Photonic crystal fibers design for 2 μm wavelenght operation

New photonic crystal fiber designs are presented and numerically investigated in order to improve the state of art of high power fiber lasers. The focus of this work is targeted on the region of 2 μm laser emission, which is of high interest due to its eye-safe nature and due to the large amount of applications permitted. Thulium doped fiber amplifiers are suitable for emitting in this region. Different fiber designs have been proposed, both flexible and rod-type, with the aim to enlarge mode area while maintaining robust single mode operation. The analysis of thermal effects, caused by the high thulium quantum defect, have been taken in consideration. Solutions to counteract issues derived by detrimental thermal effects have been implemented.

Waveguide-saturable absorber fabricated by femtosecond pulses in YAG:Cr4+ crystal for Q-switched operation of Yb-fiber laser

A waveguide-saturable absorber with low propagation loss is fabricated by femtosecond pulses in YAG:Cr4+ crystal. Q-switch operation of a Yb fiber laser with the new saturable absorber having absorption saturation parameters similar to the bulk YAG:Cr4+ crystal is demonstrated.

Crystal orientation effects in scratch testing with a spherical indenter

Spherical scratch tests were conducted in individual grains of a randomly oriented polycrystalline body-centered-cubic (bcc) Ti-Nb alloy. For each grain, scratch tests were conducted at four different levels of normal load, which resulted in varying amounts of plastic strain during indentation. The results show a dependence of the horizontal load component on the crystallographic orientation and on the amount of plastic strain. The component of the horizontal force that resulted from plastic deformation was found to correlate with the active slip systems for the particular grain orientation. © 2010 Materials Research Society.

Measured sensitivity of arc-induced long-period grating sensors in photonic crystal fibre

Reported are experimental results from investigations of the sensing properties of long-period gratings (LPGs) recorded in two different geometries of photonic crystal fibre (PCF): a large-mode area PCF and an endlessly single mode PCF. The LPGs have been characterised for their sensitivity to temperature, bending, surrounding index and strain. The LPGs in both fibres have been found to have negligible temperature sensitivity whilst exhibiting useful strain sensitivities. Strong directional bend sensitivity is shown by one PCF whilst the other shows good non-directional bend sensitivity. The fibres exhibit differing sensitivities to surrounding refractive index.

Temperature-insensitive long period grating sensors in photonic crystal fibre

A long period grating has been fabricated in endlessly single-mode photonic crystal fibre using a spatially-periodic electric arc discharge. The sensing characteristics of the grating have been studied and it was found to possess an insensitivity to temperature, a bend sensitivity of 3.7 nm · m and a strain sensitivity of -2.0 pm/µe.

Nondestructive index profiling of long period gratings in photonic crystal fibres

A numerical model of a long period grating in photonic crystal fibre fabricated by an electric arc is proposed that allows for the spectral characterisation of the grating. In the combination with the suggested model of the photonic crystal and the experimentally recorded grating growth it is used to find the index change induced by the electric arc.

Multiple period resonances in long period gratings in photonic crystal fibres

In order to characterise long period gratings fabricated in endlessly single mode photonic crystal fibres with bulk cladding we perform eigenanalysis of guided modes supported by these fibres. Resonant coupling occurs only when the beating length equals the multiple grating periods. Experimentally obtained grating spectra and sensitivity are fully explained using modified phase matching condition.

Long period gratings written into a photonic crystal fibre by a femtosecond laser as directional bend sensors

A series of symmetric and asymmetric LPGs were inscribed in photonic crystal fibre by a low repetition rate femtosecond laser system. The asymmetric LPGs were found to be spectrally sensitive to bend orientation, with some of the attenuation bands producing both red and blue wavelength shifts, whilst the symmetric devices produced only a unidirectional wavelength shift. Both sets of devices displayed strong polarisation dependence.

Creep and fatigue of a single crystal nickel base superalloy

The introduction of single crystal casting techniques has led to the development of existing nickel-base superalloys to produce materials with optimum mechanical properties in the single crystal condition. As single crystals are known to be anisotropic, a study is needed to determine the general mechanical properties of these materials, and determine the effects of crystal orientation upon them. A study has been carried out to identify the effect of orientation and temperature on the creep and fatigue properties of a development single crystal superalloy, SRR 99. Creep testing and crystal rotation experiments have been made on SRR 99 and an earlier development alloy, SRR 9. Fatigue experiments at elevated temperatures have been carried out on both notched and un-notched specimens of alloy SRR 99. To aid in this analysis, several analytical techniques have been employed including Laue x-ray orientation analysis, measurement of strain by photographic methods and microstructural examination. Crystal rotation experiments have indicated that shear of 1 precipitates by lbrace111rbrace< 112> slip systems is operative during primary creep deformation at temperatures of 750oC and 850oC. The effect of orientation variation obtained by standard casting practices was not found to be significant. Creep rupture was found to be associated with multiple crack initiation from micropores. Fatigue crack initiation in un-notched specimens was found to be related to microporosity and microstructural defects. Failure was predominantly by crystallographic crack growth on lbrace111rbrace planes. The use of linear elastic fracture mechanics to describe fatigue crack propagation in alloy SRR 99 was found to be acceptable at temperatures up to 850oC. Variation of temperature, frequency and crystal orientation was found to have only moderate effect upon crack propagation rates.

Control of the properties of micro-structured waveguides in lithium niobate crystal

We study numerically depressed-index cladding, buried, micro-structured optical waveguides that can be formed in a lithium niobate crystal by femtosecond laser writing. We demonstrate to which extent the waveguiding properties can be controlled by the waveguide geometry at the relatively moderate induced refractive index contrasts that are typical of the direct femtosecond inscription.

Influence of beam-coupling on photorefractive parametric oscillation in a dc-field-biased Bi12SiO20 crystal

The influence of beam coupling on photorefractive parametric oscillation generated in a Bi12SiO20 crystal is investigated experimentally by comparing two configurations with and without the presence of beam coupling. It is shown that beam coupling has a great influence; for example, the transversal split of the K/2 subharmonic grating is seen only in the beam-coupling geometry. A case that resembles K/4 subharmonic generation can, however, still be found in the absence of beam coupling.