Design of NO2 photoacoustic sensor with high reflective mirror based on low power blue diode laser

An NO2 photoacoustic sensor system with a high reflective mirror based on a low power blue diode laser is developed in this work. The excitation power is enhanced by increasing the number of reflections. Comparing with a traditional photoacoustic system, the pool constant is improved from 300.24(Pa·cm)/W to 1450.64(Pa·cm)/W, and the signal sensitivity of the photoacoustic sensor is increased from 0.016 μV/ppb to 0.2562 μV/ppb. The characteristics of temperature and humidity of the new photoacoustic sensor are also obtained, and the algorithm is adjusted to provide a quantitative response and drift of the resonance frequency. The results of this research provide a new method and concept for further developing the NO2 photoacoustic sensors.

Repetitive cleaning of a stainless steel first mirror using radio frequency plasma

First mirrors（FMs） are crucial components of optical diagnostic systems in present-day tokamaks and future fusion reactors. Their lifetimes should be extremely limited due to their proximity to burning plasma, greatly influencing the safe operation of corresponding diagnostics.Repetitive cleaning is expected to provide a solution to the frequent replacement of contaminated FMs, thus prolonging their lifetimes. Three repetitive cleaning cycles using radio frequency plasma were applied to stainless steel（SS） FM samples, to evaluate the change of the mirrors＇ optical properties and morphology during each cycle. Amorphous carbon films were deposited on mirror surfaces under identical conditions in three cycles. In three cycles with identical cleaning parameters, the total reflectivity was restored at up to 95%. Nevertheless, with successive cleaning cycles, the FM surfaces gradually appeared to roughen due to damage to the grain boundaries. Correspondingly, the diffuse reflectivity increased from a few percent to 20%and 27% after the second and third cycles. After optimizing the cleaning parameters of the second and third cycles, the roughness showed a significant decrease, and simultaneously the increase of diffuse reflectivity was remarkably improved.

Novel single-transverse-mode control of VCSELs with dielectric mode filter

We establish a novel method of controlling the transverse modes of vertical cavity surface emitting lasers （VCSELs） to achieve 1 mW single-fundamental-mode lasing. A dielectric mode filter is installed on top of the VCSEL. The dielectric layer （SiO2） is deposited and patterned to modify the mirror reflectivity across the oxide aperture via antiphase reflections. This mode selection is nondestructive and universally applicable for other structures under single transverse mode. Destructive etching techniques （dry/wet） or epitaxial regrowth are also not required. This method simplifies the preparation process and improves the repeatability of the device. Measurements show that under continuous-wave current injection, the side-mode suppression ratio exceeds 30 dB.

Pulse shape of ultrashort intense laser reflected from a plasma mirror

The temporal profiles of high-power short-pulse lasers reflected from self-induced plasma mirrors（PMs） were measured with high temporal resolution in the sub-picosecond window. The leading front shape of the laser pulse is found to depend sensitively on the laser fluence on the PM surface. Spectral modulation plays a key role in pulse profile shaping. Our findings will extend our knowledge on properly using PMs.

Measurement of small wavelength shifts based on total internal reflection heterodyne interferometry

This Letter presents a method of an optical sensor for measuring wavelength shifts. The system consists of a diffraction grating and a total internal reflection heterodyne interferometer. As a heterodyne light beam strikes a grating, the first-order diffraction beam is generated. The light penetrates into a total internal reflection prism at an angle larger than the critical angle. A wavelength variation will affect the diffractive angle of the first-order beam, thus inducing a phase difference variation of the light beam emerging from the total internal reflections inside the trapezoid prism. Both the experimental and theoretical results reveal that, for the first-order diffractive beam, the sensitivity and resolution levels are superior to 5°/nm and 0.006 nm, respectively, in the range of wavelength from 632 to 634 nm, and are superior to 3.1°/nm and 0.0095 nm in the range from 632 to 637 nm. For the theoretical simulation of the fourth-order diffractive beam, they are superior to 6.4 deg ∕nm and 0.0047 nm in the range from 632 to 637 nm.

Mirror that Reflects Your Future Self

在小说《道林·格雷的画像》中,一幅肖像能代替真人承担岁月和心灵的负担,使其永远保持青春貌美;而法国科学家正在发明一面“魔镜”,可以精确“预言”你五年后的体型和面容。

Measurement and analysis of restitution coefficient between maize seed and soil based on high-speed photography

The restitution coefficient is an important elementary physical parameter related to the research and development of agricultural machinery.The kinematic model of maize seed in the falling and impacting processes was developed to measure the restitution coefficient between maize seed and soil.A test bench for measuring the restitution coefficient was designed and built referred to the theory of mirror reflection.The velocities for impacting maize seed were measured and analyzed in a three-dimensional space via high-speed photography,and then restitution coefficients of in different impact conditions were obtained.On this basis,this study took flat dent seed and round seed as samples.Single factor tests were conducted to analyze the influences of these factors on the restitution coefficient.The impact angle,falling height,soil compaction,soil moisture,maize moisture content and different parts of seed were selected as test factors.The corresponding regression equations were obtained by analysis.The results showed that,as the impact angle was bigger than 25°,the restitution coefficient increased with the increase of impact angle.The restitution coefficient had a linear decreasing trend with the increase of falling height.As the soil compaction strength was 200-350 kPa,the restitution coefficient increased with the increase of soil compaction.As the soil compaction strength was larger than 350 kPa,the changing trend of the restitution coefficient was relatively stable.As the soil moisture content was 13.5%-18%,the restitution coefficient decreased with the increase of soil moisture.As the soil moisture content was 18%,the restitution coefficient was the minimum.As the maize moisture content was 11%-16%,the restitution coefficient decreased with the increase of maize moisture content.The rotational motion always occurred in falling process of flat dent seed and round seed.The probabilities of crown part and lateral part of maize seed impacting with soil were the highest,and the restitution coefficient between crown part and soil was higher than that of other parts in the same condition.

High-power Tm^(3+)-doped all-fiber laser operating at1908 nm by a master oscillator power amplifier configuration with narrow spectral linewidth

We demonstrate a diode-pump Tm^（3＋）t-doped all-fiber laser operating at 1908 nm based on a master oscillator power amplifier（MOPA） configuration. In our work, 152 W of laser output power is generated by a total incident pump power of 434 W at 790 nm, corresponding to the total optical efficiency of 35%. The laser wavelength is1908.29 nm. To the best of our knowledge, it is the highest output power reached around 1908 nm with such a narrow linewidth of 0.18 nm based on a MOPA configuration.

Diode-pumped electro-optical cavity-dumped Tm:YAP laser at 1996.9 nm

In this Letter, we demonstrate a diode-pumped electro-optical cavity-dumped Tm：YAP laser for the first time to our knowledge. A pulse width of 7.1 ns is achieved at a wavelength of 1996.9 nm. A maximum output power of 3.02 W is obtained with a pump power of 58.8 W at a repetition rate of 100 k Hz and a high-voltage time of 1000 ns, corresponding to an overall optical-to-optical conversion efficiency of 5.2%. In addition, we study the effect of repetition rate and high-voltage time on the output power characteristics of a cavity-dumped Tm：YAP laser.

Numerical modeling of thermal loading of diamond crystal in X-ray FEL oscillators

Due to high reflectivity and high resolution of X-ray pulses, diamond is one of the most popular Bragg crystals serving as the reflecting mirror and mono-chromator in the next generation of free electron lasers（FELs）.The energy deposition of X-rays will result in thermal heating, and thus lattice expansion of the diamond crystal,which may degrade the performance of X-ray FELs. In this paper, the thermal loading effect of diamond crystal for X-ray FEL oscillators has been systematically studied by combined simulation with Geant4 and ANSYS, and its dependence on the environmental temperature, crystal size, X-ray pulse repetition rate and pulse energy are presented. Our results show that taking the thermal loading effects into account, X-ray FEL oscillators are still robust and promising with an optimized design.

Efficiency-enhanced reflective nanosieve holograms

Photon nanosieves,as amplitude-type metasurfaces,have been demonstrated usually in a transmission mode for optical super-focusing,display,and holography,but the sieves with subwavelength size constrain optical transmission,thus leading to low efficiency.Here,we report reflective photon nanosieves that consist of metallic meta-mirrors sitting on a trans-parent quartz substrate.Upon illumination,these meta-mirrors offer the reflectance of〜50%,which is higher than the transmission of visible light through diameter-identical nanoholes.Benefiting from this configuration,a meta-mirror-based reflective hologram has been demonstrated with good consistence between theoretical and experimental results over the broadband spectrum from 500 nm to 650 nm,meanwhile exhibiting total efficiency of〜7%.Additionally,if an additional high-reflectance layer is employed below these meta-mirrors,the efficiency can be enhanced further for optical anti-counterfeiting.

Bending Loss Analysis of Optical Fiber with Out-Cladding of Photonic Crystals

A new type of optical fiber is presented here. It consists of a coaxial optical fiber, bounded by dielectric, multilayer and omnidirectional reflecting mirrors. Jones matrix method is used to analyze the influence of the layer number of one Dimensional (1D) photonic crystals on their reflectivity. The numerical results show that this type of fiber can be used to guide light around sharp bends whose radius of curvature can be as small as the wavelength of light without significant scattering losses.

Modal analysis of 1×3 reflective triangular gratings under normal incidence

Modal analysis of the 1×3 highly efficient reflective triangular grating operating in the 800 nm wavelength under normal incidence for TE polarization is presented in this Letter.The rigorous coupled wave analysis and simulated annealing algorithm are used to design this beam splitter.The reflective grating consists of a highly reflective mirror and a transmission grating on the top.The mechanism of the reflective triangular grating is clarified by the simplified modal method.Then,gratings are fabricated by direct laser writing lithography.