Progress towards absolute frequency measurement of the /sup 127/I/sub 2/-stabilized Nd:YAG laser at 563.2 THz/532 nm

We propose a new method to measure the absolute frequency of /sup 127/I/sub 2/-stabilized laser at 563.2 THz (532 nm), based on a frequency difference generation (DFG) of two diode lasers operating in the near infrared: a narrow free running spectral linewidth (<50 kHz) /spl alpha/-DFB diode laser (angled-grating distributed feedback) operating at 281 TPIz/1.067 /spl mu/m (DL1) and a diode laser at 192.6 THz/1.556 /spl mu/m (DL2). The resulting 88 THz-frequency radiation issued from a type IT cut AgGaS/sub 2/ crystal is compared to the third harmonic of our 29 THz-CO/sub 2//OsO/sub 4/ optical frequency standard (OFSI). On the other side, the DL2 frequency is doubled in a periodically poled lithium niobate crystal (PPLN) in order to phase lock the laser to the 385 THz rubidium optical frequency standard (OFS2). The ultimate accuracy of this measurement will be limited by the OFS2 accuracy, which is known at present time with a 3/spl times/10/sup -12/ uncertainty. The diode laser DL1 is frequency doubled in a KTP crystal placed in a ring cavity of a finesse of about 100. With 400 mW of 1.064 /spl mu/m fundamental radiation at the input of the cavity, up to 22.5 mW of green light has been recently obtained. This high level of green power, the large frequency tunability of the laser diode (more than 1 THz in the IR range) and its intrinsic spectral purity (50 kHz) open the way to an accurate spectroscopy of the I/sub 2/ molecule in the 531.5-533.5 nm range. Preliminary results on the frequency synthesis chain are presented.