest in such two-beam interferometer approaches in silicon photonics. Because the main refractive index change mechanism available in silicon (free-carrier index change) is relatively weak, such devices necessarily take substantial energy on the scale of interest here. For example, Green et al. show 5 pJ/bit at 10 Gb/s in well-optimized 100–200-μm-long devices, much larger than our target numbers.
To make compact refractive modulators, we need to use resonators or possibly slow light to enhance the effect of changing the refractive index in only a smaller length of material. The silicon microring resonator has received much attention. Such devices might be able to achieve operating energies in the range of tens of femtojoules and could be very small, e.g., a few micrometers in diameter.
The resonators need quite large quality factor (Q) (e.g., > 10,000), however, meaning that they have very narrow resonances (e.g., 0.04 nm wavelength range) that have to be precisely tuned. Often that tuning is by temperature, and that temperature would have to be stabilized precisely also to hold the device on resonance (e.g., to a small fraction of a degree Kelvin based on the 2×10⁻⁴/K temperature dependence of silicon's refractive index). The width of the resonance is also so narrow that it is one of the limits on modulation speed, though modulation above 10 Gb/s is quite possible.
An important point in the energy per bit is that the thermal tuning power must be included in estimating the total system power requirement. The required tuning power is not yet clear, but a hypothetical tuning power of 1 mW for a 10 Gb/s modulator would correspond to an additional effective 100 fJ/bit, which would take the energy out of our target range. Such devices do, however, have the advantage that they are automatically also wavelength filters and can perform WDM switching functions as well.
Related: Electroabsorptive devices