A promising method for tomography and step height measurement is proposed, which combines the high sensitivity of the frequency-shifted feedback laser and the axial positioning ability of confocal microscopy. By demodulating the feedback-induced intensity modulation signals, the obtained amplitude and phase information are used to respectively determine the coarse and fine measurement of the samples. Imaging the micro devices and biological samples by the demodulated amplitude, this approach is proved to be able to achieve the cross-sectional image in highly scattered mediums. And then the successful height measurement of nano-step on a glass-substrate grating by combination of both amplitude and phase information indicates its axial high resolution (better than 2 nm) in a non-ambiguous range of about ten microns.

We systematically investigated the power spectrum of a Nd:YAG laser with external frequency-shifted feedback and identified three factors dominating the spectrum, namely, the feedback level, the pumping level of the laser diode (LD), and the shifted frequency introduced in the external cavity. For very weak feedback, the laser power spectrum presents two peaks at frequencies of and ω r , which are the shifted frequency and relaxation oscillation frequency, respectively. When the feedback level is increased to an intermediate level, the laser presents a series of nonlinear effects to cause harmonic and parametric oscillation; for strong feedback, only harmonic peaks are observed in the spectrum. The impact of the pumping levels and the amount of frequency shifting are also experimentally investigated. Especially, even at a weak feedback level, when the frequency approaches ω r , strong nonlinear effects still appear in laser dynamics to make the laser power present only harmonic oscillation. A theoretical analysis is provided which agrees well with the experimental results.

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We describe a new optical tomography technology based on feedback of microchip Nd:YAG lasers. In the case of feedback light frequency-shifted, light can be magnified by a fact of 10(6) in the Nd:YAG microchip lasers, which makes it possible to realize optical tomography with a greater depth than current optical tomography. The results of the measuring and imaging of kinds of samples are presented, which demonstrate the feasibility and potential of this approach in the inner structure detection. The system has a lateral resolution of ~1 μm, a vertical resolution of 15 μm and a longitudinal scanning range of over 10mm.