We report the spectroscopy of crystalline waveguide amplifiers operating in the telecom C-band. Thin films of erbiumdoped gadolinium lutetium potassium double tungstate, KGd<sub>x</sub>Lu<sub>y</sub>Er<sub>1-x-y </sub>(WO<sub>4</sub>)<sub>2</sub>, are grown by liquid- phase epitaxy onto undoped potassium yttrium double tungstate (KYW) substrates and micro-structured by Ar<sup>+</sup>- beam etching. Channel waveguides with erbium concentrations between 0.45–6.35 × 10<sup>20</sup> cm<sup>-3</sup> are characterized. The transition cross-sections of interest are estimated. The effect of energy-transfer up-conversion (ETU) is experimentally investigated. Microscopic and macroscopic ETU parameters are extracted from a simultaneous analysis of 20 decay curves of luminescence on the transition <sup>4</sup>I<sub>13/2</sub> → <sup>4</sup>I<sub>13/2</sub>. The correlation between ETU and the doping concentration is studied. Pump excited-state absorption (ESA) on the transition <sup>4</sup>I<sub>11/2</sub> → <sup>4</sup>F<sub>7/2</sub> is investigated via a direct ESA measurement using a double-modulation pump-probe technique. The effect of ESA is studied for different pump wavelengths. The pump wavelength of 984.5 nm is found to be favorable for the complete range of erbium concentrations.
A potassium double tungstate layer with the composition KY<sub>0.40</sub>Gd<sub>0.29</sub>Lu<sub>0.23</sub>Tm<sub>0.08</sub>(WO<sub>4</sub>)<sub>2</sub> was grown onto a pure KY(WO<sub>4</sub>)<sub>2</sub> substrate by liquid-phase epitaxy, microstructured by standard lithography and Ar-ion etching, and overgrown by a pure KY(WO<sub>4</sub>)<sub>2</sub> layer. The end-facets were polished. Laser experiments were performed on these buried, ridge-type channel waveguides in a resonator with one butt-coupled mirror and Fresnel reflection from the other end-facet, resulting in a high output-coupling degree of 89%, compared to intrinsic round-trip losses of only 2%. By pumping with a Ti:Sapphire laser at 794 nm, 1.6 W of output power at 1.84 μm with a maximum slope efficiency of ~80% was obtained. To the best of our knowledge, this result represents the most efficient 2-μm channel waveguide laser to date. We determined the optimum Tm<sup>3+</sup> concentration in double tungstate channel waveguides to be at least 8at.% for efficient lasing. The theoretical limit of the slope efficiency depends on the Stokes efficiency which here is 43.2%, the outcoupling efficiency which here is 99%, and the pump quantum efficiency. The pump quantum efficiency of a 2-μm Tm<sup>3+</sup> laser pumped around 800 nm hinges on the efficiency of its cross-relaxation process. By fitting the macroscopic cross-relaxation parameter which linearly depends on the Tm<sup>3+</sup> concentration to concentration-dependent luminescence- decay data, calculating the overall decay rate of the pump level, and deriving the concentration-dependent pump quantum efficiency, we obtain a theoretical limit for the slope efficiency of 83% for the chosen Tm<sup>3+</sup> concentration. The experimental slope efficiency of ~80% closely approaches this limit.
A detailed study of the fabrication and continuous-wave laser operation of Ti:sapphire channel waveguides written with femtosecond (fs) and picosecond (ps) laser pulses in the bulk of a Ti:sapphire crystal is presented. The waveguides were produced using the double-line approach and the effect of parameters such as the laser pulse duration, repetition rate and interline spacing were investigated for optimizing the laser operation. Structures fabricated by fs-laser pulses (180 fs) exhibited superior performance delivering output powers up 143 mW with a slope efficiency of 23.5% and producing laser emission above a threshold of 84 mW. The emission wavelength was tuned over a wavelength range spanning from 700 to 920 nm using cavity optics with broadband transmission at the lasing wavelength in combination with a birefringent filter in an external cavity.
Air/silica Microstructured Optical Fibers (MOFs) offer new prospects for fiber based sensor devices. In this paper, two
topics of particular significance for gas sensing using air guiding Photonic Bandgap Fibers (PBGFs) are discussed. First,
we address the issue of controlling the modal properties of PBGFs and demonstrate a single mode, polarization
maintaining air guiding PBGF. Secondly, we present recent improvements of a femtosecond laser machining technique
for fabricating fluidic channels in PBGFs, which allowed us to achieve cells with multiple side access channels and low
We present results obtained from the first all-fiber, lensless, optical correlation spectroscopy gas sensor for acetylene
(C<sub>2</sub>H<sub>2</sub>). In the reported sensing configuration, hollow-core photonic bandgap fiber (PBGF) is employed to contain all gas
samples required for optical absorption measurements. This sensor relies upon comparison of the absorption spectrum of
acetylene held in a 'reference gas cell' to that of a gas sample under test, which is contained in the 'measurement gas
cell'. Ingress of the test gas mixture into the measurement cell is achieved via femtosecond laser-machined micro-channels
running from the surface of the PBGF to its hollow core. Stable, lensless optical interrogation of the
measurement cell is guaranteed by means of arc fusion splices to standard (solid-core) single-mode fiber (SMF). The
reference cell is filled with acetylene at atmospheric pressure, and is permanently sealed at both ends by splices to SMF.
Therefore, being constructed entirely from optical fiber, both the reference and measurement gas cells are inherently
compact and coilable, and dispense with the need for lenses or other free-space optics for connection to the correlation
spectroscopy system. We quantify the acetylene concentration of various test gas mixtures and compare our sensor's
measured results with computer simulations.
We present a new light source for parallel Optical Coherence Tomography (OCT) based on multiple waveguides written in Ti:sapphire. Each channel can generate a spectrum of 174 nm bandwidth centered at 772 nm, with an optical power on sample of 30 uW. A system depth resolution of 1.9 um is obtained, which correspond to 1.5 um in tissue.