Precision Measurement of Material and Optical Properties Using Interferometry

Team Members

Yang SangUk

Zhang ShunYang

Xu Zifang

List of Apparatus

Light source: 780nm laser diode (GH0781RA2C SHARP diode) with LT230-B collimation tube

Optics: optical isolator (Thorlabs IO-3D-780-VLP), beamsplitter, two 25.4mm broadband dielectric mirrors (BB1-E02-10)

Salt water of different concentrations in a transparent container (for determining the refractive index of solutions of different concentrations)

Powermeter

Introduction

Idea

We will be constructing an interferometer as a tool for precision measurement. One primary objective is to determine the refractive index of the solutions of different salt concentrations by analyzing the resulting shift in interference fringes. Additionally, the thermal expansion of the metal sample will be measured by monitoring changes in the optical path length as the temperature of the sample varies. The project will highlight the relationship between wave optics and measurable physical parameters and illustrate the advantages of high-precision experimental technique.

Theory

An interferometer is a precision scientific instrument in which a light wave is split into multiple paths and later recombined. The resulting interference depends on the relative phase difference accumulated along the different paths. Because the phase is sensitive to path length and optical properties, interferometers are widely used for precision measurements. A common example is the Michelson interferometer, which illustrates the basic principles of optical interference.

The basic version of the Michelson interferometer consists of a 50:50 beam splitter (BS) and two mirrors placed in separate arms of the interferometer. Assume that the input beam is a linearly polarized monochromatic source of wavelength $\lambda$ and field amplitude $E_0$. The light is incident on the input port of the BS, where it is divided into two beams that propagate along the two arms toward the mirrors. Each beam is reflected by its respective mirror and recombined at BS, producing an interference pattern at the output port.

The output electric field can be described by summing the two contributions from the two arms. If the path lengths are $L_1$ and $L_2$, the output field:

$E_{out}=E_1+E_2=\frac{1}{2}{E}_0{e}^{i2k{L}_1}(1+e^{i2k\mathrm{\Delta }L})$

where $\mathrm{\Delta }L=L_2-L_1$ and $k=2\pi /\lambda$. Field maxima occur whenever

$\frac{4\pi \mathrm{\Delta }L}{\lambda }=2m\pi ,$

and minima when

$\frac{4\pi \mathrm{\Delta }L}{\lambda }=(2m+1)\pi ,$

where $m$ is an integer.

Changes in the optical path length of either arm caused by displacement of a mirror, variation of refractive index in a medium, or thermal expansion will change yhr relative phase difference, leading to a shift of the interference fringes. This sensitivity forms the basis for precision measurements using the Michelson interferometer.

Experiment setup & procedures

A basic Michelson interferometer will be constructed for the experiment.

Procedure:

1. Construct the Michelson interferometer and verify the presence of interference fringes.

OI: optical isolator; M: mirror; BS: beam splitter; PM: powermeter

With Isolater

Oscilloscope Measurement without Sample (Low Pass Filter applied)

Oscilloscope Measurement with Intentional Noise created (Without Sample)

Peak -> Constructive Interference

Trough -> Destructive Interference

Bright and dark levels: use the slowly filtered signal to estimate a local bright level and a local dark level. Quadrature target: V_target = (V_bright + V_dark) / 2. Fringe half-swing: A = (V_bright - V_dark) / 2. It is the local voltage sensitivity scale. The larger A is, the more phase information is available per unit voltage change. Small-signal phase estimate near quadrature: delta_phi is approximately delta_V / A in radians.

2. Refractive index measurement Place the test solution in one arm of the interferometer. Gradually vary the solution concentration by adding either distilled water or a higher-concentration solution to the container, and observe the resulting changes in the interference pattern.

3. Thermal expansion measurement To investigate the thermal expansion of the mirror, slowly heat one of the mirrors using a heating resistor. The temperature will be monitored with a thermocouple. Record how the interference pattern changes as the temperature increases.

References

[To be added]