1. Introduction

The implementation of full duplex bidirectional transmission at 10 Gb/s in next generation extended wavelength division multiplexed – passive optical networks (WDM-PON) is currently limited by the upstream transmission capabilities of optical network units (ONU), where low cost solutions are needed. The use of reflective semiconductor optical amplifier (RSOA) at the ONU has been proposed as a low cost solution that additionally allows colourless re-modulation and broadband optical amplification [1]. However, the symmetrical intensity modulated transmission in RSOA-based ONUs is currently limited to 2.5 Gb/s due to the low bandwidth (BW) of commercially available RSOAs.

Upstream transmission at 10 Gb/s using RSOAs is presently the focus of an exhaustive investigation. Various sophisticated techniques have been proposed for single (i.e. non-duplex) upstream transmission at 10 Gb/s using RSOAs, like orthogonal frequency division multiplexing (OFDM), achieving a transmission distance of 20 km (at a Bit Error Ratio (BER) of 10^-9) [2] or offline processing with the help of Decision Feedback Equalization (DFE) together with Forward Error Correction (FEC) codes [3], achieving a maximum transmission distance of 20 km at FEC limit. An alternative low cost solution is the duobinary modulation applied in [4] where the adjustment of the RSOA’s electro-optical (E/O) response to a 2.5 GHz Bessel filter allowed 10 Gb/s transmission up to 10 km. Furthermore, in [5], the authors have demonstrated half-duplex upstream transmission up to 85 km at 10 Gb/s by means of detuned optical filtering and DFE in the OLT, using the same RSOA as in [4]. Nevertheless, none of the aforementioned techniques have been proven till now to be able to provide full duplex bidirectional transmission at 10 Gb/s using a low BW RSOA.

The work presented here focuses on full duplex bidirectional transmission at 10 Gb/s using an RSOA with a maximum BW of only 1.2 GHz at the ONU. It is proven experimentally that 10 Gb/s symmetrical full duplex bidirectional transmission can be achieved up to 25 km of standard single mode fiber when the system is assisted by optimally detuned optical filtering and electronic equalization at the OLT side.

2. Experimental set up and parameters

The experimental set-up is shown in Fig. 1(a). At the OLT, a continuous wave (CW) signal at 1532 nm is introduced into a Mach Zehnder modulator (MZM) where a 10 Gb/s non-return-to-zero (NRZ) downstream signal is applied with pseudorandom bit sequence (PRBS) of 2⁷–1.

Figure 1(b) shows the eye diagrams after 25 km of bidirectional fiber only, with the optical filter placed in its central position (up) and with optimum offset (down). The downstream and upstream signals before and after the RSOA respectively, have been acquired with a PIN diode (after removing the RSOA ASE noise using a wide BW optical filter) in order to measure the ER of the signals more accurately. The optical input power into the RSOA was -10 dBm same as in the final set up. In order to achieve full duplex transmission, the extinction ratio of the downstream signal (ER_d) has to be limited [1]. An ER_d of 1.8 dB was selected in order to achieve bidirectional transmission at 25 km and it has been maintained constant for all the studied transmission distances. Higher ER_d would be better for the downstream signal but worse for the upstream signal. Lower ER_d would make the system limited to the downstream transmission. For other distances, the optimum ERd is different but we have maintained it fixed in all the experiment. Significant cancellation of the downstream modulation because of gain saturation in RSOA is not observed.

 

Fig. 1. (a) Experimental set-up, (b) Electrical eye-diagrams after APD for bidirectional upstream transmission after 25 km of SSMF without (top) and with (bottom) optical filter (28 GHz BW) at optimum position for P_in = -10 dBm.

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An Erbium-doped fiber amplifier (EDFA) compensates the losses of the MZM and the launched optical power at the OLT output is fixed to 1 dBm in order to minimize nonlinear impairments. The fiber plant is composed of two unidirectional fibers with equal lengths (L₁ = L₃, shown in Fig. 1(a) and a bidirectional drop fiber (L₂), emulating the SARDANA architecture [1]. The fiber type used is SMF-28^TM and no optical dispersion compensation is used in the link. At the ONU, the signal is detected by an APD receiver. An RSOA module (with a gain spectrum centered at 1510 nm and an optical 3 dB BW of 25 nm) is used for upstream transmission. The upstream signal is introduced through a bias-T and a resistor-capacitor (RC) filter circuit, which in combination with the RSOA provides an enhancement of the effective BW from 1.2 to 2.3 GHz at the RSOA based ONU, when biased with 60 mA DC current and modulated with a current of ±35 mA. The input power into the RSOA is adjusted to -10 dBm by using a variable optical attenuator. The performance of the RSOA could be also better compared with the existed one by using a wavelength close to its gain peak at 1510 nm. Nevertheless, this work focuses on C-band as required by new generation PON networks such as the SARDANA network [1]. In this experiment, the transmitted wavelength is 1532 nm, which affects the chirp and the gain of the device, limiting its performance [4, 6]. Under these conditions, the optical gain of the RSOA is 10 dB.

In both receivers, an optical signal-to-noise ratio (OSNR) emulator, consisting of a variable optical attenuator and an EDFA, was used to alter the OSNR. The photo-receiver used at the OLT is also an APD whose optical input power has been fixed to -12 dBm. A JDSU-TB9 tunable optical bandpass filter (BPF), with 28 GHz BW at -3 dB and low group delay, was placed before the photodiode. After the detector at the OLT, an electronic equalization circuit is used, consisting of a 5-stage of Feed Forward Equalization (FFE), clock/data recovery and a 2-stage DFE with adjustable tap coefficients (DFE (5, 2)). The performance of the received signals was evaluated in terms of required optical signal-to-noise ratio (ROSNR) for a BER value of 10^-9. Furthermore, the BER versus OSNR has been measured for different distances with and without DFE.

The selection of ROSNR for BER of 10-9 as a performance metric was due to the fact that the proposed solution of this paper refers to WDM-PON applications in which an EDFA will be required at the OLT receiver to achieve extended reach and/or more WDM users [7]. The cost of this amplifier will be shared among the number of WDM tributaries. The optical amplifier has been used to fix a constant level for the DFE by fixing the photodetector input power instead of adding a limiter electrical amplifier after the receiver. This allows fixing the conditions in the electrical domain, thus avoiding electrical amplification and distortion, and having a more direct representation of the received signal quality.

 

Fig. 2. On the left, normalized electro-optic comparison (S21) between different cases: the response of the RSOA optimized for maximum BW (1) and optimized for maximum ER (2) in combination with the RC filter for a) detuned optical filter and b) centered optical filter; and also the original RSOA electro-optical response (without the RC filter). On the right, optical eye diagrams of the detected signal for back to back configuration for the RSOA optimized for BW and ER respectively: in a) only upstream with the filter detuned, in b) only upstream with the filter in its central position and in c) bidirectional transmission with the filter detuned and downstream with ER=1.8 dB.

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In the work presented in this paper, because of the presence of the downstream signal in the remodulated upstream signal, there is a trade-off between the maximum BW and the maximum ER that the RSOA can provide in order to be able to achieve the best performance in terms of BER. Figure 2 (left) shows the effect of the bias current in the RSOA performance, for the back–to-back configuration. Here, the electro-optic comparison (S21) among different cases is depicted. More specifically, the response of the RSOA optimized for maximum BW (1) (for I_bias = 70 mA) and optimized for maximum ER (and enough BW) (I_bias = 60 mA) (2) in combination with the RC filter for a) detuned optical filter and b) centered optical filter, is presented. Additionally, the original RSOA electro-optical response (without the RC filter) is shown in the same figure. It can be observed that for the case of maximum BW, the BW after filter detuning is 5.5 GHz, whereas for the other case, the BW is 5 GHz.

Figure 2(a) shows the received optical eye diagrams with the optical filter detuned in the optimum position only for the upstream transmission [5]. It can be observed that for the case of maximum BW, the extinction ratio of the upstream signal (ERu) was 4.2 dB, whereas for the case of maximum ER, the ERu had a value of 6.2 dB. According to Fig. 2(b), where the optical received eye diagram with the optical filter in the central position are depicted, the effectiveness of filtering detuning compared with the case of Fig. 2(a) can be easily proven.

Figure 2(c) shows the received optical eye diagrams for the case of bidirectional transmission when the ER of the downstream signal is 1.8 dB and optimum filter detuning at the receiver is used. It is evident that the quality of the eye diagrams after bidirectional transmission is decreased with respect to the eye diagrams shown in Fig. 2(a) for upstream only transmission. However, despite the degraded performance, error free (BER<10^-9) bidirectional transmission is still possible and can be further improved with the use of equalization, as it is studied next.

3. Results and discussions

For all the measurements presented below, the spectral position of the filter at the OLT with respect to the central wavelength of the transmitted signal was around 0.16 nm (blue shifted) for both cases with and without equalization of DFE (5, 2) at the receiver end. The optimum filter position has been studied extensively in [5] where it was shown that, due to the slope of the optical filter, the phase modulation generated by the RSOA’s chirp is transformed into constructive amplitude modulation when blue-shifted filter detuning is applied. In this experiment, a misalignment of ±0.01nm produces a penalty of 0.5dB in the ROSNR when the DFE is not used, while with the DFE it is necessary to detune ±0.02 nm from its optimum position to obtain this effect. Further misalignment generates strong degradation in the performance. A chirp parameter of α = 9.8 has been measured according to the methodology shown in [8].

 

Fig. 3. (a). ROSNR for BER=10^-9 versus fiber length for bidirectional upstream and (b) received (electrical) eye diagrams for various distances with detuned optical filter at the receiver (with a window width of 200 ps).

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Fig. 4. (a). Upstream comparison BER versus OSNR with 25 km bidirectional and two fibers of 25 km unidirectional (b). BER versus OSNR with 12 km bidirectional and 2×25 km unidirectional.

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Figure 3(a) presents the ROSNR for a BER of 10^-9 with respect to the bidirectional transmission length L₂, for the cases with (open symbols) and without (solid symbols) equalization, with the filter being detuned at its optimum position in both cases. The measured values in Fig. 3(a) without the equalizer have been obtained by optimizing the filter position according to the distance, which is a rather impractical approach. However, when DFE is used in combination with offset filtering, the optimum offset remains the same (~0.16nm), independent of the transmission distance, as it has been already presented in [5]. Figure 3(a) shows that for the case without equalizer the best performance is achieved around 12 km. This proves that the initial signal from the RSOA is pre-chirped, and its interaction with the CD and filtering effect smoothes out signal distortions around 12 km. Additionally, Fig. 3(b) presents the received eye diagrams, after various distances with optimum offset filtering.

Additionally, it can be observed that the additional use of DFE (5, 2) offers a significant transmission performance improvement by 6 dB in terms of ROSNR at 25 km, which increases for longer transmission distances. It is noted that the same value of ROSNR (26 dB) is needed both for a BER of 10^-9 in the back-to-back case without equalizer and for 25 km with the use of equalization. On the other hand, the BER performance is always worse than 10^-9 if only equalization is used without optical offset filtering as it can be seen in Fig. 1(b) (top).

In the measurements obtained next, the performance of single fiber bidirectional transmission is compared with the case of bidirectional transmission where separate fibers are used for upstream and downstream propagation. More specifically, Fig. 4(a) presents a comparison (in terms of BER versus OSNR) between the use of two separate fibers for up-and downstream transmission of lengths L₁ = L₃ = 25 km and the use of only one bidirectional fiber of length L₂ = 25 km. When one bidirectional fiber is used, then the ROSNR to achieve a BER of 10^-9 increases by 5 dB with respect to the case where two separate fibers are used and without considering equalization. However, with the introduction of DFE, this difference drops to 2 dB. When only one fiber is used without equalizer, there is an error floor caused, due to RB effects, which is partially eliminated when DFE equalization is applied, resulting in better ROSNR values. It is important to note that the required OSNR for all the BER values of the downstream signal is always more than 3 dB lower than the upstream case reported here. It can be also observed that when DFE is used, the linear tendency of the BER curves shows that no error floor is reached at lower BER values, which is not the case when only one fiber is employed without applying DFE.

In Fig. 4(b), the limit of the downstream transmission is observed. In this case, separated down- and upstream fiber spans, of length L₁ = L₃ = 25 km are used, with an additional span of a bidirectional fiber of L₂ = 12 km. The ROSNR to achieve a BER of 10^-9 is only 0.7 dB lower for the downstream (without DFE) than for the upstream transmission with DFE. This shows that the performance of upstream transmission in terms of ROSNR is comparable to that of downstream signal only after a total transmission distance close to 37 km (L₁+L₂), where the CD effect in combination with OSNR and the low ER of 1.8 dB limits the downstream signal quality that is detected at the ONU.

Finally, it is noteworthy to mention that, as shown in [5], it is possible to achieve a BER of 10^-9 with PRBS of 2³¹-1 when only upstream transmission is performed. However, for the measurements shown here, a PRBS of 2⁷-1 was used, as the generated patterning effects occurred after the strong BW enhancement done (with the electrical filter and the optical filter) to reach 10 Gbps operation with an RSOA of 1.2 GHz BW (biased at 60 mA) prevented the use of long PRBS. These patterning effects are associated with the combined effect of the consequent low frequency reduction (observed in Fig. 2(a)), the electronics cut-off frequency and the interference of the downstream signaling stamped in the upstream signal (which has only 6.2 dB of available ER). From a practical point of view, the chosen PRBS of 2⁷-1 is equivalent to Ethernet PON (EPON) systems’ eight to ten bit (8B10B) coding.

It is expected that new RSOAs are going to be developed in the near future, with slightly better BW like the one used in [6]. To achieve error-free full duplex bidirectional 10 Gb/s operation with a PRBS of 2³¹–1 will be feasible by using these RSOAs and applying the techniques explained in this paper.

4. Conclusions

This work has experimentally demonstrated, error free symmetrical 10 Gb/s full-duplex bidirectional transmission over 25 km by remodulation of the downstream signal by a strongly limited BW RSOAs (~1.2 GHz). It is required an OSNR of only 26 dB for a BER of 10-9 when combined with optimum offset filtering and DFE techniques at the OLT receiver. This method provides a path to upgrade next generation PONs to 10 Gb/s using low-cost RSOA-based ONUs that are nowadays rated for 1.25 Gb/s operation.