The form factors of 100G CFP optical modules can be divided into CFP/CFP2/CFP4, and they can be divided into 100GBASE-SR10, 100GBASE-LR4, and 100GBASE-ER4 according to the transmission distance. All optical module transmission distance is decided by the optical output signal OSNR tolerance and chromatic dispersion tolerance decision. Generally speaking, it is determined by the laser and its drive performance.
At present, there are two types of optical modulation methods for implementing CFP 100GBASE-LR4 transmission in the mainstream market. One is a Direct Modulation Laser Modulation (DML) mode, that is, a direct modulation laser. And another one is an Electlro -Modulation Modulation (EML) mode, that is, modulation laser.
The following post will briefly introduce some key technical points of using the TEC circuit and EML modulation mode to realize the 100G CFP / CFP2 LR4 optical transceivers.
1. TEC Circuit
EML lasers need to work at a certain wavelength, the temperature jump will lead to a temperature drift effect, resulting in wavelength instability, drifting, so the stable working conditions of the laser is a relatively stable operating temperature (steady wavelength) This requires that we have to provide a high-precision TEC circuit, that is, a semiconductor refrigerator. The TEC is actually a PN junction made of two semiconductors of different materials. When a direct current passes through the PN junction, electrons and gaps in the two materials The hole produces an endothermic or exothermic effect during movement across the PN junction, effecting a cooling or heating laser effect, and TEC heating or cooling can be controlled by changing the current direction and size.
A high-precision op amp compares the target temperature we set with the temperature fed back by the thermistor, and the error voltage is amplified by a high-gain amplifier while compensating for the network’s contribution to the hot and cold ends of the laser The phase delay is compensated to drive the H-bridge output to control the size and direction of the TEC current. When the temperature drops below our target temperature, the H-bridge will reduce the TEC current or change the direction of the TEC current for cooling. Conversely, the final control loop Road will reach a dynamic equilibrium, the temperature will stabilize. Among them, the compensation circuit is the most critical part of the TEC temperature control circuit, that is PID proportional integral differential adjustment compensation network, which determines the TEC controller response speed and regulation accuracy.
2. EAM Regulation
EML lasers are essentially integrated devices for EAM (Electroabsorption Modulator) and LD (DFB lasers). The key core is the EML chip, which is the core of an electro-absorption modulated laser based on the Stark Effect (QCSE) design. The DML laser modulates the light intensity by directly controlling the laser current, which always works in an unstable state and is easily influenced by the outside world. In contrast, the EML laser mainly controls the optical signal by controlling the EAM, the light source will be more stable, and the transmission Relatively speaking, the characteristics and transmission effects will be better, especially in high frequency modulation and long distance.
In application, the LD pin injects a constant current to the laser to make the laser emit light, and the EAM changes the ratio of the laser light to obtain different intensity of light. The development of Ethernet technology has gone through a number of development stages from low speed to high speed.
The rate of development from 1M, 10M, 100M and 1G to 10G and 100G has led to more demand for it to evolve to a higher rate. I believe there will be new technologies have emerged to meet the growing demand for optical modules.