1. INTRODUCTION
In all the applications of doped fibers, the concentration and
distribution of erbium ions play a crucial role in determining the
performances and characteristics of the devices. Actually, these ions
act as the laser active centers and their distribution inside the
matrix is fundamental in dictating the properties of the overall
system, a broad tunable laser source emitting around 1550 nm. The
control of the dopant distribution is fundamental (Othonos et al., 1995; Garcia et al., 2003) since the active
fiber works like an amplifier, in absence of the smoothing effect on
the electromagnetic field due to the optical feedback from cavity
mirrors, as it happens in lasers. In this paper, we propose an
experimental method for the measurement of dopant distribution in the
core of single-mode optical fibers (with 50 nm spatial resolution),
based on the Soft X-ray Contact Microscopy (SXCM) technique (Batani
et al., 2000, 2002; Desai et al.,
2003). The technique should provide quantitative results on
fiber doping. Its drawback is that it will be relatively slow because
of the requirement of slicing of the fiber and the use of
photoresist.
The achievable resolution of this technique (∼ 50 nm) is not as
high as electron microscopy, but better than optical microscopy (many
factors contribute to limit the resolution, that is: (i) Fresnel
diffraction; (ii) “penumbral blurring”; and (iii) shot
noise). However, the most important feature of the technique are that:
- actual fibers, and not only preforms, can be investigated. This is
important since the process of drawing the fiber from the preform can
indeed cause some redistribution in the concentration of dopants,
and
- by suitably choosing X-ray photon energy, chemical element
discrimination is possible.
Moreover, a single X-ray shot allows the simultaneous analysis of
many fiber slices, irradiated under the same conditions.
2. THE EXPERIMENT
We propose a new technique for measurement of dopant distribution in
the core of single-mode optical fibers, which is based on SXCM. The
overall picture of the proposed technique is hereafter reported,
together with its basic elements: the source, the target and its
relevant preparation problems, the fundamentals of the interaction
between the probe and the sample, and the diagnostics.
2.1. The source
A laser-plasma source produces X-rays in a window that is selected by
choosing a suitable target and filters. In particular, here we will
refer to two different sources, which are available within the frame of
the E.U. Large Facility Program: the one at Rutherford Appleton
Laboratory (RAL), at Abingdon, United Kingdom and Prague Asterix Laser
System (PALS) in Prague (Czech Republic).
The laser-plasma X-ray source at RAL (Turcu et
al., 1994) delivers an X-ray average power of ≈ 1 watt
at 1 nm wavelength (hν ≈ 1 keV) into 2π steradians, from a
point source (≈ 10 μm diameter). The source is operated in
ambient Helium gas at atmospheric pressure, which results in
contamination free delivery of X-rays to the exposure cell. The plasma
source is generated by a picosecond excimer laser system operated at
248 nm wavelength and delivering a diffraction limited beam of trains
of 16 pulses, each 5 ps long and separated by 2 ns. The lasing energy
is 350 mJ/pulse-train at a repetition rate up to 100 Hz. The RAL
laser-plasma X-ray source is the first source fully scheduled for
application experiments. It complements Synchrotron Radiation Sources
(SRS) in: (a) source geometry: 10 micron point source; (b) temporal
characteristic: pulsed ps source at 1–100 Hz; and (c) wavelength
range 0.5–50 nm.
The single beam PALS high power iodine laser (Jungwirth et al., 2001) is a
photolytically pumped gas laser with an emission wavelength at 1.315
micron. The PALS laser actually derives from the Asterix IV laser
system which was previously located in Garching. The PALS system is set
in the classical Master Oscillator Parametric Amplifier (MOPA)
configuration. The pulse to be amplified is delivered by an
acousto-optically mode locked oscillator, generating sub-ns pulses. The
amplifier chain consists of 6 amplifiers of increasing diameters and
length. The final amplifier has a clear aperture of 29 cm. The PALS
laser delivers a maximum output power of 4 TW at a pulse length of 0.3
ns, corresponding to an energy of 1.2 kJ at ω and about 450 J at
3ω.
A preliminary step is the evaluation of the performances of the two
X-ray sources showing that the microscopy experiment can be performed
in both cases.
In the case of PALS, many sample holders can be positioned into the
irradiation chamber, allowing a significant statistical analysis of the
results. Actually, using a large source with a large X-ray flux, there
is the possibility to put the microscopes (i.e., the holders for
samples and photoresist) very far from the source, typically 10 cm, as
already done in SXCM experiments of living biological cells (Batani et al., 2002). Even with a large
focal spot (400 microns, even if in reality the PALS spot size can be
reduced down to 50 microns) this reduces the penumbral blurring much
below 50 nm. Since this is the limit of the technique, imposed by
diffraction effects and shot noise, it means that penumbral blurring is
practically negligible on PALS.
In the case of the RAL source, the experiment will rely on a small
laser source; a low X-ray flux is involved so that the sample holders
can be positioned closer to the source. However, optical fibers (unlike
living biological cells) do not suffer a composition change when
exposed to X-rays for long exposure times, it will be possible to
irradiate the samples for times up to several minutes. If one
integrates the X-ray dose over a longer period of time, say 100
seconds, the 100 Hz repetition rate RAL source would possibly deliver
the same X-ray dose as the single shot 1.2 kJ pulsed laser. This
implies that this kind of experiment can possibly be done at other
facilities too.
2.2. Target preparation
A “radiography” of the sample will be recorded onto a
X-ray photoresist and analyzed, after development, by means of an
Atomic Force Microscope. The geometrical properties of the beam and the
physics of the interaction of X-rays with the sample coupled to the
photoresist pose strict conditions on the geometry of the samples
themselves. The spectral window is controlled in order to enhance the
contrast between the matrix and the dopant in the fiber.
Inter-linked parameters, such as the variation of dopant distribution
and thickness, are critical to assess X-ray absorbed doses. The
interesting region in the fiber, where relevant changes of the dopant
distribution are present and should be controlled, is the one near the
core, having a diameter of about 10 microns (see Fig. 1). The resolution ability of the proposed
technique is around 50 nm, the distribution of the relevant ions can
then be mapped and the ion concentration estimated.
Image of an Er doped fiber obtained with the Focused Ion Beam
technique (Milani et al., 2004)
having a core diameter of about 10 microns and working at wavelengths
around 1550 nm. The fiber has been both imaged and cut (in the middle)
using FIB.
To estimate the different absorption properties of relevant materials
in optical fiber systems, Figures 2a and b show, for the sake of example, the transmission of
a layer of the same thickness of Er and SiO2 in the region
around 200 and 1400 eV. These are the interesting regions in which to
perform the experiment being close to absorption edges in Erbium, and hence
allowing for a large difference between absorption coefficients of Er and
SiO2. Also the higher energy region (hν > 10 keV) is
interesting: here the large mass difference produces strong differences in
X-ray absorption.
Transmission of a 0.2 μm thick layer of Er in the region between
100 eV and 3 keV. Notice the X-ray absorption edges of Er around 140
and 1450 eV.
Transmission of a 0.2 μm thick layer of SiO2 in the
region between 100 eV and 3 keV.
Multiple targets can be simultaneously irradiated provided that a
reasonable energy is supplied of the order of Joules in hundredths of
ps on surfaces of 100 square microns. Critical parameters for the
technique are the sample thickness that rules the energy density to the
photoresist and the X-ray energy selected range to enhance, that is the
contrast between the different fiber elemental components.
2.3. The atomic force microscope
Using soft X-rays with energy around the Er absorption edge, a
natural contrast emerges in samples between core and cladding and
inside the core among regions with different dopant concentration. The
sample, or the samples (since the technique allows the simultaneous
investigation of many samples) is put in a holder directly above the
X-ray photoresist. The specimen will have the shape of a cylinder with
height as small as possible. A microradiography of the samples will be
recorded on the photoresist which can be later analyzed using a
scanning Electron Microscope, or an Atomic Force Microscope (AFM).
In particular, with AFM it is possible to envisage a
“step-by-step” development technique, in order to achieve
the best possible result. This is important because the correct
development time for the resist can be vary because of fluctuations in
the X-ray flux and in the sample height and characteristics.
4. DISCUSSION AND CONCLUSIONS
One serious problem of the proposed SXCM technique is the possibility
of avoiding X-ray emission outside the desired spectral window, which
can be probably obtained by the insertion of suitable filters. A
further critical element is represented by the structure of the sample
holder and the location of fiber slices. The experience acquired in
handling biological specimens, for SXCM experiments in particular
(Batani et al., 2002; Masini et al., 1999; Milani et al., 1999), turns out to be very
useful in solving this problem. The proposed technique appears to be
free from some bias such as the dependence on the supplier for
calibration, fluorescence signal saturation, and lack of ability in
discriminating between dopant ions that are optically active or not.
ACKNOWLEDGMENTS
We warmly thank I.C.E. Turcu, Rutherford Appleton Laboratory U.K., K.
Eidmann, MPQ Garching, Germany, and G. Spinolo, University of Milano
Bicocca, Italy for useful discussions.
