Broadband Lasing Characteristics of a Chirped InAs/InP

Quantum-Dash Laser

E. Alkhazraji, M. Talal A. Khan

and

M. Z. M. Khan

Electrical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

Keywords: Broadband Lasers, Quantum-Dot Lasers, Quantum-Dash Laser, Spectral Characteristics.

Abstract: We investigate the temperature dependent spectral characteristics of an InAs multi-stacked quantum-dash-in-

well laser. The multi-stack active medium optical transitions are dispersed by varying the thickness of

AlGaInAs barrier layers. The analysis is carried out via a Fabry-Perot 700 µm long-cavity laser with a ridge

width of 2 µm at different temperatures. A lasing bandwidth of >40 nm is observed at room temperature with

total optical power of >150 mW. Moreover, broadening of lasing spectrum is observed with increasing the

temperature, due possibly to a thermionic assisted emission and to optical pumping, enabling a full

exploitation of the inhomogeneous optical transitions within the active region which indicates an increase in

the available states in the active region. Therefore, proper optimization of the multi-stack active medium is

required to fully utilize these optical transitions while maintaining high quantum efficiency, and proper

bandgap engineering the device structure.

1 INTRODUCTION

Due to several advantageous features of self-

assembled quantum dash (Qdash) structures, they

have become a focus of research and investigation in

the past few years (Khan et al., 2013; Khan et al.,

2014; Chen et al., 2002). Among the top of said

features is the extraordinarily broad emission

displayed by structures such as chirped InP-based

InAs Qdash structures that can be exploited in several

applications such as tenable sources, sensing, and

optical communications (Khan et al., 2013; Khan et

al., 2014). Given proper engineering and

optimization, a single broadly emitting source can

replace several combined traditional sources making

for a great replacement that is eco-friendly and cost-

efficient. However, what we lack in knowledge in the

active medium of these structures and the involved

quantum processes, that take place during emission,

outweighs what we know. In order to fully exploit the

potential of these these structures, optimization and

characterization of different factors that affect their

performance become more and more significant. It

comes with no surprise that temperature is a prime

factor as it plays a significant role in the non-uniform

carriers’ distribution and transition processes that

take place within the active media of Qdash lasers. In

this work, we characterize the temperature-dependent

lasing spectral characteristics of a chirped multi-

stacked InAs/InP Qdash laser and highlight the

physics behind the emission profiles.

2 EXPERIMENT

The investigated laser diode in this work is a ridge-

waveguide 2ൈ700 µm

Fabry-Perot (FP) laser diode.

The structure of its active medium is a chirped multi-

stacked quantum-dash-in-well structure. Figure 1

illustrates the different layers within the chirped

active medium. As Figure 1 shows, this structure is

Figure 1: An illustration of the chirped active region of the

multi-stacked quantum-dash-in-well structure.

252

Alkhazraji E., Khan M. and Khan M.

Broadband Lasing Characteristics of a Chirped InAs/InP Quantum-Dash Laser.

DOI: 10.5220/0006153502520255

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 252-255

ISBN: 978-989-758-223-3

composed of four stacks of In

0.64

0.16

0.2

As non-

symmetrical 7.6 nm thick quantum-well layers.

Within each layer are embedded five mono-layers of

InAs quantum-dashes. Moreover, on top of each of

the four stacks lies a tensile-strained

0.50

0.32

0.18

As barrier-layer. The thickness of the

barrier layers has been shown in previous reports

(Chen et al., 2002; Li et al., 2015) to play a significant

role in controlling the size of the grown Qdashes and

their corresponding emission. Thus, in order to

increase the inhomogeneity of the active medium of

the structure, the thickness of these of barrier-layers

are varied from one layer to another as 20, 15, 10, and

10 nm. It has been found that Qdashes associated with

wider barrier layers have a lower average height when

compared to narrower barrier layers. A more detailed

description of this structure can be found elsewhere

(Khan et al., 2013).

For the sake of convenience, we shall denote each

Qdash stack by its top barrier layer as S20, S15, S10a,

and S10b while the last two combined shall be

represented as S10 as a single group. Due to their

varying thickness values, each stack can be associated

with a separate ground state in addition to

overlapping tail states with the neighbouring stacks.

We investigated the temperature-dependent lasing

spectral profiles of the FP laser diodes by mounting

them over a temperature controlled brass base while

injecting them under a short pulse operation of a 0.2%

duty cycle and a 0.5 µs pulse width. Thereafter, the

lasing spectra have been obtained at different current

injections. While operating under such low duty cycle

(0.2%), it is safe to assume that the junction

temperature of the laser diode is almost equal to that

of the temperature-controlled brass base.

3 RESULTS AND DISCUSSION

Figure 2 (a) shows the L-I characteristic curves of this

laser device at different temperatures, namely 15°C,

20°C (room temperature), 30°C, and 40°C, whereas

Figure 2 (b) illustrates the energy-band diagram of the

different stacks within the chirped active medium of

the laser diode in addition to the overlapping density

of states between adjacent Qdash stacks. Figure 1 (a)

indicates that increasing the temperature of the laser

device results in increasing its threshold current and

decreasing the slope efficiency, particularly for

higher values of injected current under high

temperature values, as a result of the excess losses.

Figures 3 (a)–(d) show the lasing spectral profiles

of the laser device under temperature values of 15°C,

20°C, 30°C, and, 40°C, respectively at different

Figure 2: (a) The L-I characteristics of the laser diode at

different temperatures. (b) The energy-band diagram of the

chirped multi-stacked active medium alongside the

corresponding overlapping zero-dimensional density of

states.

injection current values starting from 1I

and then

sweeping between 1.2I

and 7.6I

in steps of 0.8I

At low current injections, (3.6I

and lower), a single

emission lobe begins to emerge around 1616 nm

(shaded in green). Among the different stacks inside

the active medium of this device, this initial emission

can be attributed to the Qdashes within the stack

associated with the intermediate barrier layer

thickness and, consequently, the intermediate Qdash

height which is the S15 stack. This attribution is

based on the appreciable overlap of the ground state

of that stack with tails of the neighbouring S20 and

S10 stacks. Hence, Qdashes of the average

intermediate height are expected to be the first to

collectively achieve population inversion and

initialize the lasing process.

Moreover, as we increase the pumped current,

more Qdashes within the S15 stack and Qdashes

within the overlapped tails from the neighbouring

stacks are enabled to overcome the medium losses

which ultimately leads to broadening the emission.

However, this takes place at the expense a small red-

shift in the emission due to the small bandgap

shrinkage when the current is increased.

Nonetheless, when the injected current reaches

4.4I

, two side-lobes appear at both sides of middle

main lobe which we shortly attributed to the S15

stack. Since Qdashes within the S10 stacks are

generally of relatively larger heights, their emission

components are expected to be higher in terms of

Energy

(b)

(a)

Broadband Lasing Characteristics of a Chirped InAs/InP Quantum-Dash Laser

253

Figure 3: The lasing spectra of chirped Qdash ridge under different current injections (1.0 Ith, 1.2 – 7.6 Ith in steps of 0.8 Ith)

at different temperatures of (a) 15°C, (b) 20°C, (c) 30°C, and (d) 40°C.

energy when compared to the shorter Qdashes within

the S20 stack. In other words, the emission side lobe

of the shorter wavelengths components (shaded in

blue) can be attributed to the S20 stack while the

longer wavelength lobe (shaded in red) can be

associated with the narrow S10 Qdash stack.

Nonetheless, with more current being injected, these

side-lobes begin to merge with the main lobe into a

single lobe possibly due to phono-assisted tunnelling

that can be accelerated by high temperature values

(Jiang and Singh, 1999). Furthermore, when

comparing the progressive spectra of the low

temperature case of 15

C with higher temperatures,

it is worthy to note that a slower progressive spectrum

broadening takes place at the lower temperature

value. This is evident by the late appearance of the

emission side lobes that only appear at current

injections of 4.4 I

and beyond in the case of 15

and 20

C while they emerge at earlier current

injections of 3.6 I

in the case of 30

and 40

These observations can be possibly explained by

the increase in the junction temperature by directly

increasing the temperature or due to the indirect

temperature increase that results from the higher

influx of carriers with higher current which results in

increasing the number of Qdashes that can overcome

the medium losses due the thermionic assisted

emission that leads to increasing the density of

carriers (

Kittel, 1966)

. This, also, explains why this

merging becomes more apparent as the temperature is

increased as observed in Figure 3 (d) when compared

to Figure 3 (a).

Moreover, this thermionic emission assistance

becomes more evident when comparing the total

emission 3dB bandwidth under the different

temperature cases as it gets wider as the temperature

is increased. For instance, Figure 3 (a) indicates that

the 3 dB bandwidth is ~ 44 nm at a current injection

of 7.6I

. However, this bandwidth increases to ~ 48,

51, and 52 nm as the temperature is increased to 20

C, 30

C and 40

C, as shown in Figures 3 (b) – (d)

respectively.

In other words, for any given injection current

value, increasing the temperature aids the Qdashes

within the tails of S20 and S10 stacks in achieving

population inversion via the externally acquired

thermal energy. Consequently, this enables more

Qdashes within these stacks to overcome medium

losses which introduces more emission wavelength

30dB

1.0Ith

4.4Ith

3.6Ith

7.6Ith

(

)

(

)

(

)

(

)

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

254

components which, in turn, broadens the overall

emission.

Ultimately, the variant average heights of each

Qdash stack in addition to the individual height

variance within the stack, both contribute to

increasing the structure’s inhomogeneity and

introducing dissimilar emission components that add

up to broaden the overall emission spectrum of the

device.

Nevertheless, this broadening occurs at the

expense of a reduction in the spectral power density

of the device which translates to the progressive

reduction in slopes in Figure 2 (a) as the temperature

is increased which indicates a quenching in the

quantum efficiency. Furthermore, this quench is

evident by the signal-to-noise ratio (SNR) of this

device of ~ 27 dB under 15

C when compared with

the ~ 25 dB SNR at room temperature that is reduced

even further at higher temperatures of 30

C and 40

C as it reaches ~ 23 dB and ~ 19 dB for both

temperatures, respectively. Interestingly enough, this

quenching effect is more apparent in the short

wavelength (high energy) region of the lasing spectra

which corresponds to the S20 stack and the high

energy tails of S15 stack.

Previously in this work, we have suggested that

the excess thermal energy that results from increasing

the temperature plays a significant role in

thermionically assisting the emission. However, this

excess of thermal energy, in this case, results in

carrier leakage when acquired by carriers within

dashes of high energy states within the S20 stack and

high energy tails of the S15 stack, which are more

shallowly confined when compared to carriers within

other Qdashes. Consequently, this results in a higher

probability of carriers escaping from the potential

confinement of these Qdashes since they can easily

exhibit a thermally induced carrier spill-over due to

their shallow quantum confinement.

4 CONCLUSIONS

With all said and done, this work provides a direct

evidence to the effect of temperature on the non-

uniform carrier dynamics and, in turn, the lasing

spectral characteristics of the emission of the chirped

InAs/InP Qdash structure. The rise in the junction that

takes place with increasing the temperature directly

or indirectly, via higher current injections, introduces

a thermionic emission assistance that results in

broadening the emission spectrum as more optical

transitions become available. Nevertheless, this

broadening occurs at the expense of deteriorating the

quantum efficiency of the laser as a result of the

thermally induced carrier leakage, particularly in the

case of small-offset Qdash stacks. In other words,

optimizing the structure of the active medium of the

chirped multi-stacked laser is necessary in order to

balance out this trade-off by minimizing the medium

losses and by optimally utilizing these optical

transitions while maintaining a high quantum

efficiency.

ACKNOWLEDGMENT

The authors thank King Fahd University of Petroleum

and Minerals for the financial support through

SR141002 grant.

REFERENCES

M. Z. M. Khan, T. K. Ng, C.-S. Lee, P. Bhattacharya, and

B. S. Ooi, Effect of optical waveguiding mechanism on

the lasing action of chirped InAs/AlGaInAs/InP

quantum dash lasers, in Proc. SPIE, 2013, pp. 86400.

M. Z. M. Khan, T. K. Ng, Lee, C. S., Bhattacharya, P., &

Ooi, B. S. (2014). Investigation of chirped

InAs/InGaAlAs/InP quantum dash lasers as broadband

emitters. IEEE Journal of Quantum Electronics, 50(2),

51-61.

J. X. Chen, A. Markus, A. Fiore, U. Oesterle, R. P. Stanley,

J. F. Carlin, et al.., “Tuning InAs/GaAs quantum dot

properties under Stranski-Krastanov growth mode for

1.3 μm applications,” J. Appl. Phys., vol. 91, no. 10, pp.

6710–6716, 2002.

Li, S. G., Gong, Q., Cao, C. F., Wang, X. Z., Yan, J. Y., &

Wang, Y. (2015). Thermal coefficient of InP-based

quantum dot laser from cavity-mode measurements.

Infrared Physics & Technology, 68, 119-123.

Jiang, H., & Singh, J. (1999). Nonequilibrium distribution

in quantum dots lasers and influence on laser spectral

output. Journal of applied physics, 85(10), 7438-7442.

Kittel, Charles. Introduction to solid state. John Wiley &

Sons, 1966.

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