製品 | 2021/5/14

4. Range Limitation and its Solution (WPA Series)

Range extension (1) Rotating Polarizer vs. Rotating Phase Plate

Sensor Structure Comparison

PA Series

Integrated Polarizer array + CCD sensor
→ Same principle as Rotating Polarizer method

Rotating Polarizer method

WPA Series

Integrated Phase-Plate array + uniform polarizer + CCD sensor <3-layer unique sensor technology>

→ Same principle as Rotating Phase Plate method

Rotating Phase-Plate method

Due to the difference of principle, measurement range of WPA is twice bigger.

Range extension (1) Rotating Polarizer vs. Rotating Phase Plate

Principle: compute polarization from the brightness profile obtained when rotating filter

 ・When filter = polarizer, left-handed and right-handed states look the same

 ・When filter = phase plate, both left and right-handed states can be measured

      ↓

Measurement range for Rotating Phase-Plate is bigger

Range extension (2) Multiple Wavelength Measurement

Reason of measurement range extension

In addition to the improvement in the measurement principle, as a modification of the polarization sensor, using multiple wavelengths allows another improvement in the measurement range.

With only one wavelength, phase shifts greater than one wavelength
(modulo wavelength) look the same.
→Comparing data at multiple wavelengths allows making the difference.

Measurement with multiple wavelengths

【See figure on the left】

Let’s assume that we want to determine the value on the horizontal axis using one value, α, on the vertical axis. It is easy to convince ourselves that there are several solutions, A, A + λ、A + 2λ, … between which we cannot choose.
However, if we have several values for two different curves (for example the red curve and the green curve on the figure), we can now choose one unique correct solution.

System structure

PA System uses one single wavelength selection filter centered at 530nm, while WPA uses a revolving filter allowing switching between multiple measurement wavelengths.

Actual system: PA Series and WPA Series

Wide range measurement with WPA

Measurement of retardation > 1 wavelength is available

応力

Evolution of measured value with increasing applied force.

印加応力のライン解析結果
Line profile

Measurement data example: difference between PA and WPA

Plastic lens, birefringence ranging from 0 to approx. 800nm

位相差が数100nmのサンプル(樹脂レンズ)の測定例
Data acquired with PA
Data acquired with WPA

・With PA, data is wrapped around 100nm, while WPA allows acquiring correct values.
・Even areas with much greater values on the periphery are captured correctly with WPA.

Summary

・Principle of birefringence measurement was exposed
・The design of its key part, the polarization sensor, its structure and its operation, was explained
・We presented PA and WPA system, with sample data for comparison

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