1. Introduction

With the rapid growth of big data service and network coverage, optical access network has been developed to meet the rapidly growing demand. Existing passive optical networks using time division multiplexing (TDM) technology (i.e. Ethernet passive optical network (EPON), and gigabit passive optical network (GPON)) have a limited bandwidth due to the number of time slots that can be allocated to users [1]. Wavelength division multiplexing passive optical network (WDM-PON) is considered as one of the most promising schemes for future optical access network [2,3] due to the wide bandwidth and high reliability it offers. Different wavelength is used to differentiate each user and every subscriber can occupy the entire bandwidth of the assigned wavelength. Using colorless optical network unit (ONU) with single fiber loop-back structure in WDM-PON, maintenance is simplified and system complexity is reduced, because wavelength generation and control for ONU are all centralized at the optical line terminal (OLT) [4]. However, when bi-directional transmission in a single fiber is used, Rayleigh backscattering (RB) noise is severe and inevitable in the upstream direction [5–7]. Both the upstream receiver sensitivity and transmission distance are limited by the severe Rayleigh backscattering noise. To solve this problem, researchers use various approaches to suppress the Rayleigh backscattering noise, such as utilizing electrical filtering with DC-block [8] and high pass filter to suppress the interferometric noise at low frequency range [9,10], using central carrier suppression [11] or single side-band modulation [12] to avoid wavelength overlap in bidirectional transmission, and special signal coding to avoid optical beating noise [13,14].

In this paper, we propose and experimentally demonstrate a real-time heterodyne receiver for Rayleigh backscattering noise suppression in loop-back WDM-PON. For long distance transmission and low optical gain in ONU, carrier RB noise will be more dominant than signal RB noise [5]. Thus, we focus on mitigating the carrier RB noise in this paper. Although homodyne detection has more advantages than heterodyne detection [15], the requirement of homodyne detection is stricter than heterodyne detection, such as the need of narrow linewidth local oscillator, optical phase-locked loop (OPLL), complex optical hybrid and balanced photo detectors. While in the heterodyne detection we proposed, cost-effective distribute feedback laser (DFB) can be used as a local oscillator and a 50:50 optical coupler is used to combine the local oscillator and the optical signal. With the implementation of envelope detection in our scheme, standard photodiode is used in replace of a balanced detector and an OPLL. The proposed real-time heterodyne receiving is based on heterodyne beating to obtain the signal envelope and negative-slope transfer function of electro-absorption modulator (EAM) is exploited for Rayleigh noise suppression. In our scheme, due to the strong optical power of the local oscillator, the converted electrical power is increased dramatically that greatly improve the receiver sensitivity. In the meanwhile, the accumulated carrier RB noise is suppressed effectively by the same EAM due to the nonlinear negative-slope transfer function. This hybrid optical-electrical heterodyne receiver can be easily integrated using photonic integrated circuit (PIC) technology for cost reduction. In our experiment, we successfully perform 5 Gb/s real-time heterodyne detection in a loop-back WDM-PON with 45 km single mode fiber (SMF) transmission. The optical-signal-to-Rayleigh-noise-ratio (OSRNR) is also analyzed with different reverse voltage applied to the EAM. Compared with direct detection, our proposed scheme has higher receiver sensitivity and better tolerance to carrier RB noise.

2. Principle of carrier RB noise suppression and heterodyne detection

Working principle and experimental setup of our proposed carrier RB noise suppression scheme with heterodyne detection are shown in Fig. 1(a) and 1(b), respectively. Through heterodyne beating at the photodiode (PD 1), electrical bipolar signal that consists of high-frequency carrier is generated, while signal information is represent by the envelope. Carrier RB noise is phase-induced amplitude noise generated by upstream signal and reflected back optical carrier which may directly affects ‘mark’ symbol [9, 16]. For heterodyne detection, every ‘mark’ symbol is converted to positive and negative peaks of bipolar signal. So in each ‘mark’ level of the bipolar signal, the accumulated carrier RB noise mainly locates at the upper portion of both the positive and negative peaks, as indicated by the red regions in Fig. 1 (a). The amplified electrical bipolar signal is used to modulate an EAM for converting it into an optical single polar signal, due to the cut-off characteristic of the EAM transfer function at strong reverse bias voltage. Moreover, the transfer function of the EAM also has a flat region at low bias voltage, which corresponds to only 1 dB power deviation. Thus, the accumulated interferometric noise is suppressed by aligning the signal noise region to the zero-slope region. Figure 1(b) depicts the experimental setup for real-time heterodyne detection with carrier RB noise suppression. A low-cost DFB laser runs at wavelength λ₂ is used as the local oscillator. After passing through a 30:70 optical coupler, 30% of the local oscillator optical power is combined with the upstream data (λ₁) at a 50:50 optical coupler. Beating between the local oscillator and the upstream data happens at the PD 1, while 70% of the local oscillator power is used as the optical carrier for electrical-to-optical conversion at the EAM. Bipolar signal is generated after heterodyne beating at the PD 1, which is then amplified by a radio frequency amplifier (RF AMP). The amplified bipolar signal together with a negative DC bias voltage is used to drive the EAM via a bias-tee. Through modulation at the EAM, the bipolar signal is converted to a single polar signal and RB noise is also removed. The clean optical signal is detected by PD 2, and an RF low pass filter (LPF) is used to remove the high frequency RF carrier while retrieving the low frequency envelope of the single polar signal.

 

Fig. 1 (a) Principle of Rayleigh backscattering noise suppression; (b) Setup of real-time heterodyne receiver. PD: photodiode, RF AMP: radio frequency amplifier, PC: polarization controller, EAM: electro-absorption modulator, LPF: low pass filter.

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3. Experimental setup and results

With the experimental setup as shown in Fig. 1(b), we experimentally measured the performance of our proposed scheme. Optical spectrum of the local oscillator and the upstream data are shown in Fig. 2(a). Wavelengths of the received upstream signal and local oscillator are at 1550.46 nm (λ₁) and 1550.545 nm (λ₂), respectively, with a wavelength spacing of 85 pm (corresponding to 10.572 GHz). In order to avoid waveform distortion induced by the RF amplifier, the modulation data rate is adjusted to 5 Gb/s. A strong local oscillator is used to significantly improve receiver sensitivity during heterodyne detection. The captured time domain waveforms are shown in Fig. 2(b). A 5 Gb/s electrical signal with bit sequence of “10010110” (Fig. 2(b) 2(i) is modulated on wavelength λ₁ and acts as the upstream data for detection. After beating with the local oscillator at wavelength λ₂, a bipolar signal is generated which contains a 10.572 GHz carrier signal (Fig. 2(b) 2(ii). Through half wave rectification at the EAM [17], the electrical bipolar signal is converted to a single polar signal (Fig. 2(b) 2(iii). To remove the 10.572 GHz carrier inside the single polar signal, a RF low pass filter with 5 GHz cut-off frequency is used such that the bit sequence is successfully recovered (Fig. 2(b) 2(iv).

 

Fig. 2 (a) Optical spectrum of the upstream signal and the local oscillator; (b) Waveforms during heterodyne detection.

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As mentioned earlier, the suppression of carrier RB noise in our approach is based on the nonlinear negative-slope transfer function of EAM. The transfer function of the EAM is experimentally measured as shown in Fig. 3(a). Electro-absorption effect is significant under strong negative bias voltage, while the effect is extremely weak when the bias voltage is low or when it is slightly positive. Thus, there is a flat region at around zero bias voltage, which is used for RB noise removal. To better understand the noise suppression performance of the proposed real-time heterodyne receiver, the electrical modulation signal is disabled while the OSRNR is kept constant at 15 dB by fixing the optical power. Since carrier RB noise is mainly located at the low frequency region in electrical domain, we examine the low frequency spectral response (from 0 MHz to 50 MHz) by connecting an electrical spectrum analyzer (ESA) to PD 2 in Fig. 1(b). The measured frequency spectra at different EAM bias voltage are shown in Fig. 3(b). In Fig. 3(b), the black curve is the frequency response with the EAM bias voltage at 0 V. At this point, EAM is working in the zero-slope region, thus, noise floor of the black curve is lower than both the red and blue curves, which corresponds to bias voltage at −1V and −2 V. That is to say, noise mitigation performance at the zero-slope region is better than at the linear modulation region. As the reverse bias voltage increases (more negative), the noise floor begins to drop. When the bias voltage is set to −4 V, frequency response curve is at its lowest level. This is because insertion loss of the EAM is increased significantly at high reverse bias voltage, and the received power at PD 2 is very low.

 

Fig. 3 (a) The modulation transfer function of EAM; (b) Low frequency response of EAM.

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To evaluate the carrier RB noise mitigation performance of the proposed real-time heterodyne receiver. OSRNR is measured using the setup in Fig. 4(a). An interferometer structure is used to generate carrier RB noise. CW light at 1550.46 nm with 13 dBm output power is split into two branches by a 50:50 optical coupler. The upper branch light is modulated by a 5 Gb/s 2¹⁵ −1 pseudo-random binary sequence (PRBS) signal, while the lower branch light is launched to a 45-km single mode fiber via an optical circulator. Carrier RB noise is generated at the 45 km SMF and is propagated in the counter direction back to port 2 of the optical circulator. A polarization controller (PC) is used in the lower branch to maximize the beating effect. We use a variable optical attenuator (VOA) in the upper branch to control the power ratio between the signal and the carrier RB noise. The generated carrier RB noise is combined with the data signal by the second 50:50 optical coupler which is then detected by the proposed real-time heterodyne receiver. Figure 5(a) shows the measured power penalty at 1 × 10⁻⁹ under different OSRNR value. With heterodyne detection, error floor is observed while bias voltage is set to −1 V as shown in pink down triangular curve. By decreasing the bias voltage to −2 V, the error free transmission is achieved and OSRNR is 17.8 dB (blue triangular curve). Then, through further decreasing the bias voltage to −4 V, OSRNR arrives at 15.6 dB and the tolerance of carrier RB noise increases about 2.2 dB as shown in black square curve. It means the carrier RB noise mitigation performance is improved by biasing EAM at high reverse voltage. We also compare the performance: when direct detection is used, the OSRNR of error free transmission with no power penalty is 18.8 dB. That is to say, the total RB noise tolerance is improved by 3.2 dB using real-time heterodyne receiver in single fiber loop-back WDM-PON.

 

Fig. 4 (a) Experimental setup of OSRNR test and (b) back to back BER test. LD: laser diode, PC: polarization controller, MZM: Mach-Zehnder modulator, VOA: variable optical attenuator, SMF: single mode fiber, APC: angled physical contact, AWG: arrayed waveguide grating, OC: optical circulator, OA: optical amplifier.

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Fig. 5 (a) The measured OSRNR curves at different bias voltage and (b) Back to back BER results. Filled markers represent PRBS of 2¹⁵-1 and hollow markers represent PRBS of 2⁷-1.

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BER measurement is performed with the setup as shown in Fig. 4(b). In OLT, the downstream seeding light is generated by a laser diode (LD) and multiplexed by an arrayed waveguide grating (AWG), it is then delivered to different ONU through the transmission fiber. The lengths of the feeder fiber and distribution fiber are 45 km and 0.8 km, respectively, In ONU, we use an optical amplifier (OA) and Mach-Zehnder Modulator (MZM) with optical circulator to build a loop-back structure. The upstream signal is transmitted back to the OLT, it is then de-multiplexed by an AWG and detected by the proposed real-time heterodyne receiver. The corresponding BER results are shown in Fig. 5(b). While using direct detection with standard photodiode, error floor is observed at BER of 10⁻⁷ (pink down triangular one), the eye diagram is also appears to be very unclear (inset iii). With the proposed heterodyne receiver, receiver sensitivity of −25 dBm is achieved and a clear eye diagram is resulted as shown in inset i. If forward error correction (FEC) is applied, receiver sensitivity can be further improved to −30 dBm.

4. Conclusion

In this paper, we have experimentally realized a Rayleigh backscattering noise mitigation scheme based on the nonlinear negative-slope transfer function of EAM and real-time heterodyne receiver in loop-back WDM-PON. Compared with other noise suppression schemes based on optical/electrical filtering [18], especially the digital signal processing (DSP) technology [19], our approach can realize low cost real-time heterodyne detection with simple structure which is more appropriate for upstream transmission in loop-back WDM-PON. Both complicated off-line signal processing and expensive high speed analog-to-digital and digital-to-analog converters (ADC/DAC) are no longer needed. Thanks to the nonlinear negative-slope transfer function of EAM, the accumulated carrier RB noise at low frequency range is suppressed by the zero-slope region. Experimental results show that carrier RB noise tolerance is improved by 3.2 dB by biasing the EAM at −4 V. OSRNR is 15.6 dB with no power penalty at 10⁻⁹ under the same EAM driving condition. BER results show error free transmission is achieved with receiver sensitivity of −25 dBm.