Modulation Index Boosting -- Overcoming the Electrooptic Modulation Bottleneck
Electronic systems are dominant in information processing, display, and storage. However, optical systems are needed for broad-band long distance communication, antenna remoting, filtering, sensing, and imaging. Conventional high-performance electro-optic modulators, which place electronic information on optical waves, are the bottleneck of optical communication links and photonic time-stretch analog to digital converters, suffering a trade-off between low-voltage operation and large microwave bandwidths.
Unfortunately, the development of EO modulators has not kept up with the tremendous pace of electronics over the past decades. To be sure, access to broadband optical channels is curtailed by the limited bandwidth of EO modulators which are currently available in the 35 GHz range. Furthermore, the required drive voltage is currently in the 2-5 V range, a value that increases with speed. While EO modulators with 100Gbps capacity (~70GHz analog bandwidth) are being developed, their bandwidth is approaching the fundamental limit imposed by velocity mismatch between electrical and optical waves in the traveling wave electrode. This is in direct conflict with the trend in electronics in which the voltage swing continues to decrease with increasing transistor speed. It is then clear that the current EO modulator technology is unable to meet the requirements of future systems.
This dissertation focuses on an exciting new concept that exploits modulation instability, a nonlinear optical process, to compensate for the intrinsically weak chi^(2) electro-optic effect. The technique, known as Modulation Index Boosting or MiBo, can be exploited to boost the weak electrooptic effect, one of the most fundamental and pressing predicaments in optical communication. In the approach, modulation sidebands stimulate MI in a third order chi^(3) nonlinear optical material placed after the electrooptic device. This effect enhances the of the electrooptic modulation material and also compensates for the device related high frequency roll-off. Boosting the chi^(2) of one material with the chi^(3)-induced MI of another material is an intriguing concept that enables low voltage electrooptic modulation at ultrahigh frequencies. Such a technology could provide a tremendous capability enhancement to high-speed digital, analog, SIGINT, and radar systems.