Optical Communication is a technology that uses light waves as a transmission medium for information transmission. Usually, optical communication systems use light sources such as lasers or LEDs, and transmit large amounts of data at very high speeds through optical fibers as transmission media. Optical communication is widely used in long-distance communication, network connection of data centers, large-scale Internet transmission and other fields. The advantages of optical communication include high bandwidth, low loss, and strong anti-interference ability. 
Optical module is a key device in optical communication systems. Its function is to achieve the conversion between electrical signals and optical signals, as well as the modulation, demodulation, amplification, and transmission of optical signals. The transmitting end converts the electrical signal into a laser signal, and then modulates the laser beam sent by the laser, which is transmitted through the fiber. After receiving the laser signal, the receiving end converts it into an electrical signal, which is converted into information after modulation and demodulation. This product is widely used in scenarios such as 5G fronthaul, optical access networks, metropolitan area networks, and data centers, and is at the top of the pyramid in the field of optical communication. 
01 About "Optical Module" 
Optical modules are mainly used for the transmission, reception, and conversion of optical and electrical signals. Through them, seamless collaboration and connection with various types of devices can be achieved, including switches, routers, servers, and storage devices, with a wide range of applications. The basic principle of an optical module is to input an electrical signal with a certain code rate through the sending interface. After processing by the internal driving chip, the modulated optical signal with the corresponding code rate is emitted by the driving semiconductor laser (LD) or light emitting diode (LED). After transmission through optical fiber, the receiving interface converts the optical signal into an electrical signal through the light detection diode, and outputs the corresponding code rate electrical signal after passing through the preamplifier. 
The optical module mainly consists of a light emitter (TOSA), a light receiver (ROSA), a light source, a photodetector, a connector, and a housing. TOSA/ROSA optoelectronic devices are the core components of the optical module. 
TOSA (Transmit Optical Sub Assembly): mainly responsible for converting electrical signals into optical signals, consisting of a light source (LED or laser diode), optical interface, monitoring photodiode, metal or plastic casing, and electrical interface. Most light sources use laser diodes, which have low power consumption, high power output, and high coupling efficiency. ROSA (Receiver Optical Sub Assembly): Its main function is to convert the optical signal transmitted by TOSA into an electrical signal. ROSA includes photodiodes (PDs), optical interfaces, metal or plastic casings, and electrical interfaces. Laser, as the heart of the optical module, accounts for about 50% of the total cost of the optical module.   
The "laser" of the 02 optical module is a device that uses stimulated radiation to generate visible or invisible light. It has a complex structure and high technical barriers. It is a comprehensive system composed of a large number of optical materials and components, and occupies a central position in the entire laser industry chain. It mainly consists of four parts: optical system, power system, control system, and mechanical mechanism. The optical system is mainly composed of optical device materials such as pump source (excitation source), gain medium (working substance), and resonant cavity. The gain medium is the source of photon generation, which transitions from the ground state to the excited state by absorbing the energy generated by the pump source. Due to the unstable excited state, the gain medium will release energy and return to the steady state of the ground state. In this process of energy release, the gain medium generates photons, which have high consistency in energy, wavelength, and direction. They continuously reflect and move back and forth in the optical resonant cavity, thereby amplifying and ultimately emitting laser light through a mirror, forming a laser beam. As the core optical system of terminal devices, the performance of lasers often directly determines the quality and power of the output beam of laser equipment, and is the most critical component of downstream devices. 
The lasers in the optical module mainly include VCSEL (vertical cavity surface emitting laser), FP (Fabry Perot laser), DFB (distributed feedback laser), and EML (electroabsorption modulated laser). VCSEL belongs to the surface emission type, while FP/DFB/EML belongs to the edge emission type. 
VCSEL lasers are used in multimode optical modules, suitable for short distance transmission and have relatively high cost-effectiveness. Due to the significantly increased demand for GPU interconnection in the A1 training cluster, it is expected to grow rapidly; FP is mainly used for short to medium speed wireless access 
Distance from the market, due to significant losses 
The problem of short transmission distance is gradually being replaced by DFB laser chips in some application scenarios; DFB is mainly used for long-distance transmission, such as FTTx access networks, transmission networks, wireless base stations, and internal interconnection of data centers; EML lasers are applied in single-mode optical modules, suitable for long-distance interconnection, and are often used for upper layer switch interconnection to achieve large-scale AI clusters. 
Indium phosphide (InP) substrates are used to fabricate edge emitting laser chips for FP, DFB, and EML, as well as PIN and APD 
Detector chips are mainly used for medium to long-distance transmission in telecommunications, data centers, and other industries; Gallium arsenide (GaAs) substrates are used to fabricate VCSEL surface emitting laser chips, mainly for short distance transmission in data centers and 3D applications 
Fields such as sensing. Domestic manufacturers have mastered most of the device manufacturing technology, and some core components such as high-power semiconductor laser chips still rely on imports, while foreign laser leaders rely on full industry chain integration to achieve low-cost, high-performance, and high stability products. 
03 Packaging method of laser chip 
In order to protect laser chips from any mechanical and thermal stress, almost every diode laser or any other laser device needs to be laser encapsulated because laser materials such as gallium arsenide are very fragile. In addition, the sealed packaging method can prevent dust or other pollutants from entering the laser; Smoke, dust, or oil can cause immediate or permanent damage to the laser. Most importantly, with the advancement of technology, the emergence of high-power diode lasers requires sophisticated packaging design to help dissipate the heat generated during operation through the base and installed heat sinks. 
There are various packaging forms for semiconductor laser chips, and different packaging methods are suitable for different application scenarios to meet specific performance requirements, heat dissipation needs, and cost considerations.      
(1) The TO (Transistor Outline) packaged laser chip is mounted on a heat sink and connected to electrical pins through gold wires. It is further sealed with a metal cap and a glass cover for laser transmission. The housing of TO-CAN packaging is usually cylindrical, but due to its small size, it is difficult to embed refrigeration and heat dissipation, making it difficult to use for high-power output under high current, and therefore difficult to use for long-distance transmission. At present, the main application is still in short distance transmission of 2.5Gbit/s and 10Gbit/s. But the cost is low and the process is simple. 
(2) Sub mount (COS) packaging is a method of mounting laser chips onto a heat sink through welding, bonding, and other methods. The structure is shown in the following figure. This technology is commonly used to manufacture various electronic devices, including lasers. In the application of lasers, COS packaging can provide good thermal management and electrical performance, while reducing the overall size and improving integration. 
(3) Butterfly packaging 
Butterfly packaging is the standard packaging form for optical communication transmission and laser pump diodes. The following figure shows a typical butterfly shaped package, where the laser chip is located on an aluminum nitride (AlN) heat sink. The aluminum nitride base is mounted on a thermoelectric cooler (TEC), which is connected to a substrate made of copper tungsten (CuW), Kovar alloy, or copper molybdenum (CuMo). 
04 "Heat sink" for laser chip packaging 
Laser diode (LD) chips generate a large amount of heat during operation, which needs to be quickly dissipated to ensure the stability of the laser and extend its service life. In order to effectively manage this heat, ceramic copper-clad laminates have become a key part of LD chip packaging as heat sinks. At present, common ceramic copper-clad laminates are mainly divided into thin film ceramic substrates (TFC), thick film printed ceramic substrates (TPC), directly bonded copper ceramic substrates (DBC), active metal welded ceramic substrates (AMB), and directly electroplated copper ceramic substrates (DPC). DPC substrates are commonly used in the field of LD chip packaging due to their excellent characteristics such as high surface pattern line accuracy, vertical interconnection capability, and low production cost. 
Kyocera is a globally renowned supplier of laser chip heat sinks, mainly focusing on aluminum nitride ceramic copper-clad laminates. Aluminum nitride (AlN) has 170, 200, and 230W/mK options to choose from. The composition of the thin film includes Au based thin film and Cu based thin film, and the Au based thin film adopts LD mounting without obstacles. Cu based thick Cu (ex.50 μ m) electroplating is used, and both LD chips are mounted using Au/Sn vapor deposition technology. 
Ceramic coated copper plate not only provides structural support, but also plays a role in conducting heat from the laser chip to the external heat dissipation system, preventing heat accumulation from causing laser performance degradation and high temperature problems. Common ceramic copper-clad laminate materials include aluminum oxide, aluminum nitride, and silicon nitride. The following figure compares the parameters of several types of substrates provided by MARUWA, including alumina, aluminum nitride, and silicon nitride. Aluminum nitride, which has higher thermal conductivity, is often used in the field of high-power LD chips. MARUWA can provide three types of substrates: AlN-170/200/230. Ceramic materials are particularly suitable for thermal management in laser packaging, mainly reflected in the following aspects: high thermal conductivity: Ceramic substrates (especially aluminum nitride substrates) have high thermal conductivity, which can efficiently conduct heat from laser diode chips to heat sinks or external heat dissipation systems. The thermal conductivity of aluminum nitride (AlN) substrates is close to 200-250 W/m · K, far exceeding that of ordinary metal substrates. Therefore, ceramic substrates are an ideal choice for packaging high-power LD lasers. Excellent electrical insulation: In addition to good thermal conductivity, ceramic materials usually also have good electrical insulation properties. This enables ceramic substrates to not only carry LD chips, but also effectively manage heat while preventing issues such as current leakage and short circuits. High temperature resistance and stability: Ceramic substrates can withstand high operating temperatures and temperature fluctuations, which is crucial for laser diode chips, especially in high-power or long-term operation, to ensure that the substrate will not deform or degrade due to temperature changes. Good mechanical strength and reliability: The hardness and strength of ceramic materials give them high resistance to mechanical stress, which can protect LD chips from external physical forces. In addition, the thermal expansion coefficient of ceramic materials matches that of metal materials such as copper and aluminum, which helps to reduce stress during thermal cycling and improve the long-term reliability of packaging. So what is DPC technology? 
05 Laser chip heat sink DPC technology 
DPC technology first involves pre-processing and cleaning the ceramic substrate, depositing a Ti/Cu layer as a seed layer on the surface of the substrate using vacuum sputtering. Then, photolithography, development, and etching processes are used to complete the circuit fabrication. Finally, electroplating/chemical plating is used to increase the thickness of the circuit, and the substrate fabrication is completed after the photoresist is removed; DPC key technology: bonding strength between metal circuit layer and ceramic substrate; Electroplating filling holes. DPC Process Flow Diagram 
The front-end of DPC ceramic substrate preparation adopts semiconductor microfabrication technology (sputtering coating, photolithography, development, etc.), while the back-end adopts printed circuit board (PCB) preparation technology (graphic plating, hole filling, surface grinding, etching, surface treatment, etc.), thus having the following advantages: 1) Using semiconductor microfabrication technology, the metal lines on the ceramic substrate are more precise (line width/line spacing as low as 30 μ m~50 μ m, related to the thickness of the line layer), making DPC ceramic substrate very suitable for packaging microelectronic devices with high precision requirements; 2) By using laser drilling and electroplating filling technology, vertical interconnection between the upper and lower surfaces of ceramic substrates has been achieved, which can realize three-dimensional packaging and integration of electronic devices and reduce device volume; 3) The low-temperature preparation process (below 300 ℃) avoids the adverse effects of high temperature on substrate materials and metal circuit layers, while also reducing production costs.