Femtosecond laser direct writing in glass materials represents a simple single-step approach to generate threedimensional
(3D) optical circuits that cannot be constructed with traditional fabrication techniques.
In this paper, we present an attractive extension of such femtosecond laser processing to the writing of optical circuits
directly inside the cladding of single-mode optical fiber. To enable the formation of strongly guided and undistorted
waveguide modes within the small cylindrical fused silica volume (125 μm diameter), frequency-doubled (λ = 522 nm)
Ytterbium fiber-amplified femtosecond laser light at high repetition rate (500 kHz) was tightly focused with a high 1.25
numerical aperture (NA) oil immersion lens. In this way, low-loss waveguides could be arbitrarily located in various
cladding positions without generating ablation damage.
Basic components such as directional couplers were demonstrated that present a new means for dense integration of
optical elements that couple with the nearby fiber core. Such 3D all-fiber optical circuits represent practical tools to
bypass tedious assembly and packaging steps such as fiber pigtailing with planar lightwave components. This formation
of optical circuits directly within the cladding of optical fiber opens new prospects for manufacturing compact and
functional optical and optofluidic microsystems for Telecom, sensing and lab-on-a fiber applications.
Optical waveguide writing with femtosecond laser pulses represents a good alternative to traditional fabrication
methods thanks to its simplicity, flexibility and possibility to realize 3D structures. The direct use of a laser oscillator
allows a simpler setup, without amplification stages, greater processing speed, up to 1 cm/s, and intrinsically symmetric
waveguide cross-sections due to isotropic heat diffusion.
In this work we report on the fabrication and optical characterization of waveguides at telecom wavelengths by a
stretched-cavity (26 MHz repetition rate) Ti:Sapphire oscillator. The best results have been obtained on Corning 0211
and the previously unexplored Schott IOG10. Operation at 1.55-micron is demonstrated and a comparison between
optical properties of the waveguides on the two glasses is made. The refractive index profiles have been measured with
two different techniques: the innovative Digital Holography Microscopy (DHM), applied for the first time to optical
waveguides, and near-field refractive index profilometry (RNF). The shape of the refractive index profile was found to
depend strongly on the glass type.
We demonstrate passive photonic devices at 1.55-micron, exploiting the unique 3D capabilities of the technique. These
devices include: (i) a 1x2 splitter, obtained by writing two straight waveguides at an angle and separated by a depth
displacement; (ii) a 1x4 splitter, realized by combining 1x2 splitters on different planes in the depth; (iii) a WDM
coupler, with a good rejection of the 980-nm signal with respect to the 1550-nm one. Perspectives of the technique will
also be addressed.
Direct waveguide writing by femtosecond lasers is rapidly becoming a promising valid alternative to standard fabrication techniques. Significant research efforts are devoted to understanding the effects of interaction of the radiation with the material and determining the key parameters in the writing process. The assessment of a reliable fabrication process depends crucially also on the availability of high resolution inspection and measurement methods. In this paper we employ digital holography (DH) in a microscope configuration as the characterization tool for measuring the refractive index profile of the waveguides. The method offers the advantages of high spatial resolution, high sensitivity and it allows to determine an absolute value of refractive index change without the need of any calibration. We report on the optical characterization of optical waveguides operating at 1.5 micron wavelength in two commercial glasses written by a stretched-cavity femtosecond Ti:sapphire oscillator. Measurements made by DH have evidenced a strong dependence of the fabrication process on the type of glass substrate.