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A cost-effective structure is proposed for the optical network unit (ONU) transceivers in coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON), which is based on a single directly modulated laser (DML). This is the first time that a DML is used as both optical transmitter in upstream and local oscillator (LO) for coherent detection in downstream. The impact of extinction ratio (ER) of signal from DML is investigated and optimized by adapting the driving amplitude and bias of DML. Each UDWDM grid accommodates a pair of bi-directional signal, where heterodyne detection is used due to the Rayleigh backscattering (RB) from the bi-directional transmission. The impact of frequency offset (FO) between upstream and downstream signal is also investigated. Finally, 2.5-Gb/s bi-directional transmission of OOK signal over 60-km SSMF is experimentally demonstrated within the 12.5-GHz grid, achieving about −43 and −45.5 dBm receiver sensitivity in the downstream and upstream, respectively.
© 2016 Optical Society of America
1. Introduction
Optical coherent detection technology will enable splitter-based, optical filter-free ultra-dense wavelength division multiplexed passive optical network (UDWDM-PON) for maximum compatibility with legacy systems [1–6]. Although available state-of-the-art technologies for metro and core networks [7–9] offer sophisticated and high-performance coherent transceivers, people still intend to explore other alternatives that can tradeoff between performance and simplicity. One possible solution is to reduce the complexity of coherent receiver and number of required photon detectors (PDs) [10,11]. For instance, M. Presi et al. have reported the low-cost coherent OOK receivers based on a phase-diversity 3 × 3 coupler, three photodiodes, and basic analogue processing. Polarization independent coherent receiving can be achieved by injecting the local oscillator (or the signal) into two inputs of the 3 × 3 coupler with proper polarization states [11]. Another way to reduce the cost of coherent ONU is to employ the low cost laser/modulator, such as distributed feedback (DFB) laser and VCSELs [12–15]. Directly modulated DFB lasers (DMLs) have been proved feasible as the upstream modulator with both ASK modulation [12] and DPSK modulation [13]. Likewise, the VCSELs have also been shown feasible as laser/modulator in the low-cost coherent systems [14,15].
In our previous work, we have reported cost-effective coherent ONU structure by sharing a common laser diode as both local oscillator for downstream detection and the optical carrier for upstream modulation [16]. However, external modulation is required, which means additional external modulator, such as Mach-Zehnder Modulator (MZM), is needed. In this paper, we propose for the first time to the best of our knowledge, the use of a single low-cost DML, acting as both upstream transmitter and downstream local oscillator (LO), in a high loss budget coherent UDWDM-PON system. Heterodyne detection is employed to avoid the Rayleigh backscattering (RB) interferences in bi-directional transmission. While, polarization independency is achieved by introducing polarization-diversity coherent receiving. By optimizing the bias value and the driving amplitude of DML, we have successfully demonstrated a 2.5-Gb/s/λ bi-directional system with OOK modulation and digital coherent detection, achieving about −43 and −45.5 dBm receiver sensitivity in the downstream and upstream, respectively.
2. Principal
Figure 1(a) shows the proposed ONU structure, which is based on a common DML for both upstream and downstream. In the upstream, OOK signal is generated by a bit pattern generator (BPG). The output of BPG is used to directly drive the DML without any electrical amplifier. The optical output of DML is then split into two parts by an optical power splitter. One part is used as the local oscillator (LO) for coherent detection of downstream signal. While, the other part acts directly as the upstream signal. The inset of Fig. 1(a) also illustrates the optical spectral of generated upstream signal from DML, in which chirp exists due to the direct modulation. The downstream signal is generated by external modulation based on MZM. The downstream signal is mixed with LO, which is generated by DML, in a 90-degree polarization diversity optical hybrid with balanced detection. Due to the much lower bandwidth compared to long-haul networks, further cost reduction of the coherent receiver can be realized by the fast-growing silicon photonic technologies and high volume CMOS process [17,18]. Figure 1(b) shows the block diagram of digital signal processing for signal recovering in both upstream and downstream. First, a frequency down conversion is carried out with an estimated intermediate frequency (IF) value to move the desired signal to baseband. Then, a low pass filter (LPF) is applied to remove the out of band noise, such as Rayleigh backscattering (RB). The signal is recovered by the polarization independent envelop detection as already discussed in [16]. Note that inaccurate estimation of the IF value is acceptable, since envelop detection is robust to the frequency variations. Finally, the decision is made based on the recovered signal, and BER is calculated.
Fig. 1 (a) Proposed coherent ONU structure based on a common DML for both upstream and downstream; (b) the block diagram of digital signal processing for the proposed method.
In the coherent ONU, LO is set at an intermediate frequency from the downstream signal to avoid Rayleigh backscattering (RB) interference, since the RB from the upstream signal and LO are at the same frequency. The left part of Fig. 2(a) illustrates the scheduling of spectral for ONU. The extinction ratio (ER) of DML impacts the performance of downstream, which can be adjusted by adapting the bias and drive of DML. The right part of Fig. 2(a) shows the eye diagram with and without ER optimization. Correspondently, the left part of Fig. 2(b) illustrates the spectral scheduling for OLT. The main differences include the narrow linewidth external cavity laser (ECL) used as LO, and a low pass filter (LPF) used for upstream signal recovering. The right part of Fig. 2(b) shows the received eye diagram with and without a LPF.
Fig. 2 (a) Spectral scheduling for ONU (left), eye diagram with and without ER optimization (right); (b) spectral scheduling for OLT (left), received eye diagram with and without a LPF (right).
3. Experimental setup
Figure 3 shows the experimental setup for 2.5-Gb/s/λ bi-directional transmission, using only one DML in the ONU as both optical modulator in the upstream direction and LO of coherent detection in the downstream direction. The DML is directly driven by an arbitary waveform geneartot (AWG) operating at 2.5-GSa/s sampling rate with PRBS-15 (length: 32768). The DML’s output power is around 9 dBm, which is split into two parts by a 3-dB power splitter. One part is directly fed into an optical circulator as the upstream signal. While, the other part is connected to the LO port of a commercial integrated coherent receiver (ICR). The four outputs of the ICR are then digitally sampled by an oscilloscope (OSC) of 50-GSa/s sampling rate. Lastly, the off-line DSP is carried out using the Matlab. For each BER measurement, four PRBS-15 sequences are counted.
Fig. 3 Experimental setup for the 2.5-Gb/s/λ bi-directional transmission based on the proposed method, (a) received upstream spectral, and (b) downstream spectral with RB interference. RB: Rayleigh backscattering.
The optical distribution network (ODN) is composed of a 40-km feeder fiber, a 20-km distribution fiber and a variable optical attenuator (VOA) in between, which is used to emulate the optical power splitter and also used to dynamically adjust the link loss budget. In the OLT, a tunable ECL is employed as the LO for coherent detection of upstream signal, whose wavelength is tuned several GHz from the central wavelength of DML to form the heterodyne detection. Another ECL of 16-dBm output power is used as the optical carrier in downstream direction, which is externally modulated by a MZM. The MZM is directly dirven by 2.5-Gb/s OOK signal. The geneated signal is then fed into a ‘channel replicator’, made by a MZM biased at transmission null and driven by a CW RF tone at frequency f = 12.5 GHz. The replicator outputs consist of two identical signal copies, spaced by 25 GHz. The original signal is then recombined to its copies, in order to obtain 3 WDM channels that are 12.5-GHz spaced. Thanks to the different path lengths in the replicator, the adadjacent channels are uncorrelated. The inset of Fig. 3 also shows the measured optical spectral for 3 WDM downstream channels. The luanch power for the 3 WDM channels is around 5 dBm. No optical amplification is used in the setup.
4. Results and discussion
The performance of proposed method is first investigated in a single channel setup (replicator outputs are turned off). Figure 4 shows the measured downstream receiver sensitivity at back to back, under four different sets of parameters for the DML: a) CW mode, bias = 900mA; b) vpp = 0.5V, bias = 900mA; c) vpp = 1.0V, bias = 900mA; d) vpp = 1.0V, bias = 600mA. Note that vpp is the output peak to peak voltage of AWG, which is used to directly drive the DML. And, DML works in the CW mode, when peak to peak voltage is off. The DML’s ER for the settings of (b), (c), and (d) are 0.9 dB, 1.7 dB and 3.7 dB, respectively. Results show that the downstream sensitivity improves with the decrease of ER of output signal from DML. Considering 7% FEC overhead, the optimum sensitivity is −44.6 dBm, when DML works at CW mode of 900-mA bias. With the increase of ER, around 1.1-dB, 2-dB, and 3-dB penalty is observed, compared to the optimum case.
Fig. 4 Measured downstream receiver sensitivity under four different sets of parameters for the DML: a) CW mode, bias = 900mA; b) vpp = 0.5V, bias = 900mA; c) vpp = 1.0V, bias = 900mA; and d) vpp = 1.0V, bias = 600mA.
Figure 5 shows the measured upstream receiver sensitivity at back to back, under three different sets of parameters for the DML: a) vpp = 0.5V, bias = 900mA; b) vpp = 1.0V, bias = 900mA; and c) vpp = 1.0V, bias = 600mA. Note that DML in CW mode will cause shutdown in the upstream transmission, which is not measured. When the vpp of driving amplitude is 1.0V, an optimum receiver sensitivity of around 45.5 dBm is achieved at both 600-mA and 900-mA bias. For 0.5-V driving amplitude, less than 0.5-dB penalty is observed. Note that 600-mA bias is the typical value, while 900-mA bias is the upper bound, for the linear modulation with DML. And, the maximum output peak to peak voltage from AWG is 1 V. There is tradeoff between the upstream and downstream performance with the same set of DML’s parameter. Finally, 0.5-V driving amplitude and 900-mA bias is chosen for the DML to offer similar loss budget in both directions.
Fig. 5 Measured upstream receiver sensitivity under three different sets of parameters for the DML: a) vpp = 0.5V, bias = 900mA; b) vpp = 1.0V, bias = 900mA; and c) vpp = 1.0V, bias = 600mA.
Due to the chirp of DML, the output spectral will be broadened by large driving amplitude, as illustrated in Fig. 6. It is important to prove that DML can still be used in the ultra-dense DWDM scenario with 2.5-Gb/s bi-directional data rates for each grid. Considering the 12.5-GHz gridding, result in Fig. 6 shows that 1-V vpp seriously broadens the output spectral of the 2.5-Gb/s OOK signal. While, the output spectral at 0.5-V vpp takes less than half of the grid, which is also the reason 0.5-V driving amplitude is chosen.
The Rayleigh backscattering is found significant in the bi-directional transmission. Thus, the frequency offset (FO) between upstream and downstream will impact the BER performance in both directions. Figure 7 shows the measured curve of BER versus FO in downstream direction. The channel spacing between the 3 WDM downstream channels is 12.5 GHz, and the driving amplitude of DML is 0.5-V vpp. The BER curve is measured at around −41 dBm receiver sensitivity. The optimum FO is observed at 6.25 GHz. Smaller FO will cause RB interference with signal in the measured channel, while, larger FO will cause RB interference with signal from adjacent channel. The inset of Fig. 7 also shows the received spectrum for the middle of 3 WDM channels after coherent detection.
Fig. 7 Measured curve of BER versus FO in downstream direction, when channel spacing is 12.5 GHz and driving amplitude is 0.5 V.
Figure 8 shows the measured curve of BER versus FO in upstream direction, with the same 12.5-GHz channel spacing and 0.5-V vpp for DML. The BER is measured at around −43 dBm receiver sensitivity. The downstream launch power for the 3 WDM channels is 4.7 dBm (0 dBm for each channel), and the upstream launch power for the DML is around 5.5 dBm. We use analog low pass filter (LPF) to ‘reject’ frequency spectrum associated to the “space” symbol before digital signal processing to improve the upstream performance. The optimum FO is observed when FO is larger than 5.2 GHz and less than 7.2 GHz. Otherwise, interference will be caused between upstream signal and the RB from downstream channels. The broadening of the optimum FO range is caused by the narrowed spectral of ECL as the LO for coherent detection of upstream signal.
Fig. 8 Measured curve of BER versus FO in the upstream direction, with the same 12.5-GHz channel spacing and 0.5-V vpp for DML.
Finally, the bi-directional performance associated with receiver sensitivity is measured for both directions, as shown in Fig. 9. FO spacing of 6.25-GHz is chosen, and the DML is driven at 0.5 V with 900-mA bias. After 60-km reach and VOA, the upstream sensitivity is about −45.5 dBm for 7% FEC overhead. While, the downstream sensitivity is about 2.5-dB less than the upstream due to the use of DML, which is consider as acceptable from the perspective of cost saving and complexity.
Fig. 9 Receiver sensitivity of both upstream (US) and downstream (DS) channels in the bi-directional transmissions.
5. Conclusion
We have proposed the cost effective ONU transceiver using a single low-cost DML as both upstream transmitter and downstream local oscillator, in a high loss budget coherent UDWDM-PON system. By optimizing the bias value and the driving amplitude of DML, we have successfully demonstrated a 2.5-Gb/s/λ bi-directional system with OOK modulation and digital coherent detection within 12.5-GHz grid, achieving about −43 and −45.5 dBm receiver sensitivity in the downstream and upstream, respectively.
Acknowledgment
This work is supported by the National Natural Science Foundation of China (Grant No. 61505154).
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