Optical modulators are classified based on the modulated light parameter, material properties, and modulation technique, including amplitude, phase, polarization, absorptive, refractive, and electro-optic types.

Classification by Modulated Light Parameter

Optical modulators can be categorized according to the property of light they modify :

• Amplitude Modulators (Intensity Modulators): Control the intensity of the light beam.
• Phase Modulators: Alter the phase of the light wave, which can be converted to amplitude modulation using interferometers.
• Polarization Modulators: Change the polarization state of light.
• Frequency Modulators: Vary the frequency of the light wave.
• Spatial Modulators: Modify the spatial distribution of light, including spatial light modulators (SLMs).
• Diffraction Modulators: Control diffraction patterns for beam shaping or switching.

Classification by Material Properties

Based on the material response, modulators are divided into :

• Absorptive Modulators: Modulate light by changing the absorption coefficient of the material. Effects used include the Franz–Keldysh effect, Quantum-confined Stark effect, excitonic absorption, Fermi level changes, or free carrier concentration changes. Electro-absorptive modulators combine multiple effects.
• Refractive Modulators: Modulate light by changing the refractive index of the material. Common effects include electro-optic, acousto-optic, and magneto-optic effects, as well as liquid crystal polarization changes.

Classification by Modulation Technique

Optical modulators can also be classified by how the modulation is applied :

• Direct Modulation: Modulation is applied directly to the light source, such as a laser diode or LED.
• External Modulation: A separate modulator is used to control the light beam, often necessary for high-speed or narrow-linewidth applications.

Electro-Optic Modulators (EOMs)

A special class of refractive modulators, EOMs, use the electro-optic effect to control phase, amplitude, or polarization of light . They typically employ nonlinear crystals like lithium niobate, where the refractive index changes in response to an applied electric field. EOMs can operate at high frequencies, often in the gigahertz range, and are widely used in telecommunications, laser frequency stabilization, and high-speed pulse switching.

Additional Considerations

• Analog vs. Digital Modulation: Modulators can encode information in continuous (analog) or discrete (digital) forms.
• Spatial Light Modulators (SLMs): These modulators control light in two dimensions and are used in imaging, holography, and beam shaping applications. In summary, optical modulators are classified by the light property they affect, the material effect used, and the modulation method, with key types including amplitude, phase, polarization, absorptive, refractive, and electro-optic modulators, each suited for specific optical and photonic applications .