Integrated Optic Coupler For Interferometric Mixer

Abstract

ABSTRACT Low loss passive integrated optic interferometric sensors might successfully compete with optical fibre sensors in short path length applications such as distance measurement, force and perhaps also low performance rotation sensors. They are inherently polarization maintaining, stable, low cost and versatile.There is yet an important function which remains to be practically solved in integrated optics on passive substrates: the phaseshifting allowing a waveguide interferometer to delivering non degenerate optical power signals, ie, sine and cosine functions of the input optical phase difference.The proposed device achieving this function is a single process ion exchanged circuit on glass using a low mode number imaging waveguide The proposed configuration can be applied to all kinds of interferometers. 1. INTRODUCTION Since the early days of integrated optics the directional coupler formed by two neigh­ bouring waveguide sections1 has been considered as a key component for all singlemode fiber communication, and more recently, sensor systems. Until recently their main use was for amplitude modulation and switching purpose. There was only little concern about the re­ lationship between the power signals at the two output ports of a coupler and the optical phase at its input ports. Since the advent of coherent optical communications and, more recently, of integrated optic interferometry for sensors (e.g. in distance measurement) there has been a growing interest for such an element as a true wave mixer capable of retrieving the phase difference between two incoming beams. It is well known that a 2 x 2 port 3 dB directional coupler only delivers two degenerate power signals of the input phase shift, which means that the direction of a displacement can not be determined and that interpolation can not be performed accurately. In order to allow, e.g., phase tracking in coherent communication and fringe counting and interpolation in interferometry, the mixing coupler should supply at least two non-degenerate power signals, i.e., sine and cosine functions of the input phase difference.This is what occurs in a 3-fiber fused coupler2 delivering a three-phase output. In integrated optics the analysis of a 3-waveguide coupler was achieved3 but no experimental evidence has been published yet. Non degenerate integrated optic interferometers have been reported. Rather than being true singlemode circuits they are planarizations of bulk optic configurations using singlemode planar waveguides4 • 5 . The integration is in one dimension and leaves open the question of the phase front alteration due to various inhomogeneous disturbances within the waveguide material with possible consequences on the optical power modulation depth.Actual singlemode guided wave optics provides a perfect spatial coherence over long and topologically complex waveguide routes and offers unique features for multiple wave inter­ ference. One of these properties is that of waveguide multiple imaging first demonstrated by Ulrich6 . The phase relationship between the sub-images of a fourfold waveguide imaging section is such that the latter delivers four optical power signals in quadrature7 . This means in particular that in a balanced mixer the unknown and possibly fluctuating power signal component can be optically cancelled out. Such a waveguide section was demonstrated by means of a flat-core fibre7 .The purpose of the present contribution is to analyse the imaging capability of an integrated optic section singlemode in depth and multimode in width, hereafter called caisson. It will be theoretically and experimentally shown that a low mode number wave­ guide caisson, i.e., a rather short section (less than 10 mm), can achieve sufficiently good and low loss fourfold imaging of input singlemode waveguides to be used as an inter­ ferometric mixer for most industrial applications which nowadays use bulk interferometry. The technology used is the fairly simple and well characterized ion exchange technique in glasses.