Introduce the silicon photonic Mach-Zende modulator MZM modulator
The Mach-zende modulator is the most important component at the transmitter end in 400G/800G silicon photonic modules. Currently, there are two types of modulators at the transmitter end of mass-produced silicon photonic modules: One type is the PAM4 modulator based on a single-channel 100Gbps working mode, which achieves 800Gbps data transmission through a 4-channel / 8-channel parallel approach and is mainly applied in data centers and Gpus. Of course, a single-channel 200Gbps silicon photonics Mach-Zeonde modulator that will compete with EML after mass production at 100Gbps should not be far away. The second type is the IQ modulator applied in long-distance coherent optical communication. The coherent sinking mentioned at the present stage refers to the transmission distance of optical modules ranging from thousands of kilometers in the metropolitan backbone network to ZR optical modules ranging from 80 to 120 kilometers, and even to LR optical modules ranging from 10 kilometers in the future.
The principle of high-speed silicon modulators can be divided into two parts: optics and electricity.
Optical part: The basic principle is a Mach-zeund interferometer. A beam of light passes through a 50-50 beam splitter and becomes two beams of light with equal energy, which continue to be transmitted in the two arms of the modulator. By phase control on one of the arms (that is, the refractive index of silicon is changed by a heater to alter the propagation speed of one arm), the final beam combination is carried out at the exit of both arms. Interference phase length (where the peaks of both arms reach simultaneously) and interference cancellation (where the phase difference is 90° and the peaks are opposite the troughs) can be achieved through interference, thereby modulating the light intensity (which can be understood as 1 and 0 in digital signals). This is a simple understanding and also a control method for the working point in practical work. For example, in data communication, we work at a point 3dB lower than the peak, and in coherent communication, we work at no light spot. However, this method of controlling the phase difference through heating and heat dissipation to control the output signal takes a very long time and simply cannot meet our requirement of transmitting 100Gpbs per second. Therefore, we have to find a way to achieve a faster rate of modulation.
The electrical section mainly consists of the PN junction section that needs to change the refractive index at high frequency, and the traveling wave electrode structure that matches the speed of the electrical signal and the optical signal. The principle of changing the refractive index is the plasma dispersion effect, also known as the free carrier dispersion effect. It refers to the physical effect that when the concentration of free carriers in a semiconductor material changes, the real and imaginary parts of the material’s own refractive index also change accordingly. When the carrier concentration in semiconductor materials increases, the absorption coefficient of the material increases while the real part of the refractive index decreases. Similarly, when the carriers in semiconductor materials decrease, the absorption coefficient decreases while the real part of the refractive index increases. With such an effect, in practical applications, the modulation of high-frequency signals can be achieved by regulating the number of carriers in the transmission waveguide. Eventually, 0 and 1 signals appear at the output position, loading high-speed electrical signals onto the amplitude of the light intensity. The way to achieve this is through the PN junction. The free carriers of pure silicon are very few, and the change in quantity is insufficient to meet the change in refractive index. Therefore, it is necessary to increase the carrier base in the transmission waveguide by doping silicon to achieve the change in refractive index, thereby achieving higher rate modulation.
Post time: May-12-2025