Direct Modulation vs. External Modulation: Optical Techniques Compared

This article compares direct modulation and external modulation, highlighting the differences between these two optical modulation techniques. Direct and external modulation are primarily used in the optical domain with LED and Laser devices as methods for converting electrical data into optical form.

Direct Modulation

Figure 1: Laser diode used for direct modulation.

In direct modulation, the output power of the device is directly dependent on the input drive current. This means the device emits light when a binary “1” is transmitted and emits no light when a binary “0” is transmitted.

Figure 2: Electrical input vs. optical output characteristic for direct modulation.

Both Laser and LED devices can be directly modulated using digital and analog signals.

Advantages:

• Simplicity: This optical modulation technique is straightforward.
• Cost-Effective: It is cheaper because it doesn’t require complex circuitry during modulation.

Disadvantages:

• Slower: This method is slower compared to external modulation.
• Frequency Limitation: Usable below 3 GHz.

External Modulation

Figure 3: Laser diode used for external modulation.

In external modulation, an external device is incorporated to modulate the intensity or phase of the light source. The light source is kept ON, and the external modulator functions as a switch or shutter. This switch is controlled by the information to be transmitted.

Figure 4: Electrical input vs. optical output characteristic for external modulation.

As shown, a Continuous Wave (CW) laser diode emits light with constant power over time. The external switch emits light when the input data is a binary “1” and blocks light when the input data is a binary “0”.

Advantages:

• Faster Processing: Significantly faster compared to direct modulation.
• High Power Compatibility: Can be used with high-power laser devices.
• High-Speed Applications: Suitable for high-speed applications, such as long-haul telecom or cable TV headends.

Disadvantages:

• Expensive: More costly compared to direct modulation.
• Complex Circuitry: Requires a high-frequency RF modulation circuit, which is typically complex.

External modulation can be achieved using an integrated optical modulator employing a waveguide Mach-Zehnder interferometer. It can also be accomplished through electro-optic modulation, magneto-optic modulation, and acoustic-optic modulation processes.