As the core transmission device of the communication network, the fiber optic transceiver is used in data centers, mobile communication base stations, passive wavelength division systems, SAN/NAS storage networks and other related fields. This article hopes to introduce the whole process from design to production of the fiber optic transceiver from several parts, starting from the customer’s needs, purchasing components, selecting the technical route, determining the packaging process, and producing an optical module step by step, so as to have an overall and clearer understanding of the fiber optic transceiver.
From Requirement Input to Fiber Optic Transceiver Design Completion
The article describes the entire process of optical modules from customer requirements, to material selection, to design and production, including the analysis of many technologies in the optical module industry, and also briefly describes the development trend of the industry.
Demand Analysis
For example, a customer requirement is as follows: 500-meter transmission distance in the data center, 100G transmission rate, QSFP28 interface, considering the overall system cost. The customer’s short requirements are related to the selection of every detail in the design and production of fiber optic transceivers:
- Option 1: Packaging method and process route: hermetic packaging (TO-CAN, BOX, butterfly), non-hermetic packaging (COB, COC, etc.).
- Option 2: Optical chip type: VCSEL, DFB, EML, narrow linewidth tunable.
- Option 3: Design route: single-channel or multi-channel.
- Option 4: Modulation: NRZ, PAM4, coherent (QPSK/16QAM/64QAM, etc.).
The options are directly related to certain indicators of product performance requirements, and are also strongly related to the final product reliability and cost. The competition in the optical module industry is a process of optimizing the combination of multiple parameters. While pursuing performance (speed, miniaturization, and transmission distance), it will bring a lot of power consumption and heat dissipation issues; in order to solve heat dissipation and other problems, it will also bring cost pressure; cost control will make some risks for stability and reliability.
Therefore, although the optical module packaging, product appearance and electrical interface are standardized, the manufacturing process of the optical module includes a lot of design and process experience. Understanding customer needs and weighing indicators such as performance, power consumption, cost, and reliability are also the core competitiveness of an optical module company.
Fiber Optic Transceiver Packaging Selection
1. Hermetic Packaging
According to the needs of customers, in the case of outdoors, large changes in temperature and humidity, etc., since the laser chip is corroded by water vapor and the temperature has a great influence on the wavelength, we consider adopting the route of hermetic packaging, sealing the laser chip in a metal casing filled with inert gas.
According to the specific transmission distance, chip heat generation, cost requirements, number of channels, etc., different hermetic packaging methods can also be selected:
TO-CAN Packaging
The laser is mounted on a small heat sink and connected to the electrical pins by gold wires. A metal tube cap is repackaged with a sealed window for laser light transmission on it. The TO-CAN package is compact and relatively low in cost, but its disadvantage is that it is too small and difficult to install larger heat dissipation components. It is not suitable for high-power, high-current and long-distance scenarios. It is more suitable for 10km 10G/25G in the telecom market, such as base station front haul, home broadband and other fields.
After the TO-CAN is made, the basic laser package is available, but the spot diameter emitted by the laser is still different from that of the optical fiber. And it needs to be further coupled and aligned with the lens and optical fiber to focus most of the energy into the optical fiber. After packaging, it is made into TOSA (referring to the end of the laser emission, and the other end is ROSA, collectively referred to as OSA).
Butterfly Packaging
In order to solve the demand for high power, a butterfly package can be used to install the laser on a larger heat sink, and TEC temperature control can also be selected for higher temperature control requirements. Optical components such as lenses and isolators are also housed within the metal housing. Butterfly packaging is equivalent to packaging the laser + optical path. In terms of classification, a butterfly packaging component is actually equivalent to an OSA, which is a higher-level device than TO-CAN.
BOX Packaging
BOX package is a special form of butterfly package, mainly for multi-channel requirements. We will talk about multi-channel later, and here it is simply mentioned, integrating multiple lasers in one package, and then transmitting it externally through an optical fiber. When there are high requirements on temperature control, airtight reliability, etc., this BOX package is used.
2. Non-hermetic packaging
Since the data center market began to use optical modules on a large scale, the overall working environment has been optimized a lot compared to the telecommunications market, which is exposed to wind and sun, because the data center is equipped with air-conditioning, environmental monitoring and other equipment. At the same time, the amount of optical modules used in the data center is very large, which puts forward higher requirements for cost control, so non-hermetic packaging is gradually developed. The technology of non-hermetic packaging continues to iterate, the reliability is improving rapidly, and the scenarios that can be used are gradually increasing.
Simply speaking, non-hermetic packaging is to paste/solder the optical chip directly on the circuit board, and perform simple sealing and protection with glue such as epoxy resin, which reduces a large number of auxiliary components, saves costs, and improves integration, such as COB packaging. COB Packaging is chip-on-board packaging. A simple understanding is to adhere the laser chip on the PCB board, including the integrated package of the laser array and receiver array in a small space to achieve miniaturization. Because of the reduction of some protective measures and a large number of accessories, the cost is relatively low.
Optical modules with a rate of 25G and below mostly use single-channel TO or butterfly packaging optical transceiver components that are soldered to the PCB board to form an optical module. However, for high-speed optical modules with a rate of 40G and above, limited by the rate of the laser, it is mainly realized through multi-channel parallelization. For example, 40G is realized by 4×10G, and 100G is realized by 4×25G. The packaging of high-speed optical modules puts forward higher requirements for parallel optical design, high-speed electromagnetic interference, size reduction, and heat dissipation under increased power consumption.
Summary
The hermetic package uses metal and glass to construct a tight protection for the fragile optical chip, which can cope with various usage environments. However, more components are required, and the flexible circuit board with higher cost is required to lead high-frequency signals from the airtight casing, which leads to higher overall cost. Therefore, when the working environment is relatively controllable and the reliability can meet the requirements, the use of non-hermetic packaging can optimize the cost.
Laser Chip Selection
According to comprehensive considerations such as transmission distance, modulation method, and cost, there are a variety of chips to choose from. The selection of optical chips also needs to consider the supply chain situation. Some popular products are often out of stock in the early stage of mass production, and delivery delays are common. The scale effect of the chip industry is very significant, so many chip factories give priority to ensuring the supply of large customers. As small factories, they can only find alternative manufacturers and alternative solutions. The price of the chips they get varies greatly, which is also an important competitive factor in the optical module industry.
1. VCSEL Chip
VCSEL chip is the chip type with the lowest cost, but it emits light at a larger angle, and is generally used with a thicker multimode fiber. However, the price of multimode fiber is relatively high. Considering the total cost of the system, it is generally used in short-distance (AOC of a few meters and SR optical module of about 100 meters) scenarios.
2. DFB Chip
The DFB chip is a grating processed on the original FP laser to achieve more accurate wavelength selection and higher output wavelength accuracy. The DFB chip has a small light angle and can achieve more efficient optical path coupling, so it is widely used in medium and long distances (500 meters, 2km, etc.), and the cost is relatively moderate.
3. EML Chip
EML chips are one of the most costly chip types, consisting of a transmitting chip (which can be DFB/DBR, etc.) plus an external absorbing modulator. When working, the laser chip is always in a light-emitting state, and the signal output of the EML laser is controlled by controlling the switch of the absorbing chip.
Here is an explanation of what happens in optical signal transmission.
- The transmission speed of light of different wavelengths in the optical fiber is different;
- When the laser chip starts to emit light after voltage is applied, the emitted wavelength has a certain change (chirp) in the microscopic time dimension of the femtosecond level.
Based on these two points, the DFB chip sends out a laser signal after receiving an electrical signal, which contains a certain range of wavelengths. And dispersion occurs when it is transmitted through an optical fiber for a long distance.
Due to the different transmission speeds of different wavelengths in the optical fiber, the time it takes for the optical signal to reach the receiving end is different, which may cause signal interference. In severe cases, two pulse signals are mixed together (intersymbol interference), resulting in communication failure.
The advantage of using an EML laser is that the laser chip is in a stable working state, and the emitted wavelength is more “pure”. After modulation by an external modulator, the signal quality obtained after long-distance transmission is still high. Therefore EML is suitable for long-distance (10km, 20km, 40km or even higher) transmission applications. However, due to the addition of external absorption modulators, and for long-distance scenarios, the chip quality requirements are also higher, so the cost of EML chips at the same rate is 50% or even several times higher than that of DFB chips.
On the other hand, the response speed of the external absorption modulator is higher than that of the DFB direct modulation, and it is more suitable for use in some modulation technology fields such as PAM4.
4. Tunable Narrow Linewidth, referred to as “Tunable Laser”
As mentioned earlier, there is a problem of dispersion in long-distance transmission. EML can solve the problem caused by chirp, but the inherent emission wavelength range of the laser, that is, the “linewidth” still exists. In ultra-long distance (80km, 100km or even longer) ODN applications, the problem of dispersion still exists.
On the other hand, ultra-long-distance transmission requires the laying of a large number of optical fibers. Considering the cost of the overall system, DWDM (Dense Wavelength Division Multiplexing) technology is introduced. Transmit signals of different wavelengths in one optical fiber, greatly increase the transmission capacity of a single optical fiber, and reduce the overall system cost of ultra-long distances. These two requirements need to be met by tunable narrow linewidth lasers.
Tunable narrow-linewidth lasers have a complex structure and many schemes, including current control, temperature control, mechanical control, etc. For example, the external cavity mechanical control scheme adds a grating structure outside the ordinary laser, and adjusts the output wavelength through mechanical control.
Tunable lasers have been used in relatively few fields in the past, but with the possible introduction of wavelength division technology in 5G fronthaul, some manufacturers are also experimenting with the possibility of using tunable lasers, and the future demand may change greatly.
Summary
Choose VCSEL chips for low-cost short distances, DFB chips for medium distances, EML chips for medium and long distances and special modulation requirements, and tunable narrow linewidth lasers for ultra-long distances and some special applications.
Design Route Selection
We select the package form according to the application environment, and select the type of laser according to the transmission distance and other performance requirements. Next, we need to select the number of channels and modulation method according to the transmission rate.
At the beginning, we talked about several key indicators of the optical module that cannot be achieved at the same time. It is reflected in the design that there are many combinations, chip level (performance and cost), number of channels (miniaturization, heat dissipation, packaging difficulty), modulation method (electric chip cost, reliability, design difficulty). It is necessary to make the comprehensive choices among these combinations, and finally determine the design scheme of the optical module.
1. Single Channel
Single-channel is the simplest design method. A laser and a receiver are installed in an optical module, and a single-channel optical module is formed by adding other optical components and various electrical chips on the PCB board.
Commonly, for example, a 10G chip is made into a 10G optical module with NRZ modulation, a 10G chip is made into a 25G fronthaul optical module with overfrequency modulation, and a 50G chip is made into a 100G DR1 data center optical module with PAM4 modulation. All of these optical modules adopt single-channel design route.
2. Multi-channel
Due to the difficulty of upgrading the laser chip, the highest rate of mature lasers can achieve a single-wavelength 50G rate. However, customers’ demand for bandwidth is growing rapidly, and 400G and even 800G are on the agenda. Therefore, multiple lasers and multiple receivers are combined to form an optical module with a higher transmission rate, that is, a multi-channel design.
The multi-channel solution faces another problem: how to connect with optical fiber for transmission?
Multi-fiber Solution
The easiest way is to connect each laser with an optical fiber for direct external transmission. The advantage of the multi-fiber solution is that the internal structure of the optical module is simple, because there are relatively few components, and the cost of the optical module is low. The problem is that multiple optical fibers are required for transmission. If the distance is relatively long, the cost of the required optical fibers is also high. If the amount is large, the project will still have a lot of cost pressure. Therefore, multi-fiber solutions are mostly used in short-to-medium distance scenarios.
Single-fiber Solution
If the transmission distance is long, it is necessary to consider reducing the amount of fiber used, and then provide a single-fiber solution for customers to choose. The single-fiber solution uses the principle of wavelength division multiplexing (CWDM) to combine four lasers with different wavelengths into one optical fiber for transmission with a multiplexer (MUX). Then use the demultiplexing device (DeMUX) to separate out 4 different wavelengths and detect them separately.
There are two technical routes here, the first one is the way of using TFF filters. Four different wavelength lasers are installed in front of the corresponding wavelength filters. Another technical route uses the AWG method. Four lasers with different wavelengths are combined and demultiplexed by the AWG chip, and then coupled to an optical fiber for output.
These two technical routes have their own characteristics. AWG components are sensitive to temperature and require high temperature control capabilities. The accompanying costs are relatively high, but the production design process is relatively simple. The TFF solution is not so sensitive to temperature, but it is more difficult to design and manufacture components, and requires high technical capabilities such as design, coating, and coupling alignment.
Modulation Selection
The selection of the modulation method and the previous design route complement each other. Here we will directly compare the main modulation methods and the additional electronic chips that need to be added to realize these modulation methods.
1. NRZ Modulation
Traditional optical module modulation is mainly based on NRZ (not return to zero), and the high and low power of the laser correspond to binary 1 and 0 signals respectively.
In NRZ mode, only simple electrical chips such as basic driver chip (driver), amplifier (TIA, LA), clock repair (CDR) and main control chip (MCU or ASIC) are needed in the optical module.
2. PAM4 Modulation
Due to the difficulty and high cost of directly upgrading optical chips, PAM4 modulation technology is introduced to achieve higher speeds. The judgment of PAM4 optical signal power is divided into 4 thresholds, 00 is judged below the lowest threshold, 01 is judged between the lowest and the middle threshold, 10 is judged between the middle and the high threshold, and 11 is judged above the high threshold. Through a more intensive power determination design, it is possible to transmit twice the amount of data in the same time.
Here we need to emphasize two concepts that people often confuse. The rate of the laser chip generally refers to the baud rate, that is, how many complete pulses can be sent in one second. For example, a 25G EML chip sends 25*10^9 pulses per second, which is actually more. However, a pulse may represent several bits of binary data through high-order modulation techniques, and the actual amount of data transmitted is called the bit rate. For example, if a 25G EML chip is modulated by PAM4 to make a single-channel 50G optical module, then we directly say 50G PAM4 optical module, which means that the bit rate is 50G and the baud rate is 25G.
Since PAM4 modulation requires precise power control, the decision threshold is narrower, and the signal interference caused by fiber dispersion is more stringent, so most of them need to use EML lasers. At the same time, on the basis of the NRZ electronic chip, a DSP chip for signal processing is added to convert a set of two digital signals into an analog signal.
3. Overclocking Solution
In order to reduce costs, overclocking technology has been developed, which is to use chips with lower design rates to transmit signals at higher rates. For example, a 10G baud rate optical chip is used in the 5G fronthaul to “forcibly” add a 25G rate signal.
The laser light does not run at full capacity as soon as the voltage is turned on. There is a climbing process in the middle. When we say a 10G rate chip, it means that it can emit a complete 1010^9 pulses in 1 second. But the idea of overclocking is very simple. For this 10G chip, it is necessary to pass a 25G signal to emit 2510^9 pulses in 1 second, so that the laser power starts to go downhill before it reaches the peak. The final performance is that the signal quality of overclocking is degraded, and the recognition threshold is lowered.
At present, there are mainly two optical chip composition methods for 25G optical modules: 25G optical chip and 10G optical chip overclocking. Compared with the overclocking of 10G optical chips, 25G optical chips have the characteristics of good reliability and high stability, but their mass production process requirements are high. Although the 25G optical chip is not much different from the overclocking of the 10G optical chip in the early stage of use, its reliability in the later stage is outstanding.
4. Coherent modulation
The essence of the previous three modulations is intensity modulation, which uses the intensity of optical power or the amplitude of the sine wave (carrier) as an index to represent (modulate) the binary signal (baseband signal).
But sine waves also have phase parameters, and coherent modulation is to use the principle of coherence to modulate signals with amplitude and phase parameters.
Coherent modulation has two benefits:
- More data can be transmitted in one signal cycle (the effect is similar to PAM4);
- It can achieve super anti-interference ability. Therefore, coherent modulation has an irreplaceable advantage in ultra-long-distance transmission.
Final Thoughts
From chip selection, modulation method, to packaging process, the complete design process of an optical module is introduced. Since there are global standards for the shape and interface of fiber optic transceivers, and customers generally specify which shape they want, too. To put it simply, SFP, SFP+, SFP28, SFP56, QSFP28, QSFP-DD are all miniaturized shapes, corresponding to different interface rates. CFP, CFP2 is a relatively large form factor, which can accommodate more devices and better heat dissipation.
So far, the entire process of fiber optic transceiver from customer requirements, material selection, to design and production has been sorted out from beginning to end. The common technologies in the optical module industry have also been roughly analyzed. If there are any mistakes or omissions, please feel free to correct me.
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