TV Optical Transmitter Light Modulation Analysis

- Nov 25, 2014 -

We all know that the processing of information in the field of electricity is done in the optical fiber communication, we have to convert electrical signals into optical signals, so as to spread on the fiber. In optical fiber communication systems, the information light wave emitted by the LED or LD carried, the carrier wave is, to load the information into the process on the light wave is modulated. Optical modulator is converted from an electrical signal to optical signal devices to achieve.

Modulation is usually divided into two categories, namely analog and digital modulation.

     Analog modulation there are two types, one is the light source intensity modulation (D-IM) analog baseband signals directly; the other uses a continuous or pulsed radio frequency (RF) waves as a subcarrier, its first analog baseband signal amplitude , frequency or phase modulation, etc., and then the subject subcarrier modulation to intensity modulation source. Advantages of analog modulation is simple equipment, possession of a narrow bandwidth, but the poor performance of its interference noise when the cumulative relay.

Digital modulation is a modulation scheme main optical fiber communication, after sampling the analog signal quantized to binary digital signal "1" or "0" on-off modulated optical carrier, and pulse code (PCM). Advantages of digital modulation is the anti-interference ability, affect the noise and dispersion of the relay does not accumulate, so you can achieve long distance transmission, its disadvantage is the need wider frequency band, the device is also complex.

     Press relations modulation of the light source to points, there are two kinds of direct modulation and external modulation. The former refers to the direct electrical modulation signal to control the oscillation parameters of semiconductor light source (light intensity, frequency, etc.), to obtain the optical frequency of the AM or FM wave wave, this modulation called internal modulation, which is also known as light directly modulated transmitter optical transmitter; which is to allow light output equal amplitude and frequency constant optical carrier by an optical modulator, an optical signal through the optical modulator to achieve the amplitude, frequency and phase of the carrier is modulated and other advantages of the direct modulation of the light source is simple However, the modulation rate is limited (such as frequency chirp, etc.) degradation of carrier lifetime and high-speed rates. Require external modulation modulator, complex structure, but the availability of excellent modulation performance, especially suitable for use at high speed, this device is also known as externally modulated optical transmitter.

     Press modulated optical wave parameters point: intensity modulation, phase modulation, polarization modulation.

     Currently the most widely used in optical fiber communication is the base of the light source with direct intensity modulation, subcarrier intensity modulation and digital modulation, external modulation at high rates.

     A light source control circuit

     System requirements are high for the light source, comprising:

     1. Wavelength stability requirements: WDM light emission wavelength stability of the system has a high requirement, the wavelength shift will result in crosstalk between the channels.

     2. Power stability requirements: a channel power drift, not only affects the transmission performance of the channel, and through transient effects EDFA impact the performance of other channels.

A light source control circuit includes a temperature control and power control circuit, their role is to eliminate temperature variations and the effects of aging devices, transmitter stability performance. Other sources also slow start control circuit protection circuit, laser reverse surge current protection circuit, over-current protection circuit and laser laser shutdown circuit.

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