This paper proposes a 40Gbit/s-class-λ-tunable WDM/TDM-PON for flexible photonic aggregation networks that achieves the aggregation of a large number of users using the DWBA algorithm without an L2-SW. It also clarifies the scalability of the proposed system in terms of the transmission distance and the number of users. A λ-switching transmission experiment was conducted using a newly developed 10Gbit/s x 4λ selectable B-Tx and 4 x 4 cyclic AWG router.
© 2013 OSA
Passive optical networks (PON) are being installed throughout the world to provide broadband services. Various systems are being discussed to realize a higher-speed PON operating at over 10 Gbit/s. These systems include high-speed time-division-multiplexing (TDM)-PON, wavelength-division-multiplexing (WDM)-PON, WDM/TDM-PON, and orthogonal frequency-division-multiplexing (OFDM)-PON . For flexible next-generation optical access networks (NGOA), high scalability of the system configuration for the total bandwidth, the smooth migration from the existing PON, and the low power consumption must be considered in order to reduce the OPEX. To satisfy these requirements, we need a system utilizing wavelength (λ)-tunability in transceivers and a simple aggregation function. λ-tunability in WDM/TDM-PON can allow us to realize an incremental upgrade of the total bandwidth and flexible load balancing according to user demand [2–4]. However, the system configuration is complicated and the power consumption increases with the system scale, i.e. there are increases in the total bandwidth, the number of users, and the transmission distance, because conventional aggregation functions are implemented by an optical line terminal (OLT) and layer-2 switches (L2-SW).
This paper proposes a 40Gbit/s-class-WDM/TDM-PON for flexible photonic aggregation networks that uses the wavelength allocation providing the smooth migration. This system can eliminate the need for L2-SWs by aggregating a large number of users in line cards (LCs) and connecting directly to an edge router. It can also provide flexible bandwidth allocation by employing the dynamic wavelength and bandwidth allocation (DWBA) algorithm, and improve the usage efficiency of the total bandwidth. It also clarifies the scalability of the number of the users and the transmission distance in the proposed system. To demonstrate the feasibility of this system, a novel 10Gbit/s x 4λ selectable burst-mode transmitter (B-Tx) with a fast switching time for the upstream signals and a novel 4 x 4 cyclic arrayed waveguide grating (AWG) router operating simultaneously in both 1.3 and 1.5 μm bands as a photonic aggregator are developed.
2. Proposed system configuration and system scalability
Figure 1 shows the configuration of the proposed system. The OLT consists of LCs that output downstream signals and receive upstream signals, a DWBA controller, and a photonic aggregator using a 4 x M cyclic AWG that connects 4-LCs to M-PON branches (M-PBs). Each LC has a tunable Tx (4λ) for downstream signals and a burst-mode receiver (B-Rx) for upstream signals. The optical network unit (ONU) has a tunable B-Tx for upstream signals and a B-Rx for downstream signals. Tunable Txs in the LC and the ONU output the downstream and upstream signals while switching its output wavelength according to instructions from the DWBA controller.
Figure 2 shows an example of DWBA operation of the flexible load balancing of upstream signals with 4-LCs and 6-ONUs in 4-PBs. Upstream signals from ONU1-1 and ONU1-2 are input into IN port1 of the 4 x 4 AWG and output to each OUT port according to wavelength. The DWBA controller obtains requests for bandwidth from all the ONUs and sets the wavelength and time matrices from LC1 to LC4. In this example, the DWBA controller allocates the wavelength and timeslot to ONU1-2 so that it can send its upstream signal to the unused LC, because the total bandwidth requested from ONU1-1 and ONU1-2 in PB1 exceeds the bandwidth of LC1. In this configuration, the DWBA for downstream signals works only for a sleeping function and a protection function to the LC breakdown not for the flexible load balancing, because only one downstream signal with one wavelength is allowed to be transmitted in each PON branch. When the flexible load balancing for downstream signals is needed, the configuration equipping a tunable B-Rx in the ONU is needed.
In this system, the number of accommodated ONUs can be easily increased without any loss budget degradation, because the PBs are bundled by the AWG. Figures 3(a) and 3(b) show the numerical calculation results of scalabilities of the proposed system against the loss of the cyclic AWG and the number of accommodated users, respectively. In this calculation, used parameters are set as follow; the fiber loss in 1.3 μm was 0.5 dB/km. The loss of the power splitter was 3.5 dB. The difference between the output power from the transmitter in the ONU and minimum received power in the LC was set to 31 dB, which was reasonable value as discussed in the 10G-EPON standardization . The dynamic range of the B-Rx was 20 dB. Figure 3(a) shows the relationship between the transmission distance and the loss of the cyclic AWG when the M is from 4 to 32 and the number of ONUs( = L x M) is fixed to 64. For comparison, the calculation result in the case of the power splitter as the photonic aggregator was shown by the red line. It is observed that the cyclic AWG is more useful than the power splitter when the loss of the cyclic AWG is lower than 7 dB. The increase of the port of the AWG ( = M) leads to the reach extension. Figure 3(b) shows the relationship between the number of ONUs and the transmission distance at the loss of the cyclic AWG of 4 dB. This figure indicates the transmission distance when the number of users or wavelengths increases.
3. λ-switching transmission experiment
Figure 4 shows the experimental setup used to confirm the feasibility of the 40Gbit/s-class system. A prototype of 10Gbit/s x 4λ selectable B-Tx as the tunable B-Tx in the ONU and 4 x 4 cyclic AWG router were newly developed. The cyclic AWG router shown in Fig. 4 can multiplex/demultiplex 20-nm and 200-GHz-spaced signals in the 1.3- and 1.5-μm bands, respectively . The four wavelengths for the upstream are 1271, 1291, 1311, and 1331 nm (20-nm spacing), in which optical devices in 40 GbE market can be utilized, e.g. the optical module of the QSFP + . The wavelengths for the downstream signals are 1575.4, 1577.0, 1578.7, and 1580.4 nm (200-GHz spacing).
Figure 5(a) shows the configuration of the selectable B-Tx, which has 4-TOSAs, a 1 x 4 cross point switch (X-pt SW), a WDM filter, and a controller. Each TOSA is newly equipped with a directly modulated laser diode (DMLD) at a different wavelength and a burst-mode LD driver (B-LDD). In this configuration, the X-pt SW, which was commercially available, MAXIM 3841, outputs a 10G-data signal to the appropriate TOSA corresponding to the λ-switching signal. The controller controls the bias current and modulation current of the LD according to the Tx_enable signal and λ-switching signals expressed by 2bits, so that the selectable B-Tx can the upstream signal with the allocated wavelength and at the allocated timeslot from the OLT. Figures 5(b), 5(c) and 5(d) show the output signal waveforms. Output powers from the WDM filter with 1271, 1291, 1311, and 1331nm were + 1.3, + 1.7, + 1.5, + 1.8 dBm, respectively. The extinction ratio was 6.8 dB. The λ-switching and burst rising time was less than 30 ns. The burst turn off time was less than 10 ns. This TOSA including the B-LDD achieved the low power consumption; 434 mW. 4 TOSAs and a WDM filter are required to be integrated in one optical module for the cost reduction, e.g. the optical module used for the QSFP + . 10 Gbit/s burst-mode receivers (B-Rxs) with a 200-ns response time and a 20-dB dynamic range were used in the LC and the ONU .
With this setup, a λ-switching transmission experiment emulating the DWBA operation shown in Fig. 2 was conducted through a 10-km single mode fiber (SMF). Figures 6 (a) and 6(b) show the waveforms of 10.3125-Gbit/s upstream optical signals output from each selectable B-Tx and the waveforms of electrical signals received in each B-Rx. The transmitted burst pattern consisted of blocks of 397-ns preambles, a 1589-ns payload with a 231-1 PRBS, and a 99-ns end of burst. The selectable B-Tx1-2, which has a different signal pattern against other tunable B-Txs for the signal identification, was observed to switch its wavelength according to the λ-switching signal. Other selectable B-Txs were observed to output upstream signals at the assigned wavelength and timeslot. The optical signals multiplexed/demultiplexed by the cyclic AWG router were received instantaneously by each B-Rx and no waveform distortion was observed.
Figure 7 shows the bit error rate (BER) characteristics of the payload received in each B-Rx. The BER characteristics of all the optical burst signals (λ1, λ2, λ3, and λ4) were almost the same; the received optical power at BERs of 10−3 and 10−12 were −30 and −24 dBm, respectively. The downstream signals were observed to transmit through the cyclic AWG router.
This paper proposed a 40Gbit/s-class-λ-tunable WDM/TDM-PON for flexible photonic aggregation networks that realizes the aggregation of a large number of users without an L2-SW. It can provide an incremental upgrade of the total bandwidth and flexible load balancing with the DWBA algorithm. It also clarified the scalability of the number of the users and the transmission distance in the proposed system. A λ-switching transmission experiment was conducted using a newly developed 10Gbit/s x 4λ selectable B-Tx and a 4 x 4 cyclic AWG router emulating the DWBA operation.
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