Optical modulation is a process of modifying light waves according to high-frequency electrical signals that contain information. The transmission of the modified light wave is done through an optical fiber cable.
What is Optical Modulation?
The process of modifying information in the form of high-frequency electric signals into light signals is referred to as optical modulation.
Optical modulation is one of the important aspects of optical communication that allows the transmission of data and information through optical communication channels like optical fiber in the form of light signals.
Optical modulation typically involves modification of the properties like intensity, phase, and frequency of light signals. For this, various types of optical modulation technologies are used, such as amplitude modulation (to modify intensity), phase modulation, and frequency modulation.
In an optical communication system, optical modulation is one of the essential processes to improve the efficiency and reliability of data and information transmission over long distances.
Methods of Optical Modulation
There are two different methods available for optical modulation namely, direct modulation and indirect modulation. Let us discuss each of these methods in more detail.
1. Direct Optical Modulation
In the direct modulation technique, the information that has to be transmitted is directly modulated by the light transmitter and then the modulated signal is placed over a light stream for transmission. The direct modulation technique is commonly used in fiber-optic communication.
In direct modulation, a driving electric current is supplied to the light source (laser diode). The light signal produced by the laser diode can be varied by changing the input electric signal, which is basically a data signal. Therefore, the varying input signal changes the output optical power of the laser diode and allows the transmission of information. Direct modulation is a comparatively simpler modulation method. It is also less expensive due to no need for additional modulation components. However, it has some limitations, such as lower data transfer rates, and shorter transmission distances.
2. External Optical Modulation
External modulation is another approach for performing modulation. This method of modulation utilizes a separate optical modulator to modify the properties of a light signal.
In external optical modulation, the laser diode produces an unmodulated optical signal that is modulated by using external modulation components. In this method, an electric signal is used to control an optical modulator that modulates the light signal generated by the laser diode. This modulated light signal carries the information.
External optical modulation technique is widely used due to its advantages, such as higher data transfer rates, better transmission efficiency, and performance. Although, this method increases the complexity of the system to additional modulation components. This also results in increased cost of the system.
Types of Optical Modulators
External optical modulation is performed by utilizing a separate optical modulator. The two most commonly used types of optical modulators are namely, Electro-Optical Phase Modulators and Electro-Absorption Modulators. Let’s discuss these two types of optical modulators in detail.
1. Electro-Optical Phase Modulator
As the name implies, the electro-optical phase modulator changes the phase of the modulating optical signal in accordance with the variations in the applied electric field.
The electro-optical phase modulator has lithium niobate (LiNbO3) as its key active material. This material can exhibit an electro-optic effect on the application of an electric field. When lithium niobate is subjected to an electric field, its refractive index gets changes which result in the phase modulation of the light signal passing through it.
In the case of an electro-optical phase modulator, the optical signal enters the modulator through a beam splitter. The beam splitter splits the light signal into two halves and allows them to travel through two different paths.
The applied electric signal to the modulator changes the refractive index of the lithium niobate path, resulting in a change in the phase of one-half of the light signal. Then, both halves of the light signal are recombined through a beam combiner and transmitted through the optical communication channel. This is how an electro-optical phase modulator works.
Electro-optical phase modulator is mainly used in applications where high speed and performance in optical modulation are crucial.
2. Electro-Absorption Modulator
Electro-absorption modulator is another type of optical modulator used as an external modulator and modulates the intensity of the optical signal. The modulation of intensity depends upon the electric field applied to the modulator.
The working of electro-absorption modulators depends on the electro-absorption effect exhibited by specific semiconductor materials. The applied electric field to the semiconductor material changes its energy bandgap which causes variations in the absorption properties of the semiconductor material.
Thus, any variation in the electric field can modulate the intensity of the light signal that has to be transmitted. Electro-absorption modulator is mainly used in high-speed data transmission where fast response time and low insertion losses are desired.
Advantages of Optical Modulation
Optical modulation results in several advantages in optical communication systems. Some key advantages of optical modulation are listed below:
- Allows for transmission of data and information over long distances.
- Increases the data transfer rates that allow high-speed transmission of information.
- Helps in increasing the bandwidths capacity.
- Improves the energy efficiency of the optical communication system.
Optical modulation is an important aspect of an optical communication system. This improves the overall performance of the data transmission system. With the help of optical modulation, we can transmit information over long distances more efficiently and reliably.