Based on zero-padding and frequency-domain self-filtering, a robust and effective spectrum centroid carrier-removal method for carrier interferogram analysis is proposed. The interferogram is first spatial zero padded to increase the frequency resolution, and then the frequency domain self-filtering is carried out to suppress the noise and enhance the carrier component. Finally, the carrier frequencies are estimated by calculating the spectral centroid of the upside lobe. The simulations and experiments are carried out to testify the feasibility of this method. In addition, some factors, such as the carrier frequency values, the level of random noise, and the window size of the spectral filter, are analyzed and discussed. Compared with existing carrier-removal methods, the proposed method is effective and accurate for suppressing the carrier-removal error.
There are contradictions among speediness, anti-disturbance performance, and steady-state accuracy caused by traditional PID controller in the existing light source systems of thermal frequency stabilizing laser with double longitudinal modes. In this paper, a new kind of fuzzy adaptive PID controller was designed by combining fuzzy PID control technology and expert system to make frequency stabilizing system obtain the optimal performance. The experiments show that the frequency stability of the designed PID controller is similar to the existing PID controller (the magnitude of frequency stability is less than 10-9 in constant temperature and 10-7 in open air). But the preheating time is shortened obviously (from 10 minutes to 5 minutes) and the anti-disturbance capability is improved significantly (the recovery time needed after strong interference is reduced from 1 minute to 10 seconds).
Phase Measuring Deflectometry(PMD) is a non-contact, high dynamic-range and full-field metrology which becomes a serious competitor to interferometry. However, the accuracy of deflectometry metrology is strongly influenced by the level of the calibrations. Our paper presents a calibration-based PMD method to test optical flat surface with a high accuracy. In our method, a pin-hole camera was set next to the LCD screen which is used to project sinusoidal fringes to the test flat. And the test flat was placed parallel to the direction of the LCD screen, which makes the geometry calibration process are simplified. The photogrammetric methods used in computer vision science was used to calibrate the pin-hole camera by using a checker pattern shown on another LCD display at six different orientations, the intrinsic parameters can be obtained by processing the obtained image of checker patterns. Further, by making the last orientation of checker pattern is aligned at the same position as the test optical flat, the algorithms used in this paper can obtain the mapping relationship between the CCD pixels and the subaperture coordinates on the test optical flat. We test a optical flat with a size of 50mm in diameter using our setup and algorithm. Our experimental results of optical flat figure from low to high order aberrations show a good agreement with that from the Fizeau interferometer.
A midinfrared (mid-IR) saturable absorber mirror (SAM) was fabricated by coating aluminum film on Fe2+:ZnSe crystal, based on the vacuum evaporation method. By employing the prepared SAM, we demonstrated a high-power passively Q-switched Er3+-doped ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber laser at 2.8 μm. The highest output power in excess of 1.01 W was obtained with a pulse energy of 11.37 μJ and pulse duration of 0.73 μs, corresponding to a repetition rate of 88.84 kHz. To the best of our knowledge, these values represent the highest output power/pulse energy from a passively Q-switched ZBLAN fiber laser around 2.8 μm. Our results demonstrate that Fe2+:ZnSe SAM is a promising device for high-power/high-energy pulse generation in compact mid-IR fiber lasers.
The nonlinearity error of the dual-frequency laser interferometer with two longitudinal modes and special subsequent
electrical signal processing system was studied. The expressions for the output signal pair were derived based on the
theoretical analysis when the nonlinearity existed. From the above it was given that the nonlinearity makes the
frequency-mixing output signals shift and undulate, and its main factors are the perfection degree of the optical system
adjustment and the polarization splitting degree of the polarization beam splitter. The maximum signal shift amplitude
was close to that observed on oscillograph and it could be used to estimate the perfection degree of the optical system
adjustment and determine the maximum interpolation number in the subsequent signal processing stage.
This paper presents a laser precision ranger based on beat wave interferometry. A frequency-stabilized double
longitudinal modes He-Ne laser with thermoregulation is used as the light source. The two beams of double longitudinal
modes generated in the same resonator of the laser are naturally coaxial. They have a frequency difference of about
790MHz and a beat wavelength of 380mm. Their stability is the same as the laser, which is better than 10-7 in open air.
The node of the beat wave is used as the sampling flag. An adaptive filter and a wavelet transform program are used to
eliminate the noise and to improve the accuracy of node detection. The distance between the node and the measur ed
point is measured with a double frequency interferometer, which is incorporated in the same optical system and has a
resolution of 0.08&mgr;m. Experimental results indicate that the measuring range is 20m and the uncertainty 30&mgr;m /10m.
The frequencies of measuring signal and reference signal output from dual-frequency laser interferometer based on frequency-stabilized double longitudinal modes with thermoregulation are too high to count directly. The signal with right frequency generated by oscillator featuring high frequency stability is used to mix them respectively to get transformed signals with low frequency which can be counted. A fully alternating current system with high measuring speed is gained. This paper presents a schematic diagram of the optical system, a signal processing system of the measuring system and the basic formula for the selection of frequency of the oscillator. The mode interval of developed system is 728MHz, the used counter is 82c54, its top counting frequency is 10 MHz, the frequency of oscillator is determined for 723MHz. Experiment results from a 3m laser length measuring machine with measuring speed better than 1300mm/s, resolution 0.32 μm demonstrated that the system can effectively meet the requirements of high speed measurement and calibration.
The profilometer for on-line and non-contact measurement of the fine optical surfaces is described in this paper, which works on the principle of the differential interferometry with coaxial interference arms. The optical part of that is isolated and easy to install on the machine tools or the measuring machineries ea. This profilometer has excellent resistance to disturbance, especially to mechanical vibration, when the amplitude of vibration is about 300nm. So it still can be used to measure the micro-profile with sub-nanometer resolution. The vertical resolution of the profilometer is better than 0.05 nm rms. and need not add any condition or pretreatment
As a practical monitoring tool for measurements of surface roughness and micro-displacement, an optical implementation of the method based on light scattering for measuring surface roughness and optical triangulation for measuring micro- displacement is proposed. The technique enables evaluation of the surface roughness and micro-displacement of specimen by using only one device. The measuring principle and basic analysis applied in the design are described in detail, after which the validity of the principle is demonstrated by the results of experimental evaluations. Experimental results show that micro-displacement measuring linear range within ± 300 μm can be obtained for measurements of specimen of surface roughness with Ra from 0.005μm to 0.1 μm.