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Optical properties of an external cavity laser with a semiconductor optical amplifier and two diffraction gratings are described. The laser has been pigtailed and its emission showed two peaks in the output spectrum. The wavelength of the peaks, and in particular the spectral spacing between them, could be controlled by adjusting the reflecting elements in the system. The proposed double-frequency laser may be adopted as a signal source in terahertz generators.

Well known properties of diffraction gratings are connected with their response to wavelength and polarization state of light incident on a grating Although diffraction gratings are used as one of structures in modern optical security signs, their interesting properties seem to be rather rarely taken into consideration for distinguishing between originals and "illegal' copies of optical marks In this paper a relative simple diffraction structure and corresponding alternative method of inspection are proposed This method can be particularly useful for machine reading, although its usability for visual inspection can be also considered

The instrument for measuring the light scattered from optical surfaces is described. The measurement is carried out using the 253 nm wavelength separated from continuous spectrum of high pressure mercury lamp by a double monochromator. Output signals are detected by a photomultiplier whose sensitivity falls rapidly above 300 urn. The sensitivity of the instrument is about 0.5/3 x 10k, where the numbers 0.5 and 3 x 10 characterize the niiniml detectable output signal and the maximal one (in reflected light).