Little knowledge (forty-five) principles and methods lightwave modulation and demodulation

The concept of modulation of light waves

The information to be transmitted to the loading process of the light waves propagating in an optical information, i.e., an optical communication

Lightwave modulation principle

The method of optical modulation are divided into the direct modulation, the modulation chamber and the cavity modulation three.

Direct modulation method

The modulation scheme applied to the semiconductor light sources, this information should be transmitted is converted into a direct current signal input to the laser to obtain a modulated optical signal.

Features: Simple, economical, can be used for intensity modulation.

Intracavity modulation (see Baidu Encyclopedia: light modulation)

By changing the parameters (such as gain, the laser resonator Q value of the optical path, or the like) is achieved, mainly for Q switching, empty test chambers clamping techniques. Modulation chamber is divided into passive and active categories. Active Modulation: a modulation including mechanical, electro-optic modulators, magneto-optic and acousto-optic modulator modulation.

External cavity modulation method

This light modulator is separate from the modulation method, so called external modulation. Input signal source and an external modulator at the same time, the electrical signal to be transmitted as a modulated signal is applied to the external modulator.

Features: complex structure, it is superior to direct modulation. It can be used for intensity modulation, phase modulation and frequency

 Demodulation method: direct detection and coherent detection

Direct detection

Is an optical signal directly intensity-modulated by the envelope detection , that is intensity-modulated signal directly from the light detector can recover the original signal

Coherent detection

 Photo signal detecting intensity, phase and frequency modulation. Coherent heterodyne detection can be divided into detection and homodyne detection.

Heterodyne detection: optical signal entering the optical receiver prior to be mixed with the local oscillator laser receiving side, generating a local oscillator frequency equal to the intermediate frequency components of the difference between the laser frequency and the frequency of the original light source.

 

 

Homodyne detection: the signal light and the local light are equal frequency, then the RF signal recovered by the photodetector directly.