Optical Module Introduction

With the rapid development of optical communications, many scenes in our work and life have now realized the "advancing of light and retreat of copper". In other words, metal media 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.

Composition structure of optical module

Optical module, the English name is Optical Module . Optical means "sight, vision, optics".

To be precise, the optical module is a general term for various module categories, including: optical receiving module, optical transmitting module, optical transceiver integrated module and optical forwarding module, etc.

Optical Module: Transmitter、Receiver、Transceiver、Transpomder

What we usually call optical modules today generally refers to integrated optical transceiver modules (the same applies below).

Optical modules work at the physical layer, which is the lowest layer in the OSI model. Its function is very simple, that is, to achieve photoelectric conversion . Turn optical signals into electrical signals and electrical signals into optical signals.

Although it seems simple, the technical content of the implementation process is not low.

An optical module usually consists of an optical transmitting device (TOSA, including a laser) , an optical receiving device (ROSA, including a photodetector) , functional circuits , and optical (electrical) interfaces .

At the transmitting end, the driver chip processes the original electrical signal, and then drives the semiconductor laser (LD) or light-emitting diode (LED) to emit a modulated optical 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 passing through the preamplifier.

Packaging of optical modules

For beginners, the most frustrating thing about optical modules is its extremely complex package name and dazzling parameters.

Packaging can be simply understood as a style 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 continues to increase and their size continues to shrink, so that every few years, new packaging standards will be released. It is often difficult to be compatible between old and new packaging standards.

In addition, the application scenarios of optical modules are diverse, which is also a reason for the increase in packaging standards. Different transmission distances, bandwidth requirements, and places of use correspond to different types of optical fibers and optical modules.

Before explaining packaging and classification, let's introduce the standardization organization of optical communication . Because these packages are determined by standardization organizations.

There are currently several organizations around the world that standardize optical communications, such as the familiar IEEE (Institute of Electrical and Electronics Engineers), ITU-T (International Telecommunication Union), MSA (Multi-Source Agreement), and OIF (Optical Interconnection). Forum), CCSA (China Communications Standards Association), etc.

The most used in the industry are IEEE and MSA.

You may not be familiar with MSA. Its English name is Multi Source Agreement. It is a multi-vendor specification. Compared with IEEE, it is regarded as a private and unofficial organizational form. It can be understood as an alliance of enterprises within the industry.

Firstly, we will ignore those standards that are too old or rare, and focus on common packages.

GBIC

GBIC stands for Giga Bitrate Interface Converter.

Before 2000, GBIC was the most popular optical module package and the most widely used Gigabit module form.

SFP

Because GBIC is relatively large, SFP later appeared and began to replace GBIC.

SFP, the full name is Small Form-factor Pluggable, which is a small hot-swappable optical module. Its small size is relative to the GBIC package.

The volume of SFP is reduced by half compared to GBIC module, and more than double the number of ports can be configured on the same panel. In terms of functionality, there is not much difference between the two, and both support hot swapping. SFP supports a maximum bandwidth of 4Gbps.

XFP

XFP is a 10-Gigabit Small Form-factor Pluggable. You can understand at a glance that it is a 10-Gigabit SFP.
XFP uses a full-speed single-channel serial module connected by XFI (10Gb serial interface) and can replace Xenpak and its derivatives.

SFP+

SFP+, it is a 10G optical module like XFP.
The size of SFP+ is the same as that of SFP, and it is more compact than XFP (about 30% smaller) and consumes less power (reducing some signal control functions).

SFP28

The SFP with a speed of 25Gbps was mainly because the 40G and 100G optical modules were too expensive at the time, so we made such a compromise transition plan.

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, you can directly upgrade from 25G to 100G without going through 40G, greatly simplifying the wiring difficulty and reducing costs.

QSFP-DD was established in March 2016. DD refers to "Double Density". The 4 channels of QSFP are increased by one row of channels and become 8 channels.
It is compatible with the QSFP solution. The original QSFP28 module can still be used, just plug in another module. The number of electrical gold fingers of QSFP-DD is twice that of QSFP28.

Each channel of QSFP-DD uses 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.

Traditional digital signals mostly use NRZ (Non-Return-to-Zero) signals, that is, high and low signal levels are used to represent the 1 and 0 information of the digital logic signal to be transmitted, and each signal symbol period can be Transmit 1 bit of logical information.

The PAM signal uses 4 different signal levels for signal transmission, and each symbol period can represent 2 bits of logical information (0, 1, 2, 3). Under the same channel physical bandwidth, PAM4 transmits twice the amount of information equivalent to the NRZ signal, thereby doubling the rate.

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 SFP interface, which has larger size and supports 100Gbps data transmission. CFP can support a single 100G signal, one or more 40G signals.

The difference between CFP, CFP2, and CFP4 lies in the volume. The volume of CFP2 is one-half that of CFP, and CFP4 is one-fourth that of CFP.

CFP8 is a packaging form specially proposed for 400G, and its size is equivalent to CFP2. Supports 25Gbps and 50Gbps channel rates, and achieves 400Gbps module rates through 16x25G or 8x50 electrical interfaces.

OSFP

OSFP, Octal Small Form Factor Pluggable, "O" stands for "octal", was 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 a 25G DML laser under the modulation of PAM4), with a slightly larger size than QSFP-DD, a 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

This article mentions three types of optical modules that support 400Gbps, namely QSFP-DD, CFP8 and OSFP.
400G is currently the main competitive direction of the optical communications industry. Now 400G is also in 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 batch construction of large-scale data centers, the ICT industry's demand for 400G has become increasingly urgent.
Early 400G optical modules used a 16-channel 25Gbps NRZ implementation and were packaged in CDFP or CFP8.

The advantage of this implementation is that it can borrow mature 25G NRZ technology on 100G optical modules. But the disadvantage is that 16 channels of 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 be used to replace NRZ.

On the optical port side, 8 channels of 53Gbps PAM4 or 4 channels of 106Gbps PAM4 are used to realize 400G signal transmission, and on the electrical port side, 8 channels of 53Gbps PAM4 electrical signals are used, and the package form of OSFP or QSFP-DD is adopted.

In comparison, the QSFP-DD package size is smaller (similar to the QSFP28 package of the traditional 100G optical module), which 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 telecom applications.

The current 400G optical transceivers, no matter what kind of package they are in, are very expensive, and there is still a big gap from the user's expectations. Therefore, it is not yet possible to quickly carry out comprehensive popularization.

Another thing worth mentioning is silicon-based light, which is often mentioned as silicon light.

Silicon photonic technology is considered to have broad application prospects and competitiveness in the 400G era, and is currently attracting the attention of many companies and research institutions.

Key Concepts of Optical Modules

On the basis of the package, with some parameters, there will be a name for the optical module.
In addition to the distance and the number of channels, there is also the center wavelength.

The wavelength of light directly determines its physical properties. At present, the light we use in optical fibers has a central wavelength of 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.

By the way, CWDM and DWDM . WDM stands for Wavelength Division Multiplexing. 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 accustomed to naming by wavelength.
DWDM is dense WDM, Dense WDM. CWDM is sparse WDM, Coarse WDM. You should understand from the name that the wavelength interval in D-WDM is smaller. The advantage of WDM is that it has large capacity and can be transmitted over long distances.

By the way, BiDi, this concept is also frequently mentioned now. BiDi (BiDirectional) is a single fiber bidirectional, one optical fiber, bidirectional sending and receiving. The working principle is shown in the figure below. In fact, a filter is added. The wavelengths of sending and receiving are different, so simultaneous sending and receiving can be achieved.

Basic indicators of optical modules
The basic indicators of optical modules mainly include the following:

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. Where W or mW is a linear unit and dBm is a logarithmic unit. In communications, 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.

Maximum receiving sensitivity

Receiving sensitivity refers to the minimum received optical power of the optical module under a certain rate and bit error rate, unit: dBm .

Generally speaking, 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 devices of the optical module.

Extinction Ratio

Extinction ratio is one of the important parameters used to measure the quality of optical modules.

It refers to the minimum value of the ratio of the average optical power of the signal to the average optical power of the null signal under full modulation conditions, indicating the ability to distinguish between 0 and 1 signals. There are two factors that affect the extinction ratio in the optical module: bias current (bias) and modulation current (Mod). Let's regard it as ER=Bias/Mod.

The value of the extinction ratio is not that the larger the optical module is, the better it is, but that the optical module whose extinction ratio meets the 802.3 standard is better.

Light saturation

Also known as saturated optical power, it refers to the maximum input optical power when maintaining a certain bit error rate (10-10 ~ 10-12) at a certain transmission rate, unit: dBm.

It should be noted that the photodetector will experience photocurrent saturation when exposed to strong light. When this phenomenon occurs, the detector needs a certain amount of time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged. It will cause bit errors, and it is also very easy to damage the receiving end detector. During operation, you should try to avoid exceeding its saturated optical power.