Introduction

Broadband access network plays a crucial role in enabling various advanced technologies, including driverless cars, smarter cities, multi-broadcasting, mixed reality, and more1,2,3,4,5. To meet the growing demand for advanced communication technologies, achieving high transmission capacity is critical for both fibre networks and last-mile communications. This can be accomplished by leveraging optical fibre and optical wireless technologies through the realisation of fibre-free-space optical (FSO) communications6,7,8,9,10,11. Polarisation multiplexing technique enables the simultaneous transmission of data streams by using different polarisation states12. The use of the polarisation multiplexing technique in fibre-FSO communications results in an increase in transmission capacity. In addition, to further increase the transmission capacity, wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) modulation is adopted to meet high transmission capacity target13,14,15. However, the optical PAM4 signal holds a wider bandwidth and brings greater fibre dispersion in fibre-FSO communications. On the other hand, the optical vestigial sideband (VSB)-PAM4 signal holds a narrower bandwidth and produces smaller fibre dispersion in fibre-FSO communications16. Therefore, VSB-PAM4 modulation is used to reduce the fibre dispersion effect, bringing on better transmission performance. The combination of polarisation multiplexing technique and WDM VSB-PAM4 modulation in fibre-FSO communication systems is an advanced approach to substantially increase the transmission capacity and enhance the transmission performance.

In this study, a bidirectional WDM fibre-FSO communication is proposed and practically built, utilising the polarisation multiplexing technique and tunable optical VSB filter. It shows a bidirectional fibre-FSO communication employing optical interleavers (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 the 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 a 30 km single-mode fibre (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 the polarisation multiplexing technique with the 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 Zehnder modulators (MZM)-optoelectronic oscillators (OEOs) increases the number of 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 a 1.6 km FSO link. Utilising two cascaded MZM-OEOs and triplet lenses for bidirectional WDM fibre-FSO communications marks an important milestone. Low bit error rates (BERs) and clear VSB-PAM4 eye diagrams are achieved with 480 Gb/s high aggregate transmission capacity in both downstream and upstream transmissions.

Previous studies have shown that it is feasible to construct a combined fibre/FSO communication for long-haul wireline-wireless transmission17. However, the total transmission capacity of 56.34 Gb/s is much smaller than the associated value of 480 Gb/s operating in this proposed bidirectional fibre-FSO communication. Furthermore, due to the use of doublet lenses (rather than triplet lenses), the FSO link of 1.2 km is shorter than the related value of 1.6 km transmitting in this demonstration. A double lens contains a convex lens and a concave lens. A triple-lens includes a convex lens, a concave lens, and another convex lens. The additional convex lens in the triple-lens greatly enhances aberration correction, making it suitable for maintaining high-quality FSO links over long distances18. Moreover, this combined fibre/FSO communication is a unidirectional communication, not a bidirectional communication. The utilisation of polarisation-orthogonal modulation has been proven feasible for constructing a two-way free-space optics-based interface between fibre and 5 G communication19. However, it encounters challenges in modulating only the y-polarised light while not modulating the x-polarised light. In the combined fibre/FSO communication, the aggregate transmission capacity of 11 Gb/s is considerably lower than the related value of 480 Gb/s in such demonstrated bidirectional communication, and the 1-km doublet lenses-based FSO link is shorter than the 1.6-km triplet lenses-based FSO link in this proposed bidirectional communication. The successful establishment of bidirectional WDM fibre-FSO communication with a high transmission rate and good transmission performance marks an important step towards the implementation of bidirectional fibre-FSO convergence. It plays a crucial role in the integration of optical fibre and optical wireless communications, and the combination of polarisation multiplexing technique and WDM VSB-PAM4 modulation.

Results and discussion

Structure of two cascaded MZM-OEOs and optical spectra

Two cascaded MZM-OEOs refer to the combination of two sets of MZM-OEOs arranged in series. As depicted in Fig. 1a, each stage of MZM-OEO includes a 40-GHz MZM, a 25-GHz photodiode, two 25-GHz electrical amplifiers, a 25-GHz band-pass filter (BPF), and an optical carrier-to-noise ratio (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 millimetre-wave (MMW) signal. The modulation of the MZM results in the production of a multi-optical sideband with 25 GHz frequency separation. It is noted that as the power level of the MMW signal increases, the number of generated optical sidebands increases20,21. With the proper power level of the MMW signal modulating the MZM, multiple optical sidebands are produced at a fixed frequency separation. For the frequency separation, it is decided by the centre frequency of BPF22. A 25-GHz BPF yields a 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 delay interferometer (DI), an erbium-doped fibre amplifier (EDFA), and an optical BPF (OBPF). The DI holds a 25-GHz free spectral range, equalling the frequency of the modulated MMW signal. It operates as a comb filter, with its working wavelength range carefully shifted to align each optical sideband with DI’s passband and each valley with DI’s stopband. This arrangement attenuates the noise level between every two optical sidebands. 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 the OCNR enhancement setting is input to the second stage of MZM-OEO. The structure of the 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. The output optical field of an MZM-OEO (Eout(t)) can be described as:

$${E}_{{\mbox{out}}}\left(t\right)={E}_{{\mbox{in}}}\left(t\right)\cdot {e}^{j\beta \cos \left({\omega }_{m}t+\phi \right)}$$
(1)

where t represents time, Ein(t) is the input optical field, β is the modulation index, ωm is the angular frequency of the MMW signal, and ϕ is the phase shift. The exponential term in the output field can be expanded using the Jacobi-Anger expansion23 and the output optical field of an MZM-OEO can be obtained as:

$${E}_{{\mbox{out}}}\left(t\right)={E}_{{\mbox{in}}}\left(t\right)\cdot \mathop{\sum }_{n=-{\infty }}^{{\infty }}{J}_{n}\left(\beta \right)\cdot {e}^{{jn}\left({\omega }_{m}t+\phi \right)}$$
(2)

where Jn(β) is the nth-order Bessel function of the first kind. As the power level of the MMW signal increases, β increases, resulting in higher values of Jn(β), thus generating more optical sidebands. When the 8 optical sidebands from the first stage of MZM-OEO are used to modulate the second stage of MZM, the second stage of MZM-OEO successfully generates 16 optical sidebands with 25 GHz frequency separation.

Fig. 1: Structure of two cascaded MZM (MZM, Mach-Zehnder modulator)-OEOs (OEOs, optoelectronic oscillators) and optical spectra.
figure 1

a Each stage of MZM-OEO includes a 40-GHz MZM, a 25-GHz photodiode, two 25-GHz electrical amplifiers, a 25-GHz BPF (BPF, band-pass filter), and an OCNR (OCNR, optical carrier-to-noise ratio) enhancement setting. b Optical spectra produced by two cascaded MZM-OEOs with (w/) and without (w/o) two sets of OCNR enhancement settings. DFB LD distributed feedback laser diode, PD photodiode, DI delay interferometer, EDFA erbium-doped fibre amplifier, OBPF optical band-pass filter.

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Figure 1b 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.

Optical spectrum of downstream modulated and upstream unmodulated optical sidebands

Figure 2 shows the optical spectrum after optical circulator 2 (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 the 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. If OIL is not used and only a tunable optical VSB filter is used, a 25-GHz close frequency separation exists. In closely spaced optical sidebands, demultiplexing and distinguishing each modulated optical sideband for better transmission performance will be a huge challenge24. For cross-polarisation interferences, the cross-polarisation interferences are expected to be very small since the polarisation states are orthogonal25.

Fig. 2: Optical spectrum of downstream modulated and upstream unmodulated optical sidebands.
figure 2

Optical spectrum after OC2 (OC2, optical circulator 2), consisting of 8 downstream modulated optical sidebands with 30 Gb/s PAM4 (PAM4, four-level pulse amplitude modulation) signals in both x- and y-polarisations and 8 upstream unmodulated optical sidebands.

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BERs and PAM4 eye diagrams of bidirectional WDM fibre-FSO communications under different conditions

Figure 3a 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. Figure 3b exhibits the upstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength (sideband) of 1549.78 nm under back-to-back and through 1.6-km FSO link with 30-km SMF transmission (x-polarisation or y-polarisation; VSB-PAM4 or PAM4) conditions. It can be observed from Fig. 3a, b, in VSB-PAM4 and PAM4 modulation scenarios, the x-polarised and y-polarised BERs are almost identical, showing that there is little correlation between the BER and the polarisation state. From Fig. 3a, it is to be seen that when BER reaches 10−9, there is a 5.2-dB power penalty between back-to-back and that through 30 km SMF transmission with 1.6 km FSO link (x-polarisation or y-polarisation; VSB-PAM4). The observation from Fig. 3b 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 a tunable optical VSB filter to alleviate fibre dispersion by reducing the optical PAM4 signal’s bandwidth and filtering out self-polarisation interferences by filtering the desired optical sideband. In contrast, in the scenario of PAM4 signal transmission (without a tunable optical VSB filter at the reception side), 1.2 × 10−5 (Fig. 3a) and 1.4 × 10−5 (Fig. 3b) 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 1.2 × 10−5 and 1.4 × 10−5. 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.

Fig. 3: BERs (BERs, bit error rates) and PAM4 (PAM4, four-level pulse amplitude modulation) eye diagrams of bidirectional WDM (WDM, wavelength-division-multiplexing) fibre-FSO (FSO, free-space optical) communications under different conditions.
figure 3

a The downstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength of 1550.78 nm under different conditions. b The upstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength of 1549.78 nm under different conditions. The error bars represent the standard deviations of the measured data from three experimental trials. c Clear eye diagrams at a BER of 10−9 for downstream transmission with x-polarisation and VSB (VSB, vestigial sideband)-PAM4 modulation. d Clear eye diagrams at a BER of 10−9 for upstream transmission with y-polarisation and VSB-PAM4 modulation. e Blurred eye diagrams at an upstream BER of 1.4 × 10−5 (y-polarisation; PAM4) as the tunable optical VSB filter is not used at the upstream reception side. BTB back-to-back.

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Figure 3c, d, e show the eye diagrams of the 30 Gb/s VSB-PAM4/PAM4 signal at several BERs and polarisations. Clear eye diagrams (Fig. 3c) are achieved at a BER of 10-9 for downstream transmission with x-polarisation and VSB-PAM4 modulation. Clear eye diagrams (Fig. 3d) 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 a polarisation multiplexing technique and tunable optical VSB filter. Blurred eye diagrams (Fig. 3e) are attained at an upstream BER of 1.4×10−5 (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 a tunable optical VSB filter is not feasible.

The optical spectra of optical PAM4/VSB-PAM4 signals and the downstream y-polarised BERs at different centre wavelengths of tunable optical VSB filter

The transmission performance of bidirectional WDM fibre-FSO communications will be worsened by distortions produced by fibre dispersion over 30 km SMF transmission. Therefore, it is necessary to use a dispersion compensation device in fibre transmission to make up for fibre dispersion and correspondingly decrease the distortions caused by fibre dispersion26,27. Nevertheless, the dispersion compensation device in optical fibre transmission brings complexity to the system’s configuration. Therefore, bidirectional WDM fibre-FSO communications with simple configuration but still maintain qualified transmission performance need to be developed. A tunable optical VSB filter reduces the optical signal’s bandwidth, which is beneficial in mitigating the effects of fibre dispersion produced by 30 km SMF transmission. By narrowing the optical signal’s bandwidth, a tunable optical VSB filter minimises temporal spreading caused by fibre dispersion, thereby reducing distortions and improving communication performance. Unlike traditional dispersion compensation devices, such as tilted fibre Bragg grating-based OBPF and ring resonator-based wavelength filter28,29, tunable optical VSB filter provides a simpler and more flexible solution, making it an attractive option for practical deployment in communications. Figure 4a shows the optical spectra of 30 Gb/s optical PAM4 and VSB-PAM4 signals. The PAM4 signal exhibits a wider bandwidth compared to the VSB-PAM4 signal which has a narrower bandwidth. Over 30 km SMF transmission, since the optical PAM4 signal bandwidth becomes wider, fibre dispersion-induced distortions degrade the communication performance. Nevertheless, fibre dispersion-induced distortions will be reduced by employing optical VSB-PAM4 modulation.

Fig. 4: The optical spectra of optical PAM4 (PAM4, four-level pulse amplitude modulation)/VSB (VSB, vestigial sideband)-PAM4 signals and the downstream y-polarised BERs (BERs, bit error rates) at different centre wavelengths of tunable optical VSB filter.
figure 4

a The optical spectra of 30 Gb/s optical PAM4 and VSB-PAM4 signals. b The downstream y-polarised BERs as a function of the centre wavelength of the tunable optical VSB filter.

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To directly correlate with BERs and the centre wavelength of the filter, the downstream y-polarised BERs as a function of the centre wavelength of the tunable optical VSB filter are presented in Fig. 4b. Optimum transmission performance, featured by the lowest BERs and clear eye diagrams, is achieved when the centre wavelength shifts to 1550.28 or 1550.48 nm. Shifting the centre wavelength to 1550.28 nm or 1550.48 nm results in the removal of part of the optical PAM4 signal spectrum, thereby converting it into an optical VSB-PAM4 signal. However, when the centre wavelength shifts to 1550.38 nm, the signal remains as an optical PAM4. The BER performance degrades to 1.2×10−5, primarily due to fibre dispersion effects from 30 km SMF transmission. Shifting the centre wavelength to 1550.18 nm or 1550.58 nm results in poor transmission performance with high BERs and turbid eye diagrams, because most of the optical PAM4 signal spectrum is removed in these two scenarios. This scenario selects the centre wavelength of a tunable optical VSB filter at 1550.28 nm to achieve the lowest BERs and the clearest eye diagrams. It is to be noted that the centre wavelength shift of the tunable optical VSB filter will reduce the transmission performance. Therefore, precise filtering is crucial to maintain transmission performance. To overcome the technical challenges associated with centre wavelength drift, an OBPF (Alnair Labs CVF-300CL) with ±0.01 nm high wavelength stability and repeatability is used as a tunable optical VSB filter. The tunable optical VSB filter offers ±0.01 nm wavelength stability and repeatability, ensuring the centre wavelength remains consistent over time. Wavelength stability relies on the wavelength interference within thin films deposited on a substrate. These thin films are designed to selectively transmit wavelength within a narrow wavelength band while reflecting wavelength outside that band. The OBPF (Alnair Labs CVF-300CL) includes a mechanism to track and correct the transmitted wavelength through feedback control thereby maintaining the centre wavelength at a precise value. This mechanism is critical to achieving the specified wavelength stability and repeatability of ±0.01 nm.

Methods

Bidirectional WDM fibre-FSO communications

Figure 5a presents the structure of the bidirectional WDM fibre-FSO communication using the polarisation multiplexing technique and tunable optical VSB filter. An actual experimental setup is constructed instead of a simulated experimental setup. The experimental setup for photo capture is shown in Fig. 5b. 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 polarisation controller (PC). A 30-Gb/s PAM4 signal externally modulates the 8-even optical sidebands through a modulator driver to linearise the electrical PAM4 signal30. Afterwards, the optical PAM4 signals are split into two parts along two orthogonal polarisations utilising a polarisation beam splitter (PBS). Next, the optical sidebands in x-polarisation and y-polarisation are combined using a polarisation beam combiner. An optical delay line is introduced in one of the polarisation paths to compensate for the phase difference between two polarisation parts. Furthermore, after the output from the odd optical sideband port, 8 odd optical sidebands with 50 GHz frequency separation are generated. The 8 modulated even optical sidebands and 8 unmodulated odd optical sidebands are then combined using a 2×1 optical combiner. The combined 16 optical sidebands pass through an EDFA. A variable optical attenuator is placed at the beginning of the 30 km SMF transmission. 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. When the laser light travels through a 1.6 km long-distance optical wireless link, achieving complete coupling into the fibre ferrule becomes challenging owing to the spreading characteristics of laser light. On the reception side, the triplet lens functions as a concentrating device, concentrating the laser light precisely onto the fibre ferrule. It plays a vital role in transmitting laser light through a long-distance optical wireless link. To precisely concentrate the laser light onto the fibre ferrule, the fibre ferrule should be exactly positioned at the triplet lens focal point. In addition, after passing through a 1.6-km FSO link, the laser light diameter must be smaller than the triplet lens diameter at the reception side. With a large laser light diameter, the triplet lens will accumulate less laser light. This reduction in received optical power will substantially affect the reliability and performance of the FSO link. 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 the optical PAM4 signal into an 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 equalised with an electrical equaliser to enhance linearity and performance. After electrical equalisation, 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 equaliser, and is then detected using an error detector. Additionally, VSB-PAM4 eye diagrams for both polarisations are captured with a digital storage oscilloscope. Table 1 details the optical power levels at critical points in the structure, including two cascaded MZM-OEOs output, MZM output, EDFA output, the output after 30 km SMF transport, the output after 1.6 km FSO link, tunable optical VSB filter output, and PBS output (x-polarised /y-polarised).

Fig. 5: Bidirectional WDM (WDM, wavelength-division-multiplexing) fibre-FSO (FSO, free-space optical) communications.
figure 5

a Structure of the bidirectional WDM fibre-FSO communication using polarisation multiplexing technique and tunable optical VSB (VSB, vestigial sideband) filter. b The experimental setup for photo capture. MZM Mach-Zehnder modulator, OEOs optoelectronic oscillators, OIL optical interleaver, PC polarisation controller, PAM4 four-level pulse amplitude modulation, PBS polarisation beam splitter, PBC polarisation beam combiner, EDFA erbium-doped fibre amplifier, VOA variable optical attenuator, OC optical circulator, SMF single-mode fibre, ED error detector, DSO digital sampling oscilloscope.

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Table 1 The optical power levels at key points in the setup
Full size table

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 by the MZM. The optical PAM4 signals are split into two orthogonal polarisations by a PBS. One of the polarised signals goes through an optical delay line. The delay line is used to compensate for the phase difference that occurs between the two polarisations. The optical sidebands in x-polarisation and y-polarisation are subsequently combined using a polarisation beam combiner. After amplification by an EDFA, a variable optical attenuator optimally controls the optical powers. Through routing by OC2 and OC1, the optical signal with 8 modulated odd optical sidebands is transported over a 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 the optical PAM4 signal into an 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 equaliser. The equalised 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.

Research results based on discrete components and commercial equipment present some foreseeable challenges for future commercial use, especially the integration of the critical parts of the setup. One of the foreseeable challenges is ensuring that all components and commercial equipment can operate continuously under commercial conditions. Discrete components such as OIL, PC, MZM, PBS, polarisation beam combiner, x-polarised /y-polarised receiver, and equaliser used in transmission and reception sides are designed for commercial applications. Discrete commercial equipment of error detectors and digital sampling oscilloscopes used in the reception side can be replaced by handheld devices like mobile phones or tablets in actual scenarios. Discrete components used in optical fibre communications such as EDFA, variable optical attenuator, OC, and SMF are designed for commercial use. The discrete component of triple-lens used in the FSO link is currently an experimental prototype but is close to a commercial product. In addition, in practical scenarios, a discrete component of the tunable optical VSB filter used in the reception side can be replaced by a programmable OBPF. The research setup of bidirectional WDM fibre-FSO communications shows promise for future commercial applications, several challenges related to operational continuity, integration with handheld devices and programmable OBPF, and transition from prototype to commercial product need to be addressed. Overcoming these challenges will enable the transfer of research innovations into commercially practical applications.

Calculating the maximum FSO link with the deployment of a triplet lens

The FSO link is extended to 1.6 km with the deployment of a triplet lens on both sides. A triplet lens, with 146 mm/88.6 mm focal length/diameter, comprises three lenses (convex, concave and convex lens) that are gathered together. Given that the fibre’s numerical aperture is 0.14, the laser light diameter (d) can be obtained as:

$$d=2\cdot (146\cdot 0.14)=40.88 \, ({{\rm{mm}}})$$
(3)

Since the laser light diameter (40.88 mm) is smaller than the triplet lens 1 diameter (88.6 mm), an FSO link with triplet lens 1 is achievable. Over an l-m FSO link, the laser light diameter (dL) should be smaller than the triplet lens 2 diameter (dL < 88.6 mm) to ensure feasibility for an FSO link with triplet lens 2:

$${d}_{{\mbox{L}}}=\sqrt{{d}^{2}+{(2\theta l)}^{2}}=\sqrt{{40.88}^{2}+{(0.049l)}^{2}} \, < \, 88.6$$
(4)

where θ is the divergence angle. l is calculated as 1604, revealing the maximum FSO link is 1604 m. The FSO link performed here is 1600 m, which is less than the maximum target distance of 1604 m.

FSO link is influenced by atmospheric attenuation and turbulence when the optical signal passes through the FSO channel. Atmospheric attenuation arises from absorption and scattering by atmospheric particles and molecules. Atmospheric turbulence, in turn, stems from fluctuations in the refractive index associated with the temperature and pressure gradients along the propagation path, causing the redistribution of signal energy. Through a 1.6 km FSO link, in clear weather, the atmospheric attenuation and turbulence can be as low as 2.7 dB. In severe weather conditions like heavy fog, snow, and rain, the atmospheric attenuation and turbulence can increase to 50 dB. Severe atmospheric attenuation and turbulence seriously affect the availability of the FSO link, resulting in poor performance. To mitigate the impact of severe weather on the FSO link, however, an MMW connection can be used as a backup31. In this demonstration, the 1.6 km FSO link experiences approximately 2.7 dB of atmospheric attenuation and turbulence, and around 2.4 dB of atmospheric link loss exists because of background light noise. The additional 2.4 dB atmospheric link loss due to background light noise reduces the accessibility of the FSO link. The actual loss of the 1.6 km FSO link is 5.1 dB (2.7 dB from atmospheric attenuation and turbulence, and 2.4 dB from atmospheric link loss due to background light noise), showing that a 1.6-km FSO link utilising a couple of triplet lenses meets the target of high link accessibility.