WIDE FIELD-OF-VIEW AND UNDERWATER OPTICAL WIRELESS

COMMUNICATION SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/274,610, filed on November 2, 2021 , entitled “DUAL-WAVELENGTH LUMINESCENT FIBERS RECEIVER FOR WIDE FIELD-OF-VIEW, GB/S

UNDERWATER OPTICAL WIRELESS COMMUNICATION,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to an underwater optical wireless communication system and method, and more particularly, to an optical receiver that uses multi-wavelength luminescent fibers.

DISCUSSION OF THE BACKGROUND

[0003] Underwater wireless optical communication (UWOC) is seen as a potential technique to complement acoustic wave communication and to provide connectivity between future generation of Internet of Underwater Things (loUT) devices. One of the major challenges for UWOC systems deployment is fulfilling the pointing, acquisition, and tracking (PAT) requirements. The detection areas of commercially available high-bandwidth photodetectors (PDs) 102 shown in Figure 1 , are limited to only a few tens of mm ² , due to the limit imposed by the resistorcapacitor (RC) time constant, which results in a need to use optical focusing elements 104 to increase the received power from the light beam 106 emitted by the light source 110. The focusing elements have limited angle of view 108, which requires maintaining a perfect system alignment between the PD 102 and the light source 110. Such a condition could not be continuously ensured in a harsh underwater environment that could be subject to various propagation effects and mobility (due to the ocean swells and currents, both the receiver and transmitter may experience dramatic underwater movements which negatively affects the PAT). [0004] As discussed above, it is possible to concentrate the light 106 into high-bandwidth photodetectors and increase the signal-to-noise ratio (SNR) at the receiver using optical elements, including lenses and compound parabolic concentrators (CPCs). However, lenses and CPCs are based on reflection and refraction. They, therefore, conserve etendue, which limits the field-of-view (FoV) of the detector 102. One solution to extend the FoV of optical detectors beyond the etendue limit is through the use of fluorescent materials [1-3]. The authors of [1] reported the design of a slab luminescent solar concentrator (LSC) with a ±60° FoV and a collection gain of 12. The demonstrated fluorescent antenna was used to conduct a 190-Mb/s on-off keying (OOK) VLC transmission. Another demonstration using an LSC with a 100-MHz bandwidth and an optical gain of 3.2 reported the transmission of 400-Mb/s signals using an orthogonal frequency division multiplexing (OFDM) modulation scheme [2], The fact that two LSCs with different materials can be used to decode independent signals carried by different wavelengths enables wavelength-division multiplexing (WDM) transmission as demonstrated in [3].

[0005] Scintillating fibers have also been used in indoor optical wireless [4], and underwater wireless optical communication (UWOC) [5] scenarios to extend the FoV of optical receivers. Although many reports in the literature have shown the effectiveness of having multiple receivers to extend the FoV (i.e., receiver diversity), the increase of power consumption and computation complexity scale up with the increase of the number of receivers. Additionally, the limited spatial dimensions for underwater nodes pose more challenges in increasing the number of receivers to achieve a wide FoV. Therefore, a single receiver with a passive optical antenna to extend the FoV is highly attractive and desired.

[0006] The concept of luminescent scintillating fibers relies on the optical absorption of incoming light beam photons by dye molecules doped in the fiber core, which then emits secondary photons at longer wavelength with a similar process as the LSCs. The authors in [4] demonstrated a multi-Gbps OFDM transmission using a planar detector with an active area of 126 cm ² . The demonstration was based on luminescent scintillating fibers that absorb blue light and re-emit green light. The concept of using scintillating fibers for the design of a large area detector has been expanded in [5]. The authors reported the design of a 36 cm ² detector for UWOC applications. Restricted by the fiber type used for the detector design, the detector was used to detect UV light emitted by a 377-nm laser. The received signal is down- converted to blue light and decoded by an avalanche photodetector (APD). In one application, other type of photodetectors may be used, for example, high-speed photodiode arrays, etc. A data rate of 250-Mb/s over an underwater link was reported using an OOK modulation with a bit error ratio (BER) below the limit of forward error correction (EEC) of 3.8 x 10 ^-3 . The flexible nature of the plastic optical fibers makes it appropriate to fabricating practical shapes optimized for different applications [6, 7] along with optical wireless communication [4, 5]. The differences between the various high FoV solutions proposed in the literature are summarized in Table 1 in Figure 2. Among all the demonstrated techniques, scintillating fibers offer flexibility and can be used to obtain near 360° FoV receivers.

[0007] Thus, there is a need for a new system that is capable of delivering information underwater with high speed, a wide FoV, and without expensive PAT devices.

BRIEF SUMMARY OF THE INVENTION

[0008] According to an embodiment, there is an optical wireless underwater receiver that includes a housing configured to be transparent to light, a first scintillating fiber attached to an interior wall of the housing, a first photodetector optically coupled to both ends of the first scintillating fiber, a second scintillating fiber attached to the interior wall of the housing, and a second photodetector optically coupled to both ends of the second scintillating fiber. The first scintillating fiber is configured to receive a signal at a different wavelength than the second scintillating fiber.

[0009] According to another embodiment, there is an optical wireless underwater transceiver that includes a housing configured to be transparent to light, a first scintillating fiber attached to an interior wall of the housing and configured to receive a first light beam and generate a second light beam, which has a wavelength different from the first light beam, a first photodetector optically coupled to both ends of the first scintillating fiber and configured to transform the second light beam into a first electrical signal, a second scintillating fiber attached to the interior wall of the housing and configured to receive a third light beam and generate a fourth light beam, which has a wavelength different from the third light beam, a second photodetector optically coupled to both ends of the second scintillating fiber and configured to transform the fourth light beam into a second electrical signal, a first light source located within the housing and configured to generate a fifth light beam, and a second light source located within the housing and configured to generate a sixth light beam.

[0010] According to yet another embodiment, there is a method for dualwavelength, underwater, optical wireless communication, and the method includes the steps of receiving first and second underwater optical beams at first and second scintillating fibers, respectively, of a transceiver, generating a third optical beam, within the first scintillating fiber, having a wavelength smaller than a wavelength of the first optical beam, generating a fourth optical beam, within the second scintillating fiber, having a wavelength smaller than a wavelength of the second optical beam, transforming the third optical beam into a first electrical signal with a first photodetector, transforming the fourth optical beam into a second electrical signal with a second photodetector, emitting a fifth optical beam with a first laser source, based on the first electrical signal, and emitting a sixth optical beam with a second laser source, based on the second electrical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0012] Figure 1 is a schematic diagram of an underwater photodetector that uses optics for focusing the incoming light;

[0013] Figure 2 illustrates the performance of various detectors when working underwater in an optical, wireless communication configuration;

[0014] Figure 3 is a schematic diagram of an underwater optical wireless communication receiver that uses dual-wavelength luminescent fibers;

[0015] Figure 4A illustrates the spectra of the excitation lights for the dualwavelength luminescent fibers and Figure 4B illustrates the spectra of the emitted lights of the dual-wavelength luminescent fibers;

[0016] Figure 5A illustrates the time-resolved fluorescence measurement of the dual-wavelength luminescent fibers, when excited at the peak absorption wavelengths and Figure 5B illustrates the normalized frequency responses of the dual-wavelength luminescent fibers, showing a -3-dB bandwidth in the range of 83- to 89-MHz, respectively;

[0017] Figure 6A illustrates FoV measurements of a scintillating fiber showing omnidirectional signal reception, Figure 6B illustrates a compact underwater capsule design for wireless underwater communication with wide FoV, and Figure 6C shows the total efficiency measurements of both fibers;

[0018] Figure 7 shows an experimental setup for the dual-wavelength underwater wireless communication using scintillating fibers;

[0019] Figure 8 illustrates how one of the two scintillating fibers is coated with a material that acts as a filter for one of the incoming laser beams;

[0020] Figure 9 is a block diagram of the dual-channel communication implemented in Figure 7;

[0021 ] Figure 10A illustrates the BER versus data rate for various channel conditions when a blue channel is aggressed by a 405-nm and a 450-nm laser and Figure 10B shows the effect of an optical filter when a green channel is aggressed by a 377-nm laser;

[0022] Figures 11 A to 1 1 F show the measured eye diagram at 200-Mb/s for (a) 377-nm laser at the blue channel, (b) 450-nm laser at the green channel, (c) when the blue channel is aggressed by a 405-nm laser, (d) when the blue channel is aggressed by 450-nm laser, (e) when the filter is removed from the green channel and aggressed by the 377-nm laser, and (f) when the green channel is aggressed by the 377-nm laser in the presence of the optical filter, respectively;

[0023] Figure 12 shows the SNR of the victim blue channel as a function of the aggressor’s received power;

[0024] Figure 13 shows the bathtub curves for the blue and green channels using 377- and 405-nm transmitters; [0025] Figure 14 shows the bathtub curves for the blue and green channels using 405- and 450-nm transmitters;

[0026] Figure 15 shows a possible implementation of the receiver of Figure 3 into an underwater node;

[0027] Figure 16 illustrates the scintillating process for one of the optical scintillating fibers used herein;

[0028] Figure 17 shows another possible implementation of the receiver of

Figure 3 into an underwater transceiver node that emits parallel light beams;

[0029] Figures 18A and 18B show yet another possible implementation of the receiver of Figure 3 into an underwater transceiver node that emits non-parallel light beams; and

[0030] Figure 19 is a method for transmitting information with underwater optical beams with one of the receivers/transceivers discussed above.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a receiver placed in water and using scintillating fibers emitting at 430- and 488-nm for achieving dualwavelength, wide FoV underwater optical wireless communication. However, the embodiments to be discussed next are not limited to the wavelengths noted above or to only two light sources, but may be applied to other wavelengths or a different number of light sources, i.e., to multi-wavelength communication systems.

[0032] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0033] According to an embodiment, extending the FoV of UWOC receivers can significantly ease the need for active positioning and tracking mechanisms. In addition, by using two independent beams for transmitting the desired information along two independent channels and receivers increases the data rate to the Gbps range. In this embodiment, two bundles of scintillating fibers 304 and 306 emitting at 430- and 488-nm, respectively, are used to simultaneously receive two independent optical beams 308 and 312, from ultraviolet and visible light sources 310 and 314, and to generate two corresponding light beams 309 and 313. However, as noted above, this embodiment can be extended to three or more scintillating fibers for achieving multi-wavelength wide FOV underwater communication systems. The scintillating fibers 304 and 306 are placed in a housing 302 to form an optical antenna 307 for the receiver 300. The housing 302 may also house respective photodetectors 320 and 322, which are connected to the fibers 304 and 306 for detecting the optical light beams 309 and 313 and transforming them into electrical signals. The photodetectors are internally connected to a processor 324, which may process these electrical signals according to instructions stored in a memory 326. Light sources 328 and 330, for example, lasers, may also be located in the receiver 300 for sending information to another receiver or node, as discussed later. A zeroforcing approach is implemented for the receiver 300 to minimize interchannel crosstalk within the receiver. A net aggregated UWOC data rate of 1 Gb/s was achieved for this configuration using two wavelengths and a non-return-to-zero on-off keying scheme. The details of this embodiment are now discussed.

[0034] To select the optimal wavelengths of the light sources 310 and 314 for WDM, it is necessary to characterize both fibers 304 and 306 from an optical point of view. The emission spectra, alongside absorption spectra, provide the necessary information about the level of inter-channel crosstalk. For this embodiment, the fiber 304 was selected to be a blue fiber, i.e., it is most sensitive to incoming blue light (around 377 nm) and the fiber 306 was selected to be a green light, it is most sensitive to incoming green light (around 405 nm). For the configuration shown in Figure 3, the light beam 308 is blue and the light beam 312 is green. To verify the down-conversion process of the scintillating fibers 304 and 306, a fluorescence spectrometer was used to firstly measure the light intensity as a function of the excitation wavelength for each fiber. This absorption measurement offers information necessary for identifying each fiber’s peak excitation wavelength to obtain the photoluminescence (PL) measurement at the peak excitation wavelength. In this regard, Figure 4A shows the absorption measurements 410 and 412 for both fibers 304 and 306, respectively. The studied wavelength range was limited to between 300- and 520-nm, which covers the range of wavelengths of interest for underwater communication. It can be seen from this figure that both fibers exhibit a wide range of excitation wavelengths (i.e., the incoming light beams 308 and 312). Furthermore, the optimal (peak) excitation wavelength for the blue fiber 304 is at 377-nm, as indicated by curve 410 while the green fiber 306 has a peak excitation wavelength at 409-nm, as indicated by curve 412.

[0035] Based on these observations, the inventors have selected 377-nm and 405-nm laser sources 310 and 314 for testing (and also for the transmitter part, which is discussed later). As can be seen from Figure 4A, there is a large overlap 414 between the two absorption spectra 410 and 412 for the selected light sources. For example, using a 377-nm laser 310 can maximize the excitation of the blue fiber 304 but also causes about 78% of crosstalk with respect to the used wavelength at the green fiber 306. However, due to the presence of a long-pass filter 334, which may be placed between the two fibers 304 and 306 of the antenna 307 (see Figure 3), whose transmittance is shown in Figure 4A by the curve 416, the crosstalk 414 can be diminished. In addition, using a 405-nm laser instead of 409-nm can fully excite the green fiber 306 while also causing only about 32% of crosstalk 414. Due to the broad absorption spectra, it is possible to eliminate the induced interference by selecting a longer wavelength that does not cause crosstalk to the blue fiber 304. [0036] Based on the results shown in Figure 4A, the inventors have selected a high-power 450-nm laser (for the laser source 314) due to its availability. However, other wavelengths can be selected, maximizing the excitation without causing crosstalk, for example, in the range of 420-430 nm. The penalty for using the 450-nm laser source when compared to a laser that emits 420-430-nm to excite the green fiber 306 is that double the laser power is needed to reach the same excitation level of the blue fiber 304 when excited by a 377-nm laser. Nevertheless, by fixing the excitation wavelengths at 377-nm and 405-nm for the blue and green fibers, respectively, the PL spectra 420 and 422 were obtained for the two fibers, as shown in Figure 4B. The down-conversion process can be confirmed as the blue fiber 304 has a peak emission at 430-nm (it was excited with a blue light, which has a wavelength in the range of 450 to 495 nm) while the green fiber 306 has a peak emission at 488-nm (it was excited with a green light, which has a wavelength in the range of 495 to 570 nm). Based on the results shown in Figure 4B, it follows that the emitted photons of the blue fiber (i.e. , 430-nm) can also cause inter-channel interference with those emitted by the green fiber. This is because the trapping efficiency of the fiber is dependent on the refraction indices of the core n _C o and cladding n _ci layers (1 .6 and 1 .49, respectively). The fraction of power of the reemitted light Pt that is guided towards the ends of the fiber is given by: ncl p _t = i - cos e = 1 - — , (i) nco where Q is the maximum angle that ensures guidance. For the above-mentioned refractive indices values, the trapped light power is about 6.88% of the total generated power with a maximum angle of re-emission of 21 .4 degrees, ensuring total-internal-reflection. Therefore, the rest of the emitted energy can be lost through secondary absorption of the fluorescent molecules or escapes outside the fiber. This indicates that inter-channel crosstalk is possible when the two fibers are stacked on top of each other. Therefore, the emission of 430-nm, which is generated from the blue fiber, can affect the value of IIGB, as discussed later.

[0037] Another desired optical characterization of the fibers 304 and 306 is the Time-Resolved Photo-Luminescence (TRPL) measurement which provides an insight into the fluorescence lifetime. As shown in Figure 5A, the fluorescence lifetimes of the blue and green fibers are almost identical, with corresponding -3-dB bandwidth of 83.32-MHz and 88.92-MHz, respectively. Because the antenna’s bandwidth is determined by the rate of absorption and re-emission of the molecular dye, the travel time of the photons in the waveguide, the pulse spreading due to dispersion, and the response of the photodiode and detection electronics as discussed in [10], the inventors have confirmed that the fluorescence lifetime is the dominant factor by measuring the system’s frequency response. As shown in Figure 5B, the -3-dB of the overall system for the blue and green channels are identical to those obtained from the fluorescence lifetime. Although the -3-dB bandwidth seems limited, the frequency response of the channels decays slowly, followed by a sharp decrease at 400-MHz due to the limited bandwidth of the photodetector. Therefore, a -10-dB of about 150-MHz can still be utilized for high-speed communication.

[0038] To verify the omnidirectional capability of the scintillating fibers, the inventors measured the FoV of the fibers 304/306 shown in Figure 6A. The FoV measurement was conducted by rotating the 377-nm laser 310 around the scintillating fiber 304, which is coupled to a power meter 602. The 377-nm laser 310 was placed at different angle locations, which are indicated at the bottom of the figure. Due to the presence of the power meter 602, the FoV was measured at angles between 0-150° and 210-360°. Figure 6A shows that scintillating fibers 304/306 can exhibit near 360° FoV, depending on the design, which is highly convenient for underwater wireless optical communication. Figure 6B shows a compact WDM configuration 600 where the fibers 304 and 306 are intertwined inside each other to be fit inside a watertight capsule 604 (or housing) for deployment underwater, as discussed later.

[0039] The inventors also measured the overall efficiency of both fibers 304 and 306, which takes into account the trapping, conversion and collection efficiencies. Figure 6C shows the relationships 620 and 622 between the output power of the fibers and the incident power for the blue fiber 304 and the green fiber 306, respectively. The slope of the linear fit indicates total efficiencies of 1 .58% and 0.873% for the green and blue fibers, respectively. These measurements align well with previously measured efficiency values in the literature [4], With further improvement of the fluorescent dyes and fabrication enhancement of core and cladding, the conversion and trapping efficiencies can be improved. In addition, by engineering a specific phase plate for each fiber string, the light output from each fiber string can be focused to a single point, thereby improving the collection efficiency. Nevertheless, even with these low efficiencies, the inventors were still able to obtain high speed communication as now discussed.

[0040] To evaluate the communication speed achieved with the previously discussed two fibers 304 and 306, an experimental configuration 700 was used as illustrated in Figure 7. A 377-nm laser diode 310 was used as a transmitter alongside a 405-nm laser diode 314. Note that the figure indicates that the 377-nm laser 310 may be replaced with a 405-nm laser 310’ and the 405-nm laser 314 may be replaced with a 450-nm laser 314’. The two generated beams 308 and 312 were then collimated and combined using a beam splitter 704. For cooling, the 377-nm laser 310 was mounted on a thermoelectric-cooled laser mount, which was controlled by an electro-thermal controller. The 405-nm laser 314 was connected to an external fan powered by a power supply. An arbitrary waveform generator (AWG) 706 was used to output NRZ-OOK signals 708. The output signal 708 from the AWG 706 was then combined with a DC signal 712 from a DC source 710 via a bias-tee 714, and then fed into the 405-nm laser 314. A laser driver 715 is used to drive the laser 310. A BER tester 716 was used to output the NRZ-OOK signal to the 377-nm laser 310. The two beams 308 and 312 enter a water tank 718, which is 1 -m long, 12-cm wide, and 12-cm high, filled with pure water with an attenuation coefficient of c = 0.07 nr ¹ . At the end of the tank, the two scintillating fibers 304 and 306 (for example, fibers manufactured by Saint-Gobain Crystals, BCF-10, BCF-92) were attached to the inner sidewall of the tank 718. Each fiber was first tightly packed (as schematically illustrated in Figure 3) to form an array of scintillating fibers with a large detection area of 2.5 cm in width. The fibers were elongated to reach out of the tank and connect to corresponding silicon avalanche photodetectors 720 and 722, respectively, which were placed outside of the tank as shown in Figure 7. The facets of the fibers 304 and 306 at the APD ends were polished, for example, using sanding paper, to maximize the output light power and SNR. The cleaved fiber bundle was squeezed to form a circular array and directly coupled to the corresponding APD. Note the term fiber with regard to fibers 304 and 306 is used herein to mean one or more fibers (e.g., a bundle of fibers). The second scintillating fiber 306 (the bluegreen shifter, and denoted as the green fiber throughout the specification) was stacked underneath the first fiber 304 (UV-blue shifter and denoted as the UV fiber throughout the specification) and then coupled to its corresponding APD in the same manner.

[0041] An optical long-pass filter 334 with a cut-off wavelength of 400-nm was sandwiched between the fiber bundles 304 and 306 so that the incoming 377-nm laser beam 308 can excite the first layer of the fibers but is blocked at the second layer. However, the 405-nm laser beam 312 can penetrate through the filter 334 and excite the second layer of the fibers. Therefore, the multiplexed wavelength can be separated using the scintillating fibers with the advantage of wide FoV and omnidirectional capabilities. The optoelectrical converted signals from the APDs were then fed into a high-speed oscilloscope 724 and the BER tester 716. The connection was alternated depending on which channel is the victim and the aggressor. The term “aggressor” is used herein for the channel that interferes with a desired channel, which is the “victim.” For example, the blue channel may be the victim when the blue channel is the channel of interest for transferring data, and the green channel is the aggressor as this channel interfere with the blue channel. Note that while Figure 1 shows that the filter 334 is placed between the fiber 304 and the fiber 306, in one application, as shown in Figure 8, the filter 334 is effectively deposited on one of the fibers, as a coating layer. In this embodiment, the filter 334 is coating the second fiber 306.

[0042] To test the communication performance of the system 700, a pseudorandom binary sequence (PRBS) was generated using a linear feedback shift register, which was implemented in a MATALB platform. The PRBS sequence has a polynomial order of 17. However, the PRBS signals for the two data channels have different seeds. The data signals were generated in this embodiment according to the following polynomials:

PBRS17 _blue = % ¹⁷ + % ¹⁴ + x ⁸ + x ² PRBS17 _green = x ¹⁷ + x ¹⁴ ' ^{1 J}

[0043] These polynomials were tested to generate two sequences with a low cross-correlation coefficient of 0.0034. This is necessary to ensure the correct synchronization of the training signal, which acts as a label to extract the received sequence. The generated signals were then appended to 1% of the training symbols. Figure 9 shows a block diagram for the communication procedure. Each data stream was fed to its corresponding transmitter TB or TG where B stands for blue and G for green. The peak-to-peak voltage of each signal was identical, where the drive current of each laser was adjusted so that the received signals have equal amplitudes. Because the UV laser 310 will excite the blue fiber 304, the channel IIBB was created. Moreover, due to the presence of the optical long-pass filter 334, the UV light 308 cannot excite the green fiber 306. Therefore, the channel IIGB is close to zero. Similarly, the 405-nm laser 314 impinges on the blue fiber 304, causing crosstalk and, consequently, inter-channel interference with channel matrix IIBG. The final channel was formed when the 405-nm laser beam 312 passes through the filter 334 and excites the green fiber 306. The percentage of the excitement of each laser beam to the fibers, and hence the percentage of crosstalk, is determined by the fibers’ absorption spectra. After detecting the two signals by the corresponding APDs, they undergo a down-sampling process 902, followed by synchronization 904 using the label sequence. When the software-based equalization 906 is used, the signal is fed to an equalizer using the channel coefficients before computing the BER 908.

[0044] The results of the configuration 700 introduced above are now discussed. The effects of the crosstalk with different combinations of source wavelengths (i.e., 377, 405, and 450 nm) are first addressed. Initially, only the UV laser 310 was switched on and the BER was measured at different data rates, which serves as a reference curve. As shown in Figure 10A, an aggregated data rate of 1 Gb/s (i.e., 500 Mb/s from each fiber) with a BER value of 3.5 x 10 ^-3 was achieved, which is below the forward error correction (EEC) limit of 3.8 x 10 ^-3 . Afterward, the blue channel 308 was aggressed by the 405-nm laser. As seen from Figure 10A, significant deterioration in the BER is observed, causing a reduction in the data rate by 20%. To solve this issue, the 405-nm laser 314 was replaced with the 450-nm laser 314’. The results show that the blue channel 308 was not compromised when aggressed by the 450-nm laser.

[0045] Next, the effect of removing the filter between the two stacked fibers was investigated, so now the 377-nm laser 310 is considered to be the aggressor while the green channel 312 is treated as the victim. Figure 10B shows the effect of removing the long-pass filter 334, which separates the two fiber arrays 304 and 306. It can be seen that communication is challenging in such a scenario as the crosstalk 414 causes incorrect detection of the bit sequence. Finally, when the filter 334 is sandwiched between the two fibers 304 and 306, the green channel 312 can be demodulated correctly. This demonstrates one potential technique of achieving UWOC WDM using entirely passive optical elements with the correct selection of the source wavelengths.

[0046] Figures 11 A to 11 F show the corresponding eye diagrams 1100 for the abovementioned conditions, measured at 200-Mb/s. Figures 11 A and 11 B present the reference eye diagrams when both channels are crosstalk-free. When a 405-nm laser aggresses the blue channel 308, the eye 1110’s height collapses significantly, as seen in Figure 11 C. This is because the crosstalk level from the 405-nm wavelength to the blue channel 308 is around one-third, the eye diagram still preserves its features and could be corrected using error correction techniques. By replacing the 405-nm with the 450-nm laser, the resultant eye 1120 diagram is shown in Figure 11 D, and the eye-opening is restored as in Figure 11 A. Finally, the corresponding eye diagrams showing the effect between removing and installing the long-pass filter 334 are illustrated in Figures 11 E and 11 F. In this case, because the crosstalk level from the UV laser to the green fiber 312 is comparable to the victim’s signal, the eye 1130 diagram completely closes when no filter is present, as shown in Figure 11 E, and the eye 1140 recovers when the filter 334 is present.

[0047] To further quantify the effect of the crosstalk, the inventors measured the SNR of the victim’s signal (i.e. , blue channel 308) versus varying levels of transmit power of aggressing lasers (i.e., 450 and 405 nm). A power meter was placed in front of the fiber arrays to independently measure the incident light power from the 450-nm and 405-nm lasers 314 and 314’. Then, the drive currents for both lasers were recorded, which lead to the same incident power at the plane of the fibers. The results presented in Figure 12 show that using the 405-nm laser yields a linear decay 1210 of the victim’s SNR with a decay rate of 0.53 dB.mW ^-1 . This decay corresponds to the increased level of crosstalk, causing the eye-opening to collapse linearly, relative to the power increase from the aggressor. However, using the 450- nm laser shows a constant level of SNR 1212, confirming a crosstalk-free channel. [0048] The crosstalk in the configuration 700 can be corrected as now discussed. This method is beneficial when the receiving antennas of the receiver are designed for practical use, especially when installing a flexible filter that separates the two fibers is challenging. Such a design will be described later. Having a digital signal processing unit allows the two fibers (or more) to be mixed for compact antenna design while still achieving the WDM capability. Among all equalization techniques, the inventors selected in this embodiment a linear zero-forcing (ZF) equalizer for its efficiency. The standard 2 x 2 multi-input multi-output (MIMO) model is applicable, where the received symbols of the first and second channels are modeled as or in matrix form, equation (3) is written as: where r _t is the received symbols at the i ^th receiving antenna, hij is the channel matrix from the j ^th transmitting antenna to the i ^th receiving antenna, t, is the transmitted symbols, and n, is the noise with a Gaussian probability density function. It is noted that the off-diagonal elements of the matrix H (i.e., hij for / j) are the interfering channels that need to be eliminated. To solve this system for ti, it is necessary to find a matrix W such that WH = I. The ZF linear detector fulfill this requirement by setting I to

[0049] For real and squared matrices, this expression reduces to simply the inverse of the H matrix. Therefore, while ignoring the noise term, the estimated symbols can be written as

[0050] At this point, the task is to acquire the H matrix, which can be obtained by two methods. The first method is by referring to Figure 4A and denote the elements in the H matrix as the level of absorption of the fiber to the incident wavelength. However, this method does not account for the effect channel fading. The second method is to send each data stream with a preamble containing known bits to both the receiver and transmitter (i.e., training bits). The training bits of the dual-channel are orthogonal, allowing to extract all the elements in the channel matrix. This means that when the UV laser 310 is sending a symbol ‘1 ’, the 405-nm laser 314 sends a ‘0’ and vice versa. For example, when the UV laser 310 is in the ON state, it is possible to measure both IIBB and IIGB by computing the ratio of the transmitted voltage to the received voltage from the blue and green channels, respectively. The rest of the channel elements can be estimated when the UV laser 310 transmits a ‘0’ while the 405-nm laser 314 transmits a ‘T. This method is more accurate because it accounts for the effect of channels fading. The calculated channel matrix for the 377- and 405-nm lasers is given by: .908 0.711 0.4 0.88-1 ^{1 J}

[0051] It is noticed that the values of hGB and IIBG are quite similar to the ones in Figure 4A with a slight offset due to various attenuation factors such as the propagation in the water. After performing the equalization using equation (6), a bathtub-like curve that resembles the BER versus decision threshold values at 200 Mb/s is obtained, as shown in Figure 13. Because a total signal’s length of 1 x 10 ⁵ was transmitted, a BER of zero was plotted as 1 x 10 ^-5 to fit in the log scale. As illustrated in Figure 13, the BER curves of blue and green channels without equalization (see inserts 1310 and 1320, respectively) dip below the FEC limit 1302. However, the green curve (see insert 1320) dips further, which can be due to the higher responsivity of the APD at longer wavelengths. Insert 1310 shows the corresponding eye diagram of the blue channel. It can be noticed that there are four voltage levels, as expected. The corresponding equalized eye diagram is shown in insert 1330 for the blue channel and insert 1340 for the green insert. For these last two inserts, the eye-opening is restored, and the BER curve falls to zero. Similarly, the un-equalized eye diagram of the green channel shown in the insert 1320 was corrected in the insert 1340. Due to the high scattering property of the UV light underwater, the UV communication is preferred for non-line-of-sight. Therefore, visible wavelengths from violet to green are best suited for LOS applications. Hence, the inventors have tested the feasibility of using the 405-nm laser 310’ as a transmitter for the blue channel 308 while the 450-nm laser 314’ is used for the green channel 312. According to Figure 4A, for these wavelengths, the green channel experiences a crosstalk level of approximately 95%, while the blue channel experiences negligible crosstalk.

[0052] Figure 14 shows the BER curves for the equalized and un-equalized received signal. The corresponding eye diagram is represented in the insert 1410. Figure 14, insert 1420 also shows that the un-equalized green channel cannot sustain reliable communication due to the significant crosstalk. Because the crosstalk is about 95%, it can be observed two clear eyes stacked on top of each other in the insert 1230, which corresponds to the green channel without equalization. This is because when both the victim and the aggressor are transmitting a ‘T, the superposition of both signals generates a signal with double the voltage. While the victim transmits a ‘0’ and the aggressor transmits a ‘1 ’, it is not possible to distinguish between the genuine ‘1 ’ from the crosstalk, which means that the communication is completely perturbed. Therefore, obtaining the /-/ matrix using the training symbols is not possible because the training symbols are corrupted. In this case, the system can adopt a calibration step prior to transmission. The calibration procedure is carried out by switching on one laser while the second laser is off and measure the DC level for a long interval at the victim’s channel. The matrix coefficient is the expected value of the observations. The same procedure is applied to the second laser. Thus, the matrix elements can be obtained. With this technique, the equalized eye diagram for the green channel is drawn in the insert 1430, with a measured channel matrix given by h-GB _ F 0.385 0.9151 h _GG 1.0.0027 0.851T ^{1 J}

[0053] Based on the measurements discussed above, it is possible to implement the receiver 300 into real-life underwater communication systems as now discussed. Figure 15 shows a first possible implementation of an underwater receiver 1500. The receiver has a spherical housing 302, which is transparent to UV and visible light. The first scintillating fiber 304 may be attached (e.g., glued) to the first half 302A of the interior of the housing 302, and the second scintillating fiber 306 may be attached to the second half 302B of the interior of the housing 302, as schematically illustrated in Figure 15. Each fiber is also connected to a corresponding APD 320 or 322. Note that a scintillating fiber (304 or 306) is different from a traditional fiber as schematically illustrated in Figure 16. More specifically, a traditional optical fiber does not receive light through its cladding. However, a scintillating fiber 1600, as shown in Figure 16, receives incident light 1601 to its core 1602, through its cladding 1604, and dye molecules 1608 (only one shown for simplicity) located in the core 1602 transform (scintillate) the incident light 1601 into an output light beam 1610, where the wavelength of the output beam 1610 is different from the wavelength of the input light 1601 . Note that the traditional fibers receive the incident light 1601 at one of its end and outputs a light with the same wavelength at an opposite end. Because the scintillating fiber 1600 can receive the incident light 1601 at any point on its side surface, the FOV is large and no alignment with the source of the incident light 1601 is necessary.

[0054] Returning to Figure 15, the first scintillating fiber 304 is selected to have (1) a maximum light absorption at about 377-nm, and (2) a maximum light emission spectrum at about 430-nm, where the term “about” is understood herein to be mean up to plus or minus 10% of the reference value (i.e., 377 in this case). The second scintillating fiber 306 is selected to have (1) a maximum light absorption at about 405-nm, and (2) a maximum light emission spectrum at about 488-nm. Both ends of each fiber is coupled to the corresponding photodetector 320 or 322, thus, in this embodiment, the ends of the fibers cannot receive an incoming light beam. The receiver 1500 further includes the processor 324 for processing the data, the memory 326, and optionally light sources 328 and 330 for emitting signals if desired to work as a transceiver. The fiber 306 may be coated with a material that forms the filter 334, which has a transmittance in the 400 to 500 nm range, as illustrated in Figure 4A. Figure 15 shows only a portion of the fiber 306 being coated with the filter 334. However, the entire surface of the fiber 306 may be coated with the filter 334. If more than two fibers are used, each fiber may be coated with a different filter material to prevent the light signals from the other sources to enter them.

[0055] In another embodiment, as illustrated in Figure 17, the two fibers 304 and 306 are distributed on the interior wall of the housing 302 so that they are interdigitated. Any geometry or shape may be used for distributing the fibers inside the housing 302. Note that the light sources 328 and 330 are distributed in this embodiment next to each other and pointing in the same direction, to simulate the distribution of the lasers 310 and 314 in Figure 3. The light sources 328 and 330 may be selected to emit light beams 329 and 331 of about 377- and 405-nm, respectively, or about 405- and 450-nm respectively. The light sources may be positioned so that the emitted light beams 329 and 331 are parallel to each other, i.e. , they propagate along a common axis X. The light sources 328 and 330 may be implemented as a light emitting element (LED), or laser diodes. For this embodiment, the receiver 1700, which is an implementation of the receiver 300, is floating underwater 1702, and is configured to receive, simultaneously, first and second optical beams 308 and 312. Each of the two fibers 304 and 306 processes a corresponding optical beam and generates a corresponding third or fourth optical beam 309 and 313, which are transformed by the corresponding photodetectors 320 and 322 into corresponding electrical signals 321 and 323. These electrical signals are processed at processor 324 and then the information stored in the electrical signals is emitted as fifth and six optical beams 329 and 331 , by the corresponding light sources 328 and 330. Thus, the node 1700 acts as a transceiver which receives optical beams 308 and 312 and transmits optical beams 329 and 331 , transporting the same information. In one application, the wavelengths of the optical beams 329 and 331 are the same as the wavelengths of the optical beams 308 and 312, respectively. This means that the light sources 328 and 330 may be the same as the light sources 310 and 314, respectively.

[0056] The receiver 300 may also be implemented in a transceiver node 1800, as shown in Figures 18A and 18B. Figure 18A shows the node 1800 floating at the water-air interface 1802 and the two light sources 328 and 330 being distributed at the poles of the housing 302. More than two light sources may be used if a multiwavelength communication system is desired. Thus, one light source 330 sends the signals into the water, for the receivers present underwater, while the other light source 328 sends the signals into the air, at a communication tower or drone (not shown) so that a water-air optical communication between devices above the water and devices underwater is achieved. The node 1800 is shown in Figure 18A being in the transmission mode, with the light sources 328 and 330 emitting the light beams 309 and 313, in opposite directions. Figure 18B shows the node 1800 in the receiving mode, with external light sources 310 and 314 being located outside the node, one in air and one underwater. For both modes, the light sources 328 and 330 or 310 and 314 are configured to simultaneously emit signals. The housing of the node has a size and is made in this embodiment of a material so that the housing and its entire content floats at the water-air interface.

[0057] No matter the implementation, one advantage of the configurations discussed herein is the large FoV because of the nature of the fibers 304 and 306 (i.e. , scintillating fibers) and also because the fibers are distributed over the large internal areas of the housing 302. For example, in one application, the entire length of the fibers 304 and 306 is distributed and attached to the interior wall of the housing. Although only two fibers have been discussed herein, one skilled in the art would understand that more than two fibers may be used. In one application, to maintain the orientation of the nodes relative to the water-air interface, a weight 1810 may be added to the housing 302, for example, to the bottom, to maintain the light sources 328 and 330 to have roughly the same spatial orientation even when the water is rough.

[0058] A method for dual-wavelength, underwater, optical wireless communication using the transceiver 1700 or 1800 is now discussed with regard to Figure 19. The method includes a step 1900 of receiving first and second underwater optical beams 308, 312 at first and second scintillating fibers 304 and 306, respectively, of a transceiver 1800, a step 1902 of generating a third optical beam 309, within the first scintillating fiber 304, having a wavelength smaller than a wavelength of the first optical beam 308, a step 1904 of generating a fourth optical beam 313, within the second scintillating fiber 304, having a wavelength smaller than a wavelength of the second optical beam 314, a step 1906 of transforming the third optical beam 309 into a first electrical signal 321 with a first photodetector 320, a step 1908 of transforming the fourth optical beam 313 into a second electrical signal 323 with a second photodetector 322, a step 1910 of emitting a fifth optical beam 329 with a first laser source 328, based on the first electrical signal 321 , and a step 1912 of emitting a sixth optical beam 331 with a second laser source 330, based on the second electrical signal 323. In one application, the fifth and sixth optical beams have the same wavelength as the first and second optical beams, respectively, which means that the light sources 328 and 330 may be identical to the light sources emitting the light beams 308 and 312, respectively.

[0059] The disclosed embodiments provide an optical antenna for an omnidirectional optical receiver/transceiver that includes two or more scintillating fibers. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0060] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0061] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

References

The entire content of all the publications listed herein is incorporated by reference in this patent application.

[1 ] P. P. Manousiadis, S. Rajbhandari, R. Mulyawan, D. A. Vithanage, H. Chun, G. Faulkner, D. C. O’Brien, G. A. Turnbull, S. Collins, and I. D. Samuel, “Wide field-of- view fluorescent antenna for visible light communications beyond the etendue limit,” Optica 3(7), 702-706 (2016).

[2] Y. Dong, M. Shi, X. Yang, P. Zeng, J. Gong, S. Zheng, M. Zhang, R. Liang, Q.

Ou, N. Chi, and S. Zhang, “Nanopatterned luminescent concentrators for visible light communications,” Opt. Express 25(18), 21926-21934 (2017).

[3] P. P. Manousiadis, H. Chun, S. Rajbhandari, D. A. Vithanage, R. Mulyawan, G. Faulkner, H. Haas, D. C. O’Brien, S. Collins, G. A. Turnbull, and I. D. W. Samuel, “Optical antennas for wavelength division multiplexing in visible light communications beyond the Etendue limit,” Adv. Opt. Mater. 8(4), 1901139 (2020).

[4] T. Peyronel, K. J. Quirk, S. C. Wang, and T. G. Tiecke, “Luminescent detector for free-space optical communication,” Optica 3(7), 787-792 (2016).

[5] C. H. Kang, A. Trichili, O. Alkhazragi, H. Zhang, R. C. Subedi, Y. Guo, S. Mitra,

C. Shen, I. S. Roqan, T. K. Ng, M.-S. Alouini, and B. S. Ooi, “Ultraviolet-to-blue color-converting scintillating-fibers photoreceiver for 375-nm laser-based underwater wireless optical communication,” Opt. Express 27(21 ), 30450-30461 (2019). [6] R. Mangeret, J. Farenc, B. Ai, P. Destruel, D. Puretolas, and J. Casanovas, “Optical detection of partial discharges using fluorescent fiber,” IEEE Trans. Electr. Insul. 26(4), 783-789 (1991 ).

[7] J. Farenc, R. Mangeret, A. Boulanger, P. Destruel, and M. Lescure, “A fluorescent plastic optical fiber sensor for the detection of corona discharges in high voltage electrical equipment,” Rev. Sci. Instrum. 65(1 ), 155-160 (1994).

[8] O. Alkhazragi, A. Trichili, I. Ashry, T. K. Ng, M.-S. Alouini, and B. S. Ooi, “Wide- field-of-view optical detectors using fused fiber-optic tapers,” Opt. Lett. 46(8), 1916— 1919 (2021 ).