Optical modulators are key devices for high-speed, short-range optical communications and are one of the most important integrated optical devices. According to its modulation principle, optical modulators can be divided into electro-optic, thermo-optic, acousto-optic, all-optical, etc. The basic theory they are based on are various forms of electro-optic effects, acousto-optic effects, magneto-optic effects, and Franz- Keldysh effect, quantum well Stark effect, carrier dispersion effect, etc.
Among them, the electro-optic modulator is a device that ultimately regulates the refractive index, absorptivity, amplitude, or phase of the output light by a change in voltage or electric field. It is superior to other types of modulators in terms of loss, power consumption, speed, and integration. It is also the most widely used modulator. In the light transmission, transmission, and reception processes of the overall optical communication, the light modulator is used to control the intensity of light, and its role is very important.
The purpose of the optical modulation is to perform a formal transformation including "de-background signal, de-noise, and anti-jamming" on the desired signal or transmitted information, thereby making it easy to handle, transmit, and detect.
The basic principle of optical modulator
Depending on where the information is loaded onto the light waves, the modulation types can be divided into two broad categories:
One is to use electric signals to modulate the driving power of the light source; the other is to directly modulate the broadcast.
The former is mainly used for optical communication, and the latter is mainly used for optical sensing. Abbreviations are: inner modulation and outer modulation.
According to the modulation method, the modulation types are:
1) intensity modulation;
2) Phase modulation;
3) polarization modulation;
4) Frequency and wavelength modulation.
1.1, intensity modulation
Optical intensity modulation is based on the intensity of light as a modulation object, and uses external factors to convert the measured direct-current or slowly-changing optical signal into an optical signal that changes at a relatively fast frequency, so that it can be amplified by an AC frequency-selective amplifier. Then measure the amount to be measured continuously.
1.2, phase modulation
The principle of changing the phase of the light wave using external factors and measuring the physical quantity by detecting the phase change is called optical phase modulation.
The phase of the light wave is determined by parameters such as the physical length of the light propagation, the refractive index of the propagation medium, and its distribution. That is to say, changing the above parameters can produce a change in the phase of the light wave and achieve phase modulation.
Since optical detectors generally cannot perceive changes in the phase of optical waves, optical interferometry must be used to convert phase changes into changes in optical intensities in order to achieve detection of external physical quantities. Therefore, optical phase modulation should include two parts: First, generate optical waves. The physical mechanism of phase change; the second is light interference.
1.3, polarization modulation
The most simple method to achieve light modulation by rotating the polarized light vibration plane is to use two polarizers to rotate relative to each other. According to Marius' theorem, the output light intensity is I=I0cos2α.
Where: I0 represents the intensity of the light passing through when the principal planes of the two polarizers are identical; α represents the angle between the principal planes of the two polarizers.
1.4, frequency and wavelength modulation
The principle of measuring the physical quantity of the outside world by changing the frequency of light or the wavelength of light by an external factor and measuring the change of the frequency of light or the wavelength of light is referred to as frequency and wavelength modulation of light.
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