Realization of Four-Pass $I_{2}$ Absorption Cell in 532-nm Optical Frequency Standard

Abstract

Several innovations have been adopted in our portable 532-nm optical frequency standard. Particularly, the authors realized a four-pass scheme equivalent to 1.8-m J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption length, which gives adequate signal-to-noise ratio (~170 in a 10-kHz bandwidth). In addition, an active feedback control has been designed and implemented. The fluctuations of the residual-amplitude modulation in the electrooptic modulator have been successfully suppressed. As a result, the laser-frequency stability is greatly improved. When both lasers are locked on the same molecular iodine transition line of R (56) 32-0: alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> , the Allan deviation is lower than 2.3 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> at 1-s averaging time, reaching the flicker floor at about 4 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> after 200 s. The maximum frequency excursion at 532 nm is about 175 Hz during a continuous measuring time of 150 000 s