Compact birefringent waveplates photo-induced in silica by femtosecond laser

© 2014 by the authors; licensee MDPI, Basel, Switzerland. Recently, we showed that femtosecond laser induced "nanogratings" consist of thin regions with a low refractive index (Δn = -0.15), due to the formation of nanoporous silica surrounded by regions with a positive index change. In this paper, we investigate a wide range of laser parameters to achieve very high retardance within a single layer; as much as 350 nm at λ = 546 nm but also to minimize the competing losses. We show that the total retardance depends on the number of layers present and can be accumulated in the direction of laser propagation to values higher than 1600 nm. This opens the door to using these nanostructures as refined building blocks for novel optical elements based on strong retardance

Time-resolved plasma measurements in Ge-doped silica exposed to infrared femtosecond laser

Using a time-resolved interferometric technique, we study the laser-induced carrier-trapping dynamics in SiO 2 and Ge-doped SiO 2. The fast trapping of electrons in the band gap is associated with the formation of self-trapped excitons (STE). The STE trapping is doping dependent in SiO 2. The mean trapping time of electrons excited in the conduction band was found to be significantly lower in Ge-doped silica (75 ± 5 fs) when compared to pure silica (155 ± 5 fs). At our concentration level, this indicates that the plasma properties are determined by the presence of easily ionizable states such as the presence of Ge atoms in the glass network. Therefore, we suggest that in Ge-doped silica there exist an additional trapping pathway that leads to a significantly faster excitons trapping and a higher plasma density when compared to undoped silica. © 2011 American Physical Society

Thermal Stability of Type II Modifications Inscribed by Femtosecond Laser in a Fiber Drawn from a 3D Printed Preform

Fiber drawing from a 3D printed perform was recently discussed to go beyond the limitations of conventional optical fiber manufacturing in terms of structure and materials. In this work, the photosensitivity of silica optical fibers to femtosecond laser light, and fabricated by 3D printing a preform, is investigated. The writing kinetics and the thermal performance of Type II modifications are studied by varying the laser pulse energy and investigating the birefringence response of the femtosecond (fs)-laser written structures. Compared with a conventional telecom single mode fiber (SMF28), the fiber made by 3D printing is found to have similar writing kinetics and thermal performance. Additionally, the thermal stability of the imprinted fs-laser induced nanostructures is investigated based on the Rayleigh–Plesset equation, describing a model of nanopores dissolution underpinning Type II modifications with thermal annealing

Viscoelastic tuning of fibre Bragg gratings during regeneration

Regenerating at constant temperature under load allows inelastic changes in glass that can be exploited to tune the Bragg wavelength of a filter to any arbitrary spectral position. We have reported < 20nm of tuning of a 1 cm grating with no limit in sight. Further, regenerating under a temperature profile allows complex spectral shaping of the grating profile. As an example, we have reported broadband chirping by more than Δλ < 9 nm over 1 cm using this approach. High temperature viscoelastic tuning therefore offers a simple and powerful low cost way of tuning the properties of fibre Bragg gratings using a single phase mask. © 2014 SPIE

Applications of Raman spectroscopy, a tool to investigate glass structure and glass fiber

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Picosecond laser machining in the bulk of transparent dielectrics: critical comparison with fs-laser direct writing

Picosecond lasers for bulk machining of transparent dielectrics are assessed as an alternative to fs-lasers. Nanogratings and micro-channels by selective etching are demonstrated. Scattering and inhomogeneous etching are challenges yet to be solved

Broadband anisotropy of femtosecond laser induced nanogratings in fused silica

We demonstrate operation of femtosecond laser imprinted birefringent optical elements from 200 to 2100nm. The absorption losses in the UV can substantially reduced by the post-annealing of the structures. The birefringence is also accompanied with the weak dichroism which is related to the polarization dependent scattering. The transition from the isotropic modification to nanostructures is accompanied by occurrence of ODC(II) defect

Photo-induced densification in Er3+/Al doped silica preform plates using 193-nm laser light

The growth rate of Bragg gratings written using 193-nm light from an ArF excimer laser is shown to be non-linear with the pulse energy density for Er3+ co-doped Al silica. This yields a refractive-index increase up to a few 10-3. We then use phase-shift interferometry to measure the sample surface topography following exposure. Subsequently, we formulate an inhomogeneous stress model to analyse the observed change of volume within the core material. Below pulse energies of 550 mJ/cm2, the results show that refractive-index changes are primarily due to a densification process. At higher pulse energies material re-expansion occurs. © 2009 Springer-Verlag

Doping dependence of the femtosecond laser damage thresholds in silica glasses

We observed that the first threshold (i.e. permanent isotropic index change) is not significantly dependent on the doping whereas it is the contrary for the second threshold (i.e. permanent linear birefringence)