Effect of Temperature Change on the Performance of Laser Diode at

450 nm for Submarine Optical Communications

Chiara Lodovisi

, Silvello Betti

, Andrea Reale

and Luigi Salamandra

1,2

Department of Electronic Engineering, University of Rome Tor Vergata, 00133 Rome, Italy

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

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

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 ﬁve

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

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

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

(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

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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