In this study, a bidirectional WDM fibre-FSO communication is proposed and practically built, utilizing polarisation multiplexing technique and tunable optical VSB filter. It shows a bidirectional fibre-FSO communication employing OILs at the transmission and reception sides to separate the even optical sidebands for downstream carriers and the odd optical sidebands for upstream carriers, and employing a tunable optical VSB filter at the downstream/upstream reception side to substantially enhance the transmission performance. An OIL is used at the downstream reception side to select the unmodulated odd optical sidebands and use them for the upstream carriers. A tunable optical VSB filter at the reception side not only reduces the bandwidth of modulated optical sideband to convert the optical PAM4 signal into optical VSB-PAM4 signal and alleviate fibre dispersion, but also eliminates self-polarisation interferences caused by optical sidebands with parallel polarisation. The system demonstrates successful downstream and upstream transmissions over a total distance of 31.6 km, consisting of 30 km SMF length and 1.6 km FSO link. The implementation of such a bidirectional WDM fibre-FSO communication poses great challenges, especially for high aggregate transmission capacity and given transmission performance. By combining polarisation multiplexing technique with WDM scheme, the transmission capacity is considerably increased. Using VSB-PAM4 modulation, the transmission performance is effectively enhanced. In addition, the deployment of two cascaded MZM-OEOs increases the number of the optical sidebands, and the deployment of triplet lenses effectively extends the reach of the FSO link. This demonstration marks the first use of two cascaded MZM-OEOs to generate 16 optical sidebands and a couple of triplet lenses to achieve 1.6 km FSO link. Utilizing two cascaded MZM-OEOs and triplet lenses for bidirectional WDM fibre-FSO communications marks an important milestone. Low BERs and clear VSB-PAM4 eye diagrams are achieved with 480 Gb/s high aggregate transmission capacity in both downstream and upstream transmissions.

Figure 1 presents the structure of the bidirectional WDM fibre-FSO communication using polarisation multiplexing technique and tunable optical VSB filter. Two cascaded MZM-OEOs are used to generate 16 optical sidebands with a frequency separation of 25 GHz. Then, an OIL with 25 GHz input channel separation and 50 GHz output channel separation is deployed to split even and odd optical sidebands. After the output from the even optical sideband port, 8 even optical sidebands with 50 GHz frequency separation pass through an MZM by a PC. A 30-Gb/s PAM4 signal externally modulates the 8 even optical sidebands through a modulator driver. Afterward, the optical PAM4 signals are split into two parts along two orthogonal polarisations utilizing a PBS. Next, the optical sidebands in x-polarisation and y-polarisation are combined using a polarisation beam combiner. The 8 modulated even optical sidebands and 8 unmodulated odd optical sidebands are then combined using an optical combiner. Through OC1 and OC2, the 16 optical sidebands are transmitted through 30 km SMF and 1.6 km free-space using a couple of triplet lenses. After routing by the OC2, the optical signal with 16 optical sidebands passes through an OIL. After the output from the even optical sideband port, 8 modulated even optical sidebands are supplied in a tunable optical VSB filter to pick the desired sideband and facilitate the conversion of optical PAM4 signal into optical VSB-PAM4 signal. The 30 Gb/s optical VSB-PAM4 signal is divided into two orthogonal polarisations using a PC with a PBS. The x-polarised optical sideband is received by an x-polarised receiver and equalized with an electrical equalizer. After electrical equalization, a real-time BER analysis is performed using an error detector and the PAM4 three-eye sampling way. The y-polarised optical sideband undergoes a similar process with a y-polarised receiver and an electrical equalizer, and are then detected using an error detector. Additionally, VSB-PAM4 eye diagrams for both polarizations are captured with a digital storage oscilloscope.

Moreover, after the output from the odd optical sideband port, 8 unmodulated odd optical sidebands are used as upstream optical carriers. The 8 odd optical sidebands are sent to an MZM through a PC. A 30-Gb/s PAM4 signal is supplied in a modulator driver and subsequently driven the MZM. The optical PAM4 signals are split into two orthogonal polarisations by a PBS. The optical sidebands in x-polarisation and y-polarisation are subsequently combined using a polarisation beam combiner. Through routing by OC2 and OC1, the optical signal with 8 modulated odd optical sidebands is transported over 1.6 km FSO link and 30 km SMF transmission. Next, 8 modulated odd optical sidebands are supplied in a tunable optical VSB filter to filter the wanted sideband and convert optical PAM4 signal into optical VSB-PAM4 signal. Similar to the earlier step, the signal is split into two orthogonal polarisations after passing through a PC with a PBS. The electrical signal from the x-polarised/y-polarised receiver is passed through an electrical equalizer. The equalized electrical signal is then inputted into an error detector. The x-polarised/y-polarised sideband is next sent to a digital storage oscilloscope for VSB-PAM4 eye diagrams analysis.

Two cascaded MZM-OEOs refer to the combination of two sets of MZM-OEOs arranged in series. As depicted in Fig. 2(a), each stage of MZM-OEO includes a 40-GHz MZM, a 25-GHz photodiode, two 25-GHz electrical amplifiers, a 25-GHz BPF, and an OCNR enhancement setting. For the first stage of MZM-OEO, the noise is initially amplified by the first 25-GHz electrical amplifier, the amplified signal is then picked by a 25-GHz BPF. Subsequently, the picked-up signal undergoes further amplification by the second 25-GHz electrical amplifier. Finally, the signal is supplied in a 40-GHz MZM, which is modulated with a 25-GHz MMW signal. The modulation of the MZM results in the production of multi-optical sideband with 25 GHz frequency separation. The produced optical sidebands are then fed into an OCNR enhancement setting to enhance their OCNRs. The OCNR enhancement setting is composed of a DI, an EDFA, and an OBPF. The DI holds a 25-GHz free spectral range, equaling the frequency of the modulated MMW signal. After passing through the DI, the optical sidebands go through an EDFA to boost their power levels. The OBPF is then used to filter out the outer optical sidebands and noise. Next, the first stage of OCNR enhancement setting is input to the second stage of MZM-OEO. The structure of second stage of MZM-OEO is the same as the first stage of MZM-OEO, with a 40-GHz MZM, a 25-GHz photodiode, two 25-GHz electrical amplifiers, a 25-GHz BPF, and an OCNR enhancement setting.

Figure 2(b) shows the optical spectra produced by two cascaded MZM-OEOs with (w/) and without (w/o) two sets of OCNR enhancement settings. With two sets of OCNR enhancement settings, it is apparent that the OCNRs of optical sidebands are enhanced by approximately 6 dB. DI is operated as a noise reducer to reduce noise between adjacent optical carriers. We use an EDFA to enhance multiple optical sidebands that have passed through the DI to enhance the OCNR of each optical sideband.

Figure 3 shows the optical spectrum after OC2, consisting of 8 downstream modulated optical sidebands with 30 Gb/s PAM4 signals in both x- and y-polarisations and 8 upstream unmodulated optical sidebands. The 8 downstream modulated optical sidebands have orthogonal polarisations and are spaced 50 GHz apart. Each downstream optical sideband is 25 GHz apart from its adjacent upstream optical sideband. After passing through OC2 and being separated by an OIL at the downstream reception side, the downstream modulated and upstream unmodulated optical sidebands are dramatically separated, resulting in a frequency separation of 50 GHz. For the OIL output with even optical sidebands, the downstream modulated optical sidebands are selected to have a 50-GHz larger frequency separation. Then, the selected downstream modulated optical sidebands are supplied in a tunable optical VSB filter to pick and demultiplex the desired optical sideband, reduce the bandwidth of PAM4 signal, and filter out self-polarisation interferences. By selecting modulated optical sidebands using an OIL, filtering and demultiplexing the desired optical sideband using a tunable optical VSB filter, reducing the bandwidth of PAM4 signal and filtering out self-polarisation interferences using a tunable optical VSB filter, a WDM fibre-FSO communication with improved transmission performance is therefore achieved. For cross-polarisation interferences, the cross-polarisation interferences are expected to be very small since the polarization states are orthogonal.

Figure 4(a) shows the downstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength (sideband) of 1550.78 nm under back-to-back and through 30-km SMF transmission with 1.6-km FSO link (x-polarisation or y-polarisation; VSB-PAM4 or PAM4) conditions. And Fig. 4(b) exhibits the upstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength (sideband) of 1549.78 nm under back-to-back (BTB) and through 1.6-km FSO link with 30-km SMF transmission (x-polarisation or y-polarisation; VSB-PAM4 or PAM4) conditions. From Fig. 4(a), it is to be seen that when BER reaches 10⁻⁹, there is a 5.2-dB power penalty between BTB and that through 30 km SMF transmission with 1.6 km FSO link (x-polarisation or y-polarisation; VSB-PAM4). The observation from Fig. 4(b) indicates that at a BER value of 10⁻⁹, there is a 5.4-dB power penalty between back-to-back and that through 1.6 km FSO link with 30 km SMF transmission (x-polarisation or y-polarisation; VSB-PAM4). The observed power penalties of 5.2 dB and 5.4 dB are primarily attributed to the employment of tunable optical VSB filter to alleviate fibre dispersion by reducing optical PAM4 signal’s bandwidth and filter out self-polarisation interferences by filtering the desired optical sideband. In contrast, in the scenario of PAM4 signal transmission, 1.2x10⁻⁵ (Fig. 4(a)) and 1.4x10⁻⁵ (Fig. 4(b)) increased BERs are obtained at 1.5 and 2.6 dBm received optical powers. The 1.5 and 2.6 dBm received optical powers are used to recompense the reduction in optical signal-to-noise/distortion ratio. However, the recompense results are limited to obtaining high BERs of 1.2x10⁻⁵ and 1.4x10⁻⁵. Fibre dispersion-induced distortions from 30 km SMF transport and self-polarisation interferences from parallel polarised optical sidebands are the primary factors leading to BER performance degradation.

Figures 4(c), 4(d), and 4(e) show the eye diagrams of the 30 Gb/s VSB-PAM4/PAM4 signal at several BERs and polarisations. Clear eye diagrams (Fig. 4(c)) are achieved at a BER of 10⁻⁹ for downstream transmission with x-polarisation and VSB-PAM4 modulation. And clear eye diagrams (Fig. 4(d)) are also obtained at a BER of 10⁻⁹ for upstream transmission with y-polarisation and VSB-PAM4 modulation. The low BER of 10⁻⁹ with a clear eye diagram demonstrates the feasibility of transmitting multiple optical sidebands over bidirectional WDM fibre-FSO communications using polarisation multiplexing technique and tunable optical VSB filter. While blurred eye diagrams (Fig. 4(e)) are attained at an upstream BER of 1.4x10⁻⁵ (y-polarisation; PAM4) as the tunable optical VSB filter is not used at the upstream reception side. A high BER with blurred PAM4 eye diagrams shows that transmitting multiple optical sidebands over bidirectional WDM fibre-FSO communications in the absence of tunable optical VSB filter is not feasible.