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Abstract
Optical wireless communication (OWC) links suffer from strict requirements of pointing, acquisition, and tracking (PAT) between the transmitter and receiver. Extending the narrow field-of-view (FoV) of conventional light-focusing elements at the receiver side can relax the PAT requirements. Herein, we use all-inorganic CsPbBr3 nanocrystals (NCs) to extend various optical concentrators’ FOV to 60°, regardless of the original FOV values of the concentrators. Given the robustness of UV light against communication channel misalignment, the used CsPbBr3 NCs provide another advantage of converting transmitted UVC light into a green color that matches the peak absorption of the widely available Si-based detectors. We evaluated the feasibility of the reported wide FoV optical detectors by including them in deep UV OWC systems, deploying non-return-to-zero on-off keying (NRZ-OOK) and orthogonal-frequency division multiplexing (OFDM) modulation schemes. The NRZ-OOK and OFDM schemes exhibit stable communication over the 60° FoV, providing data transmission rates of 100 Mb/s and 71.6 Mb/s, respectively, a unique capability to the reported design.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The demands for using optical wireless communication (OWC), including the unlicensed spectrum from 400 THz to 1500 THz (200 nm to 2500 nm), have increased due to the bandwidth (BW) limitation of the radio frequency (RF) communication bands [1–3]. To support emerging applications, such as drone delivery and vehicle-to-vehicle (V2V) communication, fifth-generation (5G) and beyond technologies hold a promising implementation for OWC [4]. Visible-light communication (VLC) and ultraviolet (UV)-based communication systems can provide higher data rates as well as enhanced capacity and lower latency rates [1,2]. For example, VLC has been deployed for applications that require high data rates up to the order of gigabits per second (Gb/s) [5,6] and even included in consumer-level applications like connecting networks of toys and smartphones [7]. However, applied modulation schemes in VLC links require strict alignment between the transmitter and receiver in OWC channels.
In contrast, UV-based communication, including the bands of UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm), has gained attraction due to its robustness against channel misalignment and wavelength-beam blockage [8–10]. Compared to visible-light wavelengths, UV photons are scattered more due to Rayleigh and Mie phenomena [9,10]. Thus, the receiver side can detect the UV-diffused light through either diffuse-line-of-sight (Diffuse-LOS) and/or non-line-of-sight (NLOS) configurations [11]. Such flexibility of light detection relaxes OWC links from the strict requirement on pointing, acquisition, and tracking (PAT) between the transmitter and the receiver [9,10]. Moreover, as a result of the strong absorption by the ozone layer, UV light, especially solar-blind radiation (i.e., 100 to 280 nm), has low background noise, resulting in negligible background interference compared to visible light [9]. This property of UV light is considered an advantage for indoor and outdoor OWC applications that require strict accurate detection, such as missile tracking [12] and aircraft landing difficulty due to critical atmosphere conditions (e.g., sandy weather) [13]. Photomultipliers (PMTs), characterized by large detection spectral range and amplification factor [14], are currently the preferable optical detectors at the receiver end for detecting weak-transmitted-UV intensities. However, PMTs are bulky, expensive, and require a high operating voltage ($>500$ V) [15]. There are other developed UV- semiconductor-based photoreceiver technologies such as III-nitride $(\mathrm {Al}_x\mathrm {Ga}_{1-x}\mathrm {N}$) and III-oxide ($(\mathrm {In}_x\mathrm {Al}_y\mathrm {Ga}_z)_2\mathrm {O}_3, x + y + z = 1$) photodetectors (PDs), which have high performance and can be tuned to cover the target wavelength range. However, the growth of high-quality III-nitride and III-oxide-based PDs is a challenging process, which is mainly due to the creation of defect states and crystal dislocations [16,17]. Even though Ga$_2$O$_3$-based photodetectors exhibit a high amplification factor, particularly in the UVC region, they suffer from slow time response (>ms), which hinders the applications of high-data-rate OWC links [18–20].
Alternatively, the technology-matured, widely available, and cost-effective Si-based PDs might be an attractive solution for the detection of UV light. However, Si-based PDs suffer from low responsivity in the UV regime (<0.1 A/W), particularly for wavelengths <360 nm, due to the low penetration depth of UV-wavelength photons in Si-based materials [21]. Another general challenge for photodetectors is the trade-off between their active area and bandwidth (BW). It is known that PD’s signal-to-noise ratio (SNR) can be enhanced by increasing the active area of the optical detector. However, minimizing the photodetectors’ junction capacitance is preferable to reduce the resistance-capacitance (RC) time constant, which enables higher modulation bandwidth. Light-focusing elements, such as compound parabolic concentrators (CPC) or lenses, are used to focus the light onto small photodetectors to achieve simultaneous high modulation bandwidth and SNR at the receiver end. However, these conventional focusing components suffer from étendue conservation, where the optical gain of a concentrator can be increased by only decreasing the detector’s field of view (FoV) [22–24]. Recently, extensive research work has been conducted for using fluorescent concentrator (FC) based receivers, including Coumarin 6 (Cm6) [22,24] and SuperYellow [25], which are not limited by étendue conservation. However, such FCs show an excitonic peak absorption in the blue region of the spectrum, indicating they are unsuitable for UVC-based communication links.
In contrast, metal halide perovskites, which are down-conversion materials with the general structure of ABX$_3$, where (X= chloride (Cl), bromide (Br), or iodide (I), have been recently used in interesting optoelectronic applications such as solar cells [26], photodetectors [27,28], and LEDs [29]. Along with the halide perovskites’ time-efficient and simple process of deposition, which leads to a low cost of production, they are also wavelength tunable over a broad spectrum range [30,31]. Moreover, halide perovskites can be integrated with Si-based devices to offer highly efficient technologies [32]. Among the above possible combination of perovskite compositions, CsPbBr$_3$ nanocrystals (NCs) have the most stable structure [33]. For PD device applications, CsPbBr$_3$ NCs as a down-conversion material can efficiently convert UV-light photons into a green color that matches precisely the peak absorption of the Si-based PDs [34].
To overcome the low responsivity of Si-based PDs in UVC communication link and the narrow FoV of the conventional concentrators caused by the conservation of étendue, we report on the use of all-inorganic CsPbBr$_3$ perovskite NCs combined with optical concentrators. Subsequently, on-off keying (OOK) and orthogonal frequency-division multiplexing (OFDM) modulation schemes of communication were performed to evaluate the feasibility of the reported design for a high data rate communication link using a UV laser diode and UVC-LED, respectively.
2. Experimental results
Figure 1 shows the structural and optical characterization of the planer CsPbBr$_3$-perovskite (www.quantum-solutions.com) film. The details of synthesizing the perovskite NCs are described in Ref. [35]. In this study, all samples are characterized as a thin film of a $\sim$100-nm thickness. In particular, the single-step spin-coating process method [36] is deployed to deposit the CsPbBr$_3$ perovskite films, using 1-inch UV double-sided-polished fused silica substrates. To verify the thickness of the deposited material, we use a scanning electron microscopy (SEM) and a stylus profilometer. All substrate samples are prepared for the deposition process by subsequent cleaning with acetone, isopropanol, and deionized (DI) water, and finally the samples are dried using nitrogen (N$_2$) gas.
Fig. 1. Structural and optical characterization of the CsPbBr$_3$ perovskite NCs. (a) Photograph of the CsPbBr$_3$ NCs in solution-based form and dispersed in toluene. (b) Transmission electron microscopy (TEM) image. (c) Absorption and photoluminescence spectra. (d) Time-resolved PL decay trace for the spin-coated CsPbBr$_3$ perovskite NCs on a UV fused silica.
As shown in Fig. 1(a), the used perovskite NCs are dispersed in toluene with a concentration of up to 10 mg/mL and have a photoluminescence quantum yield (PLQY) of up to $\sim$100% and 73% as solution and thin film, respectively [34]. The degradation in PLQY is mainly due to the ligand loss which occurs during the solution-to-film transformation process [37,38]. The transmission electron microscopy (TEM) measurements show that the CsPbBr$_3$ perovskite appears to have cubic NCs with an average size of $\sim$6.5 nm, as shown in Fig. 1(b). As a light conversion material, the CsPbBr$_3$ perovskite NCs absorb UV light and convert it to green-color wavelengths. As shown in Fig. 1(c) (red line), the absorption spectrum exhibits an excitonic peak in the UVC range (230–280nm), which decreases as the incident wavelength shifts toward the perovskite bandgap. This behavior supports using UV-emitters for taking the advantage of the UVC-LEDs to produce diffused source light that can be detected through either line-of-sight or non-line-of-sight configurations and with low background noise [39]. As shown in the inset of Fig. 1(c), $I_0$ and $I$ represent the incident and transmitted light intensities, respectively. Thus, the absorption coefficient $(\alpha )$ of the perovskite films can be expressed as [40]:
In contrast, the perovskite material shows intense photoluminescence (PL) emission centered at 517 nm with a full width at half maximum (FWHM) of 20 nm (Fig. 1(c)). To investigate the time-resolved photoluminescence (TRPL) of the CsPbBr$_3$ perovskite, Horiba FluoroMax Spectrofluorometer is used. As Fig. 1(d) shows, the CsPbBr$_3$ NCs have a fast PL decay curve with a lifetime of approximately 3.8 ns under a 370-nm excitation wavelength, which results in a theoretical maximum frequency bandwidth ($f_{-3 \mathrm {dB}}$) of 42 MHz. The $f_{-3 \mathrm {dB}}$ can be calculated as follows [41,42]:
where $\tau$ is the radiative recombination lifetime.In this work, as shown in Fig. 2, three types of optical concentrators, including a compound parabolic concentrator (CPC), a plano-convex lens, and a fused fiber-optic taper (FFOT), which have different FoVs, are used with the CsPbBr$_3$ perovskite. These optical components are classified as reflection/refraction concentrators and can be used to transmit light as well as focus. However, FFOTs and plano-convex lenses can be mainly deployed for imaging transfer [43,44], while CPCs are applied for non-imaging applications [45]. These concentrators suffer from étendue conservation, which causes a trade-off between their maximum possible gain and FoV, as expressed by the following equation [46]:
where $\theta$ is the semi-angle that defines the FoV and $n$ is the concentrator’s refractive index.Fig. 2. Photograph of the used elements. CPC (left), plano-convex (middle) and FFOT (right). The CPC has input and output diameters of 14 mm and 5 mm, respectively. While the plano-convex has a diameter of 25 mm with a focal length of 50 mm and the FFOT has a ratio of 55 mm-to-10 mm.
To study the FoV improvement of each light-focusing element, two measurement cases are conducted and compared to each other. The cases include the original design where only the optical concentrators are used, without the CsPbBr$_3$ perovskite quantum dots (QDs), and the second scenario is when the CsPbBr$_3$ perovskite layer is added in front of the optical concentrators. The schematic diagram of the experimental setup is shown in Fig. 3, where a UVC-LED is driven by a DC bias to maintain the LED’s output power at 0.56 mW with a wavelength of 278 nm. The light source is followed by an objective lens to collimate the UVC-LED light. To optimize/calibrate the incident light power and diameter to match the PD active area, an iris with a controllable open diameter is used. To explore the FoV measurement, a UV-enhanced Si-based PD (Newport 818-UV) with an active area of 13 mm$^2$ is used, as well as three different optical concentrators that are alternatively placed on a rotational stage, with and without a planer CsPbBr$_3$ perovskite layer. The UV-enhanced Si-based PD is mainly used since its UV-responsivity can be measured for the two states of comparison. In addition, a 450-nm long-pass filter is added when the perovskite layer is included, to filter out the transmitted UVC light. This filter can be also placed after the optical concentrator. However, for better coupling efficiency, the optical detector should be placed as close as possible to the PD input. Prior to the experiments, to ensure that the long-pass filter is working effectively in blocking the transmitted UVC light source, which is not absorbed by the perovskite layer, a spectrometer is instead placed in the Si-based PD position and the spectrometer shows an efficient filtering out of the UVC light. It is worth highlighting that our experiment’s PD (Newport 818-UV) has a wavelength-dependent responsivity. In our experiment, the received optical power for both wavelengths, i.e., UV (without perovskite NCs) and green (with perovskite NCs and a 450-nm long pass filter), are directly acquired based on a known responsivity value of the commercial UV-enhanced Si-based PD. Therefore, for the experiment performed in Fig. 3 of measuring the FoV, the measured received optical power is independent of the responsivity of the PD used in our experiment.
The three optical concentrators including CPC, plano-convex and FFOT are characterized with/without including the perovskite layer in order to compare their FoVs. For a fair comparison, the incident beam diameter was calibrated to be smaller than the smallest input diameter among the three used optical concentrators, which is the CPC. Moreover, adjustable alignment controllers of (x-y) degrees of freedom are included to the detection side to make sure that the light-source beam always hits the center of the perovskite substrate. The same approach is also applied when the optical concentrators are only used, without the perovskite.
As mentioned in the introduction section, these components suffer from étendue conservation that limits the receiver’s FoV. To overcome this challenge, we report on the use of the perovskite film to widen the FoV of the optical detectors. As shown in Fig. 3 and Figs. 4(a), 4(c), and 4(e), the FoV measurements are conducted over a rotatable station with a step of one rotational degree for the positive and negative angles and continued untill the received optical power becomes within the background noise level of the PD. The FoV value is calculated when the received power is half of the maximum power (3-dB width). As shown in Figs. 4(a), 4(c), and 4(e), the original FoV measurements of the CPC, plano-convex and FFOT, due to étendue conservation and without using the CsPbBr$_3$ perovskite, are 28$^{\circ }$, 16$^{\circ }$ and 12$^{\circ }$, respectively. In contrast, when using the CsPbBr$_3$ perovskite, the FoV semiangle extended to 60$^{\circ }$ regardless of the optical concentrator type, which is equivalent to the original FoV value of the PD before the use of any conventional concentrator. The extension of the FoV occurs because, despite the incidence angle of the UV beam, the CsPbBr$_3$ perovskite NCs act as a secondary light source and emit the green light uniformly in 360$^{\circ }$ from the NCs. This FoV’s expansion has a substantial advantage of being able to free optical wireless communication (OWC) links from the strict alignment requirement between the transmitter and the receiver-end imposed by the requirement of small active detection areas for fast response. Additionally, it can be observed that the CsPbBr$_3$ perovskite emission, due to increasing angle, is following the Lambertian cosine distribution [47].
Fig. 4. Relative received power at the PD when recorded at different angles (a, c, e) and different translation positions (b, d, f) when using the CPC, plano-convex, and FFOT concentrators, respectively, without (black lines) and with (red lines) perovskite.
In contrast, before and after the perovskite deployment, there is no substantial difference in the relative received power when the beam arrives at different translation positions for the three concentrators (Figs. 4(b), 4(d), and 4(f)). However, each component, interestingly, has its unique detection shape which can be considered a fingerprint. This distinguishable pattern can be deployed in a mapping technique for element recognition.
For further clarification, the FoV measurement curves of the optical concentrators’ received power are fitted to the cosine law curve, as shown in Fig. 5. It is observed that, when using the plano-convex and FFOT concentrators, the received power has a non-smooth behavior curve below 15$^{\circ }$, which is attributed to the high efficiency of these concentrators to collect some inevitable UVC scattered light. However, this non-smooth peak is not limiting the FoV of the FFOT. Additionally, for a communication system, the presence of both UVC and green light at the same time should be a plus.
Fig. 5. FoV measurements using CsPbBr$_3$ perovskite with different optical detectors, as fitted to the Lambert’s cosine law.
3. High-speed modulation performance
To demonstrate the performance of the concentrators with the CsPbBr$_3$ perovskite for the use in a fast OWC communication link, two modulation schemes, on-off keying (OOK) and orthogonal-frequency division multiplexing (OFDM), are implemented. The OOK experimental setup is shown in Fig. 6. Firstly, a 375-nm laser diode (Nichia, NDU4116) is used as a transmitter. For cooling, the 375-nm laser is fixed on a thermoelectric-cooled laser mount (Thorlabs, TCLDM9), which is controlled by an electro-thermal controller (Thorlabs, L375P70MLD). An arbitrary waveform generator (AWG) (Siglent, SDG6052X) is used to output non-return-to-zero OOK (NRZ-OOK) signals. To test the communication performance, a pseudo-random binary sequence (PRBS) is generated using a linear feedback shift register, which is generated offline in MATLAB. The PRBS sequence has a polynomial order of 17. The data signals were generated according to the following polynomials.
Fig. 6. Experimental setup of the OOK modulation scheme based on a 375-nm UV-based laser diode, optical concentrators, and the CsPbBr$_3$ perovskite.
The generated data signals are then appended to 1% of training symbols for signal synchronization at the receiver end. Upon reception of the encoded optical signal, the received NRZ-OOK sequence undergoes down-sampling and synchronization process prior to the BER calculation. At the receiver side, the down-converted light is focused onto a silicon-based avalanche photodetector (APD) (Thorlabs, APD430A2) with an active area diameter of 0.2 mm, through a condenser lens. The condenser lens is chosen since it collects the highest received power compared to the other optical concentrators, which satisfies the APD requirement. The dichroic mirror is included to reflect the transmitted 375-nm-laser light that escapes from the 450-nm long-pass filter. The distance of the communication link is kept constant at $\sim$30 cm for all subsequent scenarios related to the communication experiments.
Figure 7(a) shows the BER measurement at different data rates in the presence of the perovskite sample at an incidence angle of 0$^{\circ }$. It can be observed that a data rate of 100 Mb/s can be achieved while the BER is still below the hard-decision forward error correction (HD-FEC) limit. We then investigate the maximum data rate the perovskite film can support at its maximum FoV, which is expected to be less than the 100 Mb/s data rate that lies at the edge of the HD-FEC limit at the 0$^{\circ }$ incidence angle. To demonstrate the superiority of integrating the CsPbBr$_3$ perovskite over using the conventional light concentrators in communication perspective, we measure the BER across different incident angles in the presence and absence of the perovskite sample as shown in Fig. 7(b). We sent a total bit length of 100 kbit and and set the sampling rate of the oscilloscope at 500 MSample/s to recover the transmitted signals. When using the perovskite material at a maximum incidence angle of 60$^{\circ }$, a data rate of 70 Mb/s can be supported below the FEC limit (Fig. 7(b)). In contrast, for the conventional lens without the perovskite at the same 70 Mb/s data rate and when removing the long-pass filter and dichroic mirror, a dramatic increase of the BER occurs at smaller incidence angles, owing to the limitation of the FoV.
Fig. 7. High-speed modulation performance using OOK scheme. (a) BER versus data rate of a 375-nm UV-based laser diode based on NRZ-OOK modulation scheme, yielding a data rate of 100 Mb/s when using the perovskite. (b) BER performance versus angle for the conventional lens with/without perovskite.
Subsequently, for the former communication setup that includes the perovskite material, an OFDM scheme is performed using a 278-nm UVC-LED light source following the schematic of Fig. 8(a). The signal modulation bandwidth of the CsPbBr$_3$ perovskite NCs using the 278-nm UVC LED exhibits a 3-dB bandwidth of approximately 10 MHz (Fig. 8(b)).
Fig. 8. (a) Schematic diagram of the OFDM modulation scheme measurement based on a 278-nm UVC-LED. (b) Normalized system response of the CsPbBr$_3$ perovskite NCs using the 278-nm UVC LED.
The performance of the receiver in the optical wireless communication link is tested using a DC-biased optical OFDM (DCO-OFDM). First, a uniform 4-quadrature amplitude modulation (4-QAM) signal is sent to test the channel and estimate the signal-to-noise ratio (SNR) for each subcarrier based on the error vector magnitude (EVM). The system then uses the SNR values to estimate the maximum spectral efficiency for each subcarrier. Based on the estimated SNR and the subcarrier capacity ($C_n$), the number of bits can then be allocated. The capacity of the $n^{th}$ subcarrier is obtained from the Shannon limit using the following equation:
where $B_n$ is the bandwidth of the $n^{th}$ subcarrrier (all subcarriers have the same bandwidth). Depending on that, the bit allocation scheme is designed and a new signal with bit and power loading is sent through the link. The used and the maximum spectral efficiency for each subcarrier are shown in Fig. 9(a). The spectral efficiency is the capacity divided by the bandwidth. The normalized channel capacity at different incidence angles from 0$^{\circ }$ to 60$^{\circ }$ is shown in Fig. 9(b), where the capacity values are normalized to that measured at 0$^{\circ }$ incidence angle. For each subcarrier, Figs. 9(c) and 9(d) show the SNR and power loading factor, respectively.Fig. 9. Optical modulation based on quadrature amplitude modulation OFDM (QAM-OFDM). (a) The used and maximum spectral efficiency for each subcarrier at 0$^{\circ }$ incidence angle. (b) The normalized capacity at incidence angles up to 60$^{\circ }$. SNR (c) and power loading factor (d) for each subcarrier.
The OFDM signal consists of 150 OFDM symbols, with 7 symbols used as training symbols for synchronization and equalization. The size of the fast Fourier transform is 1024. The first 5 subcarriers are neglected to avoid the low response of the electronics in the low-frequency regime. The subsequent 500 subcarriers are used to carry the signal, which is a 2$^{16}-1$ PRBS. A cyclic prefix of length 10 is used to simplify the equalization process and limit the inter-symbol interference. The sampling rate of the AWG, $f_{Tx}$, is set to 100 MSample/s whereas the oscilloscope’s, $f_{Rx}$, is set to 500 MSample/s. With this configuration, the frequencies used range from 0.5 to 49.3 MHz and the gross data rate, $D_G$, is calculated to be 81 Mb/s using:
where $N_{SC}$ is the number of subcarriers, $M_n$ is the QAM order used for the $n$th subcarrier, and $N_{FFT}$ and $N_{CP}$ are the sizes of the FFT and the cyclic prefix, respectively. After removing the training symbols and the 7% overhead needed for the FEC, the net data rate is estimated to be 71.6 Mb/s. The BER is $3.4\times 10^{-3}$, which is below the 7%-overhead FEC limit ($3.8\times 10^{-3}$). The BER for each subcarrier is shown in Fig. 10(a), where the received superimposed constellation diagrams corresponding to 2-, 4- and 8-QAM for subcarriers of the same order are included in the insets accordingly. To test the FoV of the receiver, we measure the mean SNR at different incidence angles. As can be seen in Fig. 10(b), the mean SNR does not vary substantially with the incidence angle up to 60$^{\circ }$, which confirms the FoV measurements performed using the powermeter.Fig. 10. (a) The BER for each subcarrier, yielding a net data rate of 71.6 Mb/s. The insets show the superimposed constellations of 2-, 4- and 8-QAM for subcarriers of the same order. (b) The mean SNR at different incidence angles up to 60$^{\circ }$.
The maximum distance of a communication link when using the CsPbBr$_3$ NCs relies on various parameters, including the quantum yield of the NCs (73% for a thin film [34], scattering losses, PD’s responsivity dependence on wavelength, etc. However, the FoV value is independent of the distance of the communication link because the FoV is calculated using the received powers at different incidence angles relative to that at zero incidence angle. Since the current work focuses mainly on extending the FoV of optical concentrators using a color-conversion layer, studying the impact of including the CsPbBr$_3$ NCs on the communication link distance falls out of the scope of this work.
4. Conclusion
In conclusion, we report on using CsPbBr$_3$ perovskite NCs combined with optical concentrators, including CPC, plano-convex lens, and FFOT, to overcome the narrow FoV challenge of conventional concentrators caused by the étendue conservation. The reported design offers an FoV semiangle extended to 60$^{\circ }$ regardless of the optical concentrator type. This FoV’s expansion has the substantial advantage of relaxing OWC links from the strict alignment required between the transmitter and receiver. Additionally, we highlighted that CsPbBr$_3$ perovskite NCs can be integrated with Si-based technology for high-speed UV-based communication. Using the reported design over the wide FoV, data transmission rates of 100 Mb/s and 71.6 Mb/s were achieved using (NRZ-OOK) and (OFDM) schemes, respectively.
Funding
King Abdullah University of Science and Technology (BAS/1/1614-01-01, ORA-2022-5313); King Abdulaziz City for Science and Technology.
Acknowledgments
Sultan Alshaibani gratefully acknowledges support from King Abdulaziz City for Science and Technology (KACST).
Disclosures
The authors declare no conflicts of interest. Osman M. Bakr is a founder of Quantum Solutions, a company that develops optoelectronic devices.
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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