Microwave electric-field measurement with active Raman gain

An active-Raman-gain (ARC) spectrum of Rydberg atoms is investigated and proposed to measure a microwave (MW) electric (E) field. A strong Raman field couples the ground-state transition with large frequency detuning, and a probe field drives the metastable transition, whose transmission shows a large gain. When a MW field couples the Rydberg transition, the gain peak splits. It is interesting to find that the frequency splitting of gain peaks shows a linear relationship with the MW-field strength, which can be used to measure the MW E field. Moreover, the ARC spectrum has a large peak value that is about 10 times larger than that of common electromagnetically induced transparency schemes. For weak MW fields, the Doppler effect will greatly narrow the gain spectrum if the probe and Raman fields copropagate through the atomic cell, which increases the probe sensitivity dramatically. After the Doppler averaging, the probe sensitivity is enhanced by about 10 times, and the linewidth is narrowed to 1/10 compared to the case without the Doppler effect. (C) 2019 Optical Society of America