Introduction to Optical Chips

Optical chip is a chip in the optical module that completes the conversion of photoelectric signals. It is divided into laser chip and detector chip.

The laser chip emits light based on the principle of stimulated radiation of laser. According to the type of light emission, it is divided into surface emission and edge emission: the surface emission type is mainly VCSEL (vertical cavity surface emitting laser), which is suitable for short-distance multi-mode scenarios.

The main types of edge emission are FP (Fabry-Perot laser), DFB (distributed feedback laser) and EML (electroabsorption modulation laser). FP is suitable for short and medium distance scenarios below 10G, and DFB and EML are suitable for medium and long distance scenarios. High rate scenario.

EML adds an electro absorption modulator (EAM) as an external modulator on the basis of DFB, and is currently the main light source to achieve single-channel rates of 50G and above.

There are two main types of detector chips: PIN (PN diode detector) and APD (Avalanche Diode Detector). The former has relatively low sensitivity and is used for medium and short distances, while the latter has high sensitivity and is used for medium and long distances.

On the one hand, electronic chips provide support for the operation of optical chips, such as LD (laser driver), TIA (cross impedance amplifier), CDR (clock and data recovery circuit). On the other hand, they achieve power regulation of electrical signals, such as MA (main amplifier). On the other hand, they also achieve complex digital signal processing, such as modulation, coherent signal control, serial parallel/parallel serial conversion, etc. Some optical modules also have DDM (digital diagnostic function), with corresponding MCU and EEPROM. Electric chips are usually used together, and mainstream chip manufacturers generally launch chip products targeting a certain model of optical module.

At the transmitting end, the electrical signal is modulated internally or externally through signal processing chips such as CDR and LD, driving the laser chip to complete electro-optical conversion; At the receiving end, the optical signal is converted into electrical pulses through the detector chip, and then modulated by power processing chips such as TIA and MA, ultimately outputting a continuous electrical signal that the terminal can process. The combination of optical and electrical chips achieves the main performance indicators such as transmission rate, extinction ratio, and emission power, and is the most important device that determines the performance of optical modules. The main performance indicators of optical modules can be measured through eye graph analysis, including amplitude stability, inter symbol interference, extinction ratio, jitter overshoot, and noise.

Optical chips mainly process the conversion between optical signals and electrical signals, while electrical chips mainly support optical chips, regulate electrical signal power, and process complex digital signals.

At present, the mainstream optical chips are DFB (distributed feedback laser chip), DML (direct modulation laser chip), EML (electric absorption modulation laser chip), VCSEL (vertical cavity surface emitting laser chip), and so on.

DFB lasers are suitable for medium to long-distance communication. DFB is based on FP and is currently the most commonly used direct modulation laser, mainly used in 1310nm and 1550nm band data communication, widely used in data centers, metropolitan area networks, and access networks.

EML is an integrated laser chip between DFB and EAM (Electric Absorption Modulator). Compared with directly modulated DFB lasers, EML has transmission advantages such as high power, narrow linewidth, and wide wavelength tuning range.

Compared to EML, DML has the advantages of small size, low cost, and low power consumption. Based on this, DML is more suitable for data center applications, while EML is suitable for telecom level applications.

VCSEL is mainly used in the communication field for 850nm band data transmission, and is widely used in data centers and access networks. With the breakthrough of VCSEL in the application of 3D sensing on Apple smartphones, VCSEL is expected to be widely used in emerging fields such as consumer electronics, industry, automobiles, and healthcare in the future.

Optical module chips have extremely high technical barriers and complex process flows, making them the largest part of the BOM cost structure of optical modules. The cost proportion of optical chips is usually between 40% -60%, while the cost proportion of electrical chips is usually between 10% -30%. The higher the speed and high-end of optical modules, the higher the cost proportion of electrical chips. However, the advantage of scale can increase the bargaining power of procurement.