What is an Optical Module?

When it comes to optical modules, I'm sure everyone is quite familiar with them.With the rapid development of optical communication,many scenarios in our work and life have now achieved "fiber replacing copper." That is, metal medium communication represented by coaxial cables and network cables is gradually being replaced by optical fiber media.Optical modules are a core component of optical fiber communication systems.

 

 

1. Composition of Optical Modules

 

The optical module, known as Optical Transceiver in English, is a general term for various module categories, including optical receiver modules, optical transmitter modules, optical transceiver modules, and optical forwarding modules.

Today, when we talk about optical modules, we usually mean optical transceivers (and this will be the case throughout the text).

Optical modules operate at the physical layer, which is the bottom layer of the OSI model. Its function is quite simple: it achieves photoelectric conversion. It converts optical signals into electrical signals and electrical signals into optical signals.

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

 

An optical module typically consists of an optical transmitter (TOSA, Transmitter Optical Sub-Assembly, containing a laser diode), an optical receiver (ROSA, Receiver Optical Sub-Assembly, containing 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 diode (LD) or light-emitting diode (LED) to emit a modulated optical signal.

At the receiving end, after the optical signal enters, it is converted into an electrical signal by a photodetector and then output after being amplified by a preamplifier.

 

2. Packaging of Optical Modules

 

For beginners, the most frustrating aspect of optical modules is their extremely complex packaging names and the bewildering array of parameters.

Packaging can be simply understood as a form factor standard. It is the primary way to distinguish optical modules.

The rapid development of optical fiber communication technology is the main reason for the multitude of packaging standards.

The speed of optical modules is constantly increasing, and their size is also shrinking, so new packaging standards are introduced every few years. Compatibility between old and new packaging standards is usually difficult.

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

I have listed some classification methods of optical modules, including packaging, as shown in the table below:

3. Classification of Optical Modules

 

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

Currently, there are several global organizations that standardize optical communication, such as the well-known IEEE (Institute of Electrical and Electronics Engineers), ITU-T (International Telecommunication Union), MSA (Multi Source Agreement), OIF (Optical Internetworking Forum), CCSA (China Communications Standards Association), etc.

The most commonly used in the industry are IEEE and MSA.

You might not be familiar with MSA. Its English name is Multi Source Agreement. It is a multi-vendor specification, a non-official organization form compared to IEEE, which can be understood as an industry alliance behavior.

Now, let's start introducing packaging.

First, you can take a look at the following image, which accurately describes the emergence period of different packaging and their corresponding working speeds.

 

4. Common Packaging

 

GBIC

GBIC stands for Giga Bitrate Interface Converter. Before 2000, GBIC was the most popular optical module packaging and the most widely used gigabit module form.

SFP

Due to the large size of GBIC, SFP appeared later and began to replace GBIC's position. SFP, the full name Small Form-factor Pluggable, is a small hot-pluggable optical module. Its small size is relative to GBIC packaging.

SFP's volume is reduced by half compared to GBIC modules, allowing more than double the number of ports to be configured on the same panel. In terms of functionality, both support hot-plugging. SFP supports a maximum bandwidth of 4Gbps.

XFP

XFP is 10-Gigabit Small Form-factor Pluggable. It uses a full-speed single-channel serial module with an XFI (10Gb serial interface) connection, which can replace Xenpak and its derivative products.

SFP+

SFP+ is also a 10G optical module. Its size is consistent with SFP, more compact (reduced by about 30%) than XFP, and consumes less power (reduced some signal control functions).

SFP28

SFP28 with a speed of 25Gbps was mainly because the prices of 40G and 100G optical modules were too high at the time, so this compromise transition solution was introduced.

QSFP/QSFP+/QSFP28/QSFP28-DD

Quad Small Form-factor Pluggable, a four-channel SFP interface. Many mature key technologies in XFP have been applied to this design.

QSFP can be divided into 4×10G QSFP+, 4×25G QSFP28, 8×25G QSFP28-DD optical modules, etc.

For example, QSFP28 is suitable for 4x25GE access ports. Using QSFP28, it is possible to upgrade from 25G to 100G without going through 40G, greatly simplifying wiring difficulty and reducing costs.

QSFP-DD

Founded in March 2016, DD stands for "Double Density." It increases the four channels of QSFP to eight channels.

It is compatible with QSFP solutions. The original QSFP28 modules can still be used, just insert another module. The number of electrical contacts on QSFP-DD is twice that of QSFP28.

 

QSFP-DD uses 25Gbps NRZ or 50Gbps PAM4 signal formats per channel. Using PAM4, it can support up to 400Gbps rates.

PAM4

PAM4 (4 Pulse Amplitude Modulation) is a "doubling" technology.

For optical modules, if you want to achieve rate improvement, you 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, using high and low signal levels to represent the digital logic signal's 1 and 0 information, with each signal symbol period transmitting 1 bit of logical information.

PAM4 signals use four different signal levels for transmission, with each symbol period representing 2 bits of logical information (0, 1, 2, 3). Under the same channel physical bandwidth, PAM4 transmits twice the amount of information as NRZ signals, thereby achieving a doubling of the rate.

CFP/CFP2/CFP4/CFP8

Centum gigabits Form Pluggable, a dense wavelength division optical communication module. The transmission rate can reach 100-400Gbps.

CFP is designed based on the SFP interface, with a larger size, supporting 100Gbps data transmission. CFP can support a single 100G signal, one or multiple 40G signals.

The difference between CFP, CFP2, and CFP4 lies in their size. CFP2's size is half of CFP, and CFP4 is a quarter of CFP.

CFP8 is a packaging form specifically proposed for 400G, with a size similar to CFP2. It supports channel rates of 25Gbps and 50Gbps, achieving 400Gbps module rates through 16x25G or 8x50 electrical interfaces.

OSFP

This is somewhat easily confused with the OSPF routing protocol.

OSFP, Octal Small Form Factor Pluggable, "O" stands for "octal," officially launched in November 2016.

It is designed to use eight electrical channels to achieve 400GbE (8*56GbE, but the 56GbE signal is formed by a 25G DML laser under PAM4 modulation), slightly larger than QSFP-DD, with higher-wattage optical engines and transceivers, and slightly better heat dissipation performance.

These are some of the common optical module packaging standards.

 

5. 400G Optical Modules

 

As you may have noticed, I mentioned three types of optical modules that support 400Gbps during the packaging introduction: QSFP-DD, CFP8, and OSFP.

400G is currently the main competitive direction in the optical communication industry. Now, 400G is also in the early stages of large-scale commercial use.

As is well known, due to the large-scale launch of 5G network construction and the rapid development of cloud computing and large-scale data center construction, the ICT industry's demand for 400G has become increasingly urgent.

Early 400G optical modules used a 16-lane 25Gbps NRZ implementation method, using CDFP or CFP8 packaging.

This implementation method benefits from the use of mature 25G NRZ technology developed for 100G optical modules. However, the disadvantage is that it requires 16 lanes of parallel transmission, resulting in higher power consumption and larger size, which is not suitable for data center applications.

Later, PAM4 began to replace NRZ.

On the optical side, 400G signal transmission is mainly achieved using 8 lanes of 53Gbps PAM4 or 4 lanes of 106Gbps PAM4, and on the electrical side, 8 lanes of 53Gbps PAM4 electrical signals are used, with OSFP or QSFP-DD packaging forms.

Comparatively speaking, QSFP-DD packaging is smaller (similar to the traditional 100G optical module QSFP28 packaging), which is more suitable for data center applications. OSFP packaging is slightly larger, and since it can provide more power, it is more suitable for telecommunications applications.

Currently, 400G optical modules, regardless of the packaging methods, are very expensive, far from meeting user expectations. Therefore, they cannot be quickly popularized.

 

Another noteworthy technology is silicon photonics, commonly known as silicon photonics.

 

Silicon photonics is seen as having broad applications and strong competitiveness in the 400G era, and it is getting a lot of attention from many companies and research institutions.

 

6.  Key Concepts of Optical Modules

 

After briefly mentioning 400G, let's continue with the classification of optical modules.

Based on packaging, combined with some parameters, there will be the naming of optical modules.

Take 100G for example, we often see the following types of optical modules:

The standards starting with 100GBASE are proposed by the IEEE 802.3 working group. PSM4 and CWDM4 are from MSA.

PSM4 (Parallel Single Mode 4 lanes, parallel single-mode four-channel)

CWDM4 (Coarse Wavelength Division Multiplexer 4 lanes, four-channel coarse wavelength division multiplexing)

Let's look at the naming of IEEE 802.3:

As shown in the figure above:

In the 100GBASE-LR4 name, LR means long reach, i.e., 10Km, and 4 means four channels, i.e., 4*25G, combined together to form a 100G optical module that can transmit 10Km.

The naming rules for -R are as follows:

The reason why there are IEEE's 100GBASE and MSA's PSM4 and CWDM4 is that the distance supported by 100GBASE-SR4 was too short and could not meet all interconnection needs, while the cost of 100GBASE-LR4 was too high. PSM4 and CWDM4 provided better medium-distance solutions.

In addition to distance and number of channels, let's take a look at the central wavelength.

The wavelength of light directly determines its physical characteristics. Currently, the central wavelengths of light used in optical fibers are mainly 850nm, 1310nm, and 1550nm (nm stands for nanometers).

Among them, 850nm is mainly used for multimode, and 1310nm and 1550nm are mainly used for single mode.

For more details on single mode and multimode, refer to our earlier discussion on optical fibers.

For single mode and multimode, if the bare module is not marked, it is easy to confuse.

Therefore, manufacturers generally distinguish them by the color of the pull ring:

Pull ring of blue and yellow

 

Here we also mention WDM CWDM and DWDM, which you should often see.

WDM stands for Wavelength Division Multiplexing. Simply put, it multiplexes different wavelength optical signals into the same optical fiber for transmission.

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 communication, people are accustomed to naming by wavelength.

DWDM is Dense WDM, and CWDM is Coarse WDM. From the names, you should understand that the wavelength interval in D-WDM is smaller.

The advantage of WDM is large capacity and it can be transmitted over long distances.

By the way, BiDi (BiDirectional) is unidirectional, one optical fiber, bidirectional transmission and reception. The working principle is shown in the figure below.

It is actually adding a filter. The wavelengths for transmission and reception are different, allowing simultaneous transmission and reception.

 

7. Basic Indicators of Optical Modules

 

The basic indicators of optical modules mainly include the following:

Output Optical Power

Output optical power refers to the output optical power of the light source at the optical module's sending end. It can be understood as the intensity of light, with units of W or mW or dBm. Among them, W or mW are linear units, and dBm are logarithmic units. In communication, we usually use dBm to represent optical power.

A 3dB reduction in optical power means it is halved, and 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 error rate, with units of dBm.

Generally, the higher the rate, the worse the receiving sensitivity, i.e., the larger the minimum received optical power, and the higher the requirements for the optical module's receiving end devices.

Extinction Ratio

The extinction ratio is one of the important parameters used to measure the quality of an optical module.

It refers to the minimum ratio of the average optical power of the signal under full modulation conditions to the average optical power of the space signal, indicating the ability to distinguish between 0 and 1 signals. Two factors affecting the extinction ratio in optical modules are the bias current (bias) and modulation current (Mod), which can be considered as ER=Bias/Mod.

The extinction ratio value is not necessarily higher the better; an optical module with an extinction ratio that meets the 802.3 standard is good.

Optical Saturation

Also known as saturation optical power, it refers to the maximum input optical power under a certain transmission rate while maintaining a certain error rate (10-10～10-12), with units of dBm.

It should be noted that the photodetector will exhibit a saturation phenomenon under strong light irradiation. When this phenomenon occurs, the detector needs a certain time to recover, during which the receiving sensitivity decreases, and the received signal may be misjudged, causing an error phenomenon, and it is also very easy to damage the receiving end detector. Therefore, it should be avoided to exceed its saturation optical power during use.

 

8. Industry Chain of Optical Modules

 

Finally, let's briefly talk about the industry chain of optical modules.

Currently, the optical module market is very hot, mainly because of 5G and data centers, as mentioned earlier.

 

The two most costly aspects of 5G network construction are base stations and the optical transport network. In the optical transport network, the water content of optical fibers is not much, but optical modules are quite troublesome.

At the heart of optical modules, the most expensive component is the chip. The chips in the laser and photodetector account for more than half of the cost.

As for the chip, the current situation is: foreign manufacturers have an advantage in high-end chips, while domestic manufacturers have an advantage in mid-to-low-end chips. However, domestic manufacturers are continuously making breakthroughs in the high-end market. The profit margin of high-end chips is higher than that of low-end, which is obvious.

Overall, there are over 1000 optical communication companies in China, but the profit margins are all very low. Moreover, in the industrial chain structure, facing equipment manufacturers (Huawei, ZTE), optical communication companies are also relatively "humble" and have no bargaining power.

The industry competition is fierce, and new products, high-end products, have more profit, but over time, the profit will shrink.

Anyway, it's roughly like this.