Security Risks Of High Fiber Input Power In 400Gb/s WDM Backbone Networks

As DWDM system capacity increases, the fiber power input into DWDM equipment is also increasing. Previously, when promoting G.654.E fiber, we emphasized that high input power would lead to increased nonlinear effects, thus limiting system capacity. However, when the single-wavelength rate of backbone DWDM networks increased to 400 Gb/s, we found that the nonlinear effects caused by high input power were not as serious, but fiber burning was the issue that required particular attention. Fiber burning refers to the carbonization of the pigtail jacket or bare fiber coating due to high temperatures, as shown in Figure 1.

Figure 1 - Pigtail Burning

How significant is the difference in fiber-input power between a backbone network DWDM 80×400Gb/s system (hereinafter referred to as the "400G system") and a backbone network DWDM 80×100Gb/s system and N×200G system (hereinafter referred to as the "100G system" and "200G system")?

Fiber input power of backbone network DWDM system

The fiber-input power of 100G and 200G systems is proportional to the number of channels enabled. For example, if the optical power per channel is 1.26 mW (1.0 dBm), the optical power of n channels is 1.26 mW x n, which converts to dBm as 1.0 + 101 g n. Channel utilization rates vary significantly across different
multiplex sections in backbone networks. Some sections may have 100% channel utilization, while others may only utilize a dozen or so channels. Table 1 shows the fiber-input power requirements for 100G and 200G systems using G.652.D fiber, depending on the channel.

Table 1 - Injection Power of 100G and 200G Systems

However, the fiber power of a 400G system is unrelated to the number of channels used. Regardless of whether the number of channels is 10, 20, or 80, the fiber power remains the same (e.g., approximately 25.5 dB for vendor H and 27.2 dB for vendor Z), typically exceeding 100G and 200G systems by more than 5.0 dB.

Why is the fiber power of a 400G system independent of the number of channels used? The reason lies in the SRS (stimulated Raman scattering) effect of single-mode fiber.

SRS Effect in Fiber

The SRS effect is a nonlinear optical phenomenon in optical fibers. When a high-power optical signal is transmitted through 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, resulting in a phenomenon similar to "energy extraction," as shown in Figure 2.

Figure 2 - Energy Transfer Due to the SRS Effect

Figure 3 shows the relationship between the energy transfer amplitude and the frequency difference in the SRS effect. The energy transfer amplitude is maximum when the frequency difference is 13.4 THz. The backbone network 400G system uses the C+L bands, with a total bandwidth of approximately 12 THz. The frequency difference between the shortwave (C-band) and longwave (L-band) frequencies is close to the Raman gain peak (13.4 THz), resulting in a significant increase in the energy transfer amplitude. Experiments have shown that the power transfer after single-span transmission in the C+L band can reach 7 dB, compared to only 1 dB in the traditional C-band (total bandwidth of approximately 4 THz).

Figure 3 - Relationship between Raman Gain and Frequency Deviation

The power attenuation of short-wavelength signals and the power enhancement of long-wavelength signals degrade the optical signal-to-noise ratio (OSNR) flatness of different channels at the receiver, impacting bit error rate performance. Currently, optical amplifier tilt pre-compensation technology is primarily used. This adjusts the EDFA's gain spectrum to make the C-band gain slightly higher than the L-band, compensating for the power loss caused by the transfer of C-band to the L-band during transmission, as shown in Figure 4.

Figure 4 - Pre-Compensation for Optical Amplifier Tilt

The intensity of SRS is nonlinearly positively correlated with the square of the input fiber power, with the effect becoming more pronounced with increasing power. As the number of channels activated in a system changes, the input fiber power also varies. After multiple transmission spans, power transfer accumulates across each span, further deteriorating system stability. To address this issue, the backbone 400G system employs "dummy lights" (DL) technology. This technology injects "dummy lights" to maintain full system power and avoid performance fluctuations caused by power transfer. Capacity expansion or wavelength scheduling simply requires replacing dummy lights with real lights.

In summary, the nonlinear effects of SRS are significant due to the expansion of C+L frequency bands. DL filling, a mature distortion-reduction technology, ultimately ensures that the input fiber power of 400G systems remains stable despite changes in the number of channels activated.

Hidden dangers of high fiber input power

High fiber input power can easily damage the active connector plug or burn the pigtail, leading to communication failures.

At the active connector connection, if the connector end face is contaminated, the high-intensity laser can easily generate heat, causing damage to the end face of the optical fiber active connector, as shown in Figure5.

Figure 5 - Damage to the Fiber Optic Connector Endface

Because pigtails and bare fibers are very flexible, it's common for their bend radius to fail to meet the standard (minimum 30 mm), as shown in Figure 6 (the pigtail jacket is 28.5 mm long). Consequently, high-power optical signals can leak from the fiber's cladding at bends, and localized high-power laser light can burn the fiber coating and pigtail jacket. Fiber burns observed in existing networks often occur in pigtails within base stations or at optical cable connectors close to base stations, primarily due to insufficient fiber bend radius.

Figure 6 - Current Status of Pigtail Layout in Equipment

To address the safety risks associated with high fiber input power in 400G systems, the bend radius of the pigtail and bare fiber after installation must be no less than 30mm. To prevent fiber burnout due to insufficient bend radius during installation, the optical path can be temporarily interrupted during equipment commissioning and cable repairs. The optical path will only be restored after the pigtail and bare fiber are secured as required.

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