Refracted near-field measurements of refractive index and geometry
Transcription
Refracted near-field measurements of refractive index and geometry
Refracted near-field measurements of refractive index and geometry of silica-on-silicon integrated optical waveguides Philippe Oberson, Bernard Gisin, Bruno Huttner, and Nicolas Gisin The standard refracted near-field technique for measuring the refractive-index profile of optical fibers cannot be directly used for silica-on-silicon integrated optical waveguides because of the opacity of silicon. A modified method is thus presented to characterize this kind of waveguide. The resolution it gives, both spatially and in the refracted index, is practically as good as that obtained with the standard technique for measuring optical fibers. © 1998 Optical Society of America OCIS codes: 120.5710, 230.7370. 1. Introduction 2. Measurement Principle and Setup Integrated optics on silicon has reached a high level of performance in passive optical components and circuits,1–5 with applications for telecommunications as well as for sensors. The basic physical characteristic of optical waveguides is given by the distribution of the refractive index. From this distribution all local parameters can be computed, such as the mode profile that determines the coupling losses. Measurement of the refractive-index profiles also provides useful quality control during the development and production of the components. The refracted near-field ~RNF! technique is the most advanced technique used to measure refractive-index profiles of optical fibers6,7 and glass-integrated optical waveguides.8 –10 However, because of the presence of silicon, an opaque material that creates reflections at the interface, the standard RNF technique cannot be directly used for silica-on-silicon integrated optical waveguides. In this paper we present an adaptation of the RNF technique for the measurement of refractive-index profiles and geometry of silica-onsilicon integrated optical waveguides. Section 2 gives details of the technique used for these measurements, and Section 3 presents the results. The measurement principle and setup are quite similar to those presented in Refs. 6, 7, and 9. A laser beam is focused on the sample, and the intensity of the refracted light is measured to determine the refractive index. The measurement cell is modified to accommodate the new type of waveguide. Figure 1 illustrates the new cell. It resembles the one that we used for glass-integrated optical waveguides,9 though a great deal of additional effort has been put into it to simplify the handling. The component can easily be placed in the cell, and the area to be analyzed can be centered as required. This measurement cell is a vessel containing an index-matching oil. On the bottom surface of the vessel is an aperture that is closed by a diopter made out of thin glass ~0.17 mm!. A light detector is situated 11 mm above the diopter. The integrated-optics component of silica on silicon is placed vertically, with the end face in contact with this diopter. It is necessary that the end face of the component should be well cleaved ~perpendicularly to the guide! and that the refractiveindex profile should be invariant under vertical translation ~Z direction in Fig. 1! over approximately 1 mm from this end face. Under the measurement cell is a 5-mm-diameter parallel laser beam ~wavelength, 675 nm! focused by an immersion microscope objective of 1.25 numerical aperture ~NA! on the integrated-optics component. This microscope objective is different from the one used for standard methods; the reasons for this change are explained below. Three quarters of the parallel beam are blocked in such a way as to prevent any light from The authors are with the Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland. Received 10 April 1998; revised manuscript received 8 June 1998. 0003-6935y98y317268-05$15.00y0 © 1998 Optical Society of America 7268 APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998 Fig. 1. RNF technique adapted for silica-on-silicon integrated optical waveguides. The incident beam is shaped as a quarter disk and focused through a diopter by a microscope objective ~NA, 1.25! onto the end face of the component under test. On a detector a half-disk-shaped mask is centered around the optical axis. Consequently, only the refracted light rays that have an angle larger than umin are collected by the detector. These correspond to incident rays that have an angle larger than wmin, where wmin and umin are related by Eq. ~2!. The detected intensity I~x, y! is therefore a function of n~x, y!. being reflected on the vertical face of the silicon. A drop of index-matching oil is inserted between the microscope objective and the diopter. The measurement cell can be moved with the help of a motorized XYZ microdisplacement system in steps of 0.1 mm. The Z displacement, parallel to the optical axes, allows one to adjust the position of the focal point on the section plane of the integrated waveguide. For each ~ x, y! position, the detector signal caused by the refracted light intensity I~ x, y! is collected, its value being sufficient to determine the refractive index n~ x, y!, as shown in the following paragraph. The starting point is Snell’s law ~written with vectors!: v1 z s 5 v2 z s, (1) where vi 5 nic represents both the direction of a light ray ~optical geometry! and its velocity in medium i; ni is the refractive index; and s is any vector tangent to the interface. By applying Eq. ~1! to the horizontal interface and the vertical interfaces and using s 5 ẑ, the vertical unitary vector for all the vertical interfaces, one can show that sin2 w 5 n~x, y!2 2 cos2 u, nl 2 (2) where w and u are the angles between the Z direction and a light ray entering and leaving the component, respectively, and nl is the index of oil. Equation ~2! is the fundamental relation of the RNF technique. This way of deriving Eq. ~2! shows that it is valid in full generality, without any assumption of circular symmetry or of axial rays. In particular, note that the projection of the ray on the XY plane does not necessarily follow a straight line ~lens effect!. In practice, the deviation in this XY plane does not exceed 20 –30°. For this reason only a quarter of a cylindrical beam is used to illuminate the end face of the waveguide, and a half-disk detector collects the refracted light. A circular mask, centered on the optical axis, covers the central part of the detector. This ensures that only refracted light rays with an angle larger than umin are collected. These correspond to incident rays on the component with an angle larger than wmin, where wmin is given by Eq. ~2!. Since wmin increases with the refractive index n~x, y!, the intensity I~x, y! collected at the detector decreases for increasing n~ x, y!. Hence, after calibration, n~ x, y! can be recovered from the collected intensity I~x, y!. Note that incident rays with an angle larger than wmin should reach the detector, so they should not be guided by the component. This imposes a value of umin given by umin $ arccos ~2n2min 2 n2max!1y2 , nl (3) where nmin and nmax are the minimum and maximum refractive indices that can be found in the component, respectively. Movement of the cell will cause variations in umin of the order of 0.1° and thus a slight change in the intensity @in addition to the variation due to the profile of n~ x, y!#. For the standard technique, the first order of the development of intensity I~ x, y! in the neighborhood of the central position is zero, because of the circular symmetry of the cell. With this setup the first order of x is not zero, because all the refracted light escapes in the direction of negative x. These changes in intensity can easily be corrected during calibration, since they can be measured in the oil @when n~x, y! 5 nl is constant#. Note that a firstorder correction is sufficient. The silicon part of the component causes other problems that do not appear with transparent materials such as optical fibers. Indeed, for transparent materials, any weak parasitic light from outside the beam creates only a constant additional intensity, whereas with opaque materials the collected intensity varies according to the position of the opaque obstacle and causes a perturbation of the signal. Therefore special care has to be taken to avoid all spurious reflections. In particular, with a standard objective, the Fresnel reflection on the measurement cell window followed by a reflection on the microscope objective distorted the measurement. This problem was satisfactorily solved by use of the immersion ob1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS 7269 3. Results Fig. 2. Refractive-index profile of a NPL calibrated optical fiber glued onto a silica-on-silicon substrate. This fiber was measured with the setup of Fig. 1. jective, which decreased the index steps causing the reflections. Moreover, with the standard objective ~0.85 NA, corrected for a 0.17-mm diopter! a spherical ~focal! aberration was observed when the laser beam covered the entire aperture. This aberration decreased with the immersion objective, mainly because the large NA ~1.25! allows one to use only the central part of the beam. Before measuring the integrated waveguides, we tested the setup by measuring a calibrated optical fiber provided by the National Physical Laboratory ~NPL!, Teddigton, UK. This fiber was glued to a silica-on-silicon substrate. The results are presented in Fig. 2. The values provided by NPL and the oil refractive indices are used for the calibration of n~ x, y! as a function of I~ x, y!. A simple linear calibration is used because the variation of the angle wmin is slight. Agreement with the NPL data within 5 3 1024 was obtained. Note that with this measurement method the silicon appears to have an infinitely high refractive index. Figures 3 and 5 present calibrated raster scans of two different silica-on-silicon waveguides. The guides can be seen clearly, as well as the highrefractive-index lines. These are due to the manufacturing process: The silicon is covered by two silica layers, one with a low index and one with a high index. The high-index layer is chemically removed around the guiding region, and a final low-index layer is deposited on it. From this measurement both the geometry and the refractive-index profile of the guide can be determined with an accuracy similar to that obtained for optical fibers, i.e., a few tenths of a micrometer. Note that, because of the manufacturing process, the outer surface of these components is not Fig. 3. Raster scan of a silica-on-silicon integrated optical waveguide. 7270 APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998 Fig. 4. Refractive-index profiles of the component under test in Fig. 3. The solid and dashed curves correspond to a scan perpendicular ~x scan! and parallel ~ y scan!, respectively, to the component’s surface. Fig. 6. Refractive-index profiles of the component under test in Fig. 4. The solid and dashed curves correspond to a scan perpendicular ~x scan! and parallel ~ y scan!, respectively, to the component’s surface. flat. Finally, Figs. 4 and 6 show some calibrated X–Y profiles through these waveguides. The refractive-index step and waveguide size match the manufacturer’s data well. The aberrations ~elevation of the index near the silicon! caused by the presence of silicon are present in a region of only a few micrometers around the junction. The width of the transitions visible on the figures is sometimes less than 0.5 mm, a satisfactory result when one considers that this is the minimum theo- retical diameter of the spot ~calculated from the beam aperture and the laser wavelength!. The smallest refractive-index differences observable on the figures are less than 5 3 1024. 4. Conclusions Refractive-index profiling and geometry characterization of silica-on-silicon integrated waveguides have been obtained by a modification of the standard refracted near-field technique. Problems due to Fig. 5. Raster scan of a second silica-on-silicon integrated optical waveguide. 1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS 7271 the presence of opaque silicon have been overcome by use of an immersion objective and only a quarter of the laser beam. The estimated accuracy is similar to that obtained for optical fibers: a few tenths of a micrometer for the geometry and approximately 5 3 1024 for the refractive index. Special attention has been paid to the simplification of handling procedures. This technique does not require any sample preparation; moreover, it is nondestructive and provides waveguide characterization in a few minutes. An industrial application should be a straightforward matter, as it would require only a change of objective on the measuring instrument and the use of a different measurement cell. 2. 3. 4. 5. Special thanks are due to Gilles Grand for stimulating discussions. 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