Turnkey compact frequency standard at 1556 nm based on Rb two-photon transitions

Abstract

DiCOS Technologies has developed a high coherence fully automated absolute frequency standard for metrology applications. It uses a 1556 nm narrow linewidth DFB fiber laser locked to a two -photon transition in rubidium 85 at 778 nm after second harmonic generation in a non-linear waveguide crystal. Its main features and performances are presented in this paper. lntroductinThe 778 nm rubidium two-photon transitions have been measured with great precision in both 85Rb [1] and 'Rb [2] isotopes and the frequency of the 8Rb 5Si/2 (F=3) -5Dsa (F=5) transition has been recommended for the definition of the meter in 2002 [3]. These reference transitions can be used to frequency stabilize a 1556 nm laser through second harmonic generation and provide a standard for the optical telecommunication window [4]. Such an optical frequency standard can be of great interest in high precision interferometry, radioastronomy and calibration metrology. Therefore, DiCOS Technologies has developed a commercial high performance instrument based on the use of the "Rb two -photon transitions. As an example of application, such a laser will be used as an optical synchronization signal in the Atacama Large Millimeter Array (ALMA) telescope to be deployed in Chile by the National Radio Astronomy Observatory. Considering previous work performed at Laval University [4] and the fact that a practical device needs to be automated, a simplified architecture has been designed in order to achieve a simple locking procedure. The availability of high power narrow linewidth DFB lasers and highly efficient non-linear waveguides has allowed the development of a compact frequency standard that is presented here. Descriltion of the systemIn order to get advantage of the narrow linewidth of the two -photon transition in rubidium, a laser with a linewidth in the kHz range is necessary. DFB fiber lasers that are now commercially available can meet these specifications while providing optical powers as high as 100 mW. This characteristic is very interesting because it can be used to generate several mW of 778 nm in a non-linear waveguide and allows a direct observation of the two-photon transitions in rubidium. The architecture we have selected is depicted in figure 1. A high power (100 mW) DFB fiber laser emitting at 1556 nm is used. It can be frequency tuned through its temperature (coarse and slow tuning) and through a piezoelectric actuator stretching the fiber grating (fine and fast tuning). A fiber coupler allows to pick up a fraction of the optical power to provide an unmodulated useful output while the remainder is directed toward a non-linear waveguide. This device produces a frequency-doubled beam of about 5 mW at 778 nm for a fundamental input of about 70 mW. Fig. 1: Architecture of the turnkey frequency standardThis beam is directed to a 85Rb cell heated to 1 10°C in order to achieve a high atom density. A cat's eye is used after the cell to retro-reflect the incident beam. A photo-multiplier, placed on the side of the cell, collects the fluorescen...