All-Fiber Tm-Doped Frequency Shifted Feedback Laser
He Chen, Shengping Chen, Zongfu Jiang and Jing Hou
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China
Keywords: Fibre Laser, Mode-locked Lasers, Ultrafast Lasers.
Abstract: Ultra-simple all-fiberized 2 μm Tm-doped frequency shifted feedback mode-locked fiber lasers are
demonstrated. The self-starting mode-locking is initiated by an intra-cavity acousto-optical frequency
shifter. Versatile mode-locking pulse dynamics were observed by altering the pump power, cavity structure
and intra-cavity polarization state, including picosecond single pulse sequence, pulse bundle state and
nanosecond rectangular pulse. As short as 8 ps stable pulses and up to 12 nJ, 3 ns rectangular pulses were
obtained with the aid of a nonlinear optical loop mirror. Beside the mode-locking operation, flexible Q-
switching and Q-switched mode-locking operation can also be readily achieved in the same cavity. Up to 78
μJ high energy nanosecond pulse can be generated in this regime.
1 INTRODUCTION
In recent years, continuous wave as well as pulsed
fiber lasers operating in the eye-safe 2 μm spectral
region have attracted extensive attention owing to
their numerous potential applications in areas such
as remote sensing, medicine, mid-infrared frequency
conversion, supercontinuum generation, and free-
space communication. Various technologies have
been explored in this region to produce pulses with
diverse characteristics for the applications of
different requirements, such as mode-locking, Q-
switching and gain-switching. Generally, mode-
locking is used to generate fs-ps ultrafast pulse and
Q-switching and gain-switching are exploited to
produce ns-μs high energy pulse. Currently, most
passive mode-locking techniques exploited in the 2
μm region are based on saturable absorbers, which
can be classified into two categories: real saturable
absorbers and artificial saturable absorbers. The
former is based on materials with saturable
absorbing characteristics, such as traditional
SESAM, carbon nanotube, Graphene, and various
emerging low-dimension materials. The later mainly
relies on the nonlinear optical effects, including
nonlinear polarization rotation (NPR) and nonlinear
optical loop mirror (NOLM). Although saturable
absorber can also be utilized to initiate passive Q-
switching operation, the performances and temporal
tunabilities of passively Q-switched lasers are
usually no match for actively Q-switched ones.
As far as we know, this technique has not been
exploited in the 2 μm region yet. Tm-doped fiber
laser can be an ideal platform for the realization of 2
μm FSF mode-locked laser. To thoroughly reveal the
potential of FSF in 2 μm region, in this manuscript,
we report the comprehensive investigation on all-
fiberized 2 μm Tm-doped FSF mode-locked fiber
laser. The characteristics of the laser were explored
in two novel cavity constructions. Versatile mode-
locking pulse dynamics and widely tunable Q-
switching and Q-switched mode-locking operation
were experimentally manifested and
comprehensively analyzed.
2 EXPERIMENTAL SETUP
In the experiment, two structures were adopted to
achieve mode locking based on FSF in 2 μm region.
Both of them adopted the linear Fabry-Perot cavity
structure, which included a fiberized AOM as the
frequency shifter. The difference was that the first
one was formed with a pair of fiber Bragg gratings
(FBGs), while the other one was formed with an
FBG and an NOLM as the cavity mirror and the
additional saturable absorber. The schematic
diagram of the FBG-pair-based structure is shown in
Fig. 1. The FBG pair is centered at 1980 nm. The
HR-FBG has a reflectivity of 99.1% and a 3 dB
bandwidth of 1.3 nm. The LR-FBG has a reflectivity
of 20% and a 3 dB bandwidth of 0.8 nm. A section

of 2-meters-long single mode 9/125 Tm-doped fiber
was employed as the gain medium. The core
absorption coefficient is around 10 dB/m at 1570
nm, and the numerical aperture is 0.15. The pump
light came from a 1.3 W 1570 nm home-made fiber
laser. A 2000/1570 nm WDM was inserted between
the pump source and the HR-FBG to prevent 2 μm
backward light. The AOM frequency shifter was
inserted between the gain fiber and the LR-FBG,
which had frequency shift of 80 MHz and insertion
loss of 2 dB. A matched 80 MHz radio frequency
(RF) driver was employed to activate the AOM. The
length of the passive fiber was carefully tailored to
make the AOM's shift frequency (80 MHz) to be the
integer harmonics of the inverse round-trip of the
laser, which is a critical requirement to achieve
mode-locking operation. In this case, the total cavity
length was tailored to be ~6.45 m, corresponding to
the fundamental cavity frequency of ~16 MHz (1/5
of the AOM's shift frequency). At the wavelength of
1980 nm, the cavity is abnormally dispersive, and
the total cavity dispersion was estimated to be ~-
0.98 ps2 at the wavelength of 1980 nm. It is a
notable feature that this laser is all-fiber-integrated
and compactly constructed, which is favorable in
many practical applications.
Figure 1: The schematic diagram of the FBG-pair-based
FSF mode-locked fiber laser. TDF, thulium-doped fiber;
WDM, wavelength division multiplexer; HR-FBG, high-
reflectivity fiber Bragg grating; LR-FBG, low-reflectivity
fiber Bragg grating; AOM, acousto-optical modulator; RF
driver, radio frequency driver.
A commercial autocorrelator based on 2nd harmonic
generation was used to measure the pulse width of
output pulses with maximum measurable pulse
width of 80 ps. A short Tm-doped fiber amplifier
was employed to amplify the peak power of the
output pulses to reach the measurement criterion of
the autocorrelator. The optical spectrum was
measured by a 1200 nm-2400 nm optical spectrum
analyzer. The temporal characteristics were
monitored with a 9 GHz InGaAs photodetector, a
real-time oscilloscope (1.5 GHz bandwidth, 20
GSa/s sampling rate) and a 6 GHz radio frequency
spectrum analyzer.
3 EXPERIMENTAL RESULTS
AND DISCUSSIONS
Stable single pulse mode locking could be instantly
self-started from the CW regime upon the pump
power was increased to around 200 mW. Single
pulse operation can be maintained until the pump
power was increased above 240 mW. Then the
output pulse split to amplitude-even pulse bundle.
Figure 2 depicts the measured temporal and spectral
characteristics of the FSF mode-locked laser in the
single pulse regime at the pump power of 220 mW.
The output power of 7.4 mW was obtained at this
pump power. Figure 2(a) shows the measured
autocorrelation trace with a full width at half
maximum (FWHM) of 37 ps. If a sech
2
pulse profile
is assumed, the pulse width is 24 ps. The spectrum
of the output pulse is shown in Fig. 2(b), which is
centered at 1980 nm with a 3 dB bandwidth of 0.35
nm. Then the calculated time-bandwidth product of
output pulse is 0.65, which is larger than the
theoretical value for transform-limited sech
2
pulse
indicating a small amount of negative dispersion in
laser pulses. Figure 2(c) plots the measured pulse
train with a time interval of ~62.5 ns between each
pulses corresponding to a pulse repetition rate of 16
MHz, which corresponded to the estimated cavity
round trip. The radio frequency spectrum measured
in the span of 100 MHz (resolution bandwidth 100
Hz) is shown in Fig. 2(d). The signal-to-noise ratio
of the RF spectrum is measured to be as high as 68
dB, indicating a high temporal stability. The
measured autocorrelator trace and the RF spectrum
manifested the single pulse operation. With the 16
MHz repetition rate and 7.4 mW average output
Figure 2: (a) The autocorrelator trace; (b) The optical
spectrum; (c) The oscilloscope trace; (d) The RF spectrum
of the output pulse under the pump power of 220 mW.

power, the pulse energy was calculated to be 0.46
nJ.
As mentioned above, limited by the peak power
clamping effect, the output pulses were transformed
from the single pulse state to pulse bundle state upon
the pump power was increased to around 240 mW.
With the increasing pump power, the number of the
pulses in single pulse bundle increased accordingly,
while the repetition rate of the pulse bundle
remained at the fundamental frequency 16 MHz. Up
to 5-pulse-bundle was formed with the pump power
of 410 mW. In this case, the output average power
reached 48 mW, and the corresponding energy of the
pulse bundle is calculated to be 3 nJ. The inner pulse
width is measured to be ~200 ps, which is limited by
the bandwidth of the oscilloscope. The pulse
intervals between the inner pulses are not quite
stable, dither around 2.2 ns with a timing jitter of ~
200 ps.
The various mode-locking regimes demonstrated
above were based on the frequency shifting function
of the AOM, where the modulator was not actually
modulated. When the AOM is employed as the intra-
cavity intensity modulator to switch the Q-factor of
the cavity (i.e. the cavity loss), flexible Q-switching
and Q-switched mode-locking operation can also be
facilely achieved. In this case, the FBG-pair-based
laser cavity was adopted and we used an electrical
pulse generator to modulate the power of the RF
driver which is linearly proportional to the insertion
loss of the AOM. The repetition rate and duty cycle
(laser on time/laser off time) of the modulation
signal can be tuned in wide ranges, which will
directly affect the characteristics of the Q-switched
output pulses.
Under the modulation signal of large duty cycle
(typically >50%, dependent on the repetition rate),
the laser can produce Q-switched mode-locked
pulses. Figure 3(a) shows the output pulse
waveforms under the pump power of 1.1 W and the
modulation signal of 100 kHz repetition rate, 60%
duty cycle. The pulse has a Q-switched pulse
envelope with 0.7 μs duration and 100 kHz
repetition rate, and contains a train of ~30 mode-
locked inner-pulses spaced at the laser cavity round-
trip. There are also many noisy minor pulses filled in
the spacing between the mode-locked inner-pulses.
This phenomenon was also observed in Yb-doped
FSF fiber laser (Heidt et al., 2007), as well as
common Q-switched fiber lasers and gain-switched
fiber lasers (Swiderski and Michalska, 2013, Eckerle
et al., 2012). We believe this Q-switched mode
locking regime is initiated by the combined effect of
FSF and Q-switching (Heidt et al., 2007).
Figure 3: The oscilloscope traces of the output pulse
waveforms (a) in Q-switched mode locking regime and (b)
short pulse Q-switched regime.
The mode-locking sub-pulses can be eliminated by
decreasing the duty cycle of the modulation signal
until the laser-on time is less than the building time
of the mode-locked pulses. Then the pulse
waveforms become smooth and stable, indicating a
regular Q-switching regime. In this regime, the
repetition rate follows the modulation signal, and the
pulse width is related to the repetition rate, the duty
cycle and the pump power. With the repetition rate
fixed at 1 kHz and the pump power at 1.1 W, the
shortest pulse width 35 ns can be obtained with the
duty cycle set at 1%, as depicted in Fig. 6(b). In this
case, the output power reached 36 mW. The pulse
energy was 36 μJ, and the peak power was nearly 1
kW. The highest pulse energy 78 μJ can be obtained
under the duty cycle of 50%, and the corresponding
pulse width was 47 ns, and the peak power was 1.65
kW. Under the highest pump power of 1.1 W, with
the flexible adjustment of the modulation repetition
rate and duty cycle, the repetition rate of the Q-
switched pulses can be adjusted in a wide range
(from 1 kHz to >500 kHz) and the pulse duration
can be tuned from 35 ns to 770 ns.
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