Effect of Temperature Change on the Performance of Laser Diode at
450 nm for Submarine Optical Communications
Chiara Lodovisi
1
, Silvello Betti
1
, Andrea Reale
1
and Luigi Salamandra
1,2
1
Department of Electronic Engineering, University of Rome Tor Vergata, 00133 Rome, Italy
2
ISCTI - Istituto Superiore delle Comunicazioni e delle Tecnologie dell’Informazione, Ministero dello Sviluppo Economico,
Viale America 201, 00144, Rome, Italy
Keywords:
GaN Blue Laser Diode, Underwater, Optical Communications, Junction Temperature.
Abstract:
Optical communications usually require precise temperature control systems as junction temperature may
dramatically influence the emission parameters of a laser diode. Recently, challenging optical applications,
such as micro-satellite or underwater monitoring, need for small and low power solutions making it difficult
to use complex temperature control systems. Accordingly, in this paper we explored the use of a small and
passive copper heat sink to control the temperature and stabilize the transmission of GaN laser diode emitting
around 450 nm. The results of a reliable thermal characterization for the various operating conditions showed
the effectiveness of this simple solution.
1 INTRODUCTION
In recent years underwater communications have had
increasing interest in the study and development of
wireless communication systems. The main com-
munication technologies used are based on acoustic
waves, electromagnetic waves and optical pulses. In
addition to strategies following separated approaches,
hybrid systems are being studied more and more. Ex-
amples of recent studies can be found in (Lodovisi
et al., 2018), (Farr et al., 2010), (Han et al., 2014).
These allow the communication to be adapted to the
conditions of water turbidity, so as to have the sys-
tem always functioning and with high bit rate per-
formance, above all by using optical communication
technology.
An overview of recent UOWC (Underwater Op-
tical Wireless Communication) developments is re-
ported in (Kaushal and Kaddoum, 2016). It can be
seen how the use of lasers allows high bit rates to
be reached, performance also varies in dependence on
the modulation format. An overview is also given to
lasers operating in the blue-green spectrum. The sys-
tems that implement optical modems on devices for
submarine optical communications are mainly based
on the use of LEDs(Light Emitting Diode), some
examples are reported in (Moriconi et al., 2015),
(Doniec and Rus, 2010). The LEDs allow for a wider
illumination beam, less difficulty in pointing between
transmitter and receiver, but at the expense of a max-
imum bit rate of tens Mbit/s. The use of transmitters
based on laser technology makes it possible to achieve
much higher bit rates, hundreds of Mbit/s even up to
Gbit/s.
Being able to use different technologies in a sub-
marine system, both separately and simultaneously,
allows high performance with the possibility of trans-
mitting almost in real time.
In (Lee et al., 2015),(Wu et al., 2017), some exper-
imental measurements are reported which are based
on the use of laser diodes for underwater optical com-
munications. An example of high bit rate perfor-
mance, using a GaN semiconductor laser is reported
in (Chi et al., 2016). In (Najda et al., 2016) differ-
ent applications of GaN lasers in submarine optical
communications are reported. An overview of GaN
devices and their use is given in (Akasaki, 2013).
These nitride-based devices are robust in harsh envi-
ronments and allow us to save a significant amount of
energy.
Being able to use different technologies in a sub-
marine system, both separately and simultane-
ously,allows high performance with the possibility of
transmitting almost in real time.
In the field of the optical-acoustic hybrid system,
which Venus is equipped (Moriconi et al., 2015), we
are studying the implementation of the second opti-
cal channel, consisting of a GaN semiconductor laser
40
Lodovisi, C., Betti, S., Reale, A. and Salamandra, L.
Effect of Temperature Change on the Performance of Laser Diode at 450 nm for Submarine Optical Communications.
DOI: 10.5220/0008939500400044
In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 40-44
ISBN: 978-989-758-401-5; ISSN: 2184-4364
Copyright
c
2022 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
with a wavelenght around 450 nm for transmission at
high bit rates. Advantages of the choice of a GaN
semiconductor laser are low cost, ease of implemen-
tation and high frequency modulation.
The junction temperature changes affect the per-
formance of the laser diode in different ways, act-
ing an wavelength, emitted optical power and opti-
cal spectrum in modulation regime. The reliability
of the diodes is dependent on the junction tempera-
ture. From (Lee and Velasquez, 1998) it emerges that
the optical power decreases as the junction tempera-
ture increases. This is the reason why it is necessary
a system that keeps the temperature stable and dissi-
pates the heat. In (Lee et al., 2015) the laser diode
was mounted in a TO5 package surrounded by a large
aluminum heat sink. In (Wu et al., 2017) the system
temperature control consists of a copper support, a TE
cooling device and a thermistor to maintain the tem-
perature at at 25
C to stabilize the output dynamics of
the blue LD (Laser Diode).
There are several methods for measuring junction
temperature (Siegal, 2002),(Karim, 2004) . There is
the possibility of carrying out indirect measurements
based on the optical power emitted or evaluating shifts
of wavelength. Other methods are those of determin-
ing the thermal resistance of the device or using the
transient measurement method (Hwang et al., 2007).
This laser transmitter can be used to achieve high-bit
rate communication underwater links by robot with
limited payload. For example the Venus (Moriconi
et al., 2015) is a small device for which it is neces-
sary to limit payload and consumption of the devices.
The system must be as simplified as possible in the
design. To simplify our system as much as possible,
we have realized a simple copper heat sink. In this ar-
ticle we present experimental measurements to char-
acterize the laser using the heat sink. Our aim is to
have a system that is as limited as possible with re-
spect to electronics, but capable of reaching at least
100 Mbit/s.
The article is organized as follows: section 2
presents the experimental set up; section 3 presents
the experimental results and finally the conclusions
are shown in section 4.
2 EXPERIMENTAL SETUP
This section shows the setup carried out and the mea-
surement methodology followed. For the dissipation
of heat, we have made a copper plate with a 6 cm *
6 cm size , 1 mm thich, perforated in the center to
house the laser. In the front we have fixed a copper
cylinder with a diameter of 1.6 cm and a thickness of
(a) Heat sink
(b) Thermocouple
Figure 1: Images of the heat sink and thermocouple.
6 mm, perforated to allow the emission. Conductive
paste was placed between the plate and the cylinder.
Figure 1a shows an image of the heat sink realized.
To perform temperature measurements as close as
possible to the junction we have fixed a thermocou-
ple with copper conductive adhesive tape on the pin
of the laser case, as can be seen from Figure 1b. We
constantly monitored the temperature of the device by
driving the laser with a continuous current, and by
varying the current value between 50 and 100 mA us-
ing a modular controller.
For each driving current value we took the tem-
perature measurements every minute in the first ve
minutes, and then every five minutes until the temper-
ature stabilized. At the same time, we monitored both
the wavelength laser emission on the optical spectrum
analyzer, to evaluate possible wavelength shifts, and
the dissipation of heat on the heat sink plate through a
thermo-camera, Figure 2 show the spectrum emission
of laser diode.The equipment used to record spectrum
is Ando AQ-6315A Optical Spectrum Analyzer.
Figure 3 shows the block diagram of the imple-
mented setup to evaluate the laser behaviour when the
driving current is continuous.
Effect of Temperature Change on the Performance of Laser Diode at 450 nm for Submarine Optical Communications
41
Figure 2: The spectrum emission of laser diode.
Figure 3: Block diagram of the implemented setup for mea-
surements driving the laser by controller.
We repeated the same measures described above
also moving the thermocouple inside the conductive
paste to avoid the possible temperature disturbance
due to the contact of the surrounding air. The final
temperature values reached and the trend of the tem-
perature increase were substantially the same as those
we measured with the thermocouple fixed on the case
pin.
The only difference that emerges is that the tem-
perature stabilizes after about 90 minutes, compared
to 30 minutes for the other measurement. In this case
the contact with the surrounding air could influence
the stabilization dynamics. We performed measure-
ments to evaluate possible temperature changes by di-
rectly modulating the laser, with generated signal fre-
quencies ranging between 1 and 100 MHz. At the
same time, we evaluated the value of the signal re-
ceived on an electrical spectrum analyzer. We used a
Bias Tee to directly modulate the laser. The generated
signal was a sinusoid with a variable frequency. At the
receiver site we used an inversely polarized photodi-
ode to detect the signal.
Figure 4 shows the setup scheme implemented to
perform the measurements.
Figure 4: Block diagram of the implemented set up for mea-
surements driving the laser directly.
3 EXPERIMENTAL RESULTS
In this section, the obtained results are reported and
described. The first measure performed was the cur-
rent power characterization of the laser housed in the
heat sink. Figure 5 shows the result obtained.
Figure 5: Laser response.
Figure 6 shows the junction temperature measure-
ments as a function of time by driving the laser with
a continuous current equal to 50, 60, 75 and 100 mA.
From the trend it can be noticed that the temperature
increases mainly in the first 5 minutes (about 2 de-
grees Celsius), it increases by another degree for the
next fifteen minutes, after which it tends to stabilize.
The value of the final temperature reached is indepen-
dent of the driving current of the laser, substantially
reaching the same temperature value.
Simultaneously with the junction temperature
measurements we verified the emission of the laser
peak to check eventual shifts of wavelength. Figure 7
shows the values of the laser emission wavelength, in
correspondence with the temperature values recorded
for different driving currents of the laser. From the
measurements carried out it emerges that the emis-
sion wavelength maintains substantially constant. It
slightly tends to increase as the driving current in-
creases and the temperature increases, but not signif-
PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology
42
Figure 6: Temperature trend for different driving current.
Figure 7: Wavelength vs Temperature Behaviour.
icantly or to limit the performance of the data trans-
mission system.
Figure 8 shows the images provided by the
thermo-camera to evaluate the behavior of the heat
sink, while driving the laser at 75 mA. The image is
shown with the laser off, the laser on after 5,15 and 35
minutes switching on. It can be seen how the heat is
evenly distributed and without reaching temperatures
above about 26
. In one point of the heat sink a higher
temperature was monitored, but in any case not higher
than 32
which did not affect the temperature of the
registered junction.
After having carried out an analysis on the pos-
sible effects of temperature piloting the laser with a
direct current, we carried out measurements to evalu-
ate the behavior of the laser while it was modulated.
(a) laser off (b) 5 minutes from laser
switch on
(c) 15 minutes from laser
switch on
(d) 35 minutes from
laser switch on
Figure 8: Images of the thermo-camera after different laser
lighting.
Figure 9 shows the junction temperature trend as the
frequency of the generated signal varies. Before in-
creasing the modulation frequency, we have waited
for the detected junction temperature to stabilize. The
same behavior of the laser controlled in direct current
was noticed, reaching the same temperature values.
From the measurements carried out it emerges that, at
least up to 100 MHz, once the laser stabilizes in tem-
perature, the modulation does not produce any signi-
ficative modification of the stabilized temperature of
the laser.
Figure 9: Temperature trend for different frequencies of the
generated signal.
Figure 10 shows the trend of the normalized opti-
cal power to the power transmitted as a function of the
modulation frequency. We have normalized the opti-
cal power detected on the ESA with respect to the op-
tical power transmitted at the laser bias current of 75
mA. The trend shows how, apart from an initial peak
due to of the photodiode spectral response, the power
trend slightly tends to decrease with the increase in
frequency, but remains around the same values.
Effect of Temperature Change on the Performance of Laser Diode at 450 nm for Submarine Optical Communications
43
Figure 10: Normalized optical power to the bias point
against modulation frequency.
4 CONCLUSIONS
Compact and low power systems are two fundamen-
tals requirements for systems operating in particular
scenarios such as underwater communications with
mini vehicles. To that end we assessed the feasibility
of using a simple, compact, and passive copper heat
sink to control a GaN laser diode emitting around 450
nm. Actually, the junction temperature reaches a suit-
able steady-state behavior either driving the laser with
a continuous or sinusoidal waveform.
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