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abstract: 'We have shown earlier that hyperfine spectroscopy in a vapor cell using co-propagating pump-probe beams has many advantages over the usual technique of saturated-absorption spectroscopy using counter-propagating beams. The main advantages are the absence of crossover resonances, the appearance of the signal on a flat (Doppler-free) background, and the higher signal-to-noise ratio of the primary peaks. Interaction with non-zero-velocity atoms causes additional peaks, but only one of them appears within the primary spectrum. We first illustrate the advantages of this technique for high-resolution spectroscopy by studying the $D_2$ line of Rb. We then use an acousto-optic modulator (AOM) for frequency calibration to make precise hyperfine-interval measurements in the first excited $P_{3/2}$ state of $^{85,87}$Rb and $^{133}$Cs.'
author:
- 'Alok K. Singh, Sapam Ranjita Chanu, Dipankar Kaundilya and Vasant Natarajan'
title: 'Hyperfine spectroscopy using co-propagating pump-probe beams'
---

Introduction
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High-resolution laser spectroscopy has been revolutionized in the last two decades with the advent of low-cost tunable diode lasers [@WIH91]. These diodes, when placed in an external cavity with optical feedback, have frequency uncertainty of about 1 MHz, which is small enough for hyperfine transitions in atoms to be resolved [@MSW92; @BRW01]. Hyperfine spectroscopy, particularly in the low-lying electronic 