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A new optical method based on frequency-shift feedback and laser confocal microscopy is presented to noninvasively measure a microstructure inside a sample. Due to the limit of axial resolution caused by poor signal detection ability, conventional laser feedback cannot precisely measure the microstructure. In this Letter, the light scattered by the sample is frequency shifted before feedback to the laser to obtain a magnification. Weak signals that change with the microstructure can be detected. Together with the tomography ability of laser confocal microscopy, the inner microstructure can be measured with high axial resolution.

Displacement sensor based on the polarization mixture and the cavity tuning of the orthogonal polarized He-Ne laser 1.15 μm is presented. The power tuning curves of He-Ne laser are irregular, and it is difficult to measure the change in cavity length. The distortion of the curves is caused by the higher relative excitation compared with the He-Ne laser at 633 nm. In view of its potential for the wider displacement measuring range, a new method of displacement sensing is developed. Experiments show that displacement measuring stability based on the method of the polarization mixture is better than that of the power tuning curves. The displacement sensor achieves the measuring range of 100 mm, resolution of 144 nm, and linearity of 7 × 10-6.

The cavity tuning characteristics of orthogonally polarized dual-frequency He-Ne laser at 1.15 \mu m are presented. Vectorial-extension model based on semi-classical laser theory reveals that cavity tuning characteristics are related to beat frequency, relative excitation, and type of Ne isotope. Distortions of cavity tuning curves become moderate with the increase of beat frequency because of the weakening of the crosssaturation effect. Distortions are enhanced with the increase of relative excitation because of the combined action of the self-saturation and cross-saturation effects. By adopting dual-isotope Ne instead of monoisotoplic Ne, distortions are reduced because of the misalignment between peaks of the self-saturation and net gain coefficients. The theoretical calculations are in good agreement with the corresponding experimental results.

A method is presented for in situ resolution calibration of multiple feedback interferometers (MFIs) using two lasers with different feedback levels simultaneously. The laser with weak optical feedback level generates half-wavelength optical fringes, whereas the laser with strong multiple feedback level generates optical nano-fringes. By using this method, the number of displaced optical nano-fringes can be easily counted, and the resolution of the MFIs can be accurately determined. The integrated MFIs can be used to measure displacements and calibrate other displacement sensors.