What is QSFP QSFP Working Principle and Design Structure

March 10, 2022 0 By sylvia

40G QSFP+ transceiver is now taking the place of the 10G SFP+ transceiver and becoming the mainstream in today’s fiber optic transceivers market. This article will analyze 40G QSFP+ transceivers working principle and design structures of transmitter and receiver, helping you have a full understanding of QSFP professionally.

What is a QSFP Transceiver?

QSFP stands for Quad Small Form-factor Pluggable, which is compliant with IEEE Std 802.3 ba and SFF-8436 standard. Unlike the SFP+ transceiver with only a single lane, the QSFP transceiver has four independent full-duplex transmit and receive channels, each of which supports up to 10Gbps data rates, resulting in a total of 40Gbps. QSFP transceiver modules designed for 40G Ethernet with high-density and low-power performance.

Typically QSFP Transceivers Portfolio and Spec

Table 1 — QSFP Transceivers Portfolio and Spec

How does QSFP Transceiver Work?

Next, the article will take the QSFP-40G-SR4 as the research object and introduce the working principle of QSFP 40G SR4 transceivers. At the transmitter, four-lane parallel 10 Gbp/s electrical signals are converted into optical signals through the laser driver chip. Once this process has been performed, the optical signals can then be transmitted over optical fiber to the receiver. At the receiver, the optical signals are then recovered as electrical signals via photodetector, as shown in the figure. 1.

Figure 1–40G QSFP Transceiver Working Principle

Design Structure of QSFP Transceiver

As we can see from the above diagram, the 40G QSFP+ SR4 transceivers transmitter is structured with a four-lane 850nm VCSEL array and laser driver chip while the receiver is structured with four-lane PIN photodetectors of the receiver. Then, we will analyze the transmitter and receiver structure in detail.

The Transmitter Structure of 40G QSFP Transceiver

The transmitter of the QSFP 40G transceiver consists of a circuit interface, laser driver chip, and laser. QSFP-40G-SR4, for example, is designed for short-distance transmission at a wavelength of 850nm, so it adopts VCSEL laser. Unlike LED and LD, VCSEL supports higher modulation frequency and transmission rate. Besides, VCSEL enables low driving voltage and current that greatly extends the life of VCSEL. Moreover, The production process of VCSEL is compatible with LED and enables evolution from LED production, which makes a foundation for large-scale production of VCSEL. Consequently, VCSEL enables advantages of both high reliability and cost-effectivity. The transmitter structure of QSFP-40G-SR4 is shown in figure 2.

Figure 2 — QSFP-40G-SR4 Transmitter Structure

As for the laser driver chip, which is designed to provide appropriate bias current and modulation current, meanwhile, the chip also supports current detection, temperature detection, and interrupt output. Figure 3 shows the working principle of the laser driver chip.

Figure 3–40G QSFP Laser Driver Chip Working Principle

The Receiver Structure of 40G QSFP Transceiver

The transmitter of the QSFP 40G transceiver consists of a PIN array, driven chip, and circuit interface, as shown in figure 4. After the optical signals convert into weak electric signals, it’s time for the driver chip to work. Stable and qualified electric signals output through the driver chip, which is integrated with a transimpedance amplifier (TIA), which is designed for amplifying the signals to make up for the signal transmission loss, meantime, converting the current signals into voltage signals. Figure 5 shows the working principle of the photodetector driver chip.

Figure 4 — QSFP 40G Receiver Structure
Figure 5 — QSFP 40G Photodetector Driver Chip Working Principle


As mentioned above, we have a more detailed understanding of the working principle and structure of the QSFP+ 40G module. If you have purchase demand on it, QSFPTEK will be a good choice. QSFPTEK offers a wide variety of high-density and low-power 40G QSFP+ modules for data centers, high-performance computing networks, enterprise core, distribution layers, and service provider applications. QSFPTEK 40G QSFP modules compatible with Cisco 40G QSFP. Arista, Juniper, Brocade, Intel compatibility are also available, each of which has been tested on target switches. Welcome to inquire at sales@qsfptek.com.