Integrated optics in silicon is interesting for various optoelectronic devices, since photonics and electronics could be realized together. However, to be compatible with the standard CMOS technology, optical waveguides which rely on structuring the silicon surface are inappropriate. An alternative solution is the direct structuring inside the bulk medium by using ultrashort laser pulses.
In recent years true three-dimensional photonic structures have been fabricated inside crystals and various optical glasses with this technique. Here, we demonstrate the use of femtosecond laser pulses to directly inscribe optical waveguides into the bulk of crystalline silicon. Due to the bandgap of 1.1 eV of silicon, the 800 nm pulses of the typically used Ti:Sapphire lasers cannot penetrate into the silicon. Therefore, the wavelength was converted to 2.6 μm using an optical parametric amplifier and the pulses were then focused into the bulk silicon by a Schwarzschild reflective objective. This way the laser energy was deposited in the focal region by three-photon absorption. Waveguides have been produced by translating the sample at a constant velocity of 2 mm/min. The waveguides are single-mode at the telecommunication wavelengths of 1550 nm and 1300 nm. Propagation losses were found to be less than 1 dB/cm. This technique is inherently capable of generating three-dimensional structures below the surface of silicon and therefore offers the potential to have a common platform for photonics and electronics.
The 157nm F2-laser drives strong and precisely controllable interactions with fused silica, the most widely used material for bulk optics, optical fibers, and planar optical circuits. Precise excisions of 10 to 40 nm depth are available that meet the requirements for generating efficient visible and ultraviolet diffractive optical elements (DOE). F2-laser radiation was applied in combination with beam homogenization optics and high-precision computer controlled motion stages to shape 16-level DOE devices on bulk glasses and optical fiber facets. A 128×128 pixel DOE was fabricated and characterized. Each level had distinguishable spacing of ~140 nm and surface roughness of ~38 nm. The far-field pattern when illuminated with a HeNe laser agreed well with the simulation results by an Iterative Fourier Transform Algorithm (ITFA). Improvements to increase the 1st order diffraction efficiency of 22% are offered.
F2-laser ablation at 157 nm was used for generating sub-micron surface relief structures on fused silica to define binary diffractive phase elements (DPE). A pattern array of 128 x 128 pixels was excised using the F2 laser in combination with a high resolution processing system comprising of CaF2 beam-homogenization optics and a high-resolution Schwarzschild reflective objective. A square projection mask provided precise excisions in less than 10 x 10 μm2 spots, having sub-μm depths that were controlled by the laser fluence and the number of laser pulses to provide for the required phase delay between ablated and non-ablated pixels. Thus a diffractive phase element (DPE) optimized for first order in the UV spectral range was made. A four-level DPE design computed by the Iterative Fourier Transform Algorithm (IFTA) will be described for generating an arbitrary irradiation pattern without the point symmetry of a two level design.