Modulating light with low-voltage circuits
Transmitting information using light is more energy efficient than using electrical signals. Unfortunately, for small-scale applications, light-based encoding through electro-optic modulators (EOMs) requires a trade-off between performance and compact size. Electronics, on the other hand, are already small enough to fit on a chip. Now Marko Lončar at Harvard University and his colleagues have found a way to get the best of both worlds, with new integrated EOMs that can be powered by a low-voltage CMOS circuit.
The waveguides for light in the new EOMs are made of lithium niobate (LN) crystals whose refractive index changes when a voltage is applied. That electro-optic effect also causes a voltage-dependent phase shift in light that passes through the crystal. Light entering the EOM is divided into two paths of equal length, as shown on the chip in the figure. By delaying the phase of one path while advancing the phase of the other, the researchers control whether the light interferes constructively or destructively when it recombines. That interference is what translates an input voltage modulation into an output amplitude modulation in the light.
Several innovations lie behind the Harvard group’s device. Directly patterning the optical paths onto a LN substrate supported by glass and silicon improved the confinement of light as compared with previous thin-film LN devices. The researchers’ EOM is therefore more efficient at modulating light, so it can be made smaller (10–20 mm long) and operate at lower voltages (less than 1 V). Keeping the devices small also gives them a larger bandwidth (greater than 45 GHz), which increases the speed at which they transmit information. Directly powered by a CMOS circuit, the EOM transferred data at 70 gigabits per second with an imperceptibly low error rate. Its performance demonstrates a bandwidth increase compared with other thin-film LN devices that are twice the size and use more than twice the voltage. When the researchers used a higher voltage and encoded multiple bits in each optical pulse, the data transfer rate was even faster, up to 210 gigabits per second. (C. Wang et al., Nature, 2018, doi:10.1038/s41586-018-0551-y .)