Increment of the axial region of ablation in a micromachining process is demonstrated when a refractive lens (RL) is replaced by a diffractive lens (DL). The depth of focus of a DL and a RL with the same numerical aperture are compared. For ultrashort pulses, the broadband spectrum of the laser together with the chromatic aberrations associated to the DL made its ablation region greater than the one corresponding to a RL. We measure experimentally the tridimensional ablation region for both types of lenses with 100 fs and 30 fs pulses. This study is expected to be promising to alleviate the mechanical tolerances in femtosecond micromachining with diffractive optical elements (DOEs).
The guidance of the fundamental mode of microstructured fibers with a Ge-doped core can be cut off by filling the
holes with a material, liquid in our case, that increases the refractive index in the air-holes up to a value between the
refractive index values of the Ge-doped region and the silica. A section of such a liquid-filled fiber defines a short-pass
filter widely tunable by adjusting slightly the refractive index values. Thus, a small change of temperature or a small
strain of the fiber can be used to adjust the transmittance of the filter. Alternatively, temperature changes and strain could
be determined by measuring the cutoff wavelength. A simple measurement of the power transmitted through a short
section of liquid-filled fiber can be used to monitor temperature and strain, enabling the measurement of fast transients
such as mechanical vibrations. Several Y-shaped fibers with a Ge-doped core were fabricated. This microstructure with
only three big holes surrounding the core makes straightforward the filling of the fiber with liquids. The experimental
characterization includes the measurement of the cutoff wavelength of a number of devices with different lengths,
different microstructure geometries and different refractive index liquids. The cutoff wavelength and the transmission
were measured as a function of temperature and strain. A simple theoretical simulation permits to explain the
experimental results.
Supercontinuum generation in a highly Ge-doped core Y-shaped microstructured optical fiber using long pump pulses of
9 ns duration at 1064 nm is reported. The generation of nonlinear effects takes advantage of the large nonlinear refractive
index and Raman gain of the Ge-doped core, as well as the air hole structure that surrounds the core. The fiber is easy to
fabricate due to its simple structure and shows good compatibility with standard fibers. Although the fiber was pumped
in normal dispersion far from the zero dispersion wavelength, flat and smooth supercontinuum in the fundamental mode
from 550 nm to beyond 1750 nm was generated with a value of fiber length and pump peak power product of
11.7 kWm.
Y-shaped microstructured optical fibers (MOF) are gaining increasing attention due to its simplicity, since the cross
section of this fiber is formed by a solid-core and only three large air holes in the cladding. In the fabrication process, in
order to provide a Ge-doped core, a step-index multimode Ge-doped fiber can be inserted in the interstitial hole between
the three capillaries. We have found that the filling of a Ge-doped Y-shaped MOF with a liquid of refractive index higher
than the index of silica but lower than the index of the Ge-doped core produces a cutoff of the fundamental mode. This
cutoff is very sensitive to small changes of the relative refractive index values of the liquid, as well as of in the silica and
the Ge-doped core. Finally, we use the Ge-doped fiber to inscribe fiber bragg gratings (FBG) and see the spectral shift
when the grating is put down strain and temperature changes.
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