Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm2 near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M2 ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.
We proposed and investigated a novel tapered ytterbium-doped fiber design. The tapered fiber has length of 2.1 m, single-mode thin end and thick end with core/clad diameter of about 50/430 µm. Master-oscillator power-amplifier monolithic fiber scheme have been developed using this fiber and 5 ps duration, 28 nm spectral width and 0.5 MW peak power pulses was achieved at the output. FROG measurements reveal linear chirp that allow one to expect decompression of these pulses down to sub-100 fs duration.
Theoretical aspects of microstructured fibers fabrication from preforms with sealed holes at the top end are discussed. Dependences of the holes blowing degree on their diameters, on a ratio of the preform top end temperature to temperature in the center of a furnace and on other parameters are estimated. Experimental results of different microstructured fibers drawing in such a regime are presented. We have drawn the simplest microstructured fiber with one hole from a tube with outside diameter 6.2 mm and inner diameter 4.4 mm (capillary drawing). Also we have drawn MSFs from preforms with 6 and 60 small holes. To check the results of the theoretical analysis we have prepared a preform with different size holes and then drew it into a fiber. In all cases experimental results are in good agreement with theoretical estimations. Thus this method of fabrication gives the possibility to manufacture long length microstructured fibers with stable internal structural parameters, high reproducibility, ease of controlling and changing the fibers parameters and opportunity to make structures with holes of different sizes. The observed changes of the holes blowing degree in our experimental samples can be easily compensated with the help of additional heating of the preform top end. Besides, such heating makes it possible to control and change the holes blowing degree, particularly during the fibers drawing process.
A novel design of polarization-maintaining microstructured fiber is presented. A solid core is surrounded by two or more
concentric rings of holes of constant diameter. The distance between adjacent holes in the first ring is constant except for
one or two pairs of holes, for which the spacing is increased. It is found that the fiber of such a design can demonstrate
high birefringence (up to 5x10-3) and simultaneously low azimuthal asymmetry of the mode shape. An experimental
sample was fabricated. The measured value of birefringence is in excellent agreement with the calculated one. The
results of the h-parameter measurement demonstrate the capability of the novel fiber to maintain the polarization state of
the guided light along the fiber length.
Photonic bandgap Bragg fibers are promising for designing
large-mode-area structures owing to their high bend
immunity. However, at a large core diameter, filtering of high-order modes (mainly, the LP11 mode) becomes difficult,
because the propagation constant of such modes is close to that of the fundamental LP01 mode.
In this paper, we demonstrate the possibility to suppress high-order modes in Bragg fibers by introducing low-index
inclusions into the Bragg fiber core. Numerical analysis shows that an appropriate choice of the position and types of
such inclusions allows one to increase the LP11 mode radiation loss without increasing the optical loss of the fundamental
LP01 mode. The Bragg fiber with two B-doped and two
F-doped rods in the core was fabricated and studied. The
fundamental LP01 mode at 1064 nm had a mode-field area of about 340μm2 and an optical loss below 0.2 dB/m at a
bending radius of 15 cm. The LP11 mode was not observed in both bent and straight fibers at this wavelength. Only the
LP21 mode was detected in a straight fiber; however, it was completely suppressed after propagating a length of 60 cm in
a fiber bent with a radius <50 cm.
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