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Union College - PHY 300laser-modes

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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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