Home Optics Experiments PE-0600 Optical Interferometer

Keywords:

  • Michelson Interferometer
  • White Light Interference (!)
  • Mach-Zehnder Interferometer
  • Evacuable Cell
  • Index of Refraction
  • Edlen Formula
  • Interference
  • Coherence
  • Wavelength of Light
  • Beam Expander
  • Beam Splitter
  • Translucent Image Screen

 

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

Ophthalmology

 

Introduction

How it works ...

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PE-0600 Optical Interferometer

In 1881 Albert Michelson used an interferometer to successfully disprove the theory of a universal ether that existed till then. Later on, he determined the length of the basic meter in units of light wavelengths with this set-up. Still, the use of interferometers in performing technical length measurements only reached significance after the discovery of the laser as a coherent light source. Today, high precision length measuring instruments have become an important tool for many areas of the machine building industry.

The Michelson interferometer commonly uses one moveable mirror which is attached to the object for which a path length measurement on nm scale is performed. The Mach-Zehnder interferometer has no moving parts and the  operation is based on the retardation of one beam with respect to the other by changes of the index of refraction of the probe medium.

This experiment provides both, a Michelson and a Mach-Zehnder interferometer. The Mich­elson interferometer is used to demonstrate the classical interference patterns. For the Mach-Zehnder interferometer an evacuable tube is inserted into one of the beam paths and the interference pattern visualizes the changes of the index of refraction. The measurement of the index of refraction of air as a function of the pressure is made by using the vacuum pump.

PE-0600 Optical Interferometer

The classical Michelson setup consists of the beam splitter, the mirror 1 and the mirror 2. The incident beam from either a green laser or a white light LED is split into two beams at the beam splitter. The returning beams from mirror 1 and 2 are imaged by means of a diverging lens onto a translucent screen. Mirror 2 is mounted on a translation stage for precise change of the related optical path, particularly for white light interference. The beam expander provides an enlarged beam with plane wave fronts resulting in a fringe pattern with a parallel structure. Circular rings are obtained, when the beam expander is aligned for curved wave fronts.

The great advantage of a Mach-Zehnder interferometer lies in the fact that there are no back reflections from the interferometer mirror into the light source as it is the case with a Michelson interferometer. The back reflections create undesired fluctuations and frequency hopping of the laser. That is one of the reasons why the Mach-Zehnder Interfero­meter found much more application than the Michelson setup. The beam of the green laser is enlarged by a beam expander. At beam splitter (1) the expanded beam is split into two beams (A, B) with same intensity. One propagates to the mirror (1) and the other one is deflected by 90° and travels to the mirror (2). Both beams are combined again at beam splitter (2). 50% of each beam is transmitted and reflected. The interference pattern on the screen is created by the reflected part of AR and the transmitted part of BT at exit (1). At exit (2) a combination of AT and BR is available, however not used here. The bright  to dark transitions of the interference pattern are proportional to the path or phase difference of beam A and B. In case of the Michelson, the path difference can be created by moving one of the mirrors, which is not possible with the Mach-Zehnder setup. Another way to introduce such a phase shift is to insert an optical transparent material in one arm of the interferometer and to change its index of refraction. Within this experiment a tube is used, which is filled by the surrounding air and can be evacuated. By counting the number ΔN of moved fringes for an pressure interval ΔP provides the information to calculate the index of refraction of air as function of the pressure. Additional sensors for temperature and humidity enables the comparison of the measured value for the index of refraction of air n with the results of the Edlen formula n(P,T,H), whereby P is the pressure, T the temperature and H the relative humidity.

PE-0600 Optical Interferometer

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