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Abstract
In this study, we attempted the repeated transmission of S-band signals by compensating for the loss of the transmission fiber using an optical parametric amplifier (OPA) based on a periodically poled LiNbO3 waveguide. We examined and compared the two configurations. The first method involved wavelength conversion of the signal to an idler, while the second method amplified the signal itself. In the latter case, we demonstrated repeated transmissions using external dispersion compensation. In the former case, we demonstrated that it was possible not only to compensate for fiber loss but also to reduce the accumulation of dispersion in transmission fibers by utilizing spectral inversion.
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1. Introduction
In recent years, communication traffic has increased worldwide, and there is a vital need to increase optical transmission capacity. The total transmission capacity is given by the product of spectrum efficiency and bandwidth. Although spectral efficiency has been improved using a sophisticated multilevel modulation format, it approaches its theoretical limit owing to nonlinear optical effects in the transmission fiber [1]. The primary focus of this study was to increase the transmission capacity by expanding the transmission bandwidth. Existing commercial optical transmission systems utilize erbium-doped fiber amplifiers (EDFAs) that function as repeaters in the C/L band [2]. The available bandwidth is constrained by the gain band of the EDFA. However, the low-loss window of the optical fiber covers a wide range of wavelengths from 1.3 to 1.8 µm. Therefore, optical amplification within these bands allows bandwidth expansion.
In general, the gain of a rare-earth-doped fiber amplifier (FA) is approximately 20–30 dB, and the noise figure is approximately 5–7 dB. The typical bandwidth of an FA is 30–40 nm, and the saturation power is in the range of 20–30 dBm. It is polarization independent. Owing to these excellent features, EDFAs are the most commonly used optical amplifiers in modern optical communication systems. Various wavelength ranges can be amplified by rare-earth-doped FAs such as the thulium-doped FA (TDFA) for the S-band [3] and praseodymium-doped FA (PDFA) for the O-band [4]. However, it is difficult to seamlessly cover the low-loss window of optical fibers owing to the limited choice of doped materials.
Another common optical amplifier is the semiconductor optical amplifier (SOA). The gain of the SOA is approximately 10–20 dB, and the noise figure is approximately 6–8 dB when coupled to a fiber [5]. The bandwidth can be as wide as 100 nm, and various wavelength bands can be covered by changing the composition of the semiconductor. The saturation power is approximately 0–20 dBm and it is polarization dependent. Because of their compactness, SOAs are mainly used in integrated optoelectronic devices such as laser diode arrays [6].
As the gain in optical parametric amplifiers (OPAs) using a nonlinear optical medium was not large, they have been used in cavity-based optical parametric oscillators. However, owing to recent advances in waveguide-type nonlinear media, OPAs using Si waveguides [7], Si3N4 waveguides [8], highly nonlinear optical fibers [9], and periodically poled LiNbO3 (PPLN) waveguides have been reported [10–12]. A gain of 30 dB in a PPLN-based OPA was also reported [13]. When used as a phase-insensitive amplifier, the theoretical noise figure is 3 dB in addition to the coupling loss. The saturated power is determined by the pump power and has the potential to reach 25 dBm or higher [14]. The OPA has polarization dependence, and polarization diversity has been proposed to eliminate this dependence [12]. Owing to recent progress in PPLN waveguide technologies, parametric interactions with high efficiency have been achieved [15].
Although the bandwidth of the PPLN-based OPA is approximately 10–30 nm, it can amplify bands of different wavelengths by changing the phase-matching conditions. Recently, we reported an OPA in the 1.3–1.8 µm range, where we used two separate PPLN waveguides for second harmonic (SH) pump generation and optical parametric amplification [16].
This study aimed to evaluate the performance of OPAs as repeaters for optical transmission. To achieve this, we attempted to transmit an S-band signal using an OPA repeater. An OPA has the unique feature of amplifying the signal and generating an idler, which is a phase-conjugate wave of the signal [17]. In this study, we conducted a transmission experiment using two configurations: one in which the L-band idler generated by the OPA was transmitted, and the other in which the S-band signal amplified by the OPA was transmitted. In the first configuration, we demonstrated that the signal distortion due to fiber dispersion can be suppressed by utilizing the spectral inversion property of the OPA. Although the second configuration requires separate dispersion compensation, the repeated transmission of wavelengths other than the C/L band is possible.
2. Transmission of idler utilizing spectral inversion
In this section, we describe our attempt to transmit an L-band idler generated by the OPA. The spectral inversion capability of the OPA enables the dispersion compensation of the transmission fiber. The OPA is installed in the middle of the optical fiber. The S-band signal, which is affected by the anomalous dispersion of a standard single-mode fiber (SMF), was amplified by the OPA and converted into an idler. As the direction of the idler spectrum was inverted with respect to the signal, the long-wavelength component with a slow group velocity was converted to a short wavelength, and the short-wavelength component with a fast group velocity was converted to a long wavelength. This produces the same effect as when the idler passes through a medium with normal dispersion. When this idler in the L-band is transmitted through the second half of the SMF with anomalous dispersion, the signal distortion due to dispersion can be compensated.
The experimental setup is shown in Fig. 1. An external cavity tunable laser (ECL) in the 1.48 µm band was used to generate the signal light. The signal was subjected to 20 Gbit/s quadrature phase shift keying (QPSK) modulation with an I-Q modulator and transmitted through an SMF. The output of the modulator was tapped using a fiber coupler and the time waveform was monitored using a sampling oscilloscope.
After transmission, the signal was adjusted to linear polarization using a polarization controller (PC) and polarization beam splitter (PBS) and was input to an OPA module. In that module, a PPLN waveguide was assembled with polarization-maintaining fibers (PMFs) for the 1.55 µm band and for the 0.78 µm band and a dichromatic mirror [16].
An ECL at a wavelength of 1.53 µm was used for the pump. The pump was amplified by the EDFA and was incident on the second harmonic generation (SHG) module. In that module, a multiple-quasi phase-matched (M-QPM) LiNbO3 waveguide was assembled with PMFs for the 1.55 µm band and for the 0.78 µm band and a dichromatic mirror [16,18]. The pump was converted into an SH pump and input to the OPA module.
The signal was amplified and converted to an idler in the 1.59 µm band through the optical parametric amplification/difference frequency generation (DFG) process in the OPA module. The output from the OPA module was tapped using a fiber coupler and monitored using a spectrum analyzer. After OPA/DFG, the idler and signal were transmitted through the second half of the SMF link. After transmission, the idler was extracted using a bandpass filter (BPF). The idler was input into a differential phase-shift keying (DPSK) receiver consisting of a preamplifier with an L-band EDFA, a BPF, a delayed interferometer, and a balanced photodiode. The gain of the L-band EDFA at the idler wavelength was 27.7 dB for an input of −30 dBm. An error detector was used to measure the bit error rate (BER). The time waveform of the demodulated signal observed.
Figure 2 shows the optical spectra of the signal and idler observed at the output of the PPLN waveguide. For comparison, the spectrum of the signal light without SH pump injection is shown in Fig. 2. As shown in the figure, the signal at 1483 nm was converted to an idler at 1592 nm. The parametric gain was 15.5 dB at the signal wavelength and the conversion efficiency of the idler was 16.5 dB.
From the measured gain of the OPA/DFG and the losses of the optical components, we estimated the variations in optical power in the transmission system. Figure 3 shows the power level diagrams with and without the OPA, assuming that the first and second SMFs were 50 km long. The signal quality evaluation requires at least −32 dBm of power on the receiver side. When the gain of the OPA is included, sufficient received power can be obtained even when signal degradation occurs. However, a power margin was absent when the OPA was not inserted. These results indicate that the OPA is useful for transmission in new wavelength bands.
Fig. 3. Variations in optical power in the transmission system for the L-band signal. Power levels with and without an OPA are indicated.
BER measurements were performed to check the signal quality after transmission. The length of the first SMF was set to 50 km and that of the second SMF was varied. Figure 4 shows the BER performance of the second SMF for several fiber lengths. For comparison, the BER with the back-to-back configuration is shown in Fig. 4. In this case, a tunable laser diode array (TLA), which generates the same wavelength as the idler, was used for the transmitter [6]. In the absence of fibers after the OPA, a power penalty was observed compared with the back-to-back measurement. This penalty was caused by the dispersion of the 50-km-long SMF placed in front of the OPA. However, the signal quality improved as the fiber length increased from 0 to 30 km, such that the power penalty approached 0 dB. However, when the fiber length was further increased, the signal quality deteriorated beyond 30 km. This peculiar dependence of the transmission characteristics on the fiber length is due to the group velocity dispersion (GVD) of the fiber and spectral inversion in the OPA.
The GVD of the SMF was measured at wavelengths of 1500–1600 nm. From the curve fitting and extension of the data, anomalous GVD values at the signal and idler wavelengths were estimated to be 12.3 and 18.9 ps/nm/km, respectively. Owing to spectral inversion in the OPA, the GVD of the first SMF was translated into a normal dispersion at the idler wavelength. This normal dispersion was canceled by the anomalous dispersion of the second SMF. The residual dispersion of the entire transmission system was calculated using the measured GVD values.
Fig. 5 shows the relationship between the power required to obtain a BER of 10−9 and the residual dispersion. As shown in the figure, the absolute values of the lowest residual dispersion and the received power were obtained when the second SMF was 30 km long. Notably, the dispersion of the fiber could be compensated by transmitting the idler owing to the spectral inversion effect of the OPA.
Fig. 5. Receiver sensitivity as a function of residual dispersion. The number of insets indicate the length of the second fiber.
Based on the obtained results, we conducted a transmission experiment with a total fiber length of 130 km. To minimize the residual dispersion, the lengths of the first and second fibers were set to 80 and 50 km, respectively. Figure 6 shows the BER performance after transmission over 130 km. For comparison, this figure also shows the characteristics of the back-to-back configuration at the same wavelength as that of the idler. As shown in Fig. 6, by optimizing the arrangement of the transmission fiber, we demonstrated that repeated transmission over 130 km is possible without incurring a power penalty.
3. Transmission of signal utilizing dispersion compensation
In this section, we describe a transmission experiment that uses the OPA to amplify and repeat the signal without wavelength conversion to the idler. As demonstrated in Section 2, it is possible to reduce the accumulation of the GVD in the optical fiber by alternately amplifying and converting the wavelengths of the signal and idler. However, with the recent development of digital coherent receivers, it is possible to correct the signal deterioration due to dispersion by utilizing digital signal processing. Therefore, repeated transmissions using an OPA without wavelength conversion are useful.
Figure 7 shows the experimental setup. The transmitter, transmission fiber, and OPA configurations are the same as those shown in Fig. 1. The main differences are as follows. Two dispersion compensation modules (DCMs) were added to compensate for the SMF dispersion. After transmission through the SMF link, the signal was extracted using a BPF designed for the S-band. A TDFA was used as a preamplifier to receive the S-band signal. The gain of the S-band TDFA at the signal wavelength was 27.2 dB for an input of −30 dBm.
We estimated the variations in optical power in the transmission system. Figure 8 shows the power level diagram, assuming that the first SMF is 50 km long, the first DCM is designed for a 50 km long SMF, the second DCM is designed for a 30 km long SMF, and the second SMF varies from 30 to 70 km in length.
Fig. 8. Variations of the optical power in the transmission system for S-band signal assuming different fiber lengths.
Owing to the gain of the OPA, a received power of more than −32 dBm can be obtained, even at a maximum transmission distance of 120 km. From the viewpoint of optical fiber loss compensation, the OPA is effective as an S-band repeater amplifier.
We measured the BER to verify the signal quality after repeated transmission. The length of the first SMF was set to 50 km and that of the second SMF was varied. The first DCM was designed for a 50 km long SMF, and the second DCM was designed for a 30 km long SMF. Fig. 9 shows the BER performance for several SMF lengths. A comparison of the BER with that of the back-to-back configuration is presented in Fig. 9. Compared with the back-to-back measurements, we obtained comparable BER characteristics when the total transmission distance was 80 km (50 km + 30 km) or 90 km (50 km + 40 km). These results confirm that repeated amplification using an OPA was effective. The deterioration of the BER characteristics was observed when the transmission distance exceeded 90 km. Figure 10 shows the relationship between the total transmission distance and received power required to obtain a BER of 10−9.
Figure 11 shows an example of the temporal waveform of a demodulated transmitted signal. These waveforms were measured with the received power of −29 dBm to prevent saturation of the trans-impedance amplifier built into the balanced photo diode. As shown in Fig. 11(a), a clean demodulated waveform is obtained after 80 km of transmission. This is because the total dispersion of the DCMs used in the experiment was designed to compensate for the 80 km long SMF. However, as shown in Fig. 11(b), waveform distortion was observed after a 110 km transmission. The power penalty at transmission distances greater than 100 km was due to waveform distortion caused by dispersion because the dispersion of the second SMF was not fully compensated. In this study, the availability of DCMs was limited. Therefore, a power penalty of 2.5 dB was observed at a transmission distance of 120 km. However, if appropriate dispersion compensation or digital signal processing correction is performed, transmission for up to 120 km without signal distortion is possible. In this experiment, we attempted to transmit data over a maximum distance of 120 km. This is a shorter distance than that presented in the results of Section 2; however, the difference is mainly due to the insertion loss of the DCM. Therefore, if dispersion compensation through digital signal processing is used, the difference between the two approaches may be small. However, the advantage of using an idler to suppress the dispersion accumulation is more pronounced in long-distance transmissions involving many repeating amplifiers. The noise figure of the OPA used in this study was not evaluated. Because the coupling loss between the PPLN waveguide and the fiber at the signal wavelength was estimated to be 2 dB, the noise figure was estimated to be 5 dB, which is comparable to that of a typical EDFA. The experimental evaluation of the noise figure requires detailed measurements, which is a future challenge.
4. Conclusions
In this study, we attempted the repeated transmission of the S-band signal using a PPLN-based OPA. We investigated two configurations: one involving wavelength conversion of the S-band signal to the L-band idler, and one in which only the S-band signal was transmitted. In the latter configuration, the transmission fiber loss was compensated by the OPA, and repeated transmission was demonstrated. In this case, it is necessary to compensate for the dispersion of the transmission fibers. In the former configuration, in addition to the fiber loss compensation by the OPA/DFG, dispersion compensation was demonstrated using spectral inversion. To achieve complete dispersion compensation, the fiber lengths at which the signal and idler light are transmitted must be optimized. However, for multispan repeated transmissions, a reduction in the cumulative dispersion of the transmission fiber can be advantageous, even without such optimization.
Funding
Japan Society for the Promotion of Science (JP21H01330).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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