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:
2:
3:
4:
5:
Institute of Photonics, University of Strathclyde, Glasgow, UK
School of Engineering, University of Glasgow, Glasgow, UK
POF-Application Center, Georg Simon Ohm University, Nürnberg, Germany
School of Engineering, University of Edinburgh, Edinburgh, UK
Department of Physics, University College Cork, Cork, Ireland
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.
41 090301 (2008).
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,
520, 560nm emission
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
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?
Visible-light communications
using micro-LEDs
Bandwidth of micro-LEDs
• ‘Bare’ pixels individually-addressed using a high-speed probe
• Bandwidth strongly dependent on current density
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).
Data transmission demo.
• Single micro-LED addressed using high-speed probe. Emission imaged
onto Si photodetector.
• NRZ modulation (modulation depth 2V, DC bias ~7V).
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
Visible-light communications
using CMOS-controlled microLEDs
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
1.6mm
• 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
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)
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
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.)
84µm diameter pixel, 1Gbit/s
Time (0.2ns/div)
Multi-channel transmission
•
Existing CMOS device has up to 16 data inputs – potential MIMO transmitter for highthroughput 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.
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 1346-1348, 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