1. High speed GaN micro-LED arrays for data communications
N. Laurand(1), J.J.D. McKendry(1), A.E. Kelly(2), S. Zhang(1), J. Vinogradov(3), D. Massoubre(1), B.R. Rae(4), R.P. Green(5), E. Gu(1), O. Ziemann(3), R.K. Henderson(4) and M.D. Dawson(1)
1: Institute of Photonics, University of Strathclyde, Glasgow, UK
2: School of Engineering, University of Glasgow, Glasgow, UK
3: POF-Application Center, Georg Simon Ohm University, Nürnberg, Germany
4: School of Engineering, University of Edinburgh, Edinburgh, UK
5: Department of Physics, University College Cork, Cork, Ireland

2. Introduction to micro-LEDs
0.5mm
Micro-stripes
Matrix-addressable
III-nitride wafers grown on sapphire
UV (370nm), violet (405nm) blue (450 and 470nm) and green (520nm)
Patterned by standard photolithography
M.D. Dawson and M.A.A. Neil, “Micro-pixellated LEDs for science and instrumentation” J Phys D (2008).

3. Individually-addressable micro-LEDs
16 × 16 array
72µm diameter
100µm pitch
370, 405, 450nm emission
8 × 8 array
10, 20…80µm diameter
200µm pitch
370, 405, 450, , 560nm emission

4. Maximum absolute power (mW)
CW power output
Diameter (nm)
Maximum absolute power (mW) 14
0.48 24
0.85 44
2.52 64
3.8 84
4.94
Up to 5mW from a single micro-LED pixel (450nm peak emission)
Smaller pixels – higher output power densities & current densities

5. Micro-LEDs for communications
Our GaN-based micro-LEDs emit at wavelengths corresponding to attenuation minimum in POF
Data transmission using white LEDs also topical
Possible advantages of micro-LEDs?
faster response?
multi-channel output?

6. Visible-light communications using micro-LEDs

7. Bandwidth of micro-LEDs
‘Bare’ pixels individually-addressed using a high-speed probe
Bandwidth strongly dependent on current density

8. Bandwidth of micro-LEDs
General trend that smaller micro-LEDs have higher maximum bandwidths.
Attributed to higher maximum current densities for smaller pixels (reduced current crowding and device self-heating).

9. Data transmission demo.
Single micro-LED addressed using high-speed probe. Emission imaged onto Si photodetector.
NRZ modulation (modulation depth 2V, DC bias ~7V).

10. Data transmission demo.
155 Mbit/s
622 Mbit/s
1.2 Gbit/s
520nm-emitting micro-LED, diameter 34µm
i = 35mA, output power ≈ 0.2mW
“Error-free” up to 1.1 Gbit/s

11. Visible-light communications using CMOS-controlled micro-LEDs

12. CMOS-driver array
Primarily designed for generating intense (sub)ns-duration pulses for OSL pumping
16×16 array of individually-addressable drivers, 100×100µm2, 100µm pitch
Multiple modes of operation – CW, pulsed, NRZ modulation
Each of the 16 columns may be modulated with independent data inputs (MIMO data transmission)
CMOS and micro-LED chips integrated by flip-chip bonding process
1.6mm

13. Bandwidth of CMOS-micro-LEDs
NRZ signal from BERT used to trigger CMOS drivers. Micro-LEDs bias voltage modulated between 0V and LED_VDD (variable)
Increasing V = higher bandwidth. Higher bandwidths obtained with smaller diameter pixels. Max bandwidth from single pixel ≈185MHz (450nm device)
‘Error-free’ data transmitted at up to 512 Mbit/s (450nm device)

14. Data transmission over POF
Work done in collaboration with O. Ziemann’s group, POF-AC, Nürnberg
450nm-emitting CMOS-controlled micro-LED device used
Micro-LED emission butt-coupled to 1m of 1mm diameter SI-POF
Passive equalisation, low-pass filter and electrical amplifier used

15. Data transmission over POF
Up to -3dBm of coupled CW power
“Error-free” (BER ≤ 1×10-9) data transmission at 1Gbit/s from pixel diameters ranging from 34 to 84µm
Error-free data transmission also achieved using 520nm-device at up to 500Mbit/s
Received power (a.u.)
Time (0.2ns/div)
84µm diameter pixel, 1Gbit/s

16. Multi-channel transmission
Existing CMOS device has up to 16 data inputs – potential MIMO transmitter for high-throughput parallel data transmission
Data transmission using two channels has been investigated. Using two 450nm-emitting 34µm diameter pixels, error free parallel transmission has been achieved up to 600Mbit/s (300Mbit/s per channel)
Data rate per channel limited due to EMI issues between the two channels – this issue appears to be primarily caused by the design of the interface board, not the CMOS or micro-LED arrays themselves.

17. Conclusions
Micro-LED pixels have been shown to have modulation bandwidths of up to ~450 MHz, with peak emission at wavelengths suitable for transmission over POF.
>1Gbit/s single pixel transmission demonstrated with no equalisation
CMOS control arrays have been shown to provide convenient control over micro-LED arrays, with bandwidth of up to 185MHz possible. With up to 16 independent data channels, these devices are potential MIMO transmitters for VLC.
Key Publications
Jonathan McKendry, Richard P. Green, A. E. Kelly, Zheng Gong, Benoit Guilhabert, David Massoubre, Erdan Gu and Martin D. Dawson “High Speed Visible Light Communications Using Individual Pixels in a Micro Light-Emitting Diode Array” Photon. Tech. Lett., Vol 22, No.18, pp , Sept 2010.
McKendry, J. J. D.; Massoubre, D.; Zhang, S.; Rae, B. R.; Green, R. P.; Gu, E.; Henderson, R. K.; Kelly, A. E.; Dawson, M. D.; , "Visible-Light Communications Using a CMOS-Controlled Micro-Light- Emitting-Diode Array," Lightwave Technology, Journal of , vol.30, no.1, pp.61-67, Jan.1, 2012