Interference Spectral Imaging Based on Liquid Crystal Relaxation and Its Application in Optical Component Defect Detection

In this paper, we propose a fast interference spectral imaging system based on liquid crystal (LC) relaxation. The path delay of nematic LC during falling relaxation is used for the scanning of the optical path. Hyperspectral data can be obtained by Fourier transforming the data according to the path delay. The system can obtain two-dimensional spatial images of arbitrary wavelengths in the range of 300–1100 nm with a spectral resolution of 262 cm−1. Compared with conventional Fourier transform spectroscopy, the system can easily collect and integrate all valid information within 20 s. Based on the LC, controlling the optical path difference between two orthogonally polarized beams can avoid mechanical movement. Finally, the potential for application in contactless and rapid non-destructive optical component defect inspection is demonstrated.

Spatial High Precision Scanning Control Method Based on Feedforward Closed-Loop Model Reference Adaptive

The time-modulated Fourier transform spectrometer realizes spectrum detection by scanning the optical path of the corner mirror. During the scanning process, the servo system is required to have high-precision and low-speed characteristics. Aiming at the fluctuation of scanning speed caused by spatial micro-vibration during scanning, a closed-loop model reference adaptive control algorithm based on feedforward is studied. The permanent magnet synchronous linear motor is used to drive the angle mirror to move back and forth along the guide rail to achieve large optical path and high-precision scanning with the maximum optical path difference of ± 34cm, the speed stability ≥ 99%.

Research on Dual-Wavelength Absolute Ranging Technology Based on FMCW Laser Interference

FMCW (Frequency-Modulated Continuous Wave) interferometer can achieve high-precision displacement measurement by phase discrimination of the interference signal. The phase needs to be superimposed continuously, so the optical path cannot be interrupted in the measurement process. To solve this problem, a new absolute ranging technology - decimal comparison method is proposed in the manuscript. According to the principle of FMCW interference, two DFB lasers with different central wavelengths are used to measure the same target. The sampled interference signal is processed by digital signal processing to calculate the integer and decimal of the synthetic wavelength period. The optical path difference of the target is calculated by the established mathematical model. The experimental results show that this method not only ensures the measurement accuracy, but also realizes the absolute measurement, and expands the application range of FMCW interferometry.

A stellar ranging scheme based on the second-order correlation measurement

The stellar ranging is the basis for stellar dynamics research and in-depth research on astrophysics. Parallax method is the most widely used and important basic method for stellar ranging. However, it needs to perform high-precision measurement of the parallax angle and the baseline length together. We aim to propose a new stellar ranging scheme based on second-order correlation that does not require a parallax angle measurement. We hope our solution to be as basic as the parallax method. We propose a new stellar ranging scheme by using the offset of second-order correlation curve signals. The optical path difference between the stars and different base stations is determined by the offset of the second-order correlation curve signals. Then the distance of the stars could be determined by the geometric relation. With the distance to stars out to 10kpc away, our astrometric precision can be better compared to Gaia by simulation. We also design a experiment and successfully prove the feasibility of this scheme. This stellar ranging scheme makes it possible to make further and more accurate stellar ranging without using any prior information and angle measurement.

Determination of the refractive index profile and surface topography of optically smooth objects using interference of optical vortices

In this paper, we present the results of the propagational dynamics of vortex beams in the scope of their possible applications for interferometric non-contact robust and precision optical surface profilometry with nanoscale longitudinal resolution. The result of coaxial superposition of the reference plane wave with singly charged vortex beams represents a dynamically changing intensity distribution. The nature of this changes, namely, rotational effects of intensity zeros, allows to determine directly the optical path difference which is introduced by the surfaces and internal structure of test object. We have proposed the experimental setup for examination of reflecting and transmitting objects.

Simultaneous demodulation comparison of fiber-optic Fabry–Perot sensors connected in parallel and series

In this work, the spectra of two fiber-optic Fabry–Perot sensors in parallel and series connection were studied. The spectrum of the parallel structure is a simple superposition of the two sensors’ spectrum, and that of the series structure can be regarded as the interference occurring in two Fabry–Perot sensors successively. The sensors’ optical path difference can be obtained and separated by using the theoretical formula to fit the normalized spectrum of parallel or series structure, which showed that two or more Fabry–Perot sensors can be simultaneously demodulated by the spectrum fitting method.

Comparison of corneal optical quality after SMILE, wavefront-optimized LASIK and topography-guided LASIK for myopia and myopic astigmatism

Purpose: To compare visual outcomes and corneal optical quality after small incision lenticule extraction (SMILE) , wavefront-optimized (WFO) FS-LASIK and topography-guided customized ablation treat­ment (TCAT) FS-LASIK for myopia. Methods: This prospective study included 283 eyes of 283 myopic patients who underwent SMILE or FS-LASIK according to the patient's wishes. FS-LASIK patients were randomly assigned to use WFO or TCAT ablation. There were 102 eyes, 100 eyes and 81eyes in the SMILE group, WFO group and TCAT group, respectively. The combined corneal topographer and tomographer system (Sirius) was used to measure corneal aberrations and optical quality. Visual outcomes and corneal aberrations were compared among the three groups.Results: At postoperative 1 and 6 months, there were no significant differences in uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) among the three groups (P > .05). Postoperative manifest refractive spherical equivalent was similar among the groups (P > .05). There was statisti­cally significant difference in cylinder at 1 month among the three groups, with the highest mean value in TCAT group (P < .05). The corneal optical path difference (OPD), root mean square of corneal astigmatism and strehl ratio (SR) were the most superior in the TCAT group at postoperative 1 and 6 months (P < .05).Conclusion: SMILE, WFO FS-LASIK and TCAT FS-LASIK provided similar visual results. TCAT FS-LASIK could induce fewer corneal OPD and astigmatism, and higher SR than the others. However, a better algorithm for TCAT FS-LASIK is needed to decrease postoperative residual astigmatism.

Lock-in vibration retrieval based on high-speed full-field coherent imaging

The use of high-speed cameras permits to visualize, analyze or study physical phenomena at both their time and spatial scales. Mixing high-speed imaging with coherent imaging allows recording and retrieving the optical path difference and this opens the way for investigating a broad variety of scientific challenges in biology, medicine, material science, physics and mechanics. At high frame rate, simultaneously obtaining suitable performance and level of accuracy is not straightforward. In the field of mechanics, this prevents high-speed imaging to be applied to full-field vibrometry. In this paper, we demonstrate a coherent imaging approach that can yield full-field structural vibration measurements with state-of-the-art performances in case of high spatial and temporal density measurements points of holographic measurement. The method is based on high-speed on-line digital holography and recording a short time sequence. Validation of the proposed approach is carried out by comparison with a scanning laser Doppler vibrometer and by realistic simulations. Several error criteria demonstrate measurement capability of yielding amplitude and phase of structural deformations.

Dual Tunable MZIs Stationary-Wave Integrated Fourier Transform Spectrum Detection

In order to resolve spectral alias due to under sampling in traditional stationary-wave integrated Fourier transform (SWIFT) spectrometers, an all-on-chip waveguide based on dual tunable Mach-Zehnder interferometer (MZI) stationary-wave integrated Fourier transform technology (DTM-SWIFT) is proposed. Several gold nanowires are asymmetrically positioned at two sides of zero optical path difference and scatter the interference fringes information, which can avoid aliasing of spectral signals and help to gain high spectral resolution. A systematic theoretical analysis is carried on in detail, including the optical distribution characteristics based on multi-beam interference, stationary-wave theorem and signal reconstruction method based on the FT technology. The results show that the method can complete a resolution of 6 nm for Gauss spectrum reconstruction using only 6 gold nanowires, and a resolution of 5 cm−1 for Raman spectrum reconstruction using 25 gold nanowires.