Heterodyning of AlGaAs lasers: direct frequency measurement of the isotope shift in the oxygen atom.

Semiconductor diode lasers have already been proved to be suitable sources of radiation for applications in atomic and molecular physics.' Several methods have been developed to improve their spectral purity and tunability. In particular, the sensitivity of these lasers to optical feedback has been exploited advantageously to narrow their linewidth and to control the emission wavelength by using external optical cavities.2 We have recently demonstrated the usefulness of this technique in a high-resolution study of the S 2 Pl, 2 ,3 multiplet of atomic oxygen at 777 nm.3 ' It is worth noting that, although spectral purity of the order of a few hundred kilohertz is easily achieved with this scheme, wavelength calibration with comparable accuracy is in contrast difficult. In this Letter we show how frequency-stabilized diode lasers can be combined in a heterodyne scheme to obtain precise measurements of atomic structures. We have chosen the 5 S 2 P 1 oxygen transition at 777.6 nm. Two independent lasers were locked onto the sub-Doppler resonances of different isotopes and then mixed in a fast photodiode. The direct frequency measurement of the separations allows the elimination of various unwanted effects, such as amplitude changes, tuning nonlinearities, and improper calibration of reference interferometers, that arise in optical spectroscopy when long scans of the laser frequency are performed. We used two Sharp LT024 AlGaAs/GaAs diode lasers emitting at 780 nm at room temperature. The lasers were attached to small copper blocks in thermal contact with Peltier cells; by proper insulation, a temperature stability of >10 mK was achieved. Because the lasers are provided with a reduced reflection coating on the output facet, they can be used in an extended-cavity configuration: the first-order diffracted beam from a 1200-line/mm ruled grating, mounted in the Littrow configuration, was fed back into the laser diode. This allowed us to select one of the modes of the laser cavity within a range of -10 nm, at a fixed temperature. Fine tuning to the transition of interest was then achieved by slightly changing the temperature and the injection current while monitoring the emission wavelength by means of a seven-digit wavelength meter (constructed by R. Drullinger, National Institute of Standards and Technology, Boulder, Colorado). Continuous-frequency scans of as much as -10 GHz could be accomplished by synchronously sweeping the cavity length by means of a piezoelectric transducer (PZT) on the grating and by means of the injection current. Light from an intracavity beam splitter (R 30%) was used for the experiment. Atomic oxygen was produced in two independent Pyrex cells by means of rf discharges.3 0 2-Ar gas mixtures were introduced in each cell in a ratio of 1:10; the total pressure was reduced to 1 Torr to minimize the broadening of the lines.4 The observed linewidth varied between 40 and 100 MHz, depending on the discharge conditions. To frequency lock each of the lasers onto Dopplerfree signals, a saturation spectroscopy scheme was used,5 as shown in Fig. 1. Each laser beam was split into two parts of different intensities and sent in opposite directions into the sample cell. The pump-beam power density in the discharge was -3 mW/mm2. The changes in the probe-beam intensity were detected with a good signal-to-noise ratio by using a photodiode. Derivative signals were obtained by applying a 780-Hz modulation to the PZT voltage and using phase-sensitive detection. Both firstand third-derivative signals could be detected easily. Typical signals, derived from an isotopically