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Multi‐stage all‐pass networks can be used to realise broadband phase shifters with low phase error. In this study, single‐stage and two‐stage all‐pass networks with internal switched capacitors are investigated. Potentials and limitations of using the all‐pass networks with internal switched capacitors for phase shifter design are examined. On the basis of the single‐stage and two‐stage all‐pass networks, a fully‐differential digital phase shifter with 6‐bit resolution is designed. The digital phase shifter is implemented using a 0.18‐μm complementary metal‐oxide semiconductor process. The chip area is 2.80 × 1.75 mm2. Measurement results show that minimum root‐mean‐square phase error of 2° is achieved from 2.19 to 2.82 GHz, which translates into a bandwidth (BW) of 25%. The average insertion loss is 14.6 dB at the design frequency of 2.4 GHz. Over the entire BW, the return loss is greater than 9.2 dB and the amplitude error is within ±1 dB.

In this paper, we propose and experimentally demonstrate a novel multi scrolls chaotic encryption scheme for CO-OFDM-PON. We analyze the principle of 3-dimension encryption scheme and discuss its encryption complexity. Compared with the previous hyper Chen chaotic encryption scheme, the proposed encryption algorithm can realize dynamic constellation point mapping of QAM signal with lower encryption complexity. We also compare the transmission performances of the two chaotic encryption schemes. The results show that the proposed multi scrolls scheme has better BER performance because it can decrease the peak to average power ratio (PAPR) of OFDM. What is more, the proposed encryption scheme is very sensitive to the initial secure key and a tiny discrepancy as small as 10−17 would lead to a completely different sequence. The high sensibility to the initial value can effectively increase encryption level and the key space of the multi scrolls encryption scheme is 106 times of that hyper Chen. Further, to verify the effectiveness of the proposed encryption algorithm, encrypted transmission of a digital picture in 80 km SSMF is carried out.