Extended-cavity semiconductor lasers in fundamental metrology

With the iodine-stabilized He-Ne laser at 633 nm remaining the key etalon of length and a standard for comparisons ofstabilized lasers as stated in the report of the 9th meeting of the Consultative Committee for the Definition of the Metre2there are also new optical frequencies and new laser sources being introduced into the metrological practice. The Revisionof the practical realization of the definition of the metre adopted by the by the Comite International des Poids et Mesures3recommends introduction of new standard radiations. It follows the development in laser technology and also the demandfor standardization arising from the new laser applications. Beside the Nd:YAG lasers the semiconductor lasers seem to beof growing metrological significance in these days due to their dynamic development and growing range of applications.Laser diodes became the most widespread lasers and now are available in a broad spectrum of wavelengths rangingfrom infrared to the visible region. The low power ones mainly those with the quantum well structure and gain or indexguided configuration perform a narrow linewidth and soon became a favourite tool for interferometry and spectroscopy.An effort to reduce the linewidth even further and to suppress the mode instability soon followed. The broad gain spectral