- Email: [email protected]
All-optical modulation format conversion from star-QAM to PSK and ASK signals
Optics Communications 451 (2019) 23–27
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
All-optical modulation format conversion from star-QAM to PSK and ASK signals Feng Wan, Bao-Jian Wu ∗, Feng Wen, Kun Qiu Key Lab of Optical Fiber Sensing and Communication Networks, Ministry of Education, University of Electronic Science and Technology of China, Chengdu 611731, China
ARTICLE
INFO
Keywords: Nonlinear optics Optical communications and information processing All-optical networks Four-wave mixing Highly nonlinear fiber
ABSTRACT An all-optical modulation format conversion scheme from star-QAM to ASK and PSK signals is proposed by employing four-wave mixing (FWM) in highly nonlinear fibers. The phase modulation information of star-QAM signal is firstly converted to the PSK format by a degenerate FWM process under gain saturation, and then the amplitude modulation information of star-QAM signal is mapped into the ASK format by means of a nondegenerate FWM associated with the resulting PSK signal. The detailed signal processing is presented by theory and simulation. As examples, the bit-error-ratio performances of 8QAM and 16QAM signals are evaluated by a non-degenerate FWM-based aggregation scheme for ASK and PSK formats.
1. Introduction With the rapid growth of optical communication system capacity, the maturity of coherent optical communication technology [1], and the wide application of advanced modulation formats with high spectral efficiency [2,3], all-optical modulation format conversion [4], as well as aggregation and de-aggregation [5,6] functions, will play an important role in the network switching nodes. These optical signal processing functions can usually be realized on the basis of optical nonlinear effects, in which the fiber nonlinearity have attracted more attention due to its ultra-fast response [7]. So far, many alloptical format conversion schemes have been presented, including those from NRZ-DPSK to RZ-OOK format [8], OOK to 16QAM [9], and OOK to MPSK [10]. All-optical aggregation of two QPSK signals to 16QAM [11], and the combination of ASK and QPSK into 8QAM signals [12] were also demonstrated by the experiments. Certainly, other nonlinear media can as well be used. For example, Y. GAO et al. generated MQAM signals in optical domain based on four-wave mixing effect in silicon waveguides [13]. It is well known that the advanced modulation signals require higher optical signal to noise ratio (OSNR) for transmission. Thus, in some practical applications, the advanced modulation signals have to be decomposed into multiple low-order modulation formats. Especially, in the elastic optical network (EON), the de-aggregation process enables dynamic allocation of link resources and effectively alleviates the pressure of rapid growth of communication capacity [14,15]. At present, some authors have investigated the decomposition of advanced modulation formats into low-order modulation formats, including the all-optical format conversion from 16QAM to QPSK through cascaded SOAs [16], and the multi-wavelength conversion from 16QAM ∗
to 4PAM [17]. Recently, considering that 8-ary quadrature amplitude modulated (8QAM) signal has become one of the alternatives for 400G WAN interface [18,19], Ref. [20] proposed a de-aggregation scheme for star-8QAM into QPSK and ASK signals using the fiber nonlinearity. However, the ASK signals are obtained only by the square law detection of photodetectors (PD) in electronic domain. In general, star-8QAM has two types of constellations (with 2 amplitudes), those of 8 and 4 phases. The former is widely used in the communication network due to simple IQ-modulator generation [21]. However, the latter exhibits a larger phase noise tolerance [22] and can be converted to a lower-order QPSK signal as discussed in the paper, instead of 8PSK for the former. In this paper, we put forward an all-optical modulation format conversion from any star-QAM to MPSK and ASK signals, using degenerate FWM (DFWM) under gain saturation and non-degenerate FWM (NDFWM) in the HNLFs, respectively. The NDFWM process between the input QAM signal and the resulting MPSK signal is used to generate the optical ASK signal. The scheme presented in this paper has some advantages in the scalability to higher QAM formats and the flexibility of realizing all-optical regeneration. We simulate and verify the schemes for the star 8QAM and 16QAM signals and analyze the bit-error-ratio (BER) performance before and after format conversion. Moreover, in order to estimate the format conversion performance, the resulting PSK and ASK formats are also re-aggregated to a QAM signal by a NDFWM process. Our simulations show that the error-free format conversion of an 8QAM (or 16QAM) signal can be achieved at the input OSNR of higher than 16.4 dB (or 19 dB).
Corresponding author. E-mail address: [email protected] (B.-J. Wu).
https://doi.org/10.1016/j.optcom.2019.06.014 Received 13 March 2019; Received in revised form 6 June 2019; Accepted 7 June 2019 Available online 10 June 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 1. Principle of 8QAM modulation format decomposition (a) input 8QAM, (b) converted QPSK, (c) input 8QAM and (d) converted conjugate QPSK from 1st stage, (e) converted ASK after 2nd stage.
2. The principle of all-optical star-QAM format conversion
From Eqs. (1)∼(3), the complex envelope of the converted QPSK signal 𝐸i1 can be expressed as
Our objective is to achieve the all-optical format conversion from a star-QAM signal to a QPSK signal and an ASK signal. The operating principle for a star-8QAM signal as an example is illustrated in Fig. 1. A DFWM process under gain saturation (first stage) is used to convert the star-8QAM signal into a QPSK format. Then, a part of the resulting QPSK along with the input star-8QAM signal is launched into the second stage of HNLF, and a desirable ASK signal can be obtained by the NDFWM process using a CW pump as shown in Fig. 1(c)∼(e). The QPSK and ASK signals converted in optical domain correspond to the phase and amplitude information of the star-8QAM, respectively. In what follows, we explain the signal processing processes in detail. In NDFWM, the optical fields can be represented by the complex envelopes 𝐸𝑚 = 𝐴𝑚 ⋅ exp(j𝜑𝑚 ), in which 𝐴𝑚 and 𝜑𝑚 are respectively the amplitude and phase, and the subscripts m=1∼4 represent the two pumps, signal, and idler light, respectively. Their frequency relationship and phase matching condition are respectively determined by the energy and momentum conservations, and the idler field is proportional to those of the input waves as follows [12]:
𝐸i1 = 𝐴i1 ⋅ exp(j𝜑i1 ) = 𝛼𝐴2p exp[j(2𝜑p − 𝜑s )]
𝐸4 ∝ 𝐸1 𝐸2 𝐸3∗
where 𝛼 is the saturation gain factor, related to the DFWM compression. Clearly, from Eq. (4) there is a consistent one-to-one match between the idler phase 𝜑i1 and the input signal phase 𝜑s for a given CW pump with a constant phase. In the second stage of HNLF, the NDFWM process is utilized for the conversion into an ASK signal. In the NDFWM process, the input 8QAM signal and the resulting QPSK signal act as so-called pumps and the auxiliary CW as so-called signal, represented by 𝐸a1 = 𝐴a1 exp(−j𝜑a1 ) in the paper. Thus, the complex envelope of the resulting idler 𝐸i2 can also be given as follows: 𝐸i2 = 𝐴i2 ⋅ exp(j𝜑i2 ) = 𝛼𝛽𝐴2p exp[j(2𝜑p − 𝜑s )] ⋅ 𝐴s exp(j𝜑s ) ⋅ 𝐴a1 exp(−j𝜑a1 ) [ ] = 𝛼𝛽𝐴2p 𝐴a1 exp j(2𝜑p − 𝜑a1 ) ⋅ 𝐴s (5) where 𝛽 represents the NDFWM gain factor. It can be seen from Eq. (5) that the phase information of the 8QAM signal is canceled by the conjugate relationship with the QPSK signal, and the amplitude information of the 8QAM signal is preserved for the idler as the converted ASK format after a fixed phase compensation of (2𝜑p − 𝜑a1 ). It is thus evident that Eqs. (4) and (5) is very useful for the explanation for the all-optical format conversion from the star-QAM signal to the QPSK and ASK signals.
(1)
where the superscript * indicates the complex conjugate. Eq. (1) can be further expressed as 𝐴4 ∝ 𝐴1 𝐴2 𝐴3
(2)
𝜑4 = 𝜑1 + 𝜑2 − 𝜑3
(3)
(4)
3. Simulation diagram for star-8QAM format conversion The block diagram of all-optical star-8QAM format conversion scheme is shown in Fig. 2(a), in which the converted QPSK and ASK formats are achieved by two FWM process. To further evaluate the degradation performance for the all-optical QAM format conversion scheme, an extra third-stage DFWM is also introduced to aggregate the PSK and ASK signals into the star-QAM format signals. In the simulation through VPI-Transmission Maker (VPI™ 9.1), the system symbol rate is set to be 10GBaud, and the 8QAM signal at the optical frequency of 193.1 THz is generated from the IQ modulator driven by 215 -1 pseudo-random binary sequence (PRBS) data. Then, the
A DFWM process means that the two pumps are identical in the amplitude and phase. The converted QPSK signal is implemented in the first stage of DFWM process as shown in Fig. 1(a)∼(b). The input 8QAM signal with the optical field 𝐸s = 𝐴s exp(j𝜑s ) is injected into the HNLF together with a CW pump light of 𝐸p = 𝐴p exp(j𝜑p ) and the amplitude of the induced idler light is compressed into one level in the presence of parametric gain saturation. At the same time, the phase information of the input 8QAM signal is retained onto the resulting QPSK format as the idler. 24
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 2. All-optical star-QAM format conversion scheme (a) simulation diagram of the star-QAM format conversion, (b) optical spectra after HNLF1 (c) optical spectra after HNLF2 (d) optical spectra after HNLF3. Table 1 Parameters of highly nonlinearly fibers. Parameters
HNLF1
HNLF2
HNLF3
Fiber length The loss 𝛼 Zero-dispersion wavelength 𝜆0 Dispersion slope S at 𝜆0 Nonlinear coefficient 𝛾
2000 m 0.2 dB/km 1550 nm 80 s/m3 13.1 W−1 km−1
510 m 0.2 dB/km 1550 nm 80 s/m3 10.8 W−1 km−1
400 m 0.2 dB/km 1550 nm 80 s/m3 10.8 W−1 km−1
ASE-degraded 8QAM signal is divided into two branches through a 3-dB beam splitter, launched to the first and second stages, respectively. In our simulation, the optimization of three sections of HNLFs is very important for the FWM efficiency and the gain saturation, especially for the fiber dispersion and length parameters. The HNLF parameters used in our simulation are listed in Table 1. In addition, the CW lasers with the linewidth of 10 kHz and the 3rd-order Gaussian OBPFs with the bandwidth of 20 GHz are always adopted. The noise figure (NF) is taken as 3 dB for all the EDFAs. In the first stage, the input 8QAM is amplified to 1.12 mW by an EDFA, and then coupled into HNLF1 along with the CW pump of 193.16 THz. The optical spectrum output from HNLF1 is shown in Fig. 2(b). The resulting QPSK idler is filtered out by an optical bandpass filter (OBPF1) with the central frequency of 193.22 THz. In order to achieve an identical idler output in the gain saturation region, we can optimize the corresponding power transfer function (PTF) by adjusting the star-QAM and pump powers, instead of tuning the two amplitude levels of the input star-QAM signal. The PTF curve for the DFWM process under gain saturation is shown in Fig. 3. It should be noted that, for the two-amplitude 8QAM signal, the two levels are 0.6 mW and 1.56 mW in terms of optical power. When the input pump power is 680 mW, the converted QPSK idler power is equal to 210 mW. In our scheme, the phase shift 𝛥𝜑 introduced by self-phase modulation (SPM) can be ignored owing to small input signal power, and the constellation rotation by the cross-phase modulation (XPM) can also be canceled in the receiver. In the second stage, the tunable optical delay line (TODL) is used to synchronize the 8QAM signal with the resulting QPSK signal, and the frequency and power of the auxiliary CW2 are taken as 193.04 THz and
Fig. 3. Power transfer function of the first stage FWM process.
10 mW, respectively. The 8QAM and QSPK are respectively amplified to 2 mW and 2.5 mW, and then fed into HNLF2 with CW2. The optical spectrum after the NDFWM process is shown in Fig. 2(c), and the ASK idler is output from OBPF2 with the central frequency of 193.28 THz. In theory, the TODL can be used to align the two branches, as shown in Fig. 2(a). However, an extra optical phase-locked loop (OPLL) may be introduced to compensate the slow phase fluctuation after long HNLF propagation in practical experiments. The third stage is used to aggregates the ASK and QPSK signals into a star-8QAM format by a NDFWM method, instead of the DFWM scheme as presented in Ref. [12]. Similar to Eq. (4), in the DFWM scheme, the amplitude noise of the aggregated 8QAM signal is expected to double that of the input ASK signal, and the amplitude jitter at a higher level is prone to a larger phase degradation due to the fiber nonlinearity. Therefore, the utilization of the NDFWM method may effectively reduce the extra noise introduced in the DFWM scheme. In the third-stage NDFWM process, the QPSK and ASK signals have an 25
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 4. The simulated constellations of (a) the input 8QAM, (b) QPSK signal, and (c) ASK signal, and (d) the aggregated 8QAM signal.
identical power of 5 mW and the auxiliary CW3 at 193.18 THz has a power of 10 mW. The optical spectrum output from HNLF3 is shown in Fig. 2(d) and the recovered 8QAM signal as the idler is filtered out by OBPF3 with the central frequency of 193.32 THz. In practice, the additional phase noise through the FWM process is dependent on the linewidth of CW lasers and a narrow-linewidth or injection locked laser is useful for suppressing the phase fluctuation. In our simulation, we set an identical random seed number to all the laser source for simplicity. Moreover, the phase noise accumulation due to pump-to-idler transfer in the FWM process can be eliminated by the coherent pumps generated by optical frequency comb [23]. In our simulation, the pump-to-idler phase noise transfer in the FWM process is also taken into account. 4. Results and discussion In order to evaluate the performance of the format conversion scheme, an amplified spontaneous emission (ASE) noise is added to the 8QAM signals at the point of A as shown in Fig. 2(a), and then the input OSNR can be adjusted. In addition, the optical signals are coherently received and demodulated at the positions of A, B, C, and D, respectively. The ideal homodyne coherent receiver is used in the simulation, that is, the noise introduced by photoelectric conversion is neglected. The converted electrical signals go through down-sampling, phase recovery and digital processing for the corresponding constellations. In the simulation, Monte-Carlo approach is adopted to estimate the BER performance. The obtained constellation diagrams at the points of A, B, C, and D are shown in Fig. 4, in which the corresponding EVM values are also labeled. In the simulations, the 8QAM signal with the bit rate of 30 Gbit/s is successfully decomposed into two low-order formats of 20 Gbit/s QPSK and 10 Gbit/s 2ASK. In the third stage, the two low-order formats are successfully re-aggregated into an 8QAM signal with the EVM of 12.5% as shown in Fig. 4(d). It is worth noting that the ASK signal generated by the second stage has a smaller EVM than that of QPSK after the first stage. As known from Eq. (5), the ASK signal is generated by the phase-conjugated QPSK and the initial 8QAM signal since the optical phase conjugate (OPC) can effectively suppress the nonlinear noise [24]. Then, we change the input OSNR to further investigate the BER performance for the proposed scheme. The calculated BER curves before and after the format conversion are shown in Fig. 5, including those of the back-to-back (B2B) 8QAM, converted QPSK and ASK, aggregated 8QAM, and offline synthesized 8QAM. In order to evaluate the aggregation performance, the synthesized 8QAM as a reference signal is reproduced from the generated ASK and QPSK signals directly in digital domain. In Fig. 5, the dashed line represents the 7% overhead hard decision forward error correction (HD-FEC) threshold of 3.8×10−3 . It can be known from Fig. 5 that, (i) the aggregation and offline synthesized of 8QAM format have a small difference in the BER performance, which means the NDFWM-based aggregation is desirable for the reproduction of 8QAM format; (ii) at the HD-FEC threshold, the error-free all-optical decomposition and aggregation of 8QAM format can be achieved when the input OSNR is higher than 16.4 dB, with an OSNR penalty of 2.4 dB compared with the B2B system, corresponding
Fig. 5. The BER curves for the 8QAM format conversion into QPSK and 2ASK signals.
to the case in the absence of format conversion; (iii) according to the ASK and QPSK curves in the absence of aggregation, the 8QAM format conversion is mainly determined by the noise performance of the converted ASK, instead of QPSK. Using the same conditions as the case of 8QAM, we can also simulate the 40 Gbit/s star-16QAM format conversion into 30 Gbit/s 8PSK and 10 Gbit/s 2ASK, and Fig. 6 plots the corresponding BER curves relative to the input OSNR, slightly different from that of the 8QAM format as shown in Fig. 5. By comparison of the 8QAM format, the input OSNR required for the error-free 16QAM format conversion increases to 19 dB at the HD-FEC threshold, with an OSNR penalty of ∼3.5 dB. From Fig. 6, the converted 8PSK and ASK signals have a slightly different OSNR performance at the HD-FEC threshold. Finally, it should be pointed out that the format conversion scheme proposed here is also applied to the case of converting star-QAM signals into a higher-order ASK only if the first-stage FWM process has a sufficiently wide gain-saturated region. 5. Conclusion This paper presents a novel all-optical format conversion scheme from star-QAM signals to PSK and ASK formats, as well as the aggregation process of QAM signals for evaluating the BER performance. As examples, the cases with the 8QAM and 16QAM formats are taken into account by simulation. The derivation of PSK and ASK formats from QAM signals is implemented by a gain-saturated DFWM and another NDFWM, respectively. Our simulations show that the error-free format conversion of 8QAM and 16QAM signals can be achieved when the input OSNRs are higher than 16.4 dB and 19 dB, respectively. By comparison with the B2B system, the corresponding OSNR penalties introduced by the entire format conversion are 2.4 dB and 3.5 dB, respectively. 26
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Optics Communications 451 (2019) 23–27 [7] A.E. Willner, S. Khaleghi, M.R. Chitgarha, O.F. Yilmaz, All-optical signal processing, J. Lightwave Technol. 32 (2014) 660–680. [8] T. Cao, L. Chen, Y. Yu, X. Zhang, Ultrafast gain recovery in a QW-SOA and its application for 40Gb/s regenerative format conversion from NRZ-DPSK to RZ-OOK, J. Opt. Soc. Amer. B 33 (2016) 1291–1297. [9] G. Huang, Y. Miyoshi, A. Maruta, Y. Yoshida, K. Kitayama, All-Optical OOK to 16-QAM modulation format conversion employing nonlinear optical loop mirror, J. Lightwave Technol. 30 (2012) 1342–1350. [10] K. Mishina, S. Kitagawa, A. Maruta, All-optical modulation format conversion from on-off-keying to multiple-level phase-shift-keying based on nonlinearity in optical fiber, Opt. Express 15 (2007) 8444–8453. [11] X. Wu, J. Wang, H. Huang, A.E. Willner, Experimental optical multiplexing of Two 20-Gbit/s QPSK data channels from different wavelengths onto a single 40Gbit/s Star 16-QAM using fiber nonlinearities, in: CLEO:2011 - Laser Applications to Photonic Applications, Optical Society of America, Baltimore, Maryland, 2011, p. CThH4. [12] B. Zhang, H. Zhang, C. Yu, X. Cheng, Y.K. Yeo, P. Kam, J. Yang, H. Zhang, Y. Wen, K. Feng, An all-optical modulation format conversion for 8QAM based on FWM in HNLF, IEEE Photonics Technol. Lett. 25 (2013) 327–330. [13] Y. Gao, L. Xu, S. He, Optical multi-level signal generation using four-wavemixing, J. Lightwave Technol. 29 (2011) 2166–2172. [14] O. Gerstel, M. Jinno, A. Lord, S.J.B. Yoo, Elastic optical networking: A new dawn for the optical layer? IEEE Commun. Mag. 50 (2012) s12. [15] G. Zhang, M.D. Leenheer, A. Morea, B. Mukherjee, A survey on OFDM-based elastic core optical networking, IEEE Commun. Surv. Tutor. 15 (2013) 65–87. [16] Y. Zhan, M. Zhang, M. Liu, L. Liu, Z. Liu, X. Chen, Analysis of format conversion from optical 16QAM to QPSK based on cascaded four-wave mixing in semiconductor optical amplifier, Optik 125 (2014) 1701–1706. [17] A. Fallahpour, M. Ziyadi, A. Mohajerin-Ariaei, Y. Cao, A. Almaiman, F. Alishahi, C. Bao, P. Liao, B. Shamee, L. Paraschis, M. Tur, C. Langrock, M.M. Fejer, J. Touch, A.E. Willner, Experimental demonstration of tunable optical de-aggregation of each of multiple wavelength 16-QAM channels into two 4PAM channels, in: Optical Fiber Communication Conference, Optical Society of America, Los Angeles, California, 2017, p. Th4I.6. [18] J. Zhang, J. Yu, H.-C. Chien, Single-carrier 400G Based on 84-GBaud PDM-8QAM transmission over 2, 125 km SSMF enhanced by pre-equalization, LUT and DBP, in: Optical Fiber Communication Conference, Optical Society of America, Los Angeles, California, 2017, p. Tu2E.2. [19] X. Zhou, J. Yu, P. Magill, Cascaded two-modulus algorithm for blind polarization de-multiplexing of 114-Gb/s PDM-8QAM optical signals, in: Optical Fiber Communication Conference and National Fiber Optic Engineers Conference, Optical Society of America, San Diego, California, 2009, p. OWG3. [20] H. Liu, H. Wang, M. Yu, Y. Ji, Demonstration of optical channel de-aggregation for 8QAM signal using FWM in HNLF, in: 2018 Asia Communications and Photonics Conference, ACP, 2018, pp. 1–3. [21] M. Nölle, F. Frey, R. Elschner, C. Schmidt-Langhorst, A. Napoli, C. Schubert, Performance comparison of different 8QAM Constellations for the use in flexible optical networks, in: Optical Fiber Communication Conference, Optical Society of America, San Francisco, California, 2014, p. W3B.2. [22] H. Zhang, P.-Y. Kam, C. Yu, Performance analysis of coherent optical 8-star QAM systems using decision-aided maximum likelihood phase estimation, Opt. Express 20 (2012) 9302–9311. [23] S.P.Ó. Dúill, S.T. Naimi, A.P. Anthur, T.N. Huynh, D. Venkitesh, L.P. Barry, Simulations of an OSNR-limited all-optical wavelength conversion scheme, IEEE Photonics Technol. Lett. 25 (2013) 2311–2314. [24] K.R. Bottrill, F. Parmigiani, D.J. Richardson, P. Petropoulos, FWM-based amplitude limiter realizing phase preservation through cancellation of SPM distortions, in: OFC, Optical Society of America, 2016, p. W4D. 6.
Fig. 6. The BER curves for the 16QAM format conversion into 8PSK and 2ASK signals.
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27
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
All-optical modulation format conversion from star-QAM to PSK and ASK signals Feng Wan, Bao-Jian Wu ∗, Feng Wen, Kun Qiu Key Lab of Optical Fiber Sensing and Communication Networks, Ministry of Education, University of Electronic Science and Technology of China, Chengdu 611731, China
ARTICLE
INFO
Keywords: Nonlinear optics Optical communications and information processing All-optical networks Four-wave mixing Highly nonlinear fiber
ABSTRACT An all-optical modulation format conversion scheme from star-QAM to ASK and PSK signals is proposed by employing four-wave mixing (FWM) in highly nonlinear fibers. The phase modulation information of star-QAM signal is firstly converted to the PSK format by a degenerate FWM process under gain saturation, and then the amplitude modulation information of star-QAM signal is mapped into the ASK format by means of a nondegenerate FWM associated with the resulting PSK signal. The detailed signal processing is presented by theory and simulation. As examples, the bit-error-ratio performances of 8QAM and 16QAM signals are evaluated by a non-degenerate FWM-based aggregation scheme for ASK and PSK formats.
1. Introduction With the rapid growth of optical communication system capacity, the maturity of coherent optical communication technology [1], and the wide application of advanced modulation formats with high spectral efficiency [2,3], all-optical modulation format conversion [4], as well as aggregation and de-aggregation [5,6] functions, will play an important role in the network switching nodes. These optical signal processing functions can usually be realized on the basis of optical nonlinear effects, in which the fiber nonlinearity have attracted more attention due to its ultra-fast response [7]. So far, many alloptical format conversion schemes have been presented, including those from NRZ-DPSK to RZ-OOK format [8], OOK to 16QAM [9], and OOK to MPSK [10]. All-optical aggregation of two QPSK signals to 16QAM [11], and the combination of ASK and QPSK into 8QAM signals [12] were also demonstrated by the experiments. Certainly, other nonlinear media can as well be used. For example, Y. GAO et al. generated MQAM signals in optical domain based on four-wave mixing effect in silicon waveguides [13]. It is well known that the advanced modulation signals require higher optical signal to noise ratio (OSNR) for transmission. Thus, in some practical applications, the advanced modulation signals have to be decomposed into multiple low-order modulation formats. Especially, in the elastic optical network (EON), the de-aggregation process enables dynamic allocation of link resources and effectively alleviates the pressure of rapid growth of communication capacity [14,15]. At present, some authors have investigated the decomposition of advanced modulation formats into low-order modulation formats, including the all-optical format conversion from 16QAM to QPSK through cascaded SOAs [16], and the multi-wavelength conversion from 16QAM ∗
to 4PAM [17]. Recently, considering that 8-ary quadrature amplitude modulated (8QAM) signal has become one of the alternatives for 400G WAN interface [18,19], Ref. [20] proposed a de-aggregation scheme for star-8QAM into QPSK and ASK signals using the fiber nonlinearity. However, the ASK signals are obtained only by the square law detection of photodetectors (PD) in electronic domain. In general, star-8QAM has two types of constellations (with 2 amplitudes), those of 8 and 4 phases. The former is widely used in the communication network due to simple IQ-modulator generation [21]. However, the latter exhibits a larger phase noise tolerance [22] and can be converted to a lower-order QPSK signal as discussed in the paper, instead of 8PSK for the former. In this paper, we put forward an all-optical modulation format conversion from any star-QAM to MPSK and ASK signals, using degenerate FWM (DFWM) under gain saturation and non-degenerate FWM (NDFWM) in the HNLFs, respectively. The NDFWM process between the input QAM signal and the resulting MPSK signal is used to generate the optical ASK signal. The scheme presented in this paper has some advantages in the scalability to higher QAM formats and the flexibility of realizing all-optical regeneration. We simulate and verify the schemes for the star 8QAM and 16QAM signals and analyze the bit-error-ratio (BER) performance before and after format conversion. Moreover, in order to estimate the format conversion performance, the resulting PSK and ASK formats are also re-aggregated to a QAM signal by a NDFWM process. Our simulations show that the error-free format conversion of an 8QAM (or 16QAM) signal can be achieved at the input OSNR of higher than 16.4 dB (or 19 dB).
Corresponding author. E-mail address: [email protected] (B.-J. Wu).
https://doi.org/10.1016/j.optcom.2019.06.014 Received 13 March 2019; Received in revised form 6 June 2019; Accepted 7 June 2019 Available online 10 June 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 1. Principle of 8QAM modulation format decomposition (a) input 8QAM, (b) converted QPSK, (c) input 8QAM and (d) converted conjugate QPSK from 1st stage, (e) converted ASK after 2nd stage.
2. The principle of all-optical star-QAM format conversion
From Eqs. (1)∼(3), the complex envelope of the converted QPSK signal 𝐸i1 can be expressed as
Our objective is to achieve the all-optical format conversion from a star-QAM signal to a QPSK signal and an ASK signal. The operating principle for a star-8QAM signal as an example is illustrated in Fig. 1. A DFWM process under gain saturation (first stage) is used to convert the star-8QAM signal into a QPSK format. Then, a part of the resulting QPSK along with the input star-8QAM signal is launched into the second stage of HNLF, and a desirable ASK signal can be obtained by the NDFWM process using a CW pump as shown in Fig. 1(c)∼(e). The QPSK and ASK signals converted in optical domain correspond to the phase and amplitude information of the star-8QAM, respectively. In what follows, we explain the signal processing processes in detail. In NDFWM, the optical fields can be represented by the complex envelopes 𝐸𝑚 = 𝐴𝑚 ⋅ exp(j𝜑𝑚 ), in which 𝐴𝑚 and 𝜑𝑚 are respectively the amplitude and phase, and the subscripts m=1∼4 represent the two pumps, signal, and idler light, respectively. Their frequency relationship and phase matching condition are respectively determined by the energy and momentum conservations, and the idler field is proportional to those of the input waves as follows [12]:
𝐸i1 = 𝐴i1 ⋅ exp(j𝜑i1 ) = 𝛼𝐴2p exp[j(2𝜑p − 𝜑s )]
𝐸4 ∝ 𝐸1 𝐸2 𝐸3∗
where 𝛼 is the saturation gain factor, related to the DFWM compression. Clearly, from Eq. (4) there is a consistent one-to-one match between the idler phase 𝜑i1 and the input signal phase 𝜑s for a given CW pump with a constant phase. In the second stage of HNLF, the NDFWM process is utilized for the conversion into an ASK signal. In the NDFWM process, the input 8QAM signal and the resulting QPSK signal act as so-called pumps and the auxiliary CW as so-called signal, represented by 𝐸a1 = 𝐴a1 exp(−j𝜑a1 ) in the paper. Thus, the complex envelope of the resulting idler 𝐸i2 can also be given as follows: 𝐸i2 = 𝐴i2 ⋅ exp(j𝜑i2 ) = 𝛼𝛽𝐴2p exp[j(2𝜑p − 𝜑s )] ⋅ 𝐴s exp(j𝜑s ) ⋅ 𝐴a1 exp(−j𝜑a1 ) [ ] = 𝛼𝛽𝐴2p 𝐴a1 exp j(2𝜑p − 𝜑a1 ) ⋅ 𝐴s (5) where 𝛽 represents the NDFWM gain factor. It can be seen from Eq. (5) that the phase information of the 8QAM signal is canceled by the conjugate relationship with the QPSK signal, and the amplitude information of the 8QAM signal is preserved for the idler as the converted ASK format after a fixed phase compensation of (2𝜑p − 𝜑a1 ). It is thus evident that Eqs. (4) and (5) is very useful for the explanation for the all-optical format conversion from the star-QAM signal to the QPSK and ASK signals.
(1)
where the superscript * indicates the complex conjugate. Eq. (1) can be further expressed as 𝐴4 ∝ 𝐴1 𝐴2 𝐴3
(2)
𝜑4 = 𝜑1 + 𝜑2 − 𝜑3
(3)
(4)
3. Simulation diagram for star-8QAM format conversion The block diagram of all-optical star-8QAM format conversion scheme is shown in Fig. 2(a), in which the converted QPSK and ASK formats are achieved by two FWM process. To further evaluate the degradation performance for the all-optical QAM format conversion scheme, an extra third-stage DFWM is also introduced to aggregate the PSK and ASK signals into the star-QAM format signals. In the simulation through VPI-Transmission Maker (VPI™ 9.1), the system symbol rate is set to be 10GBaud, and the 8QAM signal at the optical frequency of 193.1 THz is generated from the IQ modulator driven by 215 -1 pseudo-random binary sequence (PRBS) data. Then, the
A DFWM process means that the two pumps are identical in the amplitude and phase. The converted QPSK signal is implemented in the first stage of DFWM process as shown in Fig. 1(a)∼(b). The input 8QAM signal with the optical field 𝐸s = 𝐴s exp(j𝜑s ) is injected into the HNLF together with a CW pump light of 𝐸p = 𝐴p exp(j𝜑p ) and the amplitude of the induced idler light is compressed into one level in the presence of parametric gain saturation. At the same time, the phase information of the input 8QAM signal is retained onto the resulting QPSK format as the idler. 24
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 2. All-optical star-QAM format conversion scheme (a) simulation diagram of the star-QAM format conversion, (b) optical spectra after HNLF1 (c) optical spectra after HNLF2 (d) optical spectra after HNLF3. Table 1 Parameters of highly nonlinearly fibers. Parameters
HNLF1
HNLF2
HNLF3
Fiber length The loss 𝛼 Zero-dispersion wavelength 𝜆0 Dispersion slope S at 𝜆0 Nonlinear coefficient 𝛾
2000 m 0.2 dB/km 1550 nm 80 s/m3 13.1 W−1 km−1
510 m 0.2 dB/km 1550 nm 80 s/m3 10.8 W−1 km−1
400 m 0.2 dB/km 1550 nm 80 s/m3 10.8 W−1 km−1
ASE-degraded 8QAM signal is divided into two branches through a 3-dB beam splitter, launched to the first and second stages, respectively. In our simulation, the optimization of three sections of HNLFs is very important for the FWM efficiency and the gain saturation, especially for the fiber dispersion and length parameters. The HNLF parameters used in our simulation are listed in Table 1. In addition, the CW lasers with the linewidth of 10 kHz and the 3rd-order Gaussian OBPFs with the bandwidth of 20 GHz are always adopted. The noise figure (NF) is taken as 3 dB for all the EDFAs. In the first stage, the input 8QAM is amplified to 1.12 mW by an EDFA, and then coupled into HNLF1 along with the CW pump of 193.16 THz. The optical spectrum output from HNLF1 is shown in Fig. 2(b). The resulting QPSK idler is filtered out by an optical bandpass filter (OBPF1) with the central frequency of 193.22 THz. In order to achieve an identical idler output in the gain saturation region, we can optimize the corresponding power transfer function (PTF) by adjusting the star-QAM and pump powers, instead of tuning the two amplitude levels of the input star-QAM signal. The PTF curve for the DFWM process under gain saturation is shown in Fig. 3. It should be noted that, for the two-amplitude 8QAM signal, the two levels are 0.6 mW and 1.56 mW in terms of optical power. When the input pump power is 680 mW, the converted QPSK idler power is equal to 210 mW. In our scheme, the phase shift 𝛥𝜑 introduced by self-phase modulation (SPM) can be ignored owing to small input signal power, and the constellation rotation by the cross-phase modulation (XPM) can also be canceled in the receiver. In the second stage, the tunable optical delay line (TODL) is used to synchronize the 8QAM signal with the resulting QPSK signal, and the frequency and power of the auxiliary CW2 are taken as 193.04 THz and
Fig. 3. Power transfer function of the first stage FWM process.
10 mW, respectively. The 8QAM and QSPK are respectively amplified to 2 mW and 2.5 mW, and then fed into HNLF2 with CW2. The optical spectrum after the NDFWM process is shown in Fig. 2(c), and the ASK idler is output from OBPF2 with the central frequency of 193.28 THz. In theory, the TODL can be used to align the two branches, as shown in Fig. 2(a). However, an extra optical phase-locked loop (OPLL) may be introduced to compensate the slow phase fluctuation after long HNLF propagation in practical experiments. The third stage is used to aggregates the ASK and QPSK signals into a star-8QAM format by a NDFWM method, instead of the DFWM scheme as presented in Ref. [12]. Similar to Eq. (4), in the DFWM scheme, the amplitude noise of the aggregated 8QAM signal is expected to double that of the input ASK signal, and the amplitude jitter at a higher level is prone to a larger phase degradation due to the fiber nonlinearity. Therefore, the utilization of the NDFWM method may effectively reduce the extra noise introduced in the DFWM scheme. In the third-stage NDFWM process, the QPSK and ASK signals have an 25
F. Wan, B.-J. Wu, F. Wen et al.
Optics Communications 451 (2019) 23–27
Fig. 4. The simulated constellations of (a) the input 8QAM, (b) QPSK signal, and (c) ASK signal, and (d) the aggregated 8QAM signal.
identical power of 5 mW and the auxiliary CW3 at 193.18 THz has a power of 10 mW. The optical spectrum output from HNLF3 is shown in Fig. 2(d) and the recovered 8QAM signal as the idler is filtered out by OBPF3 with the central frequency of 193.32 THz. In practice, the additional phase noise through the FWM process is dependent on the linewidth of CW lasers and a narrow-linewidth or injection locked laser is useful for suppressing the phase fluctuation. In our simulation, we set an identical random seed number to all the laser source for simplicity. Moreover, the phase noise accumulation due to pump-to-idler transfer in the FWM process can be eliminated by the coherent pumps generated by optical frequency comb [23]. In our simulation, the pump-to-idler phase noise transfer in the FWM process is also taken into account. 4. Results and discussion In order to evaluate the performance of the format conversion scheme, an amplified spontaneous emission (ASE) noise is added to the 8QAM signals at the point of A as shown in Fig. 2(a), and then the input OSNR can be adjusted. In addition, the optical signals are coherently received and demodulated at the positions of A, B, C, and D, respectively. The ideal homodyne coherent receiver is used in the simulation, that is, the noise introduced by photoelectric conversion is neglected. The converted electrical signals go through down-sampling, phase recovery and digital processing for the corresponding constellations. In the simulation, Monte-Carlo approach is adopted to estimate the BER performance. The obtained constellation diagrams at the points of A, B, C, and D are shown in Fig. 4, in which the corresponding EVM values are also labeled. In the simulations, the 8QAM signal with the bit rate of 30 Gbit/s is successfully decomposed into two low-order formats of 20 Gbit/s QPSK and 10 Gbit/s 2ASK. In the third stage, the two low-order formats are successfully re-aggregated into an 8QAM signal with the EVM of 12.5% as shown in Fig. 4(d). It is worth noting that the ASK signal generated by the second stage has a smaller EVM than that of QPSK after the first stage. As known from Eq. (5), the ASK signal is generated by the phase-conjugated QPSK and the initial 8QAM signal since the optical phase conjugate (OPC) can effectively suppress the nonlinear noise [24]. Then, we change the input OSNR to further investigate the BER performance for the proposed scheme. The calculated BER curves before and after the format conversion are shown in Fig. 5, including those of the back-to-back (B2B) 8QAM, converted QPSK and ASK, aggregated 8QAM, and offline synthesized 8QAM. In order to evaluate the aggregation performance, the synthesized 8QAM as a reference signal is reproduced from the generated ASK and QPSK signals directly in digital domain. In Fig. 5, the dashed line represents the 7% overhead hard decision forward error correction (HD-FEC) threshold of 3.8×10−3 . It can be known from Fig. 5 that, (i) the aggregation and offline synthesized of 8QAM format have a small difference in the BER performance, which means the NDFWM-based aggregation is desirable for the reproduction of 8QAM format; (ii) at the HD-FEC threshold, the error-free all-optical decomposition and aggregation of 8QAM format can be achieved when the input OSNR is higher than 16.4 dB, with an OSNR penalty of 2.4 dB compared with the B2B system, corresponding
Fig. 5. The BER curves for the 8QAM format conversion into QPSK and 2ASK signals.
to the case in the absence of format conversion; (iii) according to the ASK and QPSK curves in the absence of aggregation, the 8QAM format conversion is mainly determined by the noise performance of the converted ASK, instead of QPSK. Using the same conditions as the case of 8QAM, we can also simulate the 40 Gbit/s star-16QAM format conversion into 30 Gbit/s 8PSK and 10 Gbit/s 2ASK, and Fig. 6 plots the corresponding BER curves relative to the input OSNR, slightly different from that of the 8QAM format as shown in Fig. 5. By comparison of the 8QAM format, the input OSNR required for the error-free 16QAM format conversion increases to 19 dB at the HD-FEC threshold, with an OSNR penalty of ∼3.5 dB. From Fig. 6, the converted 8PSK and ASK signals have a slightly different OSNR performance at the HD-FEC threshold. Finally, it should be pointed out that the format conversion scheme proposed here is also applied to the case of converting star-QAM signals into a higher-order ASK only if the first-stage FWM process has a sufficiently wide gain-saturated region. 5. Conclusion This paper presents a novel all-optical format conversion scheme from star-QAM signals to PSK and ASK formats, as well as the aggregation process of QAM signals for evaluating the BER performance. As examples, the cases with the 8QAM and 16QAM formats are taken into account by simulation. The derivation of PSK and ASK formats from QAM signals is implemented by a gain-saturated DFWM and another NDFWM, respectively. Our simulations show that the error-free format conversion of 8QAM and 16QAM signals can be achieved when the input OSNRs are higher than 16.4 dB and 19 dB, respectively. By comparison with the B2B system, the corresponding OSNR penalties introduced by the entire format conversion are 2.4 dB and 3.5 dB, respectively. 26
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Fig. 6. The BER curves for the 16QAM format conversion into 8PSK and 2ASK signals.
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