Ontheuseofatransitionsinatomic Frequencystandards

The possibility of using a three-level-A-scheme for the excitation of the clock hyper-ne transition in s’Rb is analyzed. The physics involved opens up two distinct avenues useful for the implementation of an atomic frequency standard, avenues based on two distinct phenomena: the electro-magnetically induced transparency (EIT or Dark Line) and the coherent microwave emission (Maser without population inversion). In spite of the close connection between the two phenomena, which are generated by the coherence created in the ground state by means of two coherent laser radiation fields (Coherent Population Trapping), the characteristics of an atomic frequency standard based on either of these phenomena may be quite different. The paper discusses the possibility of implementing on the basis of these two phenomena, new 87Rb atomic frequency standards having an excellent frequency stability, and makes explicit their differences. The possibility of observing atomic hyperfine resonances by means of Coherent Population Trapping (CPT) has been known for many years [ 11, while microwave emission due to the creation of a coherence in the ground state by the same CPT phenomenon was discovered more recently [2-31. The availability of laser diodes makes possible the implementation of atomic frequency standards based on either of these effects. As will be described in more details in the next section, two laser radiation fields used in a A configuration, as shown in Figure 1, create a coherence in the ground state of the atomic system leading to an electro-magnetically induced transparency effect (EIT) in the optical domain. This EIT effect has its maximum when the frequency difference of the two laser radiation fields is equal to the hyperfine splitting of the atom ground state. This signal can be used for the implementation of a frequency standard. In that case the physical construction of the system is relatively simple because no microwave cavity is needed to excite the atomic microwave transition and the detection and servo systems are basically identical to those used in the classical optically pumped rubidium frequency standard. In a previous paper, we have analyzed also the possibility of using the observed coherent microwave emission in the implementation of a new atomic frequency standard: the CPT-Maser [4]. This maser should have a frequency stability behavior somewhere between that of a classical rubidium frequency standard and a hydrogen maser. Its physical implementation may be more complex than a classical rubidium frequency standard, but simpler than a hydrogen maser. In fact, the physical packaging is not that different from a rubidium frequency standard one, while the electronics and the servo system are closer to those of a H-maser. Advantages and disadvantages of these different approaches are discussed