Jonas Burghoff

Efficient frequency doubling in femtosecond laser-written waveguides in lithium niobate

Optical waveguides in lithium niobate have been fabricated with a femtosecond laser. Different types of modifications depending on the laser parameters were observed and discussed. In these waveguides, frequency doubling of 1064nm radiation was demonstrated utilizing birefringent phase matching. A conversion efficiency of 49% was obtained in a 9.3mm long sample.

Two-dimensional soliton in cubic fs laser written waveguide arrays in fused silica

The observation of a two-dimensional discrete soliton in a cubic 5 x 5 fs laser written waveguide array in fused silica is reported for the first time. In addition to the localization the sharp edges of the array allow to study the influence of the arrays boundaries. The results are in excellent agreement with theoretical predictions and provide the basis for a variety of future applications for nonlinear two-dimensional integrated optical devices.

Discrete nonlinear localization in femtosecond laser written waveguides in fused silica

The observation of discrete spatial solitons in fs laser written waveguide arrays in fused silica is reported for the .rst time. The fs writing process permits the speci.c setting of the linear and nonlinear guiding properties of the waveguides. The results in this paper reveal a new avenue for the fabrication of various nonlinear optical devices.

Erbium fiber laser based on intracore femtosecond-written fiber Bragg grating

We report the inscription of fiber Bragg gratings (FBGs) into a nonphotosensitive Er-doped fiber by using a phase-mask scanning technique with near IR femtosecond laser pulses. A grating of 40 mm length with a period of 1.075 microm was realized. We measured transmission losses of -18.9 dB at lambda=1554.5 nm with a FWHM bandwidth of 0.15 nm. By pumping the fiber containing the fabricated FBG, it was possible to realize a fiber laser with an output power of 38 mW, a slope efficiency of 21.1%, and low noise (SNR=60 dB).

Femtosecond laser-written quasi-phase-matched waveguides in lithium niobate

We report on thermally stable optical waveguides written in periodically poled lithium niobate with a femtosecond laser using a two line approach. Both fundamental and second harmonic light at 1064 and 532nm are guided with low damping losses. In a 10-mm-long sample a conversion efficiency of 58% was achieved.

Ultrafast laser processing: New options for three-dimensional photonic structures

Abstract Using tightly focused ultrashort laser pulses allows the direct writing of three-dimensional photonic structures in different glasses and also crystalline media. One of the main drawbacks of this technology is, however, the limited writing speed achieved so far. In this paper we shall review our recent advances in the direct writing of three-dimensional integrated-optical devices and discuss a new approach using a fibre-based femtosecond laser system producing 300 fs pulses with pulse energies of 0.6 μJ at 2 MHz repetition rate. Using this laser system we fabricated low-loss waveguides (less than 0.5 dBcm−1) at writing speeds of 100 mms−1 for the first time. The influence of the writing speed on the produced structures as well as their optical properties will be discussed in detail.

Femtosecond writing of high-quality waveguides inside phosphate glasses and crystalline media using a bifocal approach

The fabrication of waveguides inside transparent media using ultrashort laser pulses has gained a lot of interest during the past years. When these intense pulses are tightly focused inside the material a refractive index increase in the focal volume can be achieved. Low-loss waveguides and true three-dimensional integrated optical devices have been produced by this direct writing technique in silicate glasses. However, other materials like phosphate glasses or crystalline quartz show a different behavior. In this case stress is induced around the focal volume leading to a refractive index increase in the surrounding areas (inverse profile). As a consequence high quality waveguides with a mode profile matched to conventional fibers cannot be fabricated. In this presentation we demonstrate a new approach to overcome this problem. The laser beam is split by a transmission grating and simultaneously focused at two different areas. When the distance between the two foci is appropriate the desired refractive index profile is obtained. We will demonstrate high quality waveguides in crystalline quartz and in phosphate glasses produced by this technique. The influence of the processing parameters is discussed in detail.

Detailed investigations on femtosecond-induced modifications in crystalline quartz for integrated optical applications

In recent years, ultrashort laser pulses have drawn increasing interest for the direct writing of photonic structures into different materials. Several optical devices have already been demonstrated, e.g. optical waveguides, waveguide amplifiers and lasers, beam splitters, couplers, stacked waveguides and three-dimensional waveguide arrays and gratings. The investigations were mainly focused on glasses where the laser irradiation causes a rise in refractive index. However, for different applications the realization of waveguides in crystalline media is interesting. Here, we present investigations on femtosecond laser induced modifications in crystalline quartz. We show that the irradiation leads to a refractive index decrease which is due to amorphization of the focal volume. A detailed analysis of the structures is performed with transmission electron microscopy and X-ray diffraction and topography. Our investigations show that the irradiated amorphous core creates a stress field in the surrounding material that possesses a positive index change and therefore supports the guiding of light. The results of the X-ray experiments allow a quantitative characterization of the stress field. We are able to simulate the stress distribution by a simple model based on the density difference between the amorphous and crystalline material. From this the refractive index profile can be calculated and compared to experimental results. The light guiding properties of the compressed regions and the fact that only one polarization is guided can be verified by the simulation results.

Fabrication of three-dimensional photonics devices using femtosecond laser pulses

Localized structural and refractive index modifications can be generated inside transparent solids by using focused ultrashort laser pulses, which allows for example the fabrication of optical waveguides. In this paper we present the fabrication of true three-dimensional integrated optical devices. The optical properties of the produced 3D structures as well as processing details and requirements on the positioning accuracy will be discussed. The experimental results will be compared with beam propagation simulations and limitations of this technique will be evaluated.

Solid phase formation of silicon nanocrystals by bulk ultrafast laser-matter interaction

Ultrashort pulse laser interaction with silica-silicon interfaces is presented as a means for all-solid-phase formation of high-purity silicon nanoparticles in the absence of ablation plumes or any substrate intermixing with surfaces in ambient air. Transmission electron microscopy and Raman spectroscopy provide definitive evidence for creation of nanocrystals in the silica host, while compressive stress in the silicon substrate corroborates the formation of optical waveguides parallel to the tracks.