Light modulators are a key component of high-speed, short-range optical communications and one of the most important integrated optics. According to its modulation principle, light modulators can be divided into electro-optic, thermo-optical, acousto-optic, and all-optical. The basic theories are based on various forms of electro-optical effects, acousto-optic effects, magneto-optical effects, and Franz- Keldysh effect, quantum well Stark effect, carrier dispersion effect, etc.
The electro-optic modulator is a device that finally regulates the refractive index, absorptance, amplitude or phase of the output light by changes in voltage or electric field, and is superior to other types of modulators in terms of loss, power consumption, speed, integration, and the like. It is also the most widely used modulator. In the process of light emission, transmission and reception of the overall optical communication, the light modulator is used to control the intensity of the light, and its role is very important.
The purpose of optical modulation is to perform a form transformation including "de-background signal, denoising, anti-interference" on the desired signal or transmitted information, thereby making it easy to process, transmit and detect.
The modulation types can be divided into two broad categories based on where the information is loaded onto the lightwave:
One is to use an electrical signal to modulate the drive 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. Referred to as: internal modulation and external modulation.
According to the modulation method, the modulation type has:
1) intensity modulation;
2) phase modulation;
3) polarization modulation;
4) Frequency and wavelength modulation.
1.1, intensity modulation
The light intensity modulation uses the intensity of light as a modulation object, and uses external factors to convert the DC or slowly changing optical signal to be measured into an optical signal that changes at a relatively fast frequency. Thus, an AC frequency selective amplifier can be used for amplification. The amount to be measured is then continuously measured.
1.2, phase modulation
The principle of changing the phase of a light wave by external factors and measuring the physical quantity by detecting a 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 phase change of the light wave and realize phase modulation.
Since the photodetector generally cannot perceive the change of the phase of the light wave, it is necessary to use the interference technique of light to convert the phase change into the change of the light intensity, so as to realize the detection of the external physical quantity. Therefore, the optical phase modulation should include two parts: one is to generate the light wave. The physical mechanism of phase change; the second is the interference of light.
1.3, polarization modulation
The simplest way to achieve light modulation by rotating the vibrating surface of the polarized light is to use two polarizers to rotate relative to each other. According to the Marlus theorem, the output light intensity is I=I0cos2α.
Where: I0 represents the intensity of light passing through the principal planes of the two polarizers; 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 by detecting the frequency of the light or the wavelength of the light by external factors, such as the frequency of the light and the wavelength modulation.
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