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Abstract
Optical phase conjugation (OPC) can be applied to boost the performance of long-haul transmission by mitigating the impairments from fiber nonlinearity. Unfortunately, noticeable nonlinear noise in the conjugator for optical orthogonal frequency division multiplexing (OFDM) systems often degrades the signal quality. In this paper, we demonstrate nonlinear distortion mitigation in OPC by introducing backward Raman amplification to the conjugator. Raman amplification allows a lower input signal power, thus suppressing the OPC distortion while maintaining the conjugated output power. We investigate the performances of Raman-enhanced OPC in both back-to-back (BTB) and transmission systems with 3 × 25 Gbaud optical OFDM signals. In the BTB OPC system, Raman amplification boosts the tolerance to system nonlinearity, achieving a 3-dB improvement in the output power, a 2.4-dB improvement in the Q factor, and a 6-dB improvement in the input dynamic range. In the transmission system with Raman-enhanced OPC, the optimum launched power is increased by 2 dB and the maximum Q factor is increased by 0.4 dB compared to direct transmission. Similar performances are observed in all the wavelengths, indicating that our scheme works well with WDM transmission systems.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Future optical network is evolving to accommodate transmission capacity over Tbit/s to meet the continuously growing traffic demand. Therefore, different advanced modulation formats have been extensively investigated to support both high spectral efficiency and long transmission distance. Optical orthogonal frequency division multiplexing (OFDM) is among the promising candidates in long-haul systems. It borrows the advantages and techniques of RF OFDM by encoding parallel data streams on orthogonal subcarriers in the optical domain. The seamlessly multiplexed subcarriers not only offer a high spectral efficiency but also enable efficient channel estimation and dynamic allocation of carrier number and data rate at negligible hardware cost. Single-wavelength 400G OFDM transmission system has been successfully demonstrated, indicating the potential capability of carrying large volume traffic by OFDM signals [1]. However, due to relatively high peak-to-average power ratio (PAPR) of optical OFDM signals, the long-haul transmission capacity is constrained by the fiber nonlinearity. Distortion arises from nonlinear interferences among closely-spaced OFDM subcarriers through self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), which become significant under a high signal launched power [2]. Therefore, to support a higher launched power which give rise to improved optical signal-to-noise ratio (OSNR) and transmission distance, nonlinearity mitigation is indispensable.
Realizing the hurdle imposed by fiber nonlinearity in optical OFDM transmission system, there has been a keen interest in developing effective approaches to mitigate the nonlinear distortion in both electrical and optical domains. Compared to its electrical counterpart, optical approaches such as optical phase conjugation (OPC) [3], phase-conjugated twin waves [4], frequency-referenced transmission [5] and phase sensitive amplification [6] avoid intensive computation brought by digital signal processing (DSP), thus showing great promise. In particular, OPC based on FWM process offers the advantages of modulation format independency, wideband operation and ultrafast processing speed. OPC has been demonstrated to significantly improve the performances of long-haul transmission systems with single carrier signals. Nonetheless, noticeable nonlinear noise was observed in the conjugator of optical OFDM systems due to interference among the multiple wavelength channels and the OPC pump [7].
The nonlinear tolerance of OPC for OFDM signals has been experimentally demonstrated and theoretically addressed in [7]. The OPC distortion is described by FWM products through modelling the OFDM subcarriers as optical comb lines. To suppress the distortion, Morshed et al. proposed to filter out undesirable cross-phase modulation products around the OPC pump at the cost of implementation complexity [8]. The OPC performance can also be improved by optimizing the pump and signal power. Unfortunately, the pump power is often limited by the stimulated Brillouin scattering (SBS) threshold of the fiber [9].
Based on fundamental difference of the mechanisms behind the generation of the conjugated signal and the nonlinear distortion products, we established a new dimension of nonlinear mitigation by introducing Raman amplification to re-engineer the power profiles of the interplaying FWM fields [10]. Our approach allowed a 10-dB enhancement in output power under the simplified context of back-to-back (BTB) OPC system for the case of a single OFDM channel. In this paper, we expand our scope of Raman-enhanced OPC to a transmission system with wavelength-division multiplexed (WDM) OFDM signals. We first investigate the BTB OPC module. Raman-enhanced OPC effectively mitigates the nonlinear impairment induced by inter-subcarrier mixing via the OPC pump. The optimum parameters obtained from the BTB experiment are subsequently applied to a transmission system. We compare the transmission performances under three scenarios: direct transmission, conventional OPC transmission, and Raman-enhanced OPC transmission. Due to the reduced nonlinear noise by Raman amplification, the launched power is enhanced by 2 dB and the maximum Q factor is improved by 0.4 dB compared to direct transmission. Similar performances are achieved in all wavelength channels thanks to the wide-band Raman amplifier. The results indicate that our approach is effective in combating optical nonlinearity in WDM OFDM transmission systems.
2. Raman-enhanced OPC: back-to-back performance
Figure 1 shows the schematic diagram of a BTB OPC module for coherent OFDM signals in three WDM channels. The baseband OFDM signal with a symbol rate of 25 Gbaud is generated offline and loaded into a 65 GSa/s arbitrary waveform generator (AWG) (Keysight M8195A). The OFDM signal consists of 256 subcarriers generated by inverse discrete Fourier transform (DFT). 212 subcarriers are encoded with quadrature phase-shift keying (QPSK) format while the DC subcarrier and the other 43 edge subcarriers are unfilled. 8 pilots are used for channel estimation and an 8-point cyclic prefix (CP) is inserted to mitigate dispersion in the transmission fibers. The WDM carriers are derived from three external cavity lasers (ECLs) with 25 kHz linewidth and frequencies at 193.7, 193.8 and 193.9 THz. They are coupled and fed into an IQ modulator driven by the baseband OFDM signal. To decorrelate the 3-WDM coherent OFDM signals, a WDM wavelength de-multiplexer is used to spatially separate the 3 channels. Three single mode fibers (SMFs) are used for channel decorrelation by providing a 5 ns delay between neighboring channels. The OPC is performed based on FWM in a highly nonlinear fiber (HNLF) with a length of 1000 m, a nonlinear coefficient of 11.7 W−1·km−1, a zero-dispersion wavelength at 1549 nm, and a dispersion slope of 0.019 ps·km−1·nm−2 at ~1550 nm. In the conventional OPC, the WDM coherent OFDM signals are amplified and directed into the HNLF with an OPC pump located at 1550.12 nm (194 THz). A variable optical attenuator (VOA) is used after the erbium-doped fiber amplifier (EDFA) to control the input signal power. The input pump power is 9 dBm to avoid SBS induced signal distortion. The conjugated idlers are generated at the HNLF output with a conversion efficiency of ~-20 dB. The idlers are extracted individually by a bandpass filter and fed into a coherent receiver. The receiver consists of a polarization-diversity 90-degree hybrid, a local oscillator (LO) with 100 kHz linewidth, four balanced detectors and a real-time oscilloscope operating at 80 GSample/s. The performances of the idler are evaluated by off-line DSP using the standard procedures described in [11].
Fig. 1 Experimental setup of back-to-back OPC module. CW: continuous wave; VOA, variable optical attenuation; HNLF: highly nonlinear fiber; LO: local oscillator; DSP: digital signal processing.
The nonlinear noises in the conjugator of an optical OFDM system can be described by higher order FWM products involving the densely spaced subcarriers and the OPC pump. The dependences of the conjugated subcarrier output and the OPC distortion on the input subcarrier power described in [7] can be extended to WDM OFDM signals when the dispersion among the channels is negligible. The conjugated subcarrier power is expressed as
and the average OPC distortion power is expressed aswhere γ is the nonlinear coefficient of the fiber, L is the fiber length, Ppump and Psc are the respective powers of the OPC pump and the subcarriers, and NSC is the number of subcarriers. As shown in Eq. (1) and (2), the conjugated subcarrier power grows linearly while the OPC distortion power grows cubically with the input signal power. Reducing the input signal power can effectively minimize the penalty caused by fiber nonlinearity, which however is obtained by sacrificing the optical signal-to-noise ratio (OSNR) of the output signal. Therefore, the tradeoff between the nonlinear penalty and the OSNR of the output signal limits the maximum achievable performance. The quality of the conjugated idler is evaluated by the Q factor derived from the error vector magnitude (EVM) using Eq. (3).To evaluate the influence of fiber nonlinearity on the OPC system, the input signal power is varied from 4.2 to 11.2 dBm using a VOA. The Q factor is plotted against the input signal power in Fig. 2 (black squares). Taking the central channel (CH2) as an example, increasing the signal power plays a positive role through achieving a higher OSNR at the receiver until the nonlinear noise in the OPC dominates. The optimum input power is measured at 7.2 dBm, while the Q factor degrades rapidly afterwards since the nonlinear noise grows cubically with the signal power.Fig. 2 Q factor against the equivalent input power in conventional OPC (black square) and Raman-enhanced OPC (red circle) at the central channel (CH2).
The tradeoff between the nonlinear penalty and the output OSNR is relieved when backward Raman amplification is introduced to the conventional OPC setup as shown in Fig. 1. Raman amplification improves the efficiency of the FWM process in the HNLF by providing a broadband gain to the interplaying light waves, which allows a lower input signal power. A continuous wave non-polarized fiber laser operating at 1455 nm serves as the Raman pump. It provides a gain profile with a 3-dB bandwidth of 24 nm from 1542 nm to 1566 nm and a peak gain at 1554 nm. We intentionally decrease the input signal power by 10 dB, theoretically giving rise to a 30-dB nonlinear noise suppression at the fiber input based on Eq. (2). The Raman pump is set to 25.6 dBm to exactly compensate the loss of the output idler power. Thus, the Q factors of the idlers in the Raman-enhanced OPC and the conventional OPC are compared at the same idler power level. We use the term “equivalent input power” to define the signal power before the power reduction. In Raman-enhanced OPC, the optimum equivalent input power is improved by 3 dB while the Q factor is improved by 2.4 dB as shown in Fig. 2. In addition, Raman-enhanced OPC significantly improves the input power dynamic range by ~6 dB. Here, the dynamic range is defined as the input power range giving rise to < 3-dB degradation of the Q-factor from its peak value. Large dynamic range is a desirable feature in today’s elastic optical network in which the signal power varies due to flexible change of light paths. Furthermore, the broadband Raman amplification makes our scheme perfectly applicable to WDM systems. Table 1 summarizes the performances of the idlers for the three wavelengths under investigation in terms of the improvement of the optimum input power and the maximum Q factor. Similar performances are achieved by Raman amplification in the three channels.
Table 1. Performance improvement in WDM channels
3. Raman-enhanced OPC: transmission performance
In the transmission experiment, three transmission scenarios are studied: (1) direct transmission without OPC module (Fig. 3(a)), transmission with conventional OPC, and transmission with Raman-enhanced OPC (Fig. 3(b)). Two spools of 90-km SMF are deployed due to the lack of recircuiting loops. An EDFA is placed before each transmission span to boost the launched power of the optical OFDM signals. The influences of the fiber nonlinearity are investigated against the input signal power. We first compare the transmission performance in the absence and presence of the conventional OPC module. In Fig. 3(a), the signals are directly transmitted through the optical fibers. While in Fig. 3(b), the signal after the first span of transmission fiber is amplified by an EDFA. A VOA is then used to fix the input power to the OPC module at 7.2 dBm, according to the optimum input power obtained in the BTB OPC experiment described in Section II. The conjugated idlers are generated in the OPC module and are subsequently directed to the second span. A VOA is used to vary the launched power before each transmission span. The powers are maintained the same ensure to be the same to introduce equivalent nonlinearity impairments in both spans. The measurement results are shown in Fig. 4 (blue triangles and black squares). In the linear transmission region, the direct transmission significantly outperforms the conventional OPC. The degradation of signal quality is attributed to two origins: 1) the nonlinearity in OPC module induces 2.6 dB Q-factor penalty; 2) the nonlinearity in the OPC module limits the output power of the conjugated idler and results in an OSNR degradation at the receiver. In the nonlinear transmission region, the optimum launched power is increased by 3 dB in the OPC transmission since the nonlinear distortion in the first transmission span is effectively cancelled in the second span. However, the peak Q factor is ~2 dB lower than the scenario without OPC. The result indicates that the distortion produced in the OPC is severe, counter-balancing the achieved benefits in the transmission by OPC. To mitigate the nonlinear distortion in the OPC, backward Raman amplification is applied as shown in the dashed box of Fig. 3(b). Adopting the BTB results in Section II, the input signal input to the OPC is set to 0.2 dBm (corresponding to 10.2 dBm equivalent input power) while the Raman pump power is 25.6 dBm. The output idler power is thus maintained to be equal to that in the conventional OPC. Figure 4 compares the transmission performance of the Raman-enhanced OPC (red dot) with the other two scenarios. It is observed that the Raman-enhanced OPC outperforms the case without OPC in both the optimum signal launched power and the peak Q factor: the optimum signal launched power is increased by 2 dB from −0.8 to 1.2 dBm, while the peak Q factor is improved by 0.4 dB. We attribute the limited peak Q factor improvement to the relatively low conversion efficiency in the OPC module. The power loss of the converted idlers is approximately equivalent to that in one transmission span. In addition, the total transmission length in our experiment is only 180 km. Therefore, the fiber nonlinear impairment is not as significant as that would occur in a long-haul transmission link. As the transmission distance increases, a more prominent performance improvement by our scheme can be expected. Comparing our scheme to conventional OPC, the Q factors are improved at both linear and nonlinear transmission regions. Our scheme not only effectively compensates the nonlinear distortion in the OPC but also improves the OSNR of the conjugated signal.
Fig. 3 Three system schemes under comparison: (a) direct transmission; (b) transmission with conventional OPC and Raman-enhanced OPC.
Fig. 4 Comparison of the transmission results in direct transmission (blue triangle), conventional OPC (black square) and Raman-enhanced OPC transmission (red dot).
Finally, the performances of the received signals at all three channels are compared under the scenarios of direct transmission, conventional OPC transmission and Raman-enhanced OPC transmission. The comparison is conducted under the same launch power of 1.2 dBm which corresponds to the optimal launch power for the Raman-OPC transmission system. The result is shown in Fig. 5(a). At the central channel (CH2) the Raman-enhanced OPC gives rise to 2.6 dB Q factor improvement compared to the conventional OPC and 3.2 dB improvement compared to direct transmission. Thanks to the wide gain bandwidth of Raman amplifier, the three channels have demonstrated very similar performance improvement in the transmission link. As a visual illustration, the corresponding constellation diagrams of CH2 in the three transmission scenarios are plotted in Fig. 5(b), confirming the improvement in Raman-enhanced OPC.
Fig. 5 (a) WDM transmission performances at three wavelengths (b) corresponding constellation diagrams at CH2.
4. Discussions
It is worth mentioning that compared to other reported OPC-based transmission systems with dedicated SBS threshold management techniques, our conventional OPC system has a relatively poor performance [12]. In [13], a strained Al-doped HNLF is utilized as the nonlinear medium which allows a stronger OPC pump to be used for high conversion efficiency. Therefore, the OPC distortion can be minimized by using a relatively low input signal power. In [14], two OPC pumps that are counter-phase modulated are applied to improve the SBS threshold, allowing higher OPC pump power to be adopted without transferring additional phase noise to the idler. In [15], a specially designed HNLF (SPINE) with stable phase-matching is utilized for improved nonlinear efficiency. In comparison, our demonstration relies on a 1000 m standard HNLF as the nonlinear medium, resulting in a much lower SBS threshold that limits the maximum OPC pump power. Since the penalty of wavelength conversion scales with the ratio of the output signal power to the input pump power [12], the SBS threshold determines the performance of the OPC system. Although in this work we have only considered the basic OPC system as our benchmark, the improvement by our approach is also applicable to other advanced schemes adopting SBS threshold control. Raman amplification is still effective in other types of HNLF (with different lengths and materials) and pump configurations (with different number of pumps and pump powers). Meanwhile, if the fiber length is shortened, the pump power should be further boosted to maintain the same conversion efficiency of the OPC. In addition, compared to EDFA, Raman amplification has a more flexible operation bandwidth which can be adjusted by the Raman pump wavelength. Thus, Raman-enhanced OPC can operate effectively in a spectral range where the OPC pump is not sufficiently strong but broadband Raman amplification is available.
5. Summary
We have demonstrated a Raman-enhanced OPC to improve the transmission performance of WDM optical OFDM signals. The Raman-enhanced OPC effectively mitigates the nonlinear impairment induced by inter-subcarrier mixing via the OPC pump in the BTB module. As a result, the input signal power increases by 3 dB and the maximum Q factor increases by 2.4 dB compared to the conventional OPC system. The reduced nonlinear impairment by Raman amplification gives rise to a 2-dB enhancement in the launched power of the transmission system and a 0.4-dB Q factor improvement. In comparison, the nonlinear noise in the conventional OPC introduces 1.1 dB Q factor penalty. Similar performances are achieved in all the wavelength channels thanks to the wide-band Raman amplification. The results indicate that our approach is effective in improvement the nonlinear tolerance of WDM optical OFDM transmission systems.
Funding
Hong Kong Research Grants Council GRF grants (14206614, 14238816, 14209517).
References and links
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