Zhang Wj

Second‐harmonic generation and sum‐frequency mixing of double‐wavelength Nd:YALO3 laser to 413.7‐nm violet coherent radiation in LiIO3 crystal

A new method to get 413.7‐nm violet coherent radiation by using second‐harmonic generation and sum‐frequency mixing of radiation from a 1341.4‐ and 1079.5‐nm dual‐wavelength Nd:YALO3 laser in a LiIO3 crystal is reported in this communication. First, the 670.7‐nm red coherent radiation is obtained by second‐harmonic generation of 1341.4‐nm radiations in a LiIO3 crystal, and then the 670.7‐ and 1079.5‐nm radiations are mixed again in a second LiIO3 crystal to get 413.7‐nm radiation. The phase‐matching angles are obtained for both nonlinear optical processes. The experimental results agree well with calculated results.

Tripling the harmonic generation of a 1341.4 nm Nd:YAP laser in LiIO3 and KTP crystals to get 447.1 nm blue coherent radiation

Abstract 447.1 nm blue coherent radiation was achieved in two kinds of crystal by sum frequency mixing of the 1341.4 nm radiation of a Nd:YAP laser and its second harmonic wave, one in a LiIO3 crystal with type I sum frequency mixing and one in a KTP crystal with type II sum frequency mixing. To our knowledge, it is the first time that 447.1 nm blue coherent radiation is obtained by tripling the harmonic generation of 1341.4 nm Nd:YAP laser.

Laser interception technique with heterodyne self-mixing interferometry

Laser interception technique is a precision measurement technology which takes advantage of laser interferometry to detect weak vibrations caused by acoustic excitation with high precision. It has been greatly valued by national intelligence agencies, with pervasive application to counter-terrorism, crime investigation and other national security and defense affairs in various countries. This paper proposes an interception scheme based on acousto-optic frequency shifting laser self-mixing interferometry. In the case of weak light, the vibration information of the target object could be acquired in real time with high precision. And it is applicable for the development of miniaturized laser interception equipment. We use self-developed acousto-optic frequency shifters to stabilize the heterodyne frequency of the two laser beams at kHz level and heterodyne beat signal as carrier signal for micro-displacement detection. Besides, the self-balanced detection technology is applied to eliminate the common-mode noise of detection system, so as to ensure the signal-to-noise ratio of weak signal detection at nano-watt-level of laser power under long-distance interception conditions. By means of modulation phase recovery technique, combined with cavity frequency modulation method, Fourier transform phase extraction principle and Kalman filter method, the program realizes high-precision recovery of self-mixing interferometric displacement under weak feedback conditions and can effectively suppress low-frequency noise of laser source.

A novel white-light interferometry using low differential-frequency heterodyne system

The optical surface profiler offers fast non-contact and high-precision 3D metrology for complex surface features, which are widely used in the field of precision machining manufacturing. The optical surface profiler traditionally adopts the white light interference (WLI) technique which mainly includes optical interference system and high-precision displacement stage. The accuracy of the displacement table determines the longitudinal resolution of the instrument. In this paper, a novel WLI technique is proposed, i.e. full-field heterodyne WLI, which combines common displacement stage, low differential-frequency heterodyne system and optical interferometry system. The low differential-frequency heterodyne system generates heterodyne signal in the range of laser coherence length. By using the digital phase shift in substitution for the mechanical phase shift, the vertical resolution increases from the sub-nanometer level to the sub-angstrom level. Due to the low difference frequency technique, the common area array detector acquisition is available. A fixed displacement stage position obtains a set of three-dimensional data cubes. Through Fourier-Transform process of the time series data, the initial phase of each pixel at a specific heterodyne frequency is calculated and transformed into surface height information. By using phase unwrapping, the object surface profile can be restored within the laser coherence length. Through digital phase-shifting, phase extraction technology replaces the intensity extraction technology, the moving distance of the displacement can be calibrated with high precision. Thus it can achieve a large range of high-precision contour measurement and reduce the cost of the instrument.

Analysis on optical heterodyne frequency error of full-field heterodyne interferometer

The full-field heterodyne interferometric measurement technology is beginning better applied by employing low frequency heterodyne acousto-optical modulators instead of complex electro-mechanical scanning devices. The optical element surface could be directly acquired by synchronously detecting the received signal phases of each pixel, because standard matrix detector as CCD and CMOS cameras could be used in heterodyne interferometer. Instead of the traditional four-step phase shifting phase calculating, Fourier spectral analysis method is used for phase extracting which brings lower sensitivity to sources of uncertainty and higher measurement accuracy. In this paper, two types of full-field heterodyne interferometer are described whose advantages and disadvantages are also specified. Heterodyne interferometer has to combine two different frequency beams to produce interference, which brings a variety of optical heterodyne frequency errors. Frequency mixing error and beat frequency error are two different kinds of inescapable heterodyne frequency errors. In this paper, the effects of frequency mixing error to surface measurement are derived. The relationship between the phase extraction accuracy and the errors are calculated. :: The tolerance of the extinction ratio of polarization splitting prism and the signal-to-noise ratio of stray light is given. The error of phase extraction by Fourier analysis that caused by beat frequency shifting is derived and calculated. We also propose an improved phase extraction method based on spectrum correction. An amplitude ratio spectrum correction algorithm with using Hanning window is used to correct the heterodyne signal phase extraction. The simulation results show that this method can effectively suppress the degradation of phase extracting caused by beat frequency error and reduce the measurement uncertainty of full-field heterodyne interferometer.

Full-field heterodyne dynamic interferometry based on hertz-level low differential-frequency acousto-optic frequency shifter

High precision measurement of optical elements with long focal length is affected by vibration, airflow and other environmental factors due to the long cavity length, which has been difficulty and hot issue in optical machining and detection. In order to overcome the difficulties of high precision measurement of optical elements with long focal length, the paper proposes a full-field heterodyne interferometric measurement technique that could effectively suppress the environmental interference. In the early related research, a series of Hertz-level high-stability, low-differential frequency acousto-optic frequency shifters were successfully developed, which could be applied to heterodyne interferometry, instead of traditional phase-shifting intererometry. On this basis, a full-field heterodyne interference measurement system is developed, using array detector with conventional frame rate for full-field detection, to solve the problem of different optical paths of reference light and measuring light in dynamic interferometers. It could effectively suppress the vibration, noise, airflow and other factors, and thus significantly improve measurement accuracy and environmental adaptability. In typical environment with vibration and airflow, our measurement system can achieve technical indicators as follows: surface measurement accuracy is better than λ/1000 and repeated measurement accuracy is better than 5λ/10000. Thereby the new full-field heterodyne interferometry could be applied to dynamic measurement of large-diameter optical components and systems quality inspection, system installation correction, on-line measurement and other areas.

Signal demodulation of Fourier telescopy based on spectrum correction

Fourier telescopy is an active unconventional imaging technique. Three or more beams from different spatially separated transmitters are pointed at a distant and faint object. The spatial Fourier spectrum of the object is carried on the reflected temporally modulated signals. The image of the target can be reconstructed from the back signals by demodulation and phase closure algorithm. The conventional demodulation processing is calculating spectrum directly by inverse Fourier transform of the signal. However spectrum estimated by inverse Fourier transform has non-negligible errors caused by frequency shift error of the Acoustic-optical modulator, the noise and the relative motion between beams and the target. An improved demodulation method based on spectrum correction of FT is proposed. The method corrects the amplitude and the phase on the demodulated frequency of the signal by which better reconstructed image can be obtained. In this paper, the effect of the frequency shift error in Fourier telescopy demodulation is investigated. The degradation of the reconstructed image is simulated. We summarize the new demodulation method based on spectrum correction and give the simulated comparison between the conventional demodulation and the developed method. The result confirms the effectiveness of the improved demodulation method.

Effect of transmitting beam position error on the imaging quality of a Fourier telescope

The effect of beam position error on the imaging quality of a Fourier telescope is analyzed in this paper. First, the origin of the transmitting beam position error and the error types are discussed. Second, a numerical analysis is performed. To focus on the transmitting beam position error, other noise sources exclusive of the reconstruction process are neglected. The Strehl ratio is set to be the objective function and the transfer function of the position error is constructed. Based on the numerical model, the features of Strehl ratio reduction caused by position error are deduced. Third, simulations are performed to study the position error effect on the imaging quality. A plot of the Strehl ratio versus the different levels of position errors is obtained and the simulation results validate the numerical model to a certain extent. According to the simulation results, a high value of the transmitting beam position error obviously degrades the imaging quality of the system; thus, it is essential to contain the position error within a relatively low level.