870 mW blue laser emission at 480 nm in a large core thulium doped ZBLAN fiber laser

Laser Physics (Impact Factor: 1.03). 04/2010; 20(4):838-841. DOI: 10.1134/S1054660X10070066
ABSTRACT
We report on high power 870 mW continuous wave stable laser operation at 480 nm in a 1000 ppm wt. Thulium doped multimode
ZBLAN up-conversion fiber laser. The fiber is pumped by a wavelength of 1064 nm generated from diode-pumped Nd:YAG laser.
A threshold of 550 mW and slope efficiency of 14% with respect to the incident pump power has been obtained. The related problem
of photo-degradation associated with formation of color centers is reported. The transparency of the darkened fiber prior
to the lasing operation is restored by circulating 514 nm through the fiber core. The time dependant oscillatory behavior
of the emitted laser is also addressed.

Full-text

Available from: N.M. Al-Hosiny
ISSN 1054660X, Laser Physics, 2010, Vol. 20, No. 4, pp. 838–841.
© Pleiades Publishing, Ltd., 2010.
Original Russian Text © Astro, Ltd., 2010.
838
1
INTRODUCTION
Continuous need for short wavelength light
sources, resulting from a huge range of potential appli
cations, comprising as various disciplines as optical
data storage, all color displays and biomedicine. Visi
ble laser emission based on the principle of intracav
ity frequency doubling at 459 and 456 nm is demon
strated [1, 2], although with a number of limitations,
with regard to, power scaling, thermal problems, and
efficiencies. A coherent blue radiation at 488 nm by
means of intracavity sumfrequency generation of
914 nm in diode pumped solid state laser is also pre
sented [3]. As an alternative rareearth ions doped up
conversion fiber lasers have attracted much attention
due to absence of thermal problems and high conver
sion efficiency. Zellmer and coworkers [4, 5] demon
strated red, green, and blue upconversion lasers in
Pr
+3
and Yb
+3
codoped ZrF
4
–BaF
2
–LaF
3
–AlF
3
NaF (ZBLAN) fiber lasers. Blue laser operation at
480 nm in Tm
+3
doped fiber lasers has been demon
strated already by several groups [6–8]. Paschotta et al.
[9] reported a 230 mW blue upconversion unstable
laser in Tm
+3
doped ZBLAN fiber lasers. Power scaling
of blue (480 nm) laser in Tm
+3
doped fiber lasers has
drawn considerable attention recently. In [10] useful
models of a wavelength 1120 nm pumped Thulium
doped ZBLAN fiber lasers have been established and
enabled the prediction of power scaling of blue laser at
480 nm up to 1 W. However, at 1000 ppm no account
was taken for ion pairing between Thulium ions due to
low dopant concentration 1000 ppm. High dopant
concentration of Tm
+3
ions in ZBLAN fiber laser is the
main cause of formation of color centers and existence
of photodegradation. This process is the main draw
back for power scaling in Tm
+3
doped ZBLAN fiber
1
The article is published in the original.
lasers [9–13]. As presented in [12], the obtained UV
laser at 284 nm was shut down after 14–17 min opera
tion due to photodegradation problem. Power scaling
can be reached with the improvement of the ZBLAN
material through the modulation of the component of
the fiber, introducing other ions into the core of the
fiber to preclude the photodegradation effect [10].
Additionally, fibers with small NA by adjusting the
refractive indices of the fiber core and cladding may
also reduce the photodarkening effect [14]. Usually,
this effect can be removed almost entirely by circulat
ing blue light or wavelength of 514 nm through the
fiber core [15, 16].
In this paper we report on 870 W blue laser emis
sion at 480 nm in a large core area Tm
+3
doped fiber
lasers, which can support, in principle, many trans
verse modes. An advantage of such large core devices
is their potential for high power operation. An addi
tional advantage of large core fibers is their compati
bility with the fiber in local area networks and fiber
sensors, where multimode fibers are sometimes pref
erable. The photocuring of the photodarkening
effect is realized by circulating a wavelength of a
514 nm thought the fiber core. Also, we showed that
the spiky behavior of the emitted laser is at stable fre
quency resulting from relaxation oscillation seeded by
noises in the high dopant concentration.
THE EXPERIMENTAL SETUP
The schematic diagram of the experimental setup is
shown in Fig. 1. A powerful transversally diode
pumped Nd:YAG laser source emitting fundamental
line at 1064 nm was used as a pump source. Nonideal
beam quality of the pump source is adequate for
pumping fiber with large core area. The pump light
was focused into the fiber with a 20
×
0.18 NA IR
FIBER
OPTICS
870mW Blue Laser Emission at 480 nm in a Large Core
ThuliumDoped ZBLAN Fiber Laser
1
R. M. ElAgmy* and N. M. AlHosiny
Department of Physics, University of Taif, PO Box 888, Taif, Saudi Arabia
*email: Redaagmy@yahoo.com
Received October 18, 2009; in final form, October 25, 2009; published online March 5, 2010
Abstract
—We report on high power 870 mW continuous wave stable laser operation at 480 nm in a 1000 ppm wt.
Thulium doped multimode ZBLAN upconversion fiber laser. The fiber is pumped by a wavelength of 1064
nm generated from diodepumped Nd:YAG laser. A threshold of 550 mW and slope efficiency of 14% with
respect to the incident pump power has been obtained. The related problem of photodegradation associated
with formation of color centers is reported. The transparency of the darkened fiber prior to the lasing opera
tion is restored by circulating 514 nm through the fiber core. The time dependant oscillatory behavior of the
emitted laser is also addressed.
DOI:
10.1134/S1054660X10070066
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870mW BLUE LASER EMISSION 839
microscope objective, having 85% calculated trans
mittance at 1.064
μ
m. We used in our experiment
1.5 m long 1000 ppm Tm
+3
doped ZBLAN fiber laser.
The core and the cladding diameters are 40 and
125
μ
m, respectively. The NA is 0.2 that would lead to
Vparameter of 53 at
λ
= 480 nm, this parameter is
typically < 2.405 in the case of single mode fibers. For
large Vparameter, large fraction of power is in core.
The pumping and its coupling can also be done very
efficiently with relatively high power pump source.
Two bulk mirrors were buttcoupled to the cleaved
ends of the fiber to form Fabry–Perot resonator. The
reflectivity of the input mirror was 98% at the 480 nm
and transmittance of 90% at the pump wavelength.
The photocuring wavelength at 514 nm from argon
ion laser was reflected by microscope slide AR coated
for the pump wavelength and injected into the fiber.
The output coupling mirrors was used with 80%
reflectivity at 480 nm and high transmittance (HT) at
1064 nm. The HT of the residual pump wavelength
was chosen to avoid scattering of blue laser and
destroying the fiber end. An index matching gel was
used at both fiber ends to minimize destroying the
fiber ends at high power.
A prism was used to separate the signal at 480 nm
and the residual pump at the output. The reflected and
filtered beam with interference filter (480
±
10 nm) on
the face of the prism is used to monitor the laser output
spectrum and temporal behavior of the emitted blue
Microscope slide AR coated
@ 1064 nm
HR@488 nm
90% T@1064 nm
80%R@488 nm
HT@1064 nm
Tm
+3
doped ZBLAN fiber
Argon laser
λ
@514 nm
Silicon
photodiode
Interference
filter
Residual
pump beam
Signal output
@ 488 nm
Nd:YAG
laser
Digital
oscilloscope
Fig. 1.
Experimental setup for the blue upconversion fiber laser.
1.064
µ
m
ESA
2
1.064
µ
m
ESA
1
Laser transition
@ 480 nm
1.064
µ
m
GSA
3
H
6
3
F
4
3
H
5
3
H
4
3
F
3
3
F
2
1
G
4
20000
15000
10000
5000
0
25000
Energy, cm
1
Fig. 2.
Energy level scheme Tm
+3
:BZLAN. The dashed
arrows represent nonradiative relaxation.
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840
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ELAGMY, ALHOSINY
laser. The prism was replaced by the interference filter
and power meter for blue laser measurements.
EXPERIMENTAL RESULTS AND DISCUSSION
The excitation scheme is shown in Fig. 2. The three
excitation steps are assumed to occur as described in
[12]. Excitation occurs in the first step by the ground
state absorption (GSA)
3
H
6
3
F
4
, two excited state
absorption (ESA) steps ESA
1
3
F
4
3
H
4
and ESA
2
3
H
4
1
G
4
. The three steps are detuned from the opti
mal wavelength by 1156, 260, and 1380 cm
–1
, respec
tively. However, the exploration of ground state
absorption GSA = 3
×
10
–23
cm
2
in [12] would lead to
85% absorption of the coupled pump in 1.5 m long
fiber of core area (1256.64
μ
m
2
). The blue laser transi
tion
1
G
4
3
H
6
is competing with
1
G
4
3
H
5
transi
tion at 780 nm (not shown in Fig. 2). The later has an
emission cross section of 3.5 times higher than blue
laser at 480 nm. However, careful selection of laser res
onators would suppress unfavorite transition at 780
nm.
A plot of the blue laser output power as a function
of pump power is shown in Fig. 3. The measurements
were carried out without by circulation of 514 nm
wavelength through the fiber core. The threshold is
reached at about 0.55 W of incident pump power; the
higher threshold is referring to competition between
gain and reabsorption losses of blue light. A maximum
output power of 870 mW with respect to 5.9 W of inci
dent pump power and 14% slope efficiency is
obtained. The comparatively low slope efficiency of
14% is partly attributed to the higher multimode oper
ation at 480 nm and large core area.
The highest emitted power operated stably over a
period of 1 hour, the fluctuation was in the range of
±
1%. Nonoptimized resonator is attributed to output
power fluctuations. The creation of color centers
affected the laser performance after about 70 min. The
laser then diminishes and shuts down. The laser recov
ered after about 30 min with no hysteresis neither
threshold nor output power. The laser restores its ini
tial conditions of threshold and output power after
allowing 30 mW of 514 nm laser beam circulation
inside the fiber core. The laser was then operates for
about 10 h at a stable level.
Despite the low dopant concentration (1000 ppm)
in our work, the evidence of ion pairing effect is pre
sented in the oscillation of the laser power. Ion paring
is known process in heavily doped fibers, where the
1000
900
800
700
600
500
400
300
200
100
01000
2000 3000 4000 5000 6000
Incident pump power, mW
Output laser at 480 nm, mW
Fig. 3.
Pump power versus output laser at 480 nm.
5
4
3
2
1
0
0 100 200 300 400 500 600 700 800 900 1000 1100
Time,
µ
s
Amplitude, arb. units
Fig. 4.
The spiky behavior of the laser output.
600
500
400
300
200
100
0 0.75 1.50 2.25 3.00 3.75 4.50 5.25 6.00
Incident pump power, W
Frequency, kHz
Fig. 5.
Oscillation frequency versus incident pump power.
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870mW BLUE LASER EMISSION 841
two adjacent ions exchange energy. Ion pairing has to
be considered in solving rate equations for power scal
ing prediction even for 1000 ppm low dopant concen
tration, which ignored in literature. Figure 4 shows
train of spikes, its frequency depending on pump
power. The measured relaxation oscillation frequency
as a function of incident pump power is shown in
Fig. 5. This shows increase in oscillation frequency
with incident pump power. A similar oscillation phe
nomenon has been observed in previous work [17, 18].
It was suggested there that the origin was a mechanism
of ion pairing. Experimental evidence was given also in
[19] confirming the prediction of the ion pair model
for the Er
+3
doped fiber laser.
CONCLUSIONS
In conclusion, we have demonstrated a stable laser
operation at 480 nm in a Tm
+3
doped multimode
ZBLAN fiber. We have obtained a 870 mW output
laser for 5.9 W of incident pump power. This presents
the highest power so far achieved from a fiber laser
operating in the 480 nm regime to the best of our
knowledge. We showed that ion pairing has to be
accounted for even low dopant concentration
1000 ppm.
ACKNOWLEDGMENTS
The author grateful thank to the Taif University
KSA for supporting this research work under award
research grant no. 3984301.
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