Jian Bai

Design of panoramic stereo imaging with single optical system

A novel panoramic stereo imaging system with double panoramic annular lenses (PAL) is introduced in this paper. The proposed system consists of two coaxial PAL units and images on one sensor. The imaging rings are concentric. The object position can be calculated by triangulation. The novelty of this system is that the central blind zone of one PAL unit is partly used by the other as an image zone. The stereo field of view is 60°~105° × 360°. The depth extracting resolution is about 500 mm at 1line/cm. The F# of the system is about 3, facilitating the system to be applied in low illumination environment. A design is presented and the depth extracting precision is analyzed.

Nanometer-scale displacement sensor based on phase-sensitive diffraction grating.

In this paper, a nanometer-scale displacement sensor based on a phase-sensitive diffraction grating with interferometeric detection is described and experimentally demonstrated. The proposed displacement sensor consists of a coherent light source, a microstepping motor controller, an integrated grating, a mirror, and a differential circuit. Experimental results show that the displacement sensor has a sensitivity of about 6 mV/nm and a resolution of less than 1 nm. This displacement measurement is an attractive technology with high sensitivity, broad dynamic range, good reliability, and immunity to electromagnetic interference.

Expanding the Detection of Traversable Area with RealSense for the Visually Impaired

The introduction of RGB-Depth (RGB-D) sensors into the visually impaired people (VIP)-assisting area has stirred great interest of many researchers. However, the detection range of RGB-D sensors is limited by narrow depth field angle and sparse depth map in the distance, which hampers broader and longer traversability awareness. This paper proposes an effective approach to expand the detection of traversable area based on a RGB-D sensor, the Intel RealSense R200, which is compatible with both indoor and outdoor environments. The depth image of RealSense is enhanced with IR image large-scale matching and RGB image-guided filtering. Traversable area is obtained with RANdom SAmple Consensus (RANSAC) segmentation and surface normal vector estimation, preliminarily. A seeded growing region algorithm, combining the depth image and RGB image, enlarges the preliminary traversable area greatly. This is critical not only for avoiding close obstacles, but also for allowing superior path planning on navigation. The proposed approach has been tested on a score of indoor and outdoor scenarios. Moreover, the approach has been integrated into an assistance system, which consists of a wearable prototype and an audio interface. Furthermore, the presented approach has been proved to be useful and reliable by a field test with eight visually impaired volunteers.

Quadriwave lateral shearing interferometer based on a randomly encoded hybrid grating.

A compact quadriwave lateral shearing interferometer (QWLSI) with strong adaptability and high precision is proposed based on a novel randomly encoded hybrid grating (REHG). By performing the inverse Fourier transform of the desired ±1 Fraunhofer diffraction orders, the amplitude and phase distributions of the ideally calculated quadriwave grating can be obtained. Then a phase chessboard is introduced to generate the same phase distribution, while the amplitude distribution can be achieved using the randomly encoding method by quantizing the radiant flux on the ideal quadriwave grating. As the Faunhofer diffraction of the REHG only contains the ±1 orders, no order selection mask is ever needed for the REHG-LSI. The simulations and the experiments show that the REHG-LSI exhibits strong adaptability, nice repeatability, and high precision.

Practical phase unwrapping of interferometric fringes based on unscented Kalman filter technique

A phase unwrapping algorithm for interferometric fringes based on the unscented Kalman filter (UKF) technique is proposed. The algorithm can bring about accurate phase unwrapping and good noise suppression simultaneously by incorporating the true phase and its derivative in the state vector estimation through the UKF process. Simulations indicate that the proposed algorithm has better accuracy than some widely employed phase unwrapping approaches in the same noise condition. Also, the time consumption of the algorithm is reasonably acceptable. Applications of the algorithm in our different optical interferometer systems are provided to demonstrate its practicability with good performance. We hope this algorithm can be a practical approach that can help to reduce the systematic errors significantly induced by phase unwrapping process for interferometric measurements such as wavefront distortion testing, surface figure testing of optics, etc.

Field-widened Michelson interferometer for spectral discrimination in high-spectral-resolution lidar: theoretical framework

A field-widened Michelson interferometer (FWMI) is developed to act as the spectral discriminator in high-spectral-resolution lidar (HSRL). This realization is motivated by the wide-angle Michelson interferometer (WAMI) which has been used broadly in the atmospheric wind and temperature detection. This paper describes an independent theoretical framework about the application of the FWMI in HSRL for the first time. In the framework, the operation principles and application requirements of the FWMI are discussed in comparison with that of the WAMI. Theoretical foundations for designing this type of interferometer are introduced based on these comparisons. Moreover, a general performance estimation model for the FWMI is established, which can provide common guidelines for the performance budget and evaluation of the FWMI in the both design and operation stages. Examples incorporating many practical imperfections or conditions that may degrade the performance of the FWMI are given to illustrate the implementation of the modeling. This theoretical framework presents a complete and powerful tool for solving most of theoretical or engineering problems encountered in the FWMI application, including the designing, parameter calibration, prior performance budget, posterior performance estimation, and so on. It will be a valuable contribution to the lidar community to develop a new generation of HSRLs based on the FWMI spectroscopic filter.

Design of a panoramic annular lens with a long focal length

A new method to improve the design of the panoramic annular lens (PAL) optical system with long focus is introduced. Cemented lenses are used in a PAL block to improve the design freedom. A multilayer diffractive optical element (MDOE) is used in relay optics to simplify the structure of the system and to increase diffractive efficiency of the design spectral range. The diffractive efficiency of MDOE in a wide spectral range is investigated theoretically. A seven piece PAL system with a total effective focal length of 10.8 mm is realized, and the diffractive efficiency of the whole design wavelength is above 99.3%. A PAL system with all spherical surfaces is described as a comparison.

Long focal-length measurement using divergent beam and two gratings of different periods

A new accurate method for long focal-length measurement based on Talbot interferometry is proposed. A divergent beam and two Ronchi gratings of different periods are employed, as the alternative of the collimated beam and two identical gratings, to achieve higher measurement accuracy. Moreover, with divergent beam, lenses of large aperture can be easily measured without scanning, which is required when it comes to traditional collimated beam. Numerical analysis and experiments were carried out. The results demonstrate the proposed method features remarkably high accuracy and repeatability.

Compact optical correlator based on one phase-only spatial light modulator

An optical correlator that utilizes one phase-only spatial light modulator (SLM) combined with a mirror is proposed and demonstrated. This system is compressed by displaying the input and filter pattern on different parts of the same SLM. The background noise is separated from the correlation signal by superimposing a high-frequency carrier with the filter pattern except in the zero-frequency regions, which will improve the signal-to-noise ratio in pattern recognition. Our architecture is compact and the cost is relatively low by utilizing only one SLM.

Common-path and compact wavefront diagnosis system based on cross grating lateral shearing interferometer

A common-path and compact wavefront diagnosis system for both continuous and transient wavefronts measurement is proposed based on cross grating lateral shearing interferometer (CGLSI). Derived from the basic CGLSI configuration, this system employs an aplanatic lens to convert the wavefront under test into a convergent beam, which makes it possible for CGLSI to test the wavefront of collimated beams. A geometrical optics model for grating pitch determination and a Fresnel diffraction model for order selection mask design are presented. Then a detailed analysis about the influence of the grating pitch, the distance from the cross grating to the order selection mask and the numerical aperture of the aplanatic lens on the system error is made, and a calibration method is proposed to eliminate the system error. In addition, the differential Zernike polynomials fitting method is introduced for wavefront retrieval. Before our experiment, we have designed several grating pitches and their corresponding order selection mask parameters. In the final comparative experiment with ZYGO interferometer, the wavefront diagnosis system exhibits both high precision and repeatability.