A HIGH-PERFORMANCE BLUE FILTER FOR A WHITE-LED-BASED VISIBLE LIGHT COMMUNICATION SYSTEM

Transcription

1 V ISIBLE LIGHT C OMMUNICATIONS A HIGH-PERFORMANCE BLUE FILTER FOR A WHITE-LED-BASED VISIBLE LIGHT COMMUNICATION SYSTEM SHAO-WEI WANG, FEILIANG CHEN, LIYE LIANG, SONGLIN HE, YIGUANG WANG, XIAOSHUANG CHEN, AND WEI LU Shao-Wei Wang, Liye Liang, Songlin He, Xiaoshuang Chen, and and Wei Lu are with the Shanghai Institute of Technical Physics and Shanghai Engineering Research Center of Energy-Saving Coatings. Feiliang Chen is with the Shanghai Institute of Technical Physics, Shanghai Engineering Research Center of Energy-Saving Coatings, and the University of Chinese Academy of Sciences. Yiguang Wang is with Fudan University. ABSTRACT A high-performance blue filter was demonstrated and employed to increase the modulation bandwidth and dramatically reduce the bit error rate of a white-led-based VLC system. It has a very wide stopband ( nm), high transmittance passband (average 97.5 percent in the blue signal range of nm), and a sharp and precise cutoff edge. Not only can the blue filter completely remove the slow phosphorescent component from modulated signals, but it can also effectively reject ambient solar radiation. Meanwhile, the blue light signals of the LED can almost be retained. This results in a high SNR and improves the performance of a VLC system, including increased modulation bandwidth and reduced BER. The BER can be dramatically reduced from to at 50 MHz bandwidth, and from to at a distance of 30 cm compared to a VLC system without our blue filter. More importantly, the stop band covers the whole response range of the receiver except for the blue signal band, which strongly increases the ability of a VLC system used in the sun or outdoors. INTRODUCTION As an exciting emerging optical wireless technology, a white-led-based visible light communication (VLC) system offers simultaneous illumination and high-speed data transmission, making it a new research hotspot in recent years [1 4]. The VLC system shows a number of benefits such as high bandwidth, freedom from licensing, low power consumption, freedom from electromagnetic interference, no harm to the human body, and high security and privacy [3 5]. All these advantages make it a prospective technology for short-range communications, especially in various specific areas [6], including aircraft [7], hospitals, underwater communications [8], and indoor navigation [9]. It is a very good complement to RF communication [6]. As for a white-led-based VLC system, both red-green-blue (RGB) LEDs and phosphorbased LEDs have been demonstrated [10, 11]. In fact, most commercially available white LEDs are phosphor-based, consisting of blue LEDs combined with a yellow phosphor coating, which offers low cost and low complexity. However, phosphor-based LEDs show a limitation in the modulation bandwidth available due to the slow relaxation time of the phosphor, which is a main challenge for high-speed communications [2, 11, 12]. One of the most effective ways to improve the modulation bandwidth is employing a blue filter in front of the receiver. The slow fluorescence component can be removed from the modulated signal with the blue filter, and the bandwidth can be increased substantially from 3 up to 20 MHz [11, 13]. However, the blue filter used in previous studies is far from the best. The transmittance of a blue filter is only 60 percent for the signal band [13, 14]; hence, 40 percent of the signal cannot reach the receiver and is wasted. The cutoff edge is not sharp and precise enough [15], which will result in more of the phosphorescent component outside the band coming in or part of the blue signal being removed by the stopband. Furthermore, the stopband is not broad enough to get rid of light from the red and nearinfrared range, where the response region of a silicon PIN or an avalanche photodiode (APD) receiver is much stronger than the signal band. Its responsivity in the blue light range is much lower than in other parts of visible light and near-infrared. The response peak always locates at between 800 nm and 900 nm. Ambient light, from sunlight, fluorescent lamps, computer or television monitors, and other light sources, becomes an inevitable strong interference source and may greatly lower the signal-to-noise ratio (SNR). This means that the ambient light can still reach the receiver and become strong background noise, deteriorating the performance of the VLC system. This has not been taken into account in previous studies. Therefore, the ideal blue filter needs to have IEEE Wireless Communications April /15/$ IEEE 61

2 Normalized intensity (a.u.) nm Phosphor-based white LED Spectral response of Si-PIN Figure 1. Emission spectrum of the phosphor-based white LED employed in our VLC system together with the typical spectral response of a Si-based receiver Responsivity (A/W) high efficiency in the signal transmission band, a very wide stopband except for the blue signal band, and a sharp and precise cutoff edge simultaneously. It is a simple and efficient way to improve the performance of a VLC system. According to the above requirements, we designed and fabricated a high-performance blue filter with a very wide stopband and sharp and precise cutoff edge, which can increase the modulation bandwidth and greatly reduce the bit error rate (BER). The latter can be reduced by more than two orders of magnitude. A common blue filter with a stopband covering only the rest of the visible region is used for comparison. DESIGN AND FABRICATION OF BLUE FILTER It is obvious that the blue filter should be designed according to the spectral characteristics of white LEDs. Figure 1 shows the emission spectrum of the phosphor-based white LED (CREE Xlamp XML) employed in our VLC system together with the typical spectral response of a Si-based receiver. The spectrum of a blue LED is a sharp peak centered at 456 nm. There is a much wider and stronger white fluorescence peak excited by blue LED covers from 485 to 800 nm, which works for lighting. It can be seen that a junction of the blue light component and excited white fluorescence component is located at 485 nm. Therefore, a short-wave-pass filter should be well designed to separate the blue signal and other light, including white fluorescence at the wavelength of 485 nm. What is more, a silicon PIN or APD photodetector used as the most common VLC receiver shows response spectrum ranging from about 400 to 1050 nm, while the peak is located between 800 and 900 nm. Its responsivity in the blue light range is much lower than in other parts of visible and near-infrared light. In order to restrain the ambient light effectively, a wide stopband ranging from 500 to 1050 nm is demanded, which is especially useful for out-door applications. Taking both the characteristics of a white LED and a Si-based receiver into account, the goal is to maximize the transmittance of the blue light signal ( nm) while minimizing the transmittance of the phosphor component and other parts of visible and near-infrared ( nm) light. A wide stopband from 500 to 1050 nm is required. In this way, the loss of received signal power can be minimized, and at the same time the phosphorescent component and ambient light will be rejected as much as possible. Therefore, an ideal blue filter should have a blue signal band with 100 percent transmittance and a very wide stopband to restrain the ambient light outside of the signal band. The cutoff edge should be sharp and precise. A high-performance optical filter can be constructed from dielectric multiple. Two kinds of materials with different refractive index are usually employed to build the multilayer structure. In this article, Ta 2 O 5 and SiO 2 are chosen as high and low refractive index materials with n H = 2.16 and n L = 1.46, respectively. These two materials have low absorption in visible light and show good mechanical property, chemical stability, as well as excellent adhesion to each other. Only one short-wavelength pass filter film with a single central wavelength is insufficient to realize such a wide stopband from 500 to 1050 nm. A cascaded multiple short-wavelength pass cut-off filter with different central wavelength is proposed to broaden the stopband effectively. The original structure of a low-pass filter is substrate (0.5LH0.5L) Air, where L and H represent low and high refractive index materials with optical of a quarter of the designed wavelength, respectively. A commercial software named OptiLayer was used to design the filters. The suitable blue filter was obtained with a structure of substrate 1.8(0.5LH 0.5L) (0.5LH 0.5L) (0.5LH 0.5L) 15 (0.5L H 0.5L) 15 Air. To optimize the optical property further, the concept of equivalent index in symmetrical thin film was used to reduce the passband ripple. Using this method, a small ripple in the passband with good optical characteristics has been obtained. The detailed parameters of the optimized multilayered structure are listed in Table 1. The blue filter is composed of 120 with total of mm. 56 are nonperiodic, and the rest are periodic. The calculated transmission spectrum of the designed blue filter and the emission spectrum of a phosphor-based white LED are shown in the blue and black lines of Fig. 2, respectively. The results show that the cutoff wavelength is precisely located at 485 nm as expected, and the stopband is wide ranging from 485 to 1050 nm. The cut-on wavelength and cutoff wavelengths are located at 484 nm and 492 nm, respectively, with a sharp transition. The average transmittance of the passband is higher than 99 percent and that of the stopband lower than 0.1 percent. Its performance is excellent for a VLC system. The designed blue filter was deposited on K9 glass substrates by electron beam evaporation. The of each layer was precisely controlled by both the quartz crystal monitoring and optical monitoring approaches. The measured transmission spectrum of the fabricated wide stopband blue filter is shown in the green line of 62 IEEE Wireless Communications April 2015

3 No. of No. of No. of No. of 1 Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Ta 2 O Ta 2 O Ta 2 O Ta 2 O SiO SiO SiO SiO Table 1. The detailed parameters of the optimized blue filter. IEEE Wireless Communications April

4 Transmittance (%) Fig. 2. The cutoff wavelength is precisely located at 485 nm as expected, and the stopband widely ranges from 500 to 1050 nm. The cut-on and cutoff wavelengths are located at 484 nm and 492 nm, respectively, which agree with the calculated spectrum very well. The average transmittance of pass band is higher than 97.5 percent while the stopband is lower than 1 percent, a little worse than the designed ones but better than those of the common blue filter. The measured transmission spectrum of a common blue filter (Abrisa Technologies, The Standard Additive Blue color coating) is also presented in the purple line of Fig. 2. The transmission of the signal band is slightly lower than our wide stopband Figure 2. Calculated and measured transmission spectra of the blue filter with wide stopband together with the measured transmission spectrum of a common blue filter, as well as the emission spectrum of a phosphorbased white LED for reference. Normalized intensity (a.u.) Blue filter with wide stopband-designed Blue filter with wide stopband-measured Normalized intensity (a.u.) Phosphor-based white LED Common blue filter Figure 3. The measured spectra of a phosphor-based white LED transmitted through our wide stopband blue filter and common blue filter, together with the normalized emission spectrum for reference Phosphor-based white LED Filtered spectrum by wide stopband blue filter Filtered spectrum by common blue filter Normalized intensity (a.u.) blue filter. Its transmission between 750 and 1050 nm is very high, and the cutoff edge is a bit away from 485 nm compared to our stopband blue filter. Their influence on the performance of a VLC system is discussed below. FILTERED SPECTRUM BY A BLUE FILTER The measured spectra of a phosphor-based white LED transmitted through our wide stopband blue filter and a common blue filter, together with the normalized emission spectrum itself for reference, are shown in Fig. 3. The signal transmitted from our blue filter is slightly higher than that of common ones due to their transmission difference in the signal band, as shown in Fig. 2. Our blue filter can transmit the signal totally while restraining the fluorescence component exactly as shown in the blue solid line of Fig. 3. An obvious difference exists in the range of 485 and 500 nm. The transmission of the common blue filter is much higher than ours, as shown in the inset of Fig. 3, which means that large leakage of the fluorescence component through the common blue filter has just resulted from a slight red shift of the cutoff wavelength mentioned above. It is worth noting that another weak difference between our blue filter and common ones is that there is a very small peak near 770 nm when transmitted through a common blue filter. This is because no stopband covers a wavelength longer than 750 nm, which is disastrous when used outdoors or in ambient light with wavelength longer than 750 nm, where the Si-based receiver will respond much stronger. In contrast, our blue filter can ensure the validity of a VLC system in any environment by restraining all light outside of the signal band. All of these advantages of our wide stopband blue filter will translate into performance improvement for VLC systems, as discussed in detail in the next sections. IMPROVEMENT OF BANDWIDTH BY A BLUE FILTER Our prepared wide stopband blue filter together with a common blue filter were employed at the downlink of the same VLC system in order to verify the function of the blue filter. The frequency responses of a white LED without filter, a white LED with a common blue filter and with our blue filter were measured and compared, as shown in Fig. 4. The results show that the bandwidth can be improved dramatically by both blue filters, due to the elimination of the slow phosphorescent component. What is more, our blue filter can improve the bandwidth better than the common blue filter. This is attributed to the more accurate cutoff wavelength and higher transmittance of the passband in blue light. The loss of the blue light signal is lower, and the slow phosphorescent component is filtered more completely. It can be seen that the higher frequency is fast fading, since equalization at the frequency domain was not applied. It should be noted that the white LED employed in our VLC system is not designed for 64 IEEE Wireless Communications April 2015

5 high-speed operation. It is very large in area, thus leading to high equivalent capacitance compared with devices used for high-speed communications [2]. However, the function of the blue filter and advantages of our blue filter are suitable for all kinds of white-led-based VLC systems if one designs the passband and cutoff edge according to the intended white LED. REDUCTION OF THE BER BY A BLUE FILTER The BER performance of a VLC system without a blue filter, with a common blue filter, and with our blue filter was compared and discussed by comparing the received data and transmitted data. The orthogonal frequency-division multiplexing (OFDM) modulation was employed to increase the data rate. At the transmitter, the input binary sequences are modulated using quadrature amplitude modulation (QAM) format, and then passed to the OFDM encoder. The electrical QAM- OFDM signals and DC-bias voltage are combined via bias tee, and applied to the LEDs serving as the transmitter [10]. The modulation format of 32- QAM was used in the experiment. First, the BER was measured vs. different data rates. During the measurements, the data rates were modulated from 200 to 350 Mb/s, while the distance between the white LED and receiver was fixed at 40 cm. The results are shown in Fig. 5a, indicating that the BER can be reduced dramatically by both of the blue filters but our filter shows a better performance. As for our blue filter, the BER is reduced by over two orders of magnitude compared with no filter under the same bandwidth. It shows that the BER performance will be degrade as the increase of the bandwidth. The BER can be below the pre-forward error correction (FEC) limit of when the bandwidth is less than 325 Mb/s with our blue filter but only 295 Mb/s for the common blue filter. Normalized response (db) Frequency (MHz) Figure 4. Normalized frequency response of a white LED without a filter, with a common blue filter, and with our blue filter. The BER was also measured vs. different distances, as shown in Fig. 5b. The distance between the white LED and receiver was changed from 30 to 60 cm, while the bandwidth was fixed at 250 Mb/s. The results show that the BER can be reduced dramatically by both of the blue filters, and our filter still shows better performance. It shows that the BER performance will degrade with increased distance. Compared to the common blue filter, the BER is reduced by nearly one order of magnitude with our blue filter under the same distance. The BER is below the pre-fec limit of after 50 cm freespace delivery with our blue filter, while it is 46 cm for the common blue filter. White LED without filter White LED with common filter White LED with our wide stopband filter 10-1 (a) 10-1 (b) 10-2 BER@3.8x BER@3.8x10-3 BER 10-3 BER Data rate (Mb/s) No filter Commercial blue filter No filter Commercial blue filter Distance (cm) Figure 5. Measured BERs of downlink with no filter, common blue filter, and our filter vs. a) data rata; b) distance. IEEE Wireless Communications April

6 100 Ocean Halogen light source HL-2000 Common blue filter No filter Common blue filter Transmittance (%) / intensity (a.u.) BER BER@3.8x10-3 LED Detector Halogen light Blue filter (a) Power of ambient light at 635 nm (mw) (b) 5 Figure 6. a) The emission spectrum of the Ocean halogen light source (used as ambient light), together with the measured transmission spectra of our wide stopband blue filter and a common blue filter for reference; b) measured BERs of downlink with no filter, common blue filter, and our filter vs. the power of ambient light at 635 nm (the light power of LED at 635 nm is 5.6 mw). The inset is the setup of the experiment; the blue filter was inserted in the front of the detector. The effect of the ambient light on the BER was also investigated to further demonstrate the functions and advantages of our wide stopband blue filter. A halogen light source (Ocean HL- 2000) was used as the ambient light, since its emission spectrum shows a broad band in the visible and near-infrared range, covering most of the response waveband of silicon PIN or APD photodetectors. The emission spectrum of a halogen light source together with the measured transmission spectra of our wide stopband blue filter and common blue filter are shown in Fig. 6a. It can be seen that the transmission of the common blue filter between 750 and 1050 nm is very high, which will leak the ambient light into the photodetector and result in increased BER. In this experiment, the ambient light was obliquely incidenced to avoid blocking the signal of LED, the data rate was fixed at 250 Mb/s, and the distance between the white LED and receiver was fixed at 40 cm. An optical power meter (Thorlabs PM100D) was used to measure the power of light. The power of ambient light at 635 nm was adjusted from 0 to 5 mw, while the light power of LED at 635 nm stays at 5.6 mw. Measured BERs of downlink with no filter, common blue filter, and our filter vs. the power of ambient light at 635 nm were shown in Fig. 6b. The results show that when no filter was used, the BER increased slowly, since the normal incidence LED light is much stronger than the oblique incident ambient light when the wider and stronger white fluorescence was not filtered out. When using the common blue filter, the BER increased rapidly with the power of ambient light and increased nearly 20 times from to 2 when the power of ambient light increased from 0 to 5 mw. That is because the leaked ambient light reduced the SNR. In contrast, when using our wide stopband blue filter, the BER shows a tiny change from to , and can always stay below the pre- FEC limit of The better performance of our blue filter in reducing BER is due to the wider stopband, which can effectively reject not only the phosphorescent component but also the ambient light, so a higher SNR can be obtained. CONCLUSIONS In this article, we have proposed and experimentally demonstrated a high-performance blue filter, which was employed to improve the performance of a white-led-based VLC system. Compared to a common commercial blue filter, our blue filter shows wider stopband, higher transmission passband, and more accurate cutoff wavelength. Due to these advantages, our blue filter can effectively reject not only the phosphorescent component but also the ambient light, so it can improve the bandwidth and reduce the BER better than a common blue filter. The results show that the BER is reduced by over two orders of magnitude compared to no filter under the same data rate and can be below the pre-forward-error-correction limit of when the data rate is less than 325 Mb/s with our blue filter. The BER with our blue filter is also reduced by nearly one order of magnitude with our blue filter under the same distance as the common blue filter. The BER is below the pre-fec limit of after 50 cm free-space delivery with our blue filter. When the VLC system is used with ambient light, our wide stopband blue filter shows a tiny change in the BER and 66 IEEE Wireless Communications April 2015

7 can work very well, unlike common blue filters, which decay dramatically. ACKNOWLEDGMENTS This work was partially supported by the Shanghai Science and Technology Foundations (12nm , 13JC ) and the National Natural Science Foundation of China ( ), and Youth Innovation Promotion Association CAS. The authors thank Prof. Nan Chi, Mr. Jianyang Shi, and Xingxing Huang of Fudan University for helping us measure the performance of a VLC system with and without our blue filter. REFERENCES [1] T. Komine and M. Nakagawa, Fundamental Analysis for Visible-Light Communication System Using LED Lightings, IEEE Trans. Consumer Electronics, vol. 50, no. 1, Feb. 2004, pp [2] D. C. O Brien et al., Visible Light Communications: Challenges and Possibilities, PIMRC, 2008, pp [3] G. Cossu et al., 3.4 Gbit/s Visible Optical Wireless Transmission Based on RGB LED, Opt. Express, vol. 20, no. 26, 2012, pp. B501 B506. [4] Y. Wang et al., Enhanced Performance of Visible Light Communication Employing 512-QAM N-SC-FDE and DD-LMS, Opt. Express, vol. 22, 2014, pp. 15, [5] D. Tsonev et al., A 3-Gb/s Single-LED OFDM-Based Wireless VLC Link Using a Gallium Nitride mled, IEEE Photonic Tech. Lett., vol. 26, no. 7, Apr. 1, [6] D. C. O Brien and M. Katz, Short-Range Optical Wireless Communications, Wireless World Research Forum, Oslo, Norway, 2004, pp [7] M. Z. Afgani et al., Visible Light Communication Using OFDM, Proc. 2nd Int l. Conf. TRIDENTCOM, 2006, p [8] Liane Grobe et al., High-Speed Visible Light Communication Systems, IEEE Commun. Mag., vol. 5, no. 12, 2013, pp [9] Y. U. Lee and M. Kavehrad, Two Hybrid Positioning System Design Techniques with Lighting LEDs and Ad- Hoc Wireless Network, IEEE Trans. Consumer Electronics, 2012, vol. 58, no. 4, pp [10] Y. Wang et al., Demonstration of 575-Mb/s Downlink and 225-Mb/s Uplink Bi-Directional SCM-WDM Visible Light Communication Using RGB LED and Phosphor-based LED, Opt. Express, vol. 21, no. 1, 2013, pp [11] J. Grubor et al., Wireless High-Speed Data Transmission with Phosphorescent White-Light LEDs, Proc. 33rd Euro. Conf. and Exhibition Opt. Commun., Sept , 2007, pp [12] A. M. Khalid et al., 1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation, IEEE Photonics J., vol. 4, no. 5, Oct [13] H. Le Minh et al., 80 Mbits Visible Light Communications Using Pre-Equalized White LED, ECOC 08, Sept. 2008, Brussels, Belgium. [14] H. Le Minh et al., 100-Mb/s NRZ Visible Light Communications Using a Postequalized White LED, IEEE Photonics Tech. Lett., vol. 21, 2009, pp [15] J. Grubor et al., Broadband Information Broadcasting Using LED-Based Interior Lighting, J. Lightwave Tech., vol. 26, no. 24, Dec. 15, BIOGRAPHIES SHAO-WEI WANG (wangshw@mail.sitp.ac.cn) received his Ph.D. (2003) degree in microelectronics and solid state electronics from Shanghai Institute of Technical Physics, Chinese Academy of Sciences, China. He is a professor of the Institute and works at the National Laboratory for Infrared Physics. His research interests include artificial photonic structures and devices, such as integrated-cavities structure and related applications, solar selective absorbers, micro and nano optical polarization structures, and optical thin films. He has published more than 40 papers and one U.S. patent. He received the Lu Jiaxi Young Talent award (2009), the Rao Yutai Basic Optical award (2007), the National Natural Science award (2014, fourth principal achiever), the National Technological Invention Award (2011, fifth principal achiever), the Shanghai Technological Invention Award (2010, seventh principal achiever), and the Shanghai Natural Science award (2007, fifth principal achiever), among others. The biographies for the other authors were not submitted. The better performance of our blue filter in reduction of BER is due to the wider stopband, that can effectively reject not only the phosphorescent component but also the ambient light, so that a higher SNR can be obtained. IEEE Wireless Communications April