With the rapid rise of cloud computing and big data, data centers and telecom operators have higher and higher requirements for the transmission rate of optical modules. Since 1998, optical modules have been continuously upgraded towards higher rates and smaller packages. Optical modules generally use technical solutions such as increasing the number of wavelengths, increasing the number of signal transmission channels, and increasing the single-channel rate to achieve higher transmission rates for optical modules. This article briefly introduces these three technical solutions.
1. Increase the number of wavelengths
The principle of increasing the number of wavelengths to achieve a higher transmission rate of the optical module is based on the wavelength division multiplexing (WDM) technology. Optical signals of different wavelengths are coupled to an optical fiber through a combiner for transmission, and then the optical signal is decomposed into the original multiple optical wave signals by a demultiplexer, which is usually called WDM technology.
WDM technology allows multiple optical signals of different wavelengths to be transmitted on the same optical fiber, thereby achieving multiplexing and improving the transmission capacity and rate of the optical fiber. When the number of wavelengths is increased, each wavelength can carry an independent data stream, so that multiple data streams can be transmitted on the same optical fiber, thereby increasing the overall transmission capacity. This method can effectively increase the transmission rate of the optical module without changing other parts of the optical module.
Through WDM technology, optical signals of different wavelengths can be transmitted independently in the optical fiber, and they will not interfere with each other. In this way, the goal of higher transmission rate of the optical module can be achieved, and the utilization rate and transmission efficiency of the optical fiber can also be improved.
Depending on the wavelength interval, optical modules will use CWDM, LWDM and SWDM technologies.
(1)CWDM coarse wavelength division multiplexing technology, the wavelength range is between 1270nm-1610nm, the wavelength interval is 20nm, and 8 to 16 wavelengths can be multiplexed on the same optical fiber. Representative optical modules include QSFP+ LR4 and QSFP28 CWDM4.
(2) LWDM fine wavelength division multiplexing technology, the wavelength range is between 1269nm and 1332nm, belonging to the O band, the wavelength interval is 4nm, and the operating wavelengths are 1295nm, 1300nm, 1304nm, and 1309nm. Representative optical modules include QSFP28 LR4, QSFP28 ER4, and QSFP28 ZR4.
(3) SWDM short wavelength division multiplexing technology, the wavelength range is between 850 and 950nm, the band interval is 30nm, and the four band windows are 850nm, 880nm, 910nm, and 940nm. The representative optical modules are multi-mode 40G SWDM4 and 100G SWDM4.
2. Increase the number of signal transmission channels
By increasing the number of signal transmission channels, multiple channels with the same wavelength are used to transmit signals, which is called parallel optical technology. The operating wavelengths are 850nm and 1310nm, which is a cost-effective solution for 4*25G, 4*50G and 8*50G. Representative optical modules include QSFP+ SR4, QSFP28 SR4 and QSFP-DD SR4.
3. Increase the single-channel rate
NRZ (non-return-to-zero modulation) and PAM4 (4-level pulse amplitude modulation) are two common modulation technologies used to increase the single-channel rate of optical modules.
NRZ technology is a common binary modulation method that sends a pulse of fixed amplitude in each clock cycle to represent the data bit. The transmission rate of NRZ technology is limited because it can only transmit one bit per unit time. However, NRZ technology is still widely used in many optical communication systems.
PAM4 technology encodes data by sending 4 pulses of different amplitudes in each clock cycle. Compared with the traditional binary modulation method, PAM4 can transmit more bits per unit time, thereby increasing the transmission rate of a single channel. PAM4 technology has been widely used in high-speed optical communications and data center interconnection.
As the next generation of high-speed signal interconnection transmission technology, PAM4 achieves a higher transmission rate per channel per unit time with its more signal levels. While ensuring that the current number of channels and existing optical devices remain unchanged, the network interface rate can be increased to twice the original by upgrading the internal electrical chip of the optical module. Representative optical modules include 50G SFP56-DD SR (1*50G PAM4), 200G QSFP56 FR4 (4*50G PAM4), and 400G QSFP-DD SR8 (8*50G PAM4).
Compared with the traditional NRZ signal, the PAM4 signal has two more level signals for signal transmission. In the same symbol period, the bit rate of the PAM4 signal is twice that of the NRZ signal.
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