Tan Yidong

Microchip Laser Feedback Interferometer with an Optical Path Multiplier

We present a microchip laser feedback interferometer with an optical path multiplier to enhance the resolution of traditional laser feedback interferometers (LFI). The optical path multiplier has a unique device, i.e. diffusive reflector. As class B microchip lasers have extremely high sensitivity to laser feedback, the diffusive reflector can easily reflect or diffuse back the laser beam without much manual adjustment to the optical system, which ensures the systems easy-adjustment and practical feature. The optical path multiplier is a two-mirror system which enables the laser beam to reflect between the two mirrors by N times. When the target shifts a distance of Δd, the variation of the optical path will be about (AN × Δd). Thus the systems resolution is about 4N times as high as the traditional LFI. Under typical room conditions, the optical path multiplier can effectively enhance the systems resolution by more than 26 times as high as a traditional LFI system and even to the level of 0.1 nm.

Real-Time Liquid Evaporation Rate Measurement Based on a Microchip Laser Feedback Interferometer

We present a novel scheme to realize the direct real-time measurement of liquid evaporation rate and nanometer order liquid level monitoring. It is based on the phase measurement technology of Nd:YAG microchip laser frequency-shifted feedback, which not only has a high resolution and precision but also ultrahigh sensitivity. The evaporation rates of four different transparent liquids and hot water are measured. Experimental results indicate the ease and convenience of measuring and present promising application prospects in non-cooperative target measurement.

Intensity modulation in single-mode microchip Nd:YAG lasers with asymmetric external cavity

Intensity modulation induced by the asymmetric external cavity in single-mode microchip Nd:YAG lasers is presented. Two kinds of experimental results are discussed based on multiple feedback effects. In one case, the intensity modulation curve is a normal sine wave, whose fringe frequency is four times higher than that of a conventional optical feedback system, caused by multiple feedback effects. In the other case, the intensity modulation curve is the overlapping of the above quadruple-frequency signal and conventional optical feedback signal, which is determined by the additional phase difference induced by the asymmetric external cavity. The theoretical analyses are in good agreement with the experimental results. The quadruple-frequency modulation of the laser output intensity can greatly increase the resolution of displacement measurement of an optical feedback system.

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.

A sensitive method of determining optic axis azimuth based on laser feedback

A sensitive method to determine the optic axis azimuth of the birefringence element is presented, which is based on laser feedback. The phase difference between the two intensities in birefringence feedback changes with the angle between the optic axis of the birefringence element and laser original polarization. The phase difference is highly sensitive to the relative position of the optic axis and the laser original polarization. This method is used to highly precisely determine the optic axis azimuth, and is able to distinguish between the fast axis and the slow axis of the birefringence element. Theoretical analysis and experimental results are both demonstrated.

Mode hopping in single-mode microchip Nd:YAG lasers induced by optical feedback

The mode hopping phenomenon induced by optical feedback in single-mode microchip Nd:YAG lasers is presented. With optical feedback, mode hopping strongly depends on two factors: the ratio of external cavity length to intra-cavity length, and initial gains of the two hopping modes. When external cavity length equals an integral multiple of intra-cavity length, there is almost no mode hopping. However, if the external cavity length does not equal an integral multiple of intra-cavity length, mode hopping occurs. The ratio of external cavity length to intra-cavity length determines the position of two-mode hopping. The initial gains of the two hopping modes determine the corresponding peak values and oscillating periods of them in the intensity modulation curves.

Measurement of Refractive Index Ranging from 1.42847 to 2.48272 at 1064 nm Using a Quasi-Common-Path Laser Feedback System*

Wavelength 1064 nm is one of the most widely used laser wavelengths in industries and science. The high-precision measurement of the refractive index of optical materials at 1064 nm is significant for improving the optical design. We study the direct measurement of refractive index at 1064 nm of lasers, including calcium fluoride (CaF2), fused silica and zinc selenide (ZnSe), whose refractive indices cover a large range from 1.42847 to 2.48272. The measurement system is built based on the quasi-common-path Nd:YAG laser feedback interferometry. The thickness can be measured simultaneously with the refractive index. The results demonstrate that the system has absolute uncertainties of ~10−5 and ~10−4 mm in refractive index and thickness measurement, respectively.

Anisotropic optical feedback of single frequency intra-cavity He--Ne laser

This paper presents the anisotropic optical feedback of a single frequency intra-cavity He–Ne laser. A novel phenomenon was discovered that the laser output an elliptical polarized frequency instead of the initial linear polarized one. Two intensities with a phase difference were detected, both of which were modulated in the form of cosine wave and a fringe shift corresponds to a λ/2 movement of the feedback mirror. The phase difference can be continuously modulated by the wave plate in the external cavity. Frequency stabilization was used to stabilize the laser frequency so as to enlarge the measuring range and improve the measurement precision. This anisotropic optical feedback system offers a potential displacement measurement technology with the function of subdivision of λ/2 and in-time direction judgment. The three-mirror Fabry–Perot cavity model is used to present the experimental results. Given the lack of need of lasing adjustment, this full intra-cavity laser can significantly improve the simplicity and stability of the optical feedback system.

Orthogonally Linearly Polarized Dual Frequency Nd:YAG Lasers with Tunable Frequency Difference and Its Application in Precision Angle Measurement

The orthogonally linearly polarized dual frequency Nd:YAG lasers with two quarter wave plates in laser resonator are proposed. The intra-cavity variable birefringence, which is caused by relative rotation of these two wave plates in laser inner cavity, results in the frequency difference of the dual frequency laser also changeable. The theory model based on the Jones matrix is presented, as well as experimental results. The potential application of this phenomenon in precision roll-angle measurement is also discussed.

Intensity Tuning in Single Mode Microchip Nd:YAG Laser with External Cavity

We investigate the characteristics of intensity tuning in a single mode microchip Nd:YAG laser with an external cavity. The undulation of laser intensity in a period of λ/2 change of the internal cavity length is observed. Two different optical feedback cases are performed. One is an external cavity reflector perfectly aligned and the other is an external cavity reflector tilted. However, the fluctuation frequency of laser intensity in a period of λ/2 change of the internal cavity length in these two cases is found to be determined by the ratio of external cavity length to internal cavity length. Meanwhile, for the tilted external cavity, the fluctuation frequency is also related to multiple feedbacks in the tilted external cavity.