1. Introduction

In recent years, Free-Space Optical (FSO) communication has attracted numerous attentions owing to its promising properties such as inherent security, unlimited bandwidth, free-licensed spectrum, and low cost [1–3]. In 2011, with successful launch of “Ocean 2” satellite, it became the first satellite-to-ground FSO two-way communication device with the highest traffic rate of 504 Mbps is designed by Harbin Institute of Technology in China [4]. In 2012, coherent optical communication technology for satellite-to-ground FSO communication has been researched by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, and in 2016 it has been demonstrated with the launching of quantum satellite (Mo-tse) [5]. In 2016, European Data Relay Satellite (EDRS) has been launched into Geosynchronous Orbit (GEO) which provides up to 1.8 Gb/s of laser bandwidth and NASA’S Laser Communication Relay Demonstration (LCRD) which has been updated, will carry out high data rate optical communication service experiments for at least 2 years [6,7]. These indicate that FSO communications have developed rapidly and have been widely applied all over the world. With the development of FSO communication technology, it is urgent to construct a Space-Air-Ground-Ocean (SAGO) optical communication network to meet the demand for space optical applications [8].

However, a number of factors like laser phase jitter, power fading and beam scintillation induced by FSO channels have a great impact on communication quality [9–11] and increase the difficulty of FSO network construction. Many methods have been proposed to overcome these problems. Mathematic distribution models such as Gamma-Gamma fading channel [12,13], Log-Normal fading channel [14,15], and Weibull fading channel [16,17], have been set up to research the atmospheric channels. It’s worth noting that it is difficult to optimize the performance of FSO communication due to the stochastic distribution of atmospheric turbulence [18]. Forwarding relays, for example mixed Radio Frequency (RF)/FSO relay [19], Amplifier Forwarding (AF) relay [20], Decode and Forward (DF) relay [21], combined with atmospheric channel analysis, are regarded as other schemes to enhance the effective communication distance. In [22], Tian et al proposed the multi-hop parallel DF-based FSO relay system and also proved theoretically that the performance can be improved effectively by the max-min criterion and the increasing number of paths with consideration of Gamma-Gamma distribution. However, this scheme is hard to be applied in the future in large capacity FSO network construction because of its signal delay, large power consumption of relay, bandwidth limitation and high system complexity. In the real application scenario, one-to-many broadcasting transmission in FSO communication can effectively reduce the times that the same signals enter the atmospheric channels. Generally, mixing RF and FSO system, which limits signal bandwidth and is insecure, is to convert optical signal to radio frequency signal and transmit to users [23]. So far, no effective, low-consumption, safe and channelized method to realize one-to-many channels broadcasting transmission for FSO communication has been proposed.

To realize one-to-many channels broadcasting transmission, optical signal with different channels can be a good choice to realize transmitting the signal at the same time. The Optical Frequency Comb (OFC) with a specific interval of wavelength multiplexing output has been widely used in the optical fiber communication or measurement, such as frequency comb metrology [24], multi-wavelengths coherent transmission [25], high-precision molecular spectroscopy [26]. In [27], Jiachuan’s team achieved 2.56 Tb/s 16QAM 792 km transmission based on OFC for optical fiber communication. In [28], the Authors used a centralized OFC for future Wave Division Multiplexing (WDM)-based hybrid optical fiber-wireless access network, and the core content of this article is optical fiber communication with laser array. However, the potential of OFC has not yet been harvested for FSO communication network construction.

In this paper, the OFC module with 100 GHz frequency interval is fabricated by our laboratory and a scheme is proposed to increase communication efficiency and reduce the times the same signals enter the atmospheric channels. Compared with the scheme using array laser that we have made in [29], the scheme based on OFC doesn’t need combiner that is used to combine multiple optical beams into a single beam, this reduces the power consumption and system complexity greatly. Meanwhile, compared with the combination of single wavelength laser and power divider, the combination of OFC and WDM can achieve channelized and wide-spectrum FSO signal transmission which is very important in communication network construction. With this scheme, the FSO communication network can be constructed, lower complexity and integrated system design can reduce power consumption, signal loss and increase the stability of system which are very important to transmitters and forwarding relays carried in mobile terminals. To demonstrate the potential of our scheme, the outdoor experiment with 12.5 Gb/s On-Off Keying (OOK) and the indoor experiment up to 21 Gb/s with one-hop are tested based on 4 channels of OFC module in the free space. The experimental results verify the feasibility of one-to-many FSO broadcasting communication and open a new platform for FSO communication network construction.

2. Optical frequency comb module

Owing to that the number of forwarding channels in the FSO communication is depend on the flatness and the comb numbers of OFC, and the frequency combs should be stable to match the channelized transmission, the wide-spectrum, ultra-stable OFC module needs to be fabricated. In order to prepare OFC module, the Fabry-Perot (F-P) Quantum Dot Mode Locked (QDML) laser based on passive mode locking technology is chosen. The QDML laser doesn’t require an external modulator because the pulse is generated under the interaction between multiple longitudinal modes and saturated absorption materials in passive mode locking technology [30]. If $E_{(t)}$ represents the pulse field of QDML laser, the formula is given that:

According to the Eq. (3), the frequency interval of OFC is equal to pulse sequence frequency of F-P QDML laser which corresponds to F-P cavity length. Based on ex-period technology of our lab [31], we fabricated the QDML laser and the control circuitry. Finally, the OFC module with a 100 GHz frequency interval was fabricated whose photographs are shown in Fig. 1(b). Compared with the previous laser modules, we improved the circuit design, adopted the current control chip (ADN8810, Analog Devices Inc.) and the temperature control chip (MAX1978, Maxim Integrated Inc.) and to operate of the OFC module. Meanwhile, we adopted the thermistor and thermoelectric cooler to keep the temperature of QDML laser stable. A high-precision resistance with 1 $\Omega$ and an Analog-to-Digital Converter (ADC) chip were used to guarantee the laser power output correctly and stably. Additionally, the central processor connected with a computer by the design of the 37-pin interface was used, the work temperature and the driver current can be set by the serial instructions. By these means, the wavelength and output power of OFC module can be set and fixed. In Fig. 1(a), the stability of output power of OFC module is tested for 3600 s, the average power is kept around 2.81 dBm with 86 mA driver current and the change of increased output power is 0.02 dB during 1 hour. In Fig. 1(b), the volume is 22 × 16 × 4 cm, the weight is less than 2 kg and the maximum power consumption is about 3 W.

 

Fig. 1 (a) The stability of output power of OFC for 3600 s and (b) the photograph of OFC module with the frequency interval of 100 GHz.

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The output wavelengths of OFC are supposed to satisfy the requirement of ITU standard because of the channelization of WDM, and in the previous research [32] we know that if the work temperature or current of OFC changes, the wavelengths will change. Their correlation is that the temperature changes by 1 $°C$ or the current changes by 10 mA, the wavelengths move by ~0.1 nm. After adjusting temperature and current to 25 $°C$ and 86 mA, the peak wavelength of OFC module is 1533.604 nm and the line-space comb of the OFC is 100 GHz shown in Fig. 2(a). In order to meet the requirements of multiple channel FSO communication, the frequency comb teeth and output power of OFC should be stable. In Fig. 2(a), the spectrum of OFC with the span of 20 nm is tested every 10 mins for 1 hour. The comb teeth jitter is hard to be observed, the uniform wavelength interval is about 0.8 nm, and the number of comb teeth is up to 25 over the span of 20 nm. In Fig. 2(b), the flatness of comb teeth with the wavelengths range from 1531 nm to 1537 nm can be limited to less than 1.5 dB.

 

Fig. 2 The spectrogram for the 100 GHz repetition rate OFC when the temperature is 25 $°C$ and the current is 86 mA with the span of 20 nm in (a) and the span of 6 nm in (b).

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3. The experimental demonstration of one-to-many FSO broadcasting communication

In the experiment, the experimental platform shown in Fig. 3 is established with the Acquisition, Pointing and Tracking system (ATPs) and three parts which is transmitter, forwarding relay and receiver respectively. The laser emits from OFC whose average output power is 2.81 dBm and every wavelengths are modulated by Intensity Modulator (IM) in Fig. 3(a). The first Erbium-Doped Fiber Amplifier (EDFA) is used to enhance the output power of OFC module to ~10 dBm to achieve a higher signal optical carrier, and the second is to compensate the power consumption caused by communication channel in the transmitter. A beam of multi-wavelength laser emits from transmitter and is converted to space light by ATP. The space light is received, injected into the fiber and divided into multiple wavelengths by 100 GHz WDM in Fig. 3(b). The multiple channels are connected with n × n Optical Switch (OS) and each carrier signal is allocated to different ATP by the controlling OS. The main power consumption occurs in WDM which is about 8 dB and the power loss of optical switch is lower than 0.3 dB. After free-space transmission, signals in each lane are detected by PD respectively and analyzed by Digital Serial Analyzer (DSA, DSA8300 Tektronix Inc.) and Error Analyzer (SHF 1125A) in Fig. 3(c). In the forwarding relay and receiver, the fixed value EDFAs our group made with less than −30 dBm power response have to be used because of the power loss caused by atmospheric turbulence. In order to achieve signal eye diagram and BER, Pulse Pattern Generator (PPG, E8403A Agilent Inc.) is used in the transmitter to generate 12.5 Gb/s, Non-Return-to-Zero (NRZ), 2¹⁵-1 Pseudo-Random Binary Sequence (PRBS) signals. Owing to that all devices in the forwarding relay are transparent devices, there is no signal delay and bandwidth limitation in the forwarding process. Meanwhile, all frequency combs carry the same signal and are allocated to different receivers by the controlling optical switch. Finally, one-to-many transmission like wireless optical broadcast for free-space optical communication network will be achieved by this scheme.

 

Fig. 3 The experimental system diagram and architecture of (a) transmitter, (b) forwarding relay and (c) receiver. OFC: Optical Frequency Comb; IM: Intensity Modulator; PPG: Pulse Pattern Generator; ATP: Acquisition, Pointing and Tracking; WDM: Wavelength Division Multiplexer; PD: Photo-detector; DSA: Digital Serial Analyzer; EDFA: Erbium-Doped Fiber Amplifier.

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To demonstrate the one-to-many channels broadcasting transmission in the FSO communication, we chose four lanes of wavelength which was 1532.796 nm, 1533.604 nm, 1534.414 nm and 1535.224 nm respectively to be testing channels and obtained BER and eye diagrams with directly detecting 12.5 Gb/s OOK modulation format signals in the outdoor experiments as shown in Fig. 4. The distance of FSO communication was about 50 meters between two buildings, the transmitter and receiver were placed together, and the forwarding relay including WDM and OS was placed in the other building. Owing to the stochastic distribution of atmospheric turbulence, the instantaneous BER is meaningless and the BER of long term measurement always depends on the worst error sampling point. We chose 300 sampling points (five points in one second) as a cumulative BER, and measured each channel lasting for half an hour. From the experimental experience in Hefei whose results have been published in [33], the operation time we preferred to measure was 8:00~10:00 am every day, and the better results are shown in Fig. 4 after a period of time. The values of BERs with four channels range from 1e-3 to 1e-2 and the jitter of BERs is caused by the unstable FSO channels. The cumulative eye diagrams are also shown in Fig. 4 and the extinction ratio range from 0.9 dB to 1.2dB.

 

Fig. 4 The BERs, eye diagrams of four channels with 12.5 Gb/s for 30 min and the wavelengths of four channels.

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In order to explore the transmission potential of OFC module in FSO communication and exclude the influences of variation of turbulence and the power loss of multi-hops, the stable free space and one hop communication channel are needed. We placed the PD at the output terminals of OS and made the indoor experiment. The indoor distance was about 10 meters and the testing results are shown in Fig. 5. At the traffic rate of 11 Gb/s, the BERs are about 3e-10. Because of the baud rate limitation of PPG whose highest rate is 13.5 Gb/s, the multiplexer (N4876A, Agilent Inc.) is used to get a higher baud rate. Two lanes of PRBSs are emitted into the multiplexer and the signal delays are adjusted manually. In order to keep consistent with 12.5 Gb/s outdoor experiment which is generated directly by PPG, we chose the 13 Gb/s signals generated by PPG and 14 Gb/s signals generated by the multiplexer. Compared with the variation of indoor turbulence which can be ignored, multiplexed PRBSs have a more impact on BERs among four channels due to manual adjustment. Thus, we chose to test BERs of four channels before changing the traffic rate. The results in Fig. 5 show a great consistency of four channels and the traffic rate of one hop FSO communication based on OFC module can reach 21 Gb/s. At the traffic rate of 21 Gb/s, BERs are about 2e-2. The reason why the sharp decline of BER curve happens in 14 Gb/s in Fig. 5 is that compared with 13 Gb/s signals, 14 Gb/s signals are generated by dual way 7 Gb/s signals, the rising and falling edges of 7 Gb/s signals are steeper and the signal quality is better. The sharp rise occurring between 14 Gb/s and 15 Gb/s is caused by the misalignment of dual way signals which is an inherent problem of the multiplexer. The misalignment results in the “big and small” eyes in the eye diagrams and the deterioration of signal quality. The BERs of eye diagrams with 14 Gb/s and 15 Gb/s are ~1e-9, ~5e-6 respectively.

 

Fig. 5 The BERs and eye diagrams of one hop FSO communication with different traffic rate based on OFC module.

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4. Conclusion

In summary, an ultra-stable OFC module with 100 GHz frequency interval has been proposed. Based on the module, we propose a one-to-many channels FSO broadcasting communication networks. Due to the precisely control circuit, the output power of OFC has a fluctuation as small as 0.02 dB and the jitter of comb teeth can be ignored. The flatness of OFC module can be limited to 1.5 dB over the span of 6 nm. Four channel wavelengths with 1532.796 nm, 1533.604 nm, 1534.414 nm and 1535.224 nm are chosen to be regarded as one-to-many communication channels. To demonstrate the one-to-many channels broadcasting optical communication in the free space, the outdoor experiment with 12.5 Gb/s OOK modulation format based on OFC module is established. Lasting for 30 min interval measurement of four channels, the values of BER ranging from 1e-2 to 1e-3 and the eye diagrams verifies the feasibility of this scheme. The indoor experiment with one-hop FSO communication is established to explore the potential transmission performance of OFC module and the highest traffic rate can reach 21 Gb/s. In this work, only four frequency comb teeth are demonstrated; however, more frequency comb teeth can be achieved by this scheme. The transfer of the technology of one-to-many channels FSO broadcasting communication based on OFC module from the research laboratory to a commercially-viable technology will depend on high-precision ATP system, the compensation of signal algorithm and atmospheric channel in receiver, and the long-term reliability. If successful, this scheme based on OFC module could leverage the benefits in one-to-many broadcasting communication and SAGO FSO communication network application.