Realization of four-pass I₂ absorption cell in 532-nm optical frequency standard

Several innovations have been adopted in our portable 532-nm optical frequency standard. Particularly, we realized a four-pass scheme equivalent to 1.8-m I₂ 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: α₁₀, the Allan deviation is lower than 2.3 × 10 ^-14 at 1-s averaging time, reaching the flicker floor at about 4 × 10^-15 after 200 s. The maximum frequency excursion at 532 nm is about 175 Hz during a continuous measuring time of 150 000 s.