Methods for Measurement and Verification of Purity of Iodine Cells for Laser Frequency Stabilization

Abstract

We present an improved technique for detection of trace impurities in iodine-filled absorption cells for laser frequency stabilization. The results of purity investigation are compared to frequency shifts measured with a set of two iodine stabilized Nd:YAG lasers. The setup for direct fluorescence measurement with an Argon-ion laser operating at 502 nm wavelength is equipped with compensation for laser power and spectral instabilities. PTICAL FREQUENCY synthesis in the field of laser metrology is becoming mainstream in these days due to the great advances in femtosecond laser technology. Frequency locking of such comb generators may be done in the radiofrequency domain to atomic clocks extending their stability into the optical spectral range or in the reverse direction to high stability lasers and also to generate rf optical clocks. Nd:YAG stabilized lasers seem to be a suitable option thanks to the relative simplicity of the stabilization scheme and stabilities that can be achieved are not far from those of the rf cesium clocks. Stabilization on the basis of saturated absorption in molecular iodine may be called a traditional method and due to good signal-to-noise ratio at the 532 nm of frequency doubled Nd:YAG laser gives very good results. Good relative stability can be easily achieved with a low-noise laser source and properly designed stabilization scheme and control electronics but the absolute value of the optical frequency is given by center frequencies of hyperfine components of the iodine transitions. Only near-ideal purity of the iodine in the absorption cell can result in optical frequencies corresponding to theoretical values. We present a technology of iodine cell preparation that is a result of a long time development and verification by an independent method based on measurement of induced fluorescence and evaluation by the Stern-Volmer formula which has been used by the Bureau International des Poids et Mesures (BIPM) so there is a chance to compare our results (1), (2), (3). The level of induced fluorescence intensity here is limited by several relaxation processes such as ionization, predissociation and collisional quenching. The quenching - nonradiative transitions - can be caused by collisions with either iodine molecules or by collisions with molecules or atoms of impurities (foreign-gas quenching). The last mentioned process is the one that reflects the purity by reducing the lifetime of a state and can be evaluated by monitoring the spontaneous emission level from the irradiated cell. The main goal of this effort is to recognize the limits of the absolute precision of the optical frequency of iodine transitions and look for further improvements in the iodine cell manufacturing technology that may lead to even smaller frequency shifts. This technique was introduced into the metrology practice with relation to He-Ne iodine stabilized lasers. With the stability that can be reached with iodine stabilized frequency- doubled Nd:YAG lasers the chance to compare the purity investigation with measured frequency shifts could go even below the kHz level (4), (5), (6). 2. MEASUREMENT OF INDUCED FLUORESCENCE