Speaking of optical modules, I believe everyone will not feel unfamiliar.
With the rapid development of optical communication, many scenes in our work and life have realized "light advancement and retreat". In other words, metal-medium communications represented by coaxial cables and network cables are gradually being replaced by optical fiber media.
The optical module is one of the core components of the optical fiber communication system.
The structure of the optical module
Optical module, the English name is Optical Module . Optical, meaning "sight, vision, optical".
To be precise, the optical module is a collective term for a variety of module categories, including: optical receiving module, optical transmitting module, optical transceiver integrated module, and optical forwarding module.
Nowadays, what we usually call an optical module generally refers to an integrated optical transceiver module (this is also the case below).
The optical module works at the physical layer, which is the lowest layer in the OSI model. Its function is simple to say, it is to realize photoelectric conversion . Turn optical signals into electrical signals, and turn electrical signals into optical signals, like this.
Although it seems simple, the technical content of the implementation process is not low.
An optical module usually consists of a light emitting device (TOSA, including a laser) , an optical receiving device (ROSA, including a photodetector) , a functional circuit, and an optical (electrical) interface .
Composition of optical module
At the transmitting end, the driving chip processes the original electrical signal, and then drives the semiconductor laser (LD) or light emitting diode (LED) to emit a modulated light signal.
At the receiving end, after the optical signal comes in, it is converted into an electrical signal by the light detection diode, and the electrical signal is output after the preamplifier.
Optical module packaging
For beginners, the most maddening thing about an optical module is its extremely complex package name and dazzling parameters.
Package name, these are just a part
The package can be simply understood as a model standard. It is the most important way to distinguish optical modules.
The reason why there are so many different packaging standards for optical modules is mainly because the development speed of optical fiber communication technology is too fast.
The speed of optical modules is constantly increasing, and the volume is constantly shrinking, so that every few years, new packaging standards will be released. It is usually difficult to be compatible with the old and new packaging standards.
In addition, the diversity of application scenarios of optical modules is also a reason for the increase in packaging standards. Different transmission distances, bandwidth requirements, and use locations correspond to different types of optical fibers, and optical modules are also different.
Mr. Xiaozao briefly lists the optical module classification methods including packaging, as shown in the following table:
Classification of optical modules
Before explaining packaging and classification, let's introduce the standardization organization of optical communication . Because these packages are determined by the standardization organization.
At present, there are several organizations that standardize optical communications in the world, such as IEEE (Institute of Electrical and Electronics Engineers), ITU-T (International Telecommunication Union), MSA (Multi-Source Agreement), OIF (Optical Interconnection Forum), CCSA (China Communications Standards Association), etc.
The most used in the industry are IEEE and MSA.
MSA may not be familiar to everyone, its English name is Multi Source Agreement (Multi Source Agreement). It is a multi-supplier specification. Compared with IEEE, it can be regarded as a non-governmental non-official organization form. It can be understood as the behavior of enterprise alliances in the industry.
Okay, let's introduce packaging.
First of all, you can take a look at the following picture, which more accurately describes the appearance period of different packages and the corresponding work rate.
We don't care about those standards that are too old or rare, and we mainly look at common packages.
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GBIC
GBIC is Giga Bitrate Interface Converter (Gigabit Interface Converter).
Before 2000, GBIC was the most popular optical module package and the most widely used Gigabit module form.
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SFP
Because of the relatively large size of GBIC, SFP appeared later and began to replace GBIC.
SFP, the full name is Small Form-factor Pluggable, that is, small, hot-swappable optical module. Its small size is relative to the GBIC package.
The volume of SFP is half that of GBIC module, and the number of ports can be more than doubled on the same panel. In terms of function, there is little difference between the two, and both support hot swap. The maximum bandwidth supported by SFP is 4Gbps.
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XFP
XFP, is 10-Gigabit Small Form-factor Pluggable, it is easy to understand at a glance, it is 10 Gigabit SFP.
XFP adopts a full-speed single-channel serial module connected with XFI (10Gb serial interface), which can replace Xenpak and its derivatives.
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SFP+
SFP+, it is a 10G optical module like XFP.
The size of SFP+ is the same as that of SFP, which is more compact than XFP (about 30% reduction), and its power consumption is also smaller (reducing some signal control functions).
You can compare the size
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SFP28
The SFP with a rate of 25Gbps was mainly because 40G and 100G optical modules were too expensive at that time, so a compromise transition plan was made.
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QSFP/QSFP+/QSFP28/QSFP28-DD
Quad Small Form-factor Pluggable, four-channel SFP interface. Many mature key technologies in XFP have been applied to this design.
According to the speed, QSFP can be divided into 4×10G QSFP+, 4×25G QSFP28, 8×25G QSFP28-DD optical modules, etc.
Take QSFP28 as an example, it is suitable for 4x25GE access ports. Using QSFP28 can directly upgrade from 25G to 100G without going through 40G, which greatly simplifies wiring difficulty and reduces costs.
QSFP28
QSFP-DD was established in March 2016. DD refers to " Double Density ". The 4 channels of QSFP are increased by a row of channels to 8 channels.
It can be compatible with the QSFP scheme, the original QSFP28 module can still be used, just insert another module. The number of gold fingers for QSFP-DD electrical ports is twice that of QSFP28.
QSFP-DD
Each QSFP-DD adopts 25Gbps NRZ or 50Gbps PAM4 signal format. Using PAM4, it can support up to 400Gbps rate.
NRZ and PAM4
PAM4 (4 Pulse Amplitude Modulation) is a "doubling" technology.
For optical modules, if you want to increase the rate, you must either increase the number of channels or increase the rate of a single channel.
The most traditional digital signal uses the NRZ (Non-Return-to-Zero) signal, that is, high and low signal levels are used to represent the 1, 0 information of the digital logic signal to be transmitted. Each signal symbol cycle can Transmit 1bit logic information.
The PAM signal uses 4 different signal levels for signal transmission, and each symbol period can represent 2 bits of logic information (0, 1, 2, 3). In the case of the same physical bandwidth of the channel, PAM4 transmits twice the amount of information equivalent to the NRZ signal, thereby realizing a rate double.
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CFP/CFP2/CFP4/CFP8
Centum gigabits Form Pluggable, dense wavelength division optical communication module. The transmission rate can reach 100-400Gbps.
CFP is designed on the basis of the SFP interface, with a larger size and supporting 100Gbps data transmission. CFP can support a single 100G signal, one or more 40G signals.
The difference between CFP, CFP2 and CFP4 is the volume. The volume of CFP2 is one-half of CFP, and CFP4 is one-fourth of CFP.
CFP8 is a package form specifically proposed for 400G, and its size is equivalent to CFP2. Support 25Gbps and 50Gbps channel rates, and realize 400Gbps module rate through 16x25G or 8x50 electrical interface .
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OSFP
This is a bit confused with the OSPF routing protocol we often say.
OSFP, Octal Small Form Factor Pluggable, "O" stands for "octal", officially launched in November 2016.
It is designed to use 8 electrical channels to achieve 400GbE (8*56GbE, but the 56GbE signal is formed by 25G DML laser under PAM4 modulation), slightly larger than QSFP-DD, higher wattage optical engine and transceiver The heat dissipation performance is slightly better.
The above are some common optical module packaging standards.
400G optical module
Everyone noticed that when we introduced packaging just now, Xiaozao Jun mentioned three kinds of optical modules that support 400Gbps, namely QSFP-DD, CFP8 and OSFP.
400G is currently the main competition direction of the optical communications industry. Now 400G is also the initial stage of large-scale commercial use.
As we all know, due to the large-scale launch of 5G network construction, coupled with the rapid development of cloud computing, and the mass construction of large-scale data centers, the demand for 400G in the ICT industry has become increasingly urgent.
The early 400G optical modules used 16-channel 25Gbps NRZ implementations and used CDFP or CFP8 packages.
The advantage of this implementation is that it can borrow mature 25G NRZ technology on 100G optical modules. But the disadvantage is that 16 signals are required for parallel transmission, and the power consumption and volume are relatively large, which is not suitable for data center applications.
Later, PAM4 began to replace NRZ.
On the optical port side, 8 channels of 53Gbps PAM4 or 4 channels of 106Gbps PAM4 are mainly used to achieve 400G signal transmission, and on the electrical port side, 8 channels of 53Gbps PAM4 electrical signals are used, using OSFP or QSFP-DD packaging.
In comparison, the QSFP-DD package size is smaller (similar to the QSFP28 package of the traditional 100G optical module) and is more suitable for data center applications. OSFP package size is slightly larger, because it can provide more power consumption , so it is more suitable for telecommunications applications.
The current 400G optical modules, no matter what kind of package, are very expensive, and there is still a big gap between the expectations of users. Therefore, for the time being, it has not been able to quickly achieve full popularization.
400G optical module price (from a manufacturer’s website, for reference only)
Another noteworthy, silicon-based light, which is often referred to silicon light .
Silicon photonics technology is considered to have broad application prospects and competitiveness in the 400G era, and is currently receiving attention from many companies and research institutions.
The key concept of optical module
We interrupted 400G, and we went back to talk about the classification of optical modules.
On the basis of packaging, with some parameters, the optical module will be named.
Taking 100G as an example, the optical modules we often see are the following:
The standards beginning with 100GBASE are all proposed by the IEEE 802.3 working group. PSM4 and CWDM4 are MSA.
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PSM4 (Parallel Single Mode 4 lanes, parallel single mode four lanes)
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CWDM4 (Coarse Wavelength Division Multiplexer 4 lanes, four-channel coarse wavelength division multiplexing)
We look at the naming of IEEE 802.3:
As shown in FIG:
In the name of 100GBASE-LR4, LR stands for long reach, which is 10Km, and 4 stands for four channels, which is 4*25G. The combination is a 100G optical module that can transmit 10Km.
The naming rules for -R are as follows:
-R noun explanation
The reason there is IEEE's 100GBASE and MSA's PSM4 and CWDM4 is because the distance supported by 100GBASE-SR4 was too short to meet all interconnection requirements, and the cost of 100GBASE-LR4 was too high. PSM4 and CWDM4 provide a better solution for middle distance.
In addition to the distance and the number of channels, let's look at the center wavelength .
The wavelength of light directly determines its physical characteristics. At present, the central wavelength of the light we use in optical fibers is mainly divided into 850nm, 1310nm and 1550nm (nm is nanometer).
Among them, 850nm is mainly used for multimode, and 1310nm and 1550nm are mainly used for single mode.
Regarding single-mode and multi-mode, Mr. Xiaozao said in detail when he introduced optical fiber before, you can refer to here: Basic knowledge of optical fiber and cable
For single-mode and multi-mode, it is easy to confuse bare modules if they are not marked.
Therefore, the general manufacturer will distinguish the color of the pull ring:
Blue and yellow
Here we mention CWDM and DWDM by the way , you should see them often.
WDM, is Wavelength Division Multiplexing ( WDM ). Simply put, it is to multiplex optical signals of different wavelengths into the same optical fiber for transmission.
Wavelength division multiplexing and frequency division multiplexing
In fact, wavelength division multiplexing is a kind of frequency division multiplexing. Wavelength × frequency = speed of light (fixed value), so dividing by wavelength is actually dividing by frequency. In optical communications, people are used to naming them by wavelength.
DWDM is dense WDM, Dense WDM. CWDM is sparse WDM, Coarse WDM. By looking at the name, you should understand that the wavelength interval in D-WDM is smaller.
The advantage of WDM is its large capacity and it can be transmitted over long distances.
By the way, BiDi , this concept is also frequently mentioned now.
BiDi (BiDirectional) means single-fiber bidirectional, one fiber, two-way transceiver. The working principle is shown in the figure below. In fact, a filter is added, and the sending and receiving wavelengths are different, which can realize simultaneous sending and receiving.
BiDi single fiber bidirectional optical module
Basic indicators of optical modules
The basic indicators of optical modules mainly include the following:
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Output optical power
The output optical power refers to the output optical power of the light source at the transmitting end of the optical module. It can be understood as the intensity of light in W or mW or dBm. Among them, W or mW is a linear unit, and dBm is a logarithmic unit. In communication, we usually use dBm to represent optical power.
The optical power is attenuated by half and reduced by 3dB. The optical power of 0dBm corresponds to 1mW.
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Maximum receiving sensitivity
Receiving sensitivity refers to the minimum received optical power of an optical module at a certain rate and bit error rate, in dBm.
In general, the higher the rate, the worse the receiving sensitivity, that is, the greater the minimum received optical power, and the higher the requirements for the receiving end of the optical module.
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Extinction ratio
The extinction ratio is one of the important parameters used to measure the quality of the optical module.
It refers to the minimum value of the ratio of the average optical power of the signal to the average optical power of the space signal under the full modulation condition, and represents the distinguishing ability of 0 and 1 signals. The two factors that affect the extinction ratio in the optical module: bias current (bias) and modulation current (Mod), tentatively regard them as ER=Bias/Mod.
The value of the extinction ratio is not that the larger the optical module, the better, but the optical module whose extinction ratio meets the 802.3 standard.
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Light saturation
Also known as saturated optical power, it refers to the maximum input optical power when a certain bit error rate (10-10～10-12) is maintained at a certain transmission rate, in dBm.
It should be noted that the photodetector will experience photocurrent saturation under strong light irradiation. When this phenomenon occurs, the detector needs a certain time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged. It can cause bit errors, and it is very easy to damage the receiving end detector. During the operation, try to avoid exceeding its saturated optical power.
The industry chain of optical modules
Finally, we briefly talk about the industry chain of optical modules.
At present, the optical module market is very hot, the main reason is mentioned above, because of 5G and data centers.
Optical module industry chain
For the entire 5G network construction, there are two places where the most money is spent, one is the base station, and the other is the optical bearer network. In the optical bearer network, the water content of the optical fiber is not much, but the optical module is relatively big.
Among the optical modules, the most expensive is the chip. The chips in lasers and photodetectors account for more than half of the cost.
As for the chip, the current status quo is: foreign manufacturers have an advantage in high-end chips, and domestic manufacturers have an advantage in low-end chips. However, domestic manufacturers are constantly making breakthroughs in the high-end market. It is obvious that the profit margin of high-end chips is higher than that of low-end chips.
On the whole, there are more than 1,000 optical communication companies in China, but their profit margins are very low. Moreover, in the industrial chain structure, facing equipment vendors (Huawei, ZTE), optical communication companies are relatively "humble" and have little bargaining power.
Industry competition is fierce. New products and high-end products have more profits, but over time, profits will shrink.
This is probably the case anyway.
Regarding the specific situation of the industry chain, due to 5G reasons, brokers are now very concerned and have output a lot of related reports. You can search and read it yourself.
Okay, that's all the content of today's article. Thank you for your patience, see you next time!
references:
1. "In-depth report on the optical module industry", Debon Securities
2. "5G Bearer Optical Module White Paper", IMT2020 Promotion Group
3. "How much do you know about 100G optical modules", specifically about optical communications
4. "Industry Graphic: 5G (Optical Module)", anonymous