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.
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.
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.
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.