Features of Optical Modulators in Telecommunication Systems

Light emitting PN junction devices such as LEDs and laser diodes are easily modulated by superimposing signals on to the injected current. This is direct modulation. Laser diodes in high-bit rate and long-span optical communication systems are often used under direct modulation. The downside of direct modulation is that, it results in chirping which limits the transmission quality because of dispersion in optical fibres. An optical modulator can modulate the light output with little or no chirping.

Optical modulation is basically performed by modulating the refractive index or optical absorption coefficient of the modulators. The devices which employ refractive index phenomenon for modulation are termed to as phase modulation type devices whereas those that use optical absorption coefficient phenomenon are called intensity modulation type devices.

We have two types of optical modulators:

• Semiconductor optical modulators.
• Optical modulators composed of dielectric materials such as lithium nitrate (LiNO₃).

Semiconductor optical modulators are PN junction diodes and they can further subdivided into two types:

• Devices used under forward bias (LEDs and laser diodes). The optical modulation in these devices is carried out by changing gain or loss within the modulators.
• Devices used under reverse bias (here photodiodes are used). Most high performance semiconductor optical modulators are used under reverse bias. The reverse bias is required to generate strong electric field.

We have a number of different types of optical modulators in use today. However, the waveguide type optical modulator is the most commonly used. Further there are several different waveguide type optical modulator structures. The figure below shows a mesa type optical modulator structure:

Also Read: What is the difference between Broadband and Baseband Transmission?