| Weikang Zhang
| Jose Figueiredo
| Jue Wang
| Henrique Salgado
| Anthony Kelly
School of Engineering, University of Glasgow, Glasgow, United Kingdom
INESC TEC and Faculty of Engineering, University of Porto, Porto,
Department of Physics, Faculty of Sciences, University of Lisbon, Lisbon,
Compound Semiconductor Technologies Global Ltd, Glasgow, United
Scott Watson, School of Engineering, University of Glasgow, Rankine
Building, Oakfield Avenue, Glasgow, G12 8LT, United Kingdom.
European Union’s Horizon 2020 Research and Innovation Programme,
Grant/Award Number: 645369
Optical modulation characteristics of resonant tunneling
diode photodetectors (RTD-PD) are investigated. Inten-
sity modulated light excites the RTD-PDs to conduct
data experiments. Simple and complex data patterns are
used with results showing data rates up to 80 and
200 Mbit/s, respectively. This is the first demonstration
of complex modulation using resonant tunneling
optical communications, photodetector, resonant tunneling diode,
complex data transmission
The demand for high speed wired and wireless communica-
tions is ever-growing and it is expected that data rates in the
order of tens of Gbit/s will be desired in the near future.
order to exploit higher frequency bands, the millimeter-wave
and terahertz part of the spectrum are being considered. The
FIGURE 1 RTD-PD epi-layer structure [Colour figure can be
viewed at wileyonlinelibrary.com]
FIGURE 2 Photo of RTD-PD oscillator with oscillation
frequency of 35 GHz [Colour figure can be viewed at
Received: 2 August 2018
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
© 2019 The Authors. Microwave and Optical Technology Letters published by Wiley Periodicals, Inc.
Microw Opt Technol Lett. 2019;1–5. wileyonlinelibrary.com/journal/mop 1
lack of practical sources has been a bottleneck for this work
but there are now a number of devices able to generate high
frequency carrier waves suitable for communications into
the terahertz bands. One example is resonant tunneling
diodes (RTDs) which have the advantage of being poten-
tially very low cost. RTDs have been the subject of research
for many years resulting in usable output powers and oscilla-
tion frequencies up to 1.9 THz.
RTDs are compact
structures, of less than a millimeter square, implemented on
monolithic microwave integrated circuit (MMIC) technology
and have been demonstrated in systems using electrical data
modulation up to 34 Gbit/s with carrier frequencies of
approximately 500 GHz.
It is possible to integrate RTDs
with antennae and other optoelectronic components such as
photodiodes. Resonant tunneling diode photodetectors
(RTD-PDs) have a number of potential uses such as the
FIGURE 3 Measured I-V characteristic of the RTD-PD. Inset: The I-V curve shifting to the right with the presence of light [Colour figure
can be viewed at wileyonlinelibrary.com]
FIGURE 4 Experimental setup for optical modulation experiments [Colour figure can be viewed at wileyonlinelibrary.com]
2WATSON ET AL.
seamless transferal of optical data onto an RF carrier and in
fiber distributed networks as a means for optical synchroni-
zation of the generated RF carrier via injection locking.
This work discusses the performance of a range of RTD-
PDs. Optical to RF data transferral experiments is demonstrated.
The use of complex modulation formats is also explored.
2|DEVICE STRUCTURE AND
Figure 1 shows the epi-layer structure of an RTD-PD and
Figure 2 shows a microscope image of the device. These
RTD-PDs have an optical window fabricated allowing direct
access for the optical signal to reach the oscillator. As seen
from the structure, they also include light absorption layers
on both sides of the double barrier quantum well (DBQW),
which increases the responsivity to light. More fabrication
details can be found in previous work.
Firstly, an I-V measurement was taken to establish
the negative differential resistance (NDR) region and this
was carried out in both light and dark conditions. The
RTD-PD is sensitive to both 1310 and 1550 nm; how-
ever, it can be seen in Figure 3 that the photocurrent is
higher for 1310 nm due to the material used for the opti-
cal absorption layer.
FIGURE 5 Output signal from the RTD-PD showing data patterns at 80 Mbit/s mixed with the oscillations [Colour figure can be viewed
FIGURE 6 Measured time domain signals at the input (yellow), before (red), and after (blue) the envelope detector [Colour figure can be
viewed at wileyonlinelibrary.com]
WATSON ET AL.3
Optical data transmission experiments were carried out using
an RTD-PD with an oscillation frequency of 35 GHz. The
oscillation frequency could be tuned by a few tens of mega-
hertz by tuning the voltage, but also by introducing different
powers of light incident on the oscillator. A pseudo-random
bit sequence (PRBS) is used to modulate the optical signal
and is sent to the oscillator via optical fiber, as seen in
Figure 4. The output time domain signal was viewed on a
fast oscilloscope so that the oscillations could be seen.
Figure 5 shows the received pattern at 80 Mbit/s, where an
oscillation relates to a “1”bit and no oscillation is a “0”bit.
Higher data rates are expected in future work and can be
achieved by optimizing the device structure for optical sig-
nals as well as improving the cut-off frequency set by the
Using an envelope detector at the output of the RTD-PD
we were able to demodulate the transmitted signal, up to
40 Mbit/s, which was limited by the envelope detector band-
width (Figure 6).
Using another RTD-PD with an oscillation frequency
around 14 GHz, complex data transmission measurements
The implemented setup used to explore complex mod-
ulation is shown in Figure 7. The complex modulations are
generated by an arbitrary waveform generator (AWG) that
sends a subcarrier modulated complex signal through the
DC port of the RTD. The output of the RTD is received by
a spectrum analyzer running a vector signal analysis
(VSA) software, used for evaluating the quality of the
transmitted signal. A signal generator was used on the RF
port side in order to lock the RTD oscillation carrier,
which allows the impact of the RTD carrier phase noise to
The signal-to-noise ratio (SNR) of the demodulated sig-
nals for both locked and unlocked regimes are shown in
Figure 8, as a function of the subcarrier frequency. In this
experiment the AWG was generating 100 Mbaud QPSK
signals. This result shows that for subcarrier frequencies
below approximately 250 MHz, locking the RTD-PD car-
rier provides a considerable advantage, while above this
value the locking does not significantly improve the results,
which shows that the RTD-PD phase noise affects mainly
the lower subcarrier frequencies due to the RTD-PD carrier
FIGURE 7 Experimental setup for advanced modulation formats
evaluation [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 8 SNR as a function of subcarrier frequency measured for 100 MBaud QPSK data transmission. Inset: Constellation of the
received signals (at 120 MHz) [Colour figure can be viewed at wileyonlinelibrary.com]
4WATSON ET AL.
proximity. The inset shows the constellation of the demo-
dulated signals for a subcarrier frequency of 120 MHz,
where the phase noise is visible in the constellation rotation
Figure 9 shows the SNR as a function of the symbol
rate measured for a subcarrier frequency of 200 MHz.
Here, the performance for lower symbol rates is poorer due
to phase noise since lower symbol rate symbols are trans-
mitted over a longer period of time and therefore experi-
ence additional phase noise.
The inset shows the
constellation of the received signals for a symbol rate of
10 MBaud, where the phase noise when unlocked is
increased compared with the previous result due to the
lower symbol rate.
This article has presented the use of RTD based photodetectors
for optical communications. Modulated optical signals were
used to excite the RTD-PDs and successful data experiments
were conducted up to 80 Mbit/s. As well as simple modulation
formats, it was shown that these oscillators can respond to
more complex data signals up to 100 MBaud (200 Mbit/s) and
work is on-going to increase data rates accordingly.
This work was performed as part of the iBROW project
which has received funding from the European Union’s
Horizon 2020 Research and Innovation Programme under
grant agreement No. 645369.
Scott Watson https://orcid.org/0000-0002-9235-9242
Joana Tavares https://orcid.org/0000-0002-7666-3738
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How to cite this article: Watson S, Zhang W,
Tavares J, et al. Resonant tunneling diode photodetec-
tors for optical communications. Microw Opt Technol
Lett. 2019;1–5. https://doi.org/10.1002/mop.31689
FIGURE 9 SNR as a function of symbol rate measured for a subcarrier frequency of 200 MHz. Inset: Constellation of the received
signals (for 10 MBaud) [Colour figure can be viewed at wileyonlinelibrary.com]
WATSON ET AL.5