What is Fiber Optical Transceiver?

1. Introduction of optical module

As an important part of fiber optic communication, optical module is an optoelectronic device that realizes the function of photoelectric conversion and electro-optical conversion. To be precise, optical module is the general name of several types of modules, including optical transmitting module transmitter, optical receiving module receiver, optical transceiver module transmitter and optical forwarding module transmitter. Generally, the optical module we call generally refers to the integrated optical transceiver module, namely, optical transceiver.

① Working principle of optical transceiver

Optical transceiver works in the physical layer of OSI model and is one of the core devices in optical fiber communication system. It is mainly composed of optoelectronic devices (optical transmitter, optical receiver), functional circuit and optical (Electrical) interface.

Components of an optical transceiver

Basic principle: the transmitting interface inputs an electrical signal with a certain code rate. After being processed by the internal driving chip, the driving semiconductor laser (LD) or light emitting diode (LED) emits a modulated optical signal with a corresponding rate. After being transmitted through the optical fiber, the receiving interface converts the optical signal from the optical detection diode into an electrical signal, After passing through the preamplifier, the electrical signal of the corresponding code rate is output.

② Key parameters of optical transceiver

The key technical indicators of the optical transceiver mainly include: transmitting optical power, receiving optical power, overload optical power, maximum receiving sensitivity and extinction ratio.

A. Transmitting optical power: refers to the optical power output by the light source at the transmitting end of the optical module under normal working conditions, which can be understood as the intensity of light, and the unit is w or MW or DBM. Where w or MW is the linear unit and DBM is the logarithmic unit. In communication, we usually use DBM to represent optical power, and the optical power of 0dbm corresponds to 1MW.

B. Receiving optical power: refers to the average optical power range that can be received by the receiving end component under the condition of a certain bit error rate (BER = 10-12). The upper limit value is the overload optical power, and the lower limit value is the maximum value of reception sensitivity.

C. Overload optical power: also known as saturated optical power, it refers to the maximum input average optical power that can be received by the receiver component when a certain bit error rate (BER = 10-12) is maintained at a certain transmission rate (unit: DBM). It should be noted that the photocurrent saturation of the photodetector will occur under strong light irradiation. Therefore, strong light irradiation should be avoided as far as possible to prevent exceeding the saturated light power.

D. Receiving sensitivity: refers to the minimum average input optical power that can be received by the receiving end component when a certain bit error rate (BER = 10-12) is maintained at a certain transmission rate (unit: DBM). Generally, the higher the rate, the worse the reception sensitivity, that is, the greater the minimum received optical power, and the higher the requirements for the receiving devices of the optical transceiver.

E. Extinction ratio (EXT): it is one of the important parameters used to measure the quality of optical modules. It refers to the ratio of the optical power P1 (when the laser emits all "1" codes) to P0 (when the laser emits all "0" codes) under the condition of full modulation, and the unit is dB. It is not that the greater the extinction ratio, the better the quality of the optical module, but that the optical module with an extinction ratio can meet the IEEE 802.3 standard is the better.

③ Example of optical transceiver information on the switch:

Cisco switch interface optical transceiver information:

Huawei switch interface optical transceiver information:

2. Classification and packaging(form-factor) of optical transceivers

① Transmission rate:

The current common types of optical modules are as follows:

400GE optical module

200GE optical module

100GE optical module

40GE optical module

25GE Optical Module

10GE Optical Module

GE Optical Module

FE Optical Module

② Form-factors:

The higher the transmission rate, the more complex the structure, resulting in form-factors. For example, SFP/eSFP, SFP+, SFP28, QSFP+, CXP, CFP, QSFP28, etc.

l  SFP (Small Form-factor Pluggable) optical module: small pluggable SFP module supports LC fiber connector.

l  SFP + (Small Form-factor Pluggable plus) optical module: refers to the SFP module with increased rate. Because of the increased rate, it is sensitive to EMI and smaller than the SFP module.

l  XFP (10GB Small Form-factor Pluggable) optical module: "X" is the abbreviation of Roman numeral 10. All XFP modules are 10GE optical modules. The XFP optical module supports LC optical fiber connectors. Compared with SFP + optical module, XFP optical module is wider and longer in size.

l  SFP28 (Small Form-factor Pluggable 28) optical module: the interface package size is the same as that of SFP +, and supports 25G SFP28 optical module and 10G SFP + optical module.

l  QSFP+ (Quad Small Form-factor Pluggable) optical module: four channel small hot pluggable optical module. QSFP + optical module supports MPO optical fiber connector, which is larger than SFP + optical module.

Fiber Mall 40G QSFP+ Optical Transceivers

l  CXP (120 GB/s Extended Capability Form-factor Pluggable) optical module: it is a hot-pluggable high-density parallel optical module standard. It provides 12 channels in the transmit and receive (TX / Rx) directions. It is only applicable to short-range multimode links.

l  CFP (CENTUM Form-factor pluggable) optical module: the size is defined as 144.75mm × 82mm × 13.6mm, which is a new optical module standard with high speed, hot-pluggable and supporting two applications of data communication and telecommunication transmission.

l  QSFP28 (Quad Small Form-factor Pluggable 28) optical module: the interface package size is the same as QSFP+, and supports 100G QSFP28 optical module and 40G QSFP+ optical module.

③ Single-mode or muti-mode?

The optical fiber is divided into single-mode optical fiber and multi-mode optical fiber. The single-mode optical module is used together with the single-mode optical fiber. The single-mode optical fiber has wide transmission frequency bandwidth and large transmission capacity, which is suitable for long-distance transmission; The multi-mode optical module is used together with multi-mode optical fiber, and multi-mode optical fiber has mode dispersion defect, and its transmission performance is worse than that of single-mode optical fiber, but the cost is low. Therefore, it is suitable for small capacity and short-distance transmission.

④ Wavelength

The central wavelength refers to the waveband used for optical signal transmission. At present, there are three kinds of common central wavelengths: 850nm, 1310nm and 1550nm.

850nm: mostly used for short distance transmission (≤ 2km)

1310nm and 1550nm: mostly used for medium and long distance transmission (>2km)

⑤ Transmission distance

According to the different transmission distance of optical module, it can be divided into:

Short distance optical module: ≤ 2km

Medium distance optical module, 10 ~ 20km

Long distance optical module: ≥30km.

The transmission distance of optical modules is limited mainly because of the loss and dispersion of optical signals during fiber transmission.

⑥ Photoelectric module

The photoelectric module is usually called electric port module or RJ45 SFP module. Unlike the optical module, the electric port module does not carry out photoelectric conversion. Through the switching of the electric module, the two optical interfaces can be connected with a network cable.

RJ45 Copper SFP Module

3. Development and evolution of optical transceivers

In the current physical architecture network of mainstream data centers, Spine-Leaf (Clos network architecture) architecture is commonly followed. Usually 10GE interface is used as the access side server docking, and 40GE interface is commonly used for the uplink on the Leaf side. In large data centers, it has been common to use 25G as the mainstream access and 100G uplink. In scenarios requiring high computation and high bandwidth, GPU servers have been using 100GE and even 200GE access. Data center switch interconnects are evolving to large-scale 400GE interconnects.

The development of Ethernet has undergone rapid changes from 1Mbit/s, 10Mbit/s, 100Mbit/s (FE), 1Gbit/s (GE), 10Gbit/s (10GE) to 40Gbit/s (40GE), 100Gbit\s (100GE), and with the rapid development of big data, smart cities, mobile Internet, cloud computing, network traffic has shown exponential growth. The thirst for continuous growth of bandwidth will require higher bandwidth rates, and optical modules will develop rapidly.