Fig. 1. Experimental setup for blue-green WDM

Fig. 2. The experimental setup of the proposed RGB LDs-based WDM UWOC system^[14]. (a) The transmitter module; (b) the receiver module; (c) the water tank

Fig. 3. The experimental setup of the proposed WDM-VLC system using transmission gratings as MUX/DeMUX^[15]

Fig. 4. The configuration of illustrated VLC system with a R/G/B five-wavelength polarization-multiplexing scheme^[16]

Fig. 5. Schematic diagram of demultiplexing achieved through filtering and polarization analysis^[16]

Fig. 6. (a) Schematic diagram of a blue-green light phased array based on HCG aperture; (b) distribution of the effective refractive index along the light propagation direction in the xz cross-section; (c) microscope image of the device taken with auxiliary illumination turned on; (d) microscope image of the device taken with auxiliary illumination turned off; (e) high-resolution photograph of the device's thin-film waveguide area and HCG aperture area^[32]

Fig. 7. (a) Simulated far-field light spot of the HCG aperture; (b) experimentally measured far-field light spot of the HCG aperture; (c) high-resolution photograph of the main lobe; (d) high-resolution photograph of the four main lobes excited simultaneously at four wavelengths: 492 nm, 506 nm, 518 nm, and 522 nm; (e) the deflection angles and divergence angles of the main lobes corresponding to the four wavelengths of 492 nm, 506 nm, 518 nm, and 522 nm^[32]

Fig. 8. The experimental setup of blue-green light phased array with HCG as a demultiplexer in a proof-of-concept demonstration of WDM system^[32]

Fig. 9. (a) Schematic diagram of a silicon nitride photonic chip with a phased array based on a fishbone-type antenna structure; (b) structural diagram of a single fishbone-type antenna in the xy cross-section; (c) distribution of the TE mode electric field for a single antenna in the yz cross-section; (d) distribution of the TM mode electric field for a single antenna in the yz cross-section^[33]

Fig. 10. (a) The microscopy images of the fabricated OPA chip based on fishbone waveguide antenna;(b) near-field field excited by the 520.5 nm laser captured by image camera; (c) measured TE mode and TM mode far-field patterns of the OPA excited by the 520.5 nm laser^[33]

Fig. 11. (a) Experimental spectra of four LD with wavelengths of 494 nm, 508 nm, 520.5 nm, and 521.7 nm; (b) simulated, experimental, and theoretical results for the deflection angles of the main lobes of the TE and TM modes excited by the aforementioned four wavelengths; (c) simulated calculation results for the optical loss of the OPA chip; (d) simulated and experimental results for the main lobe losses at the four wavelengths^[33]

Fig. 12. (a) The proof-of-concept demonstration of dense blue-green wavelength division demultiplexing via polarization-sensitive OPA, the inset is the measured far-field profile of TM modes at the wavelength of 520.5 nm and 521.7 nm; (b)(c) the measured far field profile of 494 nm TE, 508 nm TE, 520.5 nm TE, and 521.7 nm TM modes and 494 nm TM, 521.7 nm TE, 508 nm TM, and 520.5 nm TM modes^[33]

Fig. 13. (a)(b) BER for 4-channel WDM using OOK modulation before RLS adaptive equalization at various communication rates, including two WDM combinations: 494 nm TE, 508 nm TE, 520.5 nm TE, and 521.7 nm TM mode multiplexing; 494 nm TM, 521.7 nm TE, 508 nm TM, and 520.5 nm TM mode multiplexing; (c) (d) BER for 4-channel OOK-modulated WDM after RLS adaptive equalization at various communication rates, with the same two WDM combinations as mentioned above; (e) (f) amplitude distribution of the 520.5 nm signal before and after RLS adaptive equalization at a 1 Gbit/s sampling rate; (g)-(j) eye diagrams of communication signals for different modes after RLS equalization at a 1 Gbit/s communication rate: 520.5 nm TE; 520.5 nm TM; 521.7 nm TE; 521.7 nm TM^[33]