Mode-locked Thulium-Doped Fiber Lasers

based on Highly Ge-doped Fibers

D. Klimentov, V. V. Dvoyrin and I. T. Sorokina

Department of Physics, Norwegian University of Science and Technology, Høgskoleringen 5, Trondheim, Norway

1 RESEARCH PROBLEM

Thulium-doped ultrashort pulsed fiber lasers (TDFs)

attract increased attention due to their broad gain

spectrum extending from 1.8 to 2.1 μm (Hanna,

1990; Barnes, 1990), possibility to produce high

power and highly efficient tunable femtosecond

pulses (Wu, 2007; Jackson, 2007; Sorokina, 2014) in

the very interesting for many practical applications

wavelength range around 2 μm (Dvoyrin, 2014;

Sorokina, 2014), including eye-safe LIDAR,

medicine, high resolution spectroscopy and remote

sensing (Sorokina, 2014).

In the wavelength region of 2 μm silica-based

fibers typically exhibit anomalous group delay

dispersion (GDD). Therefore, without dispersion

compensation TDF lasers are restricted to soliton

operation, which has been demonstrated with

different mode-locking schemes (Nelson, 1995;

Sharp, 1996; Solodyankin, 2008). Alternatively,

elements for dispersion compensation can be

implemented in the cavity, enabling the laser to

operate in the stretched-pulse regime. This regime is

characterized by breathing dynamics that reduces the

total nonlinear phase shift and allows for relatively

high pulse energies compared to the fundamental

soliton regime, while the pulses can still be

compressed to very short durations outside the

resonator (Nelson, 1997).

Anomalous dispersion regime in the simple

dispersion uncompensated fiber laser leads to the

typical limitation of both pulse energy and duration

to picojoules and picoseconds, respectively.

However, implementation of dispersion

compensation usually requires additional

components, specialty fibers, i.e., holey fibers, or

even bulk elements. This technique leads to the

complexity of the scheme and decreases its

reliability. Such specialty fibers often have fragile or

mechanical splices with conventional silica-based

fibers; the insertion of bulk elements substantially

decreases the laser performance. The Ge-doped Tm-

codoped silica-based fibers with anomalous

dispersion have already shown to be effective as a

laser material (Rudy, 2012). The possibility of high

doping of such fibers with Tm allows improving the

quantum efficiency (Rudy, 2012) and decreasing the

length of the active fiber. Highly Ge-doped silica-

based fibers exhibit normal dispersion in the 2 μm

wavelength region. Such fibers naturally allow

constructing an all-fiber laser with normal cavity

dispersion. Excellent mechanical properties and

close relation to the conventional silica-based fibers

allow good reliable solid splices of the highly

germanium-doped fibers with the conventional ones.

The final construction is simpler and allows

producing femtosecond pulses as it will be revealed

in what follows. This makes such fibers promising

for producing of particularly short, power scalable

mode-locked pulses. The first works, on producing

stable continuous-wave lasing were reported in

(Dvoyrin, 2011; Dvoyrin, 2010).

2 OUTLINE OF OBJECTIVES

In this work, we present our first results and the

perspectives of the development of the Tm-doped

fiber laser based on active fibers of normal

dispersion. We propose the first femtosecond

SESAM mode-locked all-fiber laser based on the

highly Ge-doped thulium-doped normal dispersion

active fiber. In this article we focus our attention on

three particular configurations: laser operating in the

anomalous cavity dispersion regime, laser operating

in the regime of nearly zero cavity dispersion and

laser operating in the normal cavity dispersion

regime without the use of the additional dispersion

compensating elements. The pulses were amplified

by a compact TDF MOPA laser system.

For the nearly zero and normal cavity dispersion

regimes the femtosecond pulses with several

nanojoule energy were obtained at the wavelength of

around 1.88 nm. The pulses were compressed down

to 800 fs using a simple fiber compressor

represented by a piece of a conventional fiber.

Klimentov D., Dvoyrin V. and Sorokina I..

Mode-locked Thulium-Doped Fiber Lasers based on Highly Ge-doped Fibers.

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

3 STATE OF THE ART

There are several demonstrations of passively mode-

locked TDF lasers, using various mechanisms of

mode-locking. These demonstrations include

nonlinear polarization rotation (NPR) (Sharp, 1996;

Wang, 2010), semiconductor saturable absorber

mirrors (SESAM) (Sharp, 1996), carbon nanotube

based saturable absorbers (Solodyankin, 2008; Kieu,

2009) and graphene based saturable absorber

(Zhang, 2012). For instance, Nelson et al.

demonstrated NPR-based mode-locked thulium fiber

laser with 500 fs pulses generation (Nelson, 1995).

In another work, Sharp et al. used a semiconductor

SESAM in a TDF laser to achieve 190 fs pulses

(Sharp, 1996). More recently, Engelbrecht et al.

reported a laser with grating-based dispersion

compensation and double-clad TDF. The laser

operated in the stretched pulse regime (Engelbrecht,

2008) with pulse energy as high as 4.3 nJ and de-

chirped pulse duration around 300 fs. Mode-locked

operation of dispersion-compensated TDF lasers has

also been successfully demonstrated (Solodyankin,

2008; Haxsen, 2008).

4 METHODOLOGY

A mode-locked TDF laser has been built using the

all-fiber linear cavity configuration. The TDF was

produced by the modified chemical vapor deposition

method. It had 55% mol.% concentration of GeO

the core (Dvoyrin, 2011; Dvoyrin, 2010; Dvoyrin,

2011). The schematic of the laser and the amplifier

are shown in Fig.1.

The cavity was formed with a SESAM saturable

absorber, a fiber coupler used for launching pump

radiation into the cavity, a piece of 3 m length of the

TDF with a highly Ge-doped core of 3μm diameter

and NA of 0.49, a fiber loop mirror with the output

coupling of 5%, and a piece of the conventional

telecommunication fiber (SMF-28). Excellent

mechanical properties and close relation to the

conventional silica-based fibers allow good reliable

solid splices of the TDF with the conventional ones.

The measured splice loses were found to be 1.5 dB

per splice. The length of the SMF-28 fiber was

varied in order to investigate the laser performance

in the wide range of the cavity dispersion. It is worth

noting that all the components used in the fiber laser

had fiber outputs based on the SMF-28 fiber.

The amplifier was based on a piece of 14 m

length of the same TDF. The laser was pumped by

one or two laser diodes (Princeton Lightwave Inc.)

operating at the wavelength of 1550 nm and 1630

nm, depending on the configuration, while for the

pumping of the amplifier we used an Er-doped fiber

laser operating at the wavelength of 1.61 μm (IPG

Photonics Inc.)

Figure 1: The schematic of the TDF laser (red frame) and amplifier (blue frame).

Mode-lockedThulium-DopedFiberLasersbasedonHighlyGe-dopedFibers

5 STAGE OF THE RESEARCH

5.1 Experimental Results

5.1.1 Anomalous and Nearly Zero Cavity

Dispersion Regimes

The maximum output average power for both

anomalous dispersion configurations of the laser

oscillator was 1.9 mW with 0.7% slope efficiency.

The laser emission wavelength was centered around

1.88 μm. The dispersion of the fibers is of 35±1 and

-15±1 ps/nm*km for the SMS-28 fiber and for the

TDF, respectively, at the laser emission wavelength

(Klimentov, 2012). In our experiments the cavity

dispersion varied in the range of 0.002-0.404 ps/nm

when changing the SMF-28 fiber length from 1.35 to

12.8 m. For our particular configurations: laser

operating in the anomalous cavity dispersion regime

and laser operating in the regime of nearly zero

cavity dispersion, the cavity dispersion was

0.148±0.005 and 0.002±0.001 ps/nm, respectively.

The corresponding pulse energies inside the laser

cavity were found to be 1.6 and 3.2 nJ. These energy

values noticeably exceed the values obtained in

frames of the soliton theory, which can be estimated

as 0.11 and 2.45 nJ, but may be explained by the

stretched-pulse or “dispersion management” regime

(Tamura, 1995; Tamura, 1993), where the pulses

travel through the fiber spans of alternating

dispersion and experience stretching and

recompression in every resonator round trip. As the

average pulse duration in the resonator can be

increased, the pulse energy can be increased

accordingly.

After amplification of the pulses we achieved the

average output power of 86 mW at the repetition rate

of 12 MHz corresponding to the pulse energy of 7.3

nJ and 3.7 nJ for the longer and shorter cavity,

respectively, with the amplifier slope efficiency of

3.8%.

The spectrum and autocorrelation trace for the

cavity dispersion of 0.148±0.005 ps/nm are shown in

Fig. 2(a), indicating 1.68-ps pulses with the spectral

full width at half maximum (FWHM) of 2.7 nm. The

length of SMF-28 fiber was 5.5 m and the length of

TDF was 3 m. The time-bandwidth product was 0.43

indicating that the pulses were not transform-limited.

In that regime the laser was self-starting and the

central wavelength was 1879 nm. With the cavity

dispersion decrease the mode-locking threshold

decreased slightly (from 155 to 145 mW of the

pump power); however, the maximum output power

of the laser and the amplifier remained at the same

level as previously, while the emission spectrum was

broadened from 2.7 to 5.2 nm and the pulse duration

reduced from 1.68 ps to 900 fs.

1870 1875 1880 1885 1890

-2

-10-8-6-4-20246810

0,010

0,011

0,012

0,013

0,014

0,015

0,016

Inten sity

Delay, ps



pulse = 1.68 ps FWHM



a/c = 2.6 ps FWHM

Optical intensity, a.u.

Wavelength, nm

FWHM 2.7 nm

(a)

1870 1875 1880 1885 1890 1895

-4 -2 0 2 4

0,014

0,015

0,016

0,017

0,018

0,019

0,020

0,021

Inten sity, a.u.

Delay, ps

 pulse = 900 fs FWHM

 a/c = 1.4 ps FWHM

Optical intensity, a.u.

Wavelenght, nm

FWHM 5.2 nm

(b)

Figure 2: Spectra and autocorrelations for the mode-

locked TDF laser operating in anomalous dispersion

regime (a), and the mode-locked TDF laser with the nearly

zero cavity dispersion (b).

The spectrum and autocorrelation trace for the

nearly zero cavity dispersion of 0.002±0.001 ps/nm

are shown in Fig. 2(b), indicating 900-fs pulses with

the FWHM of 5.2 nm and the repetition rate of 23.5

MHz. In that configuration the laser produced

transform-limited pulses with the sech

pulse shape

and the time-bandwidth product of 0.315. For this

configuration the length of SMF-28 fiber was 1.35 m

and the length of the TDF fiber was 3 m.

The key feature of this work is the use of the

highly Ge-doped active fiber with normal dispersion

at 2 µm. Passive highly Ge-doped fibers can be used

for dispersion compensation of the conventional

silica-based fibers known to have high anomalous

dispersion in the 2 µm spectral region. Same fibers

doped with thulium can significantly reduce the

complexity of a laser scheme allowing achieving the

zero cavity dispersion or even normal dispersion

without the use of separate pieces of an active fiber

PHOTOPTICS2015-DoctoralConsortium

and a dispersion compensation fiber.

5.1.2 Normal Cavity Dispersion Regime

It is known that the highest pulse energies can be

obtained in the normal cavity dispersion regime. So,

we have modified the laser cavity by changing its

length thus changing the cavity dispersion from

anomalous to normal and studied the performance of

the laser.

All the SMF-28 fiber pieces were removed from the

cavity, except the short fiber ends of the couplers

and the fiber loop mirror. In this configuration the

rest of SMF-28 fiber length was 68 cm resulting in

the total cavity dispersion of -0.02±0.001 ps/nm.

The mode-locking threshold in the normal cavity

dispersion regime decreased to 90 from 145 mW.

The laser remained self-starting. The emission

central wavelength was slightly blue-shifted to 1868

nm due to change of the SESAM mirror position

during reconfiguration of the laser.

The radio frequency (RF) spectrum measured

with 200 kHz frequency span and 1 kHz resolution

Figure 3: Measured RF spectrum of mode-locked output:

with a 200 kHz frequency span and 1 kHz resolution

bandwidth (upper); with a 200 MHz frequency span and

100 kHz resolution bandwidth, showing a broad spectrum

of harmonics (lower).

bandwidth is shown in Fig. 3 (upper). The repetition

rate of the laser was 26.78 MHz. A graph at the Fig.

3 (lower) illustrates the RF spectrum recorded in the

200 MHz frequency span and 100 kHz resolution

bandwidth, showing a broad spectrum of harmonics.

No signs of dual-pulsing have been observed.

Since the intracavity pulse energy was not

limited in this regime by the fundamental soliton

energy, we increased the pump power of the laser by

adding the second pump diode. The average

maximum output power of the laser reached 2.3 mW

100 200 300 400 500 600

0,0

0,5

1,0

1,5

2,0

2,5

0,5%

Otput power, mW

Launched pump power, mW

Equation

y = a + b*x

Weight

No Weighting

Residual Sum

of Squares

0,09784

Pearson's r

0,9919

Adj. R-Square

0,98263

Value Standard Error

Intercept -0,05398 0,05626

Slope 0,00467 1,65939E-4

Figure 4: The laser slope efficiency.

0 500 1000 1500 2000

(a)

3,3%

Otput power, mW

Launched pump power, mW

Equation

y = a + b*x

Weight

No Weighting

Residual Sum

of Squares

650,64605

Pearson's r

0,97866

Adj. R-Square

0,95666

Value Standard Error

Intercept 5,60962 1,33345

Slope 0,03328 0,00113

1850 1860 1870 1880 1890

(b)

Optical intensity, a.u.

Wavelenght, nm

2.4 mW

45.5 mW

54.8 mW

58.6 mW

61.5 mW

64.5 mW

Figure 5: The amplifier slope efficiency (a) and the

evolution of the spectrum after amplification (b).

Mode-lockedThulium-DopedFiberLasersbasedonHighlyGe-dopedFibers

and was limited by the available pump only. The

laser slope efficiency is shown in Fig. 4.

After the amplification of the pulses with the

same amplifier we achieved the average maximum

output power of 70 mW with the repetition rate of

26.8 MHz corresponding to the pulse energy of 2.6

nJ with the amplifier slope efficiency of 3.3%. The

amplifier slope efficiency and the evolution of the

spectrum after amplifier are shown in Fig. 5. As it

can be seen, the amplification did not change the

shape of the spectrum.

The spectrum and autocorrelation trace for the

laser operating in normal dispersion regime with the

cavity dispersion of -0.02±0.001 ps/nm are shown in

Fig. 6, indicating 1.35-ps pulses with the spectral

FWHM of 9 nm.

It is worth noting that the launched pump power

exceeded the specification limit of the input coupler

of the amplifier. To avoid damage we placed it in a

water tank to allow a better heat dissipation. We

believe that the visible bending of the amplifier

efficiency curve is caused by the thermal effects

inside the coupler.

1840 1850 1860 1870 1880 1890 1900 1910

-4-3-2-101234

 pulse = 1.35 ps FWHM

 a/c = 2.09 ps FWHM

Intensity, a.u.

Delay, ps

FWHM 9 nm

Optical intensity, a.u.

Wavelenght, nm

Figure 6: Spectrum and autocorrelation for the mode-

locked Tm-doped laser operating in normal dispersion

regime.

5.1.3 Pulse Compression

The pulses of the TDF laser operating in normal

dispersion regime after amplification were

compressed using the fiber compressor, Fig. 1. The

compressor was formed by the SMF-28 fiber. The

optimal length of fiber for the efficient compression

was found to be 3.2 m. It was found, that elongation

of this length by several meters weakly affected the

pulse duration. The Fig. 7 shows the autocorrelations

of the mode-locked pulses before the compression,

and after the compression with two lengths of the

SMF-28 fiber of 3.2 and 8 m. With the compressor

of the optimal length the initial pulses were

compressed from 1.35 ps down to 800 fs and the

spectrum was broadened from 9 to 10 nm FWHM.

The increase of the compressor length in 3 times,

approximately, did not lead to the sufficient change

in the pulse duration; the pulses from the 8 m

compressor were only slightly longer.

-4-3-2-101234

(a)



pulse = 1.35 ps FWHM



a/c = 2.09 ps FWHM

Intensity, a.u.

Delay, ps

-3 -2 -1 0 1 2 3

(b)

 pulse = 800 fs FWHM

 a/c = 1.24 ps FWHM

Intensity, a.u.

Delay, ps

-3 -2 -1 0 1 2 3

(c)

 pulse = 910 fs FWHM

 a/c = 1.41 ps FWHM

Intensity, a.u.

Delay, ps

Figure 7: The autocorrelations of the mode-locked pulses

from the Tm fiber laser operating in normal dispersion

regime: before the compression (a), the autocorrelation

measured after the compression by 3.2 m compressor (b),

and the compression by the SMF-28 with the length 8 m

(c).

5.2 Discussion

As compared to the conventional approach, we

suggest a simpler scheme with a higher reliability.

No additional dispersion-compensation, e.g. any

PHOTOPTICS2015-DoctoralConsortium

fibers or bulk elements, was used in our laser.

However, it operated in the normal dispersion

regime. It is worth to note, that the operation near

zero cavity dispersion allowed us to produce

femtosecond pulses directly from the oscillator in

contrast to the picosecond laser with essential

anomalous dispersion. Variation of remaining length

of the pieces of the input/output conventional

anomalous dispersion fibers from the components

used in the laser naturally allowed us to carry out a

simple form of the dispersion management resulted,

however, in femtosecond pulse generation. The

compression of the pulse from the normal-dispersion

laser was naturally provided by the laser output

piece of the conventional fiber and allowed us to

achieve the shortest, 800 fs pulses. At the same time

the pulses energy was as high as 2.6 nJ. The whole

system exhibited a much simpler level of complexity

than the conventional fiber lasers with dispersion

compensation. This is a noticeable improvement of

the mode-locked fiber laser construction.

Excellent mechanical properties and close

relation to the conventional silica-based fibers allow

good reliable solid splices of the highly germanium-

doped fibers with the conventional ones. The

measured splice losses were found to be 1.5 dB per

splice. It is worth noting that a standard fusion

splicer program was used. The splice loss can be

reduced in future by modification of the electrical

fusion splicer program or use of fibers with similar

mode diameters.

5.3 Conclusion

We have demonstrated the first femtosecond

SESAM mode-locked TDF laser based on the active

fiber with highly Ge-doped core with the normal

dispersion in the spectral region near 2 µm. The

change of the length of the conventional

telecommunication fiber inside the laser cavity

allowed variation of the total cavity dispersion in the

range of -0.02-0.404 ps/nm. The stable laser

operation was obtained in three different dispersion

regimes: laser operating in the anomalous cavity

dispersion regime, laser operating in the regime of

nearly zero cavity dispersion, and laser operating in

the normal cavity dispersion regime. The lasers

operated at the wavelength of ~1.88 µm. The pulses

from the master oscillator were amplified in the

core-pumped TDF amplifier based on the same

active fiber as the laser.

The amplification resulted in the output power of

86 mW corresponding to the pulse energy of 7.3 and

3.7 nJ for the laser cavity dispersion of 0.148 and

0.002 ps/nm, respectively. In the case corresponding

to the nearly zero cavity dispersion, the laser

produced transform-limited soliton pulses with the

time-bandwidth product of 0.315 and the pulse

duration of 900 fs, with the spectral FWHM of 5.2

nm.

For the laser operating in the normal cavity

dispersion regime the amplification resulted in the

output power of 70 mW corresponding to the pulse

energy of 2.6 nJ. The laser produced pulses with the

pulse duration of 1.35 ps, with the FWHM of 9 nm.

Finally, we could compress the pulses down to

800 fs by using the all-fiber compressor, based on

the SMF-28 fiber.

6 EXPECTED OUTCOME

The laser output power is limited by the available

pump power only; we predict, based on our

calculations, that further optimization resulting in

the efficiency increase will allow us to demonstrate

more than an order of magnitude higher pulse

energies, as there is no fundamental limitations for

the pulse energy in the normal-dispersion laser

except for the thermal load and the material damage

threshold.

Evidently, all the regimes can be improved by

the reducing of the splice losses, by the use of the

highly Ge-doped active fibers with higher

concentration of Tm, and, for the normal dispersion

cavity regime, by applying higher pump power,

which is planned for the near future experiments.

Nevertheless, the present results already demonstrate

the high potential of such fibers for producing of

ultra-short femtosecond nanojoule pulses in the

vicinity of 2 µm directly from a SESAM mode-

locked fiber oscillator.

The work was supported by the NFR projects

FRITEK/191614 and MARTEC project MLR.

The expected outcome of my research is the

reliable stable ultra-short pulsed all-fiber MOPA

system operating in the 2 μm wavelength region, as

well as the power scaling of this MOPA up to 10 W

output power by adding the second fiber amplifier

stage; design and demonstration of a fiber based

supercontinuum source and tests of the developed

laser sources for practical applications, e.g.

medicine, high resolution spectroscopy and remote

sensing.

Mode-lockedThulium-DopedFiberLasersbasedonHighlyGe-dopedFibers

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