# High PowerTm3+-Doped Fiber Lasers Tuned by a Variable Reflective Output Coupler

Authors: Yulong Tang; Yong Yang; Jianqiu Xu
Journal: Research Letters in Optics
 (2008)

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## Abstract

Wide wavelength tuning by a variable reflective output coupler is demonstrated in high-power double-cladTm3+-doped silica fiber lasers diode-pumped at ∼790 nm. Varying the output coupling from 96% to 5%, the laser wavelength is tuned over a range of 106 nm from 1949 to 2055 nm. The output power exceeds 20 W over 90-nm range and the maximum output power is 32 W at 1949 nm for 51-W launched pump power, corresponding to a slope efficiency of ∼70%. Assisted with different fiber lengths, the tuning range is expanded to 240 nm from 1866 to 2107 nm with the output power larger than 10 W.


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## Body

## 1. Introduction

Cladding-pumped Tm3+-doped fiber
lasers operating in the eye-safe ~2 μm spectral region have attracted much
attention in recent years, owing to their wide applications in areas such as remote
sensing and biomedicine [1]. Benefiting from the large surface to volume ratio,
fiber-based laser sources suffer less thermal management problems, and hence
offer the prospect of higher output power and improved beam quality. Direct
pumping of double-clad Tm3+-doped fiber lasers with diode lasers at
~790 nm has achieved output power up to 85 W [2] and 120 W [3] with slope
efficiencies of around 57% with respect to the launched pump power. A slope
efficiency of 74% has also been obtained at 10-Watt level [4]. Due to the well-known
“two-for-one” cross relaxation (CR) energy transfer (3H6, 3H4→3F4, 3F4) process [2], the theoretical quantum efficiency of Tm3+-doped
fiber lasers approaches 200%.A particular attraction of thulium-doped
fiber lasers is the widely tunable region over ~1700–2100 nm due to
their very broad transition linewidth. Many wavelength tuning methods, such as
birefringent tuning plate [5], diffraction grating [6–8], fiber length
[7, 9], and Peltier plate [10] have been used to exploit the tuning capability of
thulium-doped fiber lasers. By using an external cavity containing a
diffraction grating, wavelength tuning ranges of 230 nm (1860–2090 nm) [7] and
250 nm (1723–1973 nm) [8] have
been dem-onstrated at multiwatt levels. The highest output power was around 15 W
over a 140-nm tuning range, and the longest wavelength was not longer than 2090 nm.In this study, we report efficient
operation of Tm3+-doped double-clad silica fiber lasers, pumped by
high-power diode lasers at 790 nm, with the maximum output power over 30 W and
a slope efficiency of ~70% with respect to launched pump power. In addition, by
using a variable reflective mirror (VRM) as the output coupler, the fiber laser
was tuned over a range of 106 nm from 1949 nm to 2055 nm with the output power
over 20 W in the range of 90 nm. Combined with fiber-length-tuning method, the laser
wavelength can be tuned from 1866 to 2107 nm, resulting in a tuning range over
240 nm.


## 2. Experiment and Results

In the
experiment, the double-clad Tm3+-doped silica fiber has a doped core
with the N.A. of 0.20 and diameter of 27.5 μm. High Tm3+ ions doping
concentration of 2.5 wt% is essential to facilitate the CR energy transfer
process. A small portion of Al3+ ions was also doped into the fiber to
suppress the energy transfer upconversion (ETU) processes, which may cause the quenching
of the 3F4 multiplet lifetime. The pure silica inner cladding,
coated with a low-index polymer, has a 400-μm diameter and the N.A. of 0.46. The
hexagonal cross section of the inner clad helps to improve pump absorption. The
absorption coefficient at the pump wavelength (790 nm) is ~2.8 dB/m.Figure1 shows
the experimental setup. High-power LD arrays operating at 790 nm and TM mode were
used as the pump source. The outputs from two LD arrays were polarizedly
combined to form a single pump beam. This pump beam was reshaped by a
microprism stack at first, and then focused into a circular spot using a
cylindrical lens and an aspheric lens. Through a dichroic mirror, the pump
light was launched into the fiber. The launched efficiency was measured through
a 4-cm-long Tm-doped fiber. The largest pump power of 51 W can be launched into
the fiber. The pump end of the fiber was butted directly to the dichroic mirror
with high reflectivity (>99.7%) at 2.0 μm and high transmission (>97%) at 790 nm. Both fiber
ends were cleaved
perpendicularly to the axis and polished carefully. The output coupler was formed by a
VRM or the bare fiber-end facet. The transmission of the VRM can be changed continuously
from 5% to 80% (the reflection R is changed
from ~94.8% to 18.4%) at 2 μm by simply horizontally displacing the VRM with a one-dimensional
stage.
The ends of the fiber
were clamped tightly in water-cooled copper heatsinks, and the remaining fiber
was immersed into water to achieve maximum efficiency. During the experiment, both
cavity mirrors were carefully adjusted with five-dimensional holders.Figure 1
Schematic of the experimental
setup. PP: polarizing plate; MPS: micro-prism stack; CL: cylindrical
lens; AL:
aspheric lens; HT: high transmission; HR: high reflection; VRM:
variable reflective mirror.Laser output power was measured with a thermal power
meter after unabsorbed pump light blocked by a Ge filter. Laser spectra were
measured by an InAs PIN photodiode combined with a midinfrared spectrograph with
a resolution of 0.2 nm.The lasing characteristics obtained
with relative higher output couplings in a 4-m long fiber laser are shown in
Figure2. When the VRM was moved away from the fiber end, and the bare
fiber-end facet was used as the output coupler (T≈96%), the laser
reached threshold at a launched pump power of 5.9 W and produced a maximum
output power of 32 W at 1949 nm for 51-W launched pump power, corresponding to
a slope efficiency of 69% and a quantum efficiency of 170%. The high efficiency
was attributed to high Tm3+-doping concentration, suppression of ETU
with Al3+ ions [4], and efficient fiber cooling. With T = 80% output coupling, a slightly lower
output power of 29.8 W was generated at 1970 nm, and the slope efficiency with
respect to launched pump power was ~65%. When the output coupling decreased to
60%, the output power dropped to 27.4 W at 1994 nm with a slope efficiency of
~58%. In all these cases, the output power increased linearly with the launched
pump power, suggesting that the laser can be power scaled further by increasing
the pump power. The power stability of the laser output, monitored by an InAs PIN
photodiode and a 100 MHz
digital oscilloscope, was less than 1% (RMS) at ~30 W power levels.Figure 2
Laser output power versus
launched pump power with three high output couplings.After carefully optimization the position
of the coupler, the fiber laser was wavelength tuned by simply horizontally
moving the VRM coupler. In this paper, the peak wavelength of the laser
spectrum is taken as the laser wavelength. Figure3 shows the dependence of the laser wavelength on the output coupling. When the
output coupling decreased from ~96% to 5% in the
4-m long fiber laser, the laser wavelength was
tuned from 1949 to 2055 nm with the tuning range of 106 nm. The nearly linear
dependence provides a basic knowledge to choose the wavelength from Tm3+-doped
silica fiber lasers. The phenomenon can be explained by the enhanced
reabsorption of laser in the high-Q cavity. Since the photon lifetime in the
cavity is increased with higher reflective mirrors, the photon travels more
round trips and undergoes more reabsorption before escapes from the cavity.Figure 3
Laser peak wavelength as a
function of output coupling; inset is the laser spectrum obtained with the 4-m
fiber at coupling ofT = 15%.Employing different fiber lengths
from 0.5 m to 10 m, as shown in Figure3, the laser can be tuned from 1866 to
2107 nm. The total tuning range is over 240 nm at above ten-watt levels. This
is the first demonstration of laser wavelength longer than 2100 nm from Tm3+-doped
silica fiber lasers. A typical laser spectrum obtained with the 4-m fiber at coupling
of T = 15% and 16-W output power is
shown as inset in Figure 3. The laser spectra under different couplings and
fiber lengths hold nearly identical features. The spectrum has a bandwidth (FWHM)
of ~15 nm and several lasing peaks. The multipeak spectrum indicates the laser
operated in multiple longitudinal modes.The maximum output power and launched
threshold pump power as functions of the output coupling are shown in Figure4.
When the output coupling decreased from ~96% to 5%, the threshold pump power
reduced almost linearly from 5.9 to 1.0 W, and the maximum output power dropped
from 32 W to 9.0 W. The sharp decreasing of the output power with <15%
output coupling was mainly due to low-output transmission and increased reabsorption
of laser light. Between the output coupling of 20% and 96%, the laser output
power exceeded 20 W over a tuning range of 90 nm from 1949 to 2040 nm (see Figure
3). This presents the potential of Tm3+-doped silica fiber lasers to
generate multiten watts output over a hundred nanometers tuning range.Figure 4
Maximum laser output power and
threshold launched power as functions of the output coupling.


## 3. Conclusion

Pumped by diode lasers at 790 nm, high-power widely tunable Tm3+-doped
silica fiber lasers have been demonstrated. The wavelength tuning has been
achieved by changing the output coupling with a variable reflective mirror. The
fiber laser can be tuned from 1949 to 2055 nm at multiten watts levels. The maximum output power is 32 W at 1949 nm with a
slope efficiency of ~70% relative to the launched pump power. Using different
length of fibers, the wavelength tuning range can be extended over 240 nm from
1866 to 2107 nm. The combination of high power, high efficiency, and wide tunability
of Tm3+-doped fiber lasers provides a great opportunity for
applications of eye-safe lasers.



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*Source: 919403-2008-04-21.xml*