Fast measurement of the laser beam quality factor based on phase retrieval with a liquid lens.

In this paper, a new method for measuring the beam quality (M2) of lasers based on phase retrieval with a liquid lens is proposed. With intensity profiles obtained under different focal lengths in a certain position, a variable-focus iterative retrieval algorithm is established for the reconstruction of the complex amplitude. Then M2 can be calculated with the angular spectrum theory. Feasibility of the proposed method is demonstrated with single- and multimode lasers through both simulations and experiments. Compared with the traditional liquid lens method, the M2 of lasers can be measured faster with the proposed method.

Heat suppression of the fiber coating on a cladding light stripper in high-power fiber laser.

We present a theoretical model for the thermal effect of the fiber coating on a high-power cladding light stripper, which is fabricated by chemical etching. For the input and output of the fiber coating, a novel segmented corrosion method and increasing attenuation method are proposed for heat suppression, respectively. The relationship between the attenuation and temperature rise of the fiber coating at the output is experimentally demonstrated. The temperature distribution of the fiber coating at the input as well as the return light power caused by scattering are measured for the etched fiber with different surface roughness values. The results suggest that the rise in temperature is primarily caused by the scattering light propagating into the coating. Finally, an attenuation of 27 dB is achieved. At a room temperature of 23°C and input pump power of 438 W, the highest temperature of the input fiber coating decreases from 39.5°C to 27.9°C by segmented corrosion, and the temperature rise of the output fiber coating is close to 0.

Suppressing the influence of charge-coupled device vertical blooming on the measurement of laser beam quality factor (M2) of a near-infrared laser.

In this paper, a new method, which is based on reconstructing the original intensity distribution of a laser with images captured by a charge-coupled device (CCD) in two orthogonal directions, is proposed for suppressing the influence of CCD vertical blooming on the measurement of the laser beam quality factor (M2). A simplified theoretical model for the distribution of CCD blooming is also proposed. With the proposed method and model, the influence of CCD vertical blooming on the measurement of M2 is simulated. The experimental results demonstrate that the new method can be an effective means to measure the M2 of a near-infrared laser with a silicon CCD camera. The proposed method can be applied to a beam quality analyzer in order to suppress the influence of blooming on the measurement of M2.

Determination of the laser beam quality factor (M2) by stitching quadriwave lateral shearing interferograms with different exposures.

A complete complex amplitude reconstruction method for the determination of the laser beam quality factor M2 based on the multiple exposure of a quadriwave lateral shearing interferometer (QWLSI) is presented. The theoretical analysis and simulation of the influence of the information in the small signal area on the calculation of the M2 factor is provided. The experimental results demonstrate that the new method can be an accurate means to measure the M2 factor. The proposed method can avoid the influence of phase inaccuracy in the small signal area of the interferogram, during the measurement of the M2 factor.

Radiation Temperature Measurement Technology Based on the Basis of Spectral Emissivity Function.

In recent years, with the rapid development of the national defense, industry, technology and other fields, whether it is for the power transmission systems or for the steel smelting and new high-tech industry, temperature measurement has been of important significance. Especially in the high temperature and accompanied by the demand for transient (less than lus)temperature measurement occasion, multi spectral radiation temperature measurement method has been widely used. Multi spectral thermometry is selected by the measured target multiple wavelengths radiation information, the mathematical model of emissivity and wavelength is supposed, finally the radiation temperature is obtained. At present, when the method is used to measure the temperature, the spectral emissivity is fixed with the assumption of the mathematical model, and the fixed model is unable to adapt to the target under different temperature conditions. Similarly, at different temperatures, how to calculate the final emissivity and radiation temperature has been no universal method. Based on the Planck's law of black body radiation, this paper proposes a new idea that is based on the form invariance of the spectral emissivity function under different temperatures. According to the method, the emissivity model adapte to the dynamic change of the object according to the object under different temperature conditions. At the same time, it also puts forward a general method to calculate the final emissivity and radiation temperature. Through a lot of simulations and experiments, it is proved that the method proposed in this paper is more simple and practical than the existing spectral emissivity solution, which can effectively improve the accuracy of the calculation of spectral emissivity，so as to improve the accuracy of the radiation temperature measurement. At the same time, the method proposed in this paper has the characteristics of good practicability and wide application.

Absolute flatness testing of skip-flat interferometry by matrix analysis in polar coordinates.

A new method utilizing matrix analysis in polar coordinates has been presented for absolute testing of skip-flat interferometry. The retrieval of the absolute profile mainly includes three steps: (1) transform the wavefront maps of the two cavity measurements into data in polar coordinates; (2) retrieve the profile of the reflective flat in polar coordinates by matrix analysis; and (3) transform the profile of the reflective flat back into data in Cartesian coordinates and retrieve the profile of the sample. Simulation of synthetic surface data has been provided, showing the capability of the approach to achieve an accuracy of the order of 0.01 nm RMS. The absolute profile can be retrieved by a set of closed mathematical formulas without polynomial fitting of wavefront maps or the iterative evaluation of an error function, making the new method more efficient for absolute testing.

[A Research on Filed-of-View (FOV) Widening and Thermal-Phase-Drift (TPD) Compensating Technology Applied in a Polarized Interference Imaging Spectrometer (PIIS)].

The Polarized Interference Imaging Spectrometer (PIIS), which is based on the theory of Fourier Transform Spectroscopy, is consisted of a series of birefringent crystals such as polarizers, a beam splitter as well as various lengths of birefringent crystals required to achieve large delays. The PIIS, compared with a traditional grating-based dispersion spectrometer, has various advantages of multiple-channel measurements, simultaneous information acquisition of both original images and fringes containing spectral details, large light flux, better light signal-to-noise ratio (SNR) as well as anti-vibration etc. Therefore, the PIIS has also been developed in a range of astronomy and astrophysics areas such as remote sensing, extrasolar planet radial velocity measurements, spacecraft design, lunar exploration etc. However, by reviewing of former works and references, two major drawbacks still remain in PIIS and need to be fixed. For one thing, the classic PIIS has a very limited field-of-view (FOV) around ±2°, which means the acquired fringes on the image plane will show quite strong non-linear distortion and hence degrade the accuracy of spectral reconstruction via Fourier transform. For another, the random thermal-phase-drift (TPD), given rise from both thermal expansion and birefringence variation caused by the environmental temperature fluctuation, has barely been studied before and will inevitably result in extra radial velocity error based on Doppler Spectroscopy. In this paper, a noble polarization interference imaging spectrometer with the emphasis on the FOV widening technology is introduced. This technology, using a compensated Savart plate containing a half-wave plate sandwiched between two orthogonally placed displacer plates as a compensated Savart plate, produces an angle-dependent phase shear to create parallel spatial interference fringes with a FOV around ±10°. This improvement not only enhances the accuracy of Fourier Transform algorithm but also increases input luminous flux and therefore even weak input spectrum detection and calibration results with high SNR can be fully accomplished. Also, a secondary set of birefringent plates (α-BBO and LiNbO3) with opposite thermal properties is proposed to passively diminish TPD caused by temperature fluctuation. The experiment shows that thermal-drift-phase error is perfectly restricted within 0.02 rad in the laboratory environment. As a consequence, this advanced PIIS is eligible to realize the fast and accurate measurement and calibration application in the field of large astronomical spectral instruments with ultra-high spectral resolution occasions such as Astronomical Frequency Comb.

[Influence of laser power and incident angle as well as testing distance on laser inducted breakdown spectroscopy (LIBS) technology for spectroscopy diagnosis and multi-element analysis].

Laser inducted breakdown spectroscopy (LIBS) technology has been used widely for the multi-element analysis of different samples and also an effective way to realize the spectroscopy diagnosis applied to calculating the electron temperature and vibration-rotation temperature etc of some certain elements. It is a highly effective measurement for its non-contact and nondestructive on-line analysis with the help of a high-speed response CCD camera. In the present paper, the authors tested an alley though LIBS technology to determine its elements in terms of species and quantities and also discussed some influences of the incident angle and the power changes of the laser beam as well as the measurement distance changes on the experimental results and and gave appropriate theoretical explanation.

[Development of transient pyrometer based on multi-spectral radiation technology].

In modern dynamics system, the radiant temperature of the flame, which caused by the transient plasma stimulated by high-energy-level electromagnetism field, takes an important role in the description of the flying object's status as well as cauterization of the trajectory. Due to its extremely high temperature and transient process, the radiant temperature of the flame can hardly be measured through contracted ways, either static ways such as traditional pyrophotometer or CCD arrays. In the present paper, the authors bring forward a novel pyrophotometer based on classical theory of Planck's law (blackbody radiation law) and multi-channel spectrums radiation method. With this new type pyrophotometer, any spectrum can be selected out from the wavelength of 300 to 860 nm within 2 ns. Also, the application of high-definition diffraction grating and fibers can'ensure the accuracy of selected spectrum. The results through a serial of experiments by using this theory as well as high-speed photodetector indicate that this method is valid and accurate for the measurement of the object's surface's radiant temperature.