Michael David di Rosa

Phase-locked scanning interferometer for frequency stabilization of multiple lasers.

We report a simple scheme for stabilizing and tuning the length of a conventional piezo-driven optical cavity against the resonant transmission of a master laser. In contrast with other schemes, we drive the piezo at its mechanical resonance of 5 kHz over an amplitude equivalent to one free spectral range and use a feedback circuit that incorporates phase-sensitive detection of the master-laser transmission. The bandwidth of our cavity-lock circuit is limited only by the resonance frequency of the cavity piezo and is 1.4 kHz. The stabilized mean cavity length reaches in 30 s a minimum Allan deviation of approximately 10 kHz (a length stability of 20 parts per trillion) equaling that of the polarization-stabilized He-Ne we use as our master laser. Here, we investigate the mechanical characteristics of the cavity, describe the lock circuit and its measured performance, and provide simple analytical relations between the phase-sensitive signal and cavity displacement. Our setup economizes the cost and amount of equipment necessary for stabilizing multiple continuous-wave lasers operating at different wavelengths.

Ultrasensitive detection of radioactive cesium isotopes using a magneto-optical trap

We report the first magneto-optical trapping of radioactive 135Cs and 137Cs and a promising means for detecting these isotopes to ultrasensitive levels by using a magneto-optical trap (MOT) coupled to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass separated, and implanted in a Zr foil within the MOT cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large diameter beams and a dry-film-coated cell to achieve high trapping efficiency. MOT fluorescence signals were measured for trapped-atom numbers from 104 to 107 and were found to increase linearly with the number of atoms implanted in the foil. The slope of signal versus number implanted was equal for each isotope to within 4%, signifying our ability to measure 137Cs/135Cs ratios to within 4% for MOT signal levels exceeding that associated with our present detection limit of 4000 trapped atoms. The MOT-based detection scheme was shown capable of suppressing interference from stable 133Cs by more than seven orders of magnitude. Including an isotopic selectivity of 105 of the mass separator, the overall suppression of 133Cs in the case of detecting either 135Cs and 137Cs is expected to exceed 1012. At present, the overall sample detection sensitivity is less than one million atoms.

Computational expressions for signals in frequency-modulation spectroscopy.

General expressions for the signals in frequency-modulation spectroscopy (FMS) appear in the literature but are often reduced to simple analytical equations following the assumption of a weak modulation index. This is little help to the experimentalist who wants to predict signals for modulation depths of the order of unity or greater, where strong FMS signals reside. Here, we develop general formulas for FMS signals in the case of an absorber with a Voigt line shape and then link these expressions to an example and existing numerical code for the line shape. The resulting computational recipe is easy to implement and exercised here to show where the larger FMS signals are found over the coordinates of modulation index and modulation frequency. One can also estimate from provided curves the in-phase FMS signal over a wide range of modulation parameters at either the Lorentzian-broadening or Doppler-broadening limit, or anywhere in between by interpolation.