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Abstract
In this paper, a quasi-omnidirectional transmitter is proposed and demonstrated for underwater wireless optical communication (UWOC) using the photoluminescence of perovskite quantum dots (QDs). The proposed transmitter, without complex driving circuits, is compact and reliable thanks to the lens-free design. The system performance is tested in a 50-m swimming pool with a water attenuation coefficient of 0.38 dB/m. The maximum data rates of on-off-keying (OOK) signals over 10-m and 20-m transmission distances can reach 60 Mbps and 40 Mbps, respectively. When four clients are adopted in a code division multiple access (CDMA) based UWOC network, the maximum data rates of each client can reach 10 Mbps and 7.5 Mbps over 10-m and 20-m underwater channels, respectively. The system can meet the requirements of the last meter end-user access in the Internet of underwater things (IoUT) and underwater optical cellular network systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the increasing demands for underwater exploration and marine environmental monitoring, more and more underwater sensors, and other underwater platforms are deployed. The intelligent interconnection and networking between these underwater devices, e. g. underwater wireless sensor network (UWSN) and Internet of underwater things (IoUT), have become a hotspot [1]. The traditional underwater acoustic communication (UAC), although supports a long transmission distance, suffers multipath fading, high time delay and low data rate [2]. Underwater wireless optical communication (UWOC), featuring lower delay, higher capacity and more compact antenna, is regarded as a promising solution in the underwater networks for short-range broadband communications. In the past few decades, UWOC has progressed tremendously with data rates of several Gbps [3,4] and communication distances from tens of meters to more than one hundred meters with laser diodes (LDs) and high-sensitive detectors [5–7]. However, the strongly directional beam leads to strict alignment requirement between the transmitter and the receiver, which is aggravated in the dynamic and complex ocean environment. The light source with a large divergence angle or omnidirectional illumination can relax the alignment requirement and is necessary for the underwater networks. In 2019, a large-coverage UWOC system consisting of a blue light-emitting diode (LED) and an integrated positive-intrinsic-negative (PIN) array was established with a spot of 25 cm in diameter and 11 cm in alignment tolerance at a transmission distance of 1.2 m [8]. The use of diffuser and lens can enlarge the divergence angle of the optical source, but it is unlikely to achieve an omnidirectional transmitter [9,10]. In [11], a quasi-omnidirectional transmitter was realized by three uniformly distributed high-power LED array modules. But the driving circuit of the LED array is relatively complex and bulky, hindering the miniaturization and integration of underwater equipment. A novel optical beacon realized by coupling blue (B)/green (G) light into two side-scattering fibers, was used as an omnidirectional optical source, but the output power is quite limited [12]. Compared with LEDs, an LD has the advantage of high bandwidth. Therefore, an LD-based omnidirectional light source with a high bandwidth is of great significance to relax the alignment requirement and meet the need of high data rate.
The blue-green band is adopted for traditional UWOC system due to the low absorption by pure water [13]. Most of the reported UWOC systems use blue LD as the light source for its high power and high efficiency. Actually, the absorption and scattering of light in seawater are affected by the seawater composition and suspended particles, such as marine snow, very small detritus and chlorophyll [14]. The low absorption band shifts from blue to red light as the turbidity of water increases [15,16]. Table 1 shows the optimum wavelength for UWOC for different water types, which increases from 450 nm to 700 nm as the type of water changes from Jerlov I to Jerlov 9C [17]. In particular, the wavelength of light at 575 nm performs well in many types of water. The experimental results in the sea surrounding Japan [14] and the Baltic Sea [18] show that green light is superior in transmittance to other wavelengths, which is consistent with the conclusion in Table 1. Since the turbulence is one of the important factors affecting the performance of UWOC, the green light has been proved to be more reliable than blue light in a turbulent channel [19]. Besides, the silicon-based detectors is usually more sensitive in the green band, e. g. 22% as shown in [18], than the blue one, promoting green light to be a better alternative for certain UWOC applications.
Table 1. The optimum wavelength under different water types [17].
Nevertheless, the high-performance green light is difficult to be commercialized due to the “green-yellow gap” of III–V diodes [20]. It is attractive to obtain high-quality green light with mature blue LD by wavelength conversion, which can narrow down the “green-yellow gap”. Literature [18] compared the performance of blue and converted green LEDs in UWOC, and found that the converted green LEDs perform better in the Baltic Sea and other water types with similar optical properties. The wavelength conversion materials are common rare-earth-based phosphors, e.g., yttrium aluminium garnet (YAG) [21], and some nanomaterials including cadmium selenide and zinc sulfide (CdSe/ZnS) quantum dots (QDs) [22,23], carbon QDs [24], and lead halide perovskite nanocrystals (e.g., CsPbBr3) QDs [25,26]. QDs have attracted a lot of attention because of the high bandwidth. The modulation bandwidth of the QDs-based system, is closely related to many factors, such as the particle diameter, the concentration and thickness of QDs, the injection current, and the proportion of the residual pump light [27–30]. The photoluminescence (PL) lifetime of the material is a very important intrinsic factor affecting the system frequency response. For example, the commercially available phosphor YAG has a relatively long PL lifetime to the order of milliseconds [31,32], resulting in a limited modulation bandwidth of below 1 kHz [33]. Perovskite has many excellent properties, including tunable and stable emission wavelength, high photoluminescence quantum yield (PLQY > 70%) [34], relatively short PL lifetime (minimum: 3.3 ns) [35] and high luminous efficiency, especially in the green light-emitting region [26]. A kind of perovskite nanocrystals with a modulation bandwidth of 491 MHz has been investigated and adopted for visible communication (VLC) in [26]. Perovskite can match the “low loss window” of UWOC, which makes it a unique candidate for the fast color conversion [36]. The typical characteristics of optical wireless communication (OWC) systems using perovskite materials in recent years are summarized in Table 2. The bandwidth of the QDs increases along with the proportion of pump light in the output light. To make a fair comparison, whether the pump light is filtered or not is mentioned in Table 2. To enhance the stability of perovskite, some effective strategies like surface coating and inner doping have been proposed [37]. Since the instability of perovskite is mainly caused by thermal quenching, liquid CsPbBr3 QDs are proposed and verified to have a shorter PL lifetime and better thermal diffusion properties than solid-state perovskite [38,39].
Table 2. Comparison of CsPbBr3 QDs-based optical wireless communication systems.
A mature blue pump LD and liquid CsPbBr3 QDs based transmitter is proposed for UWOC. Liquid CsPbBr3 QDs were sealed in a cylindrical bottle (2 mL) and the LD with small spot area and high energy density was used as the pump source to contribute photoluminescence. There are three advantages for the transmitter. Firstly, the omnidirectional output of liquid CsPbBr3 QDs luminescence can relax the link alignment requirement and bring convenience for underwater broadcasting and UWOC networks [40]. Secondly, the wavelength can be adjusted flexibly according to the water parameters by using different kinds of CsPbBr3 QDs, to keep the minimum link loss. Specially, the transmitter we proposed can meet two different working scenarios of short-distance omnidirectional communication (with QDs, omnidirectional light) as well as long-distance laser communication (without QDs, collimated beam) with minor adjustment of the transmitter structure. Therefore, it has strong adaptability to different work scenes Thirdly, the modulation bandwidth of CsPbBr3 QDs, which is used as the color-converting material, is limited by the spontaneous emission. But CsPbBr3 QDs have a shorter PL lifetime (∼ ns) than traditional phosphors (∼ ms), enabling an advantage in bandwidth, with a corresponding theoretical bandwidth of hundreds of MHz [25]. Considering the relatively high PLQY and narrow linewidth (can be filtered out by an optical filter), the liquid CsPbBr3 QDs should be a promising candidate for UWOC networks.
In this work, to our best knowledge, the concept of using the liquid CsPbBr3 QDs as a converter to achieve wavelength switching in underwater network is firstly proposed. The CsPbBr3 QDs used in this paper have a very wide absorption spectrum and a narrow emission spectrum with the full width at half maximum (FWHM) of 15 nm. Light with different wavelengths can be generated by switching QDs without changing the pump source to fit different types of seawater. The PL lifetime of the CsPbBr3 QDs is 45 ns, which corresponds to a 20-dB bandwidth of 353 MHz. A blue pump LD and liquid CsPbBr3 QDs based quasi-omnidirectional UWOC system is built. The proposed UWOC system employs on-off keying (OOK) signals, and successfully achieves data rates of 60 Mbps and 40 Mbps with a transmission distances over 10 m and 20 m, respectively. The output power is measured to be uniformly distributed in all measured horizontal directions, which is further demonstrated by the stable bit error rates (BERs) measured with the receiver at different yaw angles. Moreover, a four-client code division multiple access (CDMA) network is proposed and experimentally demonstrated. Each client can successfully capture and recover the data from the captured signal. And the maximum data throughputs reach 40 Mbps and 30 Mbps over 10-m and 20-m underwater channels, respectively.
The rest of this paper is organized as follows: Section 2 introduces the CsPbBr3 QDs used in this paper and their optical characteristics. The experimental setup of the proposed UWOC system is described in Section 3. In section 4, experimental results are presented and analyzed. Finally, Section 5 concludes the paper.
2. Materials and methodology
QDs are semiconductor nanoparticles which exhibit size and composition-dependent optoelectronic properties [24]. The CsPbBr3 QDs used in this paper are monodispersed cubic structures with an average diameter of 11 nm. The transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images are shown in Fig. 1(a) and (b), respectively. The electrons in the semiconductor nanoparticles are excited under the irradiation of pump light, and then decay to a lower energy level, giving off a stream of photons [41,42]. In this spontaneous emission process, the wavelength conversion is realized. The absorption and PL spectra of the CsPbBr3 QDs are shown in Fig. 1(c). It can be observed that the CsPbBr3 QDs have a very wide absorption spectrum and a narrow PL emission spectrum at 522 nm with the FWHM of 15 nm. Figure 1(d) shows the normalized time-resolved PL decay trace of the CsPbBr3 QDs, which can be fitted well with a single exponential function as shown in the red line. The fitting curve reveals that the average PL lifetime is 45 ns, which is much shorter than traditional phosphors, indicating the potential of CsPbBr3 QD as a fast color converter in UWOC.
Fig. 1. The (a) TEM image, (b) high-resolution TEM image, (c) normalized absorption and photoluminescence spectra, and (d) normalized time-resolved PL decay trace of the used CsPbBr3 QDs.
The PL lifetime is a key factor on the frequency response [43,44]. The relationship between the frequency response and PL lifetime of QDs can be simulated as follows [30]:
where $H(f)$ is the frequency response, and $\tilde{\tau }$ is the average PL lifetime of QDs. When $H(f) = \frac{1}{2}$, the corresponding 3-dB bandwidth is shown in Eq. (2),It is apparent that a shorter PL lifetime promises a faster wavelength conversion. Perovskite QDs (e.g. CsPbBr3 QDs) with a PL lifetime of nanosecond order have a higher modulation bandwidth than traditional phosphors with a PL lifetime of millisecond order [36]. The 3-dB and 20-dB bandwidth of the used liquid CsPbBr3 QDs is calculated to be 6 MHz and 353 MHz, respectively, as shown in Fig. 2.
3. Experimental setup
Firstly, the spectra of the CsPbBr3 QDs and the blue LD were measured by a spectrometer (Ocean Optics, HR4600) at 26 °C, which is shown in Fig. 3. The bias current of the laser changes from 0.2 A to 0.8 A. It can be observed that the peak of emission spectrum of the laser shifts toward red (from 439 nm to 442 nm) with the bias current increasing, while that of CsPbBr3 QDs remains stable at 522 nm. The result verifies the feasibility of using the QDs to realize a stable green optical source for UWOC.
Fig. 3. Spectra of (a) LD and (b) CsPbBr3 QDs at room temperature (26 °C) under various bias currents of LD.
The structure of the transmitter designed in this paper is shown in Fig. 4(a). The liquid QDs were sealed in a bottle with a 440-nm blue LD as the excitation source located 15 cm below. In order to reduce the thermal quenching of QDs, the bottle with liquid QDs was put in water for heat dissipation. Both the QDs and the LD were installed in an acrylic watertight cabin.
Fig. 4. (a) The structure and (b) measured optical power distribution of the proposed quasi-omnidirectional transmitter.
The output direction of emitted photons is arbitrary and uniform in the space due to the spontaneous emission process. Therefore, it is theoretically possible to form an omnidirectional light source. In order to study the optical power distribution of the transmitter, the optical power on a sphere surface 40 cm away from QDs was measured using an optical power meter (Thorlabs, PM200). The bias current of the LD was set at 0.5 A. A 495-nm long-pass filter (Thorlabs, FGL495) was mounted in front of the optical power meter to filter out the residual pump light from the LD. The measured optical power is shown in Fig. 4(b), which is approximately uniform distribution on most of the area of the sphere surface. There is a relatively higher power on the top, which is mainly caused by the residual blue light from LD and not shown in Fig. 4(b). Similarly, the relatively lower power at the bottom is due to the shelter of the LD, heat sink and the fixture. Therefore, the output light power of the transmitter presents a quasi-omnidirectional distribution, indicating an attractive optical source for UWOC network.
The experimental setup based on the proposed quasi-omnidirectional UWOC transmitter is illustrated in Fig. 5. At the transmitter side, the OOK and CDMA signals were generated offline in MATLAB, and loaded into an arbitrary waveform generator (AWG) (Tektronix, AWG70002A). For the OOK scheme, the pseudo-random binary sequence (PRBS) with a length of 216-1 was transmitted. For a four-client CDMA scheme, an orthogonal Walsh matrix space with a size of 32×32 was firstly constructed and each client was assigned with 8 orthogonal codes with the length of 32. Then four PRBSs were generated for the four clients in the network, and every 4 information bits were extracted, where the first bit decided the sign and the last 3 bits were mapped to the corresponding 8 orthogonal codes. Finally, the four mapped sequences were added together as the transmitted CDMA signal. Therefore, the coding efficiency of each client was 4/32 and the spectrum efficiency of the whole network was 4×4/32 = 1/2 bit/s/Hz. The amplitude of the electrical signal was adjusted through a power amplifier (AMP) (37 dB, 100 kHz−75 MHz) and a variable electrical attenuator (VEA) (KT2.5-90/1S-2S). The amplified signal was biased through a Bias-T, and then injected into a 440-nm blue LD. The output optical signal was transmitted in air or in a 50-m standard swimming pool. At the receiver side, the optical signal was collected by a fresnel lens with a diameter of 14 cm and focused onto a highly sensitive photomultiplier tube (PMT). Then the electrical signal was captured and recorded by a mixed-signal oscilloscope (MSO) (Tektronix MSO 71254C) and processed offline. After the synchronization and channel equalization, the OOK signals were decided and recovered into binary bits. For the CDMA scheme, the signal received by each client will be firstly correlated with the corresponding 8 orthogonal codes and then both the sign and index of the most matching one will be mapped to 4 information bits. Finally, the BERs were calculated. To measure the communication performance of the CsPbBr3 QDs, a 495-nm long-pass optical filter was installed in front of the PMT to filter out the remnant of the blue light from the LD. The transmitter and receiver cabins were fixed on the adjustable lifting tables to keep the transmitter and the receiver at the same height. Besides, the system frequency response was measured by a network analyzer (Hewlett Packard, 8753D), which is shown in Fig. 6. Limited by the bandwidth of the PMT, the 20-dB bandwidth is around 80 MHz, which is lower than that of the CsPbBr3 QDs.
Fig. 5. Experimental setup of the proposed quasi-omnidirectional UWOC system. Insets: photos of (a) the transmitter, (b) the receiver cabin in the swimming pool. (S/P: Serial/Parallel, P/S: Parallel/Serial, DC: direct current)
4. Results and discussion
To make full use of the linear region of the LD, the relationships between BER versus bias current and attenuation value of the VEA were measured, as shown in Fig. 7. The minimum BER was achieved at the bias current of 0.55 A and the attenuation value of 2 dB. The output optical power of the LD was 21.82 dBm.
To explore the impact of CsPbBr3 QDs on system bandwidth, the system frequency response with and without CsPbBr3 QDs were measured respectively. In order to measure the bandwidth impact caused by CsPbBr3 QDs more accurately, the low-bandwidth AMP and PMT in the system shown in Fig. 5 were replaced with high-bandwidth AMP (ZHL-6A+, 0.0025 to 500 MHz) and avalanche photo diode (APD) (APD210, 1-1600 MHz), respectively. As shown in Fig. 8, CsPbBr3 QDs reduce the 20-dB bandwidth of the system to 200 MHz. The 20-dB bandwidth of the system based on conventional LED is generally about 70 MHz [45,46], so the transmitter we proposed has an advantage in bandwidth. Moreover, by comparing Fig. 6 and Fig. 8, the PMT and AMP with limited bandwidth rather than CsPbBr3 QDs should bear the main responsibility for the decline of the system bandwidth. What is more, the bandwidth degradation can be alleviated by using CsPbBr3 QDs with a smaller PL lifetime and improving the structural design of the devices. Note that in subsequent swimming pool experiments, low-bandwidth devices shown in Fig. 5 were employed for higher transmitted optical power with high extinction ratio and receiving sensitivity.
Fig. 8. Normalized frequency response of the bandwidth measurement system with and without CsPbBr3 QDs.
In order to preliminary explore the performance of the proposed system, the study on the maximum data rates at different received optical powers was carried out in the air channel. A tunable optical attenuator was placed in front of the receiver to adjust the received optical power. The BER performance of the OOK scheme under different received optical powers is shown in Fig. 9. Since the 20-dB bandwidth of the system is only 80 MHz, at the same received optical power, a higher signal bandwidth means more severe inter-symbol interference (ISI), thus a higher BER. Due to the relatively high receiving signal-to-noise ratio (SNR), the achievable data rate reaches 260 Mbps at the received optical power of −25 dBm with the BER under the feedforward error correction (FEC) threshold of 3.8×10−3. When the received optical power decreases to −31 dBm, −41 dBm and −45 dBm, the achievable data rate decreases to 160 Mbps, 100 Mbps and 50 Mbps, respectively. Besides, the data rate can be further improved by replacing the QDs with lower PL lifetimes.
Similarly, the BER performance of CDMA scheme is verified in the air channel, and the results at the received optical power of −45 dBm and −40.5 dBm are shown in Fig. 10(a) and Fig. 10(b), respectively. It can be observed that four clients have similar BER performance. The maximum data rates for each client reache 17.5 Mbps and 12.5 Mbps at the received optical power of −40.5 dBm and −45 dBm, respectively. Since the signal received by each client also includes the interference from other clients, the SNR of each client is lower than that in the OOK scheme at the same received optical power. Therefore, the maximum data rate in the CDMA scheme is lower than that in the OOK scheme at the same received optical power. But the overall capacity of the CDMA scheme at the received optical power of −45 dBm is 50 Mbps, which is approximately equal to that of OOK scheme. These experimental results in the air channel verified that it is feasible to use the liquid CsPbBr3 QDs as the quasi-omnidirectional UWOC transmitter in the underwater CDMA network. The receiver sensitivity of the proposed system can reach −45 dBm, at the data rate of 50 Mbps in OOK scheme and at 12.5 Mbps for each client in the CDMA scheme, respectively, showing the potential for a long-distance UWOC. The data rate can also meet the communication needs of underwater networks, e.g. the IoUT.
Fig. 10. BER performance of CDMA scheme at the received optical power of (a) −45 dBm and (b) −40.5 dBm in the air channel.
The subsequent experiments were conducted in a 50-m swimming pool. To obtain the attenuation coefficient of the swimming pool, a power meter was used to measure the received optical power of a green LD at different transmission distances in air and water, as shown in Fig. 11. After removing the geometric loss, mainly shown as the blue curve, the attenuation coefficient of the water is calculated to be 0.38 dB/m (0.087 m−1).
In this swimming pool experiment, the distance between the transmitter cabin and the receiver cabin was 10 m or 20 m. By adjusting the data rate, the BER performance of OOK scheme over underwater channels is obtained and shown in Fig. 12. Insets are the eye diagrams at the corresponding parameters pointed by the arrows. Since the light is quasi-omnidirectional and the receiver bin has a relatively small window, the energy attenuation led by the geometric loss is as large as around 52 dB at the distance of 10 m. Moreover, the received optical power attenuates rapidly as the distance increases, leading to a lower achievable data rate. It can be observed that the maximum data rates of the OOK scheme are 60 Mbps and 40 Mbps over 10-m and 20-m transmission distances, respectively.
Similarly, the CDMA scheme was experimentally demonstrated over 10-m and 20-m underwater channels, respectively. The BER performance of each client is shown in Fig. 13. It can be observed that four clients also have similar BER performance. The maximum data rates per client in CDMA over the transmission distances of 10 m and 20 m can reach 10 Mbps and 7.5 Mbps, respectively, corresponding to a maximum total capacity of 40 Mbps and 30 Mbps.
To verify the proposed quasi-omnidirectional transmitter and the receiver could work at any covered region, the transmitter cabin was rotated counterclockwise with a step size of 60°. As shown in Fig. 4(b), the optical power is not always completely uniformly distributed. Therefore, to test the system performance at different yaw angles, for the CDMA scheme, the data rates of each client are fixed at 8.75 Mbps with 10-m underwater channel and 7.5 Mbps with 20-m underwater channel. It means that the aggregate data rates of 10-m and 20-m channels are 35 Mbps and 30 Mbps, respectively. For OOK scheme, the aggregate data rates of 10-m and 20-m channels are fixed at 50 Mbps and 35 Mbps, respectively. Then the BERs of both OOK scheme and CDMA scheme were calculated. The BER results at different yaw angles, distances and system capacities are shown in Fig. 14. Note that since the four clients have the similar performance in the CDMA scheme, only the BER of client 1 is shown. As the yaw angle changes, the BER slightly increases due to the uneven power distribution and the instability of CsPbBr3 QDs. But the fluctuation is within an acceptable range. The experimental results of Fig. 13 and Fig. 14 also indicate that the proposed quasi-omnidirectional transmitter can cover an area of 314 m2 for the 10-m transmission distance and support up to 4 clients to communicate at the data rate of 10 Mbps.
5. Conclusion
Since the green light has a relative lower attenuation coefficient in most actual sea water types than the blue one, in this study, a liquid CsPbBr3 QDs and pump LD based UWOC transmitter is proposed to realize wavelength conversion and quasi-omnidirectional emitting at the same time. The quasi-omnidirectional transmitter can relax the alignment requirements of UWOC system and be applied for underwater CDMA networks. The feasibility of the proposed UWOC system is experimentally demonstrated in a standard swimming pool with an attenuation coefficient of 0.38 dB/m. The experimental results show that the maximum data rates of OOK signal over 10-m and 20-m transmission distances can reach 60 Mbps and 40 Mbps, respectively. Besides, the multiple access applications of the proposed system are experimentally verified in a four-client underwater CDMA network, with the maximum data rates of each client reaching 10 Mbps and 7.5 Mbps over 10-m and 20-m underwater channels, respectively. The data rate can be further improved by replacing the QDs with lower PL lifetimes. This study provides a promising UWOC transmitter structure for underwater network.
Funding
National Natural Science Foundation of China (61971378); Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22030208); Zhoushan-Zhejiang University Joint Research Project (2019C81081).
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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