High Repetition Frequency Mode-locked Semiconductor Disk Laser

Yanrong Song

, Peng Zhang

, Jinrong Tian

, Zhigang Zhang

, Hark Hoe Tan

and C. Jagadish

College of Applied Sciences, Beijing University of Technology, Beijing, 100124, P.R. China

School of Electronics Engineering and Computer Science, Peking University, Beijing, 100871, P.R. China

Department of Electronic Materials Engineering, Research School of Physics and Engineering,

The Australian National University, Canberra, ACT 0200, Australia

Keywords: Lasers, Diode-pumped, Ultrafast Lasers, Semiconductor Lasers.

Abstract: A compact passively mode-locked semiconductor disk laser with a high repetition frequency of 3GHz is

demonstrated. 4.9ps pulse duration and 30mW average output power are obtained with 1.4W of 808nm

incident pump power. The gain chip consists of 16 compressively strained InGaAs symmetrical step

quantum wells in the active region.

1 INTRODUCTION

High repetition frequency pulse trains are suitable

for a wide variety of applications, such as optical

clocking (Miller, 2000), frequency conversion

(Lecomte et al., 2005), high-speed electro-optic

sampling (Weingarten et al., 1988), time-resolved

spectroscopy (Bartels et al., 1999), and the primary

light source for telecommunication systems

(Ramaswami and Sivarajan, 1998). For a long time,

mode-locked solid-state lasers and edge-emitting

semiconductor lasers were the dominators in the

field of multi-gigahertz picosecond pulse resources.

In recent years, passively mode-locked

semiconductor disk lasers (SDLs, also known as

optically pumped vertical-external-cavity surface-

emitting lasers), have been demonstrated capable to

generate multi-gigahertz pulses with high average

output power and good beam quality (Hoogland et

al., 2000); (Häring et al., 2002); (Aschwanden et al.,

2005); (Keller and Tropper, 2006). By comparison,

SDLs are cost-effective hence attractive for their

potential substitutes of mode-locked solid-state

lasers or edge-emitting semiconductor lasers in some

applications (Gherman et al., 2004); (Dupriez et al.,

2206); (Mihoubi et al., 2008).

After the first report of a passively mode-locked

SDL (Hoogland et al., 2000), performance of the

mode-locked SDL has been improved significantly:

pulse duration to 190fs at 3GHz repetition rate and

5mW output power by a fast SESAM (Klopp et al.,

2009); repetition rate to 50GHz with 3.3ps pulse

duration and 100mW output power by a low

saturation fluence quantum dot SESAM (Lorenser et

al., 2006); and average output power to 2.1W at

4GHz repetition rate and 4.7ps pulse duration by the

substrate-removing method (Aschwanden et al.,

2005). However, the lasing wavelength of a SDL

(GaAs based) redshifts at a rate of about 0.3nm/K

with increasing temperature because of the intrinsic

characteristic of the QWs (Tropper et al., 2004), and

this problem has not been solved so far. In this

paper, we demonstrate a picosecond passively mode-

locked SDL at multi-gigahertz repetition rate. The

gain chip is without any post processing (such as

substrate-removing and heatspreader-bonding), the

fabrication is quite simple and the configuration is

compact. Temperature stability of the laser is

achieved using the symmetrical step QWs in the

active region.

2 EXPERIMENTAL SETUP

Figure 1 shows the gain chip used in the

experiments. It contained three main components:

the Bragg reflector, the gain region, and the

confinement window layer. The Bragg reflector was

grown on a GaAs substrate and comprised 27

GaAs/AlAs layer pairs designed to act as a high

reflection mirror (calculated reflectivity > 99.95%).

The gain region was grown by standard metal-

organic chemical vapor deposition (MOCVD)

method which consisted of 14 InGaAs quantum

361

Song Y., Zhang P., Tian J., Zhang Z., Hoe Tan H. and Jagadish C..

High Repetition Frequency Mode-locked Semiconductor Disk Laser.

DOI: 10.5220/0004022903610364

In Proceedings of the International Conference on Data Communication Networking, e-Business and Optical Communication Systems (OPTICS-2012),

pages 361-364

ISBN: 978-989-8565-23-5

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

wells(QW) surrounded by GaAsP barrier layers for a

total thickness of 7λ/2, which allows an efficient

absorption of the pump radiation. Finally, a window

layer of AlAs was grown and the full structure was

completed by a thin InGaP capping layer (not

appeared in fig.1). The AlAs window layer

prevented the excited carriers escaping to the

surface. This surface barrier layer was transparent

for the pump wavelength.

Figure 1: The scheme of the gain structure.

The passively mode-locked SDL with the V-

shaped cavity setup is shown in Fig. 2. A

2mm×2mm unprocessed gain chip cleaved off the

wafer is directly mounted to a copper heat sink and

is temperature controlled by a Peltier cooler. The

back plate of the entire device is water-cooled. A

fiber-coupled 808nm pump radiation is collimated

and focused on the semiconductor disk at an angle of

about 45 degree, and the diameter of the pump spot

is approximately 100μm. The gain chip is used as a

folding mirror, and the laser cavity is ended by a

low-temperature grown SESAM and a highly

reflective plane-concave output coupler (OC). The

folded angle of the cavity is smaller than 10

reduce the astigmatism.

The curved OC determines a small mode area on

the SESAM, such that the saturation energy of the

absorber is lower than the saturation energy of the

gain medium, as required for pulse formation

(Paschotta et al., 2002. In the experiment, the total

cavity length is approximately same as the radius of

curvature of the OC to force the laser to operate at

the edge of the stable region. The lengths of OC arm

and SESAM arm are about nine tenths and one tenth

of the total cavity length, respectively, to produce a

laser spot about 100μm on the gain chip for

matching the pump spot. The ratio of the area

between the spot on the gain chip and on the

SESAM is about 25. With a fixed optimum laser

spot on the gain chip, we adjust the lengths of the

SESAM arm and the OC arm to obtain a stable

continuous-wave (CW) mode-locking operation.

We use an Infiniium 54833A oscilloscope

(Agilent Technologies) to monitor the mode-locked

pulse trains. The output beam is sent to a FR-103XL

autocorrelator (Femtochrome Research) to obtain the

autocorrelation trace. Meanwhile, a fast 818-BB-35F

InGaAs PIN photoelectric detector (Newport) with

10GHz bandwidth and <35ps rise time and a 40GHz

E4447A spectrum analyzer (Agilent Technologies)

are used to receive the optical pulses and obtain the

radio-frequency (RF) spectrum.

Figure 2: Cavity setup used for the passively mode-locked

SDL.

3 RESULTS AND DISCUSSIONS

Figure 3: Measured and sech

fitted autocorrelation trace

(a) and RF spectrum (b) of the mode-locked pulses. The

inset in (a) shows the optical spectrum and the inset in (b)

shows the RF spectrum on a 5GHz span. The radius of

curvature of the OC is 100mm.

OPTICS2012-InternationalConferenceonOpticalCommunicationSystems

362

Firstly, a plane-concave mirror with a radius of

curvature of 100mm is used as the OC. The

measured and sech

fitted autocorrelation trace of

the mode-locked pulse trains are shown in Fig. 3(a).

The sech

fit indicates that the ideal width (full

width at half maximum, FWHM) of the pulses is

5.0ps, which is about 4.2 times of the Fourier

transform limited sech

pulse duration of 1.2ps,

according to the inserted optical spectrum with 0.96

nm width. It is known that for a passively mode-

locked SDL, the presence of chirp in the pulses is

inevitable because of the saturation effect (also

known as phase modulation effect) of the gain

medium and the SESAM. With some dispersion-

controlled elements such as an etalon, the above

phase modulation effect can be counteracted and the

transform limited pulse trains can be produced

(Aschwanden et al., 2005).

Fig. 3(b) shows the RF spectrum of the laser.

The frequency span is 180MHz and the resolution

bandwidth (RBW) is 1.8MHz. The inset shows the

frequency spectrum on a 5GHz span. As shown in

Fig. 3(b), the fundamental frequency is 1.5GHz,

corresponding to the round-trip repetition rate of

100mm cavity. The RF spectrum trace is free from

sidebands down to the level of -35dBc,

demonstrating that the SDL exhibits stable CW

mode-locking with no Q-switching instabilities.

Figure 4: Measured and sech

fitted autocorrelation trace

(a) and RF spectrum (b) of the mode-locked pulses. The

inset in (a) shows the optical spectrum and the inset in (b)

shows the RF spectrum on a 12GHz span. The radius of

curvature of the OC is 50mm.

When the 100mm radius of curvature of OC is

replaced by a 50mm radius of curvature OC, the

autocorrelation trace and the RF spectrum of the

mode-locked laser are shown in Fig. 4(a) and (b). It

can be seen from Fig. 4(a) that the sech

fitted pulse

duration is 4.9ps, which is about 2.3 times of the

Fourier transform limited sech

pulse width of 2.1ps,

according to the inserted 0.77nm width optical

spectrum. Fig. 4(b) shows a 3GHz RF of the mode-

locked pulses, and this is consistent with the round-

trip repetition rate of 50mm cavity. The RF spectrum

trace is free from sidebands down to the level of -

45dBc, again demonstrating no Q-switching

instabilities in the CW mode-locked SDL.

Figure 5 shows the average output power versus

pump power under 3

C heat sink temperature. The

repetition is 3GHz, and the laser operates in a

circular TEM

mode. The maximum average output

power of 30mW is reached with 1.4W pump power;

the slope efficiency (SE) and the optical-to-optical

conversion efficiency are about 3.1% and 2%,

respectively. Limitations of the output power and SE

are mainly result from the thermal problem. We

soldered the gain chip to a heat-spreader to remove

the thermal and then the output laser could be higher.

Figure 5: Average output power of the mode-locked laser

at 3GHz repetition rate versus pump power at a

temperature of 3

4 CONCLUSIONS

We have demonstrated a compact passively mode-

locked SDL. The CW mode- locking operation at

3GHz repetition rate, 4.9ps pulse duration and

30mW average output power are obtained using a

low-temperature grown SESAM. The laser can be

used as gigahertz picosecond pulse source in many

applications.

HighRepetitionFrequencyMode-lockedSemiconductorDiskLaser

363

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