Magnetoelastic materials are utilized in applications under variable stress in presence of a magnetic field excitation. An accurate characterization of the material behaviour needs of magnetic and strain measurements on the material specimen, usually performed on a dynamic test machine equipped with a magnetic excitation core. Strain measurements are usually made by means of a strain gauge glued on the specimen. This method has some limitations: it requires a bonding of strain gauge for each specimen. Moreover, the strain gauge performs a local strain measurement, which is significant if the magnetization of the sample is uniform. To overcome the above-mentioned limitations of strain gauges in this application, an interferometer with a differential optical setup has been designed and tested onto a measuring machine. The design makes use of symmetric and differential paths by the reference and moving arms of the interferometer, thus compensating for drifts and dead-path of linear set-ups. The interferometer reading is zeroed or corrected afterwards by pre-registering strain of the loading cylinders at the same loading curve to be used for testing the sample. Preliminary results highlight a good repeatability of measured strain. Some further steps are envisaged by suitable control of the strain test runs.