A new passive optical network (PON) configuration and a novel silicon photonic transceiver architecture for optical network unit (ONU) are proposed, eliminating the need for an internal laser source in ONU. The Si transceiver is fully monolithic, includes integrated wavelength division multiplexing (WDM) filters, modulators (MOD) and photo-detectors (PD), and demonstrates low-cost high volume manufacturability.
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There have been growing demands on fiber-to-the-home (FTTH) in recent years. Many countries plan to complete the installation of FTTH in the coming few years. A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture. A PON configuration reduces the amount of fiber and central office equipment required compared with point to point architectures. Because of its low cost, PON configurations are used for FTTH. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. In a PON configuration, downstream signals are broadcast to each premises sharing a fiber. Upstream signals are combined using a multiple access protocol, time division multiple access (TDMA). Transceivers are key devices in the high-speed interconnects. In communication networks, a transceiver includes a transmitter which converts the electrical signals into optical signals, and a receiver which receives, amplifies and reshapes the optical signals into electrical signals. A typical transceiver has a directly modulated laser source and a photodiode which are III-V material based.
Silicon photonics receives much attention recently. Modulators, photo-detectors, filters, switches, waveguides based on silicon wire have all been demonstrated [1–3]. Silicon Photonics technology platform enables a new breed of monolithic opto-electronic devices manufactured in a low cost CMOS process. Enablence Technologies and Kotura presented a hybrid FTTH transceiver which had a III-V laser diode, two photodetectors and a preamp assembled on SOI platform with Silica PLCs [4,5]. The die was 3 mm by 12 mm. Wada et al. proposed WDM FTTH architecture based on silicon photonics, which envisions WDM filters, modulators and Ge photodetectors, on a chip . However, the lack of a silicon light source remains the show-stopper for full monolithic integration. Hybrid integration of an external III-V laser diode using assembly techniques is required, which is an immense challenge in packaging . An alternative solution is to integrate or bond a hybrid III-V laser diode on the Si-substrate . Unfortunately, CMOS and III-V process integration is hardly a trivial task.
Efforts have been made to eliminate the III-V components in transceivers to reduce the complexity and costs. Xu and Tsang proposed a scheme based on a nonreciprocal optical phase modulator and a linear optical loop mirror for sending upstream data in a WDM PON . The downstream wavelength was reused for upstream data. Non-return-to-zero (NRZ) modulation format was used for downstream data and differential-phase-shift-keying (DPSK) was used for transmission of the upstream data at 10Gbps. This ONU is compatible with monolithic integration which will help in meeting the low-cost targets for ONUs. However, the performance is compromised due to the crosstalk between upstream and downstream data.
In this paper, we propose and demonstrate a silicon based monolithic transceiver for ONUs in PON, including a Si-modulator, a Ge photodetector and a WDM DEMUX to split the wavelength for transmitter (Tx) and receiver (Rx).
2. Network design and transceiver design
In the present PON configuration, one OLT serves multiple ONUs. The downstream signals are at 1490nm wavelength while the upstream signals are at 1310nm. The downstream and upstream signals are carried in the same fiber. Each ONU has a transceiver with 1310nm laser source. To eliminate the internal laser diodes in ONUs, we propose the network configuration as shown in Fig. 1 . In this configuration, downstream and upstream lights are carried in individual fibers. The internal light source in each of the ONUs is eliminated. Instead, ONUs share a common continuous wave light source located in the OLT. The OLT TxRx remains functionally unchanged. The wavelength of OLT Tx is λ1 (1490nm) which carries the downstream data to ONUs. The OLT Rx wavelength is λ2 (1310nm). In the OLT, an extra seed laser source operates at continuous wave mode to serve the ONUs in the PON as a shared and centralized laser source at a wavelength of λ2. λ1 from OLT Tx and λ2 from the seed laser are combined by using a WDM combiner and connected to serve the multiple ONUs through the downstream fibers. The ONUs receive the data in λ1. At the same time, the ONUs encode and transmit data in λ2, which are sent back to OLT.
Since the ONUs share a seed laser source from the OLT, and the light is carried to the ONUs via the downstream fibers, the laser diodes in the ONUs can be eliminated. The immediate advantage is that the major hurdle for realizing low-cost monolithic silicon ONU transceivers is removed. Figure 2 shows the schematic of the monolithic silicon ONU transceiver. The ONU transceiver contains a wavelength-division-multiplexing (WDM) filter component for selectively guiding different wavelength components of the incoming light into different waveguides. The channel for λ1 connects to the receiver section, where the data in λ1 are detected and converted to electrical signals. The electrical signals will be amplified by trans-impedance amplifier (TIA) and limiting amplifier (LA). The channel for λ2 connects to the transmitter section where the light is modulated by upstream data from a modulator driver. The modulated optical signals in λ2 from ONUs are connected back to OLT through upstream fibers. Two splitters connect the main fibers with the fibers for each ONU. The downstream signals are broadcast to each premise. Upstream signals are combined using a multiple access protocol, time division multiple access (TDMA).
In this design, the silicon waveguide height is 220nm. The WDM filter is a three-stage Mach-Zehnder Interferometer (MZI). The width of the waveguide in the WDM filter is 400nm. In most applications in PON, upstream data is carried at 1310nm, and downstream data is carried at 1490nm. Figure 3 shows the simulated spectrum of the designed WDM filter for PON. At 1310nm, the crosstalk is less than −20dB and the bandwidth is 20nm. At 1490nm, the crosstalk is less than −35dB and the bandwidth is 20nm. For the convenience of testing with available light sources in C band, we also design a WDM filter to have the two channels in C band. The modulator is a MZI modulator with pn junction phase-shifters in both arms. It is based on the free carrier dispersion effect and operated in the carrier depletion regime  . The Ge PD is waveguide-based PD for high responsivity at 1550nm. The size of the monolithic FTTH transceiver chip is 2mm by 4mm, which is significantly smaller than the hybrid FTTH transceiver chip in [4,5]. The chip contains some unnecessary space to interface with the circuits on PCB. Potentially, it could be less than 2mm by 1mm.
GPON physical layer accepts single fiber and dual fiber based network structures. In the single fiber network, transceivers with bidirectional optical sub-assembly (BOSA) are used, while transceivers with separate transmitter optical sub-assembly (TOSA) and receiver optical sub-assembly (ROSA) are used in dual fiber network. Single fiber network with BOSA based transceivers is preferred by industry. In the proposed network topology, ONUs share one centralized light source from the OLT unit, the transceiver’s function in each ONU can be monolithically integrated on one silicon chip. By adopting this configuration, the cost structure of the transceiver will be fundamentally changed and the cost of each ONU will be significantly decreased. Although the fiber cost will be doubled in this topology, the fiber cost is a one-time investment in the network, and fibers last for tens of years. Compared to the fiber, the ONUs change or upgrade more frequently. Therefore, much lower cost at ONUs will be definitely more attractive than less fibers.
The silicon transceiver was fabricated using a SOI wafer. For the convenience of testing with available light sources in C band, we fabricated the WDM filters with the two channels in C band. Figure 4(a) and (b) show the fabricated monolithic silicon FTTH transceiver chip and the three-stage MZI based WDM filter. The parameters of the three-stage MZI are: ΔL1 = ΔL2 = ΔL3 = 40 μm. The lengths of couplers were L1 = L2 = L3 = L4 = 10 μm. Figure 5 shows the measured transmission spectra of WDM filter. The sum of insertion loss and coupling loss is about 4dB. The WDM filter has <-20dB crosstalk at 1525nm wavelength with about 0.8nm bandwidth and at 1532nm wavelength with about 1.6nm bandwidth.
The 1310nm/1490nm WDM design parameters of the three-stage MZI were: ΔL1 = ΔL2 = ΔL3 = 1.16 μm. The lengths of couplers were L1 = L2 = L3 = L4 = 10 μm. The ΔL of 1310nm/1490nm WDM is much less than that of the 1525nm/1532nm WDM. However, couplers in the both WDM filters were designed to be the same. Therefore, the overall physical size of the SOI 1310nm/1490nm WDM was slightly smaller than that of the 1525nm/1532nm WDM.
The silicon FTTH transceiver chip was assembled on a SFP + transceiver board (Fig. 6 ). One arm of the modulator was connected to the SFP + Tx + through a micro-strip line, which was data input. The modulator on the silicon photonics chip was driven by a JDS Uniphase H301 10Gbps optical modulator driver. The Ge PD was connected to a TIA (MAXIM3970). The outputs from LA (MAXIM3971A) connect to SFP + Rx + and Rx-. TIA and LA were assembled on the SFP + transceiver PCB. The modulator driver was external.
The transceiver was tested using tunable laser source (TLS) in C band. Tx and Rx were tested individually with different wavelength inputs. The light source was fed into the input of the WDM filter through a lensed fiber. In Tx functionality measurement, the CW light was guided to the MOD by the WDM filter. The MOD was reverse biased at 2V and was driven at 10Gbps using NRZ PRBS signals. The resulting eye-diagram of the modulated optical output with −9dBm optical power is shown in Fig. 7 . In Rx functionality measurement, a high speed external LiNbO3 modulator was used to modulate the continuous wave light from the TLS with 10Gbps NRZ data. The modulated signal was guided through the WDM filter into the Ge PD. The 10Gbps optical signals were recovered by the receiver. Figure 8 shows the 10Gbps electrical eye diagram from the receiver output. With −8.5dBm optical power input at WDM filter, the bit error rate (BER) of the receiver is about 2x10-10 at 10Gpbs. Figure 9 shows the BER of the receiver at 10Gbps with different input optical power.
The Ge PD sensitivity is dependent on the wavelength. It has higher responsivity at shorter wavelength (850nm/1310nm) than longer wavelength (1550nm), since 1550nm is close to the cut off wavelength in Ge absorption spectrum. In the experiment, we design the WDM near 1550nm wavelength since it’s easier to obtain light sources near 1550nm wavelength. The insertion loss of the WDM filter in front of Ge PD will also affect the receiver sensitivity. In this experiment, the WDM filter showed about 5dB insertion loss. Considering the above factors, it is potential to obtain higher sensitivity when 1490nm wavelength is used in downstream in the real PON. Besides, to minimize the insertion loss and propagation loss from the WDM filter and waveguide will help to improve the performance of the receiver.
In this work, we tested the transceiver by actively aligning the lensed fibers with the nano-tips of the waveguides on the silicon photonics chip. In order to fix the fibers with the waveguides, larger tolerance at the fiber-to-waveguide alignment would be preferred. Hence, large size mode size converter would be needed to converter the mode size of SOI the waveguide to that of a normal single mode fiber. The fiber interface would be fiber pigtail.
4. Discussion and conclusion
A new PON configuration is proposed. In this configuration, the internal light source is eliminated. Instead, ONUs share a continuous wave light source which is centralized at the OLT. A novel silicon monolithic transceiver without internal laser diode for ONU in PON is fabricated. It includes a WDM filter, a Si modulator and a Ge photodetector. The monolithic transceiver chip size is only 2mm by 4mm. The crosstalk between the Tx and Rx is less than −20dB. The transceiver chip is integrated on a SFP + transceiver board. Both Tx and Rx showed data rate capabilities of up to 10Gbps. The silicon transceiver chip can be further integrated with CMOS modulator drivers and receiver amplifiers so that the more functions can be monolithically integrated on silicon chip. By implementing this scheme, the ONU transceiver size can be significantly reduced and the assembly processes will be greatly simplified. The results demonstrate the feasibility of mass manufacturing monolithic silicon ONU transceivers using low cost CMOS processes, taking silicon photonics a step closer to practical network deployment.
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