Abstract
Mode-locking a HeNe laser by extra-cavity acousto-optic modulation
Ewuin Guatemala, Martin Cohen and John Noe,
Laser Teaching Center,
Department of Physics and Astronomy,
Stony Brook University
This project was motivated by an interest in the interaction between light and sound
(acousto-optics) and in longitudinal laser modes. In this work we have demonstrated mode
locking in a helium-neon laser through the use of an AO modulator placed outside the laser
cavity, as suggested in Ref. [1]. Mode-locking of a laser occurs when the longitudinal
cavity modes oscillate in phase. These modes generally oscillate with a random phase
relationship but when in phase the laser will output pulses of higher peak intensity than
the time-average intensity. A setup incorporating an AO modulator (AOM) can provide the
frequency and phase shifts necessary to satisfy the mode-locking condition. In an AOM a
propagating acoustic wave results in a traveling refractive index grating. The first order
diffracted light is frequency shifted by the frequency of sound propagating in the
crystal.
Our HeNe laser (Spectra Physics model 127) supports 8-11 longitudinal modes; it has a
mode spacing of 160 MHz and a cavity length L = 93.7 cm. The laser beam is incident at the
Bragg angle on an Isomet AOM (model 1205C-2-804B) driven at half the mode spacing
frequency. The first diffracted order is reflected back through the AOM to be further
frequency shifted to 160 MHz and re-diffracted back into the laser cavity creating side
band modes adjacent to the cavity modes. The optical path length (OPL) between the laser's
output mirror and the reflecting mirror is controlled by placing the latter on a
micrometer translation stage. This external cavity distance must be carefully matched to
the OPL of the laser cavity. By setting this distance and the AOM frequency, the AO
generated sidebands are coupled to the cavity modes, locking these in phase. The zero
order beam is reflected to a beam splitter, which directs it to a Fabry-Perot analyzer and
a fast Si photodetector (Thorlabs DET-210) connected to an oscilloscope (Tektronix 485) or
spectrum analyzer (HP 8566A).
The spectrum analyzer was used to tune the sidebands to coincide with the cavity mode
spacing; mirror adjustments resulted in shorter pulses with higher peak intensity. The
pulses had the expected 160 MHz spacing and a peak pulse amplitude four times higher than
the time-average intensity. Further refinements to the experiment will be aimed at finding
the optimal external cavity length. This is complicated by mechanical instability of the
mirror mount, which is very sensitive to vibrations. We hope to achieve stronger phase
locking, resulting in pulse width values more consistent with theory, which predicts
subnanosecond pulses.
We would like to thank Rich Migliaccio (East Coast Optical Technologies) for providing
the Spectra Physics laser and Sam Goldwasser for providing the FP analyzer. This work was
funded by URECA and NSF-REU (grant no. PHY-0851594).
References
[1] R.J. Jones. "Active Modelocking of a Helium-Neon Laser." Opti-511L course at the
University of Arizona,
http://www.optics.arizona.edu/opti511l/2009_files/modelocking_Fall2009.pdf
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