OFC2025|Breakthrough in Unrepeatered Fiber Transmission Using Incoherent Raman Amplification

Introduction

Unrepeatered fiber links play a crucial role in submarine communications, especially where installing powered amplifiers along the cable is impractical or prohibitively costly. Recent advances in fiber technology, amplification methods, and signal processing have enabled transmission distances exceeding 400 kilometers, supporting multi-terabit capacities. This paper discusses a significant breakthrough in unrepeatered transmission, using incoherent Raman amplification technology without remote optically pumped amplifiers (ROPA) [1].

Understanding the Challenges of Unrepeatered Transmission

The fundamental challenge of unrepeatered submarine links is signal attenuation over long distances. Traditional methods leverage ultra-low-loss fibers, specialized signal coding, and distributed Raman amplification (DRA) to maximize distances. While ROPA significantly extends transmission distances, it adds complexity and cost.

Let us examine recent advances in unrepeatered transmission capacity and distance.

Figure 1: (a) Recent throughput vs. distance in unrepeatered transmission systems. Blue circles indicate systems using ROPA; filled blue circles represent systems without ROPA. This study achieved 403 km with 10.3 Tb/s without ROPA. (b) Semiconductor optical amplifier (SOA)-based incoherent Raman pumping structure, showing seed SOA, lens coupling, and enhanced SOA.

Incoherent Raman Pumping: Innovative Approach

The key innovation of this research lies in using incoherent Raman pumping (iPUMPs) combined with conventional backward-propagating pumping. Unlike traditional coherent pumping, iPUMPs utilize amplified spontaneous emission (ASE), averaging gain contributions from multiple wavelength components and reducing relative intensity noise (RIN).

The internal structure of the incoherent pumping system (Figure 1(b)) uses semiconductor optical amplifiers (SOAs) to generate and amplify ASE. This innovative approach includes a seed SOA followed by lens coupling systems and an enhanced SOA with an isolator to prevent back reflections.

Record-Breaking Experimental Setup

Figure 2: (a) Complete experimental setup for 403.1 km unrepeatered transmission, showing signal generation, fiber link configuration, and receiver components. (b) Spectral profiles at fiber input and output. (c) Link loss characteristics showing minimum loss of 62.7 dB. (d) Spectral distribution of signals and pumps. (e) Raman gain distribution, indicating complementary forward-propagating (~10 dB) and backward-propagating (~30 dB) pump contributions, resulting in total bidirectional gain of up to 37 dB.

The transmitted signal consists of 73 wavelength channels, each carrying 49 GBaud polarization multiplexed quadrature phase shift keying (PM-QPSK) signals. Channels span 30 nm from 1547.1 nm to 1576.8 nm, covering portions of C-band and L-band, with 50 GHz spacing.

Signal generation employs a sliding test band approach, including three channels and ASE-based dummy bands. Test channels are generated by external cavity lasers (ECLs) and dual-polarization IQ modulators, driven by a 65 GSa/s arbitrary waveform generator.

The transmission link consists of a 251.9 km Sumitomo Z+ULL fiber, flanked by 50.4 km and 100.8 km segments of Corning SMF28 ULLS+ fiber, achieving a minimum loss of 62.7 dB at 1561.6 nm (Figure 2(c)).

Bidirectional Distributed Raman Amplification

To compensate for significant fiber losses, researchers employed bidirectional distributed Raman amplification. Forward-propagating direction utilized four iPUMPs at central wavelengths of 1425 nm, 1461 nm, and 1495 nm, with power between 40 nm and 90 nm. These iPUMPs were amplified by five forward-propagating coherent pumps with wavelengths ranging from 1330 nm to 1395 nm and powers up to 260 mW.

Backward-propagating direction used four traditional coherent Raman pumps at wavelengths 1426 nm, 1444 nm, 1462 nm, and 1490 nm, totaling 1.5 W. As shown in Figure 2(e), the bidirectional configuration achieved up to 37 dB gain, with backward pumping providing approximately 30 dB and forward pumping about 10 dB.

Performance Analysis and Results

Figure 3: (a) OSNR and Q-factor wavelength dependency, showing OSNR above 9.5 dB across the 30 nm range. (b) Channel throughput estimation using GMI and soft-decision FEC, with total system throughput reaching 10.37 Tb/s.

Received signals were separated into C-band and L-band components, filtered individually, and detected using a 32 GHz coherent receiver. Digital signals underwent offline processing with frequency-domain dispersion compensation and a 33-tap, 2×2 time-domain MIMO equalizer.

Figure 3(a) illustrates OSNR and Q-factor measurements across the entire spectral range. OSNR consistently remained above 9.5 dB, with all channels exhibiting Q-factors above 2.8 dB.

Figure 3(b) presents throughput performance, with nearly all channels achieving over 130 Gb/s post Forward Error Correction (FEC). Total system throughput after FEC reached 10.37 Tb/s, with Generalized Mutual Information (GMI) estimates suggesting a potential system capacity up to 11.21 Tb/s.

Conclusion

This study successfully demonstrated a record 403-km unrepeatered transmission of 10.3 Tb/s without employing ROPA. This achievement resulted from an innovative combination of bidirectional Raman amplification and forward-propagating incoherent pumping, effectively reducing signal degradation from RIN.

These findings set a new capacity-distance benchmark for unrepeatered systems exceeding 400 km without ROPA. The approach offers an efficient solution for long-distance submarine communications, particularly beneficial where simplicity, reliability, and cost-effectiveness are paramount.

With a capacity-distance product exceeding 4 Pb/s·km, this represents a significant advancement in maximizing unrepeatered fiber link transmission capacity. This technology holds considerable promise for expanding global submarine communication networks, especially in challenging routes where conventional repeatered systems are impractical.

References

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