Femtosecond inscription of the first order Bragg gratings in pure fused silica

The fabrication of sub-micron periodic structures beyond diffraction limit is a major motivation for the present paper. We describe the fabrication of the periodic structure of 25 mm long with a pitch size of 260 nm which is less than a third of the wavelength used. This is the smallest reported period of the periodic structure inscribed by direct point-by-point method. A prototype of the add-drop filter, which utilizes such gratings, was demonstrated in one stage fabrication process of femtosecond inscription, in the bulk fused silica.

Inscription of in-fibre photonic devices by an infrared femtosecond laser

In this work, a point by point method for the inscription of fibre Bragg gratings using a tightly focused infrared femtosecond laser is implemented for the first time. Fibre Bragg gratings are wavelength-selective, retro-reflectors which have become a key component in optical communications as well as offering great potential as a sensing tool. Standard methods of fabrication are based on UV inscription in fibre with a photosensitive core. Despite the high quality of the gratings, a number of disadvantages are associated with UV inscription, in particular, the requirements of a photosensitive fibre, the low thermal stability and the need to remove the protective coating prior to inscription. By combining the great flexibility offered by the point by point method with the advantages inherent to inscription by an infrared femtosecond laser, the previous disadvantages are overcome. The method here introduced, allows a fast inscription process at a rate of ~1mm/s, gratings of lengths between 1cm and 2cm exhibiting reflections in excess of 99%. Physical dimensions of these gratings differ significantly from those inscribed by other methods, in this case the grating is confined to a fraction of the cross section of the core, leading to strong and controllable birefringence and polarisation dependent loss. Finally, an investigation of the potential for their exploitation towards novel applications is carried out, devices such as directional bend sensors inscribed in single-mode fibre, superimposed but non-overlapping gratings, and single-mode, single-polarisation fibre lasers, were designed, fabricated and characterised based on point by point femtosecond inscription.

Point by point FBG inscription by a focused NIR femtosecond laser

Direct, point-by-point writing of fibre Bragg gratings in standard telecommunication fibre by femtosecond laser irradiation is demonstrated for the first time.

Modelling of femtosecond inscription in fused silica

We present theoretical and numerical analysis of the femtosecond processing of silica by laser beam with power below self-focusing threshold.

Femtosecond inscription of the first order Bragg gratings in pure fused silica

The fabrication of sub-micron periodic structures beyond diffraction limit is a major motivation for the present paper. We describe the fabrication of the periodic structure of 25 mm long with a pitch size of 260 nm which is less than a third of the wavelength used. This is the smallest reported period of the periodic structure inscribed by direct point-by-point method. A prototype of the add-drop filter, which utilizes such gratings, was demonstrated in one stage fabrication process of femtosecond inscription, in the bulk fused silica.

Waveguide inscription in YAG:Cr4+ crystals by femtosecond laser irradiation

We have observed a positive change or refractive index and formation of waveguides in YAG:Cr4+ crystals, exposed to a high-intensity femtosecond laser beam. The technique is potentially suitable for fabrication of waveguide lasers in crystal materials.

Direct femtosecond inscription of fiber Bragg gratings

Direct, point-by-point inscription of fiber Bragg gratings by infrared femtosecond laser is reported. Using this technique, highly reflective gratings can be rapidly inscribed in standard, untreated fiber. Thermal studies demonstrate increased thermal stability compared to the UV-inscribed gratings. © 2005 Materials Research Society.

Sub-critical regime of femtosecond inscription

We apply well known nonlinear diffraction theory governing focusing of a powerful light beam of arbitrary shape in medium with Kerr nonlinearity to the analysis of femtosecond (fs) laser processing of dielectric in sub-critical (input power less than the critical power of selffocusing) regime. Simple analytical expressions are derived for the input beam power and spatial focusing parameter (numerical aperture) that are required for achieving an inscription threshold. Application of non-Gaussian laser beams for better controlled fs inscription at higher powers is also discussed. © 2007 Optical Society of America.

Inscription of a 300-nm-period nanostructure in a pure fused silica

Very recently, using tightly-focused femtosecond near-IR pulses, periodical sub-micron structures have been recorded [1,2]. Such microfabrication utilizes the multi-photon approach, which allows the inscription inside various non-photosensitive optical materials. The combination of multi-photon excitation with the point-by-point technique offers the great potential of creating non-uniform chirped gratings by controlling the rate of femtosecond pulses or the sample translation speed.

Point-by-point inscription of 250 nm period structure in bulk fused silica by tightly focused femtosecond UV pulses

By conducting point-by-point inscription in a continuously moving slab of pure fused silica at the optimal depth (170νm depth below the surface), we have fabricated a 250nm period nanostructure with 30nJ, 300fs, 1kHz pulses from a frequency-tripled Ti:sapphire laser. This is the smallest value for the inscribed period yet reported, and has been achieved with radical improvement in the quality of the inscribed nanostructures in comparison with previous reports.

Nonlinear diffraction in sub-critical femtosecond inscription

Material processing using high-intensity femtosecond (fs) laser pulses is a fast developing technology holding potential for direct writing of multi-dimensional optical structures in transparent media. In this work we re-examine nonlinear diffraction theory in context of fs laser processing of silica in sub-critical (input power less than the critical power of self-focusing) regime. We have applied well known theory, developed by Vlasov, Petrishev and Talanov, that gives analytical description of the evolution of a root-mean-square beam (not necessarily Gaussian) width RRMS(z) in medium with the Kerr nonlinearity.

Characterization of nonlinear absorption in femtosecond laser inscription

Direct measurements of the absorbed energy in femtosecond laser inscription in a range of materials is performed. Key absorption parameters are characterized by fitting numerical modelling to measurements.

In-situ measurement and the reconstruction in 3D of femtosecond inscription induced complex permittivity modification in glass

We demonstrate a new approach to in-situ measurement of femtosecond laser pulse induced changes in glass enabling the reconstruction in 3D of the induced complex permittivity modification. The technique can be used to provide single shot and time resolved quantitative measurements with a micron scale spatial resolution.

Point-by-point inscription of first-order fiber Bragg grating for C-band applications

The influence of the fiber geometry on the point-by-point inscription of fiber Bragg gratings using a femtosecond laser is highlighted. Fiber Bragg gratings with high spectral quality and strong first-order Bragg resonances within the C-band are achieved by optimizing the inscription process. Large birefringence (1.2x10-4) and high degree of polarizationdependent index modulation are observed in these gratings. Potential applications of these gratings in resonators are further illustrated. © 2007 Optical Society of America.

Inscription and characterization of waveguides written into borosilicate glass by a high-repetition-rate femtosecond laser at 800 nm

A series of waveguides was inscribed in a borosilicate glass (BK7) by an 11 MHz repetition rate femtosecond laser operating with pulse energies from 16 to 30 nJ and focused at various depths within the bulk material. The index modification was measured using a quantitative phase microscopy technique that revealed central index changes ranging from 5×10-3 to 10-2, leading to waveguides that exhibited propagation losses of 0.2 dB/cm at a wavelength of 633 nm and 0.6 dB/cm at a wavelength of 1550 nm with efficient mode matching, less than 0.2 dB, to standard optical fibers. Analysis of the experimental data shows that, for a given inscription energy, the index modification has a strong dependence on inscription scanning velocity. At higher energies, the index modification increases with increasing inscription scanning velocity with other fabrication parameters constant.