Measurement of thicknesses and refractive indices of double films: The thicknesses and refractive indices of the double films consisting of an SiO, layer (x < 2) beneath an SiO 2 layer were measured on an Abbe refractometer. The method used was an extension for two layers of the method reported in Reference 8, which treats one layer. 1 2 3 - 4 waveguide length, pm Fig. 4 Insertion loss against waveguide length The data were measured by using the cut-back method with x 20 lens at 1-3 nm O sample 1 * sample 2 In the first step, we measured the refractive index of the unoxidised SiO x layer deposited on a silicon wafer using the modified method of Reference 8, thus reducing the number of unknown parameters. In the second step, we laid an oxidised sample upside down with a droplet of immersion liquid of high refractive index on the prism of an Abbe refractometer. The sample was illuminated in reflection from the side of the prism with diffuse monochromatic light from an HeNe laser (A = 0-633 nm). In the eyepiece of the instrument, sharp dark lines in a bright field are observed which correspond to reflec- tion minima and to resonances of the field inside the double film. Their positions were read on the scale of the Abbe refrac- tometer. The readings of three dark lines were put into equa- tions for the minima of the reflectance derived from the well-known optical theory of a two-layer configuration [9]. By solving this system of three equations, the thicknesses of the two layers and the refractive index of the SiO 2 layer were determined. Typical errors are ±0-3 /im for the thicknesses and ± 3 x 10" 3 for the refractive index. Table 1 REPRODUCIBILITY OF INSERTION-LOSS MEASUREMENTS Waveguide length cm 2-5 10 Sample number 1 2 3 1 2 3 1st 1-72 1-71 1-64 1 68 1-57 1-63 Losses (dB) Three measurements 2nd 1-75 1 71 1-70 1-69 1 62 1 68 per sample 3rd 1 75 1-70 1 69 1-70 1-58 1 63 M 1-74 1 71 1 68 1-69 1 59 1-65 a 001 001 003 001 002 002 The data were measured by using the cut-back method with x 20 lens at 1-3/an Conclusions: The technology for the slab silica waveguides with thermal-oxide top cladding layers is simple. Excess con- taminations in the waveguide and a rough interface between the core layer and the top cladding layer arising in all pre- vious methods have been avoided. Low propagation losses below 5dB/m have been obtained. The equality in refractive index of the top and base cladding layers leads to a better matching of the waveguide to optical fibres. Furthermore, a new method for the determination of thicknesses and refractive indices of double films on silicon wafers has been applied. © IEE 1993 22 February 1993 Q. Lai, J. Schmid, W. Hunziker and H. Melchior {Institute of Quantum Electronics, Swiss Federal Institute of Technology Zurich, CH-8093 Zurich, Switzerland) P. Pliska (Optics Laboratory, Swiss Federal Institute of Technology Zurich, CH-8093 Zurich, Switzerland) References 1 HICKERNELL, F. s.: 'Optical waveguides on silicon', Solid-State Technoi, 1988, pp. 83-87 2 KAWACHI, M.: 'Silica-based planar lightwave circuit technologies'. Proc. 17th European Conf. on Optical Communication, 8th Int. Conf. on Integrated Optics and Optical Fiber Communication, September 1991, pp. 51-58 3 GRAND, G., JADOT, J. P., DENIS, H., VALETTE, S., FOURNIER, A., and GROUILLET, A. M.: 'Low-loss PECVD silica channel waveguides for optical communications', Electron. Lett., 1990, 26, (25), pp. 2135— 2137 4 KOMINATO, T., OHMORI, Y., OKAZAKI, H., and YASU, M.: 'Very low-loss GeO 2 -doped silica waveguides fabricated by flame hydrolysis deposition', Electron. Lett., 1990, 26, pp. 327-328 5 HENRY, c. H., BLONDER, G. E., and KAZARINOV, R. F.: 'Glass wave- guide on silicon for hybrid optical packaging', J. Lightwave Technoi, 1989, LT-7, (10) 6 LAi, Q., GU, J. s., SMIT, M. K., SCHMID, J., and MELCHIOR, H.: 'Simple technologies for fabrication of low-loss silica waveguides', Elec- tron. Lett., 1992, 28, (11), pp. 1000-1001 7 MALITSON, i. H.: 'Interspecimen comparison of the refractive index of fused silica', J. Opt. Soc. Am. 1965,55, (10), pp. 1205-1209 8 LUKOSZ, w., and PLISKA, P.: 'Determination of thickness and refractive index of SiO 2 films on silicon wafers using an Abbe refractometer', Opt. Comm., 1991, 85, pp. 381-384 9 KNITT, z.: 'Optics of thin films' (John Wiley & Sons, 1976), Chap. 2 NOVEL CALIBRATION TECHNIQUE EMPLOYING A SPREAD-SPECTRUM TONE FOR MOBILE SATELLITE SYSTEMS G. E. Corazza, C. Ferrarelli and F. Vatalaro Indexing terms: Demodulation, Satellite relay systems A calibration technique based on a low-power spread- spectrum tone to assist demodulation is presented. The tech- nique does not impose any requirements on signal modulation, spectral shaping, nor on channel spacing, and is highly spectrum efficient. The performance is analysed in the presence of cochannel and adjacent channel interference, diffused-multipath fading and additive white Gaussian noise, and results are presented for M-PSK (M = 2,4, 8). Introduction: The advent of satellite land-mobile communica- tions is stimulating interest in robust and spectrum efficient techniques suited to transmission channels other than AWGN (additive white Gaussian noise), such as the Rice channel. Narrowband, diffused-multipath fading channels can be mod- elled as Rice channels, characterised by the 'Rice factor' K, i.e. the ratio of direct signal power to diffused-multipath power. A technique having good performance over a wide range of values of K consists in locating near the desired signal spec- trum suitable phase-coherent tone(s) to assist demodulation [1-3]. However, the presence of the tone(s) penalises spectrum efficiency, due to filtering needs. This Letter presents a novel technique with a spread- spectrum tone accompanying the signal. This spread tone cali- bration technique (STCT) in principle does not impose any requirements on modulation law, on signal spectrum shape nor on channel spacing, and can achieve higher values of spectrum efficiency n, than other calibration techniques pre- sented earlier. An improvement around 70% is achieved for n 716 ELECTRONICS LETTERS 15th April 1993 Vol. 29 No. 8