Receiver characteristics evaluation of over-gigabit indoor visible light transmission system with QPM device

Transcription

1 Receiver characteristics evaluation of over-gigabit indoor visible light transmission system with QPM device Takahiro Kubo 1a), Ken-Ichi Suzuki 1, Naoto Yoshimoto 1, Takeshi Umeki 2, Masaki Asobe 2, Demitar R. Kolev 3, and Mitsuji Matsumoto 3 1 NTT Access Network Service Systems Laboratories, NTT Corporation, 1 1, Hikarinooka, Yokosuka, Kanagawa , Japan 2 NTT Photonics Laboratories, NTT Corporation, 3 1 Morinosato Wakamiya, Atsugi, Kanagawa , Japan 3 Graduate School of Global Information and Telecommunication Studies, Waseda University, Nishi-Waseda, Shinjuku, Tokyo , Japan Present affiliation: Tokai University a) kubo.takahiro@lab.ntt.co.jp Abstract: This paper outlines an experimental investigation of an over-gigabit indoor visible light transmission system employing a quasiphase-matching (QPM) device. To evaluate the effect of collecting lenses with different diameters, we measured the relationship between received optical power and bit error rate by using collecting lenses with diameters of 5, 8, 10, and 15 mm. The experimental results confirmed that the relationship between the received optical power and the receiving area of the collecting lens was consistent with the analytical solution. We confirmed that a larger collecting lens is effective against the degradation caused by increased transmission distance, even if the optical output power of the wavelength converter is constant. Keywords: visible light communication, FSO, wavelength conversion Classification: Transmission Systems and Transmission Equipment for Communications References [1] G. Dede, T. Kamalakis, and D. Varoutas, Evaluation of Optical Wireless Technologies in Home Networking: An Analytical Hierarchy Process Approach, J. Opt. Commun. Netw., vol. 3, pp , [2] Y. Arimoto, Multi-Gigabit Free-Space Laser Communications Using Compact Optical Terminal with Bidirectional Beacon Tracking, IEEE ICC 2011, pp. 1 5,

2 [3] W. O. Popoola, Z. Ghassemlooy, H. Haas, E. Leitgeb, and V. Ahmadi, Error Performance of Terrestrial Free Space Optical Links with Subcarrier Time Diversity, IET Communications, vol. 6, pp , [4] Z. Wu, J. Chau, and T. Little, Modeling and Designing of a New Indoor Free Space Visible Light Communication System, NOC 2011, BAT-13, pp , [5] T. Komine and M. Nakagawa, Fundamental Analysis for Visible-Light Communication System using LED Lights, IEEE Trans. Consume. Electron., vol. 50, pp , [6] F. Wu, C. Lin, C. Wei, C. Chen, Z. Chen, and H. Huang, 3.22-Gb/s WDM Visible Light Communication of a Single RGB LED Employing Carrier-Less Amplitude and Phase Modulation, OFC/NFOEC 2013, OTh1G.4, [7] Y. Wang, Y. Shao, H. Shang, X. Lu, Y. Wang, J. Yu, and N. Chi, 875- Mb/s Asynchronous Bi-Directional 64QAM-OFDM SCM-WDM Transmission over RGB-LED-Based Visible Light Communication System, OFC/OFOEC 2013, OTh1G.3, [8] T. Kubo, T. Umeki, T. Kanai, H. Suzuki, H. Hadama, and M. Asobe, A High-Speed Visible Light Indoor Network Employing a Short Pulse Modulation and a QPM-LN Module, OFC/NFOEC 2011, JWA73, [9] T. Kubo, T. Yamada, K. Suzuki, N. Yoshimoto, T. Umeki, M. Asobe, D. R. Kolev, and M. Matsumoto, 1.25-Gb/s 2-m Indoor Visible Light Transmission Employing Wavelength Conversion with Quasi Phase Matching Device, IWOW 2012, pp. 1 3, [10] X. Fu, T. Wang, Q. Guo, Z. Chen, and F. Pang, Analyzing the Receiving Power of Rectilinear Mobile/Free-Space Optical Links by Considering the Effect of Laser Beam Divergence and Deflection Angles, Opt. Engineering, vol. 51, , [11] M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, Quasi-Phase- Matched Second Harmonic Generation: Tuning and Tolerances, IEEE J. Quantum Electron., vol. 28, pp , Introduction The optical access network has grown rapidly to meet the increasing demand for content-rich services such as video streaming, social networking, and web storage. Along with the popularization of the optical access network, highspeed indoor communication networks have been studied to make the best use of the optical access network [1]. Radio frequency (RF) communication is a promising technology that is extremely useful for indoor communication networks. However, RF signals have certain drawbacks including the available frequency range, signal interference, and security against signal interception. The alternative to an indoor communication network is free space optics (FSO) communication [2], which is a complementary technology to RF technology [3]. In addition, FSO provides directional field-of-view (FOV) communication. Therefore, FSO is effective in preventing signal interception outside a room. FSO also has less impact on the human body compared with RF signals, which emit radiation equally in all directions. In FSO communication, the infrared band (e.g nm) is a commonly used wavelength 403

3 range because of its consistency with optical fiber communication, which uses infrared optical signals. Also, the propagation loss of infrared optical signals in outdoor free space is relatively small. However, unlike research on outdoor FSO communications, indoor FSO studies employ other wavelengths (e.g. the visible band) [4]. In particular, visible light communication (VLC) with a light emitting diode (LED) is a promising technique for indoor FSO communications [5]. The VLC technique provides communication with an LED, and 3.22 Gb/s transmission experiments have been reported [6]. However, the VLC transmission speed depends on the LED modulation rate of the LED. Therefore, an over-gigabit VLC transmission requires complex transmission techniques such as wavelength division multiplexing (WDM) [6], and orthogonal frequency division multiplexing (OFDM) [7]. We have already proposed 1.25-Gb/s indoor visible light transmission employing wavelength conversion with a QPM device [8, 9]. This technique realizes broadband VLC in which an over-gigabit modulated infrared signal is converted to a visible light signal by using a QPM-LN device. Our previous work employed a collecting lens to enhance the receiver sensitivity. Although we confirmed error-free operation for a 2 m free-space visible light transmission, a more detailed study is necessary as regards the size of the collecting lens. Fu et al. analyzed the effective receiving area of the receiving lens for FSO links [10]. In this paper, we report experimental results for an indoor visible light transmission system to confirm the effects of four different collecting lenses on receiving power and sensitivity. The experimental results show that a l5 mm diameter lens is the most effective for collecting the received optical power without any sensitivity degradation in the proposed system. The experimental results also show that a larger collecting lens is effective against the degradation caused by increased transmission distance, even if the optical output power of the wavelength (λ)-converter is constant. 2 Proposed indoor visible light transmission system Figure 1 (a) shows a schematic representation of our proposed indoor visible light transmission system. In this system, a communication band infrared signal, which is passed through an optical fiber installed in a house, is converted into a visible light signal with a λ-converter. The generated visible Fig. 1. (a) Schematic representation of visible light indoor transmission system, (b) composition of λ- converter. 404

4 light signal transmits in a free space of about several meters, and is used to feed electronic devices such as televisions and mobile personal computers to realize a high-speed downstream signal. We can easily recognize a beam spot of visible light. Therefore, users can hold the receiver of their own device against the visible light to obtain high-quality reception of a visible light link, even if the spot of visible light is too narrow to cover the entire room. And a collecting lens at a receiver (Rx) improves the received optical power. Unlike conventional VLC, our proposed indoor transmission system is not equipped with an illumination function. However, the isolation of the illumination function realizes communication independent of the state of illumination. 2.1 Wavelength conversion In the proposed transmission system, the wavelength conversion from the infrared band to the visible band is accomplished by means of second harmonic generation (SHG) with a QPM device. In SHG, a fundamental wave with frequency ω 1 interacts with the second-order nonlinear susceptibility of a material to produce a polarization wave at the second-harmonic frequency ω 2 =2ω 1 [11]. QPM is a method for enabling the continuous growth of the second harmonic (SH) wave along the device. It involves the repeated inversion of the relative phase between the forced and free waves after an odd number of coherence lengths. Thus the phase is reset periodically so that on average, the proper phase relationship is maintained for the growth of the SH. In a fiber optical communication system, nm-band light is used as the carrier wave. Fortunately, this means that we can obtain nm-band visible light from the optical communication-band wave with SHG. One advantage of visible light generation with wavelength conversion is availability of an optical communication-band device with which to construct the visible light indoor transmission system. This reduces the cost of the proposed system. Secondly, SHG does not require a pump light, which is common with non-linear optical effects. Therefore, the wavelength conversion is performed without an additional laser source. This means that the λ-converter needs no power supply for the wavelength conversion as shown in Fig. 1 (b). However, we assume a downstream application for the proposed indoor visible light transmission. If the indoor visible light transmission is bi-directional, we need an additional receiver and laser source to receive the visible light at the λ-converter. 2.2 Receiving power model We describe a basic receiving power model consisting of FSO links. According to Ref. [10], if the optical spot diameter on the receiving plane is sufficiently smaller than the diameter of the collecting lens, the receiving power P re1 is given by P re1 = P tr η tr η re (1 exp[ 2]), (1) where P tr is the total transmitting power, and η tr and η re are invariable in FSO links. 405

5 On the other hand, if the optical spot diameter on the receiving plane is larger than the diameter of the collecting lens, the effective receiving area of the optical power can be equal to the area of the collecting lens. Thus the receiving power P re2 is given by [ ]) P re2 = P tr η tr η re (1 exp D2, (2) 2ω 2 L where D is the diameter of the collecting lens, and ω L is the optical spot radius on the receiving plane. By using this model, we can design the size of the collecting lens for the error free operation (BER < 10 9 )ofourproposed over-gigabit transmission system. 3 Experimental setup Figure 2 (a) shows the experimental setup we used to test our proposed technique. A 1300 nm continuous wave (CW) light was generated by a tunable laser source (TLS) and modulated with a 1.25 Gb/s pseudo random bit sequence (PRBS) in an LN-Mach-Zehnder modulator (LN-MZM). The modulation format was non-return to zero (NRZ). We adjusted the power of the modulated 1300 nm-band signals with an attenuator (ATT). The QPM-LN module converted the 1300 nm-band signal to 650 nm SH visible signals at a λ-converter. To keep the conversion efficiency constant, the temperature of the QPM-LN module was kept at 27 C with a temperature controller (TEC). The 650 nm visible light signals were transmitted via free space over 1 m and received with a silicon avalanche photo diode (Si-APD) at the Rx. To focus the 650 nm visible light signals, we used a collecting lens (BK7, φ 5, 8, 10, 15 mm), which we installed in front of the Si-APD. The Si-APD converted Fig. 2. (a) Experimental setup, (b) photograph of 650 nm visible light transmission system. 406

6 the signal from 650 nm optical signals to electrical signals. The electrical signals were then electrically amplified with a low noise amplifier (LNA) and filtered with a low pass filter (LPF) that had a 938 MHz cutoff frequency. Then the electrical signals were detected with a bit error rate tester (BERT) and a sampling oscilloscope (OSC). A photograph of the 650 nm visible light transmission system is shown in Fig. 2 (b). The experimental system was lit by the room lighting. We measured the difference between the optical received powers in the on and off states to detect the net optical received power of a 650 nm visible light signal. 4 Results and discussion Figure 3 (a) shows the relationship between the optical received power and the lens area. The solid circle shows the measured value of the optical received power for different lens areas (diameter 5, 8, 10, 15 mm). The dashed line shows an analytical solution of the optical received power based on Eq. (1). The dashed curved lines show three types of analytical solution for the optical received power based on Eq. (2), where we applied η tr and η re as From Fig. 3 (a), the optical spot radius at a distance of 1 m is approximately 7 mm. It is clear from Fig. 3 (a) that the relationship between the optical received power and the lens area is qualitatively subject to the analytical solution. Next, we measured the relationship between the bit error rate and the optical received power with different optical diameters (5, 8, 10, 15 mm) as shown in Fig. 3 (b). There was no significant difference between the other diameters (8, 10, 15 mm). This fact shows that the difference in lens size between 8 15 mm has no influence on the sensitivity of the proposed transmission system. The only exception was that the sensitivity with a diameter 5 mm was 2 db higher. A possible reason for this is that a sufficiently small lens removes excess noise. These results indicate that a larger collecting lens is effective against the degradation caused by increased transmission distance, even if the optical output power of the λ-converter is constant. Fig. 3. (a) Optical received power vs lens area, (b) bit error rate vs optical received power. 407

7 5 Conclusion In this paper, we measured the received optical power or BER of an indoor visible light transmission system with different collecting lenses (5, 8, 10, 15 mm) at the receiver. The experimental results show that the relationship between the optical received power and the lens area is qualitatively subjected to the analytical solution. The experimental results also show that a larger collecting lens is effective against the degradation caused by increased transmission distance, even if the optical output power of the λ-converter is constant. 408