The extinction ratio (ER) for the downstream and upstream transmission signals needs to be compromised for the WDM-PON systems with directly modulated lasers at the center office and reflective semiconductor optical amplifiers at the user ends. We propose to enhance the performance by adding a FP etalon before the receiver of each optical network unit (ONU). The etalon performs spectral reshaping and then waveform reshaping to the downstream signals. This allows the use of low-ER downstream signals that reduce the intensity fluctuation of RSOA-remodulated upstream signals. This approach can also extend the transmission distance by reducing the transient chirp. Colorless operation can still be obtained since the same etalon can be used to enhance multiple wavelength channels. Experimental results verify considerable performance improvement on WDM-PONs with 10-Gbps and 1.25-Gbps data rates for the downstream and upstream transmission, respectively.
© 2008 Optical Society of America
Wavelength division multiplexed passive optical networks (WDM-PONs) are emerging as the attractive solutions for next-generation optical access networks. There exist various combinations of solutions for the center office and optical network units (ONUs) [1, 2]. For the downstream transmission that requires high data rate, directly modulated lasers (DMLs) are very attractive solutions for access networks. Though the chirping effect may limit the transmission distance for DMLs, it can be mitigated with simple schemes. On the other hand, colorless and low-cost ONUs are crucial for deploying WDM-PONs. One of the promising colorless ONU schemes is the use of reflective semiconductor optical amplifiers (RSOAs) as remodulators for upstream transmission [3, 4]. Thus, WDM-PONs employing DMLs at the center office and RSOAs at the ONUs are attractive solutions for high data-rate applications. For convenience, such a scheme is called DRWDM-PON in this paper.
An array of continuous-wave (CW) WDM light sources at the OLT can be used to provide the probe light for the remodulation, but they will increase the system complexity and cost. On the other hand, the remodulation can simply be preformed by splitting a portion of the downstream signal as the probe light for the upstream transmission. With this scheme, the intensity fluctuation due to the crosstalk induced from the downstream signals needs to be minimized. The problem of intensity fluctuation can also be overcome by using special modulation formats [5–7], but simple modulation schemes are favored for access networks. The fluctuation can be suppressed by gain saturation nonlinearity of the RSOA. This requires a RSOA with low input saturation power or a high input power, which will in turn limit the downstream power budget. The problem can also be improved by using a detuned spectral filtering scheme to suppress the mark-level spectrum for the upstream signals . Here, we propose a simple scheme to enhance the downstream ER at the ONU and preserve the colorless function of the ONU.
2. Operation principle
Figure 1 shows the schematic of a DRWDM-PON system. A portion of the downstream light is coupled to a RSOA, which reflects and modulates the incident light to carry the upstream signals. The experimental system has asymmetric data rates, 10 Gbps for the downstream and 1.25 Gbps for the upstream. The data rate of the upstream is currently limited by the speed of the commercial RSOA. A higher data rate like 10 Gbps can be achieved by using an electro-optical modulator or electro-absorption modulator for carrying the upstream data . For DRWDM-PON systems, the power budget is very stringent for both the upstream and downstream transmission without using any optical amplification and/or high sensitivity receivers. Any impairment to the system performance needs to be mitigated with cost-effective solutions. Since the upstream signals are riding on the downstream signals, intensity fluctuation occurs on the upstream signals. Low power penalty is hard to obtain for the upstream signals when the downstream signals are modulated with a relatively large ER, as shown in Fig. 2. To overcome this impairment without using special coding formats, it is better to keep the ER of downstream signals below 3 dB. The low-ER penalty for the downstream signals can be reduced by using a spectral filtering scheme [9–11].
The spectral filtering can be easily carried out for DRWDM-PON systems because of the spectral broadening on the directly modulated downstream signals. Fig. 3(a) shows schematically the spectral broadening caused by the transient chirp and adiabatic chirp. In particular, the adiabatic chirp causes a separation of the mark- and space-level spectra. By using an optical filter to suppress the space-level spectrum, the ER can be improved. We propose to use a simple Fabry-Perot (FP) etalon as the optical filter because of its low cost and compactness that allows it to be easily integrated with the transceivers. The periodic spectral response of a FP etalon allows colorless operation of ONUs, since the same etalon can be used in different ONUs. This can be achieved if the channel spacing of the WDM-PON is a multiple of the FSR of the etalon. The spectral filtering scheme can also be used in the WDM-PON system that a modulator instead of a RSOA is used at the ONU to provide the remodulation onto a directly modulated downstream signal .
The low-ER modulation also helps to extend the chirp-limited transmission distance for the down streams. For example, Fig. 3(b) shows the transmission performance of a commercial available DML at 10 Gbps data rate. The transmission distance is limited to 10 km if the laser is modulated with high ER. The distance can be extended by using a low-ER modulation with a compromise on the power penalty due to low ER. By using the etalon to reshape the down stream signals, it can not only remove the low-ER penalty but also reduce the chirping effect, so the transmission distance can be further extended.
3. Performance demonstration
The performance enhancement by using a FP etalon is demonstrated by using two DMLs of 1557.48-nm (CH1) and 1558.28-nm (CH2) wavelengths to simulate multi-channel downstream transmission, as shown in Fig. 1. The DMLs used in our experiments have threshold current and slope efficiency of about 10mA and 0.11 W/A, respectively. The ER of the 10-Gbps downstream signals is 3 dB. The transmitted power for both channels is about 6.5 dBm. The downstream channels are demultiplexed by an arrayed waveguide grating (AWG) and one portion of the signal is received at the ONU by inserting a FP etalon before the detector. The other portion of signal is injected into the RSOA, modulated by a 1.25-Gbps NRZ signal, passed through the fiber and circulator, and then detected at the center office (CO). The data patterns for modulating the DML and RSOA are 231-1 and 29–1 pseudorandom binary sequences, respectively. The use of different data patterns for the upstream and down stream is just for simulating independent signal sources.
The output saturation power and the optical small-signal gain of the RSOA are +3 dBm and 20 dB, respectively. The AWGs used in the experiments are a flat-top type with a 1-dB bandwidth of about 0.5 nm. The insertion loss of the AWG is 4.5 dB. The FP etalon has a FSR of 100 GHz and a 3-dB bandwidth of 0.136 nm. The etalon is coupled to the input/output fibers by using optical lenses; and the insertion loss is measured to be 2.8 dB. The loss can be further reduced by using better alignment and packaging schemes.
Without any chirping compensation the point-to-point transmission distance achieved by modulating the DMLs at 10 Gbps with a high ER is limited to 10 km. Fig. 3(b) shows that the power penalty is insensitive to fiber length of up to 40 km by using a low ER and inserting an FP etalon before the receiver. Figure 4 shows the measured BER for the two downstream channels and the re-modulated upstream signals for the DRWDM-PON after 25 km of transmission. The power of the downstream signal injected into the RSOA is fixed to -15 dBm by using an optical attenuator. The upstream signals after 25-km transmission have a power penalty of about 1.1 dB at a BER=10-9 condition for both channels. For downstream signals, adding an etalon before the receiver can improve the power penalty by 3.0 dB for the back-to-back (B-B) case. This is mostly due to the increase in ER since the B-B case with etalon corresponds to the B-B case with high ER. After 25 km of transmission, the two channels have only 1.0 dB of power penalty. Such a performance is about 2 dB better than the B-B case without using an etalon. The eye-diagrams shown in Fig. 5 for downstream signals clearly indicate that adding a FP etalon can reshape the waveform to have an enhanced ER. Furthermore, the results in Fig. 4 verify that a FP etalon can be used to reshape different WDM channels, so each ONU is still colorless.
A well-known impairment to the bidirectional transmission of signals over a fiber is the reflection from the Rayleigh backscattering (RB) and Fresnel back reflection (FBR) effects when the same wavelength is used for both directions [12, 13]. These two effects can deteriorate the system performance of the DRWDM-PONs [13, 14]. For investigating the backreflection signals, we measured the powers at the upstream receiver when the RSOA is on and off, respectively. The measured on/off power ratio is about 22 dB, which is almost identical to the calculated optical-signal-to-Rayleigh-scattering-ratio (OSRR) . This indicates that for our experimental condition the main contribution of backreflection is from the Rayleigh scattering. Figure 6 shows the comparison of the BER between using one fiber and two fibers for the bidirectional transmission. The experimental results indicate that the induced power penalty from the backreflections is only 0.5 dB. The low penalty from backreflections may attribute to low coherence for the downstream signals due to the direct-modulation induced frequency chirping. The performance degradation from the backreflections will be smaller by using a directly modulated probe light rather than a CW one for the remodulation [16, 17]. Moreover, the RSOA is operated at a moderate gain (8-dB) that is close to the optimal condition to minimize the influence of RB and FBR .
The scheme demonstrated here is different from the detuned filtering scheme proposed by W. Lee et al. , where the filter is inserted to suppress the mark-level peak of the upstream signal spectrum. By aligning the filter response to the spectral peak corresponding to the space level of the upstream signals, the significant intensity fluctuation on the mark level can be greatly reduced. For comparisons, the BER performance of the detuned filtering approach was measured by using the same etalon for our spectral filtering scheme. The BERs for both schemes are very close by comparing the results shown in Figs. 7 and 4(b).
In spite of the similar performance, the scheme proposed in this paper provides advantages over the detuned filtering scheme. The comparisons are summarized in Table 1. Firstly, a low-ER downstream signal has a lower chirp, which can further be reduced by the etalon, so our scheme allows a relatively large span for the high speed downstream transmission. Secondly, filtering out the mark-level in the detuned filtering scheme suffers from a larger excess loss than the selection of the mark-level in our scheme. Thus, our spectral filtering scheme has a lower loss than the detuned filtering approach. The excess loss of the etalon can be critical because the system power budget for the DRWDM-PONs is usually limited by the upstream signal path. Thirdly, the DML can be operated at a larger average output power for a lower-ER modulation since the laser is usually subject to the limitation on mark-level power. This increases the power budget for both the upstream and downstream transmissions.
For the bidirectional transmission at low data rates, e.g., 1.25 Gbps for both downstream and upstream in , the link span is usually limited by the upstream transmission. Under such a limitation, the maximal loss for the optical distribution network (ODN), LODN, can be written as
Where Pt,OLT and Pr,OLT are the transmitted power and receiver sensitivity at the OLT, GRSOA is the gain of the RSOA, S is the system margin, and LDF is the loss for the detuned filtering. In our experimental conditions, the transmitted power is 6.5 dBm. The loss for the detuned filtering is 5.2 dB, including 4.2-dB extra loss from spectral filtering. With a receiver sensitivity of about -24 dBm and 3-dB system margin, it will require about 14 dB of RSOA gain for using the detuned filtering technique to transmit upstream signals over 25 km long fiber. On the other hand, the required SOA gain is about 6 dB lower for our scheme to achieve the same performance, by also accounting the larger transmitted power for lower-ER modulation.
When the downstream data rises to 10 Gbps or above, the link span is very likely limited by the chirp effects associated with the downstream signals. As demonstrated above, 10-Gbps transmission over 25 km of fiber can be achieved by using the spectral filtering technique. However, the span is limited to 10 km at this data rate when the detuned filtering approach with high downstream ER is used. Therefore, the proposed spectral filtering scheme is especially advantageous for DRWDM-PONs of high downstream data rates.
We demonstrated the performance improvement of a DRWDM-PON system by inserting a FP etalon before the receiver of each ONU. This scheme allows the use of a low-ER downstream transmission to reduce intensity fluctuations for the upstream and avoid the ER penalty on the downstream signals. The etalon can also reduce the chirping effects of downstream signals and extend the link span. Colorless operation of ONUs can still be maintained.
This work was supported in part by the National Science Council, Taiwan, under grant NSC95-2219-E-011-004 and by the Ministry of Education under the Top University Program.
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