Low-loss deposition of solgel-derived silica films on tapered fibers.
ABSTRACT Films of porous silica are deposited on the uniform waists of tapered fibers in minutes by a modified solgel dip coating method, inducing less than 0.2 dB of loss. The coated tapers are an ideal platform for realizing all-fiber devices that exploit evanescent-field interactions with the deposited porous film. As an example we demonstrate structural long-period gratings in which a periodic index variation in the film arises from the porosity variation produced by spatially varying exposure of the waist to a scanned CO2 laser beam. The long period grating is insensitive to temperature up to 800 degrees C.
- SourceAvailable from: Andrea Armani[show abstract] [hide abstract]
ABSTRACT: Ultra-low threshold lasers which operate in the telecommunications band and which can be integrated with other CMOS compatible elements have numerous applications in satellite communications, biochemical detection and optical computing. To achieve sub-mW lasing thresholds, it is necessary to optimize both the gain medium and the pump method. One of the most promising methods is to use rare-earth ions in a co- or tri-dopant configuration, where the lasing of the primary dopant is enhanced by the secondary one, thus improving the efficiency of the overall system. Here, we demonstrate an Erbium:Ytterbium co-doped microcavity-based laser which is lithographically fabricated on a silicon substrate. The quality factor and pump threshold are experimentally determined for a series of erbium and ytterbium doping concentrations, verifying the inter-dependent relationship between the two dopants. The lasing threshold of the optimized device is 4.2 microW.Optics Express 12/2009; 17(25):23265-71. · 3.55 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The high power transmission stability is characterized by launching up to 500 W at 940 and 980 nm into the fiber over a period of at least 30 min. The most critical experimental points for comparing the stability results are the reproducible stripping of the coating with a well defined crossover and the strict clean and rectangular preparation of the fiber end faces. The far field and the numerical aperture are used as an indicator for changes of the optical coating properties. Small changes in the refractive index during the power stability tests have been detected for the low index acrylate and for the hybrid material. It was tried to correlate the results with the thermochemical investigations of the used coating materials (DTA, TG). The experimental results show the limits of applicability and the potential of both types of fibers for high power applications.
OPTICS LETTERS / Vol. 29, No. 7 / April 1, 2004
Low-loss deposition of solgel-derived silica films on
G. Kakarantzas,* S. G. Leon-Saval, T. A. Birks, and P. St. J. Russell
Optoelectronics Group, Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
Received October 31, 2003
Films of porous silica are deposited on the uniform waists of tapered fibers in minutes by a modified solgel
dip coating method, inducing less than 0.2 dB of loss.
all-fiber devices that exploit evanescent-field interactions with the deposited porous film.
demonstrate structural long-period gratings in which a periodic index variation in the film arises from the
porosity variation produced by spatially varying exposure of the waist to a scanned CO2 laser beam.
long period grating is insensitive to temperature up to 800±C.
060.2310, 160.6060, 050.2770.
The coated tapers are an ideal platform for realizing
As an example we
© 2004 Optical Society of America
The solgel method1has proved to be an excellent way
to prepare optical films and coatings.
ers of precise thickness can be laid down by dip coat-
The technique involves immersing the substrate
in the solgel solutionand then removing it at a constant
speed.This method has been widely used for planar
Tapered fibers, made by heating and stretch-
ing a single-mode fiber in a heat source such as a
small flame, are a simple and effective means of
access to light in a fiber with minimal loss (typically
In a narrow waist of diameter ?10 mm,
the fundamental mode is guided as a cladding mode
at the outer boundary and can interact with external
media (such as a porous solgel-derived film) via
the evanescent field. Henry and Payne reported a
chemical sensor based on a tapered fiber coated with
a solgel film incorporating a fluorescent dye.5
process that they used to deposit the film took 2 days,
induced losses at wavelengths longer than 600 nm,
and was not capable of yielding variations in the film’s
properties along the fiber.
measure the thickness of the film.
Long-period gratings (LPGs) find use in a wide
variety of telecommunication and sensing applica-
They are typically made by periodically
changing the refractive index of a photosensitive
fiber core by exposure to an UV beam.
alternative methods were recently reported that
employ periodic mechanical deformations of the fiber
These are suitable for non-
photosensitive fibers and give gratings with much
improved thermal stability.
In this Letter we demonstrate successful deposition
of low-loss porous silica films on the uniform waists of
tapered telecommunication fiber.
with a low-power scanning CO2laser beam instead of
a furnace, we could form a complete film comprising
many layersvery rapidly, and modulating the exposure
to the beam allowed us to vary the film’s porosity from
place to place. In particular, periodic exposure as the
beam was swept along the waist yielded a thermally
stable structural LPG.
Very thin lay-
They were also unable to
By heating the film
We prepared the starting solution by mixing
tetraethyl orthosilicate (TEOS), ethanol, and deionized
water with hydrochloric acid as a catalyst.
ratio of TEOS:HCl:EtOH:H2O was 0.125:0.01:0.5:0.25,
and the solution had a pH of 2.3.
stirred at room temperature for 3 h and then drawn
into the syringe for the deposition process.
SMF-28) were tapered to a uniform waist diameter of
10 mm.In a variant of the well-known dip-coating
processes (Fig. 1), a droplet of sol suspended at the tip
of a syringe needle was passed along the taper waist,
trailing a thin layer that gelled almost immediately
on exposure to air.Eight such passes formed a film
of slightly porous silica glass that induced less than
0.2 dB of loss in the fiber.
After coating, the porous solgel-derived film is
heated to consolidate it.Although this is normally
accomplished in a furnace, the localized heat source
provided by a laser beam12has two key advantages.
First, the film may be heated much more quickly; we
could deposit and consolidate a complete silica film
in less than 10 min, compared with 2 days for Henry
Second, exposure of the film to the beam can
be varied while the beam is being swept along the
fiber [by varying the laser power and (or) the sweep
rate], giving a spatially varying porosity, for example.
Thus a pattern of varying refractive index can be
written into the film.
The solution was
Schematic diagram of the modified dip coating
0146-9592/04/070694-03$15.00/0 © 2004 Optical Society of America
April 1, 2004 / Vol. 29, No. 7 / OPTICS LETTERS
In our experiment an ?6-W cw CO2 laser beam
was focused with a 300-mm focal-length lens onto
the coated tapered fiber via a galvanometer-driven
mirror that could rapidly redirect the beam anywhere
along the taper waist.11
A computer controlled the
entire process—opening the shutter, controlling the
laser power, and scanning the beam.
it was possible to pause a rapidly scanned laser beam
at points along the taper waist for predetermined
periods of time. When a section of the porous film is
heated longer, it consolidates more than neighboring
sections and ends up with a higher refractive index
(up to the index of solid fused silica, matching the
index of the tapered fiber substrate).
of the pause and the power of the beam determine the
final porosity and index.
A sequence of 60 consolidated regions was created
at equal 400-mm intervals along the taper waist to
create a periodic structure that acted as a LPG.
notch in the tapered fiber’s transmission spectrum
grew with multiple passes of the beam until it satu-
rated at maximum coupling (Fig. 2, solid curve).
Once formed, the LPG could be erased (Fig. 2, dotted
curve) by uniform exposure to the laser beam at
higher power, completely consolidating the film and
in effect recovering a simple low-loss (,0.2-dB) taper
waist, albeit of slightly larger diameter.
the whole process identically then gave a LPG with a
50-nm shift in resonance wavelength (Fig. 2, dashed
The wavelength shift occurs because the diameter
of the substrate taper waist is bigger the second time
(by twice the thickness of the first consolidated film),
thus changing the resonance condition.
unperturbed taper waist as an ideal conducting-wall
waveguide (a reasonable approximation for a highly
multimode waveguide such as this), the propagation
constant of a low-order mode13can be written as
b ? kn 2
where k ? 2p?l, n is the index of silica, r is the ra-
dius of the tapered waist, and the modal parameter
U is approximately constant for a given mode.
this wavelength of a given LPG, the resonance can be
shown to be proportional to the square of the waist di-
ameter.From this result the thickness of the film was
deduced to be ?0.09 mm.
In a similar experiment we modified the transmis-
sion properties of a LPG by selectively erasing a num-
ber of periods with a single scan of the beam.
LPG was written as described above with 60 periods,
and then after three successive laser scans 20, 15, and
10 periods remained. As expected,7the reduction in
the grating length has a direct effect on the coupling
constant and the bandwidth of the resonance, as shown
in Fig. 3.The flexibility of this writing process may
be used to realize more complicated devices such as cas-
caded LPGs and Mach–Zehnder interferometers.14
To test the thermal response of the solgel LPGs, we
measured changes in the transmission spectrum from
room temperature to 900±C in a ceramic tube fur-
nace.The temperature dependence of the resonance
wavelength in picometers per degree Celsius is a good
indication of thermal stability.
the curve and changes in slope provide information
about the nature of the grating writing process.
resonance wavelength of a similar LPG is plotted as a
function of temperature in Fig. 4, while heating and
cooling.As expected, the resonance shifts to longer
wavelengths and the variation is linear within two
distinct ranges.The slope is 10 pm?±C from room
temperature to 450±C and 21.5 pm?±C from 450±C to
900±C. On cooling, the variation is monotonic and
linear with a slope of 15 pm?±C.
We attribute the change in slope while heating to the
continuous evolution and relaxation of the untreated
deposited porous layer with temperature, an anneal-
ing process that is reflected in small variations of its
refractive index.There was no significant change in
the notch depth up to 800±C, indicating that the film
was heated to higher temperatures in the laser beam
as the grating was written.
Also, the shape of
After 800±C, the notch
with a 60-period LPG written in the film (solid curve), after
the first LPG was erased (dotted curve), and after a second
film was deposited and an otherwise identical LPG written
Transmission spectrum of the coated tapered fiber
with a 60-period LPG written in the film (solid curve), and
after 40 periods (dotted curve), 45 periods (short-dashed
curve), and 50 periods (long-dashed curve) were erased.
Transmission spectrum of the coated tapered fiber
OPTICS LETTERS / Vol. 29, No. 7 / April 1, 2004
while heating (triangles) and cooling (circles).
Temperature dependence of the main resonance
depth started to decrease as the film started to consoli-
date further, but there was still 90% (210 dB) coupling
at 900±C. These results agree with the results of
studies of index changes in laser- and furnace-heated
While cooling, the fully annealed film
behaves in a simple manner described by known
values of the coefficient of linear expansion and
the temperature dependence of the index of fused
An approximate calculation based on the
conducting-wall model of Eq. (1) predicts a slope of
?12 pm?±C, dominated simply by the temperature
dependence of the index and in good agreement with
our measurements. In absolute terms this tempera-
ture sensitivity is among the lowest reported, which
we attribute to the fact that the taper and the film
are made from the same material and so they avoid
thermally induced stress.
low-loss (,0.2-dB) solgel-derived silica films on the
uniform waist of a tapered fiber.
with rare earths and other materials, the flexibility
of the solgel process, and the strong evanescent inter-
actions that are possible in tapered fibers can lead to
a new class of all-fiber devices for telecommunications
The ability to dope
and sensing applications.
periodically varied the porosity of a deposited film, us-
ing a scanning CO2laser beam to produce a structural
LPG that is insensitive to temperature up to 800±C.
This study was supported by the UK Engineering
and Physical Sciences Research Council.
*G. Kakarantzas (email@example.com) is now
with the Photonics Communications Research Labo-
ratory, Department of Electrical and Electronic En-
gineering, National Technical University of Athens,
15773 Athens, Greece.
As an example, we have
1. C. J. Brinker and G. W. Scherer, Sol-Gel Science (Aca-
demic, Boston, Mass., 1990).
2. I. M. Thomas, in Sol-Gel Optics Processing and Ap-
plications, L. C. Klein, ed. (Kluwer Academic, Boston,
3. R. M. Almeida, J. Non-Cryst. Solids 259, 176 (1999).
4. H. S. Mackenzie and F. P. Payne, Electron. Lett. 26,
5. W. M. Henry and F. P. Payne, Opt. Quantum Electron.
27, 185 (1995).
6. J. C. Knight, G. Cheung, F. Jacques, and T. A. Birks,
Opt. Lett. 22, 1129 (1997).
7. T. Erdogan, J. Opt. Soc. Am A 14, 1760 (1997).
8. S. W. James and R. P. Tatam, Meas. Sci. Technol. 14,
9. I. K. Hwang, S. H. Yun, and B. Y. Kim, Opt. Lett. 24,
10. D. D. Davis, T. K. Gaylord, E. N. Glytsis, S. G.
Kosinski, S. C. Mettler, and A. M. Vengsarkar, Elec-
tron. Lett. 34, 302 (1998).
11. G. Kakarantzas, T. E. Dimmick, T. A. Birks, R. LeRoux,
and P. St. J. Russell, Opt. Lett. 26, 1137 (2001).
12. G. Kakarantzas, T. A. Birks, and P. St. J. Russell, Opt.
Lett. 27, 1013 (2002).
13. A. W. Snyder and J. D. Love, Optical Waveguide Theory
(Chapman & Hall, London, 1983).
14. X. J. Xu, Opt. Lett. 23, 509 (1998).
15. D. J. Taylor, D. P. Birnie III, and B. D. Fabes, J. Mater.
Res. 10, 1429 (1995).
16. N. P. Bansal and R. H. Doremus, Handbook of Glass
Properties (Academic, San Diego, Calif., 1986).
17. G. Ghosh, IEEE Photon. Technol. Lett. 6, 431 (1994).