High-resolution force sensing by a diode-pumped Nd:YAG laser

Microchip Nd:YAG dual-frequency laser interferometer for displacement measurement

The dual-frequency laser interferometer is an indispensable instrument to measure length, position, deformation and other parameters with high precision and long measurement distance in the advanced manufacturing industry and scientific research. In general, the light source of interferometer is the dual-frequency He-Ne laser. The disadvantages of He-Ne laser include generally large size, more heat radiation to the environment, and limited service life, which limits its application. In this paper, we study a microchip Nd:YAG dual-frequency laser interferometer with a 17.4 MHz frequency difference, which is formed by the stress-induced birefringence in the microchip itself. The down-conversion mixer is designed to decrease the beat frequency to about 5 MHz in heterodyne signal preprocessing modular to meet the bandwidth of phase meter. The experimental results show that the microchip Nd:YAG dual-frequency laser interferometer has a displacement resolution of 10 nm and a measuring range of 500 mm. Due to the advantages of the microchip dual-frequency laser, such as small size (40×40×35 mm), good portability, less power consumption and almost infinite service life, the microchip Nd:YAG dual-frequency laser interferometer has broad application prospects.

Monolithic dual-polarization self-mode-locked Nd:YAG 946-nm lasers: controlling beat frequency and observation of temporal chaos

The self-mode-locked (SML) operation at 946 nm can be achieved with a monolithic Nd:YAG crystal when the pump power is above the threshold of the multiple-longitudinal-mode generation. The SML output is further found to include two orthogonal polarization components with a beat frequency coming from the birefringence effect in the laser crystal. The beat frequency can be widely adjusted in the range of 5-220 MHz by controlling the cooling temperature. The present experiment also confirms the theoretical prediction that the two-mode operation generally exhibits the chaotic dynamics when the frequency difference is sufficiently close to the relaxation frequency.

Generation of orthogonally polarized self-mode-locked Nd:YAG lasers with tunable beat frequencies from the thermally induced birefringence

The simultaneous self-mode-locking of two orthogonally polarized states in a Nd:YAG laser is demonstrated by using a short linear cavity. A total output power of 3.8 W can be obtained at an incident pump power of 8.2 W. The beat frequency Δfc between two orthogonally polarized mode-locked components is observed and measured precisely. It is found that the beat frequency increases linearly with an increase in the absorbed pump power. The origin of the beat frequency can be utterly manifested by considering the thermally induced birefringence in the Nd:YAG crystal. The present result offers a promising approach to generate orthogonally polarized mode-locked lasers with tunable beat frequency.

Effect of pump polarization direction on power characteristics in monolithic microchip Nd:YAG dual-frequency laser

The pump polarization direction can greatly influence the characteristics of the laser diode end-pumped monolithic microchip Nd:YAG dual-frequency laser. We experimentally observe the lasing thresholds and the optical powers of two splitting modes versus the pump polarization direction. The effect of the pump-induced gain anisotropy on the mode oscillation sequence is analyzed. And the effect on the intensities of these modes is also proved with a rate equation model. This study contributes to the improvement of the stability and the reliability of the Nd:YAG dual-frequency laser.

Laser frequency splitting method for high-resolution determination of relative stress-optic coefficient and internal stresses in Nd:YAG crystals

We propose a laser frequency splitting method to determine the relative stress-optic coefficient and internal stresses in Nd:YAG crystals with high resolution. In this method a mirrored Nd:YAG crystal in a circular disk shape is made into an all-internal cavity laser. Once diametral compression force is applied, due to intracavity photoelastic effect, the single laser frequency is split in two. Through measuring the beat frequency of the split frequencies, the relative stress-optic coefficient and internal stresses in the Nd:YAG crystal can be determined according to the proportional relationship. In our experiment the measured value of the stress-optic coefficient, C(111)=1.27x10(-12) m(2)/N, is close to the theoretical value; the principal stress difference at the center of the Nd:YAG crystal was also determined. The optical path retardation resolution is approximately 1.5x10(-8) lambda, and correspondingly the principal stress difference resolution is approximately 5 Pa. The resolution is approximately 5-8 orders of magnitude higher than those of conventional methods, therefore even very small residual stresses can be easily determined with this method. Although this method is a kind of point-by-point method instead of a full-field method, it is a promising novel photomechanics method with attractive advantages such as extremely high resolution.

Performance analysis of intracavity birefringence sensing

The performance of intracavity birefringence sensing by use of a standing-wave laser is theoretically analyzed when the cavity involves internal reflection. On the three-mirror compound cavity model, the condition for converting an optical path length into a laser frequency or a retardation into an optical beat frequency with good linearity and little uncertainty is derived as a function of the cavity parameters and is numerically analyzed.

Temperature characteristics and stabilization of orthogonal polarization two-frequency Nd3+:YAG microchip lasers

The temperature dependence of the beat frequency and polarization azimuth of fiber-guiding laser diode pumped Nd(3+):YAG microchip lasers emitting orthogonal linear polarization at two frequencies is investigated, and we found a resemblance to the features that result from the application of mechanical force. The experimental observations are well described by the cavity model, which indicates that temperature change induces weak phase anisotropy in addition to the original phase anisotropy with general mutually inclined axes of anisotropy. With a servo control to keep the beat frequency constant, the beat and central laser frequencies are simultaneously stabilized to within +/-0.1 MHz and +/-0.35 GHz, respectively.