In this paper, we present a novel Rayleigh backscattering (RB) noise mitigation scheme based on central carrier suppression for 10 Gb/s loop-back wavelength division multiplexing passive optical network (WDM-PON). Microwave modulated multi-subcarrier optical signal is used as downstream seeding light, while cascaded semiconductor optical amplifier (SOA) are used in the optical network unit (ONU) for suppressing the central carrier of the multi-subcarrier upstream signal. With central carrier suppression, interference generated by carrier RB noise at low frequency region is eliminated successfully. Transmission performance over 45 km single mode fiber (SMF) is studied experimentally, and the optical-signal-to-Rayleigh-noise-ratio (OSRNR) can be reduced to 15 dB with central carrier suppression ratio (CCSR) of 21 dB. Receiver sensitivity is further improved by 6 dB with the use of microwave photonic filter (MPF) for suppressing residual upstream microwave signal and residual carrier RB at high frequency region.
© 2014 Optical Society of America
Due to the rapid growth of triple play service and bandwidth-hungry applications on the Internet, various structures of optical access network have been proposed to realize fiber to the home (FTTH) service. Among various optical access networks, wavelength division multiplexing passive optical network (WDM-PON) is regarded as one of the most promising scheme to accord with the rapid growth of bandwidth requirement . Colorless ONU (optical network unit) is the key element to realize a cost-effective and simple maintenance WDM-PON system , due to its simple structure and centralized control architecture. Nowadays, different optoelectronic devices have been used in colorless ONU to realize loop-back WDM-PON structure, such as reflective semiconductor optical amplifier (RSOA), reflective electro-absorption modulator (REAM), and Fabry-Perot laser diode (FP-LD) [3–6]. With a loop-back structure in colorless ONU, all the light sources can be centralized in OLT (optical line terminal). Through delivering centralized wavelengths to specific ONU, upstream data can be modulated on those wavelengths and transmitted back to OLT for receiving. However, when a single fiber is used for bidirectional transmission in loop-back structure, Rayleigh backscattering (RB) noise is inevitable and induces serious interference in upstream direction . This drawback directly limits both the transmission distance and receiver sensitivity.
There are two kinds of RB noise in single fiber loop-back structure . They are the carrier RB noise and the signal RB noise. Carrier RB noise is generated from beating between the upstream signal and the reflected downstream signal of the same wavelength. Since beating occurs at the baseband signal, carrier RB noise is located mainly at the low frequency region. Thus, carrier RB noise can be reduced by suppressing the low frequency portion of the upstream signal. Signal RB noise comes from the loop-back upstream signal which is modulated by the colorless ONU again. Due to the re-modulation at the ONU, the signal RB noise has a wider electrical spectrum than carrier RB noise and the signal RB noise overlaps with the upstream data over the whole frequency band . Various approaches have been demonstrated to effectively suppress carrier RB noise. For example, the use of destructive port in Mach-Zehnder delay interferometer (MZDI) for demodulating upstream differential phase-shift keying (DPSK) signal that results in 19 dB tolerance enhancement of carrier RB noise , utilizing electrical filtering effect (high-pass filter and DC-block) or proper signal coding format [10–12] such that low frequency interference is successfully suppressed and tolerance to signal-to-crosstalk ratio is enhanced. For signal RB noise mitigation, a high-gain non-linear semiconductor optical amplifier (NL-SOA) with low input saturation power has been used in loop-back ONU . Meanwhile, many schemes focused on simultaneously mitigating signal and carrier RB noise, including using phase modulation for spectral broadening and wavelength shift amplitude-shift keying (ASK) modulation [14, 15], single-sideband carrier suppression scheme generated by a dual parallel Mach-Zehnder modulator (DP-MZM) in ONU [16,17], and four-wave mixing (FWM) effects [18,19].
In principle, both carrier RB noise and signal RB noise reduce receiver sensitivity in the upstream direction. However, it is worth noting that carrier RB noise plays a more important role than signal RB noise in low ONU gain situation after long distance transmission . In our system, because of the cascaded-SOA structure is operating in deep gain saturation, thus, gain provided by the ONU is reduced at this saturation state. Therefore, the main objective of this paper focuses on suppressing carrier RB noise in a low ONU gain situation. The carrier RB noise located mainly near DC region and its bandwidth is decided by the line-width of the downstream seeding light that is in the order of several MHz. Moreover, the use of non-return-to-zero (NRZ) signal results in interference at low frequency region . Therefore, suppressing low frequency component of the received signal enhances RB noise tolerance in upstream direction. In this paper, we propose and demonstrate a carrier RB noise mitigation scheme in 10 Gb/s loop-back WDM-PON. Our scheme utilizes high gain saturation effect of cascaded-SOA in ONU, such that carrier RB noise is suppressed by inducing central carrier suppression effect at the subscriber side. A 21.7 dB of central carrier suppression ratio (CCSR) is achieved which in turns eliminates low frequency interference effectively. With the use of a two-tap microwave photonic filter (MPF) in the OLT side, residual microwave signal and carrier RB noise at high frequency are suppressed. With the proposed scheme, we experimentally demonstrated approximately 45-km single mode fiber (SMF) transmission with optical-signal-to-Rayleigh-noise-ratio (OSRNR) of 15 dB, receiver sensitivity of −30 dBm, and CCSR of 21.7 dB.
2. Principle and system setup
Principle of the proposed carrier RB noise mitigation scheme is illustrated in Fig. 1. When continuous wave (CW) seeding light is used in downstream direction, upstream signal and the reflected downstream signal will interfere and beat in the low frequency region, as illustrated in the RF spectrum in Fig. 1. A thicker red arrow is used to indicate upstream signal modulation, while a thin black arrow is used to represent the carrier RB noise. In order to suppress this low frequency interference, microwave frequency f GHz is modulated onto the downstream wavelength to generate a multi-subcarrier downstream signal. Owing to the gain saturation effect of cascaded-SOA, central carrier of the upstream signal is suppressed and this avoids interfering with the reflected downstream central wavelength at low frequency region. However, interference of the RB noise with other subcarriers is increased and this interference appears at frequencies that are the integer multiples of f GHz (e.g. f GHz, 2f GHz). Thus, a two-tap MPF with periodical notches at f GHz can be used to suppress those enhanced high frequency interference as well as the residual f GHz microwave signal in the upstream direction.
The proposed loop-back WDM-PON system is shown in Fig. 2. In single fiber bidirectional transmission system, Rayleigh backscattering noise can be reduced by using different wavelengths in upstream and downstream directions. However, this approach increases both the cost and complexity for operation and maintenance. In our system, the downstream seeding lights are generated by the CW laser sources named LD 1 to LD n. All the downstream seeding lights are multiplexed at the arrayed waveguide grating (AWG) and are then modulated by an MZM with a fixed frequency at f GHz. A modulated signal with multi-subcarrier is generated. After transmitting through a 45-km SMF, the downstream seeding lights are de-multiplexed at the remote node (RN) by an AWG and are sent to the corresponding ONU. A 0.8 km SMF is connected between RN and ONU.
In the ONU, the seeding light is first passed through the cascaded-SOA for central carrier suppression, and then the upstream data is modulated onto the carrier-suppressed signal by an MZM. With the help of photonic integrated circuit (PIC) technology, 10Gb/s low cost MZM can be monolithically integrated with SOA easily. This monolithically photonic integrated circuit improves the system cost, size and power consumption to bring better support for realizing cost-effective optical access network [21,22]. The upstream signal is then transmitted back to the OLT via an optical circulator for receiving. In order to suppress the residual f GHz microwave signal and the increase RB interference at f GHz, the upstream signal is first launched to a microwave photonic filter that has a deep notch at f GHz before detecting with a photodiode (PD). Due to the wavelength independent characteristic of MPF, all upstream channels can share this MPF to remove residual interference noise without increasing system cost. Unlike MZM based carrier suppression scheme that is sensitive to bias drifting, our carrier suppression scheme utilizes gain saturation effect in SOA and is more stable over time and less sensitive to environmental temperature change. Furthermore, polarization independent central carrier suppression approach can be realized if polarization independent SOA is used in our system.
3. Experiments and results
When cascaded-SOA is used to realize central carrier suppression, the resultant central carrier suppression ratio (CCSR) depends on the gain saturation effect of SOA. Compared to single stage SOA, the overall gain saturation effect is strengthened by the cascaded-SOA structure . It has been shown that a better high-pass filtering effect can be achieved by the strengthened gain saturation effect in cascaded-SOA . In our scheme, we make use of this phenomenon such that a high suppression ratio for baseband frequency of the microwave-modulated signal is achieved. Therefore, central carrier of the multi-subcarrier optical signal is suppressed effectively after passing the cascaded-SOA.
To evaluate the high-pass filtering effect, we measure the optical-to-optical frequency response curve of cascaded-SOA under different input optical power and use single stage SOA for comparison, as shown in Fig. 3(a). The bias currents for SOA 1 and SOA 2 in cascaded-SOA structure are 280 mA and 380 mA, respectively. In single stage SOA, the bias current is fixed at 380 mA. As shown in Fig. 3 (a), the low frequency suppression ratio is enhanced significantly as the input optical power increases from −12 dBm to −4 dBm. When the input optical power is at −4 dBm, baseband suppression ratio is over 45 dB, which provides a high suppression ratio for the central carrier. Compared to single stage SOA, baseband suppression ratio is improved by ≥ 25 dB under the same input power (−4 dBm). Since the total gain saturation effect is enhanced by the cascaded-SOA, carrier lifetime of SOAs are reduced which corresponding to a deeper filtering curve in low frequency region. In our experiment, the downstream signal power is fixed around 1dBm and the input optical power to the cascaded-SOA is about −8 dBm after transmitting through the 45 km SMF. According to Fig. 3 (a), a baseband suppression ratio of 25 dB can be achieved.
The comparison of optical spectra between downstream and upstream signal is shown in Fig. 3 (b). Modulation frequency of the radio frequency (RF) source in OLT is fixed at 10 GHz, and the upstream data is a 10 Gb/s 231-1 pseudo random binary sequence (PRBS) signal. Compared with the original downstream multi-subcarrier signal, central carrier of the upstream signal is suppressed by 21.7 dB through the use of cascaded-SOA in ONU.
Figure 4 (a) and 4(b) are used for measuring OSRNR and BER performances, respectively. In Fig. 4 (a), CW light at 1551.02 nm and optical power of 13 dBm is used as the downstream light. A 10 GHz microwave signal is modulated onto this CW light through an MZM as a multi-subcarrier downstream seeding light. The downstream signal is split and combined using two optical couplers to mimic beating interference between the carrier RB noise and the upstream signal. In the lower branch, carrier RB noise is generated by launching the signal into a 45-km SMF through an optical circulator. The carrier RB noise is amplified by erbium-doped fiber amplifier (EDFA), while the corresponding amplified spontaneous emission (ASE) noise is filtered by an optical band pass filter. Since beating is a polarization sensitive phenomenon, a polarization controller is used to maximize the beating between the downstream and upstream signal. In the upper branch, we use cascaded-SOA structure to generate the central carrier suppressed signal. SOA 1 is a linear optical amplifier (LOA) and SOA 2 is a nonlinear SOA (NL-SOA). The LOA is mainly for linear amplification of the input signal and NL-SOA is mainly for gain saturation effect. A 10 Gb/s 231-1 PRBS upstream signal is modulated onto the carrier suppressed signal using an MZM. Optical spectrum of the generated upstream signal is shown by the red curve in Fig. 3 (b).
To further improve the system performance, a MPF is placed before the receiver to suppress the residual microwave carrier and RB noise at around 10 GHz. The MPF structure is shown in the inset i of Fig. 4(a), which consist of two polarization beam splitters (PBSs) and fiber delay lines. The path length difference (L1 – L2) is fixed at 50 ps which corresponding to a 10-GHz notch filter in radio frequency domain. The MPF provides over 40 dB of suppression at 10 GHz (inset ii of Fig. 4(a)). MPF can be made to be polarization insensitive  by utilizing single-mode to multi-mode combiner  instead of PBS.
Using the experimental setup as shown in Fig. 4(a), we also study both the low frequency and high frequency beat noise in our system experimentally. For comparison, we first investigate the beating effect between a CW seeding light (upper branch) and the RB noise (lower branch) without applying any RB noise mitigation scheme (i.e. without 10 GHz modulation using the MZM and cascaded-SOA). Red curve in Fig. 5(a) is the measured RF spectrum showing the level of beat noise at low frequency range from 0 GHz to 0.2 GHz. With our RB noise mitigation scheme, the low frequency beat noise is shown by the green curve in Fig. 5 (a). CCSR of the carrier-suppressed signal is 21.7 dB with saturation output power at 10 dBm. Power of the carrier RB noise is −12 dBm that corresponds to 22 dB of OSRNR. Compare with the case when CW seeding light is used, low frequency interference from 0 GHz to 0.2 GHz is suppressed efficiently using the proposed central carrier suppression scheme. Our central carrier suppression scheme significantly reduces the power of the central carrier (CCSR of 21 dB), resulting in a suppression of low frequency interference. On the other hand, due to the high-pass filtering effect of cascaded-SOA as shown in Fig. 3 (a), high frequency carriers of the multi-subcarrier signal are enhanced by the cascaded-SOA. Thus, interference at high frequency region is increased after passing through the cascaded-SOA, especially at 10 GHz where the modulation frequency is located. Figure 5 (b) shows the measured RF spectra at 10 GHz. Without applying RB noise mitigation scheme (i.e. CW seeding light is used without 10 GHz modulation and cascaded-SOA), lowest noise level at 10 GHz is achieved as shown by the red curve. When multi-subcarrier signal is used to realize central carrier suppression, high frequency interference occurs and is amplified, resulting in a strong 10 GHz RF tone with power above −20 dBm (blue curve). With the utilization of MPF with a transmission notch at 10 GHz, the residual 10 GHz RF signal is suppressed by 30 dB (black curve).
Figure 6 (a) shows the measured power penalty of the upstream data at BER of 10−9 as a function of OSRNR by using the experimental setup depicted in Fig. 4 (a). When CW light is used as the downstream light without data modulation, power penalty at BER of 10−9 is 1 dB as OSRNR reaches 22 dB (green inverted triangle curve). At this point, the receiver sensitivity is limited by the severe RB noise. With the proposed central carrier suppression scheme, error free transmission with no power penalty is achieved at OSRNR of 19.5 dB and CCSR of 8 dB (red circle curve). The RB noise tolerance is improved greatly by further increasing the CCSR value. When CCSR reaches 21 dB, error free transmission with no power penalty is achieved when OSRNR is 15 dB. Compared with the power penalty at 8 dB CCSR, RB noise tolerance is improved by 4.5 dB when CCSR is at 21 dB. That is to say, low frequency interference is reduced with an improvement in upstream RB noise tolerance through the increase of central carrier suppression. Through using the experimental setup illustrated in Fig. 4 (b), the corresponding upstream BER performance is measured as shown in Fig. 6 (b). When CW light is used without modulation, error floor is observed in the BER measurement of the upstream signal and the corresponding eye diagram is shown in Fig. 6 (b) i. For an upstream signal with CCSR of 21 dB, receiver sensitivity of −24 dBm is obtained (without using MPF). The corresponding eye diagram is shown in Fig. 6 (b) ii. Noise is found in the eye diagram due to the presence of residual downstream 10 GHz signal. With the use of MPF for suppressing the residual 10 GHz signal and residual RB noise, receiver sensitivity is improved by 6 dB. A widely open eye diagram is resulted as shown in Fig. 6 (b) iii.
In this paper, we have investigated carrier RB noise suppression performance by realizing central carrier suppression with cascaded-SOA. Our scheme is easy to realize and is suitable for enhancing loop-back WDM-PON. The MZM used for generating multi-subcarrier source, microwave source, and the MPF in OLT can be shared by all subscribers to reduce system cost. The proposed central carrier suppression scheme is based on gain saturation effect in SOA, which is stable and immune to bias voltage drift – more tolerate to environment changes. Compared with using CW light as the seeding light, RB noise tolerance is improved with the use of a multi-subcarrier light. Interference between the carrier RB noise and the upstream signal at low frequency region is mitigated successfully by central carrier suppression. Experimental results show that CCSR of 21 dB is achieved with −8 dBm input optical power and the OSRNR is reduced to 15 dB. Improvement in OSRNR is resulted from an increase in CCSR. BER performance is improved by 6 dB with the use of a microwave photonic filter in OLT, which suppresses residual microwave signal and residual RB noise at the high frequency region.
The work was jointly supported by the National Nature Science Fund of China (No. 61271216, No. 61221001, No. 61090393 and No. 60972032), the National “973” Project of China (No.2010CB328205, No. 2010CB328204 and No. 2012CB315602) and the National “863” Hi-tech Project of China and the China Scholarship Council (No.3009).
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