DUAL, FEED-FORWARD + FEED-BACK LASER FREQUENCY CONTROL FOR EFFICIENT AND CONVENIENT DIODE LASER LINE NARROWING
Résumé
Distributed feedback (DFB) diode lasers are convenient, small footprint and robust single mode laser sources. Control of the diode forward current allows for the control the frequency of the emitted laser beam. Locking the laser frequency to some optical frequency discriminator, such as an optical interferometer or an atomic or molecular line, can easily be achieved. Such a lock, with a moderate (< kHz) bandwidth can prevent long term drifts of the laser optical frequency. On the other hand, reaching high lock bandwidth (> MHz) can lead to laser line narrowing. DFB lasers have an emission linewidth in the MHz to several MHz range, which may be too large for some applications, such as cold atom physics, optical clocks, laser ranging, lidar or gas sensing...
We have tried to achieve a high bandwidth in the lock of a DFB diode laser with a feedback loop using both the laser diode forward current and an optoelectronic external modulator. A lock bandwidth in the MHz range has been obtained, and the optical spectrum shows narrowing. But a significant fraction (15-20%) of the optical power remains broadband. By contrast, much more efficient and convenient operation has been found when the external electro-optical modulator is not in a feed-back loop but in a feed-forward configuration. The optical spectrum shows a half width at half maximum of 3 to 4 kHz, with about 1% of residual broadband optical power. The system allows implementation at any wavelength, if the frequency discriminator is obtained from a Michelson interferometer with gold-coated mirrors.
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