All-optical tunable multilevel amplitude regeneration based on coherent wave mixing using a polarizer

Zahra Bakhtiari; Alexander A. Sawchuk

doi:10.1364/OE.23.010533

1. Introduction

Higher-order modulation formats are of extreme interest to the optical communications community due to their higher spectral efficiency and higher tolerance to fiber-based dispersion [1]. In particular, quadrature-amplitude-modulation (QAM) and star QAM signals can significantly increase the transmission capacity for a given symbol rate and thus enable a higher spectral efficiency [2]. On the other hand, QAM formats show a higher sensitivity to amplitude and phase noise. Amplitude noise not only reduces signal quality but may also be converted into nonlinear phase noise in a transmission line due to the Gordon–Mollenauer effect [3]. Cross-phase modulation in wavelength-division multiplexing (WDM) systems is another cause of amplitude noise to nonlinear phase noise conversion as well [4]. All-optical regenerators are expected to extend the maximum reach of high-speed transmission systems by eliminating accumulated signal impairments in transmission systems without the need for optical/electronic/optical (O/E/O) conversion. Current phase regeneration schemes are relatively complex and are limited to lower-order modulation formats [5,6]. Thus, developing all-optical tunable phase-preserving multilevel amplitude regenerator modules with bit-rate transparency is desirable. A typical method for multilevel amplitude regeneration of star QAM signals is based on a nonlinear amplifying optical loop mirror (NALM) [7,8]. There is interest in developing more tunable multilevel amplitude regeneration modules with more degrees of freedom. In this paper, we demonstrate 10 Gbaud optical multilevel amplitude regeneration of star-8QAM and star-16QAM signals with the state power ratio of 1:5 based on coherent polarization mixing using a cascaded two-stage polarization-based multilevel amplitude regenerator. The proposed polarization-based scheme provides more degrees of freedom for optimization.

2. Concept and operation principle

The operation principle of our proposed polarization based multilevel amplitude regenerator is shown in Fig. 1. First the noisy star-8QAM signal $E_{i n}$ is split into two orthogonal polarization states with a large splitting ratio $α$ using a polarization beam splitter (PBS). The splitting ratio is adjusted by a polarization controller (PC) at PBS input. Assuming $α < 0.2$ , at the PBS output, the weaker signal copy $\sqrt{α} E_{i n}$ with polarization state ${\hat{n}}_{2}$ is almost unaffected while the stronger signal copy $\sqrt{1 - α} E_{i n}$ with polarization state ${\hat{n}}_{1}$ is modified by the self-phase modulation (SPM) effect in a highly nonlinear fiber (HNLF), which converts the amplitude fluctuations into phase changes $φ$ as given by $φ = γ L P$ where P is the signal power,L is HNLF length and $γ$ is the nonlinearity factor. Note that the two signals are counter-propagating through the HNLF in order to experience the same phase shift and also to reduce the effect of cross-phase modulation (XPM). Two circulators are used to make two different paths for propagating and counter- propagating signals. At the HNLF output the stronger signal electric field $E_{s}$ is

E_{s} = \sqrt{1 - α} E_{i n} \exp (- j γ L (1 - α) {| E_{i n} |}^{2})

and the weaker signal electric field

E_{w}

is

E_{w} = \sqrt{α} E_{i n} \exp (- j γ L α {| E_{i n} |}^{2}) .

Then a custom polarization beam combiner (PBC) applies attenuation

χ

and a phase shift

θ

to the higher power signal, resulting in a considerable vector magnitude difference between the unaffected signal copy and the self-phase modulated copy that is required to get the optimum amplitude regeneration without adding a large component of phase noise. The polarization multiplexed signal

E_{m}

at the PBC output is

E_{m} \propto \sqrt{χ} E_{s} \exp (- j θ / 2) {\hat{n}}_{1} + E_{w} {\hat{n}}_{2} .

Since the self-phase modulated polarization state and the unaffected polarization state travel together throughout the system, they remain coherent with each other and a polarizer can be utilized to add the two polarization states coherently [9]. This scheme also provides extra tunability to achieve the required magnitude difference by tuning the polarizer angle. The polarizer output

E_{p}

tuned at angle

ϕ

with respect to the

{\hat{n}}_{1}

axis is

E_{p} \propto \sqrt{χ} E_{s} \exp (- j θ / 2) \cos (ϕ) + E_{w} \sin (ϕ) .

Amplitude regeneration at each amplitude level is achieved by coherent addition of the SPM-based phase-modulated polarization state and the unaffected one. To achieve significant regeneration strength on both amplitude levels of star-QAM signals, two stages of the proposed module are cascaded to get the desired regeneration results. Figure 2 is a conceptualdiagram of coherent addition at each amplitude state on a constellation plane in our proposed technique. At the left, we show three vectors corresponding to various amplitude fluctuations located in the noisy constellation point on the in phase quadrature (IQ) plane. These three arbitrary vectors

A_{1}

,

A_{2}

and

A_{3}

correspond to the lowest, the medium level and the highest power level of a single arbitrary constellation point on a constellation diagram.

{A^{'}}_{i}

is the corresponding SPM-based phase rotated copy of

A_{i}

and is proportional to

A_{i} \exp (j φ_{i})

, where

φ_{i} = γ L {| A_{i} |}^{2}

. As can be seen, the

{A^{'}}_{3}

vector has the largest SPM-based phase rotation, which results in decreasing the

A_{3}

amplitude after coherent polarization mixing, while the

{A^{'}}_{1}

vector has the smallest SPM-based phase rotation, which results in increasing the

A_{1}

amplitude level after coherent polarization mixing. The result is that the distributed noisy constellation point is mapped into a constellation point with reduced amplitude noise as shown in the right side of Fig. 2.

Fig. 1 Diagram of polarization-based phase-preserving multilevel amplitude regeneration.

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Fig. 2 Concept of multilevel amplitude regeneration: the original signal is phase-modulated based on the self-phase modulation periodic effect and is added coherently to the original signal.

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3. Setup and results

The simulation setup for the multilevel amplitude regeneration scheme is depicted in Fig. 3. We use the VPItransmissionMaker and VPIcomponentMaker software package from VPIphotonics for this work. An inphase quadrature (I/Q) modulator is driven by two electrical data streams to generate a 10- Gbaud quadrature-phase shift keying (QPSK) modulated signal at 1552.5nm with a pseudo random bit stream period (PRBS) $2^{15} - 1$ , followed by an amplitude modulator (AM) to generate a star-8QAM signal. The star-8QAM signal power ratio is 1:5 and the optical signal-to-noise ratio (OSNR) is adjusted using an amplitude and phase noise emulator. To generate a star-16QAM signal, an I/Q modulator is followed by a phase modulator. The resulting signal is then sent to an erbium doped fiber amplifier (EDFA), polarization controller (PC), and a polarization beam splitter (PBS) to generate two signal copies with orthogonal polarization states. The splitting ratio of PBS is tuned to 85:15 by adjusting the PC. These two polarization states are sent to a HNLF with zero-dispersion wavelength (ZDW) of 1552.5 nm, length of 1000 m and nonlinearity factor of 20 $1 / W - k m$ in reverse directions. These two copies are multiplexed by a custom PBC after applying attenuation and phase adjustments to the SPM modulated signal copy, then are sent to a polarizer tuned at 45 degrees with respect to the ${\hat{n}}_{1}$ polarization state for coherent polarization mixing. Note that two regenerator modules are cascaded to get the desired regeneration results for star-QAM signals with two distinct amplitude levels.

Fig. 3 Simulation setup. CW: continuous wave; PC: polarization controller; I/Q Mod: In phase/quadrature modulator; AM: amplitude modulator; PBS: polarization beam splitter; HNLF: highly-nonlinear fiber; PBC: polarization beam combiner; BPF: band pass filter; EDFA: erbium doped fiber amplifier; VOA: variable optical attenuator.

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3.1 Multilevel amplitude regeneration without a transmission line (back-to-back schematic)

Figures 4(a), 4(d) and 4(g) depict the constellation diagrams of a noisy star-8QAM signal with optical signal-to-noise ratio (OSNR) of 20 dB, 22 dB and 24 dB. Figures 4(b), 4(e) and 4(h) show the star-8QAM constellation diagrams after a one-stage amplitude regenerator. As can be seen, the first stage amplitude regenerator is intentionally tuned to apply the maximum amplitude noise reduction on the lower-amplitude level. Figures 4(c), 4(f) and 4(i) show the star-8QAM constellation diagrams after a two-stage amplitude regenerator in which the second stage is tuned to apply the maximum amplitude noise reduction on the higher-amplitude level. The performance of the system is evaluated by measuring regeneration factor versus OSNR for both amplitude levels of a star-8QAM. The regeneration factor is defined as the ratio of input amplitude noise standard deviation (STD) to output amplitude noise standard deviation. As shown in Fig. 5(a), a peak of almost 5 at 20 dB OSNR for higher-amplitude level of star-8QAM and a peak of almost 3 at 22 dB OSNR for lower-amplitude are achieved.

Fig. 4 Back-to-back constellation diagrams with OSNR 20 dB: (a) a noisy star-8QAM signal; (b) regenerated signal after one-stage amplitude regenerator; (c) regenerated signal after two-stage amplitude regenerator. Back-to-back constellation diagrams with OSNR 22 dB: (d) a noisy star-8QAM signal; (e) regenerated signal after one-stage amplitude regenerator; (f) regenerated signal after two-stage amplitude regenerator. Back-to-back constellation diagrams with OSNR 24 dB: (d) a noisy star-8QAM signal; (e) regenerated signal after one-stage amplitude regenerator; (f) regenerated signal after two-stage amplitude regenerator.

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Fig. 5 (a) Regeneration factor vs. OSNR for both amplitude levels of a regenerated star-8QAM signal after a two-stage amplitude regenerator in a back-to-back configuration; (b) Power transfer function of the one-stage polarization-based amplitude regenerator.

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Figure 5(b) shows the power transfer function of the one-stage regenerator. It is measured by applying a noisy 8-PSK signal as the input of the proposed amplitude regenerator. The power transfer function has two plateau regions with the input power difference of almost 4.7 dB. Note that we cascade two of the proposed polarization-based regenerators to get the maximum regeneration factor for both amplitude levels. The phase-preserving multilevel amplitude regeneration concept is evaluated by implementing the scheme on 10 Gbaud noisystar-16QAM signals as well. Figures 6(a) and 6(d) depict the constellation diagrams of a noisy star-16QAM signal with optical signal-to-noise ratio (OSNR) of 22 dB and 24 dB and OSNR bandwidth of 6.25 GHz. Figures 6(b) and 6(e) show the star-16QAM constellation diagrams after a one-stage regenerator. Figures 6(c) and 6(f) show the star-16QAM constellation diagrams after a two-stage polarization-based multilevel amplitude regenerator in which the first stage amplitude regenerator is intentionally tuned to apply the maximum amplitude noise reduction on the lower- amplitude level while the second stage is tuned to apply the maximum amplitude noise reduction on the higher-amplitude level. As can be seen, amplitude regeneration is achieved at both levels without noticeable phase noise.

Fig. 6 Back-to-back constellation diagrams with OSNR 22 dB: (a) a noisy star-16QAM signal; (b) regenerated signal after one-stage amplitude regenerator; (c) regenerated signal after two-stage amplitude regenerator. Back-to-back constellation diagrams with OSNR 24 dB: (d) a noisy star-16QAM signal; (e) regenerated signal after one-stage amplitude regenerator; (f) regenerated signal after two-stage amplitude regenerator.

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3.2 Multilevel amplitude regeneration in a nonlinear transmission line schematic

The effect of having a multilevel amplitude regenerator before a nonlinear transmission line to avoid nonlinear phase noise originating from the Gordon–Mollenauer effect is shown by implementing the scheme on 10 Gbaud noisy star-8QAM and star-16QAM signals with almost 16 dBm average power transmitted through a 150 km nonlinear transmission line. Thetransmission line is inserted between the polarizer and EDFA in Fig. 3. Figures 7(a) and 7(c) depict the constellation diagrams of noisy star-8QAM signals with signal-to-noise ratio (OSNR) of 20 dB and 22 dB after a 150 km transmission line with nonlinear index of $2.6 e - 20 m^{2} / W$ and core area of $80 e - 12 m^{2}$ with a two-stage proposed amplitude regenerator located at the transmission line input terminal, while Figs. 7(b) and 7(d) show the transmitted star-8QAM constellation diagrams having the same noise characteristic with no pre-regeneration. Figures 7(e) and 7(g) also depict the constellation diagrams of noisy star-16QAM signals with optical signal-to-noise ratio (OSNR) of 22 dB and 24 dB after a 150 km nonlinear transmission line with a two-stage multilevel amplitude regenerator located at the transmission line input terminal, while Figs. 7(f) and 7(h) show the transmitted star-16QAM constellation diagrams having the same noise characteristic with no pre-regeneration, showing the critical role of an amplitude pre-regeneration stage at the input of a nonlinear transmission line. To show the level of nonlinear phase noise generated by the Gordon–Mollenauer effect in a transmission line, we define the phase noise factor. The phase noise factor is defined as the ratio of transmitted signal phase noise standard deviation over initial signal phase noise standard deviation. As shown in Fig. 8(a), the phase noise factor for the higher-amplitude level of star-8QAM with no pre-regeneration has a peak value of almost 5.5, while the peak value reduces to almost 2 by applying pre-regeneration. The phase noise factors are almost in the same order for lower-amplitude with no pre-regeneration and higher-amplitude with pre-regeneration. Figure 8(b) shows the polarizer angle vs. attenuation value embedded inside the PBC at BER 10e-4. As can be seen, by applying 30 dB attenuation and tuning the polarizer angle to 45 degrees; or by applying almost 17 dB attenuation and tuning the polarizer angle to almost 13 degrees at the first stage of polarization-based regenerator, the same BER of 10e-4 can be achieved, thus showing additional degrees of freedom for tunability in our proposed regenerator.

Fig. 7 Transmitted constellation diagrams of star-8QAM and star-16QAM after 150 km transmission line with/without a pre-regeneration module.

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Fig. 8 (a) Phase noise factor vs. input signal OSNR for both amplitude levels of a transmitted star-8QAM with/without pre-regeneration module at the input of a nonlinear transmission line. (b) Polarizer angle vs. attenuation value embedded in a custom polarization beam combiner (PBC) at BER 10e-4.

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4. Summary

We described a two-stage polarization-based multilevel amplitude regenerator in which the first stage is tuned to apply the maximum amplitude noise reduction to the low-amplitude level while the second stage is set to apply the maximum amplitude noise reduction to the high-amplitude level. We applied the proposed technique to star-8QAM and star-16QAM signals and obtained a maximum regeneration factor of 5 for star-8QAM. We showed that having a pre-regeneration module before a nonlinear transmission line is crucial to avoid nonlinear phase noise generation, and can suppress nonlinear phase noise 2.5 times more than a nonlinear transmission line with no pre-regeneration.

Acknowledgment

The authors express their gratitude to the VPIphotonics University Program for their support.

References and links

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2. M. Seimetz, High-Order Modulation for Optical Fiber Transmission, Springer Series in Optical Sciences, (Springer, 2009).

3. J. P. Gordon and L. F. Mollenauer, “Phase noise in photonic communications systems using linear amplifiers,” Opt. Lett. 15(23), 1351–1353 (1990). [CrossRef] [PubMed]

4. H. Kim, “Cross-phase-modulation-induced nonlinear phase noise in WDM direct-detection DPSK systems,” J. Lightwave Technol. 21(8), 1770–1774 (2003). [CrossRef]

5. M. Gao, T. Inoue, T. Kurosu, and S. Namiki, “Sideband-assisted phase sensitive amplifiers with high phase sensitivity for efficient phase regeneration,” in 2012 Optical Fiber Communications Conference (Optical Society of America, 2012), paper OW3C.5.

6. B. Stiller, G. Onishchukov, B. Schmauss, and G. Leuchs, “Phase regeneration of a star-8QAM signal in a phase-sensitive amplifier with conjugated pumps,” Opt. Express 22(1), 1028–1035 (2014). [CrossRef] [PubMed]

7. M. Hierold, T. Roethlingshoefer, K. Sponsel, G. Onishchukov, B. Schmauss, and G. Leuchs, “Multilevel phase preserving amplitude regeneration using a single nonlinear amplifying loop mirror,” IEEE Photon. Technol. Lett. 23(14), 1007–1009 (2011). [CrossRef]

8. T. Roethlingshoefer, T. Richter, C. Schubert, G. Onishchukov, B. Schmauss, and G. Leuchs, “All-optical phase-preserving multilevel amplitude regeneration,” Opt. Express 22(22), 27077–27085 (2014). [CrossRef] [PubMed]

9. Z. Bakhtiari, J. Wang, X. Wu, J. Yang, S. Nuccio, R. Hellwarth, and A. Willner, “Demonstration of 10-40-Gbaud baud-rate-tunable optical generation of 16-QAM from a QPSK signal using a variable DGD element,” in 2011 Conference on Lasers and Electro-Optics (Optical Society of America, 2012), paper CThX5.

All-optical tunable multilevel amplitude regeneration based on coherent wave mixing using a polarizer

Abstract

1. Introduction

2. Concept and operation principle

3. Setup and results

3.1 Multilevel amplitude regeneration without a transmission line (back-to-back schematic)

3.2 Multilevel amplitude regeneration in a nonlinear transmission line schematic

4. Summary

Acknowledgment

References and links

Cited By

Figures (8)

Equations (4)

Optics Express