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We propose and demonstrate asymmetric 10 Gbit/s upstream - 100 Gbit/s downstream per wavelength colorless WDM/TDM PON using a novel hybrid-silicon chip integrating two tunable lasers. The first laser is directly modulated in burst mode for upstream transmission over up to 25 km of standard single mode fiber and error free transmission over 4 channels across the C-band is demonstrated. The second tunable laser is successfully used as local oscillator in a coherent receiver across the C-band simultaneously operating with the presence of 80 downstream co-channels.
©2012 Optical Society of America
1. Introduction
Next generation access networks (NGAN) have to meet the need of an enormous traffic growth. However the success of lighwave systems into the access segment depends on the capability to obtain scale economies. The first answer to reduce the financial investments is based on Time Domain Multiple Access (TDMA) which enables sharing the same passive infrastructure among several users and provides a dynamic time slot assignment. However the guaranteed data rate per user is limited. To overcome this limitation, wavelength division multiplexing (WDM) is proposed to provide scalable and high-capacity systems and to allow a second degree of flexibility with dynamic wavelength assignment. Furthermore, the current trend of consolidation pushes operators to reduce the number of optical line terminal (OLT), thus a mix of WDM and TDMA towards a hybrid passive optical network (PON) could be the right compromise between the high capacity provided by WDM virtual point-to-point links, and the TDMA sub-wavelength switching granularity, allowing highly shared infrastructure [1]. Cost effective migration to hybrid TDMA/WDM systems is needed therefore the same Optical network units (ONU) for all users are needed. This need for a single model of ONU is mainly driven by inventory control. On one hand, new optical devices have been developed in order to obtain high performances, colorless and low cost transmitters such as tunable lasers [2], injection-locked Fabry-Perot lasers [3], reflective EAM-semiconductor optical amplifier (SOA) [4] and reflective-SOA [5]. However reflective schemes strongly suffer from Rayleigh back scattering limiting the performance while tunable lasers still represent an expensive alternative. On the other hand, detection based on coherent technology is now catching growing attention for optical access networks, making possible colorless and filterless WDM-PONs. Any wavelength can be extracted without optical filter, by simply adjusting the wavelength of the local oscillator (LO) inside the coherent ONU [6]. However, to meet the cost requirements of access networks, all building blocks of the coherent ONU should be integrated. We believe that the silicon platform is the most promising candidate, allowing high-density integration, and high volumes production. A first milestone toward an integrated ONU was achieved when a monolithic polarization-diversity coherent receiver in a silicon photonic integrated circuit with germanium detectors was reported [7]. At that time, the prospective silicon-photonic integrated ONU was lacking a simple low-cost transmitter and a suitable LO. The LO is a continuous-wave (CW) laser that should be finely and widely tunable. Recently, we demonstrated a novel hybrid III-V/silicon continuously-tunable laser [8]. Here, we expand the work done in [9].
Two such lasers are integrated into a single chip and mounted on a high frequency submount and connected to one strip line using wire bonding. We demonstrate the generation of 10 Gbit/s non return to zero (NRZ) optical packets as upstream signals by simple direct modulation of the current of one laser, while the second laser serves as a local oscillator. The local oscillator is successfully used for the coherent detection of a 100 Gbit/s polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) downstream signal out of 80 channels. We incorporate the ONU into a hybrid WDM/TDM PON and demonstrate transmission over 25km distance, over the full C-band.
2. Network description and experimental set-up
Figure 1(a) represents the proposed hybrid WDM/TDM PON. In the optical line terminal (OLT), the light of eighty 50-GHz-spaced distributed feedback (DFB) lasers is multiplexed and passed into a PDM-QPSK modulator to produce eighty data channels at 112 Gbit/s (from 215-1 pseudo-random binary sequences, PRBS), including 12% forward error correction (FEC) and protocol overhead. The channel under test is generated out of a tunable integrated external cavity laser (linewidth < 300kHz), which replaces one of the DFBs. It is passed into a separate modulator and later combined to the rest of the multiplex, as required for realistic emulation of networks. We chose a high bit rate and continuous transmission mode to allow broadcast operation and to support bandwidth-greedy applications. The downstream multiplex is boosted through an erbium-doped fiber amplifier (EDFA) before transmission. The OLT also processes the upstream data. It incorporates a 10 Gbit/s direct-detection burst-mode receiver following a wavelength demultiplexer. A 25km-long section standard single mode fiber, possibly extended up to 100km, and a variable attenuator emulating the PON splitting module, link the OLT with the ONU, via two optical circulators which separate upstream and downstream data. The ONU is designed to be located at user side and is thus the most cost-sensitive end of the PON, which motivates the implementation of our novel chip there, integrating two hybrid wavelength-tunable silicon lasers (Fig. 1(b)) and generating the upstream 10Gbit/s-modulated data and the LO for the downstream 100Gbit/s coherent receiver. The proposed architecture enables dynamic reconfiguration of client connections with joint flexibility in time and wavelength domains. It constitutes one possible approach for telecommunication operator implemented in today Gigabit PON (GPON) infrastructure and guarantees a maximum quality of service (QoS) [10].
Fig. 1 (a) Experimental WDM/TDM PON testbed (b) LO and upstream integrated hybrid silicon sources photography
3. Coherent colorless ONU
3.1 Hybrid silicon sources
Our chip consists of a passive part in silicon and an active section based on InP. The III-V part was bonded onto the silicon wafer and then the active waveguide was processed. The modal transfer between III-V and silicon is achieved by tapers. The details of additional parameters such as the length and width of the gain region have been described in [8]. In the silicon part, two ring resonators allow single mode selection and wavelength tuning. Metal heaters are processed on the top of the ring resonators to thermally tune the ring resonator peaks wavelengths and a Peltier element is used to control the on-chip temperature. The temperature changes induced by the metal headers do not affect the performances of the gain section. This laser allows single mode operation and wavelength tunability over 45nm only using the ring resonators thermo-optical effect. The design and fabrication process have been detailed in [11,12]. The two lasers can be packaged together with only 6 electrical inputs and two optical outputs, as presented in Fig. 1(b). We coupled to the chip using standard lensed fiber pigtails with anti-reflection coating using vertical Bragg grating.
3.2 Directly modulated tunable lasers
Advantageously, we rely on direct modulation of the current of one laser to generate 10 Gbit/s signals. Direct modulation allows a simple fabrication process (cost-efficient approach) compared to the integration of an external modulator as Mach-Zehnder modulator [11]. A power sensitivity of −19.2 dBm for a FEC-compatible BER of 10−3 is obtained after upstream transmission in back-to-back (B2B) at 1544 nm as represented in Fig. 3(a). We tested four random wavelengths across the C-band in order to demonstrate the compatibility with the next generation of PON stage 2 standardization direction (NG-PON2) as initiated by the Full Service Access Network (FSAN) community in 2011 [13]. The corresponding B2B eye diagrams are shown in the inset of Fig. 3(a). Those results show that direct modulation can be obtained without affecting wavelength tunability. We noticed less than 1.4 dB of wavelength dependence of the sensitivity from 1526.6 nm to 1548.2 nm.
3.3 Local oscillator characteristics
The other laser on the chip is used as LO for the receiver. Note that our chip was not packaged and coupling loss was relatively high (~9 dB). At the LO end, we inserted an EDFA delivering 11dBm output power to exactly [8] compensate for this loss. This emulates the next level of integration, when the laser is integrated with the coherent mixer, free of any EDFA. A commercial coherent receiver using balanced photodetectors is used with off-line processing at 100 Gbit/s. The algorithms are detailed in [14].
First, we characterize the laser phase noise using the coherent receiver [15]. To do so, the laser under test beats with a continuous wave narrow linewidth (< 300kHz) external cavity laser. Figure 2(a) shows the power spectral density (PSD) of the recovered field, centered on the offset frequency. A Lorenzian best fit to the measured data is also shown. The full width half maximum of the Lorenzian shape is 2.3 MHz, which corresponds to the linewidth of the laser under test. Thanks to the carrier phase recovery mechanisms, coherent receivers are capable of tracking the low frequency components of the phase noise. It is therefore interesting to look at the PSD of the frequency noise, shown in Fig. 2(b). As it can be seen, the frequency noise appears white in frequency; we therefore expect this laser should interact with the coherent receiver similarly to more commonly used III-V lasers.
Fig. 2 (a) Power spectral density of coherently received signal complex field with a Lorentzian fit and (b) Frequency noise spectrum.
4. Transmission experiments
4.1Upstream transmission
Throughout the paper, we assume that upstream and downstream wavelengths never coincide and we leave the allocation plans open to standard bodies. As described in section 3.2, we generate 10 Gbit/s optical packets. The packets payloads contain 29-1 NRZ PRBS modulated at 10 Gbit/s (in-band FEC assumed) and the total packet length is 5 µs. The guard band between each optical packet is 200 ns long. The optical packet is represented in the inset of Fig. 3(b) . We report the measured the bit-error rate (BER) versus the received power in Fig. 3(b) for the upstream in single-channel configurations at 1544 nm. Using optimized electrical drivers, an improved power sensitivity of −21 dBm at BER of 10−3 is obtained after upstream transmission in B2B at 1544 nm. After 25km link, penalties no larger than 2.5 dB are measured, mainly caused by chromatic dispersion. Reach was also limited by the low input optical power (~-5dBm) into the optical fiber. Adequate packaging will reduce coupling loss and increase the available power budget for the PON splitters.
Fig. 3 (a) Back-to-back BER measurements and eye diagrams at λ = 1526.6, 1539.7, 1544 and 1548.44 nm and (b) Power sensitivity packets based on direct detection (upstream signal) at 1544 nm for 10Gbit/s NRZ optical signals.
4.2 Downstream colorless and filterless operation
The downstream transmission performance at 100 Gbit/s is represented in Fig. 4 . We obtained a sensitivity at BER = 10−3 of −27 dBm in B2B. After transmission, no further penalty was observed for link lengths ranging from 25km to 100 km, as accounted for by digital dispersion compensation in the receiver. We focus next on wavelength dependence of the downstream. Figure 5(a) represents the single-channel measurements of the sensitivity deviations at 10−3 BER (with respect to reference laser) versus wavelength, in B2B and after 100 km. The downstream channel as well as the LO are tuned to 10 random wavelengths across the C-band, including three consecutive wavelengths of the 50-GHz ITU grid, in order to highlight the feasibility of assigning the wavelengths closely. Less than 1 dB channel-to-channel sensitivity deviations is obtained. This shows that the ONU can be colorless over the C-band.
Fig. 4 Power sensitivity for 100Gbit/s PDM-QPSK downstream signal based on coherent detection at 1550 nm.
Fig. 5 Impact of (a) wavelength and (b) of the channel count on the received downstream data at 100 Gbit/s.
The number of channels which reach the receiver could limit the performance. We assess next the impact of neighbor channels. While monitoring the performance of one typical channel at 1550.5 nm, we increase the count of neighboring channels up to a worst case of 80, populating monotonically from the test channel outwards along the 50 GHz grid. We display the sensitivity penalty in Fig. 5(b). We find a marginal penalty of less than 1 dB at full load. The power consumption of the coherent ONU could be optimized using less complex digital signal processing [16]. Energy efficient algorithms that maintain good sensitivity are currently being evaluated.
7. Conclusion
We demonstrate a novel colorless and coherent ONU for access networks which incorporates a dual-laser chip based on hybrid III-V/Silicon technology. The hybrid Silicon lasers allow single mode operation and wavelength tunability over 45nm exploiting silicon ring resonators thermo-optical effect. We showed the excellent performances of hybrid silicon lasers as LO and directly modulated upstream transmitter. We integrated the ONU into a hybrid WDM/TDM PON with 10 Gbit/s NRZ upstream optical packets and 100 Gbit/s PDM-QPSK downstream circuit data. This network operates over the C-band and up to 25 km. For the downstream signal, no penalty is observed when increasing the number of neighbour channels allowing filterless operation. Low cost optical devices such as directly modulated tunable lasers and coherent receivers including the local oscillator could be fabricated and integrated together paving the way for the future of WDM access network. Our result constitutes an important milestone toward a fully integrated hybrid III-V/silicon colorless ONU transceiver.
Acknowledgments
This work was supported in part by the French ANR project MICROS.
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