1. Introduction

Passive optical network (PON) with flexible bandwidth and large capacity has been gaining widespread attention in recent years [1–5]. To meet the requirement, the traditional access network with time division multiplexing (TDM) is developing towards other architectures such as wavelength division multiplexing (WDM), TWDM and orthogonal frequency division multiplexing access (OFDMA) [6–8]. Meanwhile, the advanced modulation formats have received increasing interest as a method to reduce the complexity while increasing the capacity and flexibility [9–11]. Among these modulation formats, carrier-less amplitude/phase modulation (CAP) has been demonstrated to be cost-effective due to the intensity modulation and direct-detection in PON. Compared to QAM modulation, it can achieve similar performance and spectral efficiency but save components such as complex mixer and radio frequency (RF) source. However, the multi-granularity bandwidth allocation by using CAP hasn’t been discussed before.

During CAP modulation, a pair of filters is needed to produce the real-value signal. In order to increase the flexibility in bandwidth allocation, filter bank multicarrier scheme is an attractive solution [12]. It can solve the problem of spectral leakage by minimizing the side-lobes of each CAP band, which could mitigate the interference among multi-bands. Besides, it is convenient to perform filtering operation in digital domain as the development of DSP technology, which makes it easy to implement in CAP based optical access system.

In this paper, we propose a novel dynamic filter bank multi-band (FBMB) CAP PON with multi-granularity bandwidth allocation. CAP-16 and filter bank with different bandwidths are used in our scheme. In order to provide enough power margin, a synchronized transmission of energy and data is employed in this scheme, where the residual energy is used for the dynamic scheduling module at the optical distribution network (ODN). Besides, a single sideband (SSB) based on dual-driven Mach-Zenhder modulator (MZM) is proposed to mitigate the fading effect in fiber link. In the experiment, 10 × 6-band dynamic FBMB/CAP-16 downstream signals ranging from 4Gb/s per band to 19.2Gb/s per band are successfully demonstrated over 25km fiber link.

2. Architecture

The architecture of the proposed system is illustrated in Fig. 1. There are n wavelength channels at the optical line terminal (OLT), where each wavelength carries FBMB/CAP signals for several optical network units (ONUs). For each CAP band, a pair of matched orthogonal filters is adopted to generate the real-valued signal. The filter bank with multi-bands is used to produce the multi-granularity FBMB/CAP signals. In order to reduce the interference among multi-bands, we adopt finite impulse response (FIR) filters as the synthesis filter bank. The FBMB/CAP signal and its Hilbert transform (H.T.) are fed into the two arms of a dual-drive MZM, which would result in a SSB optical signal. The wavelength of λ₀ is employed as the control signal, which is in charge of the scheduling at the ODN. A pump light acts as the energy supply for the margin improvement and control module at the ODN.

 

Fig. 1 Principle of dynamic FBMB/CAP-PON with dual-driven SSB and remote energy supply for optical access network.

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An Er-doped fiber (EDF) is employed to increase the power margin of optical signal at the ODN. The scheduling module is consisting of arrayed waveguide gratings (AWG), optical switches (SW) and splitter nodes. The AWG is used to separate wavelength channels, while SW and splitter nodes are responsible for the dynamic allocation of signals. The total insertion loss of the ODN is about 6 dB without considering the split loss. By monitoring the network traffic, the ODN can assign the ONU with different spit ratios and wavelengths by changing the SW and the splitter nodes. Assuming the number of multi-bands is M and the maximum split ratio is K, the potential number of users could be n × M × K. The bandwidth or central frequency of the matched filter can be easily adjusted thanks to the conventional DSP technology. After filtering, symbol-by-symbol maximum a posteriori (MAP) detection is adopted to further mitigate the interference among symbols.

3. Experiment and result

In order to verify the feasibility, the experiment setup for 10 × 6 bands FBMB/CAP downstream over 25km fiber is illustrated in Fig. 2. Ten DFB lasers from 1548.12nm to 1551.72nm serve as the light source, where the channel space is 0.4nm. They are divided into two groups as odd and even channels, which are modulated by two dual-driven MZMs to produce the SSB optical signal. For the signal generation, 6 data sequences with different lengths are generated with CAP-16 mapping for further filtering. The filter bank is consisting of 6 pairs of FIR filters with roll-off coefficient of 0.15. After filtering, the H.T. of the signal is generated for SSB modulation. In the experiment, we assume the 6 bands get different bandwidths varying from 1GHz to 4.8GHz, which occupy a total bandwidth of 20GHz. The excess bandwidth is 12%. The bandwidths of the six bands are 4.8GHz, 1GHz, 3GHz, 3.8GHz, 3GHz and 2GHz from band 1 to band 6 respectively, which results in a rate of 70.4Gb/s. The bandwidth of each band can be dynamically adjusted according to the requirement of ONU. The adjustment is realized through DSP processing in digital domain, which would not bring any penalty to the network. Two digital to analog convertors (DACs) at 50Gs/s are adopted to produce the electrical signal. The amplitude and phase weights are calculated according to the frequency response of the DAC and applied in the frequency domain. By adjusting the weights of the filter bank, the non-flat frequency response of the DAC can be compensated. Figure 3(a) and 3(b) show the normalized amplitude and phase weights respectively. The spectra of the output 70.4Gb/s FBMB/CAP-16 signal before and after compensation are shown as inset in Fig. 2. The control channel with wavelength of 1552.52nm (λ₀) is intensity modulated and combined with the odd and even data channels before amplification, where the optical spectrum is shown in Fig. 3(c). The output optical power of the EDFA is about 6dBm. A 1480nm pump light is coupled with the downstream signals before launched into the 25km single mode fiber (SMF).

 

Fig. 2 Experimental setup (MAP: maximum a posteriori).

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Fig. 3 (a) amplitude compensation weight; (b) phase compensation weight; (c) optical spectrum of combined signal at OLT; (d) optical spectrum at the ODN.

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At the ODN, an 8m-long EDF is used for remote amplification of the optical signal, which is followed by an optical filter to separate the pump light from the downstream signal. After the optical filter, an effective signal power of about 13.5dBm is observed in the experiment. The optical spectrum before filtering is shown in Fig. 3(d). When the pump is off, the signal cannot be amplified and would experience performance deterioration since the power margin has reduced. The residual energy is fed into the photoelectric cell, which provides power supply for the control circuit. The controls channel of λ₀ carriers the scheduling information for different wavelengths. As a proof-of-concept, we adopt a 10 × 10 SW and splitters with ratio of 1:4 and 1:8 to simulate the dynamic scheduling. It means that the potential number of users would be 10 × 6 × 8 = 480 in the experiment. The SW can lead different wavelengths to different splitters. When the traffic flow changes, the wavelength can alternate between 1:4 and 1:8 split ratios. The control circuit is used to extract the scheduling information and send corresponding command to the SW. At the ONU, a 0.3nm tunable optical filter (TOF) is employed to suppress the ASE noise before detection. Direct detection is adopted for optical signal and the photodiode (PD) is with a 3dB bandwidth of 25GHz. Then the signal is sampled by an analog to digital convertor (ADC) with 100Gs/s for offline processing. In practical use, the cost of the ADC chip would be greatly reduced as the development and mass production of the high-speed ADC chip. It provides a good solution for the high-speed ADC application in ONU. The sampled signal is demodulated with the matched filter at the designated band, where the symbols are reconstructed before decoding. Then the symbols are down-sampled for MAP decoding, where the information and the interference experienced from the neighboring symbols are both considered. In the experiment, we assume that the ONU has known the dedicated band assigned for it. When the baud rate and band are changed, the OLT could inform the ONU by preamble sequence or upper layer protocol.

The back-to-back (b2b) bit rate error (BER) for the 6 bands on 5th wavelength channel is shown in Fig. 4. It can be seen that the performance of the 6 bands is similar after compensation. The BER of 5th and 6th bands deteriorates a little compared with bands 1-3, which is mainly due to the imperfect compensation of frequency response for DAC.

 

Fig. 4 The measured BER performance for the 6 bands in b2b case.

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Figure 5(a) shows the BER performance of the 10 × 6 bands at a fixed received optical power of −16dBm after 25km transmission, where the 10 wavelengths are all assigned with 1:4 split ratio. It can be seen that all the channels are below FEC limit of 1 × 10⁻³. We also show the BER of band 5 and 6 without compensation in Fig. 5(b), where the BER is beyond 1 × 10⁻¹. The constellation maps before and after compensation are also shown as insets.

 

Fig. 5 the measured BER (a) for the 6 bands of 10 wavelength channels at received optical power of −16dBm at the ONU; (b) for band 5 and 6 without compensation.

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We have also measured the BER performance when the SW is dynamically switching between 1:4 and 1:8, where band 1, 2, 3 and 6 on 5th wavelength channel are tested in the experiment. The measured BER is shown in Fig. 6. Compared to the continuous case with 1:4 split ratio, the power penalties for band 1-3 and band 6 are below 0.5dB at BER of 1 × 10⁻³. It is mainly due to the response time of the optical switching. Although the response time is very short compared with the symbol time duration, it would cause the performance deterioration.

 

Fig. 6 The measured BER performance in continuous and dynamic switching cases.

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4. Conclusion

We have proposed and experimentally demonstrated a novel dynamic FBMB/CAP PON with remote energy supply and dual-driven SSB modulation. 10 × 70.4Gb/s dynamic FBMB/CAP-16 signals from 4Gb/s per band to 19.2Gb/s per band are successfully achieved over 25km SMF fiber, which could have 480 potential users. The remote energy supply can support dynamic scheduling module at ODN as well as improve signal power margin in PON. The results suggest a prospective solution of future access network to achieve greater efficiency.