Internal vs. External Light Modulation: Advantages and Disadvantages

This article compares internal light modulation and external light modulation, highlighting their key differences, advantages, and disadvantages. Understanding these techniques is crucial for various applications, including fiber optic communications, optical switching, and Q-switched lasers.

Introduction:

Light modulation involves altering the irradiance (amplitude, phase, pulse width, etc.) or direction of light as it travels through a medium. Different light modulation techniques cater to diverse application needs.

Internal vs. External Modulation: A Comparison

• Internal Modulation: In this approach, the light output from sources like LEDs or lasers is directly controlled by injecting or driving current. It utilizes a simpler circuit for this purpose.

• External Modulation: This method modulates both the amplitude and phase of light from external sources. Processes like electro-optic, magneto-optic, or acousto-optic effects are employed. The polarization status changes due to the presence of external force fields (electric, magnetic, or acoustic). Consequently, the phase or irradiance of the light beam is modified.

optical direct modulation

optical external modulation

Advantages and Disadvantages of Internal Light Modulation

Benefits (Advantages):

• Uses a simple circuit for modulation.
• It’s a very cost-effective method.

Drawbacks (Disadvantages):

• Usable up to a frequency of about 40 GHz.
• Shifts the emission frequency by changing the driving current.
• Limited modulation, primarily achieved through amplitude variation.

Advantages and Disadvantages of External Light Modulation

Benefits (Advantages):

• Higher device speed, controlled by the modulator’s properties.
• Emission frequency remains unaffected, unlike internal modulation.
• Both amplitude and phase modulation are possible.

Drawbacks (Disadvantages):

• Larger modulator size compared to micro-electronics standards.