1. Introduction

Time division multiplexed-Passive optical networks (TDM-PON) such as EPON and GPON are being deployed around the world instead of other access technologies such as xDSL and HFC networks. Though currently this optical technique is considered as the most suitable technology for last mile broadband access, even it will be pushed to its limits due to the limited capacity per customer as a single wavelength channel is shared among all users. Hence, 10 Gb/s TDM-PONs including 10 Gb/s EPON and wavelength division multiplexed (WDM) PONs are being investigated in order to increase the capacity per customer [1–2]. In TDM-PON the upstream transmission is operated under burst mode due to the multiple access scheme employed. 10 Gb/s burst mode transceivers can be uneconomical in the near future and hence the 10 Gb/s downstream and 1 or 2.5 Gb/s upstream asymmetric transmission are also being considered for these systems. Such systems will limit the available upstream capacity for users in TDM-PON. High upstream capacity will be important in the future internet when individual users become active participants in providing information to the outside world.

In this paper we propose a novel technique for increasing the number of upstream wavelength channels in TDM-PON thus increasing the upstream capacity avoiding higher upstream burst mode data speeds. Multiple-wavelength TDM-PON transmission schemes have been reported either using tunable lasers or tunable filters at the optical network units (ONU) [3–4]. The use of tunable lasers or multiple CW laser sources with tunable filters can be costly and complex to implement and also would require external modulators. In this work we present a single Fabry-Perot Laser Diode (FP-LD) as the channel selector and burst mode transmitter for the upstream transmission in a multiple-wavelength TDM-PON. The FP-LD locks onto only one of many strategically separated seeding wavelengths from the optical line terminal (OLT) with minimal temperature tuning. The paper presents experimental results for the multiple-channel operation at 2.5 Gb/s per channel. Investigation of the locking properties of the FP-LD in the experiment shows that possibly up to 9, 2.5 Gb/s upstream channels can be utilized with the FP-LD with a free spectral range (FSR) of 140 GHz.

2. The proposed scheme for upstream channel selection by FP-LD transmitters

 

Fig. 1. Schematic for proposed multi-wavelength upstream transmission scheme employing FP-LD. The concept is explained using spectral diagrams in the bottom figure.

Download Full Size | PDF

The schematic of the proposed scheme is shown in Fig. 1. Multiple seeding wavelengths (together with downstream channel) are transmitted downstream at a channel separation of Δλ(1+1/n), where ‘Δλ’ is the mode spacing (the FSR) of the FP-LD used at the ONUs and ‘n’ the number of seeding wavelengths. Initially, all FP-LDs will transmit at a single wavelength such as λ₁ through locking onto this wavelength as shown in the bottom of Fig. 1. When the upstream transmission capacity is pushed to the limit in this virtual channel or when some customers require higher upstream capacity, the OLT can send a request to those ONUs to tune their FP-LD transmitters to another seeding wavelength and join another separate upstream virtual channel (λ₂ or λ₃). The temperature tuning of the FP-LD as observed in our experiments can be achieved in a fraction of second and after this tuning, the ONU will join the second or third upstream virtual channel by the standard discovery process. The temperature range to tune the FP-LD across a full FSR (140 GHz) was observed to be only 9.1 °C. The FP-LD transmitters at the ONUs should include a look up table to achieve the necessary wavelength shift and this can be done at the manufacturing stage. It will enable fast switching between channels with simple electronics. The seeding wavelengths can be depolarized CW light or finite bandwidth optical noise sources to overcome polarization sensitivity in the injection locking process [5].

3. Experiment and results

We used a 1550 nm FP-LD with a mode spacing of ~140 GHz and threshold of 18 mA. An optical filter with 0.8 nm bandwidth was used at the upstream receiver and the signals were transmitted through 21 km of single mode fiber and variable optical attenuator to mimic the losses at the passive splitter and couplers. The initial experiment was carried out with CW seeding sources with -13 dBm injection power and continuous data streams. The burst mode operation of the FP-LD required for this application was also confirmed through experiments. The burst mode operation of FP-LDs has also been shown by other research groups earlier [6]. The temperature was controlled by an LDT-5412 ILX Lightwave thermoelectric temperature controller with +/- 0.05 kΩ accuracy. The FP-LD was initially biased at 25 mA and three seeding channels were used at wavelengths of approximately 1553 nm (channel 1), 1554.35 nm (channel 2) and 1555.7 nm (channel 3). The channels were separated approximately by 170 GHz (1.21×Δλ). When the temperature was tuned to 22.6 °C the FP-LD locks only onto channel 1 and the other channels are suppressed by 18 dB. In this paper we will define this suppression of the unused seeding wavelengths as the suppression of other seeding wavelengths (SOSW) and the suppression of other longitudinal mode of the FP-LD due to injection locking as the side mode suppression ratio (SMSR). The worst case SOSW was observed to be around 15 dB (including the contribution from the laser front facet reflection), which was at low laser bias (20mA). At higher laser bias (40 mA) the SOSW can be larger than 20 dB. The SMSR was observed to be > 35 dB. When the laser is tuned to 24.7 °C it locks onto only the second channel with the other seeding wavelengths suppressed. And when the temperature was 26.7 °C, the FP-LD locks only onto channel 3 with channels 1 and 2 suppressed. The optical spectra are shown in Fig. 2 for the three channel operation of the FP-LD. We used only a 30 GHz separation between the other seeding channels and the closest unlocked modes of the FP-LD showing that a high channel count can be supported. In experimental characterization we observed that even a 15 GHz separation between the unlocked laser modes and the other seeding wavelengths will give acceptable SOSW (> 15 dB) hence the potential to support up to 9 upstream channels for 140 GHz mode spacing. For example if the 9 seeding wavelengths are selected to have a frequency offsets of 0, 75, 155, 230, 310, 385, 465, 540 and 620 GHz, they can be guaranteed a minimum separation of 15 GHz from each mode at each tuning point. Here we have selected a minimum of half the FSR separation between the seeding wavelengths. The ultimate number of upstream channels will also depend on the power penalty due to the accumulated reflections from all seeding channels (which will be unmodulated light) due to the finite SOSW. However, as observed in the BER measurement this penalty is not significant, indicating the limit on the number of wavelengths would mainly dependent on the minimum seeding wavelength separation.

 

Fig. 2. Time averaged optical spectra showing injection locking of FP-LD with three seeding channel injection under different operation temperatures.

Download Full Size | PDF

Figure 3(a) shows the BER vs received optical power at 2.5 Gb/s (2³¹-1 PRBS) for two channels before and after 21 km of fiber transmission with -13 dBm injection power per seeding wavelength. An APD with clock recovery circuit was used at the receiver. For both channels the receiver sensitivity at BER of 10^-9 was below -32 dBm and the power penalty after transmission was ~0.25 dB. This penalty can be attributed to the Raleigh backscattering. The inset shows the optical eye at error free operation. Fig. 3(b) shows the BER vs received optical power for three upstream channels at 2.5 Gb/s with and without other two seeding wavelength channels. The results are after 21 km single mode fiber transmission. No penalty due to the reflected residual components from other seeding wavelengths is observed. Note, in full system implementation the other upstream channels which are simultaneously transmitted are filtered out at the OLT receiver and hence no penalty is expected from the other upstream channels. The slight difference in the receiver sensitivity of the three channels might be due to the differences such as the gain spectrum of the laser at the three independent channel wavelengths [7].

 

Fig. 3. (a) BER against received optical power for two channels before and after 21 km fiber transmission. (b) BER against received optical power for the three channels with and without the other seeding wavelength channels.

Download Full Size | PDF

4. Locking Characteristics and Burst Mode Operation

Figure 4 shows the minimum wavelength separation between the suppressed laser modes and unlocked seeding wavelengths against laser bias. The minimum separation is defined as the smallest wavelength separation where the SOSW is within 3 dB of the best SOSW for the respective laser bias. The SOSW was observed to be almost flat between the modes until the seeding wavelength is closer than 0.2 nm to the laser mode. The inset of Fig. 4 shows the seeding wavelength detuning across a laser mode. The injection powers were kept constant at -13 dBm. The longer and shorter wavelength denotes when the seeding wavelength is on the longer wavelength side of a mode (right of mode) or shorter wavelength side of a mode (left of mode) respectively. On the shorter wavelength side the SOSW is maintained within 3 dB until the seeding wavelength is only 0.12 nm (15 GHz) away from the cavity mode in 25–30 mA bias setting. On the longer wavelength side an even smaller minimum separation of 10 GHz is observed. When the laser is biased lower than 25 mA the suppression degrades a little quicker when the seeding wavelength get closer to the lasing mode due to weak cavity effect. While for higher bias current, the minimum separation is also increased due to the enhanced relaxation oscillation frequency. We also note marginally better suppression is achieved on the longer wavelength side from each mode which is due to laser injection locking characteristics [8].

 

Fig. 4. Minimum separation between the laser mode and unlocked seeding wavelengths against laser bias. The inset shows the change in SOSW when the seeding wavelength is detuned across a laser mode.

Download Full Size | PDF

Fig. 5 shows SOSW relative to the locked wavelength against injection power. The bias current was maintained at 25 mA. As observed the SOSW is better with lower injection power into the laser due to lower backward reflection from both fiber and laser. This is desirable since in TDM-PON the injection power will be low due to the high loss in the passive splitter. The Fig. 5 also shows the SMSR against the injection power. As expected the SMSR is worse at lower injection powers since the suppression of other cavity modes degrades with lower injection power. However, the SMSR is still larger than 25 dB with injection power as low as -20 dBm. In experiments we also confirmed error free operation with injection powers of less than -20 dBm and the lowest injection power for error free operation was observed to be -24 dBm.

 

Fig. 5. Suppression of other seeding wavelengths and side mode suppression ratio (SMSR) relative to the locked mode vs injected optical power.

Download Full Size | PDF

Next the experiments were carried out to observe the burst mode transmission by modulating the laser with packet data. Fig. 6 shows the data pattern recovered with burst mode operation. As before three seeding wavelengths were used. An extinction ratio of >20 dB was achieved showing burst mode operation of the laser under injection locking conditions.

 

Fig. 6. Data traces in burst mode operation.

Download Full Size | PDF

5. Conclusion

We demonstrated a scheme for multi-wavelength upstream transmission in a TDM-PON using a single FP-LD at the ONU. It increases the effective upstream capacity by allowing the users to join more than one upstream transmission channels through defined small temperature tuning of the FP-LD transmitter. Multiple channel experiments at data rates of 2.5 Gb/s confirmed the feasibility of the scheme. The effect on transmission performance from reflections from the other unused seeding wavelengths alongside the modes of the laser was observed to be negligible. In the experimental demonstration, the seeding wavelength separation can be made as small as 15 GHz showing a high channel count can be supported.