ALP Seminar: Multichannel optogalvanic frequency stabilization of diode lasers using an audio interface

An inexpensive USB audio interface combined with a direct digital synthesizer and a DSP lock-in amplifier, both implemented as open source computer software, are used to perform FM laser spectroscopy of ions produced in a hollow cathode lamp's discharge. The output of the audio interface modulates a diode laser's injection current which induces optical frequency variations. When the laser is tuned to an ion transition, the optogalvanic effect causes a small modulation of the hollow cathode lamp's impedance isochronous to the applied optical frequency changes. The magnitude and the phase of this impedance modulation are detected by the DSP lock-in amplifier and fed into a digital control loop that stabilizes the laser's average optical frequency to a setpoint within the spectral line, using a thermoelectric cooler attached to the laser diode to tune it.

By using orthogonal modulation frequencies, two lasers at different wavelengths can be combined on a dichroic optical component and elegantly locked to a single hollow cathode lamp, using the two output channels available on a typical stereo audio interface.

The idea was first demonstrated with telecommunication DFB lasers on the low-lying transitions of doubly-ionized lanthanum near 1390nm and 1410nm. These transitions happen to be conveniently aligned to the ITU CWDM grid; as a result, high-quality lasers and optics are widely available for these wavelengths and are easy-to-use and inexpensive. The hyperfine structures of these transitions were easily observed at the output of the lock-in amplifier while sweeping the laser temperatures, and both lasers were succesfully locked simultaneously to their corresponding spectral lines using a single lanthanum hollow cathode lamp and a single stereo audio interface.

The idea was then applied to inexpensive Fabry-Perot laser diodes near 493nm and 650nm, corresponding to barium ion transitions. The diodes were operated free-running without an external cavity, after manually identifying regions of operation where the diodes were mostly single-frequency and free from mode partition noise. After compensating in software for the photoelectric effect caused by the 493nm light, both lasers were again successfully locked to a barium hollow cathode lamp.

While it is noisier than typical stabilized laser systems used in quantum research, the present system is likely able to perform barium ion Doppler cooling and repumping on its own, and distinguishes itself by its extremely low cost, radical simplicity, ease of construction, and immunity to vibration.