Interferometry is a technique used in order to diagnose the properties of two or more waves by observing their interference pattern. The instrument used to make the waves interfere is called and interferometer.

There are many different types of interferometers, but for our experiments we used what is known as a Mach-Zehnder interferometer. The Mach-Zehnder interferometer, named after physicists Ludwig Mach and Ludwig Zehnder, is a device used to observe interference. In this type of interferometer, a collimated beam is split by a half-silvered mirror (beam splitter cube) and the two resulting beams each travel a distinct path. Each beam then hits a mirror which reflects the beam onto a second half-silvered mirror where the beams are recombined.

This type of interferometer is a little harder to set-up then some others, but it is easier to understand some of the more complicated ideas behind the experiments using this type of interferometer. A difference, for example, is that in contrast to the Michelson interferometer, the Mach-Zehnder interferometer has two output ports.



  • Non-Polarizing Beam Splitter – (ThorLabs BS014) – We used non-polarizing beam splitters to make the paths indistinguishable. This is important because in order for the photon to interfere itself we must NOT be able to find out which path it traveled.
  • Stages – Used to make adjustments to path lengths
    • Manual Stage – used for coarse adjustments on Path 1. Adjusts up to +/- 1 inch around its center
    • Electronic Stage – Driven by a Piezo bridge, used for finer adjustments. We can make adjustments that are accurate up to as fine as 20 nanometers.
  • Broadband Dielectric Mirrors – (ThorLabs BB1-E03) - effective for a range of wavelengths from approximatly 650 nm to 1200 nm
  • Pick Off Mount - (ThorLabs NX1F)– Used to redirect down converted light into interferometer. As we have two experiments being run simultaneously we opted to use removable mirror mounts in order to prevent interference and still maintain alignment.
  • Red Tide Spectrometer – Used to find interference in white light


  1. We began by first mounting all optic equipment.
  2. We decided to use a Mach-Zender Interferometer because the two distinct paths traveled are more obvious for educational purposes.
  3. We used a HeNe laser for preliminary alignment of the interferometer for two reasons: because it has a much longer coherence length, which makes observing the interference pattern much easier and because it is visible to the naked eye, which makes alignment possible.
    • We first ensured that the beam was parallel to the table using two mirror and two irises
    • We started alignment of the interferometer by placing the beam splitter so that part of the beam traveled straight through and part was reflected by 90 degrees
    • Then we inserted the piezo driven stage at a distance of 15 cm from beam splitter 1
    • Parallel the beam that reflects from mirror 3 on the piezo stage with the table and with the beam that travels along path B.
    • Insert manual stage 15 cm from Beam Splitter 1 along path A
    • Parallel the beam with the table
    • The beams should now intersect with each other in the air at a distance of 15 cm from each stage.
    • We can now insert the second beam splitter at this point, which will recombine the beams.
    • Used the manual stage and beam splitter 2 to adjust the beams to be perfectly collinear.
    • Pair the stage with aligning the beams close to the beam splitter and the beam splitter to align the beams when far from the beam splitter
    • We new can look at the interference pattern by passing it through a diverging lens in order to enlarge it and project it onto a piece of paper.
    • As we turn the screw of the stage, we can see the fringes move in either direction, depending on which way we turn the screw.
    • We now aligned the detector by attaching a fiber optic cable and maximize the light coming through the fiber. (For further hints see the section on the Optics Playground)

White Light Fringes

  1. To find white light interference we began by inserting a white light source into the path. We used a small Maglight with the top completely unscrewed.
    • It does not matter that the light will go everywhere, as the only light that will enter the detector is the light that travels through the path of the interferometer.
  2. We then attached spectrometer to the fiber optic cable attached to the detector. (we needed an FC-SMA adapter) The Spectrometer was attached to computer using a USB cable
    • MAKE SURE NO LASER LIGHT IS ON as it could burn out the spectrometer
  3. We used LoggerPro to observe the light spectrum seen by the detector
    • We were able to observe interference in the white light. We determined that there was interference by seeing fringes across our spectrum of white light by making small adjustments to the mirrors we were able to see the fringes spread out till they were smooth. This is what we call t=0 interference.
    • The series of photos below show the progression of fringes as we smooth them out to t=0.
    • t=0 means total destructive interference
  • This means the interferometer is properly aligned and we can move on to the down converted light

Down Converted Light


  1. The first step in getting our down converted light into our interferometer is to send a diode laser backwards through the interferometer and line the light up with the path our down converted light has traveled
    • To do this we must have iris set up defining the path our light travels (this is done by sending the diode laser back from the detector in our down converted light experiment after it has been maximized). We can use the iris set up for that experiment to align the pick off mirror that will send our invisible downconverted light through our interferometer.
  2. Once the diode laser has been aligned we can carefully attach our detector to the SPCM, and maximize the down converted light using first the pick off mirror.

After we obtain a maximum we can start collecting interference data from our interferometer.

Data Collection

  • We collected data using this standard method:
  1. Make sure to follow all Safety Procedures
  2. Warm up the laser and Piezo stage for a good period
  3. Zero the stage following the instructions found here (LINK)
  4. Using the LabView program take 30 1 second runs at each position of the stage (this automatically saves your data into a text file which you can later transfer to Excel)
  5. Move the stage 20 nanometers between each experiment
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