Iodine Frequency Stabilized Lasers in CMI

Abstract

The paper describes recent development and improvement of optical frequency standards used in CMI for length metrologyiodine stabilized lasers 543nm and 532nm. The iodine cells, experimental set-up, operating parameters and results of comparisons are summarized together with absolute frequency measurements of these lasers and 633nm standard. Introduction Frequency stabilized lasers represent standards that provide traceability in length measurements. Since 1978 CMI developed several iodine stabilized He-Ne lasers 633nm, 612nm, 543nm and SHG Nd:YAG laser 532am. These wavelength standards were studied for optimization of stability and reproducibility and were successfully internationally compared for many times, values and repeatability always safely fall within the CCL recommended [1] ones. We present design of external iodine cells, set-up and properties of lasers with external cell and absolute measurements of laser frequencies. Status of 1542nm standard development is briefly mentioned. Cells Except of 633nm lasers we use 50cm long external iodine cells made and filled in ISI Brno. These cells have perpendicular windows with broadband AR coatings from inside and outside and aperture about 2cm. Double-stage TE (Peltier) cooler was selected to reach temperatures -15'C reliably. According to advice of L. Vitushkin [2] the TE cooler is soldered both to radiator and copper cap for cell finger by low-melting point Wood metal. We extended this approach even to contact copper cup to glass finger itself instead of using traditional paste. PU mounting foam prevents water condensation and insulates the TE cooler and central part (40%) of cell wall. We can reach cell finger temperature below -30°C temporarily and steadily to about -20°C at room temperature enviroament.The cell finger temperature stability is in the order of 1OmK, but the total uncertainty is about lOOmK due to residual gradient in copper cup. The purity of iodine cells was measured in BIPM 2001 and both cells CMI-ISI-7 and CMI-ISI-9 were found very pure (Stern-Volmer factor ko=l.04 and 1.23 respectively).13xperimental setup For locking 532 am and 543 am lasers we use sub-Doppler spectroscopy arrangement with isolation by changing the sense of circular polarization (similar to e.g. [3], scheme on figure 1). Polarization of laser radiation can be selected by half-wave plate (V2); laser is isolated from residual back-reflections by optical isolator (ISO). Laser beam is deflected by mirrors and passes through polarizing beam-splitter (P). By means of quarter-wave plate (V4) polarization changes to circular and beam passes through cell and part of it also back as probe. The returning probe beam polarization is converted to linear rotated by 90°and signal beam is thus reflected to detector (D).We use 532 nm Nd:YAG laser Prometheus 20 from INNOLIGHT (CMI YAGI). Lasers 543 am use internal mirror He-Ne tubes: Siemens in PLG1 (22Ne) and REO in PLG2 (20Ne); Single longitudinal mode operation is selected and kept for one linear polarization by permanent magnetic field...