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Annual Meeting Optical Society of America

Passive pulse train generation from a frequency modulated diode laser

J. Menders, Kyunam Choi, Eric J. Korevaar

Published Jan 1, 1992• 0 citations

Abstract

By sweeping the frequency of a diode laser beam near the cesium resonance frequency, followed by propagation through a short length of cesium vapor, we obtained periodic peaks with peak-to-average ratios near 5 at pulse repetition rates up to a gigahertz. Light in the wings of a strong atomic absorption line of an alkali metal vapor encounters large group velocity dispersion. A frequency modulation in an off-resonance cw beam passing through the vapor consequently develops an amplitude modulation. For certain combinations of laser and vapor parameters, a sinusoidally frequency modulated beam can be made to develop peaks. We generated pulse trains in a frequency modulated single mode diode laser tuned to the cesium absorption line near 852 nm, which we observed by using a fast p-i-n photodiode and a sampling oscilloscope. The frequency of a diode laser was sinusoidally modulated with a sweep of about a gigahertz and a period of a few nansoseconds by introducing a small modulation into the laser pump current. We observed the formation of sharp peaks with a width of about one-tenth of the modulation period and a peak-to-background ratio of approximately 5.

DOI: 10.1364/oam.1992.wu4

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By sweeping the frequency of a diode laser beam near the cesium resonance frequency, followed by propagation through a short length of cesium vapor, we obtained periodic peaks with peak-to-average ratios near 5 at pulse repetition rates up to a gigahertz. Light in the wings of a strong atomic absorption line of an alkali metal vapor encounters large group velocity dispersion. A frequency modulation in an off-resonance cw beam passing through the vapor consequently develops an amplitude modulation. For certain combinations of laser and vapor parameters, a sinusoidally frequency modulated beam can be made to develop peaks. We generated pulse trains in a frequency modulated single mode diode laser tuned to the cesium absorption line near 852 nm, which we observed by using a fast p-i-n photodiode and a sampling oscilloscope. The frequency of a diode laser was sinusoidally modulated with a sweep of about a gigahertz and a period of a few nansoseconds by introducing a small modulation into the laser pump current. We observed the formation of sharp peaks with a width of about one-tenth of the modulation period and a peak-to-background ratio of approximately 5.

Using a technique similar to chirped pulse compression, we have compressed the 50-mW cw output of a diode laser into pulses of greater than 500-mW peak power and less than 400-ps duration. By applying a small current modulation to the diode, we induced a small wavelength modulation in the vicinity of the 6s(1/2)-to-6p(3/2) cesium resonance transition at 852 nm. Group-velocity dispersion on propagation through a cesium vapor cell then led to pulse compression. We developed a simple model to make predictions of output pulse shapes by using different modulation waveforms.

The velocity of a cesium atomic beam was controlled using two broadband frequency-modulated, external-cavity diode lasers. One laser beam was used to decelerate faster atoms and the other was used to accelerate slower atoms. The central velocity of the atoms was readily tuned in a wide range while maintaining the width and peak height of a velocity distribution. In addition, the velocity distribution of cesium atoms was calculated on the basis of the scattering force caused by two frequency-modulated laser beams. The calculated peak ratio was in good agreement with the experimental result.

Laser Generation without Population Inversion in the Red Wing of the

{D}_{1}

-Line of Alkali Metal Atoms under Resonant Diode Pumping

A new method of laser radiation generation without population inversion in the red wing of the

D_{1}

-line of alkali metal atoms under resonant absorption of broadband radiation of pump laser diodes at the

D_{1}

transition is studied theoretically. The appearance of laser generation is caused by the fact that the probability of stimulated emission in the red wing of the spectral line exceeds the probability of absorption if homogeneous broadening due to interaction between active particles and the buffer gas significantly exceeds the natural broadening (under high pressures of the buffer gas). Analytical formulas representing the operation of a transversely diode-pumped alkali metal vapor laser are obtained. It is found that cesium atoms are the most promising object for observation of laser generation without population inversion in the red wing of the

D_{1}

-line of alkali metal atoms. In a sufficiently long active medium (the length of the medium is 50 times its width), the efficiency of pumping radiation conversion into laser radiation can reach 57% at a buffer gas pressure of 5 atm, a pump diode radiation intensity of 5 kW/cm

{}^{2}

, and a half-width of its spectrum of 1 cm

{}^{-1}

. The laser radiation frequency can be tuned by several tens of cm

{}^{-1}

.

A simple control method for the velocity distribution of cesium atoms is demonstrated using a frequency modulated diode laser with a wider spectrum. Current to the diode laser was modulated and the laser spectrum was broadened to overlap the Doppler broadening of the atomic beam. The reduction of the atom velocity was achieved on the basis of the laser cooling technique and a single narrow peak at approximately 45 m/s was observed. From a view point of a novel purification process, the possibility to deflect this cooled cesium atomic beam was discussed to spatially separate the beam from impurities.

We have cooled and stopped a beam of cesium atoms using frequency chirped diode lasers. In this technique, a moving atom absorbs and reemits many photons from a counter propagating laser that is frequency swept to keep it in resonance with an atomic transition. The resulting momentum transfer slows the atom. The basic approach is identical to that developed by Hall and co-workers, however, the use of diode lasers allows us to obtain the necessary frequency modulation simply by sweeping the laser injection current. We use two diode lasers tuned to each of the 6s hyperfine ground state levels to drive the 6s—6p3/2 transitions. Each laser is simultaneously swept over the Doppler profile of the 100°C thermal cesium beam to bring a substantial number to a stop. Using additional fast frequency modulation, the same lasers are used to probe the resulting velocity distribution. This technique allows us to bring ~106 atoms/cm3 to a temperature of 1 K. This method is an extremely simple and inexpensive way to produce cold atoms.