The noncooperative and high sensitivity optical displacement measurement technology is very relevant to the study and the determination of high-precision thermal expansion coefficients (TECs) of materials. This paper describes a measurement technology based on Nd:YVO<sub>4</sub> laser feedback systems, which can realize fully non-
contact measurement of many kinds of materials with surface reflectivity greater than 10<sup>-5</sup>. A muffle furnace is
designed with two coaxial holes opened on the opposite furnace walls. This length determination technique is
based on the frequency-shifted optical feedback effects and the heterodyne phase measurement technique. For
validation, the samples are determined in the temperature range 298 to 748K, confirming high sensitivity non-
contact measurement of the materials and demonstrating TEC-measurement capabilities with uncertainties in the range of 10<sup>-7</sup> or less.
A method is presented in this paper for resolution calibration of the laser feedback displacement sensor based on Fabry-Perot (F-P) high-order feedback cavity using a conventional feedback system to measure the same displacement with it simultaneously. By calibrating the ratio of the intensity modulated curves gotten by this two different systems, the accurate optical resolution of the integrated system can be obtained. Without using any other subdivision method, the optical resolution can be about 11nm, which is traceable to light wavelength. By adding 20 times electric subdivision, the final resolution of this system is about 0.55nm. The integrated system can fit the requirement of the industrial application, and can also be used for nanometrology.
We present the experimental observation of a phenomenon in which the reflection loss, induced by an uncoated glass sample placed in a laser cavity, significantly reduces at a series of incident angles. The light amplification condition for a laser to work can be satisfied by means of this phenomenon, though the gain is less than the loss when the sample is placed in the normal incidence. The angle ranges for the laser can keep working are intermittent, and both of the lasing range and no-lasing range become narrow with the incident angle increasing. Six kinds of optical glass samples and one birefringent sample have been tested, and three types of lasers are used to confirm this phenomenon. This phenomenon may make the anti-reflection film be not necessary for a transparent sample in some techniques or instruments based on the characteristics of laser resonant cavity. Principle and properties of this phenomenon are analyzed, and the theoretical analyses are coincident to the experimental observations. Three conditions for this phenomenon to occur, as well as the potential applications, are given finally.
A simple and effective displacement sensor based on external anisotropic feedback in Nd:YAG lasers has been presented
and demonstrated. When the system operates in anisotropic feedback induced by placing a birefringence element with
phase difference about 45 degree(such as a wave plate) in the external cavity, both the laser intensities in two orthogonal
directions are sinusoidal-modulated by the external reflector with a period of half wavelength displacement, but with a
phase difference about 90 degree between them. When threshold intensity is introduced, a period of intensity fringe can
be divided into four equal zones. Each zone corresponds to &lgr;/8 displacement of the external feedback reflector.
According to the appearing sequence of the four states, the movement direction of external reflector can be discriminated.
Thus, a novel displacement sensor with a resolution of up to 133 nm, as well as a function of direction discrimination, is
believed to be achieved. The chief advantages of this sensor are that it is compact, small size, flexible, low cost, and
robust. Most importantly, this sensor has a great potential to be improved in resolution by electric subdivision methods
applied in the grating encoders. Experimental results have shown that the uncertainty (3&sgr;) of displacement measurement
is 0.2&mgr;m in a 7mm range, and the linearity is better than 2.5710<sup>-5</sup>.
The characteristic of laser intensity modulation in microchip Nd:YAG lasers with anisotropic feedback is presented, on
which a force measurement scheme based is demonstrated. The measurement system is composed of a microchip
Nd:YAG laser, a birefringence element (BFE), and an external feedback mirror. Due to the birefringence effect of BFE,
the external cavity modulates the laser intensities in two orthogonal directions with a phase difference (PF), which is
twice as large as that of the BFE. If a photoelastic element with force loaded on is served as BFE, the PF between two
in-quadrature laser intensities is proportional to the force loaded on the photoelastic element. Thus, the force can be
easily and conveniently obtained from the PF between two in-quadrature laser intensities. A theoretical model is put
forward and is in good agreement with the experimental results. Moreover, the results here can also be applied to
displacement measurement. Our researches broaden the optical feedback in application for precision measurement.