Short Pulse Generation in Erbium-Doped Fiber Lasers Using

Graphene Oxide as a Saturable Absorber

Catarina S. Monteiro

1,2 a

, Rosa P. Herrera

3,4 b

, Susana Silva

and Orlando Frazão

INESC TEC—Institute for Systems and Computer Engineering, Technology and Science,

Rua do Campo Alegre 687, 4150-179 Porto, Portugal

Department of Engineering Physics, Faculty of Engineering, University of Porto,

Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

Department of Electrical, Electronic and Communication Engineering,

Public University of Navarra, 31006 Pamplona, Spain

Institute of Smart Cities (ISC), Public University of Navarra, 31006 Pamplona, Spain

Keywords: Fiber Laser, Graphene Oxide, Saturable Absorption.

Abstract: The use of graphene oxide (GO) as a saturable absorber for short pulses generation in an Erbium-doped fiber

laser was studied and demonstrated. The saturable absorber consisted of a thin GO film, with a high

concentration of monolayer GO flakes, spray-coated on the end face of a ferrule-connected fiber. By including

the saturable absorber in the laser cavity and controlling the intra-cavity polarization, the generation of short-

pulsed light was achieved under mode-locking and Q-switching operations. Under mode-locking operation,

it was observed a pulse train with a fundamental repetition rate of 1.48 MHz, with a working wavelength

centered at 1564.4 nm. In the Q-switch operation, a pulse train with a 12.7 kHz repetition rate and a 14.3 µs

pulse duration was attained for a 230-mA pump current. Further investigation showed a linear dependence of

the repetition rate with the pump power, attaining frequencies between 12.7 and 14.4 kHz.

1 INTRODUCTION

Ultrafast fiber lasers are capable of delivering pulses

in the order of pico- or femtoseconds in a compact

and align-free structure (Cheng et al., 2020), making

these devices a powerful tool for applications in

nonlinear optics (M. Liu et al., 2020), precision

metrology (Oh & Kim, 2005), industrial applications

(Wang et al., 2022), and medical treatments (Hoy et

al., 2014), for example. The most typical approach to

producing short and ultrashort pulses is through Q-

switching and mode-locking. Q-switching and mode-

locking can be achieved passively, using a saturable

absorber, creating a fiber laser with a more compact

and simpler design (Ahmad et al., 2016). To passively

initiate the generation of short and ultrashort pulses,

different saturable absorber materials and methods

have been explored such as the well-established

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conductor saturable absorber mirrors (SESAMs)

(Keller et al., 1996), the nonlinear polarization

rotation technique (X. M. Liu & Mao, 2010), or using

carbon nanotubes (Set et al., 2004), for example.

More recently, graphene has attracted much

attention due to its unique optical and electrical

properties such as high carrier mobility, low

saturation power, and broadband wavelength

tunability (Steinberg et al., 2018). Graphene can be

synthesized through chemical vapor deposition or

mechanical exfoliation, yielding graphene samples

with different physical, optical, and electrical

properties. The two techniques present different

advantages and disadvantages, but both lack

scalability for mass production. Graphene oxide and

reduced graphene oxide exhibit comparable

properties with faster synthesis methods. In particular,

graphene oxide exhibits saturable absorption, which

Monteiro, C., Herrera, R., Silva, S. and Frazão, O.

Short Pulse Generation in Erbium-Doped Fiber Lasers Using Graphene Oxide as a Saturable Absorber.

DOI: 10.5220/0011700400003408

In Proceedings of the 11th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2023), pages 78-81

ISBN: 978-989-758-632-3; ISSN: 2184-4364

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

makes it suitable for ultrafast pulse generation using

mode-locking (Zhao et al., 2011). Femtosecond pulse

generation has already been demonstrated using GO

as a saturable absorber (Huang et al., 2014; Ko et al.,

2017; J. Liu et al., 2012; Xu et al., 2012; Yap et al.,

2022; Zhou et al., 2012).

This communication demonstrates Q-switching

and mode-locking operation using a GO-based

saturable absorber. The thin GO film was deposited

on the end face of a fiber using spray coating and was

placed between two fiber ferrules. The GO thin film

was added to the erbium-doped fiber laser cavity and,

by controlling the intra-cavity dispersion and

polarization state, mode-locking and Q-switching

were achieved. The mode-locking operation was

studied in the wavelength and temporal domain,

attaining a fundamental repetition rate of 1.48 MHz.

In the Q-switching regime, a pump power-dependent

pulse train was attained with a repetition rate between

12.7 and 14.4 kHz.

2 EXPERIMENTAL

CONFIGURATION

The experimental configuration was composed of an

erbium-doped fiber amplifier (EDFA), schematically

demonstrated in the grey box in Figure 1. The EDFA

(IPG Laser HBM, model EAD-1K-C3-W) is capable

of outputting a maximum power of 1 W. The isolator

of the EDFA guarantees unilateral light propagation,

and the polarization controller (Thorlabs FPC030)

can adjust the traveling light’s polarization state. The

output coupler, with a 5% output ratio, enables to

monitor the extracted light. The output spectrum and

pulse train were attained using an optical spectrum

analyzer (OSA – Advantest Q8384) and

photodetector (Thorlabs PDA10CS-EC) connected to

a 2GHz sampling rate oscilloscope (GWinstek

GDS-2304A), respectively. A single-mode fiber

(SMF) coil of 100 m was added to the laser ring

cavity to control the dispersion and total cavity length.

The total length of the ring was measured using an

OTDR, attaining an approximated value of 130 m.

The GO film serving as a saturable absorber was

deposited on the end face of a fiber terminated with

an FC/PC connector using spray coating. The GO

solution was acquired from Graphene, in a water

suspension with a concentration of 4 mg/mL with a

monolayer content greater than 95%. Prior to spray

coating, the GO solution was sonicated to decrease

the agglomerates of the GO flakes.

Figure 1: Experimental setup of the EDFL based on

graphene oxide saturable absorber.

3 EXPERIMENTAL RESULTS

AND DISCUSSION

3.1 Mode-Locked Operation

In passive mode-locking operation, the saturable

absorber attenuates weak longitudinal modes, while

high-intensity modes are passed through it. This

stimulates other modes with phase-locking, resulting

in the formation of periodic short pulses. To exclude

the possibility of self-mode-locking or self-Q-

switching, the output spectrum of the cavity with no

GO saturable absorber was attained using the OSA.

By varying the pump power and changing the

polarization of the light traveling in the laser cavity

using the polarization controller, no mode-locking

was observed as presented in Figure 2, where only the

laser's peak is observed with a central wavelength of

1564.4 nm with an FWHM of 1.5 nm. Therefore, no

self-start mode-locking operation induced by non-

linear polarization rotation was achieved.

Figure 2: Output spectrum at different pump powers.

1560 1565 157

-60

-50

-40

-30

-20

-10

Optical Power (dBm)

Wavelength (nm)

No SA

200 mW

350 mW

500 mW

Short Pulse Generation in Erbium-Doped Fiber Lasers Using Graphene Oxide as a Saturable Absorber

Figure 3: Output pulse train in mode-locking operation.

By introducing the GO saturable absorber into the

setup, increasing the pump power to a value greater

than 200 mW, and controlling the polarization state,

the mode-locking operation was achieved as shown in

Figure 2, where Kelly’s sidebands start to appear,

which are characteristic of the soliton operation of a

fiber laser (Bao et al., 2009). The acquired pulse train

is presented in Figure 3 with a pulse interval of

approximately 688 ns, corresponding to a

fundamental repetition rate of 1.48 MHz which is in

accordance with the total length of the cavity.

3.2 Q-Switched Operation

In Q-switch operation, the losses of the cavity are

temporarily increased by introducing an attenuator

that prevents laser action. This will allow a population

inversion that far exceeds the case where no

attenuator is present. By rapidly decreasing the

attenuation, the laser will exhibit gain that far

surpasses the losses, leading to the release of the

stored energy as a short and intense light pulse (Svelto,

2010).

For the Q-switch operation, the experimental

setup was adjusted to reduce the number of fiber

connectors. The erbium-doped fiber amplifier was

replaced by an in-house amplifier, with a 4 m erbium-

doped fiber. Moreover, the SMF coil was removed

from the fiber cavity. Adjusting the polarization state,

it was possible to observe the Q-switching operation

on the laser. Figure 4 presents the typical Q-switching

pulse train, achieved for a pump current of 230 mA

with a repetition rate of 12.7 kHz and a pulse duration

of 14.3 µs.

Figure 4: Q-switching operation: pulse train for a pump

power of 230 mA, with a repetition rate of 12.7 kHz.

The evolution of the repetition rate of the

Q-switched pulses with the pump power was studied

for a fixed polarization state. For a pump current

between 230 and 250 mA, the frequency of the

Q-switched pulses varies between 12.7 and 14.4 kHz

as presented in Figure 5.

Figure 5: Evolution of the repetition rate with the increase

of pump current.

4 CONCLUSIONS

In this work, the generation of short pulses in an

erbium-doped fiber laser was studied using GO as

saturable absorber. The saturable absorber was spray-

coated on an end face of a ferrule-connected fiber,

after sonication of the GO solution, avoiding

agglomerates of GO flakes. The saturable absorber

was inserted on the erbium-doped fiber laser cavity,

-4 -2 0 2 4

0.00

0.06

0.12

0.18

0.24

0.30

Intensity (a.u.)

Time (μs)

688 ns

-100 -50 0 50 100

Intensity (a.u.)

Time (μs)

79 μs

230 235 240 245 250

12.74

13.15

13.57

13.98

14.42

Repetition Rate (kHz)

Pump current (mA)

PHOTOPTICS 2023 - 11th International Conference on Photonics, Optics and Laser Technology

resulting in both mode-locking and Q-switching

operation after intra-cavity polarization control.

Under mode-locking operation, a fundamental

repetition rate of 1.48 MHz was attained for pump

power values higher than 200 mW for a central

wavelength of 1564.4 nm. In the Q-switching

operation, short pulses were attained with a repetition

rate between 12.7 and 14.4 kHz. At the lower

repetition rate, a pulse duration of 14.3 µs was

achieved.

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Short Pulse Generation in Erbium-Doped Fiber Lasers Using Graphene Oxide as a Saturable Absorber