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A novel multifunctional frequency-doubling optoelectronic oscillator (FD-OEO) mainly based on a Mach-Zehnder modulator (MZM) cascaded with a phase modulator (PM) and a subsequent optical bandpass filter (OBPF) is proposed. We experimentally demonstrate simultaneous operations of frequency-doubled optical clock (FD-OC) recovery, low-duty-cycle dual-wavelength prescaled OC recovery and error-free fourfold time division demultiplexing with the proposed OEO injected with a 4 × 25-Gb/s optical time-division-multiplexing (OTDM) signal. We show that the proposed FD-OEO operates well for both the differential phase shift keying (DPSK) and on-off keying (OOK) modulation formats. The extracted dual-wavelength prescaled OC is proved to be nearly transform-limited with Gaussian-like shape. Furthermore, all four 25-Gb/s tributaries can be selectively demultiplexed by adjusting the phase shifters in the OEO loop. The power penalty at a bit error rate (BER) of 10−9 is measured to be 2.4 dB, 1.2 dB for the best channel for DPSK signal and to be 2.5 dB, 1.1 dB for the best channel for OOK signal. In addition, as an extra benefit of the OEO, low-phase-noise prescaled electrical clock (EC) is also extracted.
© 2014 Optical Society of America
1. Introduction
Clock recovery (CR) with high spectral purity and low phase noise from a transmitted phase or amplitude-modulated optical data stream is critical at nodes of the optical networks in optical communication system. It is one of the essential technologies to enable optical signal processing involving optical logic gate, optical signal regeneration, wavelength conversion, synchronous modulation, add-drop multiplexing and/or demultiplexing of an OTDM signal with a high symbol rate.
In past decades, a great amount of schemes have been reported to perform CR. Among the various methods, both the phase-locked loop (PLL) and optoelectronic oscillator (OEO) based ones are promising candidates for the remarkable ability of the simultaneous operation for CR, modulation format conversion and/or demultiplexing of a high-speed OTDM signal [1–6], which may extremely facilitate the signal processing at nodes or terminals of the networks. However, in addition to a costly voltage-controlled oscillator (VCO) mandated for guaranteeing sustained oscillation, the PLL-based schemes usually require a sophisticated phase comparator and an extra mode-locked laser diode to perform simultaneous demultiplexing operation, dramatically increasing the system cost and system complexity. Consequently, OEO-based schemes are preferable for the compact structure and flexible tunability [5–9].
A conventional OEO features a single-loop structure mainly composed of an electrooptical modulator (EOM) such as Mach–Zehnder modulator (MZM) or electroabsorption modulator (EAM), a photodetector (PD), a high-Q electrical bandpass filter (EBPF) and a phase shifter (PS). When injected with a continuous wave (CW) laser, the OEO will be a self-starting oscillator oscillating at one of its eigenmodes fixed by the equivalent electrical path length of the whole loop and center frequency of the EBPF. Further, if injected with a data signal with frequency component around the free oscillation frequency, the OEO can select out this frequency component and feed it back to the EOM to form a closed loop, and then, adjusting the phase shifter in the loop, the OEO can be injection locked by the selected frequency component with sustained positive feedback. Finally, single frequency oscillation with high spectral purity and low phase noise is obtained. However, these kinds of conventional OEOs can only operate with a maximum frequency confined by the limited bandwidths of the devices such as the EOM, PD and radio frequency (RF) electronics involved in the OEO loop.
In order to improve the maximum working frequency of the OEO comprised of low-frequency devices, FD-OEOs have been proposed [10–15]. Previously reported outstanding FD-OEOs are mainly based on MZM, dual-parallel Mach–Zehnder modulator (DPMZM) or polarization modulator (PolM). FD-OEOs based on DPMZM cascaded with an additional chirped fiber Bragg grating (CFBG) or a segment of high nonlinear dispersion-shifted fiber (HNL-DSF) are demonstrated to be effective to generate FD signal, however, the dispersion of the required CFBG must be accurately selected to ensure the sustained oscillation, while the involved HNL-DSF is a power-consumption medium requiring an extra high-power erbium-doped fiber amplifier (EDFA). Besides, the three direct current (DC) biases applied to the DPMZM must be carefully controlled to maintain the carrier phase-shifted double sideband (CPS-DSB) modulation, seriously increasing the system complexity and decreasing the system stability. Recently, FD-OEOs based on PolM have been reported for simultaneous operation of high-quality prescaled and FD-OC recovery, mainly relying on the principle of polarization control, however, in addition to the special PolM, these kind of FD-OEOs always require extra polarization controllers and polarizers to implement double sideband modulation, resulting in a complex and environmental sensitive structure, which is a little hard to align. In addition, schemes mentioned above are not demonstrated to be capable of realizing simultaneous multi-fold demultiplexing of an OTDM signal with a high aggregated rate and providing multi-wavelength Gaussian-like OC with high extinction ratio (ER) and low duty cycle.
In this study, we propose and experimentally demonstrate a multifunctional FD-OEO comprised of a conventional MZM in tandem with a PM and an OBPF. Injected with a 4 × 25-Gb/s OTDM-DPSK or OTDM-OOK signal together with only one CW laser, the proposed FD-OEO enables simultaneous operations of FD-OC recovery, dual-wavelength prescaled OC recovery and error-free fourfold OTDM demultiplexing. When the FD-OEO is injected with DPSK or OOK signal, the extracted FD-OC and the dual-wavelength prescaled OC feature duty cycles of 33% and ~17%, and ERs of 25 dB and 30 dB, respectively. The time-bandwidth products (TBPs) of the extracted dual-wavelength prescaled OC are measured to be around 0.45, demonstrating the extracted dual-wavelength OC is nearly transform-limited with Gaussian-like shape. By adjusting the phase shifters in the OEO loop, all four 25-Gb/s channels can be selectively demultiplexed. The power penalties at a BER of 10−9 for the four demultiplexed channels are measured to be between 1.2 dB and 2.4 dB for the DPSK signal and between 1.1 dB and 2.5 dB for the OOK signal. Furthermore, integrating from 100 Hz to 10 MHz, the root-mean-square (RMS) timing jitters of the extracted EC are measured to be 129.3 fs and 166.6 fs for DPSK and OOK signal, respectively.
2. Principle of the proposed OEO
As can be seen in Fig. 1, the proposed FD-OEO is a dual-loop feedback structure mainly composed of a conventional MZM, a PM, an OBPF, a PD, a high-Q EBPF and two electrical amplifiers (EAs). The EDFA in the loop is employed to provide optical gain to ensure the net gain of the loop is higher than loss; the PD is implemented to convert the optical signal into electrical signal; the two EAs are used to amplify the converted electrical signal to drive the involved MZM and PM, respectively, forming the dual closed loop with sustained positive feedback. The key part of the proposed FD-OEO is illustrated in Fig. 2.
As shown in Fig. 2, when the MZM is injected with a CW laser, its output can be expressed as:
Where Ein represents complex amplitude of the injected CW laser; VπMZM represents the half-wave voltage of the MZM; f and V represent the frequency and amplitude of the RF signal applied to the MZM, respectively; Vbias represents the DC bias voltage.Equation (1) reveals that when the MZM is biased at the peak transmission point and is driven by RF signal with amplitude of VπMZM and frequency of f, the injected CW laser will be carved into a chirp-free pulse train with a repetition rate of 2f, substantially a FD on-off window, by the MZM, as shown in Fig. 3(a).
Fig. 3 Simulated results of the waveform (blue) and chirp (red) of the (a) FD pulse train generated by MZM and (b) phase-modulated FD pulse train; (c) spectrum of the phase-modulated FD pulse train (red) and the response of the employed OBPF (blue); (d) pulse train with a repetition rate of f generated by red-shifted filtering.
The generated FD pulse is further phase modulated by the subsequent PM driven by RF with a frequency of f, which is the same to that applied to the MZM. The output of the PM can be expressed as:
Where PMindex represents phase modulation index of the PM; φ represents the phase difference between RF applied to MZM and PM, and can be tuned by the phase shifter between MZM and PM.As can be seen from Eq. (2), the adjacent pulse peaks of the Eout1, for example pulse peaks 1, 2, 3 and 4 shown in Fig. 3(a), can be phase modulated with alternate minimum chirp (peaks 1 and 3) and maximum chirp (peaks 2 and 4) by adjusting φ, as shown in Fig. 3(b), which means the spectral components of the pulse 1 and 3 will be shifted towards red sides, while spectral components of the pulse 2 and 4 will be shifted towards blue sides. Thus, if we employ an OBPF offsetted at red or blue-shifted spectral sides immediately after the PM, pulse 1 and 3, or 2 and 4 can be selected out, respectively, resulting in pulse train with a repetition rate of f. When the PMindex is 3π, the phase-modulated spectrum after PM is shown in Fig. 3(c). The blue line in Fig. 3(c) represents the response of the employed 1-nm OBPF allocated at red-shifted side. Figure 3(d) shows the waveform of the pulse train (pulse 1and 3) selected out by red-shifted filtering, and analogously, pulse train composed of pulse 2 and 4 can also be selected out by blue-shifted filtering. In addition, the ER of the filtered pulse train is decided by PMindex of the PM and frequency response of the OBPF. According to theoretical calculation, when the PMindex is larger than 0.8π, the spectrum of the injected FD pulse can be effectively broadened, and then with the help of appropriate offset filtering, pulse train with ER beyond 27 dB can be obtained.
3. Experimental results and discussion
Using the afore-mentioned theoretical analysis as a guideline, we perform a corresponding experiment. Illustrated in Fig. 1, 25-GHz 2-ps optical pulse generated by a home-built optical pulse generator [16] is modulated by a MZM driven by a 25-Gb/s electrical pulse pattern generator (PPG) with a pseudo random binary sequence (PRBS) length of 231-1, and then 25-Gb/s return-to-zero DPSK or OOK signal is obtained. The generated 25-Gb/s signal is further multiplexed by a passive polarization-maintaining 1 × 4 optical multiplexer (OMUX), and then 100-Gb/s OTDM-DPSK or -OOK signal can be achieved.
The proposed FD-OEO is first test with the 100-Gb/s OTDM-DPSK signal. As shown in Fig. 1, in order to extract OC, the 100-Gb/s DPSK signal together with a 6-dBm 1544.8-nm CW laser is launched into the proposed FD-OEO. The MZM with a 5-V VπMZM is driven by EA with a 10-V peak-to-peak output voltage and biased at the peak transmission point; the PM with a 3.5-V half-wave voltage is driven by another EA with an 15.9-V peak-to-peak output voltage, corresponding to a PMindex of 2.3π; the OBPF1 in the loop is used to select the modulated DPSK signal and to eliminate the modulated CW laser; the bandwidth and center frequency of the EBPF are 10 MHz and 25 GHz, respectively. According to the theoretical analysis mentioned above, by carefully adjusting the phase shifters, the OEO can be injection-locked by the injected DPSK signal, and then, FD-OC (50 GHz) recovery (by carving the injected CW laser) with simultaneous twofold demultiplexing (50 Gb/s) can be achieved by the 33% FD on-off window provided by the MZM. The OBPF2 outside of the loop is used to separate the extracted FD-OC and twofold demultiplexed tributary. Further, dual-wavelength prescaled OC recovery and fourfold demultiplexing can be respectively realized with the red or blue-shifted spectral filtering performed by OBPF3 outside of the loop.
Figure 4(a) shows the eyediagram and spectrum (inset) of the injected 100-Gb/s DPSK signal measured with an optical sampling oscilloscope and an optical spectrum analyzer, respectively. The slight amplitude imbalance among the multiplexed pulses is inevitable due to the non-ideal OMUX. Figures 4(b) and 4(c) show the twofold demultiplexed tributary (50 Gb/s) and the extracted FD-OC (50 GHz), respectively. The ER and duty cycle of the FD-OC are measured to be 25 dB and 33%, respectively. Figure 4(d) presents the spectrum of the extracted FD-OC, from which we can see the intensity of the 1st-order sideband is 28-dB lower than that of the carrier, demonstrating the 50-GHz frequency spacing.
Fig. 4 Eyediagrams and spectra (insets) of the (a) injected 100-Gb/s DPSK signal and (b) twofold demultiplexed DPSK tributary; (c) eyediagram of the extracted FD-OC; (d) spectrum of the extracted FD-OC.
With the help of the ~1-nm Gaussian OBPF3 allocated at the red or blue-shifted spectral sides of the phase-modulated FD-OC and the twofold demultiplexed tributary, prescaled OC (25 GHz) on two wavelengths and fourfold demultiplexed tributary (25 Gb/s) can be filtered out, respectively. Figures 5(a) and 5(b) show the eyediagrams of the filtered prescaled OC at 1543.5 nm and 1545.9 nm, respectively. The blue open cycles in Figs. 5(a) and 5(b) represent the Gaussian fitting of the extracted prescaled OC. The duty cycle, ER and TBP are measured to be 15.8%, 30 dB and 0.462 for the OC at 1543.5 nm, and to be 17.5%, 30 dB and 0.446 for the OC at 1545.9 nm, respectively, demonstrating the nearly transform-limited Gaussian-like shape of the extracted dual-wavelength OC.
Fig. 5 Eyediagrams and spectra (insets) of the extracted prescaled OC by (a) blue and (b) red-shifted filtering.
All four channels can be successfully demultiplexed by adjusting the phase shifters in the OEO loop. Lacking of a balanced receiver, we perform BER measurements using a single-ended receiver for all the four demultiplexed 25-Gb/s DPSK tributaries after demodulated with a commercial demodulator, as shown in Fig. 6. As a reference, BER measurement for 25-Gb/s back-to-back (B2B) DPSK signal is also performed. As can be seen from Fig. 6, error-free detections without error floor are obtained for all demultiplexed tributaries and B2B signal, and the power penalty at a BER of 10−9 resulting from OTDM multiplexing and demultiplexing is 2.4 dB, 1.2 dB for the best channel. Furthermore, we should note that due to the non-balanced detection, the receiving sensitivity is not improved by theoretical 3 dB compared to the results of OOK signal shown latter.
Fig. 6 BER curves for the demultiplexed 25-Gb/s DPSK tributaries (Demux. 1, 2, 3 and 4) and back-to-back (B2B) DPSK signal.
Prescaled EC (25 GHz) is also extracted with the proposed FD-OEO. Figure 7(a) presents the electrical spectrum of the extracted EC with 1-MHz resolution bandwidth (RBW). The inset in Fig. 7(a) is a zoom-in view of the same electrical spectrum with 1-kHz span and 1-Hz RBW. Figure 7(b) shows the single-sideband (SSB) phase noise spectrum of the extracted EC measured with a RF spectrum analyzer. As references, SSB phase noise spectra of the 25-GHz RF source and 25-GHz optical pulse source used for DPSK signal generation are also measured and shown in Fig. 7(b). Integrating from 100 Hz to 10 MHz offset frequency, the RMS timing jitters for the extracted EC, RF source and optical pulse are measured to be 129.3 fs, 107 fs and 223.5 fs, respectively.
The proposed FD-OEO is further tested with a 100-Gb/s OTDM-OOK signal. Figure 8(a) presents the eyediagram of the extracted FD-OC with 25-dB ER and 33% duty cycle. Figure 8(b) shows the extracted prescaled OC with red-shifted filtering and corresponding Gaussian fitting. The duty cycle, ER and TBP of the extracted prescaled OC are measured to be 17.6%, 30 dB and 0.446, respectively. Figure 8(c) presents the BER measurements for all the four demultiplexed 25-Gb/s OOK tributaries and 25-Gb/s B2B OOK signal, demonstrating power penalties between 1.1 dB and 2.5 dB at a BER of 10−9. Figure 8(d) shows the SSB phase noise spectra of the extracted prescaled EC and 25-GHz RF source. The RMS timing jitters (100 Hz to 10 MHz) of the extracted EC and RF source are measured to be 166.6 fs and 107 fs, respectively. We should note that the timing jitter of the prescaled EC extracted from the OOK signal is a little higher than that extracted from the DPSK signal presented above, mainly resulting from the fact that the OOK format has a varying intensity envelope while the DPSK format has a constant one.
Fig. 8 Eyediagrams of the (a) FD-OC (50 GHz) and (b) prescaled OC with spectra (insets) extracted from the injected OOK signal; (c) BER curves for the demultiplexed 25-Gb/s OOK tributaries (Demux. 1, 2, 3 and 4) and back-to-back (B2B) OOK signal; (d) SSB phase noise spectrum of the extracted 25-GHz EC.
Finally, it should be noticed that the OEO is a versatile CR device. Besides of the OTDM tributary CR capability demonstrated in this work, it is also shown to recover base rate clock from injected non-return-to-zero (NRZ) signal [17]. Thus in principle, the proposed FD-OEO can be used to recover clock for the currently widely-used 25GBaud/s dual-polarization differential quadrature phase shift keying (DP-DQPSK) system, and further, it can also be deployed after wavelength division demultiplexing in the wavelength division multiplexing (WDM) system. However, regarding to the Nyquist-TDM system, the effectiveness of the scheme should be experimentally evaluated further. Actually, it is one of our on-going projects.
4. Conclusion
The proposed FD-OEO capable of performing simultaneous multifunctional operations may effectively increase the system flexibility and decrease the system cost. FD-OC and dual-wavelength prescaled OC with low duty cycle and high ER can be simultaneously extracted from the injected 100-Gb/s OTDM-DPSK or OTDM-OOK signal. Error-free fourfold OTDM demultiplexing is also done simultaneously with power penalties between 1.2 dB and 2.4 dB for DPSK signal and between 1.1 dB and 2.5 dB for OOK signal at a BER of 10−9. Furthermore, as an intrinsic property of OEO, low-phase-noise prescaled EC is also extracted.
Acknowledgments
This work was supported in part by the National Science Foundation of China (No. 61077055 and No. 61275032), the “973” Major State Basic Research Development Program of China (No. 20llCB301703) and the Foundation for the Excellent Doctoral Dissertation of Beijing (No. YB20091000301).
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