A transmitter is a device that can transmit a signal at a certain frequency. It is a relatively general concept. It is widely used in various civil and military equipment such as television, broadcasting, communication, alarm, radar, remote control, telemetry, and electronic countermeasures. Transmitters can be divided into four categories: frequency modulation (FM), amplitude modulation (AM), phase modulation (PM) and pulse modulation according to modulation methods. They are divided into analog and digital. Generally, the transmitter includes three parts: a high frequency part, a low frequency part, and a power supply part. The high frequency part generally includes the main oscillator, buffer amplifier, frequency multiplier, intermediate amplifier, power amplifier drive stage and final stage power amplifier. The main vibrator’s role is to generate a carrier with stable frequency. In order to improve the frequency stability, the main oscillator stage often uses a quartz crystal oscillator, and a buffer stage is added after it to weaken the influence of the latter stage on the main oscillator. The low-frequency part includes a microphone, a low-frequency voltage amplifier stage, a low-frequency power amplifier stage, and a final low-frequency power amplifier stage. The low-frequency signal is gradually amplified to obtain the required power level at the final stage power amplifier to modulate the high-frequency final stage power amplifier. Therefore, the final low-frequency power amplifier stage is also called a modulator. Modulation is the process of loading the information to be transmitted onto a certain high-frequency oscillation (carrier frequency) signal. So the final high-frequency power amplifier stage becomes a modulated amplifier.
We all know that the processing of information is done in the field of electricity. In optical fiber communication, we must convert electrical signals into optical signals so that they can be propagated on optical fibers. In the optical fiber communication system, the information is carried by the light waves emitted by the LED or LD. The light wave is the carrier wave, and the process of loading the information on the light wave is modulation. An optical modulator is a device that realizes the conversion from an electrical signal to an optical signal.
Modulation methods are usually divided into two categories, namely analog modulation and digital modulation.
There are two types of analog modulation. One is to use analog baseband signal to directly modulate the intensity of the light source (D-IM); the other uses continuous or pulsed radio frequency (RF) waves as the subcarrier, and the analog baseband signal first modulates its amplitude. , Frequency or phase, etc., and then use the modulated subcarrier to intensity modulate the light source. The advantage of analog modulation is that the equipment is simple and the occupied bandwidth is narrow, but its anti-interference performance is poor, and noise accumulates when relaying.
Digital modulation is the main modulation method of optical fiber communication. After the analog signal is sampled and quantized, the optical carrier is modulated on and off with a binary digital signal “1” or “0”, and pulse coding (PCM) is performed. The advantage of digital modulation is that it has strong anti-interference ability, and the influence of noise and chromatic dispersion does not accumulate when relaying, so long-distance transmission can be realized. Its disadvantage is that it requires a wider frequency band and the equipment is complicated.
According to the relationship between the modulation mode and the light source, there are two types: direct modulation and external modulation. The former refers to the direct use of electrical modulation signals to control the oscillation parameters (light intensity, frequency, etc.) of the semiconductor light source to obtain optical frequency amplitude modulation waves or frequency modulation waves. This modulation is also called internal modulation; the latter is the amplitude and frequency of the light source output The constant optical carrier passes through the optical modulator, and the optical signal modulates the amplitude, frequency and phase of the optical carrier through the modulator. The advantage of direct modulation of the light source is simple, but the modulation rate is affected by the carrier lifetime and high rate. Limitations of performance degradation (such as frequency chirp, etc.). The external modulation method requires a modulator and has a complex structure, but can obtain excellent modulation performance, which is especially suitable for high-speed applications.
In OOK, which only considers the light amplitude as the information carrier, by using the phase of the light wave to encode data, new possibilities can be realized, and this method will also bring certain technical challenges.
When transmitting OOK signals, only a laser source directly modulated by electrical signals is needed. The resulting optical signal has a binary intensity. But if the phase needs to be modulated, such simple and low-cost methods are difficult to implement.
Using electro-optic effect to control the phase of optical signal
The good news is that despite the increasing complexity of transmitters, we don’t need to worry about dispersion compensation for network transmission. The signal processing algorithm at the receiving end can manage the dispersion loss, so the dispersion compensation module is no longer needed, and the cost of building a new optical network is also significantly reduced.
When constructing a phase modulator, we can take advantage of the “electro-optical effect” in which the refractive index n of a specific crystal (such as lithium niobate) is affected by the strength of the local electric field. This phenomenon is called the “electro-optical effect”.
How does this effect help phase modulation? Assuming that n is related to the field strength, then the propagation speed and wavelength of light in the crystal are also related to the field strength. Therefore, if a voltage is applied to the crystal, the wavelength of the light passing through the crystal is reduced, and the phase of the emitted light can be controlled by selecting an appropriate voltage

 

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