Efficient ~ 3 µm dysprosium-doped ZBLAN fibre laser
pumped at ~ 1.1 µm Yb fibre laser
Atalla E. El-Tahera , Yuen H. Tsanga, David J. Binksa, Terence A. Kinga, and
Stuart D. Jacksonb
a
Lasre Photonics Research Group, School of Physics and Astronomy, University of
Manchester, Manchester M13 9PL, UK;
b
Optical Fibre Technology Centre, Australian Photonics CRC, The University of
Sydney, 206 National Innovation Centre, Australian Technology Park, 1430 Eveleigh,
Sydney, Australia
atama@fs2.ph.man.ac.uk
Abstract. Operation of a single-clad Dy3+-doped ZBLAN fibre laser operating around 3µm is
presented. The laser is pumped by a Yb3+-doped silica fibre laser centred at 1088nm. For 45cm
fibre length, an output of 90mW with a slope efficiency of up to ~ 20% with respect to
absorbed pump power was measured. The experimental slope efficiency is four times higher
than the previous demonstration, and was ~ 60% of the Stokes efficiency limit. The efficiency
was improved by using cavity mirrors of reflectivities of 99% and 50%. The laser wavelength
is found to be dependent on the pumping power, shifting towards longer wavelength when the
pump power is increased. Self-pulsation behaviour was observed for all power levels.
1. Introduction
Laser emission in the 3 m region of the spectrum is of particular interest for medical applications
because photons at these energies are strongly absorbed by water [1, 2]. Optical pumped rare earth
doped fibre lasers have advantages of low costs, high efficiency and compactness. Due to the low
transmission loss in ZBLAN glass for the wavelengths >2 µm, it is used as a host material for the rare
earth doped fibre lasers operating beyond 2 µm. There are a number of ionic transitions emitting near
3 µm in rare earth doped ZBLAN, for example the 4I11/2  4I13/2 transition of Er3+ [3,4], the 5I6  5I7
transition of Ho3+ [5,6] and 6H13/2  6H15/2 transition of Dy3+ [7,8]. Dy3+ has shown a wide emission
spectrum in BaYb2F8 host material ranged from ~2.78 m to ~3.55 m [9]. Although lasers at 3 µm
region based on trivalent dysprosium (Dy3+) 6H13/2  6H15/2 transition have been reported in various
crystalline hosts, e.g., Dy3+ in BaYb2F8 host material at 3 µm and 3.4 m [9,10]. However the
optically pump Dy-fibre laser offers higher pump and signal overlap and large surface area to volume
ratio that leads to high output power with higher conversion efficiency.
The energy levels and the absorption spectrum of Dy-ZBLAN are shown in Fig. 1 and Fig. 2. It shows
four pump absorption band: the 6H15/2  6H7/2 , 6F9/2 transition centred at 1.1µm, the 6H15/2  6H9/2 ,
6
F11/2 transition centred at 1.3 m, the 6H15/2  6H11/2 transition centred at 1.7 m and the respective
laser transition 6H15/2  6H13/2 is centred at 2.84 m. Up to date only two pumping routes for a ~ 3 μm
laser using Dy3+ in fibre configuration have been demonstrated. Recently, efficient 6H13/2  6H15/2
~3µm laser emission of Dy3+ has been achieved by using a ~1.3 µm Nd: YAG laser and a slope
efficiency up to 20% with respect to absorbed pump power has been demonstrated [8]. The diode
pumped Yb-fibre laser is a better pump source compared to the ~1.3 µm flashlamp pumped Nd:YAG
laser as the Yb-fibre laser has advantages of high power output, high overall efficiency, high beam
quality, water cooling free and compactness [11].
4I
15/2
4F
9/2
2nd and 3rd
Pump ESA
~0.58 m
6F
3/2
6F
5/2
6F
7/2
6H
5/2
6H , 6F
7/2
9/2
6H , 6F
9/2
11/2
1st Pump ESA
6H
11/2
6H
13/2
Pump at ~ 1.1  m
Laser ~3 m
6H
15/2
Figure 1Energy levels of the Dy3+ion. The pump GSA at 1.1 μm, pump ESA and laser transition are
shown.
Figure 2 The measured absorption spectrum for Dy 3+-doped ZBLAN bulk sample in the range 1-3.5 µm.
The advantages of a fibre laser-pumped-fibre laser system have been previously demonstrated in [1214]. However the previously demonstrated continuous wave 2.9 m Dy-ZBLAN fibre laser using a
1.1 m Yb3+-silica fibre laser pump only showed approximately 5% slope efficiency and relatively
high lasing threshold of ~1.78 W [7]. Here experiments are aimed to enhance the performance of the
~1.1 µm pumped Dy-fibre by optimising the cavity losses. As demonstrated in these development an
efficient cw Dy-fibre laser operating at ~3 µm is able to be achieving with slope efficiency up to 20%.
2. Experimental set-up
975 nm pump
Focusing lens
High reflector
Focusing lenses
Dy – doped fibre
Yb – doped fibre
High
reflector
50 %
reflector
Figure 3 Experimental set-up for CW Dy3+-doped ZBLAN fibre laser pumped at ~ 1.1 µm by a Yb fibre
laser.
A schematic of the experimental set-up is shown in Fig. 3. The pump source for the experiment was a
Yb3+-doped silica double-cladded fibre laser, which was pumped with a high power 975 nm diode
laser, the fibre had 25 µm circular core diameter with a NA of 0.14. The Yb3+-doped silica fibre laser
resonator was formed by a flat dichroic input mirror (>99% reflectance between 1050 nm and 1150
nm, 93% transmittance at 975nm) butted to the pump end of the fibre and ~4% Fresnel reflection from
another perpendicular cleaved fibre end. This Yb3+-doped silica laser delivers a maximum available
incident pump power of 9 W in the range 1080 to1100 nm. The ~1.1 µm output from the Yb-fibre
laser was collimated and focused with a pair of microscope objective lenses (×10, numerical aperture
0.25). The launch efficient to the Dy3+-ZBLAN fibre was up to 21%. The Dy3+-ZBLAN fibre had a
Dy3+ concentration of 1000 ppm (or 1.7×1025 m-3), a core diameter of 15 µm, and a NA of 0.13. The
fibre supported single transverse mode operation down to 2.55 µm. The fibre had a measured intrinsic
loss of 20 dB/km at 2.1 µm. The Dy3+-ZBLAN fibre laser cavity was formed by two flat reflectors.
The perpendicularly cleaved fibre ends were placed directly against the dichroic mirrors. The pump
end mirror had ~99% reflection at ~3 µm and output couplers with various reflectivities were applied
to the fibre output end to find the optimum cavity coupling. This included output couplers reflectivities
of 50%, 70% and 95% at ~3μm. A Ge filter, which had very low transmission at wavelengths shorter
than 1.8µm and transmitted about 57% at wavelengths longer than 2µm, was used to block out the
transmitted pump power. The output from the Dy3+-doped ZBLAN fibre laser was collimated with a
CaF2 lens into a thermo-electric power meter or a liquid nitrogen cooled InSb photo-detector. The
spectra were measured by a computer controlled auto-scan monochromator. By measuring the
transmitted 1.1µm laser from a silica fibre, core diameter of 17.6µm and a NA of 0.13, the launch
efficiency into this fibre was calculated.
3. Results
The Dy-ZBLAN fibre laser output power as a function of the absorbed pump power is shown in Fig 4
with 50% and 70% reflectivity output couplers. It can be observed that an unsaturated output power of
90 mW is measured for a total absorbed pump power of 0.6 W by using the 50% output coupler. The
threshold absorbed power for this fibre laser arrangement was 0.3 W. The ~20% slope efficiency of
this fibre laser with respect to the absorbed pump power is 60% of the Stokes efficiency limit of
~37%. The laser performance with respect to the absorbed pump power for different output couplers
are given in Table 1.
Output power (W)
0.1
70% output coupler
0.08
50% output coupler
0.06
0.04
0.02
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
Absorbed pump power (W)
Figure 4 Dy- ZBLAN laser output power as a function of the absorbed pump power for a fibre laser
cavity 50% and 70% output coupler reflectivies.
Output coupler reflectivity (%)
Threshold (mW)
Slope efficiency (%)
50
70
95
~290
~250
< 150
~20
~7
<2
Table 1: The performance of the Dy-ZBLAN fibre lasers with respect to absorbed pump power for
different output coupler reflectivities.
20000
Intensity (a. u.)
20000
(a)
18000
16000
16000
14000
14000
12000
12000
10000
10000
8000
8000
6000
6000
4000
4000
2000
2000
0
0
2750
(b)
18000
2800
2850
2900
2950
3000
3050
3100
2750
2800
2850
2900
2950
3000
3050
3100
Wavelength (nm)
Figure 5. Measured spectrum of Dy3+-doped ZBLAN laser output for a fibre laser cavity that
incorporated 50% mirror output coupling, for launched pump power (a) = 0.4 W, (b) = 1 W.
The emission spectrum of the Dy-ZBLAN fibre laser is shown in Fig. 5. The optical spectra
of the laser output covered the range from 2.88 µm to ~3.00 µm. The measured spectrum was
time dependent. The use of a wavelength selective element, prism or grating, inside the cavity
would be useful in narrowing the output spectrum and making it more stable. The spectral
range increases with the incident power. Increasing from (2890 - 2960 nm) with 0.4 W
launched pump power to (2840 – 3020 nm) at 1 W launched pump power as evident in Fig. 5
shown. The output spectrum is also output reflectivity dependent. The lasing wavelength with the
Intensity (a. u.)
70% reflectivity output coupler has a central wavelength ~25 nm longer than that of the 50 % output
coupler for the same pumping power. The spectrum is shifted to longer wavelengths due to the higher
reabsorption loss at short wavelengths in the low loss cavity.
0.08
0.06
0.04
0.02
0
0
20
40
60
80
100
Time (µs)
Figure 6. Typical output temporal behaviour of the 0.46 cm Dy 3+-doped ZBLAN fibre for a fibre laser
cavity that incorporated 50% mirror for 0.6 W launched pump power and generating an average power
of 45 mW.
The temporal behaviour of the output from the cavity 99%-50% has also been investigated as shown in
Fig. 6. As for other rare earth doped fibre lasers the output power from the Dy-ZBLAN fibre laser
exhibited a high degree of self-pulsation. It is suggested that the self-pulsation are due to saturable
absorber effects caused by the under-pumped section near the distal end of the fibre in the three-level
fibre laser system [15]. This self-pulsation effect can be reduced or even eliminated by reducing cavity
loss or pumping rates, so that a another words, more stable output can be achieved with higher
pumping power or with higher reflectivity output couplers.
4. Discussion
The slope efficiency with respect to the absorbed pump power of the three-level system is dependent
on several efficiency factors, the pump absorption efficiency, the quantum defect, the quantum
efficiency of the gain medium and the output coupling efficiency of the laser cavity. Although the
slope efficiency is dependent on the output coupler reflectivity, a certain value of the output coupler
reflectivity will optimize the overall power efficiency of a laser. Generally the high gain fibre laser,
e.g. the quasi-four-level Yb-fibre laser, will have lower optimal output coupler reflectivity, typically
~5%. In contrast, relatively low gain fibre laser, e.g. three-level system Tm3+ [12] or Dy3+ [7] doped
fibre lasers will have higher optimal output coupler reflectivity, typically greater than 50%. The
quantum efficiency depends on the gain medium and the possible electronic transitions for the laser
ions. The quantum efficiency is defined as the fraction of the absorbed pump photons which contribute
to the lasing transition 6H13/2 - 6H15/2. Some undesirable pump ESA will introduce losses inside the
cavity and reduce the quantum efficiency and the overall slope efficiency. The coincidence between
the pump energy and energy difference between (6H13/2 - 6F5/2) and (6F5/2 - 4F9/2) produces pump ESA
processes and emission of a yellow light at a wavelength of ~575 nm, as shown in Fig. 1. The
observation of a yellow fluorescence is due to the transition between 4F9/2  6H13/2 energy levels in the
Dy-ZBLAN fibre with cavity reflectivities of (99%-~5%) when strongly pumped by 1.1 m laser, this
indicates that excitation to the 4F9/2 multiplet exists [7]. It shows the Dy ions undergo strong pump
ESA in this cavity, these transitions cause reduction of both the population inversion and the amount
of useful pump light and leads to lower laser performance. However a higher output coupler
reflectivity can be used to reduce pump ESA and gives higher efficiency [8]. The use of a high
reflection output coupler results in a higher density of stimulated photons at ~3 m. In this case the
Dy3+ ions in the 6H13/2 excited state will prefer to de-excite to the 6H15/2 state by emitting a ~3 m
photon. Therefore further excitation to 1st pump ESA by absorbing another pump photon can be
suppressed under higher flux of stimulating photons ~3 m. The slope efficiency can there be
enhanced by reducing ESA losses in the low loss cavity. From the measured absorption spectrum of
the Dy-ZBLAN fibre shown in Fig. 2, the absorption cross section for the pump wavelengths ~1.1 µm
is nearly equal to the absorption cross section for laser wavelengths ~2.9 µm, this leads to large
ground state absorption at 2.9 µm and increases the power required to reach threshold. Then laser
threshold of the Dy3+ fibre laser is generally high compared with four-level lasers. However, the pump
threshold can be reduced by increasing the output coupler reflectivity as shown in Table 1. The lasing
threshold reduces as cavity loss reduces.
5. CONCLUSIONS
A continuous wave Dy3+-doped ZBLAN fibre laser operating at ~3m has been demonstrated using an
Yb3+-doped silica double-cladded fibre laser operating at ~1.1 m as the pump source. The laser
performance achieved with laser cavity reflectivities of (99%-50%) showed a threshold of 290 mW
and a slope efficiency of ~20% with respect to the absorbed pump power. The current slope efficiency
is ~60% of its Stokes efficiency limit. The maximum output power achieved is about 90 mW. The
improvement in the slope efficiency and threshold is attributed to optimization of the output coupling
and leads to reduced levels of pump ESA.
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