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Abstract
The last decade has witnessed considerable progress in underwater wireless optical communication in complex environments, particularly in exploring the deep sea. However, it is difficult to maintain a precise point-to-point reception at all times due to severe turbulence in actual situations. To facilitate efficient data transmission, the color-conversion technique offers a paradigm shift in large-area and omnidirectional light detection, which can effectively alleviate the étendue limit by decoupling the field of view and optical gain. In this work, we investigated a series of difluoroboron β-diketonate fluorophores by measuring their photophysical properties and optical wireless communication performances. The emission colors were tuned from blue to green, and >0.5 Gb/s data transmission was achieved with individual color channel in free space by implementing an orthogonal frequency-division multiplexing (OFDM) modulation scheme. In the underwater experiment, the fluorophore with the highest transmission speed was fabricated into a 4×4 cm2 luminescent concentrator, with the concentrated emission from the edges coupled with an optical fiber array, for large-area photodetection and optical beam tracking. The net data rates of 130 Mb/s and 217 Mb/s were achieved based on nonreturn- to-zero on-off keying and OFDM modulation schemes, respectively. Further, the same device was used to demonstrate the linear light beam tracking function with high accuracy, which is beneficial for sustaining a reliable and stable connection in a dynamic, turbulent underwater environment.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The significance of optical communication extends beyond free space applications, as the vast expanse of water covering over 70% of the Earth’s surface, including unexplored regions in the deep sea, has sparked considerable research interest in underwater wireless optical communication (UWOC) [1]. With larger modulation bandwidth than acoustic communication and lower attenuation than radio frequency (RF), this technology has facilitated advancements in marine exploration, underwater environmental monitoring, and off-shore industries [2]. However, the underwater environment presents a highly dynamic and unpredictable feature, posing challenges for achieving precise point-to-point optical signal transmission and reception [3]. In this regard, the utilization of color-converting materials holds promise in developing omnidirectional transmitter [4,5] and large-area high-speed photoreceivers [6,7] that can relieve constraints of positioning, acquisition, and tracking (PAT).
Typically, the luminescent-based photoreceiver module relies on the luminescent solar concentrators (LSCs) and the scintillating fibers. Manousiadis et al. introduced the concept of using a planar fluorescent antenna for achieving wide field-of-view (FoV) detection in high-speed visible light communication [8]. Subsequently, the concept was enhanced by developing a high-gain broad-FoV demultiplexer through the integration two overlapped fluorescent antennas [9]. Dong et al. utilized SuperYellow to fabricate nanopatterned LSC with a parabolic shape, and achieved high optical gain with a wide FoV [10]. Kang et al. developed an ultraviolet (UV)-to-blue color-converting photoreceiver based on plastic scintillating fibers, offering large-area, high-speed, and omnidirectional photodetection [11]. In this work, we shed light on a multifunctional photoreceiver made of fiber-coupled LSC, which performs high-speed underwater optical signal transmission and simultaneous light beam tracking with enhanced detection area and FoV. By incorporating beam tracking, the color-conversion technique also has the potential to mitigate the link blockage and ensure the stability and reliability of the UWOC channel, facilitating swift and efficient communication even for autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), and other machinery networks [12]. To realize this vision, there is a pressing need to explore color-converting materials possessing short photoluminescent (PL) lifetimes (typically < 10 ns), high PL quantum yield (PLQY), high absorption coefficient, and a large Stokes shift [13].
Difluoroboron $\beta$-diketonate (BF2bdk), a widely recognized BF2 chelate compound, possesses relatively high quantum yield, large Stokes shift and molar extinction coefficient, diverse chromism, high photostability and tunability [14]. These unique photophysical features contribute to their widespread utilization in the fields of luminescent sensing [15–17], data encryption [18], and molecular logic gates [19]. By utilizing a series of BF2bdk (dBF1-4) with tunable photophysical properties by employing a fine-structure modification approach, we successfully demonstrate, for the first time, their exceptional performance in UVA-based optical wireless communication (OWC) systems, both in free-space and underwater scenarios. In free space, tens-of-megahertz bandwidth and more than 0.5 Gb/s net data rate have been achieved with direct current-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) for single channel. Then, the dBF4, which exhibited the best performance, was selected and fabricated as a 4$\times$4 cm$^2$ luminescent concentrator coupled with silica fiber bundle to be used in the UWOC system. The net data rates of 130 Mb/s and 217 Mb/s were obtained with non-return-to-zero on-off keying (NRZ-OOK) and DCO-OFDM modulation schemes, respectively. With the proof-of-concept demonstration of light beam tracking, this material and technology showed great potential for high-speed multi-wavelength optical signal transmission and large-area photodetection.
2. Material synthesis and characterization
The chemical structure of the fluorophores we used in this work is shown in Fig. 1(a), with the detailed synthesis procedures of BF2bdk powder (dBF1-4) provided in Supplement 1. To ensure durability and flexibility in the OWC operation, the synthesized powder was dissolved in chloroform and mixed with polysulfone (PSF). After dropping the viscous solution on a quartz plate and letting the solvent evaporate slowly, the films were peeled off from the quartz plate to form transparent and free-standing color converters. To select the samples with the optimized concentration, we measured the PL with varying BF2bdk concentration in PSF from 0.1wt% to 1.4wt%, and the weight percentage with the highest PL intensity was selected for the following experiments. As shown in Supplement 1 Fig. S1, the optimized weight percentages of samples dBF1-4 are 0.3wt%, 0.5wt%, 0.5wt%, and 0.7wt%, respectively. By changing the substitutes, the absorption peak was adjusted in the range of 380-410 nm, and the PL peaks were tuned from blue (454 nm) to green (510 nm), as plotted in Figs. 1(b) and 1(c), which are well-matched with the ’blue-green’ window (i.e., 450-550 nm) of the seawater [20]. Such alignment allows for their potential application as color-converting layers in underwater transmitters for low-loss long-distance light signal transmission. A broad spectra coverage with full width at half maximum (FWHM) larger than 50 nm offers compatibility with different optical systems and applications. And the Stokes shift can be as large as 102 nm with dBF4, which minimizes the reabsorption of emitted photons, allowing for efficient light emission and reducing energy loss.
Fig. 1. (a) Chemical structures, (b) normalized absorption spectra, and (c) normalized PL spectra of 0.3 wt% dBF1, 0.5wt% dBF2, 0.5 wt% dBF3, and 0.7wt% dBF4 in PSF.
The time-resolved PL of these color converters was measured with a time-correlated single-photon-counting (TCSPC) system under the excitation of 400-nm femtosecond pulses (see Supplement 1). The emission intensity at the peak emission wavelength was collected over time, as shown in Figs. 2(a)-(d). The average PL lifetimes were obtained by using exponential decay fitting for time-resolved PL decay traces (see Fig. 2(e)). The fitting parameters are listed in Supplement 1 Table S1. Among them, dBF1-3 exhibit double-exponential decay with the average lifetimes of around 2 ns, indicating multiple decay pathways within the material. In the meantime, the dBF4 was fitted well by single-exponential decay with a 3.19-ns lifetime. The measured PLQY of each sample is shown in Fig. 2(f), which presents an opposite trend of the PL lifetime, which could be attributed to the suppression of the non-radiative pathways when adjusting the chemical structure.
Fig. 2. The PL emission decay profiles measured at the peak emission wavelength with corresponding exponential decay fittings of (a) 0.3wt% dBF1, (b) 0.5wt% dBF2, (c) 0.5wt% dBF3, and (d) 0.7wt% dBF4 in PSF. (e) Average PL lifetime and (f) PLQY of each sample.
3. UVA-based OWC in free space
Recognizing the significant influence of modulation bandwidth ($f_{-3{\rm dB}}$) on communication performance, especially its inverse relationship to the average PL lifetime ($\tau _{\rm ave}$) as shown in Eq. (1) [13]:
we conducted measurements of the frequency response within an OWC channel to gain insights into the impact of PL lifetime on the system’s performance. The schematic of the optical path is shown in Fig. 3(a). A 375-nm UVA laser diode (LD) (Nichia, NDU4116) was used as the transmitter. The diode was installed in a laser diode mount (Thorlabs, LDM56F/M), including an integrated thermo-electric cooler, temperature controller, and bias-tee. The temperature of the LD operation is maintained at 21 $^{\circ }\textrm {C}$ with a DC current of 80 mA. The RF signal at different frequencies was generated by a vector network analyzer (VNA) (Agilent Technologies, E5061B) to modulate the driving current of LD. The modulated laser beam was then collimated and guided into an integrating sphere to excite the color converters. Then, the re-emitted light was collected by two aspheric condensers and an objective lens. A 400-nm long-pass (LP) filter (Thorlabs, FELH0400) was mounted between the two aspheric condensers to ensure that no unabsorbed photons emitted by the excitation source are detected by the photodetector. A silicon-based avalanche photodetector (APD) (Thorlabs, APD430A2/M) with an active diameter of 0.2 mm and –3-dB bandwidth of 400 MHz was mounted after the objective lens as a receiver. The electrical signal from the APD was analyzed by the VNA to obtain the frequency response information. The small-signal frequency responses of each sample were obtained by sweeping the modulation frequency from 300 kHz to 250 MHz.Fig. 3. (a) DCO-OFDM implementation: The schematic of optical path with signal modulation and demodulation hardware. The block diagram shows the signal processing steps. (b) The normalized small-signal frequency response of dBF1-4.
After normalizing the frequency response with respect to its peak value, the –3-dB bandwidths for dBF1-4 were extracted, as shown in Fig. 3(b). The highest bandwidths for blue and green colors are 36.2 MHz (dBF1) and 40.0 MHz (dBF4). While other samples exhibit slightly lower modulation bandwidths around 20 MHz, which are within the range defined by Eq. (1). However, it can be observed that the inverse proportional trend between the average lifetime and –3-dB bandwidth is not consistently applicable to single-exponential and double-exponential decays. Among the samples exhibiting double-exponential decay (dBF1-3), the –3-dB bandwidth experiences a reduction as the average lifetime increases. These samples also follow a similar response trend from low to high modulation frequencies. As for dBF4, which features single-exponential decay, demonstrates a higher –3-dB bandwidth with a longer average lifetime as compared to the others, and the trend of frequency response is also different from dBF1-3. Beyond 70 MHz, dBF1 surpasses dBF4 in frequency response, followed by dBF2 at around 130 MHz. The possible reason for this phenomenon could be that for samples with single-exponential decay, there is only one dominant radiative recombination pathway. Once the recombination process fails to complete within a modulation period, a rapid decrease in response will occur after a specific modulation frequency. In comparison, samples with double-exponential decay might exhibit a slower radiative recombination pathway dominating at lower frequencies, while the faster one gains prominence at higher frequencies. This characteristic has the potential to impact the usable bandwidth during OFDM implementation. Further in-depth investigation is required to understand the intricate relationship between frequency response and the distinct temporal decay components.
It is worth noting that the modulation bandwidth of the LSC could also be affected by its dimension, which is quantified by critical length L$_{c}$ [21]:
where $c = 3 \times 10^8$ m/s, denoting the speed of light in vacuum, and $n_{\rm LSC}$ represents the refractive index of the LSC. Given that dBF1-4 were incorporated into PSF at substantially low concentrations in this study, the refractive index of the LSC can be approximated as $n_{\rm LSC} = n_{\rm PSF} = 1.633$. Consequently, the critical lengths of dBF1-4 can be computed as 1.23 m, 1.70 m, 1.76 m, and 2.22 m, respectively. Therefore, only in the case of light propagation through an LSC with a very large dimension, does the size of the LSC become a primary limitation on bandwidth.Before using the color converter for underwater scenarios, their communication performance was investigated in free space using DCO-OFDM modulation, as shown in the block diagrams in Fig. 3(a). The length of the communication link is 0.5 m. On the transmitter side, the DC was set to be 100 mA and the input RF signal with 10-V peak-to-peak voltage was provided by an arbitrary waveform generator (AWG) (Siglent, SDG6052X). The transmitted sequence is a 2$^{16}$-1 pseudorandom binary sequence (PRBS) that is converted into an array whose size is defined by the number of used subcarriers (500) and the number of OFDM symbols (150) to be transmitted after performing quadrature amplitude modulation (QAM). On the receiver side, the output signal was recorded by a mixed-domain oscilloscope (Tektronix, MDO3104), and then processed offline by MATLAB. The signal-to-noise ratio (SNR) was first estimated by sending a 4-QAM uniform test signal through the communication channel. Then, the error vector magnitude of each subcarrier was calculated based on the position of the received and transmitted symbols on the complex plane. The deviation between two positions was used to estimate the power of the noise, while the signal power was extracted from the position of the transmitted symbol. The estimated SNRs are shown in Supplement 1 Figs. S2(a)-(d). The spectral efficiency was calculated based on SNR using
The bit loading scheme is a simplified scheme that follows the estimated spectral efficiency of each subcarrier. The signal amplitude at each subcarrier was then adjusted based on the SNR to minimize the bit error ratio (BER). No bits were allocated to subcarriers with SNR below 3 dB. Under the irradiance of 828 mW/cm$^2$ (26-mW power generated by a circular beam of 2-mm diameter), the high PLQY allows for a more than 20-dB SNR with a broad usable bandwidth until 150 MHz for dBF1, 2, 4, and 74 MHz for dBF3. The high SNR also eliminates the need for external amplifiers, which potentially lowers the cost and power consumption further. Subsequently, the gross data rate was calculated using Eq. (4):
Figure 4 shows the impact of PLQY and –3-dB bandwidth on the final net data rate. The material with higher PLQY and –3-dB bandwidth tends to have a higher data rate. On an independent note, the high quantum efficiency significantly enhances the SNR, and the increased SNR will provide a larger margin against noise, allowing for the transmission of more bits per symbol. On the other hand, once the number of subcarriers is fixed, a broader modulation bandwidth contributes to wider subcarriers with more symbols to be transmitted per unit time, leading to a higher symbol rate. Moreover, the varying responsivity of silicon-based APD at different wavelengths should also be considered, as it exhibits higher responsivity in the green range compared to the blue and UV regimes [22]. Therefore, dBF4 outperformed others with the highest overall data rate of 536.8 Mb/s, which is comparable to other emerging fast-acting down-conversion counterparts such as perovskites [23–25], metal-organic frameworks [26–28], and organic dyes [29–31].
4. UWOC and linear beam tracking with fiber-coupled LSC photoreceiver
Among all samples, dBF4 exhibits the best communication performance in the free space with the highest –3-dB bandwidth, PLQY, and largest Stokes shift. Therefore, dBF4 was selected to be fabricated for a fiber-coupled LSC (4 cm$\times$4 cm$\times$60 $\mathrm {\mu }$m) and applied in the UWOC system for simultaneous high-speed large-area photodetection and real-time beam tracking. The schematic diagram of the communication link is shown in Fig. 5. The 375-nm laser was utilized as a transmitter to excite the LSC screen after passing through a 1.2-m water tank with an irradiance of 13 mW/cm$^2$. The total internal reflection within the high-refractive-index LSC ($n_{\textrm{PSF}}$ =1.633) will guide the fluorescent green light to the edge with a theoretical geometric gain G of 167 (G = input area/output area) [32]. The light output from the edges will be further coupled into a bunch of silica fibers (Thorlabs, FP400URT) attached to all edges (5 fibers at each edge) with another end bundled and aligned with an APD (Thorlabs, APD120A2/M). The overall coupling efficiency was 0.83%, which was calculated by the re-emitted power from the LSC and the power received from the total 20 fibers. When transmitting with the high-scattered UVA photons and receiving with a large-area LSC-enabled receiver module, a diffused-line-of-sight (DLOS) or non-line-of-sight (NLOS) communication can be more easily achieved, which therefore obviates the need of strict alignment when using high-speed APD with a smaller detection area [33].
Both NRZ-OOK and DCO-OFDM modulation schemes were conducted. When performing the NRZ-OOK, a 2$^{16}$-1 PRBS signal was sent by the same AWG with varying sampling rates to change the frequency. The signal received from the APD was analyzed by the same oscilloscope with a fixed sampling rate of 250 MSamples/s. The BER was calculated offline at different data rates, as shown in Fig. 6(a). The eye diagrams obtained at the data rates of 90 Mb/s and 110 Mb/s are shown in Fig. 6(b). Regarding the DCO-OFDM modulation, the AWG and oscilloscope sampling rates were set to 100 MSamples/s and 250 MSamples/s, respectively. With the FFT size of 1024, cyclic prefix length of 10, and 10 training symbols. The achieved gross and net data rates were 251.3 Mb/s and 217.0 Mb/s, respectively with the average BER of 3.7$\times$10$^{-3}$ (see Supplement 1 Fig. S3). In summary, efficient data transfer speeds of 130 Mb/s and 217 Mb/s were successfully achieved with NRZ-OOK and DCO-OFDM modulation schemes, respectively. Although within a short distance for lab demonstration, DCO-OFDM can take advantage of the high SNR to achieve higher data rate than NRZ-OOK, when confronted with real-world conditions encompassing extended distances and increased turbulence, the efficiency of OFDM will drop significantly. Conversely, NRZ-OOK exhibits higher resilience and robustness with less complexity in such circumstances. Undoubtedly, enhanced communication performance can be achieved by applying equalization for OOK. Other modulation techniques with high spectral efficiency, such as PAM-M, can also be chosen based on the system’s performance requirements and available resources. Moreover, these results leave space for enhanced data rate if the coupling losses between LSC/fiber and fiber/APD interfaces could be better addressed. With this design, the fiber bundle can also be split to transfer the signal to multiple users with similar levels of power.
Fig. 6. (a) Net data rate versus BER by implementing NRZ-OOK modulation scheme. (b) Eye diagrams obtained at 110 and 90 Mb/s.
For a proof of concept, light tracking on the x-axis was demonstrated with the illustration in Fig. 5. Aside the fiber array used for UWOC, two additional optical fibers were attached on the left and right sides of the LSC at the same height in the middle between the top and bottom edges and connected to PD1 and PD2 (Thorlabs, DET10A). The height of the tracking fibers was calibrated to be zero for the y-axis, and the x-axis is also located in the middle between the left and right edges, as shown in Fig. 7. Both PDs were connected to an oscilloscope at channel 1 (CH1) and channel 2 (CH2) to convert the optical power received by PDs into voltage. The 375-nm LD was mounted on a transition stage to change the pumping position of the optical beam perpendicularly focused on the LSC, which provided 7.2-mW/0.4-mW power before/after the water tank. When moving the laser, the average voltages of both channels ($V_{1}$, $V_{2}$) were recorded to correlate the pumping position and received power. The light output from the edge of the LSC depends on the propagation distance of the light by following the Beer-Lambert law [34]. In this study, it can be expressed as
where $\alpha _1$ and $\alpha _2$ are the effective absorption coefficients representing the combination of matrix absorption and self-absorption coefficients. $A_1$ and $A_2$ are constants. $x_1$ and $x_2$ are the distances from the light beam to the corresponding fiber end, which are correlated to each other with respect to the side length a and can be denoted by the actual position x as $x_1=a/2-x$ and $x_2=a/2+x$.In the ideal case of a perfectly uniform LSC, $\alpha _1$ and $\alpha _2$ are supposed to be the same. While in practice, a slight inhomogeneity in film thickness, surface roughness, or dye aggregation will lead to a deviation between both values. To this end, we correlated the output from both channels by dividing $V_{1}$ by $V_{2}$ and dividing $V_{2}$ by $V_{1}$ after normalizing the individual by its maximum value. The simplified equations after fitting were obtained with respect to the position on the x-axis (y = 0):
During the fitting process, half of the collected data points were used for fitting and the other half for verification. The fitting curves are shown in Fig. 8(a). It is worth noting that when plotting the ratios in log scale, an obvious deviation between the actual and fitted values can be found, and it became larger especially when the ratios of $V_{1}$/$V_{2}$ or $V_{2}$/$V_{1}$ decreased to less than 1, which will substantially influence the accuracy of tracking at these positions. Therefore, for the points located at x<0, we used the fitting of $V_{1}$/$V_{2}$ to estimate the actual position; the points located at x$\geq$0 were estimated by the fitting curve of $V_{2}$/$V_{1}$. As a result, this model exhibits a good linear relationship between the actual positions measured in the experiment and the estimated positions, as shown in Fig. 8(b). A low root-mean-square error (RMSE) of 0.045 cm was calculated with this fitting, which is in the range of the uncertainty caused by manual operation and the ruler, and this linear dynamic range could be as large as possible when enlarging the area of the LSC. It is important to highlight that correlating $V_{1}$ and $V_{2}$ with their ratios can effectively compensate for potential errors arising from fluctuations in received power, which may be attributed to factors such as variations in distance, partial blockage, laser performance degradation, or instability. Compared with conventional light tracking systems by using PD array, charge-coupled device, or position-sensitive detector [35], this tracking scheme holds the advantages of high-speed data transmission with large detection area, and simpler signal processing. Furthermore, using the same method in the y-axis, precise light tracking in both x- and y-directions could be realized. If another fiber-coupled LSC could be assembled directly behind, even the angle of the beam can be accurately recognized. This photoreceiver module featuring beam tracking capabilities can be seamlessly integrated into various underwater machinery, offering many advantages. Firstly, the acquired position data can be utilized within a feedback system to precisely fine-tune the module’s alignment with either the photoreceiver or the optical charging module, leading to enhanced signal and energy transfer. The real-time position information can also be transmitted to facilitate more efficient communication with other underwater systems, promoting synergy and coordinated operations in challenging underwater networks.
Fig. 8. Result of optical beam tracking: (a) Exponential fitting for beam tracking. Both $V_{1}$/$V_{2}$ and $V_{2}$/$V_{1}$ are plotted in log scale. (b) The actual positions measured in the experiment versus the positions estimated by fitting.
5. Conclusion
In summary, the utilization of the LSC for reliable optical wireless communication and light beam tracking in underwater is very attractive, although the scintillating fibers are still dominant in this field, as listed in Table 1. By tuning the material structure of BF2bdk fluorescent compounds, high-speed OWC in free space with multiple wavelengths was achieved. With the properties of wide modulation bandwidth, relatively high PLQY, large Stokes shift, and high operational stability, the fast-acting material with green-color emission was applied to the UWOC system and made as an LSC with the light output from the edges collected by optical fiber bundles. The communication performance of this fiber-coupled LSC was investigated by implementing two different modulation schemes of OOK and OFDM, which provided promising net data rates of 130 Mb/s and 217 Mb/s, respectively, over 1.2 m of water. The light beam tracking in one dimension was incorporated simultaneously without sacrificing the communication performance, which can provide a precise location estimation with a low RMSE of 0.045 cm. This integration is significant because it could enhance the reliability and stability of communication links in dynamic underwater environments and overcome the line-of-sight limitation by enabling high-speed multi-user communication with precise localization. The potential impact of this research extends to various other underwater applications, including underwater imaging, real-time video streaming, and communication between autonomous underwater vehicles and sensor networks.
Table 1. Comparison of color-converting based optoelectronic component for UWOC.
Funding
Office of Naval Research Global (N62909-19-1-2079); King Abdullah University of Science and Technology (BAS/1/1614/01/01, ORA-2022-5313).
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.
Supplemental document
See Supplement 1 for supporting content.
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