MICROWAVE OPTICS - Granular Materials Laboratory

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Microwave Optics012-04630FExp 9– Michelson InterferometerNotes – on Procedure1-4 Best results are obtained when the mirrors are botha significant distance from the central beamsplitter. Ifeither mirror is too close to the center, the maximasplits into two peaks due to secondary interference effects.n Refl. Pos. _1 75.02 76.4 1.43 77.9 1.54 79.4 1.55 80.8 1.46 82.2 1.47 83.7 1.58 85.1 1.49 86.6 1.510 88.0 1.411 89.4 1.412 90.9 1.513 92.3 1.414 93.8 1.515 95.2 1.416 96.6 1.417 98.1 1.518 99.5 1.419 101.0 1.520 102.4 1.421 104.0 1.6average: 1.45From this we can calculate the wavelength as being2.90 cm.By this method, we calculate the wavelength as2.89 cm.Answers – to Questions1 The limit of resolution for distance measurements witha Michelson interferometer is roughly 1/4 the wavelengthof the light used. Thus with these microwaves,we can measure distance changes of about 7 mm.With a visible-light interferometer and a wavelength of633 nm (HeNe laser light) we can measure distancechanges of only 158 nm.ä NOTE: An alternate method of analysis is toplot the reflector position versus fringe number.The slope of this line will be half the wavelength.42
012-04630F Microwave OpticsExp 10 – Fiber OpticsNotes – on Procedure2 The meter reading with the “optical fiber” bag in placecan more than double the unobstructed meter reading.This is because the bag prevents the normal spreadingof the beam, and directs all the microwave radiationinto the receiver horn.3 The bag can direct the beam through a full 90° turnwithout measurable attenuation, if the curvature isgradual.4 The signal begins to be attenuated with a radius of curvatureof about 5 cm, and drops off rather suddenly fromthere. It is difficult to get consistent results, though.Answers – to Questions1 Theoretically one can use the radius at which the microwavesbegin to “leak” to determine the index ofrefraction of the material. In reality, this is quite difficult.Our values for n using this method range from1.1 to 1.4.2 The styrene pellets are very small, compared to themicrowave wavelength. Compared to visiblewavelengths, the pellets are enormous. Because ofthis size difference, optical radiation is scattered bythe pellets and microwave radiation is transmitted.Additional IdeaThis apparatus may also be used as a demonstrationof how the plane of polarization can be rotatedby multiple reflections. Rotate the transmitteror receiver 90° to each other, so that no signal getsthrough. Now put the bag of pellets between thetwo. If the bag is held straight, there will be a zerometer reading; but if the bag is curved into a spiral,there will be a non-zero reading.Exp 11– Brewster's AngleNotes – on Procedure1-5Answers – to Questions1 Glare off water from a low source is primarily horizontalin polarization, so sunglasses should be designedto block horizontally polarized light.2 Theoretically, one could do this by finding thepoint at which there was no transmission of verticallypolarized light. We have not been able to getgood results in this experiment, however.General Notes1 The index of refraction of the polyethylene is 1.5.(Calculated from the dielectric constant at 10 GHz)2 One must be careful on this experiment to note thatthere are actually two effects which are being measured.In addition to the Brewster’s angle reflection,there is a certain amount of interference between thefront- and rear-surface reflections from the polyethylene.This interference causes extra peaks at 13.8° and67.2°, and a local minimum at 43.6°. The interferenceis notable enough that you may want to demonstratethat, instead of the Brewster’s angle.43
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