Skip to content 
As DWDM system capacity increases, the input power of DWDM equipment also increases. Previously, when promoting G.654.E fiber, we always emphasized that higher input power would lead to increased nonlinear effects, thus limiting system capacity. However, when the backbone network DWDM single-wavelength rate increased to 400Gb/s, we found that the nonlinear effects caused by high input power were not as severe; rather, fiber burn-out was what required special attention.
Fiber burn-out refers to the carbonization of the pigtail sheath or bare fiber coating due to high temperatures, as shown in Figure 1.
How different is the fiber input power of the backbone network DWDM 80 x 400Gb/s system (hereinafter referred to as “400G system”) from that of the backbone network DWDM 80 x 100Gb/s system and Nx200G system (hereinafter referred to as “100G system” and “200G system”)?
Ingress Power of Backbone DWDM Systems
The input fiber power of 100G and 200G systems is directly proportional to the number of channels activated in the system. For example, if the optical power per channel is 1.26 mW (1.0 dBm), then the optical power of n channels is 1.26 mW x n, which translates to 1.0 + 10lgn in dBm.
The channel utilization rate varies greatly between different multiplexing sections in the backbone network; some sections may have 100% channel utilization, while others may only use a dozen or so channels. Table 1 shows the input fiber power of 100G and 200G systems using different channels when using G.652.D fiber.
| Item | 80x100G(QPSK) | 64x200G(QPSK) | |
| Single wavelength power (dBm) | 1.0 | 3.0 | |
| Input fiber optical power (dBm) | 20 wavelength | 14.0 | 16.0 |
| 40 wavelength | 17.0 | 19.0 | |
| 64 wavelength | 19.1 | 21.1 | |
| 80 wavelength | 20.0 | – | |
However, the input power of a 400G system is unrelated to the number of active channels. Whether the number of active channels is 10, 20, or 80, the input power remains the same (e.g., around 25.5 dB for Huawei and around 27.2 dB for ZTE), typically more than 5.0 dB higher than 100G and 200G systems.
Why is the input power of a 400G system unrelated to the number of active channels? This has to do with the SRS (stimulated Raman scattering) effect in single-mode fiber!
The SRS effect in optical fibers
The SRS effect is a nonlinear optical phenomenon in optical fibers. When a high-power optical signal propagates in an optical fiber, the short-wavelength (high-frequency) signal acts as a pump source, transferring energy to the long-wavelength (low-frequency) signal through Raman scattering, creating a phenomenon similar to “energy extraction,” as shown in Figure 2.
Figure 3 shows the relationship between the power transfer amplitude and frequency difference in the SRS effect. The power transfer amplitude is largest when the frequency difference is 13.4 THz. The backbone 400G system uses a total bandwidth of approximately 12 THz in the C+L bands. The frequency difference between the shortwave (C band) and longwave (L band) is close to the Raman gain peak (13.4 THz), resulting in a significant increase in the power transfer amplitude. Experiments show that the power transfer after a single-segment transmission in the C+L band can reach 7 dB, while the traditional C band (total bandwidth of approximately 4 THz) only has 1 dB.
The attenuation of short-wavelength signal power and the enhancement of long-wavelength signal power will lead to the degradation of the optical signal-to-noise ratio (OSNR) flatness of different channels at the receiver, affecting the bit error rate performance. Currently, the main technology used is optical amplifier tilt pre-compensation, which adjusts the gain spectrum of the EDFA so that the C-band gain is slightly higher than that of the L-band, to compensate for the loss of C-band power transferred to the L-band during transmission, as shown in Figure 4.
The strength of SRS (Short-Range Spectrum Distortion) is non-linearly positively correlated with the square of the input power, and the effect is more significant with higher power. When the number of channels activated in the system changes, the input power changes accordingly. After multiple transmission segments, power transfer accumulates segment by segment, further deteriorating system stability. To address this issue, the 400G backbone system employs dummy light (DL) technology, injecting dummy light to maintain a full-wavelength state and avoid performance fluctuations caused by power transfer. Capacity expansion or wavelength scheduling only requires replacing the dummy and true light sources.
In summary, it is precisely the expansion of the C+L (Continuous + Long-Range) frequency bands that makes the SRS non-linear effect non-negligible. DL filling technology, as a mature anti-distortion technique, ultimately ensures that the input power of the 400G system does not change with the number of activated channels.
Hidden dangers of high fiber input power
When the fiber input power is high, the plug of the movable connector can easily be damaged or the pigtail burned, leading to communication failure.
At the movable connector connection, when the connector end face is contaminated, high-intensity laser light can easily generate heat, damaging the end face of the fiber optic movable connector, as shown in Figure 5.
Because pigtails and bare fibers are very flexible, it is very common for their bending radii to not meet the standard (not less than 30 mm), as shown in Figure 6 (the pigtail sheath length in the figure is 28.5 mm). As a result, at fiber bends, high-power optical signals may leak from the fiber cladding, and localized high-power lasers can burn off the fiber coating and pigtail sheath. The fiber burning observed in the current network mostly occurs in pigtails within the central office or at fiber optic cable joints close to the central office, primarily due to insufficient fiber bending radius.
To address the safety concerns associated with high fiber input power in 400G systems, the bending radius of pigtails and bare fibers after installation and fixation should be no less than 30mm. To prevent fiber burn-out due to insufficient bending radius during construction, the optical path can be temporarily interrupted during equipment commissioning and fiber optic cable repair. The optical path should only be restored after the pigtails and bare fibers are properly secured.
