Optical material stress measurement using two orthogonally polarized sinusoidally intensity-modulated semiconductor lasers

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Abstract
A novel, compact, robust and highly versatile polarization-modulated electro-optical instrument for measuring material properties, fluid flow parameters, stress, strain and molecular structure of optically anisotropic materials has been developed in this paper. The new instrumentation uses two polarized laser beams. Each beam is linearly polarized with the two polarization states orthogonal to each other. The laser beams are sinusoidally intensity modulated with 180° phase difference by two laser drivers and a signal inverter connected to the output of one of the laser driver circuits. The anti-phase intensity modulation of each orthogonal polarization increases the instrument's sensitivity through the use of heterodyning signal analysis techniques with a single lock-in amplifier (LIA). When the two semiconductor laser beams are optically combined, the result produces a laser beam with a constant optical power level comprised of time-varying power levels in each orthogonal polarization state. The polarization state of the laser light is modulated without the use of a traditional modulator. The instrument photodetector produces a direct-current signal along with a periodic signal at the modulation frequency that is recovered by a LIA tuned to the modulation frequency. By combining these signals in the appropriate relationship, a material's phase retardance or average molecular orientation angle may be measured. The main advantages of this technique over existing methods are lower cost due to the lack of an optical modulator, small size when compared to a photoelastically modulated system and improved sensitivity over continuous-wave laser crossed-polarizer instruments.