The invention relates to an optical multi-gate element with an acousto-optical modulator (AOM), which exhibits at one side at least two optical waveguide connections, in particular monomode optical waveguides. The end surfaces of the waveguides are disposed in the focal plane of a lens situated between the optical waveguides and the acousto-optical modulator.
Such an arrangement is known from "Journal of Lightwave Technology", Vol. Lt-2, No. 2, pages 108 to 115. In this case, a gradient lens is coaxially associated with each individual optical waveguide forming a port connection. A transformation of the field widths of the Gaussian beams is caused by the lenses. In consequence of this, the field width of the beams on the path is increased by the AOM as compared with the field width in the optical waveguides.
In the case when the AOM is not excited acoustically, the rays are not diffracted and further transmitted on a direct path. In the case of acoustic excitation, beams impinging at the Bragg angle to the acoustic wavefront are deflected by twice the Bragg angle. On account of this effect, an optical switch can be constructed with an AOM, as in the known case. Since the frequency of the deflected beam is also additionally increased or reduced by the acoustic frequency, an AOM multi-port element is also suitable for the formation of a local oscillator beam (LO) for optical heterodyne reception. The combined use of an AOM as switch and for the formation of an LO beam is known from the DE-OS No. 3,506,884 in an application for a heterodyne OTDR.
An OTDR (Optical time domain reflectometer) is a device with which the attenuation behaviour of an optical waveguide can be measured from one end, whereby the components of an optical transmission signal (laser pulse) which are backscattered by the longitudinal positions of the optical waveguide are evaluated. In order to achieve an OTDR with a large range, its optical components should have little attenuation, so that the intensity of the back-scattered signals, which decreases with increasing distance from the OTDR, is still sufficiently large to ensure an adequate signal-to-noise ratio.
A multi-port element of the initially mentioned type consists of numerous individual elements, which necessarily result in further losses. Besides the unavoidable material-dependent transmission attenuations, there are insertion attenuations, which are dependent upon the quality and geometric accuracy of the coupling-in of the optical beams. Thus, a careful relative adjustment of the individual components is essential for a low-attenuation multi-port element of the initially mentioned type. Furthermore, long free beam paths through air should be avoided, since, on the one hand, a small overall length of the multi-port element is, of course, to be aimed at, and since, on the other hand, irradiation losses, in particular due to beam broadening, unavoidably arise on the free beam paths.
Without particular measures, the direct beam and the beam deflected in the case of acoustic excitation can be detected separately only at a relatively large distance from the AOM by the lenses which cannot be constructed arbitrarily small, because the Bragg angle is very small. In order to avoid an excessively large distance, a deflecting prism is provided, in the known case mentioned initially which prism, however, causes additional attenuation losses and requires a high degree of expenditure on adjustment.