Inverse-designed free-form nanophotonic structures have shown great potential in designing ultra-compact integrated photonic devices, but strict fabrication requirements may hinder further applications. We propose here a photonic-crystal-like (PhC-like) subwavelength structure, which is insensitive to the lag effect that is the most common fabrication error. A colorless 3 dB coupler employing such a structure is designed, fabricated, and characterized. With only one-step etching, the coupling region of our final device occupies a compact footprint of $2.72\times 2.72\mathrm{\mu m}$. The simulated insertion loss of each output port is about 3.2 dB over 100 nm bandwidth around 1550 nm, and the measured insertion losses of both ports are 3.35 dB, on average, over the observable 60 nm bandwidth with a near zero loss imbalance.

We propose a novel method to simultaneously improve the physical layer security and the transmission performance of the orthogonal frequency division multiplexing (OFDM) passive optical network system by using chaos and fractional Fourier transform (FrFT) techniques. The designed 3-D chaotic sequences are used to form the training sequence for time synchronization, to perform the OFDM subcarriers masking, and to control the fractional order of the FrFT operation. The analyses show that the whole key space size of the proposed scheme could be beyond 10 ⁵⁰ , and the peak-to-average-power-ratio of the transmitted OFDM signal can be decreased by about 0.5 dB. Furthermore, we successfully demonstrate an 8.18 Gbps 16-quadrature-amplitude- modulation (QAM)-OFDM data transmission experiment with chaotic and FrFT operations over 25 km single mode fiber. The results show that the proposed scheme could effectively enhance the system security and the transmission performance without additional bandwidth requirement.

A dual-field encryption scheme of an orthogonal frequency division multiplexing passive optical network (OFDM-PON) is proposed based on analog–digital hybrid chaotic system. In our scheme, the downstream data are encrypted in optical field, and the upstream data are encrypted in electrical field. The hybrid chaotic system circumvents the security vulnerabilities of both analog and digital chaos, e.g., the periodicity of digital chaos is masked by the analog one, and the time delay signature of the analog system is concealed by injecting digital chaos. Meanwhile, the key space dimension of the system is enlarged. Simulation results show both the feasibility and security of the proposed OFDM-PON system.

With the development of highly densified photonic integrated circuits, the optical cross nodes number exhibits dramatically increasing. Not only efficient but also ultra-compact waveguide crossings are required to materialize the full potential of silicon photonics for on-chip optical intercross connect. In this work, we proposed several inverse-designed 4 × 4, 5 × 5 and 6 × 6 star-crossings based on the photonic-crystal-like (PhC-like) subwavelength structures, which have ultra-high port density of about 7.1 μm²/port, 5.83 μm²/port and 7.3 μm²/port respectively. Moreover, the star-crossings are practically fabricated and experimentally characterized. The average measured insertion losses (ILs) are less than 0.75, 0.9 dB and 1.5 dB, while the crosstalks are sub-22.5 dB, −20 dB and −18 dB for other output ports over 60 nm bandwidth centered at 1550 nm wavelength.

We propose and numerically investigate a chaotic optical coherent secure communication scheme, which supports long-haul secure transmission for signals in advanced modulation formats. A hybrid optical chaos system is designed with coordination of digital and analog signals. The hybrid entropy source provides a broadband analog optical chaos signal, which could serve as the carrier to load quadrature amplitude modulation (QAM) data. Simultaneously, a digital binary signal generated from the entropy source is transmitted to establish long-haul chaotic synchronization. Coherent detection is utilized at the receiver, and a digital signal processing (DSP) algorithm is adopted to reduce transmission distortion. A 5 Gbaud 16QAM signal is encrypted by a phase chaos carrier with the effective bandwidth of 5.8 GHz. A bit error rate (BER) below forward error correction (FEC) can be achieved after transmitting over 1600 km based on digital-signal-induced chaos synchronization technology. Optimal launch power is investigated to minimize nonlinear effects of transmission links. System security is guaranteed by the high dynamical complexity of the chaotic source and the sensitive time delay as the secret key.

We propose and experimentally demonstrate a novel physical layer encryption scheme for high-speed optical communication. A 10 Gb/s on-off keying signal is secretly transmitted over 100 km standard single-mode fiber. The intensity-modulated message is secured by the encryption mechanism, which is composed of an external noise source and an internal time-delayed feedback loop. The external noise serves as an entropy source with sufficient randomness. The feedback loop structure in the transmitter introduces a time-domain encryption key space, and a corresponding open-loop configuration at the receiver side is used for synchronization and decryption. Experiment results show the effectiveness of the proposed scheme. For a legitimate terminal, bit error rate below ${10}^{-8}$ can be obtained. Decryption degradations with the mismatch of different hardware parameters are researched. The time delay in the feedback loop provides a sensitive encryption key. For other hardware parameters, the system is robust enough for synchronization. Meanwhile, the time-delay signature of the loop is able to be well concealed by the external noise. Moreover, the proposed scheme can support density wavelength division multiplexing transmission with a relatively simple structure. This work also provides a new concept to establish optical secure communication by combining a time-delayed feedback chaotic system and random noise.