A wave plate is a commonly used optical element in optical experiments. The phase retardation of a wave plate is an important value that influences the measurement results significantly. In this report, an interferometric method is presented to measure the phase retardation of a wave plate intuitively. We achieve measuring the phase retardations of a half-wave plate and a quarter-wave plate using a Mach-Zehnder interferometer. When we rotate the half-wave plate’s c-axis form the vertical to the horizontal directions, or vice versa, the phase retardations of the two orthogonally polarized beams are observed to be exchanged between 180° and -180°. In addition, we also evaluate the results in theory by Jones calculus in order to check the experimental results.
Experimental Setup and Principle of Measurement
The schematic diagram and photograph of the experimental setup are shown in Fig. 1. The output light of a He-Ne laser is linearly polarized with an angle of 45° with respect to the horizontal direction, and is of wavelength at 632.8 nm. The laser beam passes through a beam splitter and is divided into two parts. The reflected light passes through the free space and the transmitted light passes through a test wave plate with c-axis in the x or y direction, and a mirror mounted on a piezoelectric transducer (PZT) which is driven by a triangle-wave alternating voltage of low frequency. In this way the phase of the transmitted light could be modulated linearly. Both of the reflected light and the transmitted light are incident on the second beam splitter and interfere with each other. Then the interfering light is divided into two orthogonally polarized parts by a polarizing beam splitter. The plane-polarized transmitted light passes through an objective lens and arrives at a photodetector. The surface-polarized reflected light passes through another objective lens and then is measured by another photodetector. The two sinusoidally oscillating optical intensities are recorded by an oscilloscope, which reveals the phase retardation produced by the test wave plate.
The theoretical analysis of the principle of measurement is derived as follows. At first, the He-Ne laser beam passes through a beam splitter and is divided into two parts. The electric field of the transmitted beam passing through the test wave plate and the mirror mounted on a PZT is expressed by
The βx and βy are the propagation constants of the two orthogonally polarized modes in the principal axes of the wave plate. The β0 is the propagation constant of light in free space. The x̂ and ŷ are the unit vectors along the two birefringent principal axes. The z0 is the total distance of the optical path. The L is the thickness of the test wave plate. The coefficient a is the varying rate of the optical path induced by the linear displacement of the vibrating PZT. The electric field of another beam passing through the free-space optical path is
Finally, the two optical beams recombine in the second beam splitter and interfere with each other component at the reflection or the transmission ports. One of the two interfering beams will be divided into the plane-polarized and the surface-polarized beams by a polarizing beam splitter, with optical intensities
Thus, the phase delay between the two sinusoidally oscillating optical intensities Ix(t) and Iy(t), which is induced by the optical path vibration due to the PZT driving, is given by
As a consequence, the phase delay ϕ between Ix(t) and Iy(t) happens to equal the phase retardation of the two orthogonally polarized fields produced by the birefringent wave plate, which is to be determined. As long as the magnitude of the PZT driving voltage is large enough to make Ix(t) and Iy(t) oscillate by a phase change more than 2π in the time of half a period of the PZT alternating signals, we can easily measure the phase delay between the two sinusoidally oscillating scan traces on the oscilloscope, which represent the signals from the two photodetectors.
Measurement of the phase retardations of various wave plates
Figure 2 shows the measured results of the two sinusoidally oscillating optical intensities on the oscilloscope. The phase retardation of the two orthogonally polarized interfering beams is 0°, when there is no any wave plate placed in the set up. The phase retardation becomes 90° when a quarter-wave plate is inserted, and the phase retardation becomes 180° when a half-wave plate is tested. Hence we have achieved measuring the phase retardations of wave plates using a Mach-Zehnder interferometer in a more intuitive way.
Measurement of the phase retardations of a rotated half-wave plate
Figure 3 shows the theoretical and experimental results of the phase retardation of the two orthogonally polarized beams when we rotate the half-wave plate by an angle of 22.5° successively. We can see that the phase retardations of the two orthogonally polarized beams are exchanged between 180° and -180° when we rotate the half-wave plate’s c-axis form the vertical to the horizontal directions, or vice versa. The experimental results agree quite well with the theoretical analysis performed by the MATHEMATICA software, as shown in the Appendixes.
The experimental results and theoretical analysis show good agreement and prove that using a Mach-Zehnder interferometer is a good technique to measure the phase retardations of wave plates easily and quickly. This system can be applied in an optics teaching laboratory to explore the birefringence characteristics of anisotropic materials in a more intuitive manner.
We acknowledge the financial support from the National Science Council, Taiwan, through project NSC-95-2112-M-415-004.