Introduction

In an optical communication link, the signal-to-noise ratio (SNR) is a critical factor, as higher SNR enables greater transmission capacity according to the Shannon theorem. However, in practical systems, the SNR is inevitably deteriorated due to the non-ideality of transmission links, which ultimately limits the achievable transmission performance. Current optical fiber communications based on conventional single-mode fibers (SMFs) face challenges in increasing the transmission capacity because the signal SNR is degraded by amplifier noise and fiber nonlinearities1. As a promising technology to overcome the capacity limit of SMF transmission systems, space division multiplexing (SDM) has an additional new noise component degrading the signal SNR, which is the spatial crosstalk, such as the inter-core crosstalk in multi-core fibers (MCFs), resulting from mode coupling between cores2,3,4. Despite the use of weakly-coupled schemes in SDM systems, such as uncoupled multi-core fibers5 and weakly-coupled few-mode fibers6,7, which can reduce spatial crosstalk to some extent, the SNR of SDM systems is still negatively impacted by the noise from optical amplifiers. Moreover, compared to unidirectional transmission systems, bidirectional transmission systems based on MCFs not only offer higher capacity but also substantially conserve fiber resources8. This advantage is notably apparent in scenarios such as submarine cables, where substantial transmission capacity is required, yet the deployment and maintenance costs of optical cables are very high. However, in bidirectional transmission systems, the SNR is severely constrained by Rayleigh backscattering noise9,10. Accordingly, techniques to improve SNR via mitigating amplifier noise, nonlinear phase distortion, and Rayleigh backscattering noise are of critical importance for further enhancing the transmission throughput in optical fiber communications and thus have attracted much research attention over the past few decades. In particular, all-optical approaches are highly desired due to the attractive feature of wideband operation and high energy efficiency11,12.

It is well recognized that phase-sensitive amplifiers (PSAs) by means of optical parametric amplification (OPA) in highly nonlinear media are capable of SNR enhancement in an all-optical manner and are very promising for improving the transmission performance of optical communications. In a PSA, coherent superposition of a signal and its conjugated copy (i.e., idler) via nonlinear wave mixing can provide an additional gain for the deterministic, correlated waves over that for the uncorrelated, stochastic fields, such as vacuum fluctuations13 and amplified spontaneous emission from other amplifiers14. This gain difference gives rise to low-noise amplification with 0-dB noise figure (NF) limit outperforming erbium-doped fiber amplifiers (EDFAs)15,16. In particular, a frequency-non-degenerate PSA (FND-PSA)17,18, in which the signal and idler are at separate optical frequencies, can reach a NF limit of −3 dB for the signal, meaning that ideally the signal SNR can be enhanced by 3 dB. Another benefit from the PSA is that the coherent superposition of the signal and idler waves enables mitigation of phase distortions induced by fiber nonlinearities19. Mitigating nonlinear distortions caused by intra-channel20,21,22 and inter-channel effects23 has been demonstrated using the FND-PSA scheme, which is format-independent and compatible with wavelength-division-multiplexing (WDM) systems.

A variety of fundamental investigations on utilizing PSAs in fiber transmission links have been reported, aiming for throughput increase and extension of transmission distance, for instance, the amplifier system design for broader bandwidth24,25,26, higher gain18,27,28, and lower crosstalk29,30, optimization of nonlinear distortion compensation31, etc. Long-haul transmission of a single-channel 10-Gbaud QPSK signal benefiting from the simultaneous low-noise amplification and nonlinearity mitigation of PSA has been reported, showing a 5.6-times reach improvement compared to a transmission link using conventional in-line EDFA20. Shimizu et al. demonstrated WDM transmission with an 8-Tb/s total throughput by using a wideband PSA based on periodically poling lithium niobate (PPLN) waveguides, which has an over-100-nm bandwidth outperforming the EDFA bandwidth32. Nevertheless, these successful attempts of phase-sensitively amplified transmission are implemented in the laboratory. While the pioneering field trial by Slavik et al. has demonstrated PSA implementation for phase-shift keying signal regeneration33, their approach exhibits inherent limitations for broader practical applications of PSA, including modulation format constraints and single-wavelength operation that hinders its extension to high-capacity WDM systems.

There have been no field trial demonstrations of phase-sensitively amplified WDM fiber transmission to provide convincing verifications towards practical applications. In addition, previous studies focused on the PSA performance in unidirectional SMF transmission systems, there have been limited investigations on the SNR enhancement and system performance improvement enabled by PSAs in a practical same-wavelength bidirectional transmission link, which has great potential to increase the capacity of current optical communication systems.

Here we present the field trial of high-capacity phase-sensitively amplified transmission in a deployed optical fiber cable. Same-wavelength bi-directional transmission of WDM signals is utilized to increase the transmission capacity. A PPLN-based wideband PSA is used at the receiver side as a pre-amplifier, which not only increases the signal power but also improves the SNR by lowering the interference crosstalk primarily caused by Rayleigh backscattering along the fiber. A net transmission capacity of 10.944Tb·s−1 and a net capacity-distance product of 541.728Tb·s−1·km are achieved, exceeding the results reported in the literature for phase-sensitively amplified transmission demonstrations. These results confirm the favorable performance of PSA in SNR improvement for high-capacity fiber transmission in real deployment.

Results

Field-trial phase-sensitively amplified bi-directional transmission system

Figure 1 displays the schematic of a field-trial bidirectional transmission link with a PSA pre-amplified receiver. At the transmitter, 38-channel single-polarized WDM signals densely allocated in a 50-GHz grid from 191.45 THz to 193.3 THz were injected into an optical phase conjugator (OPC) stage to generate idler lights (phase conjugation of the signals). The WDM signals included a test channel and 37 dummy channels, each modulated with distinct 45-GBaud 16QAM data streams, yielding a total raw rate of 6.84 Tb·s−1. The idlers were generated by cascaded second-harmonic generation and optical parametric amplification (cSHG/OPA) in a fiber-pigtailed module consisting of two PPLN waveguides, in which a continuous-wave at 1550.12 nm was used as the pump. The programmable optical filter (POF) is set at the transmitter for power equalization and pre-compensation of the dispersion in the following transmission link. To transmit the same wavelength sets of lights from both terminals of the bidirectional transmission system, the signal-idler pairs and the pump after the OPC stage were split into two equal beams by a 3-dB coupler. One beam was fed into the transmission link in Direction 1 via an optical circulator (CIR1), and the other beam was fed into the transmission link in Direction 2 via CIR2.

Fig. 1: Schematic diagram of field-trial phase-sensitively amplified bi-directional transmission system.
figure 1

a Transmitter and receiver of PSA-based bidirectional transmission system. Insets (I)–(IV) are conceptual optical spectra for the OPC input, OPC output, PSA input, and PSA output, respectively. b Deployed fiber cable encircling the Guangzhou Higher Education Mega Center. Insets show the cross section of seven-core fiber (top) and the armored cable (bottom). The map depicting the Guangzhou Higher Education Mega Center is based on the exact map presented in ref. 5. ECL external cavity laser, WDM wavelength division multiplexer, AWG arbitrary waveform generator, IQM in-phase quadrature modulator, EDFA erbium-doped fiber amplifier, POF programmable optical filter, PC polarization controller, OC optical coupler, OPC optical phase conjugator, CIR circulator, PSA phase sensitive amplifier, OBPF optical bandpass filter, LO local oscillator, RTO real-time oscilloscope, DSP digital signal processing.

Full size image

The transmission link is based on a field-trial testbed that consists of a field-deployed armored optical fiber cable encircling the Guangzhou Higher Education Mega Center along the underground tunnel5, as shown in Fig. 1b. The optical cable consists of four 16.5-km uncoupled seven-core fibers. Three of these fibers are connected using fan-in/fan-out (FI/FO) devices to establish a 49.5-km transmission link in our experiment (see Supplementary Note 1). Each silica-core seven-core fiber has a hexagonal structure with a central core and six surrounding cores, featuring a cladding diameter of 200 ± 5 μm for each core and a pitch of 62 ± 2 μm between the cores, as illustrated in the top inset of Fig. 1b. The central core of the deployed fiber was used for the following bidirectional transmission experiments, while no signals were loaded in the other cores. The central core has an average splicing loss of about 0.3 dB per point and a transmission attenuation of roughly 0.2 dB·km−1 at 1550 nm. The total loss of the bidirectional transmission link was 20 dB, including the fiber loss, splicing loss, the insertion loss of the FI/FO components, and the circulators.

At the receiver, the signal-idler pairs and the pump were injected into a PSA stage for SNR enhancement of the signals. The PSA stage employed another PPLN module similar to that used in the OPC stage, offering wideband phase-sensitive amplification with a flat gain of 16.8 dB and a residual wavelength-dependent gain of 1.8 dB within a 12.5-THz band from 1500 to 1600 nm (see Supplementary Note 3). After the PSA, the signal from the test channel was first filtered and detected using a standard coherent receiver, then captured on a real-time oscilloscope for subsequent offline digital signal processing (DSP). A more detailed description of the transmitter and receiver setup is presented in Supplementary Note 4.

SNR improvement by PSA

In contrast to unidirectional transmission, the signal quality in a same-wavelength bidirectional transmission link is mainly limited by intra-core Rayleigh backscattering crosstalk (RBXT) (see Supplementary Note 1). To demonstrate both the impact of RBXT and the enhancement afforded by the PSA, we measured the optical spectra of all transmitted waves and captured the constellation diagrams of the signal at 192.4 THz at several key points within the bidirectional transmission system (as shown in Fig. 2a). In our bidirectional transmission experiment, the same set of signal-idler pairs and pump, as depicted in Fig. 2b-II, were transmitted in both directions of the fiber. Consequently, not only the transmitted lights but also the reflected lights caused by Rayleigh backscattering were observed at the link output, as indicated by the blue and red curves in Fig. 2b-III. Note that the reflected signals were measured when only the optical waves in Direction 2 were transmitted. It is evident from Fig. 2b-III that the power of the reflected signals is only 14.6 dB lower than the signal power after the 49.5-km transmission in the deployed fiber cable. After the PSA, this optical power difference increases to approximately 17.1 dB, as shown in the optical spectrum in Fig. 2b-IV. This increase indicates that the optical SNR of the WDM signals is improved by 2.5 dB, aligning well with the 3-dB theoretical value (for details, see the Methods section). The signal constellation diagrams at 192.4 THz before and after the PSA, displayed in Fig. 2c-III and 2c-IV, respectively, demonstrate a clear improvement in signal quality.

Fig. 2: Measured optical spectra and constellation diagrams in the PSA-based bidirectional transmission system.
figure 2

a Schematic of the PSA-based same-wavelength bidirectional transmission system. b Experimental optical spectra of the transmitter output, OPC output, transmission link output, and PSA output, corresponding to points I~IV in a, respectively. c Constellation diagrams of the signal at 192.4 THz measured at the transmitter out, OPC output, transmission link output, and PSA output, corresponding to points I~IV in a, respectively.

Full size image

Transmission performance

To demonstrate the large capacity of the phase-sensitively amplified bidirectional transmission in the field test, the BER values of the received signals were measured for all the wavelength channels transmitted simultaneously in both directions. The link input power for each wavelength channel was set at −6 dBm, as Rayleigh noise is the primary SNR-limiting factor in the EDFA-preamplified bidirectional system at this power. Figures 3a and 3b show the measured BER results as a function of signal frequency for both directions. The crosstalk noise from reflected lights in the bidirectional transmission leads to high BER values for the signals (indicated by the red rectangles), which mostly exceed the 20% soft-decision forward error correction (FEC) threshold of 2.4 × 10−2, indicating unsuccessful transmission. Note that in this case without using the PSA, the signal was pre-amplified by an EDFA with the same gain as the case with PSA. When the PSA was used as the pre-amplifier for the signal, the BER decreased to approximately 1.0 × 10−2 for all signal channels in both directions, well below the FEC threshold. The results indicate that by using the PSA-based pre-amplifier, successful transmission was achieved with an aggregate capacity of 13.68 Tb·s−1 (net 10.944 Tb·s−1) and a capacity-distance product of 677.16 Tb·s−1·km (net 541.728 Tb·s−1·km), showing the great potential of PSA for SNR improvement in high-capacity bidirectional transmission.

Fig. 3: Bit-error rate (BER) results of 38 channels in both directions for PSA pre-amplified and EDFA pre-amplified same-wavelength bidirectional transmission.
figure 3

a Direction 1. b Direction 2. The blue circles are the PSA pre-amplified results, while the red squares indicate EDFA pre-amplified results. The black dotted line is the BER threshold of 2.4 × 10−2, which is the 20% pre-FEC limited.

Full size image

Discussion

This work presents a demonstration of PSA enabling bidirectional same-wavelength DWDM transmission in a deployed fiber link, verifying the SNR enhancement capability of the PSA in practical optical links. By implementing a wideband PSA in the bidirectional DWDM system, an aggregate capacity and capacity-distance product are achieved that outperform previous phase-sensitively amplified unidirectional transmission demonstrations20,32,34,35, as shown in Fig. 4. This pioneering field trial of phase-sensitive amplification for bidirectional WDM transmission in a deployed fiber cable validates the effectiveness of PSA in enhancing SNR for practical optical systems, establishing PSA as a potential solution for high-capacity optical communication systems.

Fig. 4
figure 4

Representative demonstrations of fiber transmission utilizing PSA as inline amplifier or pre-amplifier.

Full size image

Constrained by the operational wavelength range of the POF in our experimental setup, the signal bandwidth in the transmission experiment only covers 1.9 THz of the C-band. If the operational wavelength range limitation of the POF was disregarded, the signal bandwidth could be expanded to the entire C-band, as the PSA has a flat gain crossing the S + C + L bands, totaling 12.5 THz. Under such circumstances, we could transmit signals and idlers across the entire PSA gain bandwidth, thereby increasing the transmission capacity by more than three times.

In the FND-PSA, the signals and the conjugated idlers undergo coherent addition, while the noise is added incoherently, resulting in improved SNR. One concern is that the idlers need to be transmitted simultaneously with the signals, which means they occupy additional bandwidth. One approach to mitigating this limitation is to employ advanced on-chip integrated nonlinear waveguides to achieve ultra-broadband parametric amplification, thereby expanding the working bandwidth of PSA. Currently, chip-based parametric amplification can be extended to cover over 300 nm36, implying that PSA has the potential to operate across a wider bandwidth than that achieved in this study.

It is also worth noting that the results are demonstrated for single-polarized signals. This is because PPLN waveguides require careful polarization alignment to achieve high-efficiency PSA. Notably, the BER data for 38 channels in each transmission direction in Fig. 3 were collected within a relatively short time frame (less than 30 minutes). During this period, the polarization alignment and phase locking of the PSA remained highly stable, and no performance degradation was observed under potential environmental disturbances (e.g., vibrations caused by subway trains). However, it should be noted that long-term stability may still be affected by factors such as polarization drift and environmental vibrations. In principle, a polarization-diversity configuration could eliminate the polarization alignment requirement, thereby enabling the mitigation of Rayleigh noise for polarization-division-multiplexing (PDM) signals. Such polarization-diversity configurations have already been demonstrated34. In this approach, PDM signals with backscattering noise are split into two orthogonal polarization components in the polarization-diversity configuration and then processed independently by the PSA. In this way, bidirectional transmission of PDM signals can be achieved while reducing the impact of Rayleigh noise.

Although our bidirectional transmission experiments were conducted in the central core of the seven-core fiber, a further notable enhancement in transmission capacity can be achieved by utilizing bidirectional transmission in the outer cores of the seven-core fiber. Because the deployed seven-core fiber is weakly coupled and has negligible inter-core crosstalk, the transmission performance in each core would be mainly affected by the RBXT. It is worth noting that actual multi-core fibers exhibit variations in RBXT between different cores, which will affect the final transmission capacity. The seven-core fiber we used faced issues with crosstalk variation among cores, which ranged from −15.2 dB to −6.9 dB. Such differences among the cores are attributed to the varying levels of transmission loss within each core (see Supplementary Note 1 for more details). In practical applications, we can adjust the modulation format and baud rate of the signals based on the RBXT of each core, thereby achieving the maximum transmission capacity.

Furthermore, while this demonstration primarily addressed the use of PSA in bidirectional transmission dominated by Rayleigh backscattering noise, the technique also has the potential for application in SDM transmission systems, such as those based on coupled-core multi-core fibers37, few- and multi-mode fibers38,39. Since the light coupled from other cores (modes) is out of phase with the transmitted signal and treated as stochastic noise, this noise experiences a 3-dB lower gain in the PSA process compared to the signal, leading to an improvement in SNR. It can mitigate the impact of spatial crosstalk, thereby avoiding complex intermodal multiple-input-multiple-output (MIMO) digital signal processing.

In summary, we have demonstrated high-capacity, phase-sensitively amplified, same-wavelength bidirectional WDM transmission in a field-deployed fiber cable. We achieved SNR improvement for bidirectionally transmitted broadband WDM signals by mitigating Rayleigh backscattering using a wideband phase-sensitive preamplifier realized by using PPLN waveguides. The field trial demonstrated bidirectional transmission of C-band WDM channels over 49.5 km in a deployed fiber cable, reaching a net transmission capacity of 10.944Tb·s−1 and a net capacity-distance product of 541.728Tb·s−1·km, which exceeds the results of previous phase-sensitively amplified transmission demonstrations. This field test confirms the feasibility of using PSA as a pre-amplifier in large-capacity optical communications in realistic environments. The unique advantages of PSA for SNR improvement enable an increase in transmission capacity and an extension of reach. The results of this work contribute to the field of optical fiber communication and demonstrate the great potential of PSA for practical applications.

Materials and methods

PSA operation

The PSA configuration we consider in this work is an FND-PSA in which the signal and idler are at different frequencies and phase-sensitively amplified via an optical parametric amplification process based on \({\chi }^{(2)}\) nonlinear effect. The FND-PSA scheme allows all the phase states of the signal to have low-noise amplification and thus is independent of modulation format40. It also enables simultaneous amplification of multi-channel WDM signals, which is preferred in high-capacity transmission. In particular, the FND-PSA in this work is implemented using \({\chi }^{(2)}\)-based PPLN waveguide, which has the advantage of providing flat and broad gain bandwidth.

The principle of PSA can be explained by the following transfer matrix41:

$${\left[\begin{array}{l}{A}_{s}\\ {A}_{i}^{\ast }\end{array}\right]}_{out}=\left(\begin{array}{cc}\mu & \nu \\ {\nu }^{\ast } & {\mu }^{\ast }\end{array}\right){\left[\begin{array}{l}{A}_{s}+{N}_{s}\\ {A}_{i}^{\ast }+{N}_{i}^{\ast }\end{array}\right]}_{in}$$
(1)

where \({A}_{s,i}\) represents the signal and idler complex amplitudes, and \({N}_{s,i}\) represents the noise field at the PSA input. The subscripts \(s\) and \(i\) denote signal and idler waves, and the superscript * denotes complex conjugate. The complex transfer matrix coefficients \(\mu\) and \(\nu\) depend on pump power, nonlinear interaction strength and phase-matching condition, and satisfy the relation \({|\mu |}^{2}-{|\nu |}^{2}=1\). When the idler is absent at the input, the signal will be phase-insensitively amplified with a gain of \(G={|\mu |}^{2}\). In our experiment, the signal-idler pair are power-equalized and phase-correlated, which can be phase-sensitively amplified with a gain of approximately \(4G\) thanks to coherent addition of signal and the conjugated idler. In contrast, the noise fields, dominated by the reflected lights caused by the lights propagating in the opposite direction, only obtain a gain of \(2G\) since \({N}_{s}\) and \({N}_{i}\) are uncorrelated. Thus, the PSA operation provides a 3-dB less gain to the noise than that on the signal, which implies that the output SNR of the signal can be improved by 3 dB.

Experimental setup of OPC and PSA

Figure 5 shows the experimental setup of the OPC stage at the transmitter side and the PSA stage as the pre-amplifier at the receiver side. In the OPC stage, a narrow-linewidth fiber laser (NLFL) at 1550.12 nm with a linewidth of ~100 Hz was used as C-band fundamental pump light. The 14-dBm output power of the NLFL was divided by a 40/60 coupler as the fundamental pump for OPC stage, and the seed light was used for optical injection locking (OIL) in the subsequent PSA stage. The fundamental pump light was amplified by a high-power EDFA (HP-EDFA) with a maximum output power of 5 W and then coupled into a PPLN waveguide (SHG-PPLN) for conversion to second-harmonic light through SHG. Consequently, the SH pump at 775.06 nm, with an output power of 1.03 W, was combined with the transmitted signals in a polarization-maintaining WDM (PM-WDM) and injected into another PPLN waveguide (OPA-PPLN) for the generation of idlers. An on-off gain of 9.0 dB over a 3-dB gain bandwidth exceeding 14.2 THz was achieved, with a conversion efficiency (CE), defined as the ratio of output idler power to output signal power, measured at approximately −0.6 dB (see Supplementary Note 2). The generated signal-idler pairs then passed through a POF for power equalization and pre-compensation of the dispersion in the following transmission link. The seed light at 1550.12 nm was combined with the signal-idler pairs by a WDM and co-transmitted to the link. The power of the seed light was adjusted to −1 dBm by a VOA while the signals and idlers are −6 dBm per wavelength channel at the transmission link input.

Fig. 5: Detailed experimental setup.
figure 5

a OPC stage. b PSA stage. OPC optical phase conjugator, NLFL narrow linewidth fiber laser, PC polarization controller, HP-EDFA high-power erbium-doped fiber amplifier, PPLN periodically poling lithium niobate, DCM dispersion compensation module, SHG second harmonic generation, VOA variable optical attenuator, DFB distributed feedback, PZT piezoelectric-transducer, PD photo detector, OPLL optical phase-locked loop.

Full size image

After undergoing same-wavelength bidirectional transmission, the pump, signals, and idlers, which were affected by Rayleigh backscattering noise, were injected into the PSA pre-amplifier, as depicted in Fig. 5b. A dispersion-compensation module (DCM) based on fiber Bragg grating was utilized to synchronize the pump light and all signal-idler pairs to achieve the maximum phase-sensitive gain over a broad bandwidth. Then, a WDM was used to separate the pump light and signal-idler pairs. The weak and noisy pump light was injected into the OIL stage via a circulator. The OIL stage contains a distributed feedback fiber (DFB) laser diode, which was controlled by an integrated laser driver and temperature controller. The recovered low-noise pump light from the OIL stage was amplified by an HP-EDFA for SH pump generation in another PPLN module. The PPLN module comprises an OPA-PPLN waveguide that offered a phase-insensitive parametric gain of 10.5 dB and a 3-dB gain bandwidth exceeding 14.2 THz with 1.03-W SH pump power (see Supplementary Note 2). A piezoelectric-transduce (PZT)-based optical phase-locked loop (OPLL) compensated the relative phase drift between the pump and the signal-idler branches to reach a maximum and stable phase-sensitive gain. The OPLL monitors a 1% tapped signal at 192.4 THz to lock the relative phase for optimized phase-sensitive gain.