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Direct modulation vs External modulation | Difference between Direct modulation and External modulation

This page compares direct modulation vs external modulation and mentions difference between direct modulation and external modulation types. The direct and external modulations are used in optical domain mainly in LED and Laser devices.

These are two ways electrical data can be converted into optical form. These methods are described as follows.

Direct Modulation

optical direct modulation

The figure-1 depicts Laser diode used as direct modulation. In this modulation type, output power of device depends directly on input drive current. This means light is emitted from the device when "1 (binary one)" is being transmitted and no light is emitted when "0 (binary zero) " is being transmitted.

direct modulation input/output characteristic

Figure-2 shows electrical input vs optical output characteristic for direct modulation method. Both the Laser and LED devices can be directly modulated using digital as well as analog signals.

Advantages: This optical modulation is simple. It is cheaper as no complex circuitry is involved during modulation process.
Disadvantages: This method is slower compare to indirect or external modulation type. It can be used below 3 GHz.

External Modulation

optical external modulation

The figure-3 depicts Laser diode used as external modulation. In this method, external device is incorporated to modulate the intensity/phase of the light source. Here light source is kept ON and external modulator is used which functions as switch/shutter. This switch is controlled by information to be transmitted.

external modulation input/output characteristic

Figure-4 shows electrical input vs optical output characteristic for external modulation method. As shown, CW laser diode is emitting light whose power vs time graph is constant. The second component i.e. switch emits light when input data is binary "1" and blocks light when input data is binary "0".

Advantages: It is much faster in processing. It can be used with high power laser devices. It can be employed in high speed applications e.g. long haul telecom or cable TV head ends.
Disadvantages: It is more expensive. High frequency RF modulation circuit is required for operation which usually will be complex.

External modulation can be accomplished using integrated optical modulator which uses waveguide Mach-Zehnder interferometer.

Optical Components

Optical Isolator
Optical Circulator
Optical Amplifier
Optical Filter
Optical Coupler
Optical Equalizer
Optical Switch
Wavelength converter

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