Laser Modes
Morgan Clark, Mark Mininberg, Simon Wallace, Nate Venuto
June 20, 2017
Abstract
In this experiment, we investigated He-Ne lasers by looking at the laser modes, and
cavity lengths of an open-frame He-Ne l
...
Laser Modes
Morgan Clark, Mark Mininberg, Simon Wallace, Nate Venuto
June 20, 2017
Abstract
In this experiment, we investigated He-Ne lasers by looking at the laser modes, and
cavity lengths of an open-frame He-Ne laser as well as the intensity of the beam of a
closed frame He-Ne laser as a function of its diameter, and used this to calculate the
divergence angle of the laser. In the first part of the experiment, we set up an openframe laser, and a high reflective mirror which functioned as a secondary mirror of the
laser, and a piezo detector which provided us with a spectrum of the beam. Our goal for
this part of the experiment was to measure the length of the laser cavity at two different
cavity lengths in two different ways: using a ruler to directly measure the cavity, and
calculating the length of the laser cavity using the difference between the peaks of our
spectrum and the Free Spectral Range (FSR) of our laser. Using a ruler, we measured
the lengths of our cavities to be 38.42±0:5cm and 44.4 ±0:5cm. Calculating the length
with the FSR of the laser and the difference between the peaks, we determined the
lengths to be 38.46 ±0.1 cm and 44.38±0.1cm, which are within 0.1% and 0.04% of
the measured values, respectively. In the second part of the experiment, we used a
closed-frame He-Ne laser and five mirrors to create a long path length of 740 ±2 cm,
and a beam with a diameter of about 2.3cm, then used a power meter to measure the
intensity of our beam as a function of diameter. We fit a Gaussian curve to this data
and used the curve to measure the divergence angle of the beam, which we determined
to be 35.4o ± 0:5o, using the Full-Width Half-Max calculated from the σ value from
our fit.
1 Introduction
The characteristics of lasers are important to understand in order to properly use them.
A laser is a device that emits coherent light, meaning that all of the light is in the same
phase. The name laser comes from the acronym "light amplification by stimulated emission
of radiation". The properties of this coherent light are based on its spatial coherence and
temporal coherence. The spatial coherence of laser light allows it to be focused tightly and
for the light to remain in a narrow beam over long distances. The temporal coherence of laser
light allows the light to be emitted in a very narrow spectrum, meaning it can be emitted
at a specific wavelength. In order for a laser to emit laser light, two major conditions must
be met. The first condition is simply that the power of the stimulated radiation be greater
than the loss of power from the absorption and scattering in the laser device. The second
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