Guanghua Cheng

Micro Fabry-Perot interferometers in silica fibers machined by femtosecond laser

Micro Fabry-Perot (F-P) interferometers (MFPIs) are machined in a single-mode fiber (SMF) and a photonic crystal fiber (PCF) by using a near-infrared femtosecond laser, respectively. The strain and temperature characteristics of the two MFPIs with an identical cavity length are investigated and the experimental results show that the strain sensitivity of the PCF-based MFPI is smaller than that of the SMF-based MFPI due to their different waveguide structures, while the two MFPIs have close temperature sensitivities due to their similar host materials. These MFPIs in silica fibers are compact, stable, inexpensive, capable for mass-production and easy fabrication, offering great potentials for wide sensing applications.

Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials.

Femtosecond laser processing of bulk transparent materials can generate localized positive changes of the refractive index. Thus, by translation of the laser spot, light-guiding structures are achievable in three dimensions. Increasing the number of laser processing spots can consequently reduce the machining effort. In this paper, we report on a procedure of dynamic ultrafast laser beam spatial tailoring for parallel photoinscription of photonic functions. Multispot operation is achieved by spatially modulating the wavefront of the beam with a time-evolutive periodical binary phase mask. The parallel longitudinal writing of multiple waveguides is demonstrated in fused silica. Using this technique, light dividers in three dimensions and wavelength-division demultiplexing (WDD) devices relying on evanescent wave coupling are demonstrated.

Ultrafast laser photoinscription of polarization sensitive devices in bulk silica glass.

Ultrashort pulsed laser irradiation of bulk fused silica may result under specific energetic conditions in the self-organization of subwavelength material redistribution regions within the laser trace. The modulated structures have birefringent properties and show unusual anisotropic light scattering and reflection characteristics. We report here on the formation of waveguiding structures with remarkable polarization effects for infrared light. The photoinscription process using 800 nm femtosecond laser pulses is accompanied by third harmonic generation and polarization dependent anisotropic scattering of UV photons. The photowritten structures can be arranged in three-dimensional patterns generating complex propagation and polarization effects due to the anisotropic optical properties.

Nanosize structural modifications with polarization functions in ultrafast laser irradiated bulk fused silica

Laser-induced self-organization of regular nanoscale layered patterns in fused silica is investigated using spectroscopy and microscopy methods, revealing a high presence of stable broken oxygen bonds. Longitudinal traces are then generated by replicating static irradiation structures where the nanoscale modulation can cover partially or completely the photoinscribed traces. The resulting birefringence, the observed anisotropic light scattering properties, and the capacity to write and erase modulated patterns can be used in designing bulk polarization sensitive devices. Various laser-induced structures with optical properties combining guiding, scattering, and polarization sensitivity are reported. The attached polarization functions were evaluated as a function of the fill factor of the nanostructured domains. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.

Control of ultrafast laser-induced bulk nanogratings in fused silica via pulse time envelopes

Employing a method of in-situ control we propose an approach for the optimization of self-arranged nanogratings in bulk fused silica under the action of ultrashort laser pulses with programmable time envelopes. A parametric study of the influence of the pulse duration and temporal form asymmetries is given. Using the diffraction properties of the laser-triggered subwavelength patterns we monitor and regulate the period and the quality of the periodic nanoscale arrangement via the effective nonlinear excitation dose. Periodicity tuning on tens of nanometers can be achieved by pulse temporal variations, with a minimum around 0.7 ps at the chosen powers. Equally, strong sensitivity to pulse asymmetries is observed. The driving factor is related to increasing carrier densities due to nonlinear confinement and the development of extended nanoroughness domains upon multiple exposure, creating a pulse-dependent effective accumulation dose via a morpho-dimensional effect. The result may impact the associated optical functions.

Polarization behavior of femtosecond laser written optical waveguides in Ti:Sapphire

Ultrashort pulsed laser photoinscription of Ti:Sapphire crystals may result in the self-organization of nanoscale material redistribution regions in regular patterns within the laser trace and stress-induced birefringence around the laser trace. We report on the formation of anisotropic optical waveguides in Ti:Sapphire by a procedure that involves femtosecond laser inscription of adjacent nonguiding birefringent traces with nanopatterned crosssections and the accumulation of stress birefringence in the region between. Double parallel line structures with a separation of 25μm with vertical and horizontal nanoscale arrangements were written with a choice of orthogonal polarizations. Due to anisotropic light scattering on periodic nanostructures and stress-induced birefringence in the central zone, remarkable polarization dependent guiding effects were observed as a function of the microscopic geometry of the structures. Building on this polarization sensitivity, several structure such as 3 × 3 waveguide arrays, diamond and hexagon patterns are also investigated.

Large-mode-area infrared guiding in ultrafast laser written waveguides in Sulfur-based chalcogenide glasses

Current demands in astrophotonics impose advancing optical functions in infrared domains within embedded refractive index designs. We demonstrate concepts for large-mode-area guiding in ultrafast laser photowritten waveguides in bulk Sulfur-based chalcogenide glasses. If positive index contrasts are weak in As2S3, Ge doping increases the matrix rigidity and allows for high contrast (10(-3)) positive refractive index changes. Guiding with variable mode diameter and large-mode-area light transport is demonstrated up to 10 μm spectral domain using transverse slit-shaped and evanescently-coupled multicore traces.

Large mode area waveguides with polarization functions by volume ultrafast laser photoinscription of fused silica

We present optical designs allowing large mode area light guiding by ultrafast laser photoinscription of bulk fused silica. If usual concepts are based on large core and depressed cladding, evanescently coupled multicore waveguides with coherent mode superposition can be effective solutions, where the introduction of nanostructured defects determines additional polarization functions.

Formation of nanogratings in a transparent material with tunable ionization property by femtosecond laser irradiation

Irradiation inside some transparent materials such as fused silica can induce nanograting structures at the focal area. Here, we investigate experimentally how the nanograting formation can be influenced by tuning the ionization property of the transparent material, which is achieved by irradiation inside a porous glass immersed in water doped with NaCl at variable concentrations. Our results show that the doping of NaCl not only reduces the threshold fluence of optical breakdown, but also leads to nanograting structures with shorter periods. These effects may be attributed to the enhanced photoionization in water doped with NaCl.

Stressed waveguides with tubular depressed-cladding inscribed in phosphate glasses by femtosecond hollow laser beams

We report on the single-step fabrication of stressed optical waveguides with tubular depressed-refractive-index cladding in phosphate glasses by the use of focused femtosecond hollow laser beams. Tubelike low index regions appear under direct exposure due to material rarefaction following expansion. Strained compacted zones emerged in domains neighboring the tubular track of lower refractive index, and waveguiding occurs mainly within the tube core fabricated by the engineered femtosecond laser beam. The refractive index profile of the optical waveguide was reconstructed from the measured transmitted near-field intensity.