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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 3 — Feb. 10, 2014
  • pp: 2830–2838
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Visible light communications: real time 10 Mb/s link with a low bandwidth polymer light-emitting diode

Paul Anthony Haigh, Francesco Bausi, Zabih Ghassemlooy, Ioannis Papakonstantinou, Hoa Le Minh, Charlotte Fléchon, and Franco Cacialli  »View Author Affiliations


Optics Express, Vol. 22, Issue 3, pp. 2830-2838 (2014)
http://dx.doi.org/10.1364/OE.22.002830


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Abstract

This paper presents new experimental results on a polymer light-emitting diode based visible light communications system. For the first time we demonstrate a 10 Mb/s link based on the on-off keying data format with real time equalization on a field programmable gate array. The 10 Mb/s transmission speed is available at a bit error rate less than 4.6 × 10−3, which is the limit for forward error correction. At a BER of 10−6 a transmission speed of 7 Mb/s is readily achievable.

© 2014 Optical Society of America

1. Introduction

Organic and polymer light-emitting diodes (PLEDs) have been gaining substantial attention in recent years due to their outstanding potential for future lighting and display applications [1

1. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]

]. Advantages of PLEDS include low-cost solvent-based processing which in turn means large area devices are palpable with relative ease in comparison to inorganic LEDs.

2. Production and characterization of the polymer light-emitting diodes under test

A schematic of the PLEDs used in this work is illustrated in Fig. 1
Fig. 1 A schematic of the PLED used in this work. The devices are composed of a stack of several thin polymeric layers encapsulated between two planar electrodes. The anode is a transparent conductive layer of ITO deposited on a glass substrate via a sputtering process. A hole injection layer made of a conjugated polymer poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (the mix is referred to as PEDOT:PSS) is in contact with the anode. On top of it, the conjugated polymer poly[(9’9’-dioctylfluorene-alt-N-(4-butylphenyl)diphenylamine] (TFB) acts as electron-blocking/hole-transporting interlayer [46]. The emissive polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) is deposited on top of the TFB and is in direct contact with the metallic calcium cathode which is in turn covered by a layer of aluminum as a protection against oxidation.
. PLEDs were prepared starting with a transparent anode comprised of a thin layer (~120 nm) of indium tin oxide (ITO) deposited via a sputtering process on a glass substrate. The ITO surface was cleaned in an acetone and isopropanol sonication bath followed by an oxygen plasma treatment [7

7. N. Johansson, F. Cacialli, K. Z. Xing, G. Beamson, D. T. Clark, R. H. Friend, and W. R. Salaneck, “A study of the ITO-on-PPV interface using photoelectron spectroscopy,” Synth. Met. 92(3), 207–211 (1998). [CrossRef]

,8

8. T. M. Brown and F. Cacialli, “Contact Optimisation in Polymer LEDs,” J. Polym. Sci. Pol. Phys. 41, 2649–2664 (2003). [CrossRef]

]. Immediately after the oxygen plasma treatment, we spin coated (4,500 rpm for 60 s plus 5,000 rpm for 10 s in air) a dispersion 2.8% w/w in H2O of the polymer PEDOT:PSS (Sigma-Aldrich) to obtain a highly conductive polymeric film approximately 80 nm thick. The sample was then annealed at 140 °C for 600 s in a nitrogen atmosphere. A solution 2% w/w in p-xylene of the polymer TFB (American Dye Source) with a molecular weight Mw = 68,000 is then spin coated (2,500 rpm for 60 s under nitrogen atmosphere) on the sample followed by annealing (140 °C for ~1 hour) and slow cooling to increase the crystallinity of the TFB layer. The amorphous portion of TFB is then removed via spin rinsing (1,000 rpm for 30 s and 4,500 rpm for 10 s) with p-xylene in which the solvent was added drop-by-drop while spinning.

To deposit the active layer we spin coated (1,800 rpm for 60 s) a solution of poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with a Mn of ~23,000 g/mol (Sigma-Aldrich) 1% w/w in toluene. A metallic calcium cathode 30 nm thick was evaporated onto the active layer and subsequently covered with a 150 nm layer of aluminum as a protection against oxidation. For the evaporation of the cathode we used a mask to produce eight different pixels, see Fig. 1. The active area of each pixel is of about 3.5 mm2 and it is given by the intersection between the ITO stripe and the calcium layer. The corresponding energy levels are shown in Fig. 2
Fig. 2 The energy-level diagram, relative to vacuum, of the isolated materials used in the fabrication of the PLED. HOMO and LUMO stand for ‘highest occupied molecular orbital’ and ‘lowest unoccupied molecular orbital’ respectively. They indicate the two energy levels of the molecule that are responsible for its semiconductor behavior in the same way as valence and conduction bands in inorganic semiconductors. The HOMO and LUMO values for TFB and MDMO-PPV are measured by a combination of cyclic voltammetry and optical absorption [9, 10]. The Fermi levels of the electrodes are also reported [8].
.

The normalized optical emission intensity for each polymer were measured using an Andor spectrometer (Shamrock 163 spectrograph with an Andor Newton EMCCD camera) and are shown in Fig. 3(a)
Fig. 3 The PLED characteristics: (a) the normalized optical spectra and the responsivity of the ThorLabs PDA36A PD, and (b) the JLV relationship, with VON at ~2 V; note the semi-logarithmic axes.
. The PLEDs have a peak wavelength of 630 nm, with a pronounced shoulder at ~595 nm. The voltage-current and voltage-optical power (JLV) relationships were measured using a Keithley 2400 voltage source, which supplied and measured the drive voltage and current. A Keithley 2000 digital multi-meter is used to measure the voltage from the photodetector, which was converted to the received optical power in MATLAB using the responsivity curve of the silicon photodetector. The JLV response was measured from 0 to 8 V as shown in Fig. 3(b). The operating voltage during the transmission tests was set at 8 VDC as this value is well above the turn-on for luminescence, therefore offering a milder non-linearity and less distortion to the transmission signal. Although at the limit of the range shown in Fig. 3(b), we did not observe any significant degradation in the device operation during our experiments.

The equalization of the Fermi levels of the electrodes generates a built-in voltage (VBI) across the semiconductor layers inside the device. When the voltage supplied to the device is above VBI a bipolar injection into the emitting polymer occurs and electroluminescence ensues. The device used in this work shows a peak external quantum efficiency of 1.9% when driving the LED with 72 mA/cm2 current density and an applied voltage of 7.2 V as shown in Fig. 4(a)
Fig. 4 The PLED: (a) current efficiency (cd/A) and external quantum efficiency (%) as a function of the current density and (b) the device frequency response (red) and the noise profile (black).
. Finally, the device bandwidth was measured by transmission of a frequency swept sinusoid (20 kHz – 1MHz) under the following operating conditions: 8 VDC, 4 VAC. At the receiver an Agilent N9010A electrical spectrum analyzer measured the magnitude response of the received sinusoid over the given frequency range. Subsequently the light was switched off and a noise measurement was made over the same range. The bandwidth and noise measurements are illustrated in Fig. 4(b) along with the 270 kHz 3-dB point.

3. Experimental Test Setup and Least Mean Squares Equalizer

The schematic block diagram of the experimental test setup is illustrated in Fig. 5
Fig. 5 Block diagram of the experimental test setup.
.

No focusing optics were used in the measurements and the distance between PLED and a ThorLabs PDA36A PIN photodetector (with 5.5 MHz bandwidth with an inbuilt transimpedance gain of 10 dB) was ~0.05 m. This is a very short distance in comparison to a full room scale and the reason is because the experiment was performed using singular pixels (~3.5 mm2) where the brightness was relatively low. To increase the transmission distance (> 1 m) for future applications, the solution is to simply scale up the amount of PLEDs until the minimum desired optical power is collected at the receiver. There are several sources of noise n(t) including ambient, thermal and shot noise. To minimize the ambient noise, the experiment was conducted in a pitch black laboratory. The thermal and shot noise sources are assumed to be additive white Gaussian noise (AWGN) as stated in [11

11. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85(2), 265–298 (1997). [CrossRef]

,12

12. Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling (CRC Press INC, 2012).

].

The received signal y(t) = Gℜ[h(t) * s(t) + n(t)] is captured and sampled by a Tektronix MDO4104-6 real time oscilloscope with the output given by:
yi=G[yih0+j=jiyjhij+ni]
(2)
where ℜ is the photodiode responsivity, G is the 10 dB transimpedance gain, hi is the sampled channel impulse response, i is the current sampling instance, j represents the contributions of the inter-symbol interference (ISI) and ni is a zero mean Gaussian random variable with variance N0/2 representing the noise at each sample. The data yi is acquired by a PC via a LabVIEW script where synchronization with the transmitted data (clock synchronisation in Fig. 5) is carried out before being passed through a low pass filter (LPF) to remove the high frequency noise components. Both synchronisation and LPF are not performed in the FPGA domain; this is to ensure that any errors introduced in the system are due to the equalizer in this first demonstration of such a link.

3. Results

Thus it is necessary to introduce a BER limit for forward error correction (FEC) at 4.6 × 10−3, at the cost of 7% increase in the overhead [14

14. F. Chang, K. Onohara, and T. Mizuochi, “Forward error correction for 100 G transport networks,” IEEE Commun. Mag. 48(3), S48–S55 (2010). [CrossRef]

], as is a common practice in high speed VLCs [3

3. G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, and E. Ciaramella, “3.4 Gbit/s visible optical wireless transmission based on RGB LED,” Opt. Express 20(26), B501–B506 (2012). [CrossRef] [PubMed]

, 15

15. J. Vucic, C. Kottke, S. Nerreter, K. Habel, A. Buttner, K. D. Langer, and J. W. Walewski, “230 Mbit/s via a wireless visible-light link based on OOK modulation of phosphorescent white LEDs,” in Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), 2010), 1–3.

]. The limit is indicated by the dashed line in Fig. 7 and it is clear that data rates can be increased to 10 Mb/s using N = 20 and 25 tapped weight coefficients. The performance of an equalizer with N = {5; 7; 10; 15} taps show BER of ~0.006 – 0.007 and thus slightly exceeding the FEC limit, while N = 3 taps fails to converge to the target value. The predicted SNR at a frequency of 10 MHz is < 5 dB, where the performance is decending into the noise floor, which is benefiting from the long training length with numerous tapped weight coefficients in order to compose an accurate account of the channel. Having a FEC overhead of 7% means that the line rate of 10 Mb/s would be reduced to 9.3 Mb/s.

Overall this demonstration of a real time PLED-VLC system operating at an overall data rate of 10 Mb/s is the first real landmark in high speed organic based VLC systems. This work represents three separate increases in the current state-of-the-art data rates over the 2.7 Mb/s reported in [2

2. P. A. Haigh, Z. Ghassemlooy, I. Papakonstantinou, and H. Le Minh, “2.7 Mb/s With a 93-kHz White Organic Light Emitting Diode and Real Time ANN Equalizer,” IEEE Photon. Technol. Lett. 25(17), 1687–1690 (2013). [CrossRef]

]. Firstly by using a custom produced PLED with ~3 times larger bandwidth a data rate of 3 Mb/s was possible with simple threshold detection. Secondly using the FPGA based LMS equalizer at a BER target of 10−6 a data rate of 7 Mb/s can be readily achieved. Finally by introducing the FEC BER limit of 4.6 × 10−3 an overall transmission speed of 10 Mb/s could be achieved. Removing the 7% redundancy gives an overall information rate of 9.3 Mb/s, or an increase over [2

2. P. A. Haigh, Z. Ghassemlooy, I. Papakonstantinou, and H. Le Minh, “2.7 Mb/s With a 93-kHz White Organic Light Emitting Diode and Real Time ANN Equalizer,” IEEE Photon. Technol. Lett. 25(17), 1687–1690 (2013). [CrossRef]

] by ~3.5 times whilst using a significantly less computationally complex equalizer.

7. Conclusion

Acknowledgments

The authors would like to acknowledge C. Soos of the European Organization for Nuclear Research (CERN), R. Bouziane and I. Darwazeh of University College London (UCL) for fruitful discussions on the subject of VHDL and best practices and providing the test and measurement equipment, respectively. This work was supported by the EU COST Action IC1101, the EPSRC and the EU FP7 Marie Curie ITN GENIUS - grant number PITN-CT-2010-264694.

References and links

1.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]

2.

P. A. Haigh, Z. Ghassemlooy, I. Papakonstantinou, and H. Le Minh, “2.7 Mb/s With a 93-kHz White Organic Light Emitting Diode and Real Time ANN Equalizer,” IEEE Photon. Technol. Lett. 25(17), 1687–1690 (2013). [CrossRef]

3.

G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, and E. Ciaramella, “3.4 Gbit/s visible optical wireless transmission based on RGB LED,” Opt. Express 20(26), B501–B506 (2012). [CrossRef] [PubMed]

4.

R.-Q. Png, P.-J. Chia, J.-C. Tang, B. Liu, S. Sivaramakrishnan, M. Zhou, S.-H. Khong, H. S. Chan, J. H. Burroughes, L.-L. Chua, R. H. Friend, and P. K. Ho, “High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains,” Nat. Mater. 9(2), 152–158 (2010). [CrossRef] [PubMed]

5.

G. Lazzerini, F. Di Stasio, C. Fléchon, D. Caruana, and F. Cacialli, “Low-temperature treatment of semiconducting interlayers for high-efficiency light-emitting diodes based on a green-emitting polyfluorene derivative,” Appl. Phys. Lett. 99(24), 243305 (2011). [CrossRef]

6.

J.-S. Kim, R. H. Friend, I. Grizzi, and J. H. Burroughes, “Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes,” Appl. Phys. Lett. 87, 023506 (2005).

7.

N. Johansson, F. Cacialli, K. Z. Xing, G. Beamson, D. T. Clark, R. H. Friend, and W. R. Salaneck, “A study of the ITO-on-PPV interface using photoelectron spectroscopy,” Synth. Met. 92(3), 207–211 (1998). [CrossRef]

8.

T. M. Brown and F. Cacialli, “Contact Optimisation in Polymer LEDs,” J. Polym. Sci. Pol. Phys. 41, 2649–2664 (2003). [CrossRef]

9.

O. Fenwick, S. Fusco, T. N. Baig, F. D. Stasio, T. T. Steckler, P. Henriksson, C. Fléchon, M. R. Andersson, and F. Cacialli, “Efficient red electroluminescence from diketopyrrolopyrrole copolymerised with a polyfluorene,” APL Materials 1(3), 032108 (2013). [CrossRef]

10.

B. W. D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov, and M. E. Thompson, “Relationship between the ionization and oxidation potentials of molecular organic semiconductors,” Org. Electron. 6(1), 11–20 (2005). [CrossRef]

11.

J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85(2), 265–298 (1997). [CrossRef]

12.

Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling (CRC Press INC, 2012).

13.

S. U. H. Qureshi, “Adaptive equalization,” Proc. IEEE 73(9), 1349–1387 (1985). [CrossRef]

14.

F. Chang, K. Onohara, and T. Mizuochi, “Forward error correction for 100 G transport networks,” IEEE Commun. Mag. 48(3), S48–S55 (2010). [CrossRef]

15.

J. Vucic, C. Kottke, S. Nerreter, K. Habel, A. Buttner, K. D. Langer, and J. W. Walewski, “230 Mbit/s via a wireless visible-light link based on OOK modulation of phosphorescent white LEDs,” in Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), 2010), 1–3.

OCIS Codes
(000.2700) General : General science
(250.3680) Optoelectronics : Light-emitting polymers
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Optical Communications

History
Original Manuscript: October 2, 2013
Revised Manuscript: November 8, 2013
Manuscript Accepted: November 9, 2013
Published: January 31, 2014

Citation
Paul Anthony Haigh, Francesco Bausi, Zabih Ghassemlooy, Ioannis Papakonstantinou, Hoa Le Minh, Charlotte Fléchon, and Franco Cacialli, "Visible light communications: real time 10 Mb/s link with a low bandwidth polymer light-emitting diode," Opt. Express 22, 2830-2838 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-3-2830


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References

  1. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]
  2. P. A. Haigh, Z. Ghassemlooy, I. Papakonstantinou, H. Le Minh, “2.7 Mb/s With a 93-kHz White Organic Light Emitting Diode and Real Time ANN Equalizer,” IEEE Photon. Technol. Lett. 25(17), 1687–1690 (2013). [CrossRef]
  3. G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, E. Ciaramella, “3.4 Gbit/s visible optical wireless transmission based on RGB LED,” Opt. Express 20(26), B501–B506 (2012). [CrossRef] [PubMed]
  4. R.-Q. Png, P.-J. Chia, J.-C. Tang, B. Liu, S. Sivaramakrishnan, M. Zhou, S.-H. Khong, H. S. Chan, J. H. Burroughes, L.-L. Chua, R. H. Friend, P. K. Ho, “High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains,” Nat. Mater. 9(2), 152–158 (2010). [CrossRef] [PubMed]
  5. G. Lazzerini, F. Di Stasio, C. Fléchon, D. Caruana, F. Cacialli, “Low-temperature treatment of semiconducting interlayers for high-efficiency light-emitting diodes based on a green-emitting polyfluorene derivative,” Appl. Phys. Lett. 99(24), 243305 (2011). [CrossRef]
  6. J.-S. Kim, R. H. Friend, I. Grizzi, J. H. Burroughes, “Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes,” Appl. Phys. Lett. 87, 023506 (2005).
  7. N. Johansson, F. Cacialli, K. Z. Xing, G. Beamson, D. T. Clark, R. H. Friend, W. R. Salaneck, “A study of the ITO-on-PPV interface using photoelectron spectroscopy,” Synth. Met. 92(3), 207–211 (1998). [CrossRef]
  8. T. M. Brown, F. Cacialli, “Contact Optimisation in Polymer LEDs,” J. Polym. Sci. Pol. Phys. 41, 2649–2664 (2003). [CrossRef]
  9. O. Fenwick, S. Fusco, T. N. Baig, F. D. Stasio, T. T. Steckler, P. Henriksson, C. Fléchon, M. R. Andersson, F. Cacialli, “Efficient red electroluminescence from diketopyrrolopyrrole copolymerised with a polyfluorene,” APL Materials 1(3), 032108 (2013). [CrossRef]
  10. B. W. D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov, M. E. Thompson, “Relationship between the ionization and oxidation potentials of molecular organic semiconductors,” Org. Electron. 6(1), 11–20 (2005). [CrossRef]
  11. J. M. Kahn, J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85(2), 265–298 (1997). [CrossRef]
  12. Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling (CRC Press INC, 2012).
  13. S. U. H. Qureshi, “Adaptive equalization,” Proc. IEEE 73(9), 1349–1387 (1985). [CrossRef]
  14. F. Chang, K. Onohara, T. Mizuochi, “Forward error correction for 100 G transport networks,” IEEE Commun. Mag. 48(3), S48–S55 (2010). [CrossRef]
  15. J. Vucic, C. Kottke, S. Nerreter, K. Habel, A. Buttner, K. D. Langer, and J. W. Walewski, “230 Mbit/s via a wireless visible-light link based on OOK modulation of phosphorescent white LEDs,” in Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), 2010), 1–3.

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