We investigate a novel WDM-PON system by using self-seeded Reflective Semiconductor Optical Amplifier (RSOA) both in downstream and upstream. The transmission performance is evaluated and reported for the first time and meets carrier level requirements.
©2012 Optical Society of America
Passive optical network (PON) such as Ethernet PON (EPON) and Gigabit PON (GPON) has been widely used in fiber-to-the-home (FTTH) deployment nowadays. Many advantages have been shown for PON system, such as passive infrastructure all the way, no line interference, high bandwidth, and etc. However, as applications with high bandwidth draw customers’ attentions, EPON and GPON will be not sufficient to provide enough bandwidth for new services such as backhaul of Common Public Radio Interface (CPRI) protocol data of wireless distributed sites, high definition video, cloud computing, and etc [1,2]. To solve this problem, EPON and GPON are upgraded to 10G EPON and 10G GPON respectively . Since the downstream and upstream comply broadcast and TDMA, there are always security problems and bandwidth seizing problem among different end users. Wavelength division multiplexed (WDM) PON was proposed to provide high dedicated bandwidth [4–7]. Both security and bandwidth are guaranteed as different users are separated by different wavelengths. The key technology for WDM-PON is colorless transmitter to achieve convenience of installation and low inventory. Currently, some colorless transmitters are demonstrated using tunable lasers, modulation of sliced external broadband light source (BLS) or re-modulation of downstream signal light [8–14] These solutions encounters poor performance, large footprint of BLS components as well as relatively high cost. Other ideas like Self-seeding reflective semiconductor amplifier (RSOA) scheme were proposed later on [15–17]. In this paper, a novel WDM-PON with self-seeded RSOAs both in downstream and upstream is demonstrated and system performance with 32 and 16 channels is evaluated, showing that it meets carrier level requirements for the first time.
2. Principle of self-seeded WDM-PON
Figure 1 shows the schematic diagram of self-seeded WDM-PON system. Tx/Rx at left side can be considered as optical line terminal (OLT), while gain medium RSOA and Rx at right side is optical network unit (ONU). AWG1 and AWG2 connect to OLT and ONU respectively which are used to combine different wavelengths, launch to feeder fiber and also forward appropriate wavelength from the feeder fiber to different receivers. Faraday rotator mirrors (FRM) with partial reflectivity are located next to AWG and the reflective insertion loss of FRM is about 2dB. The insertion loss of the Gaussian athermal AWG is about 3dB for both 100GHz and 200GHz channel spacing. With the insertion loss of the fiber and WDM-PON couplers altogether, insertion loss of the total round trip is about 10dB. As for the RSOA used in the experiment, the small signal gain of TE mode in room temperature is about 22dB and the gain of TM mode is about 19dB. The light originally coming out of gain medium at transmitter is broad amplified spontaneous emission (ASE) light in C band as shown in Fig. 1. When the ASE is transmitted into one port of AWG2, only the light with wavelength corresponding to the port will reach the mirror due to the filtering effect of AWG, and get reflected partially. The reflection rate is carefully adjusted to keep balance between injected power into gain medium and the output power from FRM. The reflected light then passes through AWG again and gets amplified by the gain medium. Basically the rear end of gain medium, AWG2 and FRM form an external cavity fiber laser, and the wavelength is determined by the wavelength of the AWG port. Figure 2 demonstrates the oscillation process of the external cavity fiber laser. As long as the gain of the gain medium is larger than the total loss of the round trip, the oscillation will happen to stimulate gain medium to continuously emit light of certain wavelength instead of broad light. The mirror is a passive device so that the system is very neat and convenient for engineering, and also promising for very low cost.
The Faraday rotator mirror is a combination of a mirror and a Faraday rotator which rotates the polarization axis by π/4, and has been used to compensate for birefringence of fibers in fiber interferometers and fiber amplifiers. The Fabry–Perot cavity composed of FRMs is known as a unique configuration to stabilize the polarization by using non-PM fibers . By using the FRM as the common mirror of the self-seeded lasers, the polarization of the signal injected to the RSOA is always orthogonal to that of original output signal. Based on this principle, the FRM not only stabilizes the polarization of the injected signal, but also decreases the polarization dependent gain requirement.
is the photon roundtrip time along the whole external cavity, is the cavity roundtrip loss for the ith mode, is the photon emission factor in the front facet of RSOA. The fourth item in the right of Eq. (2) and (3) denotes the self injection effect. When the FRM is used in the common port of the AWG, due to the FRM always rotates the polarization axis of the reflected signal by π/2, the TE mode is partially injected into the TM mode by a time delay, similarly the TM mode is partially injected to the TE mode by the same time delay .
Figure 3 shows the operating simulation of the self-seeded laser. The parameters used in the model are given in Table 1 . At first, the RSOA emits a broad band ASE spectrum, as Fig. 3(a) shows. When the RSOA is connected to one channel of the AWG, the modes in the pass band of the AWG channel will be partially reflected back to the RSOA. So after one roundtrip, the modes in the AWG pass band obtain more power than the modes outside. After five roundtrips, the modes in the AWG pass band increase dramatically while the modes outside the pass band are deeply suppressed. After 100 roundtrips, the spectrum is quite similar with that after five roundtrips, only the side mode suppression ratio (SMSR) increases a bit further. The AWG is in the system is a Gaussian AWG with 100 GHz spacing; the FRM has an 80% reflectivity and 20% transmission ratio. From Fig. 3, we can see that the self-seeded laser can reach the steady state after several roundtrips.
In the test bed, the gain medium is RSOA with build-in modulation function. In the 32 channel WDM-PON system, an odd 100GHz AWG and an even 100 GHz AWG are combined with an interleaver to a “50 GHz” spacing AWG. Standard Gigabit Ethernet package is applied in the evaluation test bed. The operation wavelengths are set in C band from 1531 nm to 1560 nm. 32 ONUs are connected to the AWG ports in C + band, with the upstream wavelength from 1547.72 nm~1561.02 nm, while the downstream wavelengths are from 1531.5 nm~1543.73 nm in C- band. The test bed devices are shown in Fig. 4 . In the OLT, two WDM-PON cards with 16 GE ports on each are used to communicate with the ONUs. The transmission distance is 20km normally without further notice. In the test bed, only 27 ONUs are activated for the 32 channel system, and the other 5 ONUs are activated for the 16 channel system with 200 GHz AWG. The physical bit rate is 1.25 Gbit/s and the payload data rate of all the channels is up to 1 Gbit/s.
Figure 5 shows the back-to-back spectra coming out of FRM1 and FRM2 for upstream and downstream signal composing of all the 27 out of 32 channels in total. The total power at the output of FRM1 and FRM2 are both about 5 dBm. The space among the first several channels is left for the 5 channels of 16 channel system, so the mode spacing for the first 5 modes is 0.8 nm, while for the left 22 modes is 0.4 nm. Figure 6 shows the spectra after 20 km transmission for upstream and downstream signal in which no obvious deterioration has been observed. The reason ONU’s transmission spectra is wider than OLT’s ones is that there is 1 km fiber cavity between each ONU and FRM2. Therefore, the oscillation between ONU and FRM2 will suffer more loss and have worse side mode suppression effect. This piece of 1 km fiber is used to simulate scenarios in real applications where certain distance may exist between users and AWG.
Figure 7(a) shows the RSOA self-seeded performance of Channel 17 in 32 channel system and 7(b) shows the eye diagram after the FRM2. It is obvious that the broadband ASE turns into stimulated signal with wavelength aligned with AWG passband. The colorless technique is achieved in a very economic way and the whole oscillation process only takes several roundtrips’ time, which is usually no more than several hundred microseconds after the RSOA is powered on. From the eye diagram, the extinction ratio is roundabout 6dB. Due to the residual data noise in the reflected signal, the extinction ratio is always limited and hard to increase further. However, it’s sufficient already for a system level’s transmission when the FEC function is open.
16 channel WDM-PON test bed with 200 GHz AWG is also demonstrated for superior performance, in which 5 channels are activated as examples. Figure 8 shows the spectra of OLT from FRM1 for (a) back-to-back and (b) after 80 km transmission respectively. There is still no packet loss detected for 80 km transmission. The distance between ONU and FRM2 is set to different values to identify its impact on the transmission performance. 1st spectrum from left in Fig. 9(a) represents several meters distance while 2nd, 3rd ones are 1 km and 4th, 5th ones are 5 km. ONUs with 5 km and 1 km distance have worse performance after 80 km transmission. Figure 9(b) shows the uplink eye diagram for this 16 channel WDM-PON system when the distance between the ONU and FRM2 is 1km. Compared with Fig. 7(b), the 16 channel system have better performance than 32 channel system, for the output power is a little higher, lip of eye diagram is less thick . This is because the 200 GHz AWG has larger bandwidth and a little lower insertion loss, so there can be more ASE power reflected to the RSOA and then stronger self injection effect.
Figure 10(a) shows the BER power penalty for 32 channel system and (b) the BER power penalty for 16 channel system, which is measured by an Anritsu’s BERT at 1.25 Gb/s bit rate and the data pattern is the Pseudorandom Bit Stream (PRBS) with 231-1 length. The distance between ONU and FRM2 for both systems is 1km and all the BER is measured with the last channel, which is also the worst channel (The channel with the longest wavelength is far away from the gain peak of RSOA and also the most vulnerable to noise, hence has the worst performance). After 20 km, the power penalty is only 0.5 dB at 1e-10 for 32 channel system and 0.3 dB at 1e-10 for 16 channel system. With FEC function open, the uplink receive sensitivity can reach −33 dBm after 40 km for both 32 and 16 channel system. Due to the larger bandwidth, lower insertion loss of 16 channels AWG (channel spacing is 200 GHz) and consequently stronger self-injection effect, the 16 channel system has smaller power penalty and better performance than 32 channel system.
The distance between ONU and FRM2 is also an important factor which affects the system performance. The gain medium in the ONU optical module, AWG channel and FRM2 constitute a self-seeded laser. Here we denote the distance between the ONU and the FRM2 as the “Cavity length”. Figure 11 shows the performance variation at different cavity length after 20km for both 32/16 channel systems. All the BER is measured when the received power before APD is −28 dBm. The BER increases monotonically with the cavity length from several meters to 5 km. When the cavity length increases, the insertion loss becomes larger, the roundtrip time also increases consequently. Both these factors decrease the self-injection effect hence the spectrum becomes wider and BER becomes larger. When the cavity length increases to long enough, the gain of the RSOA can’t overcome the roundtrip insertion loss and the ASE noise of other modes. The whole system would fail to work. As shown in Fig. 11, both 32/16 channel systems can support at least 5 km cavity length after 20 km with the FEC function open. In practical application scenarios, the distance between the ONU and FRM is rarely over 5km. Therefore, no further test is done for cavity length longer than 5 km.
Moreover, throughput test is performed to further prove the performance for both 32 and 16 channel systems with 20 km fiber. 5 channels of 16 channel system and 27 channels of 32 channel system are tested together with maximum data rate allowed by the test instrument, in which the maximum payload data rate for a channel is 1 Gbit/s. The received power for every channel is about −25 dBm in 32 channel system, and −20 dBm in 16 channel system (No interleaver in 16 channel system). No packet loss has been observed during more than 14 hours with the FEC function open, indicating that the system works with error free.
This paper proposes an economic WDM-PON system using self-seeded colorless lasers both in OLT and ONU. And for the first time the system level performance of self-seeded WDM-PON is investigated and evaluated. The test result shows that self-seeded WDM-PON system can meet carrier level requirements.
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