Silicon Photonics: Fundamentals and Devices

7.1 Introduction

Light sources like LEDs and lasers emit radiation continuously with a constant power in which one needs to control the output power of the sources. The most noteworthy example is the modulation of light in communications systems. The signals to be transmitted from one end of the fiber to the remote end are usually audio and video, which are basically analog signals. In modern telephony, the analog signals are converted into digital signals which are then used to modulate the light signal from the source. In most situations, on-off keying is employed, that is, the light signal is transmitted or blocked according to the bit pattern derived from the original analog signals. The total bit rate of the multiplexed signal is at least 10 Gb/s, whereas 40 Gb/s bit rate systems are not uncommon. In addition to digital modulation, analog modulation of light signals is also used in some niche applications. The prime requirement for the modulators in communication is the high speed of operation, preferably in the GHz range.

Another important area regarding control of light: either on-off switching or continuous change is in optical interconnects (ICs) between chips or within a single chip. The interconnection is the standard wiring using copper as the interconnecting metal. As the device dimension shrinks to produce higher and higher speeds for the ICs, the delay time arising from resistor–capacitator (RC) paths and the noise, both due to electrical interconnects, rise to a ...