In the pursuit of higher capacity and longer transmission distance in modern optical communication systems, noise, as a fundamental physical limitation, has always constrained performance improvement.
In a typical EDFA erbium-doped fiber amplifier system, each optical transmission span generates approximately 0.1dB of accumulated spontaneous emission noise (ASE), which is rooted in the quantum random nature of the light/electron interaction during the amplification process.
This type of noise manifests as picosecond level timing jitter in the time domain. According to the jitter model prediction, under the condition of a dispersion coefficient of 30ps/(nm · km), the jitter increases by 12ps when transmitting 1000km. In the frequency domain, it leads to a decrease in optical signal-to-noise ratio (OSNR), resulting in a sensitivity loss of 3.2dB (@ BER=1e-9) in the 40Gbps NRZ system.
The more severe challenge comes from the dynamic coupling of fiber nonlinear effects and dispersion – the dispersion coefficient of conventional single-mode fiber (G.652) in the 1550nm window is 17ps/(nm · km), combined with the nonlinear phase shift caused by self phase modulation (SPM). When the input power exceeds 6dBm, the SPM effect will significantly distort the pulse waveform.
In the 960Gbps PDM-16QAM system shown in the above figure, the eye opening after 200km transmission is 82% of the initial value, and the Q factor is maintained at 14dB (corresponding to BER ≈ 3e-5); When the distance is extended to 400km, the combined effect of cross phase modulation (XPM) and four wave mixing (FWM) causes the eye opening degree to drop sharply to 63%, and the system error rate exceeds the hard decision FEC error correction limit of 10 ^ -12.
It is worth noting that the frequency chirp effect of direct modulation laser (DML) will worsen – the alpha parameter (linewidth enhancement factor) value of a typical DFB laser is in the range of 3-6, and its instantaneous frequency change can reach ± 2.5GHz (corresponding to chirp parameter C=2.5GHz/mA) at a modulation current of 1mA, resulting in a pulse broadening rate of 38% (cumulative dispersion D · L=1360ps/nm) after transmission through an 80km G.652 fiber.
Channel crosstalk in wavelength division multiplexing (WDM) systems constitutes deeper obstacles. Taking the 50GHz channel spacing as an example, the interference power caused by four wave mixing (FWM) has an effective length Leff of about 22km in ordinary optical fibers.
Channel crosstalk in wavelength division multiplexing (WDM) systems constitutes deeper obstacles. Taking the 50GHz channel spacing as an example, the effective length of interference power generated by four wave mixing (FWM) is Leff=22km (corresponding to fiber attenuation coefficient α=0.22 dB/km).
When the input power is increased to+15dBm, the crosstalk level between adjacent channels increases by 7dB (relative to the -30dB baseline), forcing the system to increase the forward error correction (FEC) redundancy from 7% to 20%. The power transfer effect caused by stimulated Raman scattering (SRS) results in a loss of approximately 0.02dB per kilometer in long wavelength channels, leading to a power dip of up to 3.5dB in the C+L band (1530-1625nm) system. Real time slope compensation is required through a dynamic gain equalizer (DGE).
The system performance limit of these physical effects combined can be quantified by bandwidth distance product (B · L): the B · L of a typical NRZ modulation system in G.655 fiber (dispersion compensated fiber) is approximately 18000 (Gb/s) · km, while with PDM-QPSK modulation and coherent detection technology, this indicator can be improved to 280000 (Gb/s) · km (@ SD-FEC gain 9.5dB).
The cutting-edge 7-core x 3-mode space division multiplexing fiber (SDM) has achieved a transmission capacity of 15.6Pb/s · km (single fiber capacity of 1.53Pb/s x transmission distance of 10.2km) in laboratory environments through weak coupling inter core crosstalk control (<-40dB/km).
To approach the Shannon limit, modern systems need to jointly adopt probability shaping (PS-256QAM, achieving 0.8dB shaping gain), neural network equalization (NL compensation efficiency improved by 37%), and distributed Raman amplification (DRA, gain slope accuracy ± 0.5dB) technologies to increase the Q factor of single carrier 400G PDM-64QAM transmission by 2dB (from 12dB to 14dB), and relax the OSNR tolerance to 17.5dB/0.1nm (@ BER=2e-2).
Media ContactCompany Name: Hangzhou Softel Optic Co., Ltd.Email: Send EmailPhone: +86 571 8898 9381Address:708-709 Haiwei Building, Binjiang District City: HangzhouCountry: ChinaWebsite: https://www.softeloptic.com/
