Large-capacity wireless data transmission systems are demanded along with the development of multimedia services, video-based interactions, and cloud computing in the era of big data. Compared with radio-frequency communication systems, free-space optical (FSO) signal transmission technology has the merits of high data rate, great flexibility, less power consumption, high security, and large license-free bandwidths [1–3], which has been widely applied in terrestrial transmission [4], last mile solutions [5], ground-to-satellite optical communication [6], disaster recovery [7], and so on. To date, up to 10 Gbit/s FSO communication system has been realized for transmission distance over 1000 km of star-ground or inter-star communications [8], and 208 Gbit/s terrestrial communication is also reported at 55 m transmission distance [9]. Wavelength-division multiplexing (WDM) technology is commonly employed to improve data transmission capacity in fiber communication systems, which would be more effective in FSO communication systems benefitting from very weak non-linear cross talk between different frequency channels in free space. Based on a simulation platform, a WDM FSO communication system could boost the signal transmission capacity to 1.28 Tbit/s by modulating 32 optical channels with dual-polarization 16 quadrature amplitude modulation signals [10]. To date, beyond 10 Tbit/s FSO communication systems have been experimentally demonstrated recently using WDM technology [11,12]. However, a WDM communication system becomes power-hungry and bulky with the increase of transmission channels while traditional distributed feedback lasers are used as optical carriers. In addition, more rigorous requirement is imposed on the frequency tolerance of carrier lasers to avoid channel overlap with the decrease of channel frequency interval.

The invention of microresonator-based optical frequency combs provides novel integrated optical laser sources with the natural characteristic of equi-spaced frequency intervals which can overcome the challenge of massive parallel carrier generation [13–19]. In particular, the spontaneously organized solitons in continuous-wave (CW)-driven microresonators provide a route to low-noise ultra-short pulses with a repetition rate from 10 GHz to beyond terahertz. Soliton microcombs (SMCs) are typical stable laser sources where the double balances of non-linearity and dispersion as well as dissipation and gain are reached in microcavities. Meanwhile, the linewidth of the comb lines is similar with the pump laser, which enables low power consumption and costs multiwavelength narrow-linewidth carriers for a wide range of applications. Through designing the scale of microresonators, the repetition rate of SMCs could be compatible with dense wavelength-division multiplexing (DWDM) communication standard. To date, several experiments have demonstrated the potential capacity for ultra-high-speed fiber communication systems using SMCs as multiwavelength laser sources [20–30]. For instance, a coherent fiber communication system has improved the transmission capacity up to 55 Tbit/s using single bright SMCs as optical carriers and a local oscillator [20]. And dark solitons and soliton crystals are also employed as multiwavelength laser sources for WDM communication systems [27–30]. However, few studies have carried out massive parallel FSO communication systems using the integrated SMCs as laser sources.

In this paper, we experimentally demonstrate a massive parallel FSO communication system using an SMC as a multiple optical carrier generator. 102 comb lines are modulated by 10 Gbit/s differential phase shift keying (DPSK) signals to boost the FSO transmission rate up to beyond 1 Tbit/s. The transmitter and receiver terminals are installed in two buildings at a distance of ∼1km, respectively. Using a CW laser as reference, the influence of optical signal-to-noise ratios (OSNRs) on the bit error rate (BER) performance is experimentally analyzed. Our results show an effective solution for large-capacity spatial signal transmission using an integrated SMC source which has potential applications in future satellite-to-ground communication systems.

Figure 1(a) shows one typical application scenario of the proposed massively parallel FSO communication system which uses an SMC as multiple wavelength optical carriers. To verify the feasibility, two buildings with a straight-line distance of 1 km are chosen to settle the transmitter and receiver terminals. For an FSO communication system, the power penalty is mainly caused by the laser divergence angle. Two optical lenses with a 25 mm aperture are employed for laser alignment and collection at the transmitter and receiver terminals, respectively. Meanwhile, an auto laser tracking system is used to maintain the system stability. The measured power penalty of the communication link is ∼40dB during our long-time experiments. On the transmitter side, a coherent broadband SMC is used as equal frequency interval optical carriers for massively parallel data transmission. The SMC is demultiplexed using a WDM device and modulated by high-speed signals with a non-return-to-zero differential phase shift keying (NRZ-DPSK) format. All the modulated comb lines are multiplexed together by another WDM device for parallel signal transmission. To compensate power loss of the space link, the optical signals are amplified using an erbium-doped fiber amplifier (EDFA) before transmission. On the receiver side, the parallel optical signals are demultiplexed by a WDM device and demodulated using a delay-line interferometer (DLI) which is composed of an unequal arm Mach–Zehnder interferometer. The delay time equals the signal period. A balanced photodetector is employed to detect the interference signals, and the differential data is reshaped using a clock and data recovery circuit before data error measurement.

To verify the feasibility of the parallel FSO communication system, the single SMC state is employed as a multiple wavelength optical source for the merits of broadband, smooth spectral envelope, and inherently low phase noise. The SMC is generated in a high-index doped silica glass microring resonator (MRR) using the well-developed auxiliary laser-assisted thermal balance scheme. Figure 2(a) shows the packaged MRR which has a free spectral range of ∼48.97GHz and a quality factor of 1.7 million. A laser module with a typical linewidth of 100 Hz is used as the pump, which ensures the high coherence character of the SMC teeth. A computer program is used to control the generation of single SMC, which is also used to maintain the single SMC state through tuning the pump laser frequency according to the SMC power, as well as the auxiliary laser frequency on the basis of the beating tone between the auxiliary laser and SMC [31–34]. Figure 2(b) shows the typical optical spectrum of a single SMC, which spans over the C and L bands. Once the SMC is formed, the index of the MRR will be modulated through cross-phase modulation (XPM) effect which results in an XPM comb generation. A periodic intensity modulation is added to the optical carriers due to the beating between the SMC and XPM comb lines. Fortunately, the power modulation amplitude is ∼1% as the XPM comb power is about 20 dB lower than the power of the SMC [32]. Therefore, the impact of carrier intensity modulation can be neglected in the proof-of-principle experiment. Figure 2(c) compares the phase noise spectra of the CW laser (1551.72 nm) with an optical linewidth of about 10 kHz and a filtered comb line (1560.626 nm) with an optical linewidth of about 500 Hz. These narrow linewidths have negligible effects on the performance of the communication system, according to the relevant DPSK transmission theory [35]. The OSNR is also measured at the transmitter and receiver terminals to evaluate the OSNR influence of the FSO link. The under test optical signals are amplified to 15 dBm, and the amplifier spontaneous emission (ASE) noise of the EDFAs is filtered out using narrow bandpass optical filters. Figure 2(d) presents the measured optical spectrum of an optical carrier (1560.626 nm) at the two terminals, which indicates about 8.28 dB degeneration of the OSNR which mainly caused the noise of the EDFA. The measured results indicate that the optical link has slight influence on the optical signal OSNRs.

First, we evaluate the FSO communication system performance through comparing the BERs when the SMC comb lines and a separate continuous-wave laser diode are used as optical carriers. As the SMC has a sech2 spectral envelope, the OSNR of the comb lines decreases when the frequency is far away from the central frequency. To evaluate the influence of optical carrier OSNR, the comb lines of 1559.093 nm, 1553.554 nm, 1548.054 nm, and 1537.171 nm are selected as optical carriers, whose OSNRs are 45.77 dB, 42.95 dB, 39.27 dB, and 29.17 dB, respectively. The optical carriers are modulated by NRZ-DPSK signals at a rate of 10 Gbit/s. The modulated optical signals are demodulated using a DLI after 1 km transmission. Figure 3(c) shows the measured eye diagrams for different optical carriers when the detected power is −26dBm. A clock and data recovery circuit is used for signal re-shaping before BER measurement, and region vi in Fig. 3(c) shows the typical eye diagram of the recovered signal. Figure 3(a) presents the measured BERs versus received power for different comb lines and a CW diode. Using 10−3 as a reference, the receiver sensitivity is about −36dBm for the CW diode and comb line of 1559.093 nm. The CW diode has a higher BER decrease rate, which is mainly related to the OSNR of optical carriers. The comb line of 1537.171 nm has lower sensitivity due to the poor OSNR. According to the DPSK transmission theory, the relationship of the BER and OSNR can be expressed as BERDPSK=12erfc(OSNR·BrefRs), where RS, Bref are the symbol rate and reference bandwidth, respectively [20,36]. In our experiments, RS and Bref are 10 Gbit/s and 12.5 GHz, respectively. Figure 3(b) presents the curves of BERs versus OSNR at the received power of −32dBm. The theoretical curve shows that the BER monotonically decreases with the OSNR, and an error-free operation (BER less than 10−9) is achieved when the OSNR is beyond 11.57 dB. Compared with the theoretical result, a much higher OSNR is demanded for error-free operation in actual optical links due to the system noises. For example, an OSNR of 27.7 dB is required even for a back-to-back measurement using a CW laser diode. For an FSO communication system, there is a large OSNR penalty due to the signal quality deterioration caused by the atmospheric scattering, turbulence, and background radiation from natural and artificial sources [37].

To demonstrate the massively parallel FSO communication system, 102 comb lines (1528.726–1568.279 nm) are selected and transmitted over ∼1km between two buildings. Due to the frequency interval mismatching between the repetition rate of the SMC and the standard of the International Telecommunication Union, a wavelength selective switch (WSS) rather than a standard WDM device is employed for exact carriers selection. To simplify the verification experiment, the 102 comb lines are separated into two groups, where one comb line is individually modulated, and the other 101 comb lines are modulated together using another modulator to emulate the WDM FSO transmission. All the 102 comb lines are individually modulated in turn to evaluate communication system performance. Figure 4(b) shows the measured optical spectra of the 102 individually modulated comb lines. The two groups of modulated optical signals are multiplexed together using another WSS and amplified up to 19.8 dBm by a commercial EDFA. Figure 4(a) shows the cumulative BERs (1 min) of all the 102 optical channels. For the carrier wavelength from 1551.586 nm to 1568.279 nm (42 channels), the BERs are less than 10−9. Different from optical fiber communication systems, the FSO communication system is more impressionable to atmospheric scattering, turbulence, and background radiation, which results in real-time variation of the measured BER. The inset of Fig. 4(a) shows the real-time BER for an optical channel of 1551.192 nm over 30 min, where the red solid line is the measured BERs with counting time circle of 10 s, and the green dotted line shows the corresponding cumulative BER.

For the optical carrier wavelength shorter than 1551.192 nm, the measured BERs are higher than 10−9 and gradually increase up to 4.0×10−3 at a wavelength of 1528.726 nm. It is mainly caused by the OSNR decrease. As the SMC has a sech2 optical spectral envelope, the power of the comb lines gradually decreases from the central wavelength of 1561.885 nm, which is about a 1.6 nm redshift from the pump due to Raman self-frequency shift. The lower power induces ASE noise when the optical carriers of shorter wavelengths are amplified using an EDFA. The ASE noise not only induces the OSNR decrease, but also reduces the usable signal power at the receiver terminal. Therefore, increasing the power comb lines is important for the parallel FSO communication system. Intuitively, the SMC power can proportionally increase with the pump power. However, the available pump power will be limited for a practical system. Perfect soliton crystals can improve the power of comb lines by N2 times, where N is the soliton quantity [32]. However, the optical carriers reduce by N times, which reduces the available optical channels. Actually, the poor OSNRs of optical carriers appear at the wavelengths far away from the pump due to the sech2 optical spectral envelope. A more reliable method is improving the flatness of microcombs, such as dark pulses or platicon in microresonators with normal and near-zero dispersion, respectively. Meanwhile, the total communication capacity can be further improved while more optical carriers of the SMCs are employed and modulated with higher baud rate or high-order modulation format. Finally, the transmission distance can also be increased using larger-aperture optical lenses as transmitter and receiver antennas.

A DWDM FSO communication is demonstrated over two buildings at a distance of ∼1km using an SMC as an integrated multiwavelength laser source. The total transmission capacity is up to 1.02 Tbit/s with 10 Gbit/s per optical channel, which can be further improved using more optical channels and higher modulation rate. The proposed FSO communication system can be miniaturized along with the development of a photonic integrated circuit, which would satisfy the requirement of rapid large-capacity communication system deployment for disaster recovery, defense, and so on.

Acknowledgment. The authors thank R. K. X. for providing high-resolution images of the experiment scenario.