
Spectral analysis of short-wavelength emission
by up-conversion in a Tm
3+
:ZBLAN
dual-diode-pumped optical fiber
M. Juárez-Hernández
1,2,
* and E. B. Mejía
1
1
Centro de Investigaciones en Óptica, A.C., 37150 Guanajuato, México
2
División de Ciencias e Ingenierías, Universidad de Guanajuato, 37150 Guanajuato, México
*Corresponding author: mjuarezh@fisica.ugto.mx
Received January 17, 2020; accepted April 3, 2020; posted online May 26, 2020
The fluorescence evolution along Tm
3+
-doped ZrF
4
–BaF
2
–LaF
3
–AlF
3
–NaF (ZBLAN) optical fibers, as well as
amplified spontaneous emission in the UV-IR region with emphasis on 350 nm, 365 nm, and 450 nm, is studied,
estimating optimal fiber lengths for amplification within the region. The fibers were diode-pumped with single
and double lines (687 and/or 645 nm). Double-line pumping presents a quite superior efficiency for producing
UV-blue signals with the benefit of requiring very short fibers, around 20 cm, compared to single-line pumping
requiring more than 50 cm. A virtual cycle in which the pumps enhance each other’s absorption is the key to these
systems.
Keywords: nonlinear optics; fiber; up-conversion; fluorescence and luminescence.
doi: 10.3788/COL202018.071901.
The need for obtaining short-wavelength operation sources
is constantly increasing. These types of light sources are
already highly used in an ample range of disciplines and
applications such as interferometry, laser microscopy, bio-
medicine, photolithography, optical data storage, pump
sources, laser projection displays, among others. As a gen-
eral rule, shorter-wavelength sources focus on smaller spots
and resolve finer structures in imaging applications.
These sources are often obtained from frequency
doubling or tripling lasers emitting around 600–1100 nm.
The most common lasers used for this purpose are
Nd∶Y
3
Al
5
O
12
(Nd:YAG) (946 nm to obtain 473 nm
and 1064 nm to get 355 nm)
[1]
,Nd∶YVO
4
(914 nm to
obtain 457 nm and 1064 nm to obtain 532 nm)
[2]
,
Nd∶GdAl
3
ðBO
3
Þ
4
(Nd:GAB) (1062 nm to get 531 nm)
[3]
,
and Nd∶YAlO
3
(930 nm to get 465 nm)
[4]
. Nevertheless,
these kinds of lasers are usually bulky and hence usually
require external cooling systems. Their short absorption
lengths translate into low efficiencies. All of these qualities
together make them expensive with relatively complicated
operation and maintenance.
As an alternative to the bulk solid-state laser systems,
the up-conversion fiber lasers doped with rare-earth ions
are excellent candidates. They have many properties that
make them suitable for generating short wavelengths in
the visible (VIS) and the ultraviolet (UV) regions. At
moderated powers (up to some watts), they do not usually
present thermal issues and have superior conversion effi-
ciencies. Among the rare-earth ions, thulium (Tm
3þ
)asa
dopant in ZrF
4
–BaF
2
–LaF
3
–AIF
3
–NaF (ZBLAN) glass is
capable to convert low-energy infrared (IR) photons into
high energy photons (in the VIS and UV regions); the
reason is that fluorozirconate glasses based on ZBLAN
have more metastable levels compared to silica glasses,
expanding in this way the number of radiative transitions
for most rare-earth dop ants.
Since the first report of VIS up-conversion lasing in fluo-
ride glass fibers
[5,6]
, short-wavelength laser operation in
Tm
3þ
:ZBLAN has been demonstrated by many groups
in the past, usually obtaining 480 nm pumped by IR laser
sources (generally at 1064 nm with Nd:YAG lasers)
[7–10]
.
Also, wavelengths in the UV region (284 nm, 293 nm,
351 nm, and 360 nm) have been achieved by pumping with
solid-state lasers such as Nd:YAG at 1064 nm
[11]
, with an
Ar
þ
laser at 485 nm and a dye laser at 585 nm
[12]
. However,
these systems often require absorption of three photons,
which often have a tradeoff between ground-state absorp-
tion (GSA) and excited-state absorption (ESA) in addition
to having small absorption [less than 5 dB/(km·ppm)
(ppm, parts per million) at 1064 nm and 10 dB/(km·ppm)
for 485 nm]. They also present a factor that may reduce the
efficiency of their systems; when being pumped with IR
sources, it may produce a photodarkening effect
[13,14]
.
An alternative arrangement for obtaining short-
wavelength emission in Tm
3
:ZBLAN is pumping with a
dual-wavelength configuration
[15,16]
. Since the strongest
absorption band of thulium-doped fluorozirconate is
centered at around 690 nm (
3
H
6
→
3
F
3
transition), we
proposed to use this wavelength as GSA
[17]
. Using this
wavelength, we can expect an optimal absorption
[≥35 dB∕ðkm·ppmÞ]
[18]
; this represents a factor of
3.5 to 7 better than that of IR and some other VIS
(485 nm or 585 nm) pump sources. Additionally, using
∼645 nm (
3
F
4
→
1
D
2
) as ESA, it is then possible to
obtain a variety of radiative transitions in the UV
(360 nm from
1
D
2
→
3
H
6
) and in the blue-VIS region
(450nm from
1
D
2
→
3
H
4
and 475 nm from
1
G
4
→
3
H
6
).
An additional advantage is that these kinds of pump
COL 18(7), 071901(2020) CHINESE OPTICS LETTERS July 2020
1671-7694/2020/071901(5) 071901-1 © 2020 Chinese Optics Letters