We demonstrated robust and bend insensitive fiber delivery of high power laser with diffraction limited beam quality for two different kinds of hollow core band gap fibers. The light source for this experiment consists of ytterbium-doped double clad fiber aeroGAIN-ROD-PM85 in a high power amplifier setup. It provided 22ps pulses with a maximum average power of 95W, 40MHz repetition rate at 1032nm (~2.4μJ pulse energy), with M<sup>2</sup> <1.3. We determined the facet damage threshold for a 7-cells hollow core photonic bandgap fiber and showed up to 59W average power output for a 5 meters fiber. The damage threshold for a 19-cell hollow core photonic bandgap fiber exceeded the maximum power provided by the light source and up to 76W average output power was demonstrated for a 1m fiber. In both cases, no special attention was needed to mitigate bend sensitivity. The fibers were coiled on 8 centimeters radius spools and even lower bending radii were present. In addition, stimulated rotational Raman scattering arising from nitrogen molecules was measured through a 42m long 19 cell hollow core fiber.
An investigation of the use of hollow-core photonic bandgap (PBG) fiber to transport high-power narrow-linewidth light is performed. In conventional fiber the main limitation in this case is stimulated Brillouin scattering (SBS) but in PBG fiber the overlap between the optical intensity and the silica that hosts the acoustic phonons is reduced. In this paper we show this should increase the SBS threshold to the multi-kW level even when including the non-linear interaction with the air in the core. A full model and experimental measurement of the SBS spectra is presented, including back-scatter into other optical modes besides the fundamental, and some of the issues of coupling high power into hollow-core fibers are discussed.
Fabrication of a truly single mode, low loss and polarization maintaining HC-PCF is reported. This fiber has a 50 nm
wide strictly single mode region with good polarization holding (h-parameter below 10<sup>-4</sup> m<sup>-1</sup>) and low loss (<20 dB/km).
1178 nm single-frequency amplification by Yb-doped photonic bandgap fiber has been demonstrated. 24.6 W
output was obtained without stimulated Brillouin scattering. 1.8 dB suppression of Brillouin gain by an acoustic
antiguiding effect has been found in the low-index core antiresonant reflecting optical waveguide.
We present two low-loss 7-cell core hollow-core photonic crystal fibers (HC-PCF) with intrinsic single mode properties
around 1550 nm. By reducing the number of surface modes within the bandgap these fibers can be operated close to the
short wavelength bandgap edge. It is well known that by omitting a core tube in HC-PCF fabrication of a surface mode
free bandgap can be achieved. We found by experimental as well as numerical, investigation that using a core tube with
a wall thickness reduced to between 60-70 % is sufficient to have a surface mode free short wavelength bandgap edge.
The transmission and mode properties of the fabricated fibers are examined experimentally and compared to numerical
Ytterbium-doped photonic-bandgap fiber sources operating at the long-wavelength edge of the ytterbium gain band are
being investigated for high power amplification. Artificial shaping of the gain spectrum by the characteristic distributed
filtering effect of the photonic bandgap enables power scaling free of amplified spontaneous emission. As high as 167 W
power and 16 dB saturated gain at 1178 nm have been demonstrated. Single-pass frequency doubling to 14.5W 589nm
light was also demonstrated with 34% conversion efficiency.
Several 7-cell core hollow-core fibers with photonic bandgap spectral positions between 1.4 μm and 2.3 μm were
fabricated. The loss follows the ≈ λ<sup>-3</sup> dependency previously reported<sup>1</sup> with a minimum measured loss of 9.5 dB/km at
We demonstrate suppression of amplified spontaneous emission at the conventional ytterbium gain wavelengths around
1030 nm in a cladding-pumped polarization-maintaining ytterbium-doped solid core photonic crystal fibre. The fibre
works through combined index and bandgap guiding. Furthermore, we show that the peak of the amplified spontaneous
emission can be shifted towards longer wavelengths by rescaling the fibre dimensions. Thereby one can obtain lasing or
amplification at longer wavelengths (1100 nm - 1200 nm) as the amount of amplification in the fibre is shown to scale
with the power of the amplified spontaneous emission.