nanometers laser embodiments include external cavity tunable diode lasers emitting in the uv nir wavelength range 200 1700 nm the laser power is approximately 15 mw however laser embodiments include lasers operating at powers of approximately 5 mw or above the laser controller is a programmable power source compatible with the laser diode and may be controlled by the computer the laser beam is directed into the electro optic eo phase modulator where its polarization state is modulated in an exemplary embodiment phase modulator has a resonant frequency of approximately 20 mhz and is driven sinusoidally by an external function generator the phase modulator will typically operate at or above 1 mhz and may be driven by an external function generator embodiments also include broadband phase modulation including dc bias operation once the laser beam exits from the phase modulator it may have a small u201c residual amplitude modulation u201d induced by the phase modulator the phase modulator design minimizes residual amplitude modulation however any residual amplitude modulation produces a constant or sloping background which may be removed the polarization modulated laser beam is then focused onto the sample surface at an angle of incidence of approximately 45 u00b0 the optical system comprises various optical elements including focusing and collimation lenses and mirrors all optical elements are matched to the laser source wavelength once the laser beam is reflected from the sample surface it has sources of amplitude modulation at the modulation frequency from the difference in reflectivity due to the polarization state change including any polarization induced anisotropy in the semiconductor optical response and any residual amplitude modulation the reflected beam is collected and focused onto the photodiode fig3 shows the polarization state of the laser beam as it passes through the optical system.