PUBLISHED VERSION
Lancaster, David George; Gross, S.; Ebendorff-Heidepriem, Heike; Kuan, K.; Monro, Tanya Mary; Ams, M.;
Fuerbach, A.; Withford, M. J.
Fifty percent internal slope efficiency femtosecond direct-written Tm3+:ZBLAN waveguide laser, Optics Letters,
2011; 36(9):1587-1589.
© 2011 Optical Society of America
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Fifty percent internal slope efficiency femtosecond
direct-written Tm
3
:ZBLAN waveguide laser
D. G. Lancaster,
1,
* S. Gross,
2
H. Ebendorff-Heidepriem,
1
K. Kuan,
1
T. M. Monro,
1
M. Ams,
2
A. Fuerbach,
2
and M. J. Withford
2
1
Institute for Photonics and Advanced Sensing, School of Chemistry & Physics, University of Adelaide,
Adelaide, South Australia, 5005, Australia
2
Centre for Ultrahigh bandwidth Devices for Optical Systems, MQ Photonics Research Centre, Department of
Physics and Astronomy, Macquarie University, North Ryde, New South Wales 2109, Australia
*Corresponding author: david.lancaster@adelaide.edu.au
Received February 18, 2011; revised March 24, 2011; accepted March 25, 2011;
posted March 30, 2011 (Doc. ID 142923); published April 22, 2011
We report a 790 nm pumped, Tm
3þ
doped ZBLAN glass buried waveguide laser that produces 47 mW at 1880 nm, with
a 50% internal slope efficiency and an M
2
of 1.7. The waveguide cladding is defined by two overlapping rings
created by femtosecond direct-writing of the glass, which results in the formation of a tubular depressed-
index-cladding structure, and the laser resonator is defined by external dielectric mirrors. This is, to the best of
our knowledge, the most efficient laser created in a glass host via femtosecond waveguide writing. © 2011 Optical
Society of America
OCIS codes: 140.0140, 130.2755.
Direct writing of waveguides (WGs) in crystals and
glasses is an emerging technology that enables the rapid
fabrication of WG lasers in a range of rare-earth doped
hosts. The advantages of these directly written WGs over
other WG fabrication techniques are the single-step
optical processing and consequent geometrical flexi-
bility. The ultrafast direct-write process uses focused
femtosecond (fs) laser pulses to induce a permanent
refractive index change in dielectric media [1]. WGs
written into bulk laser materials improve laser perfor-
mance over unguided lasers due to the intrinsic pump
and laser mode overlap. The tailored WG modes can give
diffraction limited beam quality and allow convenient
pigtailing to fibre optics.
Heavy-metal fluoride glass is well known for its high IR
transparency, especially in its most common composi-
tion, ZBLAN (ZrF
4
-BaF
2
-LaF
3
-AlF
3
-NaF), thereby making
it an attractive host for mid-IR emitting rare-earth ions
[2]. A previous study on ultrafast direct writing in ZBLAN
glass fibers with a focused low repetition rate (RR) and
high pulse energy fs laser demonstrated fiber Bragg
gratings based on reduced refractive index regions of
Δn ¼ −1 × 10
−3
[3]. Other work using high pulse energies
demonstrated the writing of damage lines that guided
light with increased refractive index [4] and self-
channelled plasma filament WGs [5]. Our initial direct-
write investigations of ZBLAN, using both low RR/high
pulse energy and high RR/low pulse energy fs lasers, re-
sulted in regions of reduced refractive index, consistent
with [3]. This confirmed that a depressed-index-cladding
geometry is required to demonstrate light guidance and
lasing in ZBLAN for our direct-write regime.
The most efficient directly written WG laser in a glass
host reported to date is a 1: 5 μmEr
3þ
,Yb
3þ
codoped
phosphate glass laser achieving a 21% slope efficiency
and power of 50 mW [6]. In YAG, fs laser irradiation
has been shown to induce defects, enabling a depressed
cladding structure to be written, and the first Nd
3þ
laser
reported achieved 170 mW with an 11% internal effi-
ciency [7]. A 75% slope efficiency and 0:8 W power
was recently reported in a stress-induced Yb:YAG
WG [8].
The first reported direct-written 2 μmTm
3þ
doped WG
laser was in a germanate glass host with a direct-write
WG channel pos sessing a positive index change and
demonstrating an incident slope efficiency of <2% [9].
We report what we believe to be the most efficient
(50% slope efficiency) direct-write depressed cladding
WG laser in a glass host. This is also the first report of
a direct-write ZBLAN glass WG laser.
The WGs were fabricated with a commercial ultra-
fast Ti:sapphire oscillator (FEMTOSOURCE XL 500—
Femtolasers GmbH, 800 nm center wavelength, 5:1 MHz
RR, 550 nJ pulse energy, 50 fs pulse duration), which was
focused into the bulk sample using a 1.25 NA 100× oil
immersion objective, while the sample was translated
using a set of computer controlled XYZ air-bearing trans-
lation stages. The combination of high NA focusing and
high RR causes cumulative heating followed by heat dif-
fusion [10 ]. This results in structures of quasi-circular
cross section with diameters of up to 50 μm. The depos-
ited heat causes a change in the glass structure asso-
ciated with a relative drop in n of ∼1:5 × 10
−3
.
The ZBLAN glass is doped with 2:0 mol: % TmF
3
,
(3:72 × 10
26
ions=m
3
of Tm
3þ
) to allow efficient two for
one cross relaxation of the Tm
3þ
ion when pumped at
790 nm. The ZBLAN samples were fabricated in a
controlled atmosphere glass melting facility using 50 g
batch sizes [11]. For this work the WG substrates were
diced using a CNC diamond saw into chips measuring
9 mm long, 8 mm wide, and 2 mm high. The top face of
each sample was polished to optical grade, thereby
allowing the ultrafast direct-write laser to be focussed
through this surface. Each chip was inscribed by the
fs laser with up to 42 WGs at a depth of 150 μm. After
WG writing, the end faces were polished back by
∼250 μm to reveal the WG ends.
Microscope images of the end views of three WG
geometries are shown in Fig. 1. These structures
approximate the well-known W WG geometry (see inset
May 1, 2011 / Vol. 36, No. 9 / OPTICS LETTERS 1587
0146-9592/11/091587-03$15.00/0 © 2011 Optical Society of America
of Fig. 3)[12], which can support guided modes provided
that the depressed region of the cladding is sufficiently
wide for the given index contrast. Structures with in-
creasing complexity [Figs. 1(a)–1(c)] were written with
the aim of achieving a more uniform reduced refractive
index in the cladding layer and increased cladding dia-
meters. Note that while all structures guided light, only
the structure in Fig. 1(c) achieved lasing (this is consis-
tent with the numerical modeling predictions below).
The WG in Fig. 1(a) is composed of six cylinders ar-
ranged in a hexagon around the unexposed core and
written using 60 nJ pulses; Fig. 1(b) is composed of 12
overlapped cylinders (65 nJ), while Fig. 1(c) is 24 cylin-
ders formed from two partially overlapping rings of 12
cylinders each (50 nJ pulses) with a core diameter of
∼30 μm. The depressed claddings for all WGs were
written sequentially from the bottom to the top to avoid
focusing through previously modified glass, while the
sample was moved at 1000 mm= min. The stress fracture
apparent in Figs. 1(a) and 1(c) does not appear to affect
the guiding behavior, and we attribute it to the high
density of the devices, which have a separation of
just 150 μm.
To explore the effect of writing depressed cylinders in
close proximity to each other (e.g., two overlapping
rings), high resolution refractive index profiles were
taken at 637 nm with a refractive index profilometer
(RINCK Elektronik). The main image in Fig. 2 shows
the absolute refractive index profile of a 24-cylinder
WG structure fabricated at 50 nJ, with the inset showing
an optical microscope image of the same WG. The n data
reveals a net Δn change in the ring structures of ∼ − 1 ×
10
−3
to −1:5 × 10
−3
, as well as localized regions with
slightly increased n, possibly due to stress. Further stud-
ies will be necessary to map the Δn as a function of pulse
energy and cylinder overlap, as well as optimizing the
placement of the direct-write cylinders to reduce confine-
ment losses.
To understand the guiding behavior of these WGs, we
investigated the confinement loss of idealized W WGs as
a function of cladding diameter and index contrast. In
general W type WGs support “leaky” modes in which the
imaginary propagation constant is associated with the
confinement loss of the guided mode.
Numerical modelling results, using the exact electro-
magnetic solution to a circularly symmetric W refractive
index profile, are shown in Fig. 3 for two representative
cladding Δn’sof−1 × 10
−3
and −1:5 × 10
−3
and a core di-
ameter of 30 μm. As shown in Fig. 3, the fundamental
mode (FM) confinement losses at λ ¼ 1:89 μm decrease
strongly with increasing Δn contrast and cladding width,
with the first higher order mode having a substantially
higher loss (∼25 × FM loss) for a cladding width of 23 μm,
which is the approximate cladding width of the structure
in Fig. 1(c), thus enabling such structures to be “effec-
tively single mode.” At a cladding width of 23 μm, the pre-
dicted FM losses are sensitive to Δn and are 0:018 dB =cm
(0.7% per round trip) for Δn ¼ −1: 5 × 10
−3
, increasing to
0:14 dB=cm (5.6% per round trip) for Δn ¼ −1 × 10
−3
.
On modelling the WG shown in Fig. 1(b), FM losses
are substantially higher at 1 to 3:5 dB=cm for Δn’sof
−1:5 × 10
−3
to −1 × 10
−3
, respectively. This corresponds
to predicted roundtrip cavity losses of 34% (for Δn ¼
−1:5 × 10
−3
) to 77% (for Δn ¼ −1 × 10
−3
), which would
explain the nonlasing observed from the WG shown in
Fig. 1(b). We attribute the nonlasing of the WG shown
in Fig. 1(a) to the partially overlapped cylinders leading
Fig. 2. (Color online) Absolute refractive index profile at
637 nm of the WG formed from 24 partially overlapping
cylinders direct written at 1 m= min. Inset shows corresponding
optical microscope image.
Fig. 3. (Color online) Predicted confinement loss at
λ ¼ 1:9 μm for a W depressed cladding WG. Loss of the FM
and first higher order mode as a function of cladding width.
Fig. 1. (a)–(c) Range of waveguide structures fs laser written
in ZBLAN glass. Structures with increasing complexity were
produced to explore waveguide losses as a function of cladding
structure and thickness. The writing laser beam entered from
the top of the images. Scale bar corresponds to 50 μm.
1588 OPTICS LETTERS / Vol. 36, No. 9 / May 1, 2011
to regions of narrow cladding, and a low average Δn.If
we assume Δn ¼ −1 × 10
−3
and a cladding width of
15 μm, the predicted loss is 11 dB=cm, indicating that
laser operation of this WG is not feasible. (While the
W model is clearly an approximation of the geometry
of this WG, it provides useful insight into the conditions
required for lasing to occur.)
To pump the 9 mm long, 30 μm diameter Tm
3þ
doped
WG shown in Fig. 1(c) , 150 mW of 790 nm diode laser
light was delivered by a 25 μm diameter, 0.10 NA fiber.
This was free-space imaged into the WG using a pair
of f ¼ 20 mm aspheric lenses. The laser resonator was
formed using external dielectric coated mirrors that were
butted up to the uncoated WG ends. The input mirror was
highly reflecting at 1:9 μm and highly transmitting at
790 nm. The 1:9 μ m output couplers (OCs) available
ranged from 4% to 33%, and the 790 nm reflection for each
OC was measured. To remove residual pump light after
the WG, an uncoated Si window was used (T ¼ 53%
at 1880 nm).
The measured slope efficiencies as a function of
absorbed power for a range of output couplers are shown
in Fig. 4 (α
Tm
3þ
:ZBLAN; λ¼790 nm
¼ 5:1 dB=cm). The best re-
sult was achieved using the 30 μm diameter WG and
an R ¼ 77% OC. This gave a 50% internal slope efficiency,
21 mW threshold, and 47 mW of output. The free running
laser spectrum was measured to be centered at λ ¼
1880 nm with a broad 5 nm bandwidth (Fig. 4 inset).
To estimate the WG propagation loss, we performed a
Findlay–Clay analysis on the lasing data plotted in Fig. 4,
which gave an estimated loss of 0:22 0:06 dB=cm. This
value should be considered an upper limit, since it
includes ground state absorption losses due to the
three-level nature of the 1:9 μm transition in thulium.
The beam quality was measured by determining the
focused beam widths on an array sensor (Spiricon
Pyrocam) and was measured to be M
2
¼ 1:7 0:2.A
Gaussian beam profile in the far field was observed that
would be expected for the fundamental mode. The
non-diffraction-limited beam quality we attribute to non-
uniformities in the cladding Δn and the noncircular
waveguide geometry. The beam quality will be further in-
vestigated in future work.
We expect the efficiency to improve by fabrication of
appropriate dielectric coatings on the slab and by
optimizing device length, WG confinement, and dopant
concentration. This result indicates that ZBLAN is a
promising host glass for efficient depressed cladding
WG lasers, and its midinfrared transparency and low
phonon energy should allow access to longer wavelength
laser transitions.
In conclusion, we have demonstrated a 790 nm
pumped thulium 1:9 μm WG laser that has a 50% internal
slope efficiency, M
2
of 1:7 0:2 and pump-power-limited
output of 48 mW. To our knowledge, this depressed
cladding WG laser is the most efficient fs direct-write
glass WG laser reported to date, has achieved the highest
power at λ > 1:6 μm, and is the first ZBLAN direct-write
WG laser.
This work was produced with the assistance of the
Australian Research Council (ARC) under the Centres
of Excellence and Linkage Infrastructure, Equipment
and Facilities programs, as well as the South Australian
Premier’s Science and Research Fund. The authors ac-
knowledge S. Warren Smith for the MATLAB code for
the exact EM solution for the WG W model and
A. Dowling for technical support. T. Monro acknowl-
edges the support of an ARC Federation Fellowship.
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May 1, 2011 / Vol. 36, No. 9 / OPTICS LETTERS 1589
