Talk:Advanced Projects Lab

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The goal of the spectroscopy project was to create a michelson, mach-zehnder and sagnac Interferometer. Each setup utilized a 632.8nm Helium-Neon laser.

In order to reduce fringe-drift, the michelson interferometer employed both an optical iris and optical isolator. The setup of the michelson interferometer is shown below:

The Michelson interferometer was used to measure both the "voltage-to-expansion" ratio of a diode and the coherence length of the He-Ne laser.

To measure the "voltage-to-expansion" ratio, the diode was wedged in the track of the adjustable mirror. By increasing the voltage across the diode, and subsequently counting the number of fringes that passed an arbitrary point on the projection screen, it was possible to measure the expansion distance of the diode with the relation d=m*lambda/2. Multiple measurements were recorded and plotted in the graph shown here:


The slope of the graph indicates the diode expands at a rate 100nm/Volt.

To measure the coherence length of the He-Ne laser, the mirror located perpendicular to the laser was gradually moved backwards until the interference pattern was no longer visible. The coherence length <l>, is given by the relation <l>=2*d, where d is difference in interferometer arm lengths. The measured coherence length was 78cm.

The mach-zehnder interferometer setup is shown below:

As with the Michelson interferometer, an optial iris was employed to minimize outside noise. However, the optical isolator proved unnecessary with the Mach-Zehnder interferometer as no fringe drift was observed.

A diagram of the setup of the sagnac interferometer is shown below:


The extra mirror placed above the beam splitter was added to fit the interferometer onto a small, rotatable board. The small board was fastened beneath another small board that contained both the power supply and the digital oscilloscope. This entire setup was duck-taped to a rotating stool.

The sagnac interferometer was intended to verify the wavelength of the He-Ne laser using the relationships z=(4*omega(t)*A)/(lambda*c) and P(t)=(Pin/2)*(1+cos(z(t)). Here z is the phase change, omega is the angular velocity at which the stage is rotated, A is the area enclosed by the interferometer's arms, P(t) is the photocurrent of the outgoing laser, and Pin is the laser photocurrent without rotation.

Unfortunately, it was not possible to measure the photcurrent of the laser due to outside noise. Noise could be reduced in future experiments by means of a lock-in amplifier.