Master the knowledge of optical modules in one article and become a necessary skill for network engineers

In this era of information explosion, data transmission has become an indispensable part of our lives. Among the many data transmission methods, optical fiber communication has become the mainstream of modern communication with its high speed, high bandwidth and low loss. Behind this optical fiber communication, there is a magical device that plays a role silently, that is, the optical module. So, what exactly is an optical module? How does it work? This article will unveil the mystery of the optical module for you.

Composition and structure of optical module

Optical module, in English, is called Optical Module. Optical means "visual, visual, optical".

To be precise, optical module is a general term for multiple types of modules, including: optical receiving module, optical transmitting module, optical transceiver integrated module and optical forwarding module.

Nowadays, the optical modules we usually refer to are generally optical transceiver modules.

Optical modules are mainly responsible for the work of the physical layer, which is the basic layer in the OSI model. Its main function is to perform optical-to-electrical conversion, which is a basic data transmission process. Specifically, optical modules are able to convert optical signals into electrical signals and vice versa. This conversion process plays a vital role in many modern communication systems.

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

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

• Optical receiving assembly TOSA: laser, metal structure and ceramic ferrule, etc.
• Optical receiving assembly ROSA: PIN or APD detector, preamplifier and other structural parts
• BOSA: laser, detector, optical filter, metal parts, ceramic sleeve and ferrule

Optical module packaging

There are many packaging methods for optical modules, the most common ones are SFP, SFP+, XFP, QSFP+, QSFP28, etc.

The function of optical module packaging is to solder the electrical chip and optical transceiver components to the PCB board to form an optical module, while protecting the electrical chip and optical transceiver components so that they have good mechanical strength and electrical performance.

The application scenarios of optical modules are indeed very diverse, which is also an important reason for the diversification of packaging standards. Different transmission distances, bandwidth requirements and usage locations require different types of optical fibers and optical modules.

The following table briefly lists the classification of optical modules including packaging:

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 factor.

SFP

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

SFP, the full name of Small Form-factor Pluggable, is a small hot-pluggable optical module. Its smallness is relative to GBIC packaging.

The volume of SFP is half that of GBIC module, and the number of ports can be configured on the same panel more than twice. In terms of function, the two are not much different, and both support hot plugging. The maximum bandwidth supported by SFP is 4Gbps.

XFP

XFP, which stands for 10-Gigabit Small Form-factor Pluggable, is easy to understand at a glance, it is 10-Gigabit SFP.

XFP is a full-speed single-channel serial module using an XFI (10Gb serial interface) connection, which can replace Xenpak and its derivatives.

SFP+

SFP+, like XFP, is a 10G optical module.

The size of SFP+ is the same as SFP, which 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 developed mainly because the prices of 40G and 100G optical modules were too expensive at the time, so such a compromise transition solution was developed.

Key parameter standards for optical modules

The key performance indicators of optical modules include: transmit optical power, receive optical power, receive sensitivity, extinction ratio and overload optical power, etc.

• Transmitted optical power: refers to the optical power output by the transmitting end light source of the optical module under normal working conditions, which can be understood as the intensity of light, with units of W or mW or dBm. W or mW is a linear unit, and dBm is a logarithmic unit. In communications, we usually use dBm to represent optical power, and 0dBm optical power corresponds to 1mW.
• Received optical power: refers to the average optical power range that the receiving end component can receive under a certain bit error rate (BER=10-12) of the optical module. The upper limit is the overload optical power, and the lower limit is the maximum receiving sensitivity.
• Overload optical power: Also known as saturated optical power, it refers to the maximum input average optical power that the receiving end component can receive when maintaining a certain bit error rate (BER=10-12) at a certain transmission rate, unit: dBm.
• Receive sensitivity: refers to the minimum average input optical power that the receiving end component can receive when maintaining a certain bit error rate (BER=10-12) at a certain transmission rate, unit: dBm.

How are optical modules named?

Each optical module manufacturer has its own naming rules. Some manufacturers' product models can directly show some basic information of the module, for example: SFP-GE-MM-850-SR. Among them, SFP means the package type of the optical module is SFP; GE means the transmission rate of the optical module is 1G; MM means the connection distance of the optical module is multi-mode; 850 means the transmit optical power of the optical module is 850mW; SR means the module is hot-swappable.

Example of viewing optical module information on a switch

Optical module information on Huawei device interfaces: Run the display transceiver [ interface interface-type interface-number | slot slot-id ] [ verbose ] command to check information about optical modules on device interfaces.

 100GE1/0/0 transceiver information: ------------------------------------------------------------------- Common information: Transceiver Type :100GBASE_CWDM4 Connector Type :LC Wavelength (nm) :1301 Transfer Distance (m) :2000(9um/125um SMF) Digital Diagnostic Monitoring :YES Vendor Name :XXX Vendor Part Number :XXXX Ordering Name : ------------------------------------------------------------------- Manufacture information: Manu. Serial Number :XXXXX Manufacturing Date :2017-11-2+08:00 Vendor Name :FINISAR CORP. ------------------------------------------------------------------- Alarm information: Non-Huawei-Ethernet-Switch-Certified Transceiver ------------------------------------------------------------------- Diagnostic information: Temperature (Celsius) :38.64 Voltage (V) :3.25 Bias Current (mA) :40.23|36.02 (Lane0|Lane1) 36.68|36.97 (Lane2|Lane3) Bias High Threshold (mA) :55.00 Bias Low Threshold (mA) :25.00 Current RX Power (dBm) :-1.05|-1.02 (Lane0|Lane1) 0.14|0.01 (Lane2|Lane3) Default RX Power High Threshold (dBm) :3.50 Default RX Power Low Threshold (dBm) :-16.00 Current TX Power (dBm) :0.72|-0.14 (Lane0|Lane1) -0.48|0.55 (Lane2|Lane3) Default TX Power High Threshold (dBm) :5.50 Default TX Power Low Threshold (dBm) :-8.00 -------------------------------------------------------------------

Detailed description of individual parameters: