The advancement of digital holography in the past 2 decades has enabled precise capturing of three-dimensional (3-D) images of physical objects. This important technology has been widely applied in numerous industrial sectors such as, but not limited to remote sensing, metrology, bio-medical imaging, advertising, and entertainment. Most of the hologram acquisition techniques developed to date are employing digital cameras for recording the hologram, hence imposing rigid restrictions on the size and resolution of the captured 3-D image. This limitation, however, is not found in optical scanning holography (OSH). Based on a scanning mechanism and a single pixel sensor, OSH is capable of capturing digital holograms of both macroscopic and microscopic, as well as fluorescent objects with high precision. Since its invention in the late 70s, numerous research works have been conducted to enhance this technology, optimizing important factors such as acquisition speed, precision, data size, and security. The objective of this review paper is to provide a walkthrough of the state-of-the-art of the OSH technology, from its original principle, to different variants that have been developed over the years with emphasis on their feature extraction capabilities under the incoherent mode of operation. Whenever possible, we shall provide the key formulations of each approach, and experimental outcomes for demonstrating the pros and cons of the method.

Employing a single-pixel digital holographic recording technique called optical scanning holography (OSH), we accomplish the formidable task of sectioning with edge extraction in three-dimensional (3D) optical incoherent imaging. OSH is a special variant of generalized two-pupil heterodyning image processing, where one of the pupils used is a delta function with the other being a uniform function. In this study, we investigate the use of an annular pupil and a random-phase pupil for edge extraction during sectioning of a 3-D object. Novel simulation results indicate excellent edge extraction of a multi-section object with good sectioning capability, i.e., with each focused edge-extracted section out-of-focused haze has been eliminated.

An important factor restricting the development of fiber loop ring-down spectroscopy (FLRDS) is that real-time continuous monitoring of the inherent system loss is inconvenient and time-consuming. However, the inherent system loss will vary across environments. To ensure measurement accuracy, real-time monitoring is essential. This paper proposes a method for real-time monitoring of the inherent system loss to improve the accuracy of FLRDS-based gas sensors. A precise dual-wavelength tuning of the laser diode can be achieved by changing the injection current, in which the emission wavelength outside the absorption region can be used to acquire the inherent system loss value fast and conveniently. The advantage of this method has been verified by testing the acetylene concentration of a calibration gas with 1% acetylene. The average measurement error can be decreased from 1.393% to 0.037% through detection and correction the inherent system loss.

This article proposes a frequency-division multiplexing(FDM) laser dispersion spectroscopy (LDS) method to implement high-precision water vapor temperature measurement in a dynamic combustion environment. Two laser beams at 7185.6 and 7444.4 cm1 pass through the flame simultaneously along a common path through the intensity-modulated FDM approach. A pair of lock-in amplifiers (LIAs) are used to demodulate the dispersion phase signals of the two laser beams. Then, the two-line thermometry is used to obtain the water vapor temperature in the flame. Static and dynamic experiments were carried out on a McKenna burner and a kerosene-fueled multiswirl staged combustor, respectively. The in situ and time-resolved measurement results show that the method is more immune to laser power fluctuation than direct laser absorption spectroscopy (LAS) and the measurement results obtained by using the proposed method are more precise than those by using the time division multiplexing (TDM) method. Therefore, the proposed method is more suitable for dynamic combustion monitoring.

In many industrial applications, temperature and mechanical vibration are closely coupled but measured separately. A novel, to the best of our knowledge, method for simultaneous telemetry of temperature and vibration parameters was proposed in this work from laser dispersion spectroscopy profiles at two different central wavelengths. The temperature was extracted from the peak-to-peak ratio of these two absorption spectra. The vibration amplitude as well as its frequency were derived from the time-varying baselines of the two spectra. A telemetry sensor was designed and evaluated on a thermal vibration coupled experiment platform. The extracted temperatures agree well with the readings of a reference thermocouple, and the signal-to-noise ratio is at least 18 dB higher than those by classical direct laser absorption spectroscopy (DLAS). The extracted vibration frequencies are the same as the outputs of a commercial laser Doppler vibrometer (LDV), and the sensitivity of the extracted vibration amplitudes is 3.64 micrometers, in terms of the Allan variance.

A harsh environment-oriented distributed multipoint fiber optic gas sensor system realized by automatic gain control (AGC) technology is proposed. To improve the photoelectric signal reliability, the electronic variable gain can be modified in real time by an AGC closed-loop feedback structure to compensate for optical transmission loss which is caused by the fiber bend loss or other reasons. The deviation of the system based on AGC structure is below 4.02% when photoelectric signal decays due to fiber bending loss for bending radius of 5 mm, which is 20 times lower than the ordinary differential system. In addition, the AGC circuit with the same electric parameters can keep the baseline intensity of signals in different channels of the distributed multipoint sensor system at the same level. This avoids repetitive calibrations and streamlines the installation process.