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Bi-directional Free-Space Visible Light Communication (VLC) Supporting Multiple Moveable Clients Using Special Light Diffusing Optical Fiber (LDOF)

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23 April 2023

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24 April 2023

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Abstract
Visible light communication (VLC) can offer the advantages of license and electromagnetic interference (EMI) free wireless transmission. As optical signal does not interference with the radio-frequency (RF) signal, VLC can be used to augment RF wireless communication to provide extra communication capacity while without degrading the performance of both signals. In order to achieve high performance VLC transmission, optical alignment between the optical transmit (Tx) and receiver (Rx) is very critical to enhance the received signal-to-noise ratio (SNR). Optical beam-steering at the Tx can be utilized to ensure narrow optical beam can reach the Rx; however, complicated and active tracking are required. Lenses or compound parabolic concentrators (CPCs) can be install in front of the Rx for focusing to enhance the SNR. However, these will limit the Rx field-of-view (FOV) and making the VLC transmission more subjected to misalignment issue. Hence, many creative optical antennas have been proposed and demonstrated using special optical materials as well as special Rx to enhance the FOV of VLC systems. However, they have their limitations, such as data rates and FOVs. In this work, we put forward and demonstrate a bi-direction free-space VLC system supporting multiple moveable Rxs using a light-diffusing optical fiber (LDOF). The downlink (DL) signal is launched from an head-end or central office (CO) far away to the LDOF at the client side via a free-space transmission. When the DL signal is launched to the LDOF, which acts as an optical antenna to re-transmit the DL signal to different moveable Rxs. The uplink (UL) signal is sent via the LDOF towards the CO. In a proof-of-concept demonstration, the LDOF is 100 cm long and the free space VLC transmission between the CO and the LDOF is 100 cm. 210 Mbit/s DL and 850 Mbit/s UL transmissions, meeting the pre-forward-error-correction bit error rate (pre-FEC BER = 3.8 × 10−3) threshold are achieved.
Keywords: 
Subject: Physical Sciences  -   Optics and Photonics

1. Introduction

Due to the great demands for wireless communication bandwidth for applications, such as Internet-of-Things (IoT), online gaming and conferencing, cloud-based storage and processing, etc, the radio-frequency (RF) spectrum has been exhausted. Utilizing the optical frequency spectrum for the future wireless communication, which is known as optical wireless communication (OWC), could be a promising solution [1,2,3,4,5]. Visible light communication (VLC) [6,7,8,9,10,11,12,13,14,15] is one implementation of OWC using the visible light spectrum. VLC has been developing rapidly in the past decades to provide both communication and illumination simultaneously since it can integrate with the existing light-emitting-diode (LED) illumination infrastructure. Besides, it can offer the advantages of license-free and electromagnetic interference (EMI)-free wireless transmission. As optical signal does not interference with the RF signal, VLC can be used to augment RF wireless communication to provide extra communication capacity while without degrading the performance of both signals. In the future 6G wireless system, VLC is also considered as one of the promising candidates [16,17]. In addition, VLC could also provide many value-added functions of lighting, including underwater optical wireless communication (UWOC) [18,19,20,21], visible light positioning (VLP) [22,23,24,25], and optical camera communication (OCC) [26,27,28,29].
In order to achieve high performance VLC transmission, optical alignment between the optical transmit (Tx) and receiver (Rx) is very critical to enhance the received signal-to-noise ratio (SNR). At the Tx side, optical beam-steering can be utilized to ensure narrow optical beam can reach the Rx. Optical beam-steering schemes based on mechanical [30], tunable laser [31], diffractive [32], spatial light modulator (SLM) [33], as well as active optical phased array (OPA) [34] approaches have been proposed and demonstrated. Apart from using optical beam-steering schemes, lenses or compound parabolic concentrators (CPCs) [35] can be install in front of the Rx for focusing to enhance the SNR. However, these will limit the Rx field-of-view (FOV) and making the VLC transmission more subjected to misalignment issue, which is the étendue limit issue.
Many creative optical antennas have been demonstrated using special optical materials as well as special Rx to enhance the FOV of VLC systems. In 2016, Peyronel et al. proposed and demonstrated a tight array of polystyrene fibers doped with an organic dye (Saint-Gobain BCF-92), forming a rectangular detector with increased detection area [36]. They used optical waveguides doped with wavelength shifting dyes. The incident modulated optical signal was absorbed by the dye molecules independently of incidence angle of the optical signal and subsequently re-emitted at a different wavelength. In 2018, Ishibashi et al. proposed and demonstrated a free-space optical communication (FSOC) system for industrial vehicles using two types of optical fibers (i.e., light-diffusing fiber and wavelength shifting fiber) providing both downlink (DL) and uplink (UL) transmissions [37]. In 2019, Kang et al. reported a large-area scintillating-fiber-based Rx using ultraviolet (UV)-to-blue color conversion for underwater wireless optical communication [38]. In this scheme, a large-area and wide FOV Rx was achieved to establish a reliable communication link under a turbulent underwater environment. In 2020, Manousiadis et al. fabricated a wide FOV and high gain fluorescent optical antenna [39]. The structure consisted of a fluorescent material sandwiched between two glass layers. By using different dyes dispersed in transparent epoxy, wavelength division multiplexing (WDM) operation can be realized. In 2020, Riaz et al. demonstrated a 240o wide FOV VLC Rx for smart phone using a fluorescent fiber [40]. The fluorescent fiber used had a 3-dB response of 80 MHz. By using decision feedback equalization (DFE) with 40 feed-forward and 20 feed-backwards taps, the intersymbol interference (ISI) of the on-off-keying (OOK) signal was mitigated, achieving 1.1 Gbit/s operation. In 2022, Tsai et al. proposed and illustrated a 360o wide FOV optical camera communication (OCC) system using a phosphor-coated light diffusing fiber [41]. When blue laser diode (LD) was coupled at one end of the light diffusing fiber, blue light was scattered and emitted at the fiber circumference. The yellow phosphor converted the blue light to yellow light; hence, white light was produced by combining the blue and yellow lights. In this system, rolling shutter camera was used; and the data rate was 3.3 kbit/s. Table 1 summaries the performance comparison of recent VLC systems using advanced optical antennas to enhance the FOV.
Table 1. Performance of VLC systems using advanced optical antennas to enhance the FOV.
Table 1. Performance of VLC systems using advanced optical antennas to enhance the FOV.
Year Optical Antenna Modulation Data Rate Antenna Length FOV Feature
2016 Polystyrene fiber array (Saint-Gobain BCF-92) OFDM 2.1-Gbit/s 3.6 × 35 cm 59.4o Omni-directional detection potential [36]
2018 Light diffusion fiber + wavelength-shift fiber (BCF-92) OOK 100-Mbit/s (DL) + 100-Mbit/s (UL) 50 m (DL), 25 m (UL) 360 o For industrial vehicles [37]
2019 Scintillating fiber array (Saint-Gobain BCF-10) OOK 250-Mbit/s 1.2 × 30 cm 360 o Underwater wireless optical communication [38]
2020 Fluorescent layer sandwiched by 2 glass layers OOK 12-Mbit/s - 60 o 2-color WDM [39]
2020 Fluorescent fiber (Saint-Gobain BCF-20) OOK 1.1-Gbit/s 7.57 cm 240 o For smart-phone Rx [40]
2022 Phosphor-coated light diffusion fiber OOK 3.3 kbit/s 100 cm 360 o OCC [41]
This work Light diffusion optical fiber (LDOF) OOK 210-Mbit/s (DL) + 850-Mbit/s (UL) 100 cm (DL and UL) 360 o Bidirectional + FSO capability
From Table 1, we can observe that in order to provide hundreds Megabit/s, wide FOV and large VLC detection area, light diffusing fiber can be a promising candidate by allowing 360o wide FOV VLC detection around the fiber circumference. In this work, we put forward and demonstrate a bi-direction free-space VLC system supporting multiple moveable Rxs using a light-diffusing optical fiber (LDOF). The downlink (DL) signal is red color at wavelength of 633 nm, which is launched from an head-end or central office (CO) far away to the LDOF at the client side via a free-space transmission. When the DL signal is launched to the LDOF, which acts as an optical antenna to re-transmit the DL signal to different moveable Rxs. The uplink (UL) signal is green color at wavelength of 520 nm. It is sent via the LDOF towards the CO. In a proof-of-concept demonstration reported here, the LDOF is 100 cm long and the free space VLC transmission between the CO and the LDOF is 100 cm. 210 Mbit/s DL and 850 Mbit/s UL transmissions, meeting the pre-forward-error-correction bit error rate (pre-FEC BER = 3.8 × 10−3) threshold are achieved.
This paper is organized as: in Section 1, the motivation of the proposed bi-directional free-space VLC system supporting multiple moveable clients using LDOF is introduced. A performance comparison of different recent VLC systems using advanced optical antennas to enhance the FOV is presented. In Section 2, the design and structure of the LDOF acting as the omni-directional optical antenna is discussed. The system architecture, experiment, results and discussion will be presented in Section 3 and Section 4 respectively. Finally a conclusion will be given in Section 5.

2. Design and Structure of the Light-Diffusing Optical Fiber (LDOF)

Traditional optical fiber is used to deliver optical carrier containing data information from one end to the other end. As shown in Figure 1a, as the refractive index of the fiber core is higher than the fiber cladding, light is refracted and restrained in the fiber core due to total internal reflection (TIR). The traditional optical fiber is not typically considered suitable for use as an extended light source. By introducing nanostructure scattering centers in the fiber core, very efficient light scattering through the circumference sides of the optical fiber can be achieved as shown in Figure 1b. The proposed LDOF [42,43] has a silica glass core and acrylate polymer cladding with diameters of 170 μm and 230 μm respectively as shown in Figure 1c. By using lower index acrylate polymer cladding, the numerical aperture (NA) of the LDOF is about 0.46. The uniformity of the extracted light in the circumference can be adjusted by controlling the number of scattering sights in the fiber core. These nanostructure scattering centers range in size from 50-500 nm; hence, they can scatter effectively the transmitting light in the visible wavelengths. The scattering loss is about 5 dB/m, while the bending loss is small with minimum bending radius of 5 mm. Table 2 summaries the characteristics of the LDOF used in the experiment.
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Figure 1. (a) Traditional optical fiber is used to deliver optical carrier containing data information from one end to the other end. (b) LDOF can provide efficient light scattering through the circumference. (c) Cross-section of the LDOF with nanostructure scattering centers in the fiber core for efficient light scattering.
Figure 1. (a) Traditional optical fiber is used to deliver optical carrier containing data information from one end to the other end. (b) LDOF can provide efficient light scattering through the circumference. (c) Cross-section of the LDOF with nanostructure scattering centers in the fiber core for efficient light scattering.
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Table 2. Characteristics of the LDOF.
Table 2. Characteristics of the LDOF.
Optical or Mechanical Properties Feature
Diffusion Length 1 m
Numerical Aperture (NA) > 0.46
FOV Around Fiber Circumference 360o
FOV Along Fiber 120o
Operating Wavelength 420
Core Diameter 170 ± 3 μm
Clading Diameter 230 + 0/ - 10 μm
Proof Test: Tensile Strength 100 kpsi
Operating Temperature - 20 to + 105 oC

3. Architecture and Experiment of the Bi-directional Free-Space VLC

Figure 2 shows the proposed system architecture of the bi-directional free-space VLC system, in which the LDOF acts as an optical antenna supporting multiple moveable clients. In order to increase the VLC system flexibility, the LDOF could be installed at a remote location, and the DL data is sent from the head-end office or CO via free-space VLC. The LDOF at the client side acts as an optical antenna to re-transmit the DL signal to different moveable clients. In principle, the system can support a large number of Rxs simultaneously as long as there are enough space along the LDOF circumference. The UL signal at another wavelength is sent via the LDOF and free-space towards the CO.
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Figure 2. System architecture of the bi-directional free-space VLC system, in which the LDOF acts as an optical antenna supporting multiple moveable clients.
Figure 2. System architecture of the bi-directional free-space VLC system, in which the LDOF acts as an optical antenna supporting multiple moveable clients.
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Figure 3 shows the experimental setup of the free-space VLC system with bi-direction transmissions supporting multiple moveable Rxs. The LDOF is manufactured by Corning®. As discussed in Section 2, nanostructures are added to the inner core to produce light diffusion. The DL and UL Txs are a 633 nm red LD (Thorlabs®, HL63163DG) and a 520 nm green LD (Thorlabs®, PL520) respectively. Two pulse-pattern generators (PPGs) are used to drive the DL and UP LDs at the same time to produce optical on-off-keying (OOK) signals via bias-tees with proper direct-current (DC) biases. At the CO, a dichroic mirror (DM) is employed to separate the red DL and green UL signals. Collimators (Col.) are used to couple optical signals into and out of the LDOF. At the CO, the green UL signal is received by an avalanche photodetector (APD, Thorlabs®, APD210). At the client side, APD (Thorlabs®, APD110A) with a red optical filter (OF) is employ to receive the red DL signal from the LDOF. The client APD can slide along the whole LDOF to receive the DL signal, and the performance will be discussed in next Section. As discussed above, in principle, the system can support a large number of client APDs simultaneously as long as there are enough space along the LDOF circumference. Finally, the received DL and UL OOK eye-diagrams are captured by a digital sampling oscilloscope (DSO) (Agilent®, 86100A); and their BER are measured by a BER tester (Anritsu®, MP1800A).
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Figure 3. Experimental setup of the free-space VLC system with bi-direction transmissions supporting multiple moveable Rxs. PPG: pulse-pattern generator; APD: avalanche photodetector; DM: dichroic mirror; OF: optical filter; DSO: digital sampling oscilloscope.
Figure 3. Experimental setup of the free-space VLC system with bi-direction transmissions supporting multiple moveable Rxs. PPG: pulse-pattern generator; APD: avalanche photodetector; DM: dichroic mirror; OF: optical filter; DSO: digital sampling oscilloscope.
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4. Results and Discussion

Figure 4 illustrates the optical powers measured by an optical power meter (Thorlabs®, PM100D) when sliding along the 100-cm LDOF. it can be observed that the light intensity is quite uniform in the 20 - 80 cm range with an average optical power of 25 μW, illustrating that clients Rx locating at 20 - 80 cm range can receive similar optical signals. It is also worth to point out that as the LDOF is designed to diffuse light 360o around the fiber circumference, the measured optical powers are nearly the same around the LDOF circumference. Figure 5 illustrates the optical spectra of the DL and UL signals emitted by the 633 nm and the 520 nm LDs measured by a spectrometer (Ocean® Insight USB2000). It can be observed that both the DL and UL signals both have narrow linewidths and high side-mode suppression ratios.
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Figure 4. Experimental measured optical powers by a power meter at different positions along the LDOF.
Figure 4. Experimental measured optical powers by a power meter at different positions along the LDOF.
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Figure 5. Experimental optical spectrum of both DL and UL signals.
Figure 5. Experimental optical spectrum of both DL and UL signals.
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Figure 6a–c illustrate the photographs of the LDOF without, with the red light, and with the green light launchings respectively. We can observe uniform light around the fiber circumference in all cases. The optical signal emitted via the LDOF is safe for human eyes. We purposely make turns in the LDOF to illustrate the flexibility of the LDOF as optical omni-directional antenna.
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Figure 6. Photographs of the LDOF (a) without, (b) with the red light, and (c) with the green light launchings.
Figure 6. Photographs of the LDOF (a) without, (b) with the red light, and (c) with the green light launchings.
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Figure 7 illustrates the experimental photographs of the CO, in which a directly modulated red LD is used to provide the DL data, and a APD is used to receive the UL green data. The DM is used to separate the red DL and green UL signals from the wavelength multiplexed signal, and a lens is used to focus the UL signal into the APD. Figure 8 illustrates the experimental photographs of the client side at different viewing angles. Two collimators at each side of the LDOF are used to couple optical signals into and out of the LDOF. The client APD mounted on a sliding stage can slide along the whole LDOF to receive the DL signal. As discussed above, we purposely make turns in the LDOF to illustrate the flexibility of the optical antenna. Yellow color emitted via the LDOF can be observed when both red and green lights are launched and combined in the LDOF.
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Figure 7. Experimental photographs of the CO. APD: avalanche photodetector; DSO: digital sampling oscilloscope; DM: dichroic mirror.
Figure 7. Experimental photographs of the CO. APD: avalanche photodetector; DSO: digital sampling oscilloscope; DM: dichroic mirror.
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Figure 8. Experimental photographs of the client side at different viewing angles. APD: avalanche photodetector; Col.: collimator; LDOF: light-diffusing optical fiber.
Figure 8. Experimental photographs of the client side at different viewing angles. APD: avalanche photodetector; Col.: collimator; LDOF: light-diffusing optical fiber.
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Figure 9 shows the DL BER measurements via the LDOF from data rates 100 Mbit/s to 220 Mbit/s measured at the client side. From data rate 100 Mbit/s to 190 Mbit/s, it is error-free. BER starts to increase at data rate of 200 Mbit/s, and the BER is 6.50 × 10-7. BER of 3.69 × 10-4 is measured when the DL data rate is 210 Mbit/s, satisfying the 7% pre-FEC threshold (BER = 3.8 × 10−3). Figure 10 shows the corresponding received DL OOK eye-diagrams at different data rates. Clear eye-diagrams can be observed at data rates up to 180 Mbit/s.
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Figure 9. DL BER measurements via the LDOF at the client side.
Figure 9. DL BER measurements via the LDOF at the client side.
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Figure 10. Received DL OOK eye-diagrams.
Figure 10. Received DL OOK eye-diagrams.
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Figure 11 shows the UL BER measurement from data rates 100 Mbit/s to 1,000 Mbit/s measured at the CO. From data rate 100 Mbit/s to 600 Mbit/s, it is error-free. BER starts to increase at data rate of 700 Mbit/s, and the BER is 1.14 × 10-6. BER of 2.15 × 10-3 is measured when the UL data rate is 850 Mbit/s, satisfying the 7% pre-FEC threshold. Figure 12 shows the corresponding received UL OOK eye-diagrams at different data rates. Clear eye-diagrams can be observed at data rates up to 800 Mbit/s.
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Figure 11. UL BER measurement at the CO.
Figure 11. UL BER measurement at the CO.
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Figure 12. Received DL OOK eye-diagrams.
Figure 12. Received DL OOK eye-diagrams.
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Figure 13 shows the DL BER measurement curves at different data rates and at different positions of the 100-cm LDOF optical antenna. We can observe that at date rates of 100, 150 and 190 Mbit/s, error-free detection can be achieved at positions from 10 to 90 cm even the light is not quite uniform along the LDOF as shown in Figure 4. At data rates of 200 and 210 Mbit/s, we can observe that pre-FEC BER detection (BER = 3.8 × 10−3) can be achieved for the whole range of LDOF, with the highest BERs are measured at 90 cm locations with BER = 7.60 × 10-5 and 1.30 × 10-3, respectively. It is also worth to mention that in this proof-of-concept demonstration, a 1-m LDOF is employed since it is available in the laboratory. 10-m long LDOF is also commercially available, and it can extend the available number of Rxs at the client side. In future work, silicon photomultipliers (SiPM) could also be used to increase the free-space distance between LDOF and the Rx.
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Figure 13. DL BER measurement curves at different data rates and at different position of the 100-cm LDOF optical antenna.
Figure 13. DL BER measurement curves at different data rates and at different position of the 100-cm LDOF optical antenna.
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5. Conclusions

In this work, we put forward and demonstrated a bi-direction free-space VLC system supporting multiple moveable Rxs using a LDOF. The proposed LDOF had a silica glass core and acrylate polymer cladding with diameters of 170 μm and 230 μm respectively. The NA of the LDOF was about 0.46. The uniformity of the extracted light in the circumference was adjusted by controlling the number of scattering sights in the fiber core. These nanostructure scattering centers ranged in size from 50-500 nm. The LDOF can provide field-of-views of 360o around the fiber circumference and 120o along the LDOF. In the proposed bi-directional system, the DL signal was launched from a CO away to the LDOF at the client side via a free-space transmission. When the DL signal was at wavelength of 633 nm, and was launched to the LDOF, which acted as an optical antenna to re-transmit the DL signal to different moveable Rxs. The optical signal emitted via the LDOF was safe for human eyes. The UL signal was at wavelength of 520 nm and was sent via the LDOF towards the CO. In a proof-of-concept demonstration, the LDOF was 100 cm long and the free space VLC transmission between the CO and the LDOF was 100 cm. In principle, the system can support a large number of Rxs simultaneously as long as there are enough space along the LDOF circumference. 210 Mbit/s DL and 850 Mbit/s UL transmissions, meeting the pre-forward-error-correction bit error rate (pre-FEC BER = 3.8 × 10−3) threshold were achieved.

Author Contributions

Data curation, Y. H. Chang, Y. Z. Lin, Y. H. Jian, C. C. Wang; Funding acquisition, C. W. Chow; Investigation, Y. H. Chang, C. W. Chow, Y. Liu, C. H. Yeh; Writing – original draft, Y. H. Chang; Writing – review & editing, C. W. Chow, Y. Liu, and C. H. Yeh.

Funding

This paper was supported by National Science and Technology Council, Taiwan, under Grant NSTC-110-2221-E-A49-057-MY3, NSTC-109-2221-E-009-155-MY3.

Conflicts of Interest

The authors declare no conflict of interest.

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