Gb/s Single-LED OFDM-based VLC Using Violet and UV Gallium Nitride μLEDs

Ricardo Ferreira1, Dobroslav Tsonev

2, Jonathan McKendry

1, Stefan Videv

2, Scott Watson

3,

Alexander Griffiths1, Erdan Gu

1, Anthony Kelly

3, Harald Haas

2 and Martin Dawson

1.

1 — Institute of Photonics, University of Strathclyde, U.K.2 — Institute for Digital Communications, The University of Edinburgh, U.K.3 — School of Engineering, University of Glasgow, U.K.

IEEE Summer Topicals 20151

• Motivation • White light generation • Wavelength Division Multiplexing (WDM)

• UV / Violet µLEDs • Concept • Properties

• Data transmission • On-Off-Keying (OOK) • Orthogonal Frequency Division Multiplexing

(OFDM) • Conclusion

2

Outline

3

Shorter λs are better to pump colour

converting materials*

high quality white light**

Visible Light Communications (VLC)

*M. Park et al., J. Electroceram. 33 (2014) **Bai et al., Nat Commun 5 (2014)

*** Ian Watson & C.J. Humphreys, MRS Bulletin, 33, 59 (2008)

Internal Quantum Efficiency peaks at the UV/Violet

wavelengths***

4

Visible Light Communications (VLC)

Wavelength Division Multiplexing (WDM)

3-Gb/s with a single 450nm µLED

• Multi-wavelength system extends colour gamut

• Data streams divided into discrete λs/µLED

10 11 10 λ1

PD

λ2

λ3

10 11 10

Towards 10-Gb/s WDM link

5

< 100µm

200µm

200µm

µLEDs

Schematic of a flip-chip µLED

‣ micrometer-sized pixels ‣ Improved thermal management ‣ High current densities > 1kA/cm2

‣ High frequency response

Optoelectronics

6

V-I-L Emission Spectra

• Optical power up to 1.4 & 2.5 mW (CW) • Optical power densities:

‣ UV = 42 W/cm2 ‣ Violet = 201 W/cm2

Current (mA)0 20 40 60

Voltage (V)

1

2

3

4

5

6

7

Luminescence (mW)

0

0.5

1

1.5

2

2.5

UV Ø60µm

Violet Ø40µm

Wavelength (nm)360 380 400 420 440 460 480

Intensity

0

0.2

0.4

0.6

0.8

1 UV

Violet

Frequency Response

7

V

(DC)bias–tee

(DC+AC)

NetworkAnalyser

(AC)

(AC)

LED Photodiode

J (A/cm2)0 1000 2000 3000 4000

E-E

Band

widt

h (M

Hz)

40

60

80

100

120

140 UV Ø60µm Violet Ø40µm

‣ Current densities > 1kA/cm2

‣ Electrical Bandwidths > 100MHzFrequency (MHz)

100 101 102 103

Ampl

itude

(dB)

-30

-20

-10

0

UV Violet

-3dB

8

Data Transmission OOKDC bias

pattern generator

(2Vpp)

network analyzer

(clock)

error detector

amplifier

pre-amp.50 cm

µLED PIN

DC ACDC+AC

bias-tee

scopeprobed µLED chip

Error detector

Pattern generator

DC bias

pre-amplifier

amplifier

PIN (1.4GHz)

9

Data Transmission OOKUV 370nm Violet 405nm On-Off-Keying (OOK)

Aggregated 910Mb/s with direct modulation

Time (µs)0 0.5 1

Ampl

itude

-0.5

0

0.5

Time (ns)0 2 4 6

Ampl

itude

-0.5

0

0.5

Time (ns)0 2 4

Ampl

itude

-0.5

0

0.5

155-Mb/s

300-Mb/s

400-Mb/s

Time (µs)0 0.5 1

Ampl

itude

-0.2

0

0.2

Time (ns)0 2 4 6

Ampl

itude

-0.2

0

0.2

Time (s)0 2 4

Ampl

itude

-0.2

0

0.2

155-Mb/s

300-Mb/s

400-Mb/s

Received optical power (dBm)-16 -14 -12 -10 -8 -6

BER

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

UV 400 Mb/s Violet 510 Mb/s error-free

10

Data Transmission OFDM

DC bias

AWG Computeramplifier

50 cm

µLED PIN

DC ACDC+AC

bias-teescope

‣ Orthogonal Frequency-Division Multiplexing (OFDM);

is widely used in communications (DSL, 4G, etc…)

‣ Multiple-subcarrier modulation scheme

‣ Amplitude and phase

‣ Total bandwidth is divided in frequency sub-bands

‣ Carrier signals are orthogonal to each other

‣ Quadrature amplitude modulation (QAM)**

‣ High spectral efficiency*

4-QAM

*Tsonev et al., JLT, 2013 **Armstrong, JLT, 27(3) 2009

11

OFDM — UV (370nm)

f (MHz)0 100 200 300 400 500

Bits

0

2

4

6

8 bit loading capacity

f (MHz)0 100 200 300 400 500

Ener

gy A

lloc

atio

n

0

1

2

3

4

} Vpp = 2V, Ibias = 15 mA

} Used bandwidth = 390 MHz

} 4-QAM to 64-QAM

} Data Rate = 1.41 Gb/s

} BER = 2.46 x 10-3

} After 7% overhead,

Data Rate = 1.31 Gb/s

12

OFDM — Violet (405nm)

f (MHz)0 200 400 600 800 1000

Bits

0

5

10 bit loading capacity

f (MHz)0 200 400 600 800 1000

Ener

gy A

lloc

atio

n

0

1

2

} Vpp = 3V, Ibias = 25 mA

} Used bandwidth = 770 MHz

} 4-QAM to 256-QAM

} Data Rate = 3.57 Gb/s

} BER = 2.1 x 10-3

} After 7% overhead,

Data Rate = 3.32 Gb/s

13

Results

‣ Optical power up to 1.4 & 2.5mW (CW)

‣ Current densities above 1 kA/cm2

‣ Electrical Bandwidths > 100MHz

‣ Direct modulation: 910-Mb/s (aggregated)

‣ OFDM after Forward Error Correction (FEC)

• UV: 1.31-Gb/s

• Violet: 3.32 Gb-s

• Aggregated: 4.63-Gb/s

14

Conclusion‣ Shorter λs are more efficient to pump colour converters

‣ µLEDs show fast response at 370nm & 405nm

‣ Gb/s links with UV & Violet µLEDs are demonstrated

✦ Record data rates for UV/Violet wavelengths

‣ 370nm & 405nm join longer λs for WDM working

towards to 10-Gb/s

http://up-vlc.photonics.ac.uk/

Special thanks to David Massoubre for the µLED fabrication