Using Brewster's Angle

To investigate the transverse mode patterns of the He-Ne laser, we rotated a thin piece of glass thru the lasing cavity. Of course, when the laser struck the glass at Brewster's Angle, the "He-Ne" lased as normal. However, when the glass was slowly rotated away from Brewster's Angle, specific mode patterns could be seen. The glass interfered with the lasing, and thus eliminated certain modes from reaching the lasing threshold. Consequently, the modes that were visible  we able to reach the lasing threshold.

As discussed in the theory section, the modal patterns are of the form:

H2.txtgr3.gif (1920 bytes)

Below, we show examples of modal patterns seen using the He-Ne laser. The first two patterns are pure modal patterns, thus they correspond to specific Hermite polynomials. The rest of the patterns are combinations of modes.

TEM (1,0)

TEM (0,1)

TEM (1,1)

TEM (0,2)

?? TEM (2,0) ??

As the above pictures show, the modal patterns take many amazing shapes. It is also evident that pure modal patterns are a lot harder to get than combinations of patterns.