Today you will build a simple interferometer.  You will split the output beam of a He-Ne laser into two beams which will move along different paths, but eventually recombine.  After recombining, beams will produce an interference pattern on a screen.

The light from the He-Ne laser is a periodic electromagnetic wave with a wavelength of 633 nm.

The electric field of this wave increases and decreases as a function of position and time.  We picture such a wave as a series of crests and troughs.

Two or more waves traveling through the same region of space can pass through each other.  In regions where they overlap we only observe a single disturbance.  We observe interference.  If two waves with equal amplitudes overlap in phase, i.e. if crest meets crest and trough meets trough, then we observe a resultant wave with twice the amplitude.  We have constructive interference.  If the two overlapping waves, however, are completely out of phase, i.e. if crest meets trough, then the two waves cancel each other out completely.  We have destructive interference

In this lab, if the two recombining He-Ne laser beams would travel exactly collinear, then we could observe completely constructive or destructive interference, i.e. we could see either a bright or a dark spot.  Changing the path length of one beam by 1/2 wavelength with respect to the path length of the other beam would switch the spot from bright to dark or from dark to bright.

If there is a very small angle between the directions of travel of the two beams, then we observe interference fringes. 

Different parts of the beams interfere constructively or destructively.  If the angle increases, the fringe spacing becomes smaller and smaller and the fringes quickly smear out.

Equipment needed:

• 1 He-Ne Laser
• 3 mirrors
• 1 beam splitter
• 4 post holders

Procedure:

In all optics experiments it is very important to align things properly and to tighten things properly.

• Turn on the laser.
• Reflect the laser beam from the middle of mirror 1 approximately 50 cm away from the output hole back onto the laser output hole.  Use a mirror with a center-of-rotation adaptor so you later can turn it easily, and use a collar to fix the height of the mirror, so the height does not change when you loosen the screw in the post holder to turn the mirror.

• Use lens paper in the beam path to observe the incident as well as the reflected beam and adjust the mirror so that the reflected beam overlaps the incident beam.

• Mount the beam splitter approximately 10 inches from mirror 1 at a right angle to the laser - mirror 1 line, and mount each of the other two mirrors without center-of-rotation adaptors approximately 8 inches from the beam splitter as shown in the figure below.

• Rotate mirror 1 so that one of the laser beams strikes the center of mirror 2, and then rotate the beam splitter so that the other laser beam strikes the center of mirror 3.
• Cover mirror 3 with an index card and adjust mirror 2, so that the laser beam reflects from mirror 2 back onto the laser output hole.  Use lens paper to help you view the reflected beam.
• Cover mirror 2 with an index card and adjust mirror 3, so that the laser beam reflects from mirror 3 back onto the laser output hole.
• Uncover both mirror and observe fringes on an index card.  Magnify the fringes using a lens.

• Observe how the fringe pattern changes when you touch the mirrors, touch the table, disturb the air in the path of one of the laser beams, etc.  Every relative movement of 1/2 wavelength (1/2 *633 nm) will change a bright fringe into a dark fringe and vice versa.
• Place various combinations of polarizers and quarter-wave plates into the path of one or both of the laser beams and observe changes in the interference pattern.

Interferometer are optical instruments of high precision and versatility.  They are generally used in investigations that involve small changes in optical path lengths.  One can produce circular and straight-line fringes and use these fringes to make an accurate comparison of wavelengths, measure the refractive index of gases and transparent solids, and determine small changes in length quite precisely. Interferometers are also used for measuring defects in optical components such as lenses, prisms, plane-parallel windows, laser rods, and plane mirrors.