Matthias Will

Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.

Sub-diffraction limited structuring of solid targets with femtosecond laser pulses

Possibilities to produce sub-diffraction limited structures in thin metal films and bulk dielectric materials using femtosecond laser pulses are investigated. The physics of ultrashort pulse laser ablation of solids is outlined. Results on the fabrication of sub-micrometer structures in 100-200 nm chrome-coated surfaces by direct ablative writing are reported. Polarization maintaining optical waveguides produced by femtosecond laser pulses inside crystalline quartz are demonstrated.

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.

Laser helical drilling of silicon wafers with ns to fs pulses: Scanning electron microscopy and transmission electron microscopy characterization of drilled through-holes

Electron microscopic methods (scanning electron microscopy and transmission electron microscopy) are used to characterize the precision and quality of microthrough-holes produced in 0.4 mm-thick silicon wafers by applying the pulsed laser helical drilling technique. The primary aim of the present work is to investigate how the mechanisms of material removal and redeposition change when the pulse width is systematically varied from nanosecond (ns) to femtosecond (fs) range (8 ns to 160 fs). Under the chosen processing conditions (Ti:sapphire laser, pulse energy: 0.3 mJ, beam spot diameter: 40 μm, resulting fluence: 24 J/cm2, processing time: 120 s) optimal drilling results, i.e., smooth holes being free of recast and free of thermally and mechanically driven structural damage, are achievable by using laser pulses with a width of 10 ps. On the contrary, drilling with ns pulses is associated with thick melt redepositions, high thermal load, and formation of microcracks, while processing with fs pulses suffer...

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.

Waveguides produced by ultrashort laser pulses inside glasses and crystals

Using tightly focused femtosecond laser pulses waveguides are fabricated inside glasses and crystalline materials. The guiding and attenuation properties at different wavelengths as well as the micro morphology of the irradiated samples are studied. We demonstrate the fabrication of single- and multi-mode waveguides with damping losses well below 1 dB/cm in fused silica. In crystalline quartz we found that the irradiated area has become amorphous due to the absorption of the laser radiation. In this case waveguiding is observed in a stress-induced region surrounding the irradiated, amorphous area.

Single- and multimode waveguides in glasses manufactured with femtosecond laser pulses

In this paper we demonstrate the possibility to produce single- and multimode waveguides with a well-defined number of guided modes in different doped and undoped glasses using fs laser pulses. In fused silica waveguides with damping values below 0.8 dB/cm have been realized. Moreover symmetrical beamsplitters and waveguides in different doped materials have been manufactured. Based on measurements of the generated refractive index changes calculations of the near-field intensity distribution of the guided light are performed and compared to experimental results.

Cutting of optical materials by using femtosecond laser pulses

In the past years, ultrashort pulse lasers have been established as precise and universal tools for the microstructuring of solid materials. Since thermal and mechanical influences are minimized, the application of this technology is also suitable for the structuring of optical materials and opens new possibilities. In this paper, the influence of pulse duration, pulse energy (fluence) and polarization on the cutting quality for glass and silicon will be discussed. As a concrete application, the cutting and micromarking of dielectric coated mirrors for high power fiber lasers will be highlighted.

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.