Photoelectric modulation principle

When an electric field is applied to a crystal or liquid, the refractive index of the crystal or liquid changes, which is called electro-optic effect. Electro-optic effect has many important applications in engineering technology and scientific research. It has a short response time and can be used as a shutter in high-speed photography or a beam chopper in light speed measurement. After the appearance of laser, the research and application of electro-optic effect have developed rapidly, and electro-optic devices are widely used in laser communication, laser ranging, laser display and optical data processing. The electro-optic modulation system proposed in this paper is based on the electro-optic effect of crystal to verify the electro-optic modulation principle.

Electro-optic modulation principle of 1

Electro-optic modulation works by using the electro-optic effect that the refractive index of some crystal materials changes under the action of external electric field. According to the direction of electric field applied to the crystal and the direction of light beam propagation in the crystal, it can be divided into longitudinal modulation and transverse modulation. The direction of electric field is parallel to the propagation direction of light, which is called longitudinal electro-optic modulation; The direction of electric field is perpendicular to the direction of light propagation, which is called transverse electro-optic modulation. The advantages of transverse electro-optic modulation are low half-wave voltage, low driving power and wide application. This electro-optical modulation system takes the transverse modulation of lithium niobate crystal as an example. Figure 1 is a schematic diagram of transverse electro-optical modulation.

The electric field is applied along the z direction, and the light passing direction is along the Y' direction of the induction spindle. After passing through the polarizer, the included angle between the vibration direction of light and the Z axis is 45. After light enters the crystal, it will be decomposed into two components that vibrate in the x' and z directions, and the difference between the refractive indices of the two components is. Assuming that the length of the crystal in the light passing direction is L and the thickness is D (that is, the distance between the two poles), when the applied voltage is V=Ezd, the phase difference between the two beams of light emitted by the crystal is:

From the formula (1), it can be seen that as long as the crystal and transmission wavelength λ are determined, the magnitude of phase difference △φ depends on the applied voltage V, and changing the applied voltage V can make the phase difference △φ change in direct proportion to the voltage V. One of the main characteristics of commonly used electro-optic crystals is to use a half-wave voltmeter to characterize them (when the phase difference between two light waves is π radians, the required applied voltage is called a half-wave voltage).

2 overall design of electro-optic modulation system

This electro-optic modulation system is designed based on the electro-optic modulation principle, which is used to study the physical process of the interaction between electric field and light field, and is also suitable for the experimental research of optical communication and physics. The structure of the electro-optic modulation system is shown in Figure 2.

2. 1 working principle

The laser power supply provides voltage for the normal operation of the laser to ensure the stable operation of the laser. The laser generated by the laser is linearly polarized after passing through the polarizer. When linearly polarized light passes through the electro-optic crystal, a voltage is applied to the electro-optic crystal, and this voltage is the signal to be modulated. When a voltage is applied to the electro-optic crystal, the refractive index and optical properties of the crystal change, changing the polarization state of light waves, and linearly polarized light becomes elliptically polarized light. In order to select a suitable modulation working point, a λ/4 wave plate is inserted behind the electro-optic crystal to delay the phase of two beams of light passing through the electro-optic crystal by π/2, and the modulator works in the linear part. The analyzer detects the polarization direction of the output light. Finally, the modulated optical signal is detected by photoelectric detector, and converted into electrical signal to be observed by oscilloscope.

2.2 Laser and laser power supply

In this system, the laser is He-Ne laser. He-Ne laser tube is a special gas discharge light source. Compared with other light sources, it has excellent monochromaticity, high coherence and strong directivity (small divergence angle). The laser power supply firstly raises the input voltage of 220 V to 1 000 V through the transformer, then raises the voltage to about 5 000 V through the voltage doubling circuit, and then directly supplies power to the laser tube through the current limiting resistor. When the power switch is turned on, the gas in the laser tube has not been ionized, and the internal resistance is equivalent to infinity. At this time, the power supply outputs a high voltage of about 5 000 V, which is the ignition voltage of the laser tube, which ionizes the gas in the laser tube, and the laser tube starts to work, and the resistance of the laser tube will be greatly reduced. That is to say, with the increase of load current, the output voltage of laser power supply will also decrease.

2.3 Lithium Niobium Electro-optic Crystal

Lithium niobate crystal has excellent piezoelectric, electro-optic, acousto-optic and nonlinear properties. LN electro-optic crystal is used in this system. LN crystal is a cubic crystal, n 1=n2=no, n3=ne.

Before applying the electric field, the refractive index ellipsoid of LN is:

In this system, the Y-axis light is transmitted and the Z-axis electric field is applied, that is, E 1=E2=0 and e3 = e. Then, after the electric field is applied, the refractive index ellipsoid is:

Equation (4) shows that when an electric field is applied to the LN crystal along the z axis, a transverse electro-optical effect can be generated, but a longitudinal electro-optical effect cannot be generated.

After passing through the crystal, the phase difference between O light and E light is:

2.4 signal source

The structure of the signal source system is shown in Figure 3. The signal source provides modulation voltage for the electro-optic crystal, which enables the system to access audio signals. The power supply part can output several DC stabilized power supplies at the same time and supply power to each module of the signal source at the same time; The signal generation module generates sine wave and square wave with continuously adjustable frequency and amplitude; The power amplification module amplifies the input sine wave, square wave and audio signal to tens of volts, and then adds them to the electro-optic crystal to modulate the laser passing through the electro-optic crystal; The demodulation module demodulates and amplifies the weak signal input by the detector, drives and amplifies the input weak audio signal, and then releases the sound through the sound box; The bias high voltage module generates DC high voltage with continuously adjustable amplitude instead of λ/4 wave plate as the half-wave voltage of the modulation crystal.

Application of electro-optic modulation in optical communication

In this system, sound information is transmitted by light waves. The laser generated by the laser becomes linearly polarized light after passing through the polarizer, and then becomes circularly polarized light through the λ/4 wave plate, so that the two polarized components (O light and E light) have a π/2 phase difference before entering the electro-optic crystal, and the modulator works in an approximate linear region. When the laser passes through the electro-optic crystal, an external voltage is applied to the electro-optic crystal, which is the sound signal to be transmitted. When a voltage is applied to the electro-optic crystal, the refractive index and other optical properties of the crystal change, thus changing the polarization state of light waves. Therefore, circularly polarized light becomes elliptically polarized light, and then becomes linearly polarized light through the analyzer, and the light intensity is modulated. At this time, light waves carry sound information and spread in free space. At the receiving end, the modulated optical signal is received by the photodetector, and then the circuit is converted into an electrical signal, and the sound signal is restored by the demodulator, and finally the optical transmission of the sound signal is completed. The applied voltage is a transmitted sound signal, which can be the output of a tape recorder or the output of a tape recorder. In fact, it is a voltage signal that changes with time.

4 conclusion

Through the above electro-optical modulation system, it is verified that electro-optical modulation technology is feasible for laser communication. This communication mode has the advantages of fast transmission speed, strong anti-interference ability, good confidentiality, simple structure, low cost and easy implementation.