As shown recently, a long telecommunication fibre may be treated as a natural one-dimensional random system, where
lasing is possible due to a combination of random distributed feedback via Rayleigh scattering by natural refractive
index inhomogeneities and distributed amplification through the Raman effect. Here we present a new type of a random
fibre laser with a narrow (~1 nm) spectrum tunable over a broad wavelength range (1535-1570 nm) with a uniquely flat
(~0.1 dB) and high (>2 W) output power and prominent (>40 %) differential efficiency, which outperforms traditional
fibre lasers of the same category, e.g. a conventional Raman laser with a linear cavity formed in the same fibre by adding
point reflectors. Analytical model is proposed that explains quantitatively the higher efficiency and the flatter tuning
curve of the random fiber laser compared to conventional one. The other important features of the random fibre laser like
"modeless" spectrum of specific shape and corresponding intensity fluctuations as well as the techniques of controlling
its output characteristics are discussed. Outstanding characteristics defined by new underlying physics and the simplicity
of the scheme implemented in standard telecom fibre make the demonstrated tunable random fibre laser a very attractive
light source both for fundamental science and practical applications such as optical communication, sensing and secure
transmission.
We experimentally demonstrate a Raman fiber laser based on multiple point-action fiber Bragg grating (FBG)
reflectors and distributed feedback via Rayleigh scattering in a ~22 km long optical fiber. Twenty two lasing lines with
spacing of ~100 GHz (close to ITU grid) in C-band are generated at Watts power level. In contrast to the normal cavity
with competition between laser lines, the random distributed feedback cavity exhibits highly stable multiwavelength
generation with a power-equalized uniform distribution which is almost independent on power. The current set up
showing the capability of generating Raman gain of about 100-nm wide giving the possibility of multiwavelength
generation at different bands.
Wavelengths around 1.15 μm, 1.3 μm and 1.7 μm can be used to pump Dy-doped ZBLAN fibre in order to generate ~3
μm with high efficiency. Previously the generation of 2.9 μm from the Dy-ZBLAN fibre was demonstrated by pumping
with 1.1 μm Yb-silica fibre laser sources. The laser slope efficiency and lasing threshold demonstrated was about ~5%
and ~1.78 W. In this investigation, the longer wavelength absorption band (6H9/2 , 6F11/2) centred at 1.3 μm of Dy3+-doped
ZBLAN is utilised and the lasing transition around ~3 μm takes places from 6H13/2 → 6H15/2. With this pumping scheme
the Stokes' efficiency is expected to be up to ~45%. A quasi-continuous wave Dy3+-ZBLAN fibre laser pumped by a
~1.3 μm Nd:YAG laser and operating at 2.96 μm with a bandwidth (FWHM) of ~14 nm has been demonstrated. For a
60cm fibre length, a threshold of 0.5W and a slope efficiency of ~20% with respect to the absorbed pump power was
observed. The overall pump absorption in the fibre was around 84%. The cavity reflectivities at 2.9 μm were 99% and
50%. The demonstrated slope efficiency was 45% of the Stokes' limit. The slope efficiency was around four times
higher and the threshold around 3.6 times lower than the previous performance demonstrated by using the 1.1 μm Yb
fibre laser pumping scheme. The higher performance achieved compared to the 1.1 μm pump scheme is due to the higher
Stokes' limit, lower pump ESA losses and higher cavity reflectivity. About 590 cm-1 Raman Stokes shift has also
detected by using 514.5 nm and 488 nm Ar ion laser as excitation pump sources.
Intracavity second harmonic generation (ISHG) of a continuous wave, diode-pumped, broadband Yb-doped fibre laser has been investigated. Frequency doubling of the fibre output and of the residual diode pump light, and sum frequency mixing (SFM) between the fibre output and the pump light were achieved simultaneously, resulting in three colour operation in the blue-green region.
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