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Abstract
A fluorescent fiber can be used as an optical antenna in visible light communication (VLC) for simultaneous optical filtering and light concentration and therefore to build a compact receiver. Since its light concentration principle is based on fluorescence, it can exceed the étendue limit and achieve both a high concentration gain and a wide field-of-view (FOV). In addition, because the photoluminescence (PL) lifetime of the fluorophore is typically only several nanoseconds, it can support high-speed data transmissions. When a fluorescent fiber antenna is used in a white light-emitting diode (LED)-based VLC system, the transmission performance highly depends on how the absorption and emission spectra of the fluorophore are associated with both the spectrum of the LED and the responsivity of the silicon photodetector. In this paper, we analyze the performance of several different commercially available fluorescent fibers. We show that, when the data rate is low or the transmission distance is long, since the light emitted from a red fluorescent fiber is associated with high silicon responsivities, it can result in high signal-to-noise ratios (SNRs) at the receiver output and therefore lead to low transmission error rates. In contrast, when the data rate is high or the transmission distance is relatively short, the bandwidth dominates the overall performance and consequently the green fluorescent fiber has better performance since it only absorbs the light emitted from the blue LED rather than the light emitted from the yellow phosphor.
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1. Introduction
Visible light communication (VLC) is an emerging technique which uses energy-efficient white light-emitting diodes (LEDs) for both indoor illumination and wireless data transmission. It can ideally augment the existing RF techniques (such as WiFi) to support a large number of Internet of Things (IoT) devices used in future smart home environments [1]. Currently, there are mainly two different types of white LED. One type uses a combination of multiple LEDs with different colors (e.g., red, green, blue) to produce white light. When this type of white LED is used in VLC, it can support transmission techniques such as wavelength-division multiplexing (WDM) or color shift keying (CSK) [2,3]. However, these LEDs are usually used in display applications rather than lighting infrastructures. In the current market, most commercially available white LEDs used for lighting are made of a blue LED coated with a yellow phosphor. The phosphor coating converted part of the blue light to yellow so that the overall emitted light can be perceived as white. However, the phosphor typically has a long light emission lifetime which limits the modulation bandwidth and consequently affects the overall achievable transmission data rate. The common solution to this problem is to place a blue filter in front of the receiver [4]. However, the use of optical filtering only allows a portion of the light to be detected which affects the signal-to-noise ratio (SNR) at the receiver output. Therefore, an additional optical concentrator is usually needed to increase the received signal strength. However, typical concentrators, such as the lens or the compound parabolic concentrator (CPC), have a trade-off between the concentration gain and the field-of-view (FOV) known as the étendue limit [4,5]. A high concentration gain means that the FOV of the receiver is small which limits the mobility of the device. More importantly, the use of an optical concentrator together with a filter also makes the receiver expensive and bulky and can hardly be incorporated into compact IoT devices.
In recent studies, a new approach of using fluorescent optical antennas in VLC to achieve both optical filtering and light concentration has shown very promising performance [6–15]. This type of optical antenna is made of fluorescent materials which can absorb light within certain ranges of wavelengths and then quickly emit light with longer wavelengths. Therefore, they are capable of optical filtering. Also, the photoluminescence (PL) lifetime of the fluorophore is usually only few nanoseconds and consequently they can support high data rate transmissions. In addition, because it is designed to have both the light propagation layer and the cladding layer, a large number of emitted photons can be trapped within the antenna and waveguided to the photodetector. Therefore, it is also an optical concentrator. More importantly, because its light concentration principle is based on fluorescence rather than reflection and refraction, it can exceed the étendue limit and achieve both a high concentration gain and a wide FOV [7,16]. In [7], one type of fluorescent optical antenna which contains a thin fluorescent film sandwiched between two microscope slides was introduced. The thin film is made of a fluorescent dye (coumarin-6) which has strong absorption of blue light. Its performance was studied in a white LED-based VLC system for increasing the transmission bandwidth. This structure was later extended in [11] to include a second thin film layer made of a different fluorescent dye (DCM) which has strong absorption of green light. Using this approach, the fabricated antenna can support WDM to boost the transmission data rate. However, the drawback of this structure is that the antenna has four wide rectangular edges which are difficult to be coupled with many commercially available photodiodes and consequently many photons which are guided to the antenna edges cannot be detected. In [8,13,17–19], another type of optical antenna made of fluorescent doped plastic optical fibers is studied. The advantage of using a fluorescent fiber as an optical antenna is that the end of the fiber can be easily coupled with most photodiodes so that the leakage of the photons from the edge is very small. Also, the advanced multiple-layer cladding techniques used in fiber fabrication can increase the photon trap efficiency. Moreover, the use of plastic means that the antenna is flexible in shape and can be cut into different lengths to be used in devices with different sizes. Now, various types of fluorescent fibers are commercially available from many companies including Kuraray [20]. However, despite the many advantages of using a fluorescent fiber antenna in VLC, it is only considered in the cases when lasers were used as the transmitters and many of its important properties are still not well studied.
In VLC, the transmission performance highly depends on the wavelength of the light. However, in a white LED-based VLC system, there is a mismatch in the optimal wavelength which should be used for transmission between the transmitter and the receiver. At the transmitter, the blue light emitted from the GaN LED has a much shorter emission lifetime compared to the yellow light emitted from the phosphor. Consequently, when only the blue part of the light is used for data transmission, the modulation bandwidth is much higher compared to the case when the whole white spectrum is used. In contrast, at the receiver, the photodiodes made of silicon typically have higher responsivities for longer wavelengths of light and the use of red/infrared light can typically provide higher SNRs for a given overall received optical power. When fluorescent antennas are used in VLC, this mismatch problem becomes particularly interesting but not well-explored. This is because the absorption spectrum of the fluorophore determines which wavelengths of the light emitted from the LED are used for data transmission. At the same time, the emission spectrum of the fluorophore affects the wavelength of the light which is eventually detected by the photodetector.
In this paper, we consider several different commercially available fluorescent fibers and investigate their performance in a white LED-based VLC system. In particular, we study how the mismatch in the optimal wavelength between the transmitter and the receiver can affect the system’s transmission performance. The considered fluorescent fibers have different absorption and emission spectra and thus are perceived with different colors. We show that the error rate performance of the system highly depends on the color of the fluorescent fiber and the best choice of the fiber is related to the data rate and transmission distance. When the data rate is low and/or the transmission distance is long, the use of red fluorescent fibers can provide higher received signal strength and consequently result in low bit error rates (BERs). However, when the data rate becomes high and/or the transmission distance is short, the achievable bandwidth dominates the overall transmission performance and green fibers achieve better performance. In future smart home environments, the wireless network not only needs to connect high-speed Internet user equipment but also needs to support a large amount of low-speed IoT devices. This means that when the devices require related low-speed data rates or occupy low-frequency subchannels, the red fiber is a suitable option. However, if the wireless devices require high-speed data rates or need to occupy high-frequency subchannels, the green fibers should be used since they can absorb the blue light directly emitted from the GaN LED which leads to high modulation bandwidths.
2. Optimal wavelength mismatch
In this section, using the white LED and the photodetector considered in this work as examples, we briefly highlight the mismatch problem on the choice of the optimal transmission wavelength considering the spectrum of the LED and the responsivity of the Si photodetector. In this work, the measured optical spectrum of the white LED (LXML-PWC2, 5650K) is shown in Fig. 1. It can be seen that the plot peaks at 450 nm and 550nm which are associated with the blue light emitted from the GaN LED and the yellow light emitted from the phosphor respectively. This means the emitted blue light around 450 nm is suitable for achieving high modulation bandwidth. At the receiver, a Si APD (Thorlabs APD130A) was used to detect the light intensity signal and its responsivity is also shown in Fig. 1. It can be seen that, within the visible light range (below 700 nm), the responsivity becomes higher for a longer wavelength. It can be noticed that the blue light emitted from the LED is only associated with a low responsivity of 2 A/W. This means there is a mismatch in the optimal wavelength which should be used for transmission. In a fluorescent antenna-based VLC system, this mismatch is highly related to the optical properties of the fluorophore.
Fig. 1. The spectrum of the considered white LED and the responsivity of the avalanche photodiode (APD). The spectrum of the white LED was measured using a photonic multichannel analyzer (Hamamatsu, PMA-12) and the responsivity of the APD was obtained from the datasheet [21].
3. Fluorescent fiber antenna in VLC
In this paper, we focus the study on how the color of the fluorescent fiber can impact the transmission performance in a white LED-based VLC system. A photo of the three types of considered fluorescent fiber manufactured by Kuraray is shown in Fig. 2. To illustrate how the fluorescent fiber antennas can be used in a VLC system, Fig. 3 shows one particular application scenario when the fluorescent fiber antennas are incorporated into several wireless devices including a smartphone, a laptop and a TV. In this scenario, the fluorescent fiber antennas are placed on the top edge of the devices to collect light. Different from some typical VLC systems in which the photodiodes are usually pointing directly towards the light sources, the photodiodes in these cases are coupled to one end of the fluorescent fiber antennas.
Fig. 2. A photo of the considered fluorescent fibers under 400 nm light. The three fibers include a green (YS-2) fiber, an orange (O-2) fiber and a red (R-3) fiber. They are manufactured by Kuraray [20].
Fig. 3. A possible application scenario of using fluorescent fiber antennas in an indoor VLC system to support wireless devices.
3.1 Physical processes within the fiber
When the incident light arrives at the fiber, various physical processes occur. As illustrated in Fig. 4, an incident photon can be reflected by the fiber surface or can be transmitted through the fiber without reflection and absorption. In contrast, many photons can be absorbed within the fiber. The absorption may be relaxed non-radiatively or result in the emission of photons with longer wavelengths. In the case of emission, the photons can be emitted into different directions and undergo different processes which are summarized into three cases as shown in Fig. 4. In the first case, the emitted photons can escape the fiber when the incident angle of the emitted light is less than the critical angle. In the second case, the emitted photons can be re-absorbed and cause the re-emission of photons with even longer wavelengths. In the third case, due to the internal reflection, a large amount of emitted photons will be trapped within the fiber and waveguided to one end of the fiber where a photodiode is placed. Overall, due to the above physical processes, the fluorescent fiber can be used as an optical filter since it only absorbs light within a certain wavelength range. At the same time, it is a wavelength converter due to the difference between the absorption spectrum and the emission spectrum. Also, since many emitted photons can be trapped within the fiber, it is also an optical concentrator.
Fig. 4. A schematic diagram of the physical processes of the light within a fluorescent optical fiber. The cladding of the fiber is made of polymethylmethacrylate (PMMA) with a refractive index of $1.49$ and the core of the fiber is made of polystylene (PS) with a refractive index of $1.59$.
3.2 Absorption and emission spectrum
The absorption and emission spectra of different colors of the fluorescent fibers are shown in Fig. 5. Compared Fig. 5 with Fig. 1, it can be noticed that the green YS-2 fiber only absorbs the blue part of the light emitted from the white LED. The orange O-2 fiber absorbs a small portion of the blue light and also a part of the yellow light emitted from the phosphor. The red R-3 fiber mainly absorbs the yellow part of the light emitted from the phosphor. This information suggests that, in a white LED-based VLC system, the use of the YS-2 fiber can lead to a higher modulation bandwidth. As mentioned previously, the performance of a VLC link not only depends on the absorption spectrum but also is related to the emission spectrum. As shown in Fig. 5, the light emitted from the YS-2 fiber peaks approximately at 480 nm which is associated with very low silicon responsivity coefficients. For O-2 fiber, its emission spectrum peaks at 550 nm. For the R-3 fiber, its emission spectrum peaks at 610 nm which is associated with much higher silicon responsivity coefficients as shown in Fig. 1. Also, it can be seen from Fig. 1 that the yellow light emitted from the phosphor occupies a large portion of the spectrum and therefore the R-3 fiber can absorb a large portion of the light emitted from the white LED. These factors mean that the use of the R-3 fiber can potentially lead to higher SNRs at the receiver output.
Fig. 5. The absorption and emission spectrum of the light using different Kuraray fluorescent fibers (a) Green (YS-2) (b) Orange (O-2) (c) Red (R-3). These plots were extracted from the Kuraray fiber datasheet [20].
4. Experiment setup
The setup of the experiment is shown in Fig. 6. The transmitted signal was first generated off-line using MATLAB and then uploaded into an arbitrary waveform generator (AWG, Siglent SDG2082X). The output signal from the AWG was first amplified using an electrical amplifier (Mini-circuits, ZHL-32A-S) and then superimposed onto a DC current using a bias-T (Mini-circuits, ZFBT-4R2GW) to drive an white LED luminaire (LXML-PWC2) which was placed on the top of the fiber antenna. At the receiver, a commercial APD (Thorlab APD130A) was coupled to one end of a fluorescent plastic optical fiber and detected the light intensity signal. Finally, the received signal was captured by an oscilloscope (LeCroy, 204Xi-A) and processed off-line using MATLAB.
5. Experiment results
To investigate how the color of the fluorescent fiber affects the VLC transmission, various system performance metrics, including the FOV, the frequency response, the 3 dB bandwidth, the received signal strength, the BER, were considered in the experiment and the measured results are discussed in the following sections.
5.1 FOV measurement
The FOV of the receiver was measured using the configuration shown in Fig. 7(a). In the measurements, a red fluorescent fiber with a length of 25 cm was considered since it can provide high signal voltages at the output of the receiver. Different light incident angles were obtained by varying the position of the LED. As shown in Fig. 7(a), the middle of the fiber was set as the rotating origin and the distance between the LED transmitter and the origin was fixed at 40 cm. The LED transmitted a sinusoidal wave of 1 MHz. The detected peak-to-peak voltage was measured for different values of $\varphi$ and the definition of $\varphi$ is shown in Fig. 7(a). For any value of $\varphi$, the LED was always pointing directly to the origin so that the emergence angle of the light was zero degrees. Also, since the fiber has a relatively longer length compared to other types of concentrators, the actual incident angles of the light arriving on different parts of the fiber were different. The result in Fig. 7(b) shows that the receiver can achieve a very wide FOV. When the incident angle is less than 20 degrees, the received signal strength is almost not affected. When the incident angle becomes 80 degrees, the light can still be detected by the photodetector and the signal intensity becomes approximately $25\%$. Note that, in Fig. 7(b), the value of $\varphi =90^{\text {o}}$ was not considered. This is because, when $\varphi =90^{\text {o}}$, the light detected by the photodetector is collected by the fiber end rather than the fiber surface.
Fig. 7. (a) The measurement configuration, and (b) the measured voltage at different incident angles.
5.2 Frequency response and bandwidth
The frequency response of a VLC link can potentially be affected by several factors including the capacitance of the electronic components, the emission lifetime of the blue LED, the PL emission lifetime of the optical components (the yellow phosphor, the fluorescent fiber) and the transmission delays in different optical paths. In this work, the electronic devices were all selected with high bandwidths so that they are not the limiting factors. Also, in this specific experiment setup, the transmission link mainly depends on the line-of-sight (LOS) channel and the diffuse component of the received light is much weaker. Therefore, the multipath effect does not impact the frequency response either. It was studied in [8,13] that, when a laser is used as the transmitter and the transmission data rate is several hundred Mbps or several Gbps, the symbol duration is shorter than both the fluorescence lifetime and the transit time of the light within the fiber. In this case, both the fluorescence lifetime and the fiber length affect the frequency response. However, this is not the case in this work. In this work, we focus on the use of a white LED and the transmission data rates limited to several tens of Mbps. Also, the considered fiber length is only several tens of cm. Therefore, the duration of individual transmitted signal samples is much longer than the fluorescence lifetime of the fiber and the transit time of the light within the fiber. Consequently, in white LED-based VLC systems, both the fluorescent lifetime and the light transit time within the fiber antenna will not dominate the frequency response.
In this work, similar to other white LED-based VLC systems, the transmission bandwidth is either dominated by the emission lifetime of the yellow phosphor without blue filtering or the emission lifetime of the blue LED with blue filtering. However, unlike other VLC systems, since a fluorescent fiber antenna is used, the transmission bandwidth also depends on how the absorption spectrum of the fiber is associated with the spectrum of the white LED. During the light emission processes in the GaN blue LED or the yellow phosphor, similar to a simple RC circuit [12], the excited state population normally decays exponentially and therefore each of them causes an individual frequency response given by
where $\tau$ is the emission lifetime of the blue LED or the emission lifetime of the yellow phosphor. The 3 dB frequency, $f_{\text {3dB}}$, is related to $\tau$ byThe measured frequency responses when different colors of fiber were used are shown in Fig. 8. It can be seen that when there is no fiber placed in front of the receiver, the 3 dB bandwidth is only 3.5 MHz. When a red R-3 fiber is used, the frequency response remains unchanged. This is because R-3 cannot absorb the blue part of the light and only absorb the yellow light converted from the phosphor. Therefore, the frequency response is not affected very much. Based on (2), this suggests that the PL lifetime of the yellow phosphor is approximately or greater than 45 ns. When an orange O-2 fiber is used, the 3 dB bandwidth is slightly increased since the fiber can absorb part of the blue light. Figure 8 also shows that the transmission bandwidth is significantly increased to 8 MHz when a green YS-2 fiber is used. This is because, as shown in Fig. 5, the YS-2 fibers only absorb the blue part of the light and therefore the transmission bandwidth becomes much higher. Based on (2), it can be estimated that the light emission lifetime of the blue LED is approximately 19 ns.
5.3 Color and signal strength
When a fluorescent fiber is used as an optical antenna in VLC, the optical power of the collected light highly depends on the length of the fiber. A longer fiber has a larger light collection area and therefore can trap more photons and waveguide them to the photodetector. At the same time, as explained previously, the signal strength at the receiver output is also affected by color of the fluorescent fiber depending on how its emission spectrum is associated with the responsivity of the photodetector. In this section, we investigate how the length of the fiber and the color of the fiber can affect the received signal strength.
Figure 9 shows the measured peak-to-peak values of the detected voltage signals when different frequencies of sinusoidal waves were transmitted by using fibers with different lengths. First, it can be seen that the received signal strength increases when a longer length of fiber is considered. The relationship between the detected voltage and the fiber length can be well fitted using linear lines. Second, it can be noticed that when the frequency of the signal is 1 MHz, the green fiber has the lowest signal strength. However, when the considered frequency is increased to 5 MHz, the green fiber achieves higher signal strengths compared to the orange fiber. When the frequency is further increased to 10 MHz, the differences between the green fiber and the red fiber become smaller. Third, compared to the case of using a white LED, the red fiber can provide an equivalent level of signal strength when its length is approximately 20 cm regardless of the considered frequency. For the orange fiber and the green fiber, the required fiber length to obtain the same signal strength as the white LED case depends on the frequency of the transmitted signal. For example, the green fiber with a length of 30 cm provides the same signal strength as the white LED at 5 MHz and this length is reduced to be 20 cm at 10 MHz.
Fig. 9. The measured peak-to-peak voltage versus the length of the fiber. The marks show the considered fiber lengths and the associated measured signal strength. The dash lines are the fitting plots obtained using the linear regression principle. In this measurement, three different frequencies of the transmitted signal were considered: (a) 1 MHz, (b) 5 MHz, (c) 10 MHz.
5.4 Data rate and BER
This section discusses the data transmission results when different fluorescent fibers were used in the measurement. In this experiment, orthogonal frequency-division multiplexing (OFDM) was used as the modulation method [22]. The transmitted OFDM signal is pre-clipped to avoid extremely high peaks so that the signal dynamic range provided by the AWG for a given peak-to-peak voltage can be well utilized. Also, both the top clipping ratio and the bottom clipping ratio are considered to be 10 dB so that significant signal peaks can be removed but the clipping signal distortion is very minor [23]. Other related parameters are listed in Table 1.
Table 1. Transmission parameters
Similar to other communication systems, in VLC, the transmission error performance depends upon a combination of the channel bandwidth and the SNR at the receiver output. When a fluorescent antenna is used in VLC, the bandwidth and the SNR are related to the absorption and emission spectra of the fluorophore. In this section, we show the measured BERs and study how they are related to the color of the fiber antenna. Also, note that the BER is zero if the result is not marked in the plots. In the measurement of the BER, fibers with a shorter length of 25 cm were first considered. Figure 10 shows the measured BERs at different transmission data rates when the distances between the LED and the fiber antenna are 0.3 m and 0.2 m. Note that, in this proof-of-concept setup, the considered distances between the LED and the fiber were relatively short considering some real-life applications. However, in a commercial VLC system, this distance can be significantly increased by using multiple LEDs in parallel and/or using custom-designed electronics. First, it can be seen that, when the transmission data rate is low and less than 10 Mbps, the use of a red fiber can lead to the lowest BER. This is because when the transmission data rate is low, the bandwidth is not the dominating factor and a higher signal strength obtained by the red fiber can result in a higher SNR and thus a lower BER. In contrast, in these low data rate cases, the use of a green fiber gives the highest BERs. This is because the fluorescent fibers only absorb a portion of the light and the emission of the green light is also not sensitive to the Si photodetector. Even though the green fiber can absorb and concentrate the blue light emitted from the LED to the photodetector, the overall received signal strength is still lower compared to other cases including the case when no fiber is used. Next, it can be noticed that when the transmission data rate is high (e.g., more than 15 Mbps in Fig. 10(a) or more than 10 Mbps in Fig. 10(b)), the green fiber can achieve the lowest BERs. This is because the bandwidth dominates the overall performance when the transmission data rate is high and the green fiber can provide a higher modulation bandwidth as shown in Fig. 8. In the case of Fig. 10(b), when the forward error correction (FEC) limit is considered to be $3.8 \times 10^{-3}$ [24], the green fiber can support the highest transmission data rate of 30 Mbps. Next, we considered a longer length of fiber of 35 cm and the obtained results are shown in Fig. 11. Similar to the observations obtained from Fig. 10, it can be seen that when the transmission data rate is less than 10 Mbps, the BERs obtained using the red fiber are the lowest. When the data rate is more than 10 Mbps, the green fiber can support the best performance.
Fig. 10. The measured BER plotted as a function of the transmission data rate when the length of the fiber is 25 cm and (a) the transmission distance is 0.3 m and (b) the transmission distance is 0.2 m.
Fig. 11. The measured BER plotted as a function of the transmission data rate when the length of the fiber is 35 cm and (a) the transmission distance is 0.35 m and (b) the transmission distance is 0.3 m.
As shown in Fig. 10 and Fig. 11, if the best achievable transmission data rate is the only considered performance metric, the green fiber should be the choice. However, in a smart home environment, VLC techniques are expected to not only support applications which require high-speed data rates but also connect a large number of low-speed IoT devices. In this case, when frequency-division multiplexing (FDM) is used to support multiple low-speed users, the red fiber should be used for providing a high SNR for the user devices assigned with the low-frequency subchannels. However, for the IoT user devices assigned with high-frequency subchannels, the green fiber should be used since it can respond to higher-frequency signal changes.
Next, we mainly investigate how the distance between the LED transmitter and the fluorescent fiber affects the transmission performance. Figure 12 shows the measured BERs plotted as a function of the data rate when different transmission distances were considered. It shows that, for any given data rate, the BER becomes higher when the transmission distance is increased. In the case of using a green fiber antenna, all measured BERs are above the FEC limit of $3.8 \times 10^{-3}$ when the distance is increased to be greater than 0.5 m. When a red fiber was used, all BERs are above the FEC limit when the transmission distance is greater than 0.9 m. Figure 13 shows the highest achievable transmission data rates at different distances considering different fiber antennas. It can be seen that, when the distance is less than 0.4 m, the green fiber supports the highest transmission data rates. This is because, when the distance is relatively short and the signal intensity is relatively high, the bandwidth dominates the achievable transmission data rate and consequently the green fiber supports the best data rate. In contrast, when the transmission distance is greater than 0.4 m, the signal intensity becomes low and the SNR dominates the overall performance and the red fiber can support higher transmission data rates. Also, since the use of the red fiber antenna can enhance the overall received signal strength, it can be seen that the use of a 25 cm red fiber antenna always outperforms the case when no antenna is used.
Fig. 12. The measured BER plotted as a function of the data rate when different transmission distances were considered and using, (a) no antenna, (b) a green (YS-2) fiber antenna, (c) an orange (O-2) fiber antenna, (d) a red (R-3) fiber antenna.
Fig. 13. The achievable transmission data rate at different transmission distances using different fiber antennas.
6. Conclusion
In VLC, a fluorescent fiber can be used as an optical antenna and placed in front of the photodetector for light collection. Compared to typical VLC setups using a blue filter and a light concentrator, there are several significant advantages of using a fluorescent fiber antenna. First, it can be used to build a compact receiver since the optical filtering and the light concentration are achieved simultaneously using a single fiber. Second, the receiver has a wide FOV since it can exceed the étendue limit. Third, since the fiber antenna can be bent or cut into different lengths, the design of the receiver as well as the position of the photodetector can be flexible. Moreover, the cost of the fluorescent fiber antenna is potentially very low since it is made of a short length of plastic fiber. When it is used in a white LED-based VLC system, the transmission performance is highly related to the optical properties of the fluorescent dyes doped into the fiber. In particular, the performance depends on how the absorption and the emission spectra of the fluorophore are associated with the spectrum of the white LED as well as the responsivity of the Si photodetector. In this paper, several different commercially available fluorescent fibers are considered to investigate how the optical properties of the fluorescent fiber can affect the overall transmission performance. The measurement results show that when the data rate is low or the transmission distance is relatively long, the use of red fibers can result in high received signal strength and therefore low error rates. In contrast, when the data rate is high or the transmission distance is short, the green fibers have much better performance since it mainly absorbs the blue light emitted from the LED which results in a high modulation bandwidth.
Acknowledgment
We want to thank Kuraray Co., Ltd for providing us with the fluorescent fibers used in the experiment. We also want to thank JAIST for supporting this research with an internal research grant (JAIST research grant).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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