Effective optical clock recovery and simultaneous fourfold demultiplexing of OTDM signal using an optoelectonic oscillator

We propose and demonstrate a novel scheme for clock recovery and simultaneous fourfold optical time-division demultiplexing using a dual-parallel Mach-Zehnder modulator based optoelectronic oscillator. 25GHz prescaled optical clock with a 23% duty cycle and a 22-dB extinction ratio is successfully extracted from both 100-Gb/s on-off keying (OOK) and differential phase-shift keying (DPSK) optical time-division-multiplexing (OTDM) signal. The timing jitters (100 Hz to 10 MHz) are measured to be 195.9 fs and 125.6 fs for the optical clock extracted from the 100-Gb/s OOK and DPSK signal, respectively. Error-free optical time-division demultiplexing is also achieved simultaneously with clock recovery. By adjusting the phase shifter in the OEO loop, all four channels can be selectively demultiplexed. The power penalties at a bit error rate (BER) of 10 for the four demultiplexed channels are measured to be between 0.8 dB and 1.2 dB for the OOK signal and between 0.9 dB and 1.5 dB for the DPSK signal. ©2013 Optical Society of America OCIS codes: (250.4110) Modulators; (230.4910) Oscillators; (060.2360) Fiber optics links and subsystems. References and links 1. N. Calabretta, J. Luo, J. Parra-Cetina, S. Latkowski, R. Maldonado-Basilio, P. Landais, and H. J. S. Dorren, “320 Gb/s all-optical clock recovery and time demultiplexing enabled by a single quantum dash mode-locked laser Fabry-Perot optical clock pulse generator,” in Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2013), paper OTh4D.5. 2. B. K. Mathason and P. J. Delfyett, “Pulsed injection locking dynamics of passively mode-locked external-cavity semiconductor laser systems for all-optical clock recovery,” J. Lightwave Technol. 18(8), 1111–1120 (2000). 3. C. Boerner, V. Marembert, S. Ferber, C. Schubert, C. Schmidt-Langhorst, R. Ludwig, and H. G. Weber, “320 Gbit/s clock recovery with electro-optical PLL using a bidirectionally operated electroabsorption modulator as phase comparator,” in Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper OTuO3. 4. S. Pan and J. Yao, “Optical clock recovery using a polarization-modulator-based frequency-doubling optoelectronic oscillator,” J. Lightwave Technol. 27(16), 3531–3539 (2009). 5. H. Chou, Z. Hu, J. E. Bowers, D. J. Blumenthal, K. Nishimura, R. Inohara, and M. Usami, “Simultaneous 160Gb/s demultiplexing and clock recovery by utilizing microwave harmonic frequencies in a traveling-wave electroabsorption modulator,” IEEE Photonics Technol. Lett. 16(2), 608–610 (2004). 6. N. Jia, T. Li, K. Zhong, J. Sun, M. Wang, and J. Li, “Simultaneous clock enhancing and demultiplexing for 160Gb/s OTDM signal using two bidirectionally operated electroabsorption modulators,” IEEE Photonics Technol. Lett. 23(21), 1615–1617 (2011). 7. T. Miyazaki and F. Kubota, “Simultaneous demultiplexing and clock recovery for 160-Gb/s OTDM signal using a symmetric Mach–Zehnder switch in electrooptic feedback loop,” IEEE Photonics Technol. Lett. 15(7), 1008– 1010 (2003). 8. E. S. Awad, P. S. Cho, and J. Goldhar, “Simultaneous four-wave mixing and cross-absorption modulation inside a single EAM for high-speed optical demultiplexing and clock recovery,” IEEE Photonics Technol. Lett. 17(7), 1534–1536 (2005). 9. S. Pan and J. Yao, “Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1460–1468 (2010). #195414 $15.00 USD Received 7 Aug 2013; revised 14 Nov 2013; accepted 20 Nov 2013; published 27 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.030000 | OPTICS EXPRESS 30000 10. H. Tsuchida, “Subharmonic optoelectronic oscillator,” IEEE Photonics Technol. Lett. 20(17), 1509–1511 (2008). 11. H. Tsuchida, “Simultaneous prescaled clock recovery and serial-to-parallel conversion of data signals using a polarization modulator-based optoelectronic oscillator,” J. Lightwave Technol. 27(17), 3777–3782 (2009). 12. J. Yu, K. Kojima, and N. Chand, “Simultaneous demultiplexing and clock recovery of 80 Gb/s OTDM signals using a tandem electro-absorption modulator,” in Proceedings of Lasers and Electro-Optics Society, (Institute of Electrical and Electronics Engineers, San Diego, 2001), pp. 358–359. 13. L. Yan, W. Jian, J. Yu, K. Deming, L. Wei, H. Xiaobin, G. Hongxiang, Z. Yong, and L. Jintong, “Generation and performance investigation of 40GHz phase stable and pulse width-tunable optical time window based on a DPMZM,” Opt. Express 20(22), 24754–24760 (2012). 14. L. Huo, H. Li, Q. Wang, and C. Lou, “4 x 25-GHz 2-ps multicolor ultrashort pulse generation with a single phase modulator and Mamyshev reshaper,” in Conference on Lasers and Electro-Optics, Technical Digest (CD) (Optical Society of America, 2012), paper JTh2A.122.


Introduction
High speed clock recovery (CR) is one of the key enabling technologies in optical communication systems.The recovered clock may be preferred in either optical or electrical forms, or in both, depending on the requirements of the system design.Generally speaking, optical clock (OC) with high extinction ratio (ER), low duty cycle and low timing jitter is desirable for all-optical signal processing functions such as 3R regeneration, modulation format conversion, add-drop multiplexing and demultiplexing of OTDM signal, etc. Electrical clock (EC) with high spectral purity and low phase noise or timing jitter is essential to driving electric-optic (EO) devices in optical networks to carry out functions such as synchronous modulation, high-speed switching driving and also demultiplexing of OTDM signal, etc. CR has been demonstrated in quite a number of schemes in past decades, e.g., by mode-locked lasers (MLLs), self-pulsing lasers, phase-locked loops (PLLs) and optoelectronic oscillators (OEOs) [1][2][3][4].Among various technologies, PLL and OEO based schemes draw more attention for the most outstanding feature of simultaneous operation for CR and demultiplexing of OTDM signal [5][6][7][8][9][10][11][12], which effectively release the complexity and increase the flexibility of an OTDM receiver.
Compared to OEO-based schemes, the PLL based ones require a costly voltage-controlled oscillator (VCO) providing clock signal to guarantee sustained oscillation.Furthermore, a complex phase comparator and an extra ultrashort optical pulse source are usually indispensable for simultaneous demultiplexing operation [7,8], which increase cost and decrease stability and flexibility of the system.As a consequence, OEO based schemes are more attractive for CR and simultaneous demultiplexing due to the inherent merits of robustness, simplicity, self-starting oscillation and low cost.Polarization modulator (PolM) and conventional Mach-Zehnder intensity modulator (MZM) based OEOs have been reported to perform CR and simultaneous demultiplexing [9][10][11].However, the PolM-OEO operates on principle of polarization control, which is sensitive to environmental disturbance, whereas the MZM-OEO requires the involved MZM to be biased at its maximum transmission point, which presents strict requirements on DC bias voltage and radio frequency (RF) power applied to MZM.And essentially, only a twofold demultiplexing is achieved by such OEOs due to the relatively wide on-off window generated by PolM or MZM, which is unsuitable for OTDM signal with a higher aggregated rate.As an improvement, electroabsorption modulator (EAM) based OEOs can be implemented to realize simultaneous CR and effective demultiplexing thanks to the narrow enough on-off window generated by EAM [12].Tandem-EAM-OEO with a dual-loop structure is demonstrated to realize simultaneous CR and demultiplexing.Driven by 20-GHz and 10-GHz RF, respectively, the cascaded EAMs provides an on-off window with a 9-ps 3-dB width, which is narrow enough for eightfold demultiplexing of 80-Gb/s OTDM signal.However, this OEO requires a frequency multiplier to form a dual-loop structure, which is costly and hard to align, and moreover, in order to compensate the high power consumption caused by EAMs, a semiconductor optical amplifier (SOA) is necessary between the two EAMs.Furthermore, these reported OEO based schemes are not demonstrated to be capable of providing a practically useful OC with a high ER and a low duty cycle.
In this study, a compact and cost-effective dual-parallel Mach-Zehnder modulator (DPMZM) based OEO is proposed and demonstrated to perform effective OC recovery and simultaneous demultiplexing of 100-Gb/s OTDM signal.Compared to PolM or MZM, a standalone DPMZM can conveniently provide an on-off window which is narrow enough for fourfold demultiplexing.We show that the proposed DPMZM-OEO works for both OOK and DPSK modulation formats.25-GHz 9.2-ps OCs with 23% duty cycle and 22-dB ER are successfully extracted from 100-Gb/s OOK and DPSK OTDM signal, respectively.The rootmean-square (RMS) timing jitters (100 Hz to 10 MHz) are measured to be 195.9fs and 191.8 fs for OC and EC extracted from 100-Gb/s OOK signal, respectively, and to be 125.6 fs and 122.6 fs for OC and EC extracted from 100-Gb/s DPSK signal, respectively.OTDM demultiplexing simultaneous with CR can be also obtained for both the OOK and DPSK format, respectively.By adjusting the phase shifter in the OEO loop, all four 25-Gb/s tributaries can be obtained.The power penalties at a BER of 10 −9 for the fourfold demuliplexing are measured to be between 0.8 dB and 1.2 dB for the OOK signal and between 0.9 dB and 1.5 dB for the DPSK signal.The schematic diagram of the proposed DPMZM-OEO is shown in Fig. 1.It is a ring structure mainly composed of a DPMZM, an erbium-doped fiber amplifier (EDFA), a photodetector (PD), an electrical band-pass filter (EBPF), a phase shifter and an electrical amplifier (EA).The EDFA and the EA are used to provide optical and electrical gain to guarantee the oscillation condition that the gain of the loop is higher than loss.The freerunning oscillation frequency of the OEO is determined by the equivalent electrical path length of the loop.Usually it has multiple free-running modes even with the spectral confinement of the EBPF.However, the OEO can be injection-locked by an external frequency which is close to free-running frequency or harmonics of the free-running frequency.When the OEO is injection-locked, only one frequency with high spectral purity and low phase noise is obtained.

Principle of the proposed OEO
The key device in the system is the DPMZM.It is an integrated optical device composed of two child MZMs (MZM1 and MZM2) embedded in a parent MZM.By appropriately setting the bias voltages and RF driving powers of the child MZMs and parent MZM, the DPMZM can generate an on-off window [13].In our experiment, the DPMZM (Fujitsu FTM7961EX) has a half-wave voltage (V π ) of 6 V at 25 GHz.MZM1 is driven by a 25-GHz EA with an output power of 21 dBm (corresponding to a peak-to-peak voltage of 1.18V π ) and is biased at its 80.62% transmission point; MZM2 is open circuit for RF and is biased at its 89.35% transmission point; the parent MZM is biased around its null point.Then as Fig. 2  The optical bandpass filter 1 (OBPF1) in the loop is used to suppress the potential freerunning oscillation and to make sure that the OEO is injection-locked by the injected OTDM signal.If a continuous wave (CW) laser is launched into the OEO together with the OTDM signal, the DPMZM on-off window with low duty and high ER will be thus applied to the CW laser, and then, OC with the same duty cycle and ER as that of the on-off window will be achieved.By carefully adjusting the phase shifter, OTDM demultiplexing can also be achieved simultaneously, also attributed to the low enough duty cycle and high ER of the DPMZM on-off window.Tuning the center wavelength of the OBPF2, the generated OC or the demultiplexed OTDM tributary can be selected, respectively.

Experimental results and discussion
As Fig. 1 shows, the transmitter used for generation of 100-Gb/s RZ-OOK or RZ-DPSK signal mainly consists of a home-built 25-GHz 2-ps optical pulse source at 1551 nm [14], a 25-Gb/s electrical pulse pattern generator (PPG) for pseudo random binary sequence (PRBS) generation with a length of 2 31 -1, a MZM for modulation and a passive polarizationmaintaining 1 × 4 optical multiplexer (OMUX).Firstly, the CR by the DPMZM-OEO is tested with 100-Gb/s OOK signal.100-Gb/s OOK signal together with a CW laser at 1546 nm is launched into the DPMZM-OEO.The optical powers of the OTDM signal and the CW laser are set as 6 dBm and 3 dBm, respectively.Inside of the OEO loop, a 2.8-nm OBPF1 is used to select the 1551-nm OTDM signal.The center frequency and bandwidth of the EBPF are 25 GHz and 10 MHz, respectively.Figures 3(a 4(a) is a zoom-in view of the same electrical spectrum with 1-kHz span and 1-Hz RBW.As can be seen, the spectrum is highly pure with the intensity of the 25-GHz clock tone standing more than 61.5 dB higher than that of the background noise.Single-sideband (SSB) phase noise spectrum of the 25-GHz EC is also measured, as shown in Fig. 4(b).As references, SSB phase noise spectra of the 25-GHz RF source used for OTDM tributary generation and the extracted OC after converted into RF signal with a 50-GHz PD are also measured and plotted.Phase noises at 10 kHz frequency offset for the 25-GHz RF source, OC and EC are −92.7 dBc/Hz, −95.3 dBc/Hz and −92.5 dBc/Hz, respectively.SSB phase noise spectra show that the corresponding RMS timing jitters (from 100 Hz to 10 MHz offset frequency) are 104 fs, 195.9 fs and 191.8 fs, respectively.We also test the DPMZM-OEO with PRBS OOK signal with different code length.When the PRBS code length is 2 23 -1, 2 15 -1 and 2 11 -1, the RMS timing jitters of the measured EC remain to be around 192 fs with 2-fs variation.If the code length is 2 10 -1 and 2 7 -1, the RMS timing jitters are decreased to be around 124 fs, approximate equal to that of the EC extracted from the DPSK signal (given in the following parts).In addition, the DPMZM-OEO is also test with special repeated patterns with an increasing ratio of zeros for the CR of OOK signal.The average optical power of the repeated pattern is fixed as 6 dBm.When the repeated pattern is composed of 2 5 -4, 2 6 -4 or 2 7 -4 consecutive zeros followed by 4 consecutive ones, the RMS timing jitters are measured to be 206.4fs, 263.6 fs and 657.2 fs, respectively, demonstrating the phase noise deteriorates seriously as the ratio of the zeros increases.6(a).As a reference, BER measurement for the 25-Gb/s back-to-back (B2B) signal (before multiplexing) is also performed.Error-free detection without error floor is achieved for all four channels.As can be seen from Fig. 6(a), the receiving sensitivity at a BER of 10 −9 is −31.3 dBm for the 25-Gb/s B2B signal, and the power penalty resulting from OTDM multiplexing and demultiplexing is 0.8 dB for the best channel and 1.2 dB for the worst one.Then, the DPMZM-OEO is further tested with 100-Gb/s RZ-DPSK signal with an average optical power of 6 dBm and a PRBS length of 2 31 -1. Figure 7(a) shows the waveform of the OC extracted from the injected 100-Gb/s DPSK signal.The corresponding 3-dB width and ER are measured to be 9.2 ps and 22 dB, respectively.Figure 7 (b) shows the demultiplexed 25-Gb/s tributary from 100-Gb/s DPSK signal, from which we can see a clear eye diagram.All four demultiplexed 25-Gb/s DPSK tributaries are further demodulated for BER measurements by adjusting the phase shifter in the OEO loop.As a reference, BER measurement for 25-Gb/s B2B DPSK signal is also performed.As can be seen in Fig. 6(b), the power penalty at a BER of 10 −9 is between 0.9 dB and 1.5 dB.8(a) is a zoom-in view of the same electrical spectrum with a 1-kHz span and 1-Hz RBW.The intensity of the 25-GHz clock tone is more than 64.2 dB higher than that of the back-ground noise.Figure 8(b) shows the SSB phase noise spectra of the 25-GHz RF source, OC and EC, respectively.The corresponding RMS timing jitters (integration from 100 Hz to 10 MHz offset frequency) of the 25-GHz RF, OC and EC are measured to be 104 fs, 125.6 fs and 122.6 fs, respectively.The timing jitter of the clock extracted from DPSK signal is lower than that extracted from the afore-mentioned OOK signal, mainly attributing to the fact that the DPSK signal has a constant intensity envelope while the OOK has a varying one.

Conclusion
The proposed DPMZM-OEO is demonstrated to be capable of recovering both EC and OC from 100-Gb/s OOK and DPSK OTDM signal, respectively.Compared to the reported schemes with similar functionality, the proposed one may be more compact, more economical, and more effective.Prescaled OC with a 23% duty cycle and a 22-dB ER is achieved, and simultaneous fourfold OTDM demultiplexing is also successfully performed.Error-free detection is obtained for all channels with power penalties between 0.8 dB and 1.5 dB.
(a) shown, an on-off window with a 9.2-ps 3-dB width (equivalent to 23% duty cycle) and a #195414 -$15.00USD Received 7 Aug 2013; revised 14 Nov 2013; accepted 20 Nov 2013; published 27 Nov 2013 (C) 2013 OSA 23-dB ER is generated by the DPMZM.Figure 2(b) presents the corresponding optical spectrum of the generated on-off window.
) and 3(b) show the waveform and optical spectrum of the extracted 25-GHz OC, respectively.The pulsewidth and ER are measured to be 9.2 ps and 22 dB, respectively, with a 500-GHz optical sampling oscilloscope.The spectral width of the OC measured from the #195414 -$15.00USD Received 7 Aug 2013; revised 14 Nov 2013; accepted 20 Nov 2013; published 27 Nov 2013 (C) 2013 OSA optical spectrum is 50.21GHz, and the corresponding time-bandwidth product is 0.462, indicating that the extracted OC is nearly transform-limited with Gaussian shape.

Figure 4 (
Figure4(a) shows electrical spectrum of the extracted 25-GHz EC with 1-MHz resolution bandwidth (RBW).The inset in Fig.4(a) is a zoom-in view of the same electrical spectrum with 1-kHz span and 1-Hz RBW.As can be seen, the spectrum is highly pure with the intensity of the 25-GHz clock tone standing more than 61.5 dB higher than that of the background noise.Single-sideband (SSB) phase noise spectrum of the 25-GHz EC is also measured, as shown in Fig.4(b).As references, SSB phase noise spectra of the 25-GHz RF source used for OTDM tributary generation and the extracted OC after converted into RF signal with a 50-GHz PD are also measured and plotted.Phase noises at 10 kHz frequency offset for the 25-GHz RF source, OC and EC are −92.7 dBc/Hz, −95.3 dBc/Hz and −92.5 dBc/Hz, respectively.SSB phase noise spectra show that the corresponding RMS timing jitters (from 100 Hz to 10 MHz offset frequency) are 104 fs, 195.9 fs and 191.8 fs, respectively.We also test the DPMZM-OEO with PRBS OOK signal with different code length.When the PRBS code length is 2 23 -1, 2 15 -1 and 2 11 -1, the RMS timing jitters of the measured EC remain to be around 192 fs with 2-fs variation.If the code length is 2 10 -1 and 27 -1, the RMS timing jitters are decreased to be around 124 fs, approximate equal to that of the EC extracted from the DPSK signal (given in the following parts).In addition, the DPMZM-OEO is also test with special repeated patterns with an increasing ratio of zeros for the CR of OOK signal.The average optical power of the repeated pattern is fixed as 6 dBm.When the repeated pattern is composed of 2 5 -4, 2 6 -4 or 2 7 -4 consecutive zeros followed by 4 consecutive ones, the RMS timing jitters are measured to be 206.4fs, 263.6 fs and 657.2 fs, respectively, demonstrating the phase noise deteriorates seriously as the ratio of the zeros increases.

Figure 5 (
b) shows the eye diagram of a typical 25-Gb/s tributary demultiplexed from the 100-Gb/s OOK signal.BER curves for all the four channels are measured and the results are shown in Fig.

Figure 8 (
Figure 8(a) shows electrical spectrum of the 25-GHz EC extracted from the 100-Gb/s DPSK signal.The inset in Fig. 8(a) is a zoom-in view of the same electrical spectrum with a 1-kHz span and 1-Hz RBW.The intensity of the 25-GHz clock tone is more than 64.2 dB higher than that of the back-ground noise.Figure 8(b) shows the SSB phase noise spectra of the 25-GHz RF source, OC and EC, respectively.The corresponding RMS timing jitters