COL 11(10), 102301(2013) CHINESE OPTICS LETTERS October 10, 2013
Buried waveguide in neodymium-doped phosphate glass
obtained by femtosecond laser writing using
a double line approach
Xuewen Long (
999
ÆÆÆ
©©©
)
1,2
, Jing Bai (
xxx
¬¬¬
)
1,2
, Xin Liu (
444
!!!
)
1,2
,
Wei Zhao (
ëëë
¥¥¥
)
1
, and Guanghua Cheng (
§§§
111
uuu
)
1∗
1
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and
Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
∗
Corresponding author: gcheng@opt.ac.cn
Received June 18, 2013; accepted August 2, 2013; posted online September 29, 2013
We fabricate a buried channel waveguide in neodymium-doped phosphate glass using a double line approach
by femtosecond laser writing. Raman spectra reveal an expansion of the glass network in the laser irradiated
region. Given the stress-induced positive refractive index change, waveguiding between two separated
tracks is demonstrated. The refractive index difference profile of the waveguide is reconstructed from
the measured near-field mode. Propagation loss is measured by scattering technique. Microluminescence
spectra reveal that the Nd
3+
fluorescence prop erty is not significantly affected by waveguide formation
process, which indicates that the inscribed waveguide is a good candidate for active device.
OCIS codes: 230.7380, 140.7090.
doi: 10.3788/COL201311.102301.
Since the pioneering work of Davis et al.
[1]
femtosec-
ond (fs) direct laser writing (FDLW) has attracted
considerable interest for integrated, miniaturized three-
dimensional (3D) photonics in transparent mater ials. fs
lasers can induce permanent, highly localized regions
of refractive index change suitable for the formation of
3D photonic devices including splitters
[2]
, dire c tional
couplers
[3]
, gratings
[4,5]
, magneto-optical waveguides
[6]
,
and waveguide lasers
[7,8]
in a range of glasses and crys-
tals. This technique enables the reliable, fast, and low-
cost fabrication of 3D photonic devices without the
design and fabrication of a complex mask in contrast
to most standard fabrication techniques. Accordingly,
FDLW has gradually become one of the most popular
methods in rece nt years.
Phosphate glasses are important substrates because
they can incorporate high c oncentrations of rare-earth
ions. The ability to po ssess a high g ain per unit length
is crucial to miniaturized active devices. Therefore, ul-
trafast laser 3D photo-inscription in phosphate glasses
deserves investigation. Phosphate glasses exhibit posi-
tive or negative changes to the index of refra c tion inside
the fs-laser irradiated region depending on initial glass
composition
[9]
. When writing waveguides in transpar-
ent materials, two distinct regimes exist. In type I, the
modified region exhibits a localized increase in refractive
index, and the optical waveguide can be directly gen-
erated along the straight line of modificatio n. In type
II, the mode is guided in the virtually pristine ma te-
rial neighboring laser damage track. Therefore, type-II
waveguides preser ve the properties of the or iginal mate-
rial. Type-II waveguides have be en mainly demonstrated
in a ra nge of crystals
[10−13]
.
In this letter, we inscribed a buried type-II waveguide
in neodymium-doped phosphate glass by writing two
parallel lines in clos e separation. Ra man microscopy was
used to analyze the laser written track. The Raman
peak associated with P-O bonds exhibits a negative shift
in wavenumber, which reveals long e r P-O bonds and
an expansion of the glass network in the fs-laser irr adi-
ated region
[9]
. The stress-induced increase in refractive
index between pa irs of tracks and depressed refractive
index within tracks support efficient light propagation.
The waveguide fabricated using a do uble line approach
exhibits propagatio n loss of 1.2 dB/cm at 980-nm wave-
length. The refractive index profile of the buried waveg-
uide is reconstructed. Microluminescence experiments
reveal tha t the original fluorescence of the neodymium-
doped glasses is preserved in the guiding regio ns.
Figure 1 is a schematic of the experimental setup. A
transverse writing configuration with the sample trans-
lation perp e ndicula r to the laser propagation axis was
used throughout the study. To structure the phosphate
glass, a commercial amplified Ti:sapphire laser system
(Spitfire, Spectra Physics) was used. The sys tem was op-
erated at a r e petition rate of 1 kHz and provided pulses
at a c e ntral wavelength of 800 nm with a minimum
pulse duration of 120 fs and pulse e nergy up to 1 mJ. A
Fig. 1. Schematic representation of the femtosecond laser
transversal writing geometry.
1671-7694/2013/102301(4) 102301-1
c
2013 Chinese Optics Letters