TECHNIQUES FOR REFRACTIVE INDEX MEASUREMENTS ON SEMICONDUCTOR WAVEGUIDES: PRISM COUPLING AND MODAL CUT-OFF SPECTROSCOPY

摘要

Integrated optical waveguides on semiconductor materials are in rapid development, for the great potential offerd by integrated optoelectronics, both in optical communications and in optical data processing. Waveguide attenuation, in both GaAs and InP based materials, has dropped well below 1 dB/cm, and is practically equal to the values exhibited by lithium niobate and other waveguide materials: this makes it practical to consider integration of several components and devices, requiring relatively long paths, on a single chip, without incurring in intolerable losses or requiring on-chip amplification. The design of sophisticated components needs accurate knowledge of the refractive indices both of the substrate and of the layers involved in the propagation of radiation (waveguide, cladding and cover layers etc.), which depend not only on the composition of the material, but also on the doping levels; unfortunately, so far the available data on this aspect are not as rich as needed, from the point of view both of quantity and precision, and device design relies more on estimates from theoretical models than on precise experimental determinations of these physical parameters. In this paper we present results based on two techniques developed to improve the knowledge of these refractive indices: the first one is an extension of the well-known prism coupling technique to high index materials, and uses a silicon prism to determine the refractive index of substrate material or of epitaxial layers, with an accuracy to the fourth decimal place. The second method, modal cut-off spectroscopy (MCS), yields the refractive index of waveguides at the wavelengths where cut-off of a guided mode is detected, if the substrate refractive index is known; the accuracy reaches the third decimal place. The use of a prism to couple radiation into waveguides is well known [1], and is widely applied to the measurement of modal indices in planar guides. However, so far this technique has not been used, to our knowledge, in the case of semiconductors, probably due to their high refractive index and brittleness. We have used a silicon prism, suitable for InP and a range of InGaAlAs and InGaAsP compositions, due to its high index of refraction; in our case the prism data are: refracting angle α=64.25°, refractive index at 1.31 μm n=3.5039, calculated from a Herzberger interpolation formula [2] (direct measurement of this value by minimum deviation is in progress); presently a diode laser source at 1.31 μm is used, but extension to other wavelengths with a color center laser is planned.