Development of Ultra-Stable Rb-Referenced 420nm Optical Frequency Standard

We built a 420nm interference filter configuration external cavity diode laser with a Lorentzian linewidth of 24kHz. Taking the transition line of 85Rb 5S1l2 F=3→6P3/2 F'=4 as reference, we directly stabilized the laser frequency on the reference employing the excellent-performance modulation transfer spectroscopy locking technique and fast feedback mechanism. The frequency fluctuation amplitude of the locked residual error signal is about 100Hz. According to the residual error signal, we evaluated the stability of the Rb-referenced 420nm optical frequency standard system which is $4.1\times 10^{-15}/\sqrt{\tau}$ , showing a great superiority among all compact optical frequency standards in the absence of Pound-Drever-Hall (PDH) frequency stabilization system. This ultra-stable 420nm diode laser system is a good candidate of pumping light source for 1367nm Rb four-level active optical frequency standard.