The invited paper explains the transmission properties of a range of near-, mid-, and far-IR optical fibres for their
applications in chemical and biological sensing. Methods for the fabrication of single and multiple-core mid-IR fibres are
discussed in view of controlling the thermal and viscosity properties for fibre drawing. In particular, the need for
removing impurity bands in the 5000 to 1000 cm<sup>-1</sup> range is explained. The importance of engineering multi-core fibres
is also discussed for simultaneous measurements of Raman, IR and surface plasmon enhanced modes together with say,
temperature using a mid-IR transmitting tellurite fibre e.g. in a chemical process. The paper explains the principles and
advantages of evanescent wave coupling of light at the resonant frequency bands for chemical sensing using a fibre
evanescent wave spectroscopic sensor having a GeTeSe chalcogenide fibre. Using fibre based techniques, measurements
for Cr<sup>6+</sup> ions in solution and As<sup>3+</sup> and As<sup>5+</sup> in solids have been characterized at visible and mid-IR regions, respectively.
In this paper we also explain the importance of using mid-IR fibres for engineering novel laser and broadband sources
for chemical sensing.
We present efficient CW lasing Tm<sup>3+</sup>/Ho<sup>3+</sup>/Yb<sup>3+</sup>-triply-doped tellurite fibre at ~2.1 μm. Two different pump schemes
have been demonstrated for this laser: a 1.088 μm
Yb<sup>3+</sup>-doped silica fibre laser simultaneously pumping the Tm<sup>3+</sup>: <sup>3</sup>H<sub>5</sub>,
Ho<sup>3+</sup>: <sup>5</sup>I<sub>6</sub> and Yb<sup>3+</sup>: <sup>2</sup>F<sub>5/2</sub> levels, and a 1.6 μm
Er<sup>3+</sup>/Yb<sup>3+</sup>-doped silica fibre laser directly pumping the Tm<sup>3+</sup>: <sup>3</sup>F<sub>4</sub> level. For
the 1.6 μm pumping, a slope efficiency of 62% has been achieved in a 76 cm long fibre which is close to the Stokes
efficiency limit of ~75%. An output power of 160 mW has also been achieved, but with no signs of saturation or fibre
damage suggesting that higher output powers should be possible. For the 1.088 μm pumping there is very strong pump
ESA resulting in bright blue (480 nm) and near-IR (800 nm) fluorescence due to the <sup>1</sup>G<sub>4</sub> → <sup>3</sup>H<sub>6</sub> and <sup>3</sup>H<sub>4</sub> →
transitions of Tm<sup>3+</sup>, respectively, and this limits the achievable slope efficiency, which in this case was a maximum of
25% for a 17 cm long fibre. With this pump scheme, the highest observed output power was 60 mW, and further power
scaling was limited due to the intense ESA and thermal damage to the pump end of the fibre. We also present results on
the active Q-switching of the 1.6 μm pumped fibre laser using a mechanical chopper operating at 19.4 kHz. Average
powers of 26 mW and pulse energies of 0.65 μJ were measured with pulse widths in the range 100-160 ns.
<sup></sup>Near- and mid-infrared fibre lasers find many applications in areas such as remote and chemical sensing, lidar and
medicine, and tellurite fibres offer advantages over other common fibre glasses such a lower phonon energy and higher
rare-earth ion solubility than silicate glasses, and greater chemical and environmental stability than fluoride glasses. Rate
equation modelling is a very useful tool for the characterisation and performance prediction of new rare earth transitions
in these novel fibre materials. We present the numerical rate equation modelling results for a ~2 μm Tm<sup>3+</sup>-doped tellurite
fibre laser when pumped with a 1.6 μm Er<sup>3+</sup>/Yb<sup>3+</sup>-doped double-clad silica fibre laser. A maximum slope efficiency of
76% with respect to launched pump power was achieved in the experimental fibre laser set up with a 32 cm long fibre.
The high slope efficiency is very close to the Stokes efficiency limit of ~82% which is due to the in-band pumping
scheme employed and the lack of pump excited state absorption. The two-level rate equations involving absorption and
emission between the Tm<sup>3+</sup>: <sup>3</sup>H<sub>6</sub> and <sup>3</sup>F<sub>4</sub> levels have been solved iteratively using a fourth-order Runge-Kutta algorithm
and the results compared with the experimental results. For the 32 cm fibre with output coupler reflectivities of 12%,
50%, 70% and 90%, the respective theoretical slope efficiencies of 73%, 64%, 53% and 29% are in very good agreement
with the experimentally measured values of 76%, 60%, 48% and 33%.
Various lengths of Yb:Er:YVO4 were end pumped by a quasi-continuous wave 967 nm diode laser. The best slope
efficiency with respect to absorbed pump power for gain switched operation was 8 % for a 5 mm long crystal.
Co:MgAl<sub>2</sub>O<sub>4</sub> saturable absorbers of 98 % and 93 % initial transmission were used to passively Q-switch the cavity. For
the 98 % initial transmission absorber, average pulses energies of 44 µJ were measured. The average pulse width and
repetition rate were ~256 ns and 36 kHz, respectively. For the 93% initial transmission absorber, a single output pulse of
37 µJ energy and 22 ns duration per pump pulse was measured when the crystal was pumped for a pumping duration of
We report the active Q-switching of a Yb:Er:YVO4 laser for the first time. A Yb:Er:YVO4 crystal was end pumped by a
quasi-continuous wave laser diode emitting at 967 nm with a peak power of up to 48 W. The laser cavity was actively Qswitched
using the spinning disc technique. At a repetition rate of 19.2 kHz, the Q-switched slope efficiency and
threshold were 4 % and 62 mJ respectively. In comparison, the same system had a slope efficiency of 5% and a threshold
of 75 mJ without mechanical Q-switching. Single pulse of energy up to 90 μJ and duration as short as 110 ns were
obtained for the single output pulse per pump pulse operation.
The first demonstration of a pulsed Nd:YCOB laser at 1060 nm is reported, with results for both gain switching and Qswitching
presented. Active Q-switching is achieved using the spinning disc technique pulses of 50 ns duration with
pulse energies up to 0.6 mJ are obtained. Optimisation is performed for both pulse energy and slope efficiency of the
laser. A Q-switched slope efficiency of 56% is achieved.
We report a Tm3+/Yb3+-doped tellurite fibre laser operating at wavelengths in the range 1879 - 1994 nm. Two different pump schemes have been demonstrated for this laser: a 1088 nm Yb3+-doped silica fibre laser simultaneously pumping the Tm3+: 3H5 and Yb3+: 2F5/2 levels, and a 1610 nm Er3+/Yb3+-doped silica fibre laser directly pumping the Tm3+: 3F4 upper laser level. For the 1610 nm pumping, a slope efficiency of 76% has been achieved in a 32 cm long fibre which is very close to the Stoke efficiency limit of ~80%. An output power of 283 mW has also been achieved, but with no signs of saturation or fibre damage suggesting that higher output powers should be possible. For the 1088 nm pumping there is very strong pump ESA resulting in bright blue (480 nm) and near-IR (800 nm) emission and this limits the achievable slope efficiency, which in this case was a maximum of 10% for a 16 cm long fibre. With this pump scheme, the highest observed output power was 67 mW, and further power scaling was limited due to the intense ESA and thermal damage to the pump end of the fibre. Lasing has been achieved in <10 cm lengths of this fibre making this material a promising candidate for ultra compact medium power mid-IR laser sources for range-finding, medical and atmospheric monitoring and sensing applications.
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.
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 (<sup>6</sup>H<sub>9/2</sub> , <sup>6</sup>F<sub>11/2</sub>) centred at 1.3 μm of Dy<sup>3+</sup>-doped
ZBLAN is utilised and the lasing transition around ~3 μm takes places from <sup>6</sup>H<sub>13/2</sub> → <sup>6</sup>H<sub>15/2</sub>. With this pumping scheme
the Stokes' efficiency is expected to be up to ~45%. A quasi-continuous wave Dy<sup>3+</sup>-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<sup>-1</sup> Raman Stokes shift has also
detected by using 514.5 nm and 488 nm Ar ion laser as excitation pump sources.