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Abstract
A high-sensitivity and large-capacity free space optical (FSO) communication scheme based on the soliton microcomb (SMC) is proposed. Using ultra-large bandwidth stabilized SMC with a frequency interval of 48.97 GHz as the laser source, 60 optical wavelengths modulated by 2.5 Gbit/s 16-Pulse position modulation (PPM) are transmitted in parallel. A corresponding outfield high-sensitivity 150 Gbit/s FSO communication experiment based on the SMC was carried out with 1 km space distance. Our experimental results show that the best sensitivity of the single comb wavelength which has higher OSNR can reach −52.62 dBm, and the difference is only 1.38 dB from the theoretical limit under the BER of 1 × 10−3 without forward error correction (FEC). In addition, at BER of 1 × 10−3, 16-PPM has a higher received sensitivity of 6.73dB and 3.72dB compared to on-off keying (OOK) and differential phase shift keying (DPSK) respectively. Meanwhile, taking the advantage of multi-channel SMC, 60 × 2.5 Gbit/s can achieve 150 Gbit/s large-capacity free-space transmission. For comparison, commercially available single-wavelength laser based FSO communication system have also been performed in the outfield. The outfield experimental results demonstrated the feasibility of high-sensitivity, large-capacity PPM FSO communication based on SMCs and provided a new perspective for the future development of large-capacity, long-haul FSO communication.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the spectrum resources of RF communication are becoming more and more scarce, free- space optical communication, as an alternative solution for wireless communication, has received widespread attention due to its strong anti-electromagnetic interference capability, negligible spectrum resource issues, large transmission capacity and wide bandwidth [1–4]. Considering the tremendous promising potential of space laser communication, worldwide countries have spared no efforts to develop space laser communication technology, which include, for example, the United States’ s LLCD and LRCD [5–7], Europe’ s EDRS [8], Japan’ s JDRS [9], and China’ s Motse quantum satellite [10]. However, the challenges of large capacity and high sensitivity will be faced in the future when space laser communication moves towards deep space and networking [11–13]. Thus, the establishment of large-capacity and high-sensitivity transmitter terminal has become urgent.
In order to realize the development of high-sensitivity and large-capacity FSO communication while minimizing the size, weight, and power consumption (SWaP) of the system, a promising approach is to parallelly transmit multi-channels optical signals through optical frequency combs (OFCs). A diversity of OFCs suitable for large-capacity data transmission have been developed, such as mode-locked fiber laser comb [14], electro-optical (EO) modulation comb [15,16], non-linear broadened comb [17,18], semiconductor gain-switched laser comb [19] and microresonator-based SMC [20,21]. Therein, SMCs are more suitable as integrated coherent light sources due to their chip-scale size, ultra-low noise and ultra-high repetition frequency, therefore we have chosen the SMC as a compact coherent light source for our experiment. SMCs with wavelength-specific multiplexed output intervals have been widely used in fiber optic communications [22], microwave photonics [23], frequency synthesis [24], optical ranging [25]. So far, the representative applications of SMCs are mainly in fiber optic communication. In [26] and [27], 44.1 Tbit/s and 50 Tbit/s large communication capacity based on SMCs were achieved, respectively, both featuring coherent modulation. Also worthy of intense attention is the recent advances in intensity modulated direct detection large-capacity data transmission based on SMCs for applications such as data centers [28,29]. However, the potential of SMCs in FSO communication has not yet been explored.
The performance of FSO communication is affected by the fading and turbulence of the atmospheric channel. The PPM modulation method can effectively increase the peak optical power of the transmitted pulse by changing the duty cycle, thus improving the optical signal-to-noise ratio (OSNR) and reducing the impact of fading and turbulence on the communication system [30,31]. Unfortunately, the PPM sacrifices bandwidth in exchange for a higher peak power, and how to balance the relationship between them is a question worth exploring. Fortunately, benefiting from SMCs’ ultra-large bandwidth characteristics, it effectively solves the disadvantage that PPM cannot achieve high-capacity data transmission due to low spectral efficiency. Therefore, the PPM high-sensitivity modulation format based on SMCs for large-capacity data transmission is promising for future high-speed, and integrated FSO communications.
In this paper, an outfield large-capacity 150 Gbit/s (60 comb lines and with 2.5 Gbit/s 16-PPM) FSO communication experiment based on the SMC was carried out with 1 km space distance. The SMC with a frequency interval of 48.97 GHz is realized using a thermally controlled microring resonator (MRR) based on a high refractive index doped silica glass platform. Further, three different wavelengths of the SMC 1561.037 nm, 1550.766 nm, 1542.936 nm were selected for 16-PPM FSO communication field experiments at 2.5Gbit/s, 1.2Gbit/s, 625Mbit/s, and compared with commercial high-performance lasers under the same experimental conditions to evaluate the communication performance of the FSO communication system. In addition, the comparison results with OOK and DPSK modulation formats show that the 16-PPM sensitivity increases by 6.73 dB and 3.72 dB respectively at BER = 1 × 10−3. The BER curves were obtained by offline digital signal processing (offline DSP). PPM large-capacity transmission scheme based on SMCs has the characteristics of high sensitivity, large capacity, low power consumption and miniaturization, which are important for developing future large-capacity FSO communication networks.
2. Principle
2.1 Principle and generation of soliton microcomb
Large-capacity parallel optical transmission links require SMCs with stable power, enormous bandwidth, and high-performance operation, i.e., meeting the requirements of FSO communication systems when experimental conditions are hostile in the field. SMC is generated by cascade four-wave mixing process based on the third-order nonlinearity of microcavity media [32]. When continuous wave (CW) pump laser is coupled into the microcavity and the energy of the light field is above the parametric threshold, modulation instability gain is generated in the microcavity. Under the condition of phase matching, the pump laser firstly generates the comb sideband near its center frequency through degenerative four-wave mixing. As the detuning of pump frequency and microcavity resonance decreases, more comb sidebands are generated by the cascade of degenerative and non-degenerative four-wave mixing. Finally, when both the intracavity dispersion balance the nonlinearity, and the pumping balance the loss, the SMC with low noise mode-locked state will be formed in the microcavity. The Lugiato-Lefever (LLE) equation of nonlinear dynamic light field evolution in the resonator is expressed as follows [33,34],
The schematic diagram for the SMC generation is shown in Fig. 1(a). The monolithic integrated four-port high-Q (1.6 × 106) MRR with thermal electric cooler (TEC) in a compact butterfly package, as shown in the Fig. 1(b), is the first advantage that enables the stable operation of the SMC. This allows precise regulation of the MRR operating temperature by an external TEC controller, while maintaining a stable SMC state isolated from temperature fluctuations in the external environment. Another advantage is the use of laser-assisted internal cavity thermal balancing scheme to achieve SMC in a deterministic manner. In our previous experiments [35–37], the ring radius was ∼ 592.1 µm, corresponding to a free spectral range (FSR) of ∼ 48.97 GHz, and the pump laser was a high-performance laser manufactured by NKT (NKT Photonics, Koheras BASIK X15) with a fixed wavelength of 1560.2 nm and a linewidth of 100 Hz, auxiliary lasers was initialized to 1562.972 nm, and empirically initialize the TEC to 55°C to ensure that the pump and auxiliary laser are on the blue detuned side of the relevant resonance. On-chip power of pump and auxiliary lasers are similar (∼400 mW). As shown in Fig. 1(c), during the single SMC generation process, there are four typical microcomb states: pump out of resonance (I), modulational instability microcomb (II), multiple SMC (IV), and single SMC (IV). IV shows a typical optical spectrum of a single SMC we used for FSO system, which exhibits a standard squared hyperbolic secant envelope. As shown in Fig. 1(d), in order to develop the SMC as an ideal broadband coherent laser source for field transmission experiments, the stability of output power of the SMC module is tested for 3600 s, the average power is kept around 3 dBm after filtering pump, and the output power fluctuated within a range of no more than 1% over 1 hour. Actually, the soliton power can be increased by increasing the power of EDFA properly. Further observation and measurement of the frequency stability of single SMC was implemented, with a peak frequency change of 3.03 kHz and a standard deviation of 0.7782 kHz in 1 hour, all of which are critical to our FSO communication field experiments.
Fig. 1. a) Experimental setup for single SMC generation. An auxiliary laser-assisted intracavity thermal-balanced scheme is adopted for single SMC generation. (b) Image of the butterfly-packaged MRR. (c) The typical optical spectra of different microcomb states. I, pump out of resonance; II, modulational instability microcomb; III, multiple SMC; IV, single SMC. (d). The stability of output power of single SMC for 1 hour. (e). The repetition rate fluctuation of single SMC for 1 hour.
2.2 Principle and theory performance analysis of 16-PPM
PPM is an orthogonal high energy-efficiency modulation technique, which transmits information from different positions of the time slot where the pulse is located, and has a strong channel anti-interference capability. Compared with other modulation formats, L-PPM signal has higher peak power at the same average transmit power [31,38]. By virtue of its high energy efficiency, high-order PPM modulation can effectively mitigate the impact of atmospheric turbulence on laser communication transmissions, which are extremely suitable for FSO communication [39–42]. L-PPM is a single pulse signal that maps n bits of binary data to one of the $L = {2^n}$ equal time slots. Figure 2(a) represents the structure of the L-PPM symbol. The number of time slots for L-PPM symbols is a fixed L, only one time slot has a pulse signal “1’’. Consequently, its information bits transmitted are $lo{g_2}L$ per symbol. For a n-bits data set $M = ({{m_1},{m_2}, \ldots ,{m_n}} )$, for which the optical pulse is in time slot l, the corresponding mapping coding relation is $l = {m_1} + 2{m_2} + \ldots + {2^{n - 1}}{m_n} \in \{{0,1, \ldots ,n - 1} \}$. For a 16-PPM modulation with 4 bits per symbol, there are 16 representations. As shown in the Fig. 2(b), the time slot positions where the pulses are located are listed as “0”, “5”, “10”, “15” corresponding to the PPM frame transmission information of “0000”, “0101”, “1010”, “1111” cases, where P represents the pulse peak optical power and T is the time-slot width.
Fig. 2. (a) Structure of the LPPM symbol. (b) Examples of 16-PPM symbols correspond to “0000”, “0101”, “1010”, “1111” respectively. ${P_0}$ stands for the transmitted light intensity in an ON slot.
The theoretical BER equation for L-PPM modulation format in the absence of atmospheric turbulence in a Gaussian noise channel is expressed as follows [43–45],
The BER versus SNR for 2-PPM, 4-PPM, 8-PPM, 16-PPM, OOK, DPSK, BPSK and QPSK modulation schemes in absence of turbulence is given in Fig. 3(a). The required SNR for 16PPM at BER = 1 × 10−3 is 0.77 dB. The required SNR for NRZ-OOK/RZ-OOK/PAM4/BPSK/DPSK/QPSK at BER = 1 × 10−3 is 15.82 dB, 12.80 dB, 22.35 dB, 6.79 dB, 9.80 dB, 7.33 dB, which is higher than 16PPM by 15.05 dB, 12.03 dB, 21.58 dB, 6.02 dB, 9.03 dB, 6.56 dB, respectively. It can also be seen that the sensitivity gain of L-PPM increase with the value of time slots. It is fairly obvious that 16-PPM has the best BER performance; hence it is the most power efficient modulation scheme in the above schemes. The bandwidth efficiency of the L-PPM for different time slots and its power efficiency with respect to the NRZ-OOK signal is shown in Fig. 3(b), where the bandwidth efficiency is defined as the ratio of the transmittable bit rate ${R_b}$ to the required bandwidth ${B_{req}}$ in bit/s/Hz,
Fig. 3. (a) BER performance of L-PPM, OOK, DPSK, BPSK, and QPSK modulation schemes. (b) The theoretical bandwidth efficiency and the normalized power requirement of L-PPM.
3. Experimental setup
Figure 4 shows the outfield experiment setup of a high-sensitivity, large-capacity 16-PPM FSO communication system based on SMC. As shown in Fig. 4(a) at the transmitter of the system, the pump and the auxiliary light are simultaneously amplified to the same power ∼900 mW (on-chip power ∼400 mW) by erbium-doped fiber amplifiers (EDFAs, Connet, MFAP-1550-M-SF-MP-050) and the polarization is aligned to the same direction to ensure that both beams are in the blue detuned region of the resonance peak. Lowering the temperature of the MRR by tuning the TEC shifts the resonance peak in the direction of the two beams of light. Eventually, when a certain pumping optical power and intracavity temperature conditions are fulfilled, an SMC is generated. Subsequently, 60 combs with better OSNR are filtered out by the wavelength selection switch (WSS, Finisar, DWP-EP-C00) for FSO communication field experiments in the wavelength range of 1542.95 nm to 1566.23 nm. Since the single comb power of the SMC is too low to accommodate FSO communication, we amplify them with additional EDFA and filter the out-of-band noise with an optical bandpass filter (OBPF, EXFO, XTA-50/S) with adjustable center wavelength. The amplified and polarization-adjusted optical signal enters the Mach-Zender modulator (MZM, iXblue, MXER-LN-20) to achieve the 16-PPM format. The DSP process is performed by generating a 27-1 pseudo-random binary sequence (PRBS), encoding and mapping it into 16 time slots of a single signal symbol, after which amplifying it through a RF amplifier. Finally, load it onto the MZM to achieve 16-PPM format. Given the large free-space channel attenuation and space-fiber coupling losses, the modulated PPM optical signal power is not sufficient to support the power requirements of FSO communication, so we additionally used a high-power amplifier (HPA, MFAS-C-Er-PA-M) to reinforce the 16-PPM optical signal, while adding an adjustable attenuator to evaluate the performance at different transmission powers. The amplified optical signal is coupled into the transmitting telescope through optical fiber for FSO transmission. The transmitting telescope with an optical aperture of 25 mm. It should be noted that during the experiment we modulated a single wavelength with 16-PPM, while the remaining 59 wavelengths were modulated with 16-PPM simultaneously using a single MZM, due to the restriction of the number of modulators available. This allows the communication performance of each channel wavelength to be verified by transmitting a single wavelength with 16-PPM format at a time, and the simultaneous 16-PPM modulation of the remaining 59 wavelength channels as well as paralleled transmission can prove its ability to achieve 60-channel wavelength division multiplexing (WDM). Each of the 59 simultaneously modulated spectral lines can be filtered out for demodulation by means of an OBPF at the receiver. Figure 4 background image shows the field experiment diagram for FSO transmission performed during the experiment, with a 1 km space transmission distance. It should be noted that the experimental procedure was obtained under the same experimental conditions, i.e., measurements made under the same atmospheric channel conditions.
Fig. 4. Experimental setup for 16-PPM FSO communication. WSS: Wavelength selective switch, OBPF: Optical bandpass filter, MZM: Mach-Zehnder modulator, AWG: Arbitrary waveform generator, RF AMP: Radio frequency amplifier, LNA: Low noise amplifier, APD: Avalanche photodiode, DSO: Digital sampling oscilloscope, Offline DSP: Offline digital signal processing. (a) Transmitter. (b) Receiver.
As shown in Fig. 4(b), the optical signal, received by the receiving telescope, is amplified by a low-noise amplifier (LNA, KEOPSYS, CEFA-C-HG-SM-50-M201) and filtered before feeding into the avalanche photodiode detector (APD, Discovery Semiconductors, Inc., DSC-R402), filtering out-of-band noise. The receiver adopts the direct detection demodulation method, and the electrical signal after the photoelectric conversion is sampled and stored by a real-time oscilloscope (DSO, Tektronix, DPO 72304DX) with a sampling rate of 50 GSa/s for further analysis of offline DSP. The offline DSP for BER measurement is performed mainly by a conjunction of hard and soft decision. First, the independent clocks at the transmitter and receiver are synchronized by a bit synchronization algorithm, followed by the execution of a hard and soft threshold decision that is highly reliable with strongly noisy degraded signals, and normalization of the data. Next, dividing the data according to the 16-PPM time slot and finding the maximum amplitude point in each symbol as the signal value. Finally, BER measurement is performed by data decoding and comparing with 27-1 PRBS at the transmitter.
4. Results and discussion
Figure 5(a) shows the wavelengths of 60 optical frequency comb channels selected by WSS during the field experiment, which range from 1542.95 nm to 1566.23 nm and encompass most of the spectrum in the C-band. It’s quite obvious that the variance in OSNR of the comb limited to 12.7 dB and the OSNR of each comb is higher than 33.7 dB, which is naturally suitable as a multi-wavelength light source due to its flat spectrum and excellent OSNR performance. For field transmission demonstration, we coded the observed 60 combs with 16-PPM at 2.5 Gbit/s and calculated the BER of each comb wavelength at the receiver through offline DSP. It is worth emphasizing that the modulation extinction ratio of the 16-PPM format achieved in the experiment is as high as 38.6 dB, which is very desirable for achieving saturated amplification of the optical signal at the transmitter and improving the signal quality of FSO communication. Figure 5(b) shows the BERs of all 60 comb channels, indicating that all 60 comb carriers achieve BER < 1 × 10−3 without FEC. This result shows that even with the IM-DD approach, an ultra-lager transmission capacity of up to 2.5 Gbit/s × 60 can be achieved.
Fig. 5. (a) Spectrum of 60 comb wavelengths selected from single SMC. (b) BER of a total of 60 comb wavelengths.
We further tested the BER curves of OOK, DPSK, and 16-PPM modulation formats at the comb wavelength of 1561.037 nm, as shown in Fig. 6(a). It is quite clearly seen that the received sensitivity of 16-PPM is higher than that of DPSK and OOK for the same BER. At BER = 1 × 10−3, the received sensitivity of 16-PPM is 3.72 dB and 6.73 dB better than that of DPSK and OOK, respectively, while the sensitivity is only degraded by 1.38 dB compared with the theoretical BER. Figure 5(b) shows the spectrum of the three comb wavelengths circled by ellipses in Fig. 5(a), whose central wavelengths are 1561.037 nm, 1550.766 nm, and 1542.936 nm, respectively. Undesirably, the OSNR of the comb wavelengths deteriorates with respect to the commercial laser (RIO PLANEXTM Laser, RIO0195-5-01-4-W1), and gets worse when the comb wavelength further away from the center of the comb. This also interprets the difference in received sensitivity at different wavelengths in the following discussion. Figure 6(c) presents the comparison of the received sensitivity with the theoretical limit at the comb wavelength of 1561.037 nm and bit rates of 2.5 Gbit/s, 1.2 Gbit/s and 625 Mbit/s, respectively. The sensitivity penalty is 1.38 dB, 3.57 dB, 5.96 dB, respectively, when the BER is 1 × 10−3. The experimental results reveal that the sensitivity difference among the three rates in the experiment is smaller than that in the theoretical case, which mainly stems from the fact that the extinction ratio at the transmitter is high enough but not the ideal infinity, and the turbulence during the experiment leads to a relatively serious degradation of the received signal, making it not conducive to offline DSP. The inset in Fig. 6(c) shows the received 16-PPM eye diagram at three rates. The BER performance of our measured commercial RIO laser, and three selected SMC wavelengths, 1561.037 nm, 1550.766 nm, 1542.936 nm, at signal rates 2.5 Gbit/s under the same free-space channel is shown in Fig. 6(d). It can be seen intuitively that as the wavelength of the comb wavelengths moves away from the center wavelength, the received sensitivity gradually decreases. This is principally because the squared hyperbolic secant (sech2) envelope of the SMC, resulting in a power variation of the channel wavelengths, which leads to a greater reduction in the OSNR for channels located further away from the central wavelength, ultimately leading to a deterioration in BER performance. However, the sensitivity penalty is 0.53 dB, 2.83 dB, 6.22 dB, respectively, at BER = 1 × 10−3 with respect to the RIO laser, and the ultra-high sensitivity of −52.62 dBm is obtained at 1561.037 nm, which quite directly demonstrates that the performance of the SMC is quite competitive with that of the RIO laser. The inset of Fig. 6(d) shows the 16-PPM eye diagrams received at the receiver terminal, and the superior carrier performance of the SMC enables that the difference in their eye diagrams is not significant.
Fig. 6. (a) Measured BER at 2.5 Gbit/s for the modulation format of 16-PPM, OOK, DPSK, respectively. (b) Spectrogram for the RIO wavelength of 1550.724 nm, comb wavelengths at 1561.037 nm, 1550.766 nm and 1542.936 nm. (c) Theoretical and experimental measurements of BER curves of 1560.037 nm comb wavelength at 2.5 Gbit/s, 1.2 Gbit/s, and 625 Mbit/s. (d) Comparison of received sensitivity between three selected comb wavelengths and RIO laser at 2.5 Gbit/s.
During our experiments, it is worth noting that FSO links undergo substantial optical signal losses caused mainly by atmospheric absorption, scattering and turbulence [46,4]. Atmospheric turbulence typically has the greatest impact on FSO communications owing to the random variation/decay in irradiance (scintillation) and phase experienced by optical radiation traversing a turbulent atmosphere. Scintillation may seriously affect the performance of FSO communication systems, leading to degradation of the communication link, i.e. an increased probability of error in the received signal. However, measuring turbulence intensity in real atmospheric conditions is very challenging because weather effects in the outdoor environment may be a mixture of atmospheric conditions (e.g. rain, turbulence, smoke, dust, etc.). The effect of atmospheric turbulence on FSO communication links has been studied in many Refs. [47–49]. Therefore, the effect of atmospheric turbulence on transmission is beyond the scope of this paper. In our experiments, we minimize the effect of atmospheric turbulence on FSO transmission by carrying out FSO communication measurements during periods of low wind speed and high visibility (sunny day).
In conclusion, the above experimental results show that 150Gbit/s large capacity and high sensitivity FSO communication is realized by combining single SMC and high pulse peak of 16-PPM.
5. Conclusions
Summarily, we have demonstrated outfield experiments of 150 Gbit/s 16-PPM high-capacity FSO transmission based on single SMC, with all 60 comb lines exhibiting a BER of less than 1 × 10−3 without FEC. To the best of our knowledge, for the first time to achieve PPM FSO communication based on single SMC at this large capacity. The 16-PPM FSO communication based on a comb wavelength of 1561.037 nm, demonstrated in the field experiment, differs from the theoretical limit of BER of 1 × 10−3 by only 1.38 dB without FEC, and the received sensitivity is 6.73 dB and 3.72 dB higher than that of OOK and DPSK modulation formats, respectively, at BER = 1 × 10−3. Furthermore, three selected comb wavelengths were compared with commercial laser at a signal rate of 2.5 Gbit/s showing similar performance, and an ultra-high sensitivity of −52.62 dBm was obtained at the comb wavelength of 1561.037 nm with the BER of 1 × 10−3. Experimental results show that the system has enormous potential for the future ultra-long range, ultra-large data capacity FSO communications.
Funding
National Natural Science Foundation of China (61231012, 62075238, 91638101); National Key Research and Development Program of China (2018YFC0307904-02).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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