Modulation resonance in a laser II. The case of atomic and cavity frequency coincidence

Савинов Константин Николаевич, Nickolay N. Golovin, Ilya D. Dneprovsky +2 more

For the first time, a series of CPT resonances under the simultaneous exposure of microwave and VHF modulation of the injection current of a diode laser with an external cavity, represented by diffraction gratting, was observed. The frequency spacing between resonances was coincided with half of the VHF modulation frequency. The contribution to pumping in this case comes from pairs of VHF components located in different spectral bands and separated from each other by the frequency of hyperfine splitting of ground state of rubidium-87 atom (about 6.8 GHz). The use of multifrequency pumping will make it possible to reduce the light shifts of the observed CPT resonances due to averaging, and the use of a semiconductor laser with an external cavity opens up the possibility of creating a combined optical and radio frequency standard. The dependences of the CPT parameters of resonances on the intensity of the incident radiation are also studied in this work, and the threshold observed in this case is consistent with the results described in the literature. The dependences of the amplitude of CPT resonances on the deviation of the applied VHF modulation have also been studied. The VHF modulation frequency remained unchanged at 34 MHz (1/200 of the clock transition frequency).

R. N. Shakhmuratov, A. R. Kessel

L. Aubrecht

Stephen H. Chakmakjian, Karl W. Koch, Carlos R. Stroud +1 more

We report our results on measurement of the total fluorescent power radiated by two-level atoms excited by a 100% amplitude modulated or bichromatic laser field. For the case of single-frequency excitation, it is well known that the total fluorescent power monotonically saturates as the intensity of the field is increased. This occurs because the inversion of the atoms approaches zero at high field strengths. In contrast, when the field is bichromatic the total fluorescent power displays resonant peaks as the strength of the driving field is increased. This occurs because there is enhanced absorption when the modulation frequency coincides with the Rabi freqeuncy or one of its subharmonics.

David D. Smith

We analyze the effect of a highly dispersive element placed inside a modulated optical cavity on the frequency and amplitude of the modulation to determine the conditions for cavity self-stabilization and enhanced gyroscopic sensitivity. Hence, we model cavity rotation or instability by an arbitrary AM/FM modulation, and the dispersive element as a phase and amplitude filter. We find that anomalous dispersion may be used to self-stabilize a laser cavity, provided the magnitude of the group index of refraction is smaller than the phase index of refraction in the cavity. The optimal stabilization is found to occur when the group index is zero. Group indices with magnitudes larger than the phase index (both normal and anomalous dispersion) are found to enhance the sensitivity of a laser gyroscope to rotation. Furthermore, our results indicate that atomic media, even coherent superpositions in multilevel atoms, are not useful for these applications, because the amplitude and phase filters work against one another, i.e., decreasing the modulation frequency increases its amplitude and vice versa, with one exception: negative group indices whose magnitudes are larger than the phase index result in negative, but enhanced, beat frequencies. On the other hand, for optical resonators the dispersion reversal associated with critical coupling enables the amplitude and phase filters to work together under a greater variety of circumstances than for atomic media. We find that for single over-coupled resonators, or in the case of under-coupled coupled-resonator-induced absorption, the absorption and normal dispersion on-resonance increase the contrast and frequency of the beat-note, respectively, resulting in a substantial enhancement of the gyroscopic response. Moreover, for cavity self-stabilization, we propose the use of a variety of coupled-resonator induced transparency that is accompanied by anomalous dispersion.

Dae Kim, Marlan O. Scully