A Refresher on QSFP 40G Transceivers

Optical modules (transceivers) provide data exchange in telecommunication networks. The main task of transceivers is to convert electrical signals into optical signals for further information transmission via a fiber-optic communication line (FOCL).

The 40G QSFP optical module is a QSFP based product designed for high-density applications. Compared to standard SFP + optical modules, the port density is higher, and the overall system cost is lower. It has four data transmission channels, and the transmission rate of each channel is about 10Gbps. Simultaneous transmission of four channels can achieve a transmission speed of 40 Gbps.

Network transceivers are used to minimize traffic latency and provide high throughput. The need to solve such problems often arises in large data centers and high-performance computing networks. QSFP is proving to be very useful for operators willing to upgrade their existing 10G networks to 40G and beyond. The following exhibit shows two possible options for upgrading from SFP+ to QSFP.

• Providing optical and electrical interface at a speed of 40 Gbps;
• High-speed optical QSFP modules have increased port density - this figure has grown by 3-4 times compared to previous generations
• The length of the communication line of the QSFP transceiver reaches 40 km, which is four times longer than that of its predecessors (previous modifications could give no more than 10 km)
• Hot-swap support - a unique feature that allows you to replace an optical module without necessarily shutting down or rebooting the equipment
• Online monitoring is carried out using DDM - a unique function that allows you to control the power of the incoming (Rx) and outgoing (Tx) channels, including temperature indicators during the operation of the transceiver
• Affordable price of QSFP modules - the cost is justified by the device's capabilities.

QSFP MTP Transceivers 

On the transmit end, 40G QSFP MTP modules use a laser array to convert electrical signals into optical signals. The conversion of the parallel optical signal into parallel electrical signals is done through a photodetector array on the receiving end. Then, these parallel optical signals are transmitted via MTP parallel cable.

Four streams of 10G serial data are passed to laser drivers on the transmit side in these transceivers. The laser divers regulate directly modulated lasers (DMLs). Then the output from the four DMLs is multiplexed onto a fiber cable via LC connector, and it is transmitted as a single, 40G optical signal. Each wavelength of light is received by a PIN and then forwarded as an electrical signal on the receiving end. A transimpedance amplifier or TIA then amplifies the received electrical signals. Unlike MTP QSFP transceivers, LC QSFP transceivers cannot be used in applications where splitting of 40G signal into 4 x 10G signals because they use four different wavelengths on a single pair of SMFs.

High-speed 40G QSFP modules are produced in various types, including; 

1. QSFP SR2 (BiDi) 

This model differs from the usual 40G module as it utilizes two wavelengths simultaneously for data transmission. QSFP BiDi optical modules require MMF and can achieve a maximum transmission rate of 20Gbps per channel. This type of transceiver uses 800/950 nm wavelength for transmission. 

2. QSFP PSM4

The device can split 40G into 4 x 10G, transmitting the signal over a distance of up to 10 km. The transceiver is equipped with four lasers with a wavelength of 1310 nm, as well as an MPO / APC connector (an 8xLC cable is used for splitting into 10G)

3. QSFP ER4

The module wavelength ranges from 1270 to 1330 nm. For a 40G channel, you will have to organize two single-mode optical fibers. In this case, the use of QSFP + ER4 will cost significantly more than installing four 10G channels. If you use 10G SFP + modules, then one fiber will be enough to provide high-speed communication

4. QSFP-DD

This unusual solution was created in order to maintain backward compatibility - the QSFP-DD ports are designed to work with the usual QSFP28. The model is equipped with an extended platform with an additional row of contacts, which can work with eight channels. Actually, because of these characteristics, the device was named Double Density.

The availability of 40G QSFP + optical modules in various types makes its integration easier for network operators. 40G QSFP+ is a viable intermediate solution for those planning to upgrade their existing 1G and 10G networks to 40G and beyond. Different versions of these transceivers are available for SMF and MMF.