In this paper, a novel profiling method, Laser Confocal Feedback Profilometery (LCFP), combining quasi-common-path
heterodyne phase detection with laser confocal feedback technology, is proposed. A microchip Nd:YAG laser emits
1064nm laser, which passes through a pinhole and frequency shifter, and is focused onto the sample surface. The
reflected light is coupled back to the microchip laser cavity and forms the frequency shifted feedback light, causing the
laser intensity modulation. When the sample is scanned laterally, its surface height variation changes both the phase and
strength of the feedback light. LCFP then extracts both the amplitude and phase information out of the laser intensity
modulation to determine the integral and fractional number of half laser wavelengths contained in the height variation of
two points on the sample surface. LCFP can thus overcome the half-laser-wavelength limit of phase measurement in the
axial direction. The high sensitivity of microchip laser to feedback light makes LCFP able to measure samples with very
low reflectivity. The LCFP experimental setup is built, and it has successfully measured the height of the stages on a
glass-substrate grating. The current performances of LCFP are as followed: the axial resolution is better than 2nm, the
axial range about 5μm, and the detectable reflectivity as low as 10-9. Due to its direct traceability to laser wavelength,
LCFP can potentially be used as the metrology standard of small-scale features.
The principle for utilizing a full-external-cavity He-Ne laser as a displacement sensor is presented. Inserting a quartz plate into the cavity, we split one laser frequency into two orthogonally polarized beams (o light and e light). When one cavity mirror is moved along the laser axis, we obtain power-tuning curves for o light and e light, in which equal-intensity points appear periodically, one period corresponding to /2 displacement. Moreover, four different polarization states in the laser output appear periodically. Attaching the moving mirror to the measured object, we realize displacement measurement with resolution of /2 through counting the number of equal-intensity points. We measure displacements less than /2 by means of a PZT mounted to the other cavity mirror. An increasing voltage is applied to the PZT to displace the mirror. Once the two beams reach the neighboring equal-intensity point, we note down the voltage variation of the PZT to get the true displacement. We discriminate displacement directions by sensing the order of appearance of the four polarization states. This system can be expected to afford a measurement range of 35 mm and a resolution of 10 nm, and has the capability of self-calibration. The potential error factors are also discussed.
The influence of optical feedback on the longitudinal mode stability of microchip Nd:YAG lasers is demonstrated. Under optical feedback, the longitudinal mode output of microchip lasers relies strongly on the external cavity length, and mode suppression can be achieved. The intensity modulation waveform varies at different external cavity lengths. At higher feedback levels, the laser flips between orthogonal polarizations. Three important factors determining the influence of optical feedback are summarized, which are the external cavity length, longitudinal mode competition, and the external reflectivity.
In this paper, we demonstrate the evolution of the self-mixing phenomena of dual-polarization microchip Nd:YAG lasers as we change the feedback strength and the frequency difference. At high feedback level the microchip laser's polarization flips orthogonally as the external mirror moves. Especially, we observed that the intensity modulation amplitude varies periodically with the frequency difference of orthogonal polarizations, and that with different types of targets the periodic change always exists. A qualitative model is put forward and is in good agreement with the experimental results. The results can be applied to the self-mixing sensitivity enhancement, and also presents a novel method of absolute distance measurement.
We demonstrate a method of displacement measurement based on polarization hopping of laser with optical feedback. The measurement system is composed of a half-intracavity He-Ne laser, a quarter wave plate and an external feedback mirror. When the conditions of the second polarization flipping are satisfied, the frequency of intensity modulation can be doubled, and one polarization switching corresponds to displacement of external mirror. Meanwhile, the intensity transfer between two polarization states will come out, i.e., an increase of one polarization light intensity always accompanies a decrease of another polarization light intensity. One period of intensity transfer can be divided into four domains: e-light, e and o-light, o-light, and no light, and each domain corresponds to change of external cavity length. According to appearing sequence of the four domains, the movement direction of external mirror can be distinguished. Our method can improve the resolution of displacement measurement 8 times that of conventional optical feedback, and reaches 40nm for a laser wavelength of 632.8nm.