High-precision single-pixel 3D calibration method using pseudo-phase matching.

Compressive sensing makes it possible to explore two-dimensional spatial information using a single-point detector. However, the reconstruction of the three-dimensional (3D) morphology using a single-point sensor is largely limited by the calibration. Here we demonstrate a pseudo-single-pixel camera calibration (PSPC) method using pseudo phase matching in stereo, which can perform 3D calibration of low-resolution images with the help of a high-resolution digital micromirror device (DMD) in the system. In this paper, we use a high-resolution CMOS to pre-image the DMD surface and successfully calibrate the spatial position of a single-point detector and the projector with the support of binocular stereo matching. Our system achieved sub-millimeter reconstructions of spheres, steps, and plaster portraits at low compression ratios with a high-speed digital light projector (DLP) and a highly sensitive single-point detector.

High-precision nanosecond detection of a gas absorption spectrum based on optical frequency comb time-frequency mapping.

There is an increasing demand for high-precision gas absorption spectroscopy in basic research and industrial applications, such as gas tracking and leak warning. In this Letter, a novel, to the best of our knowledge, high-precision and real-time gas detection method is proposed. A femtosecond optical frequency comb is used as the light source, and a broadening pulse containing a range of oscillation frequencies is formed after passing through a dispersive element and a Mach-Zehnder interferometer. Four absorption lines of H13C14N gas cells are measured at five different concentrations within a single pulse period. A single scan detection time of only 5 ns is obtained along with a coherence averaging accuracy of 0.0055 nm. High-precision and ultrafast detection of the gas absorption spectrum is accomplished while overcoming complexities related to the acquisition system and light source that are encountered in existing methods.

Arbitrary distance measurement without dead zone by chirped pulse spectrally interferometry using a femtosecond optical frequency comb.

We demonstrate an arbitrary distance measurement method by chirped pulse spectrally interferometry (CPSI) using femtosecond optical frequency comb (OFC). In this paper, the chirped fiber Bragg grating (CFBG) is used to investigate the mapping relationship between displacement and the center frequency of the chirped spectral interferogram. We overcome the direction ambiguity of dispersive interferometry (DPI) ranging and expand the range of distance measurement to 18 cm. Besides, we achieve a full range of dead-zone free ranging by introducing a variable optical delay line (VODL). And through principles simulation and experiment, it is demonstrated that the measurement accuracy is 12 µm in comparison with an incremental He-Ne laser interferometer and the minimum Allen deviation is 52 nm at an average time of 1.76 ms. Similarly, in the experiment with long-distance of ∼30m, the accuracy reaches 20 µm, and 2.51 µm repeatability is achieved under harsh environment.

Nonequal arm surface measurement of femtosecond optical frequency combs using the Savitzky-Golay filtering algorithm.

In precision machining, the surface geometry of a device is one of the important parameters that directly affects the device performance. This paper proposes nonequal arm surface measurement of femtosecond optical frequency combs (OFCs) using the Savitzky-Golay filtering algorithm, which uses the high spatial coherence of OFCs to realize high-precision, nonequal surface measurements. The Savitzky-Golay filtering algorithm and a high-order polynomial envelope fitting algorithm are used to smooth and denoise the interference signals to improve signal quality and measurement accuracy. The experiments are carried out under the condition of nonequal arms, and the results show that the repeatability is 28.6 nm for 20 consecutive measurements on the step surface of a 0.5 mm gauge block. The frosted glass surface is measured 20 times, and the measurement repeatability at the center position is 89.6 nm, which verified the system capability of nonequal arm high-precision measurement under different reflective surfaces.

Improvement of Distance Measurement Based on Dispersive Interferometry Using Femtosecond Optical Frequency Comb.

Since the dispersive interferometry (DPI) based on optical frequency combs (OFCs) was proposed, it has been widely used in absolute distance measurements with long-distance and high precision. However, it has a serious problem for the traditional DPI based on the mode-locked OFC. The error of measurements caused by using the fast Fourier transform (FFT) algorithm to process signals cannot be overcome, which is due to the non-uniform sampling intervals in the frequency domain of spectrometers. Therefore, in this paper, we propose a new mathematical model with a simple form of OFC to simulate and analyze various properties of the OFC and the principle of DPI. Moreover, we carry out an experimental verification, in which we adopt the Lomb-Scargle algorithm to improve the accuracy of measurements of DPI. The results show that the Lomb-Scargle algorithm can effectively reduce the error caused by the resolution, and the error of absolute distance measurement is less than 12 µm in the distance of 70 m based on the mode-locked OFC.

Fast HDR image generation method from a single snapshot image based on frequency division multiplexing technology.

Traditional high dynamic range (HDR) image generation algorithms such as multi-exposure fusion need to capture multiple images for algorithm fusion, which is not only slow but also occupies a lot of storage space, which limits the application of multi-exposure fusion technology. In this paper, the frequency division multiplexing method is used to separate the sub-images with different exposure values from a single snapshot image successfully. The resolution of HDR images generated by this method is almost the same as that of the traditional multiple exposure methods, the storage space is greatly reduced and the imaging speed is improved.

Precision and repeatability improvement in frequency-modulated continuous-wave velocity measurement based on the splitting of beat frequency signals.

The basic principle of frequency-modulated continuous-wave lidars is to measure the velocity of a moving object through the Doppler frequency shift phenomenon. However, the vibration generated by the moving object will cause the spectrum to broaden and the precision and repeatability of speed measurement to decrease. In this paper, we propose a speed measurement method based on H13C14N gas cell absorption peak splitting the sweep signal of a large bandwidth triangular wave modulated frequency laser. This method obtains the speed of a continuously moving target by re-splicing an accurately-split frequency sweep signal, which effectively solves the problem of simultaneous processing of excessive amounts of data when measuring the speed of a continuously moving target. At the same time, the H13C14N gas cell absorbs the spectra of specific wavelengths, which reduces the phase delay of the beat signal corresponding to the up- and down-scanning, thus reducing the signal spectrum broadening caused by frequency deviation, and improving the speed measurement resolution and range effectively. The experimental results show that for speeds of up to 30mm/s, the mean error was less than 23µm/s and the mean standard deviation was less than 61µm/s.

Nonlinear calibration of frequency modulated continuous wave LIDAR based on a microresonator soliton comb.

Traditional frequency modulated continuous wave (FMCW) LIDAR ranging is based on heterodyne detection, calculating unknown distance by extracting the frequency of the interference signal, while the main error source is frequency modulation (FM) nonlinearity. In this paper, a ranging system based on a microresonator soliton comb is demonstrated to correct the nonlinearity by sampling the ranging signals at equal frequency intervals, producing a ranging error lower than 20 µm, while at the range of 2 m. Advantages of fast data acquisition, light computation requirements, and a simple optical path, without long optical fiber, give this method a high practical value in precision manufacturing.

Performance Evaluation and Compensation Method of Trigger Probes in Measurement Based on the Abbé Principle.

Trigger probes are widely used in precision manufacturing industries such as coordinate measuring machines (CMM) and high-end computer numerical control(CNC) machine tools for quality control. Their performance and accuracy often determine the measurement results and the quality of the product manufacturing. However, because there is no accurate measurement of the trigger force in different directions of the probe, and no special measuring device to calibrate the characteristic parameters of the probe in traditional measurement methods, it is impossible to exactly compensate for the measurement error caused by the trigger force of the probe in the measurement process. The accuracy of the measurement of the equipment can be improved by abiding by the Abbé principle. Thus, in order to better evaluate the performance parameters of the probe and realize the accurate compensation for its errors, this paper presents a method which can directly measure the performance parameters of the trigger probe based on the Abbé measurement principle, expounds the measurement principle, the establishment of the mathematical model, and the calibration system, and finishes with an experimental verification and measurement uncertainty analysis. The experimental results show that this method can obtain the exact calibration errors of the performance parameters of the trigger probe intuitively, realize the compensation for the errors of the probe in the measurement process, and effectively improve the measurement accuracy.

Microdeformation of RBCs under oxidative stress measured by digital holographic microscopy and optical tweezers.

This paper utilized digital holographic microscopy and optical tweezers to study microdeformation of red blood cells (RBCs) dynamically under oxidative stress. RBCs attached with microbeads were stretched by dual optical tweezers to generate microdeformation. Morphology of RBCs under manipulation were recorded dynamically and recovered by off-axis digital holographic microscopy method. RBCs treated with H2O2 at different concentrations were measured to investigate the mechanical properties under oxidative stress. Use of optical tweezers and off-axis digital holographic microscopy enhanced measuring accuracy compared with the traditional method. Microdeformation of RBCs is also more consistent with the physiological situation. This proposal is meaningful for clinical applications and basic analysis of Parkinson's disease research.

Nonlinear error correction for FMCW ladar by the amplitude modulation method.

FMCW ladar is a kind of absolute distance measurement technology with high spatial resolution. However, the advantage of high spatial resolution is significantly covered up by the non-linearity of laser frequency sweep. One of the typical approaches for the nonlinearity is resample technology, which has residual phase error from the sample time delay mismatch between the clock signal and the measurement signal. We have proposed and demonstrated a novel amplitude modulation method for correcting the nonlinear error of FMCW technology. The optical structure of the method is comprised of two tandem fiber interferometers. The first interferometer is used to produce a carrier signal and the second one is used to load the range information on the amplitude of the carrier signal. In the end, the experimental result verifies that the nonlinear error can be suppressed effectively, the phase error from the mismatch has been eliminated observably, and the range resolution can be notably improved to 69µm; the stability is 2.9µm and the measurement precision is 4.3µm.

Micron-precision measurement using a combined frequency-modulated continuous wave ladar autofocusing system at 60 meters standoff distance.

In this paper, we propose a novel combined frequency-modulated continuous wave (FMCW) ladar autofocusing system and a fast compensation method for dispersion mismatch, which could allow high-precision ranging to be performed at a long distance. By using the dual-beam laser autofocusing system based on a liquid lens, this system can quickly complete a measurement with high-precision. The experimental results showed that the precision was below 126 µm in a range up to 60m, corresponding to a relative precision of 2.1 × 10 -6, compared to a reference interferometer.

Laser interference-based technique for dynamic measurement of single cell deformation manipulated by optical tweezers.

A laser interference-based method was proposed to measure the deformation response of cell manipulated by optical tweezers. This method was implemented experimentally by integrating a laser illuminating system and optical tweezers with an inverted microscope. Interference fringes generated by the transmitted and reflected lights were recorded by a complementary metal oxide semiconductor camera. From the acquired images, cell height was calculated and cell morphology was constructed. To further validate this method, the morphological analyses of HeLa cells were performed in static state and during detachment process. Subsequently, the dynamic deformation responses of red blood cells were measured during manipulation with optical tweezers. Collectively, this laser interference-based method precludes the requirement of complex optical alignment, allows easy integration with optical tweezers, and enables dynamic measurement of cell deformation response by using a conventional inverted microscope.

Absolute distance measurement by multi-heterodyne interferometry using an electro-optic triple comb.

We experimentally demonstrate a method for absolute distance measurement using a triple-comb-based multi-heterodyne interferometer which has the capacity to simultaneously balance the non-ambiguous range, resolution, update rate, and cost. Three flat-top electro-optic combs generated via cascaded intensity and phase modulators are adopted to form a measurement scheme including rough and fine measurements, and the unknown distance is determined by detecting the phase changes of the consecutive synthetic wavelengths. Experimental results demonstrate an agreement within 750 nm over 80 m distance at an update rate of 167 µs.

Digital micromirror device camera with per-pixel coded exposure for high dynamic range imaging.

In this paper, we overcome the limited dynamic range of the conventional digital camera, and propose a method of realizing high dynamic range imaging (HDRI) from a novel programmable imaging system called a digital micromirror device (DMD) camera. The unique feature of the proposed new method is that the spatial and temporal information of incident light in our DMD camera can be flexibly modulated, and it enables the camera pixels always to have reasonable exposure intensity by DMD pixel-level modulation. More importantly, it allows different light intensity control algorithms used in our programmable imaging system to achieve HDRI. We implement the optical system prototype, analyze the theory of per-pixel coded exposure for HDRI, and put forward an adaptive light intensity control algorithm to effectively modulate the different light intensity to recover high dynamic range images. Via experiments, we demonstrate the effectiveness of our method and implement the HDRI on different objects.

High-precision frequency estimation for frequency modulated continuous wave laser ranging using the multiple signal classification method.

The long fiber frequency sampling method is used to eliminate the nonlinearity of laser tuning in the frequency-modulated continuous-wave laser detection and ranging (FMCW ladar) technique. However, although it has high precision, it is affected by the picket fence effect and spectrum leakage. In this paper, we propose a novel frequency estimation method, multiple signal classification (MUSIC), to be used instead of the conventional fast Fourier transform (FFT)-based algorithm in order to obtain better range precision. The proposed method was verified by experiments. In the experiments, when the distance was up to 3.814 m and chirped bandwidth was equal to 20 nm (2.5 THz), the full width at half-maximum of the range peak, which represented the estimated precision of frequency obtained by MUSIC, was 20 µm, and it was improved by 7 times compared to the FFT-based method. Meanwhile, to evaluate the performance of the proposed method, the frequency estimation according to the Cramer-Rao lower bound (CRLB) was also performed. The experimental results have shown that the mean square error of distance estimation based on the MUSIC algorithm is 0.56 µm, which is much closer to the CRLB of 0.18 µm than the mean square error of the conventional FFT-based method. Furthermore, we demonstrated that the MUSIC estimator has an unparalleled advantage over other estimators in the high-precision ranging fields.

Absolute distance measurement with correction of air refractive index by using two-color dispersive interferometry.

Two-color interferometry is powerful for the correction of the air refractive index especially in the turbulent air over long distance, since the empirical equations could introduce considerable measurement uncertainty if the environmental parameters cannot be measured with sufficient precision. In this paper, we demonstrate a method for absolute distance measurement with high-accuracy correction of air refractive index using two-color dispersive interferometry. The distances corresponding to the two wavelengths can be measured via the spectrograms captured by a CCD camera pair in real time. In the long-term experiment of the correction of air refractive index, the experimental results show a standard deviation of 3.3 × 10-8 for 12-h continuous measurement without the precise knowledge of the environmental conditions, while the variation of the air refractive index is about 2 × 10-6. In the case of absolute distance measurement, the comparison with the fringe counting interferometer shows an agreement within 2.5 µm in 12 m range.

Long distance measurement using optical sampling by cavity tuning.

We experimentally demonstrate a method enabling absolute distance measurement based on optical sampling by cavity tuning. The cross-correlation patterns can be obtained by sweeping the repetition frequency of the frequency comb. The 114 m long fiber delay line, working as the reference arm, is actively stabilized by using a feedback servo loop with 10-10 level stability. The unknown distance can be measured via the instantaneous repetition frequency corresponding to the peak of the fringe packet. We compare the present technique with the reference incremental interferometer, and the experimental results show an agreement within 3 µm over 60 m distance, corresponding to 10-8 level in relative.

Glass thickness and index measurement using optical sampling by cavity tuning.

In this paper, we describe a method based on optical sampling by cavity tuning, which is capable of high-accuracy glass thickness and index measurement. By tuning the repetition frequency of the frequency comb, a series of cross-correlation patterns can be obtained that correspond to the front and rear surfaces of the specimen and the co-operation mirror. Both the geometrical thickness and the optical thickness of the specimen can be measured via the cross-correlation patterns, and consequently, the glass refractive index can be determined at the same time. The comparison with the reference value shows an agreement within 1.3 µm for the thickness measurement, and within 5×10-4 for the refractive index measurement.

Absolute distance measurement by multi-heterodyne interferometry using a frequency comb and a cavity-stabilized tunable laser.

In this paper, we develop a multi-heterodyne system capable of absolute distance measurement using a frequency comb and a tunable diode laser locked to a Fabry-Perot cavity. In a series of subsequent measurements, numerous beat components can be obtained by downconverting the optical frequency into the RF region with multi-heterodyne interferometry. The distances can be measured via the mode phases with a series of synthetic wavelengths. The comparison with the reference interferometer shows an agreement within 1.5 µm for the averages of five measurements and 2.5 µm for the single measurement, which is at the 10-8 relative precision level.