1. Introduction

Passive optical networks (PONs) have being triggering tremendous interest for being considered as an economic and future-proof strategy to meet the rising demand of bandwidth-hungry multi-media applications in multi-user access networks. Optical orthogonal frequency-division multiplexing (OFDM) is a promising multi-user access technology due to its high spectrum efficiency and robustness to chromatic dispersion and it has been extensively used in PON systems [1–3]. In a typical direct-detect OFDM-PON system, radio frequency (RF) upconversion is a necessary process and it creates a frequency guardband between the DC component and the OFDM sideband, thus preventing the transmitted signal from significant deterioration by non-coherent mixing products [4].

Since cost-effectiveness is one of the most attractive advantages of the PON system, many investigations have been done about reducing the cost of OFDM-PON system, such as signal remodulation technique using electroabsorption modulator (EAM), reflective semiconductor optical amplifier (RSOA) or injection-locked Fabry–Pérot laser diode (FP-LD) [5–7] and ONU-side optical source delivering technique [1, 8]. These techniques are mainly aimed to make each ONU free of optical source in the optical domain, while RF source or a broadband ADC/DAC is needed for each ONU in a typical up-converted OFDM-PON system and it inevitably aggrandizes the ONU-side cost for the commercial RF source is quite expensive, let alone the broadband ADC/DAC. Ref [9]. has proposed that coherent ONU-side detection with low-speed ADCs/DACs/DSP and off-the-shelf RF components can address the optical signal to noise ratio (OSNR) and high-speed electronics challenges, but this approach is still relatively complicated and expensive. To the best of our knowledge, no research focusing on reducing the RF source cost in each ONU using polarization multiplexing (PolMUX) has ever been reported and it is worth further study. As an effective solution to further increase spectrum efficiency, PolMUX has been widely employed in the transmission of high-speed and large-capacity OFDM signals [10–12]. Consequently, applying PolMUX to the up-converted OFDM-PON system offers the possibility to lower the cost of RF source in each ONU, and it results in novel source-free ONUs both in the optical and the electric domain, thus with enhanced cost-efficiency and off-the-shelf polarization components it can provide the implementation of an extremely cost-effective optical OFDM-PON system.

In this work, a novel cost-effective bidirectional up-converted OFDM-PON system with source-free ONUs utilizing PolMUX is proposed and successfully verified. With downlink two pure optical bands and up-converted OFDM signal carried in two orthogonal states of polarization (SOPs), an available RF source can be obtained and then employed to facilitate the downconversion of downlink OFDM signal and the upconversion of uplink OFDM signal. Only one single 10 GHz RF source is required at the OLT in the whole system and with the help of the 10 GHz RF source generated in the ONU, downstream (DS) 10 Gb/s 16 quadrature amplitude modulation (QAM)-OFDM signal and upstream (US) 8 Gb/s 16 QAM-OFDM signal are successfully transmitted for back-to-back (B2B) and 20 km standard single mode fiber (SSMF), respectively. With US optical carrier suppression (OCS) modulation and coherent detection (CD), the impact of the Rayleigh backscattering (RB) effect on the transmission performance of the proposed system is mitigated [1, 13].

2. Principle of the proposed optical OFDM-PON system

Figure 1 depicts the principle of our proposed optical OFDM-PON system. In the OLT, a continuous-wave (CW) optical source λ₁ is firstly split into two streams and after passing a polarization controller (PC) each stream is used to drive an intensity modulator (IM). After passing through two optical filters (OFs) at the IM outputs, two single sideband (SSB) signals are generated. One is modulated by a 10 GHz up-converted OFDM signal while another is modulated by a pure 10 GHz sinusoidal wave, and the two modulated SSB signals are combined by a polarization beam combiner (PBC), placing them on two SOPs. Then another CW optical source λ₂ with the pure SSB signal is coupled together with the two SSB signals, and the coupled signals are launched into fiber after a circulator. The corresponding frequency-domain description of the downlink transmitted signal generation is shown in the inserts (a), (b), (c) and (d) of Fig. 1. In the ONU, the λ₂ band is filtered out of the received signals by an OF and it is used as the US CW optical source. The two SSB signals are next separated by a polarization beam splitter (PBS) and each enters into a photodiode (PD) for direct detection. The SSB signal modulated by a pure 10 GHz sinusoidal wave generates a pure 10 GHz RF source, and another SSB signal modulated by 10 GHz up-converted OFDM signal generates the DS 10 GHz RF-OFDM signal after PDs. The extra beating terms caused by polarization rotation could lead to certain degradation at the DS PDs and in order to eliminate the degradation, these terms are confined to the DC region and the 20 GHz region and so would fall outside of the PD bandwidth. Thus utilizing the generated 10 GHz RF source, the DS RF-OFDM signal is down-converted to the baseband OFDM signal while the US baseband OFDM signal is up-converted to 10 GHz RF-OFDM signal. Via an IM, the US 10 GHz RF-OFDM signal is modulated on the US optical source and then launched into fiber after a circulator. The US OFDM signal is coherently detected in the OLT with the help of the λ₂ laser and the pure 10 GHz sinusoidal wave. In the whole optical OFDM-PON system, only one single 10 GHz RF source is adopted. The principles of baseband OFDM modulation and de-modulation blocks in Fig. 1 are shown in Figs. 2(i) and 2(ii), respectively. For the baseband OFDM modulation, serial input data is firstly converted to parallel. After using QAM to map bit stream data onto each OFDM subcarrier, the modulated subcarriers are then presented as an input of the inverse fast Fourier transform (IFFT). The cyclic prefix (CP) is added to the output of the IFFT and the parallel subcarrier are converged by parallel to serial conversion, thus producing an I/Q output. As to the baseband OFDM de-modulation, the I/Q input is firstly converted to parallel and the CP is removed. After the fast Fourier transform (FFT), the output of FFT is equalized. Then each OFDM subcarrier is de-modulated and converted to serial and the final serial output data is generated.

 

Fig. 1 Principle of the proposed bidirectional up-converted optical OFDM-PON system and the corresponding frequency-domain description of the downlink transmitted signal generation. C: combiner, S: splitter.

Download Full Size | PDF

 

Fig. 2 Principles of the baseband OFDM modulation (i) and de-modulation (ii) in Fig. 1.

Download Full Size | PDF

3. Experimental setup, results and discussion

Figure 3 illustrates the detailed experimental setup of our proposed bidirectional up-converted OFDM-PON system. In the OLT, a narrow-linewidth distributed feedback (DFB) laser at λ₁ = 1549.60 nm with 80 kHz linewidth and 10 dBm launch power is employed as the DS CW optical source and it is split into two streams and each enters into an IM after passing a PC. A 10 Gb/s baseband 16 QAM-OFDM signal is generated off-line with the FFT size N = 128, 7% forward error correction (FEC) overhead, 6% training sequence overhead and 3.125% CP overhead. The baseband OFDM signal is firstly uploaded into a Tektronix arbitrary waveform generator (AWG7102) at 10 Gsample/s with 8 bits resolution, and then it is up-converted to 10 GHz RF using an analog IQ-mixer and a 10 GHz RF source. The 10 GHz RF-OFDM signal is modulated onto one of the CW laser stream via an IM and a SSB signal is generated after the modulated signal passing an optical interleaver as the band-pass OF with the central wavelength of 1549.64 nm and the bandwidth of 10 GHz as shown in Fig. 4 (1). Another CW laser stream is modulated by the pure 10 GHz RF source and a pure SSB signal is generated after the modulated signal passing another band-pass OF with the central wavelength of 1549.56 nm and the bandwidth of 10 GHz as shown in Fig. 4(2). A PBC is adopted to combine the two generated SSB signals and place them on two orthogonal SOPs as depicted in Fig. 4(3). After coupled with another narrow-linewidth DFB laser at λ₂ = 1549.76 nm with 80 kHz linewidth and 10 dBm launch power, the DS signal is finally formed as given in Fig. 4(4) and it is transmitted through 20 km SSMF and a variable optical attenuator (VOA) at 12 dB after a circulator.

 

Fig. 3 Experimental setup of the proposed optical OFDM-PON system.

Download Full Size | PDF

 

Fig. 4 The corresponding spectra of the proposed optical OFDM-PON system in Fig. 3.

Download Full Size | PDF

In the ONU, the DS signal is firstly amplified by an erbium-doped fiber amplifier (EDFA) to improve its power level and then an OF is used to separately filter the DS SSB signals and US carrier band out of the received DS signals as shown in Fig. 4(5) . The DS SSB signals are next split away by a PBS to separate the two SSB signals on different SOPs as illustrated in Figs. 4(6) and 4(7), and then each SSB signal enters into a 20 GHz PD for direct detection. A pure 10 GHz RF source at −20 dBm power is generated by a heterodyne of two amplified pure optical bands after PD, and it is sampled by a 20 GHz Tektronix real-time oscilloscope (DSA72004C) at 50 Gsample/s. Then the generated 10 GHz RF source is utilized to down-convert the generated RF-OFDM signal in an analog IQ-mixer. After downconversion and analog to digital conversion (ADC), the produced DS baseband 16 QAM-OFDM signal is analyzed off-line to get the DS bit-error-rate (BER) performance. The generated 10 GHz RF source is still used to up-convert the US 8 Gb/s baseband 16 QAM-OFDM signal in an analog IQ-mixer. The US RF-OFDM signal drives an IM which is biased at a minimum transmission point to realize OCS modulation to modulate the filtered US carrier band and generate the US transmitted signal as depicted in Fig. 4(8). The US signal is transmitted through 20 km SSMF and a VOA at 12 dB after a circulator. The received US signal is coherently detected with the help of the 1549.76 nm laser and the pure 10 GHz sinusoidal wave. Then the produced US baseband 16 QAM-OFDM signal is also analyzed off-line to get the US BER performance. Figure 5 gives the waveform of the finally achieved 10 GHz pure RF source in the ONU side. Although certain deterioration occurs, the generated RF source is pure and effective enough to assist both DS and US signals to fulfill the RF conversions.

 

Fig. 5 The waveform of the ONU-side achieved 10 GHz pure RF source.

Download Full Size | PDF

 

Fig. 6 Measured BER curves and normalized constellations of (i) DS B2B and 20 km SSMF; (ii) US B2B and 20 km SSMF for the proposed optical OFDM-PON system.

Download Full Size | PDF

Figures 6(i) and 6(ii) plot the BER versus received optical power (ROP) performance for both downlink and uplink of our proposed optical OFDM-PON system after back-to-back (B2B) and 20 km SSMF transmission, respectively. The FEC limit of BER = 10⁻³ is also plotted in Figs. 6(i) and 6(ii) for reference. As for the downlink transmission, a ROP of about −16.4 dBm is needed to reach the FEC limit, while the ROP needed to reach the FEC limit for uplink transmission is about −17 dBm. Moreover, from the comparison of the B2B and 20 km SSMF transmission cases, the fiber dispersion induced power penalty is almost negligible for both downlink and uplink transmission. Figures 6(i) and 6(ii) also express the normalized constellations of bidirectional transmission of 16 QAM-OFDM signals after B2B and 20 km SSMF. It is clear that the 16 QAM constellations are effectively recovered both in downlink DD reception and uplink CD reception.

4. Conclusion

We have proposed and successfully demonstrated a novel cost-effective bidirectional up-converted optical OFDM-PON system. In our proposed system, all ONUs are source-free not only in the optical domain but also in the electric domain. PolMUX is utilized in the downlink transmission to generate the ONU-side 10 GHz pure RF source. With the help of the achieved RF source, each ONU can take the RF conversions for both downlink and uplink OFDM signals free of the initially equipped local RF source and therefore the overall cost of each ONU is significantly reduced. As such, the introduced cost-effective bidirectional up-converted optical OFDM-PON system can be considered as a very promising candidate for the next-generation broadband optical access networks.