Matti J. Leppihalme

Efficient Bragg waveguide-grating analysis by quasi-rigorous approach based on Redheffer's star product

We introduce a computationally efficient quasi-rigorous method for the analysis of corrugated planar waveguide structures. The method is based on rigorous diffraction theory of gratings. The computational efficiency is achieved by using Redheffers star product and the so-called binary method for the involution of the transfer matrix. The developed method enables efficient rigorous analysis of corrugated waveguide structures without any limitations for the corrugation depth. Comparison with the thin-film stack method shows that the proposed method gives similar results for Bragg grating for the fundamental mode when the corrugations are shallow, but the results differ significantly when the corrugations are deep. Furthermore, the quasi-rigorous method also facilities the analysis of the coupling of light from the fundamental mode into the higher waveguide modes.

Development of silicon-on-insulator waveguide technology

An overview of the present silicon-on-insulator (SOI) waveguide technology is given and supplemented with an extensive set of theory and simulation results. Characteristics of slab-, rectangular- and ridge waveguides in SOI are explained. In particular, the number of modes and the single-mode conditions are carefully analyzed. Experimental work with straight and bent 8 to 10 μm thick SOI ridge waveguides and a very fast thermo-optical switch are reported. Propagation loss in a very long spiral waveguide down to 0.3 dB/cm, waveguide birefringence below 10-4, and a switching frequency up to 167 kHz were obtained. A very promising multi-step patterning principle for SOI waveguides is described together with many practical application examples.

Fabrication and characterization of waveguide structures on SOI

A survey of the most common silicon-on-insulator (SOI) substrates and waveguide structures, as well as an evaluation of their applicability in optical telecommunication at the 1550 nm wavelength is presented. The design, fabrication and characterization of straight and bent SOI waveguides, as well as a thermo-optical SOI switch are described. The propagation loss of the realized SOI waveguides is below 0.25 dB/cm and thermo-optical switching is demonstrated at 10 kHz. The effect of cladding material on top SOI ridge waveguides on the polarization properties of straight and bent waveguides, as well as on directional couplers, is discussed. Both polarization independent and polarization maintaining waveguides are demonstrated. Finally, a basic principle of multi-step SOI waveguides is proposed. As examples of the potential in multi-step processing, efficient coupling between different rectangular, ridge and photonic crystal waveguides, ultra-small bends, waveguide mirrors, and extremely short multi-mode interference couplers are described.

Thin-film chemical sensors with waveguide Zeeman interferometry

We describe a highly sensitive chemical sensor scheme using a channel waveguide with a selective surface coating based on polarimetric Zeeman interferometry. The sensing is based on measurement of the phase difference between TE and TM modes propagating in the anisotropic waveguide structure under exposure to toluene vapour. A real-time and reversible response at low ppm level is observed. Modelling results of the sensor structure to further increase its sensitivity are presented.

Si-based integrated optical and photonic microstructures

The design, fabrication and properties of various types of optical waveguides on silicon substrates as well as Fabry- Perot devices accomplished mainly at the Microelectronics Center of VTT Electronics are introduced. Different waveguides, directional couplers, multimode interference couplers, switches and power splitters have been studied. For waveguide fabrication on silicon substrates principally three materials were used depending on the application: silicon oxynitride, silicon on insulator (SOI). Silicon waveguides with large Si-core and a thermo-optical switch have been fabricated by SOI technology. Silicon Fabry-Perot structures have been fabricated for wavelength scanning applications in instrumentation and to measure chirp properties of lasers used in optical communications.

Optical waveguides for 1064 nm and chip pigtailing with PM fibers

The purpose of this project was to develop optical waveguide technology to offer polarization maintaining (PM) components for a Nd:YAG laser wavelength at 1.064 μm. The silicon oxynitride waveguide layers were grown with a PECVD method on a silicon wafer. The waveguide height and width are 3.5 μm and 6.0 μm, respectively. A well balanced 3 dB operation was achieved with a 2´2 MMI coupler. The excess loss of the coupler was 2 dB. The polarization maintaining properties were studied by measuring the polarization extinction coefficient (PXR) of the straight waveguides and pigtailed chips. Typical PXR for a 15 mm waveguide was <18 dB. Waveguide crossings at different crossing angles were characterized in terms of excess loss and PXR. Both passive and active fiber alignment was studied to achieve low-loss pigtailing with a polarization maintaining fiber.

Thin film waveguide sensors for chemical detection

A chemical sensor scheme, based on selective sensing surfaces and highly sensitive integrated optical transduction is presented. Self-assembly techniques are used to covalently attach species selective films onto the surface of silicon nitride waveguides. Exposure to targeted analytes results in selective absorption of these molecules onto the waveguide surface, causing a change in the effective refractive index of the guided modes. These relative changes in effective refractive indices of TM and TE modes are measured using Zeeman interferometry. The measurements demonstrate reversible, real time sensing of volatile organic compounds at ppm level. Improvements in the waveguide design are proposed to further increase the sensor performance.

Optical and Electrical Properties of a-Si:H prepared by Reactive Sputtering

Hydrogenated amorphous silicon films have been prepared by reactive sputtering under various deposition conditions. The films have been systematically studied by means of conductivity, ESR, optical and IR-absorption and magnetoresistance measurements. The optical and electrical properties of the films were found not to be exclusively determined by hydrogen concentration, but also strongly dependent on deposition temperature.

Magnetoresistance in Sputtered Hydrogenated Amorphous Silicon

Magnetoresistance (MR) measurements have been made on sputtered hydrogenated amorphous silicon films for magnetic fields up to 1 T in the temperature range 100-300 K. The effect of a systematic increase of the hydrogen content in the films has been studied. In α-Si films with no hydrogen MR was always negative and it increased with decreasing temperature. In hydrogenated films MR was positive and its magnitude about a decade larger than the negative MR. The positive MR showed a maximum at low fields ~ 0.2 T. Most of the observed features in the temperature range 200 K < T < 300 K can be explained by a hopping model in which MR is a result of the modification of the spin-flip relaxation time by the external magnetic field. It is concluded that the change of sign in MR with increasing hydrogen content is most probably due merely to the decrease in the density of states in the gap which causes a narrowing in the distribution of the hopping relaxation times. The obtained results support the idea that the variable range hopping mechanism dominates the electrical conduction in sputtered α-Si below room temperature. Further, the results indicate that spin dependent effects have a strong influence on the electrical transport in amorphous semiconductors.