Along with the growth in AI computing power demands, the global optical communication industry is under increasing pressure to enhance data transmission rate. 800Gbps technology has already been gradually commercialized, and recently, industry leading players such as NVIDIA, Broadcom, Marvell, and Cisco have successively released 1.6Tbps products. At the same time, companies are actively investing in research and development to achieve higher transmission rate.

Currently, the mainstream approach for achieving 1.6Tbps data transmission rate is based on 200Gbps per lane. To achieve higher data rate in the future, there are three key approaches: improving modulation scheme, increasing symbol rate, and enhancing transmission channels by WDM or increasing fibers. We are going to explore the potential challenges in technical implementation and in testing accordingly.

1. Adopting Higher-Order Modulation to Increase Bits per Symbol

In current commercial applications, PAM4 modulation has been adopted widely.  It effectively doubling the data transmission rate comparing to NRZ modulation. At present, PAM4 plays a significant role in achieving 200Gbps per lane data transmission rate. However, to enhance data transmission rate in the future, it will be necessary to explore higher-order modulation schemes. PAM6 and PAM8 are the potential candidates under investigation, along with the possibility of utilizing QAM and other advanced modulation techniques.

PAM6 utilizes 6 power levels, with each symbol carrying approximately 2.585 bits, while PAM8 utilizes 8 power levels, carrying 3 bits per symbol. Compared to PAM4, which carries 2 bits per symbol, these higher-order modulation schemes offer an increasement in data transmission rate. However, as the difference between modulated signal power levels become smaller, a higher SNR is required to ensure reliable transmission and demodulation.

Typical SNR Requirements for Different Modulation Schemes:

Therefore, higher requirements are imposed on the overall system design. Not only does the hardware need to be comprehensively enhanced in terms of performance, but error correction algorithms must also be integrated to ensure reliable system operation.

Higher-order modulation schemes such as 64QAM or higher have been adopted in RF transmission and long-haul backbone coherent optical communication systems where cost and power consumption are less sensitive. However, in short distance optical communication systems, there are still many uncertainties and challenges that need to be addressed and improved. To test the optical communication systems adopting higher-order modulation schemes, the instruments have to minimize its own noise as much as possible to prevent the impact to the system noise level. The figure below shows an ideal eye diagram of PAM6. We can imagine the impact to the eye diagram if the test instrument introduced the extra noise. during the test.

Meanwhile, as data transmission rate increase, higher requirements are placed on the clock recovery unit to obtain a synchronized clock signal. If the synchronization clock experiences drift or unlock, it will be impossible to get a correct eye diagram.

2.   Increasing Symbol Rate

Where Rₛ is the symbol rate, and α is the roll-off factor ranging from 0 to 1.From this, we can see that the system's spectrum bandwidth increases as the symbol rate increases, which imposes higher requirements on device and system design. Additionally, higher symbol rate make signals more sensitive to dispersion and polarization mode dispersion. Achieving higher transmission rate requires system-level optimization of both hardware and software, along with the adoption of higher-order modulation schemes. Industry leading players expect to reach 200GBaud transmission rate by 2025.

And for testing, the test instruments must have enough bandwidth to accommodate higher symbol rate measurements. This imposes higher requirements on the hardware capabilities of the instruments to ensure accurate and reliable testing.

3.  Increasing the Number of Wavelengths per Fiber or Expanding the Number of Fibers

In addition to adopting higher-order modulation schemes and increasing the symbol rate, enhancing overall data transmission rate in optical communication systems can also be achieved through WDM (wavelength division multiplexing) and parallel fiber transmission.

The WDM (wavelength division multiplexing) technology, is to transmit multiple wavelength carriers within a single optical fiber to increase the overall data transmission rate. Based on the wavelength spacing, WDM can be categorized into Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

According to Shannon's formula

By directly increasing the number of physical channels, the system capacity can be multiplied. In current commercial optical communication systems, 8 or 16 parallel optical fibers are commonly used, with future expansions expected to reach 32 fibers or even more. To accommodate the increasing number of fibers, system design must consider how to efficiently manage optical signal switching.