Using a Circular Aperture to Observe Axial and Transverse Modes:
A circular aperture was used in conjunction with the piezoelectric device to observe axial and transverse modes of the HeNe laser. The digital oscilloscope allowed observation of the modes as they appeared and disappeared. The aperture was adjusted by moving a small lever which opened and closed a shutter so that one or more modes could pass through. By closing the aperture, one effectively creates loss for certain modes. For example if we compare the TEM40q mode and the TEM00q mode, we can see that closing the aperture may block out part of the TEM40q while allowing the TEM00q mode to pass through entirely(See figure below).
The loss introduced by the aperture causes the TEM40q to stop lasing, so we only see the TEM00q. Images of axial modes were captured from the digital oscilloscope using a computer for three different cavity lengths. For an intermediate cavity length, the aperture was closed until only the TEM00q modes appeared and then was slowly opened to allow more transverse modes through.
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