In this letter, we discuss the increase in the average cluster size by lowering the stagnation temperature of the methane (CH4) gas. The Coulomb explosion experiments are conducted to estimate the cluster size and the size distribution. The average CH4 cluster sizes Nav of 6 230 and 6 580 are acquired with the source conditions of 30 bars at 240 K and 60 bars at 296 K, respectively. Empirical estimation suggests a five-fold increase in the average size of the CH4 clusters at 240 K compared with that at room temperature under a backing pressure of 30 bars. A strong nonlinear Hagena parameter relation (\Gamma *\propto T^{-3.3}) for the CH4 clusters is revealed. The results may be favorable for the production of large-sized clusters by using gases at low temperature and high back pressures.

Detection of underwater bubbles is one of the key issues of the research of ocean-atmosphere flux exchange. Digital holographic experiment is carried out based on Mach-Zehnder digital holographic system, to detect the distribution of bubbles. Holographic images of the dynamical bubble fields are recorded by the chargecoupled device (CCD) video system and the tomographic images at different depth are reproduced. The distribution of sizes and densities of the bubbles is obtained through following steps as denoising, edgedetection, and bubble-recognition using Hough transform. Through the experiments, the efficiency and applicability of the digital holographic detection of underwater bubble fields are tested and verified.

In order to test convex aspheric surfaces without the aid of other null optics, a novel method combined sub-aperture stitching and interferometry called SSI (sub-aperture stitching interferometry) is introduced. In this letter, the theory, basic principle, and flow chart of SSI are researched. A synthetical optimization stitching mode and an effective stitching algorithm are established based on homogeneous coordinate's transformation and simultaneous least-squares fitting. The software of SSI is devised, and the prototype for testing of large aspheres by SSI is designed and developed. The experiment is carried out with five subapertures for a convex silicon carbide (SIC) aspheric mirror with a clear aperture of 130 mm. The peak-to-valley (PV) and root-mean-square (RMS) error are 0.186 \lambda and 0.019 \lambda, respectively. For the comparison and validation, the TMA system which contained the convex asphere is tested by interferometry. The wavefront error of the central field of the optical system is 0.068 \lambda RMS which approaches to diffraction limitation. The results conclude that this technique is feasible and accurate. It enables the non-null testing of aspheric surfaces especially for convex aspheres.

The hysteresis nonlinearity of piezoelectric actuator is one of the main defects in the control of deformable mirror which is widely used as a key component in adaptive optics system. This letter put forward a modified Prandtl-Ishlinskii (PI) model in order to precisely describe the hysteresis nonlinearity of piezoelectric actuator. With this proposed model, an inverse-model based controller used for trajectory tracking in open-loop operation is designed to compensate the hysteresis nonlinearity effect. Then, some tracking control experiments for the desired triangle trajectory are performed. From the experimental results, we can see that the positioning precision in open loop operation is significantly improved with this inverse-model based controller.

In a certain amplification system, signal laser can be amplified from 1 nJ to 5 J. To realize a high quality imaging transfer, meet beam diameter expansion requirement, and get good filtering effects, three spatial filters are designed and assembled. Lenses in these spatial filters can be assembled and adjusted by their reflection spots to meet wave-front requirements. In this letter, we analyze precision of the assembling, and make a ray-tracing simulation to discuss the relation between assembling precision and lenses parameters. On the optimized distance of 1000 mm, adjusting precision of lens tilt can reach 5 mrad.

Three-dimensional (3D) simulations and theoretical analyses on super-short pulse generated using freeelectron lasers (FELs) at perfect synchronism are carried out with the help of our 3D OSIFEL code. The evolution of longitudinal pulse width in the Japan Atomic Energy Research Institute (JAERI) experiment is simulated. The results show that the optical pulse is compressed on successive passes due to the slippage between the optical and electron beams, and an ultra-short 221-fs optical pulse is finally obtained, which agrees with the experiment. Furthermore, to shorten wavelength such as soft ultraviolet (SUV) spectrum range, an ultra-short pulse generated at perfect synchronism is analyzed and studied. Finally, the relationship between the optical pulse length compressed and the peak electron beam current is calculated. It shows that the higher the electron beam current, the shorter the output FEL width length, due to the higher gain.

In this letter, the influence on the beam quality due to cumulative effect of the inner channel thermal deformation in the high energy laser system with unstable resonator is researched. Firstly, three typical laser powers of 50, 100, and 150 kW are selected to analyze thermal deformation of mirror by the finite element analyze of thermodynamics instantaneous method. Then the wave front aberration can be calculated by ray-tracing theory. Finally, Strehl ratio and Zernike aberration coefficients of the vacuum far-filed beam can be calculated and comparably analyzed by Fresnel diffraction integration. The theory and simulation results show that due to the effect of inner channel thermal deformation, eccentric phenomenon and astigmatism of far-filed beam emerge, and peak power and the focused ability decrease. With the increasing of reflective times, Strehl ratio decrease, and tilt, astigmatism and coma of x direction gradually increase, which become the main aberration. The results can provide the reference to the thermal aberration analysis for high energy laser system and can be applied to the field of laser nuclear fusion and laser weapon, etc.

A mode locked Er-doped fiber laser based on a single-wall carbon nanotube saturable absorber is demonstrated. A high quality single-wall carbon nanotubes (SWCNTs) absorber film is fabricated by a polymer composite. The pulse duration is 488 fs with 9.6-nm spectral width at the center of 1564 nm. The repetition rate is 30.4 MHz. The maximum output power is 3 mW. And the single pulse energy is 0.1 nJ.

A method to shape the incident laser beam into a concentric multi-ring pattern with different intensity distribution is presented based on geometrical transform method and energy conservation. The output two and three rings are designed as examples to verify the validity of the method. The real shaped rings are produced by the spatial light modulation (SLM) and the experimental results show that the shaped laser beam can satisfy the design requirements.

We present a novel technique to generate an orthogonally polarized optical single sideband (OSSB) generated by a tunable bandpass filter (TBF).When the OSSB passes through the other polarization modulation (PolM) which is polarization dependent, the phase shift of the optical carrier and first-order sideband is different under different bias. As a result, a wideband tunable phase shifter is realized by adjusting the bias applied to the polarization modulator.

A novel fiber Bragg grating (FBG) sensing system based on code division multiple access (CDMA) technology is proposed. CDMA is used to separate each reflected sensor. Simulation of experimental results indicates the CDMA technology combines with optical fiber grating sensing system together successfully. Furthermore, the system using semiconductor optical amplifier (SOA) is experimented. The experimental results show that theory and simulation are correct.

In this letter, the theoretical model and rate equations of Ytterbium-doped double-clad fiber amplifiers are introduced. The output performance of fiber amplifiers is analyzed through numerical simulations considering three main factors: active fiber length, pump power, and dopant concentration. It is found that the output signal power experiences a stable growth and then decreases rapidly as the active fiber length becomes longer. It is also revealed that the dopant concentration shows similar effects as active fiber length on the output signal power of fiber amplifiers.

A novel kind of octagonal dual-concentric-core photonic crystal fiber (PCF) is calculated with the finite element method (FEM) and proposed for broadband compensation. In order to realize highly negative dispersion, the hole diameter of the second ring is reduced relative to conventional PCFs. Numerical simulation results show that when the diameters of large holes and small holes are 0.77 \mu m and 0.5 \mu m, respectively, and the pitch of the adjacent rings is 1.1 μm, the negative dispersion can achieve about –840 ps.nm-1.km-1 at 1 550 nm and the dispersion slope can match with single mode fiber (SMF-28) perfectly. This PCF can compensate the positive dispersion and also its slope of about 50 times its length of SMF-28 fiber, which is suitable for C-band optical fiber telecommunication as a dispersion compensation fiber.

Self-pulsation effects of cladding-pumped Erbium-Ytterbium co-doped fiber laser (EYDFL) at around the lasing threshold are investigated. It is demonstrated that laser output of the Erbium-Ytterbium co-doped fiber under the bi-directional pumping regime is more stable than that under the unidirectional pumping one due to the relatively uniform pumping of the fiber. Mechanisms of self-pulsations in the laser system are discussed and possible techniques to avoid self-pulsing and stabilize the laser are proposed.

Packaging of Distributed feedback (DFB) laser array based on reconstruction-equivalent-chirp (REC) technology is a bridge from chip to system, and influences the practical process of REC chip. In this letter, DFB laser arrays of 4 channel @1 310 nm and 8 channel @1 550 nm are packaged. Experimental results show that both 4 channel @1 310 nm and 8 channel @1 550 nm have uniform wavelength spacing and average side mode suppression ratio (SMSR)>35 dB. When I=35 mA, we get the total output power 1 mW of 4 channel @1 310 nm, and 227 \mu W of 8 channel @1 550 nm, respectively. The high frequency characteristic of the packaged chips is also demonstrated, and the requirements of 4 \times 10 G or even 8\ teims 10 G system can be reached. we demonstrate the practical and low cost performance of REC technology and indicates its potential application in the future fiber-to-the-home (FTTH).

Light chirp is a major issue in optical fiber links. This letter deals with precise characterizations of the frequency chirp parameters of reflective semiconductor optical amplifiers (RSOAs). The RSOA chirp properties are represented by transient and adiabatic chirps, whose parameters are characterized utilizing a ratio between the phase and the amplitude modulation depths of the RSOA when modulated with sine waves. Utilizing a high-resolution optical spectrum analyzer, a RSOA linewidth enhancement factor \alpha and an adiabatic factork are obtained experimentally, based on which the influence of chirp parameters on the transmission performance of non-return-to-zero (NRZ) signals can be analyzed.

Infrared thermography determines the surface temperature of an object or human body. It is a promising imaging technology for medical and biological observations due to its contactless and completely noninvasive properties. However, traditional two-dimensional (2D) infrared thermography cannot retain the spatial information, and thus provides only qualitative diagnosis information. A novel real-time three-dimensional (3D) infrared imaging system which takes full advantages of high-speed, high-quality, high-sensitivity, and low-cost in 3D thermograph is presented. We demonstrate the real-time 3D thermal imaging at the speed of 24 frames per second (fps), with resolution of 640 \times 480 points. Experimental results demonstrate quantitatively measurement of temperature distribution of 3D surfaces in real-time is realized with this system.

In this letter, a new single three-dimensional (3D) laser projector is proposed. As liquid crystal (LC) can produce two image patterns with orthogonal polarization states at 120 Hz, only one projector is required in this approach for reconstruction of a 3D object. The light source is made up of RGB (red, green, and blue) lasers because laser has lots of advantages such as longer life, higher brightness, and larger color gamut. A novel diffusive media with good polarization-maintaining quality is used as rear projection screen for its high transmission efficiency (～90%) and low reflection efficiency. When laser incidents into the diffusive media, which contains lots of spherical particles with sizes between 2 and 15 \mu m, laser is scattered randomly and the laser speckle is reduced. A spatial phase mask is also inserted into the optical path to reduce speckle. With these techniques, the speckle contrast is reduced to 0.1 and the quality of image patterns has been greatly improved.

A novel laser-electro magnetic acoustic transducer (EMAT) system for nondestructive testing NDT surface crack of continuous casting billet (CCB) is provided. Rayleigh wave generated by line laser source is used to detect the surface crack of CCB. According to the principle of mode conversion from Rayleigh wave to shear wave, the defect signal is received using the shear wave EMAT sensor in a non-contact way. Experiments are carried out on the steel sample with size 30 × 0.2 × 0.2 (mm) of crack. Further, the influences of life off value and distance between EMAT sensor and laser beam on the testing sensitivity are discussed, respectively. It is found that the life off value is the main factor that effects sensitivity of the proposed method. There is a clear prospect of the method applied to test continuously cast bloom at high temperature.

A tunable pulsed laser induced photoacoustic (PA) measurement set-up is established in the forward mode to monitor in vitro glucose concentration. A series of experiments are investigated to verify the feasibility of this set-up and scheme. Peak-to-peak values (PPVs) of several glucose aqueous solutions are recorded and averaged 512 times at each wavelength. Experimental results demonstrate that the time-resolved PA profile of glucose solutions has a good agreement with the PA theories. The characteristic wavelengths of glucose solution are determined via differential method. The root-mean-square error (RMSE) of predicted concentration reaches 3.15 mg/dl at the optimum wavelength of 1 510 nm via least square (LS) fitting algorithm.

In order to couple into or out of a silicon photonic waveguide on silicon on insulator (SOI) substrate from optical fibers, we present a simple but practical method to design a grating coupler. The grating is periodic with fully etched slots; strong reflection between the fully etched grating and the waveguide is avoided by adding an antireflection interface. Theoretical coupling efficiency up to 43% is demonstrated. A taper waveguide used to link the grating and waveguide is also designed.

Using the first-principles plane-wave pseudopotential method, based on the density function theory, the electron structure and optical properties of GaAs (100) \beta 2(2 \times 4) and GaAs (100) (4 \times 2) reconstructions are calculated. The formation energy of As-rich \beta 2(2 \times 4) reconstruction is minus and the formation energy of Ga-rich (4 \tiems 2) reconstruction is positive; As-rich \beta 2(2 \times 4) reconstruction is stable and Ga-rich (4 \times 2) reconstruction is unstable. Ga-rich (4 \times 2) reconstruction owns lower work function. The electrons at two reconstructions both move into the bulk and form a band-binding region. Both the absorption and the reflectivity of As-rich 2(2×4) reconstruction are smaller than the Ga-rich (4×2) reconstruction. Asrich \beta 2(2 \times 4) reconstruction is more benefit for the movement of photos through the surface to emit photoelectrons.

A ring-cavity synchronously-pumped optical parametric oscillator (OPO) is investigated based on periodically poled KTi:OPO4 (PPKTP). The wavelength of the signal wave covers from 1 000 to 1 500 nm, the output power is 32.3 mW, and idler wave spectrum range from 1 800 to 2 500 nm is detected. By inserting a BBO or BIBO crystal respectively, a stable and adjustable range from 450 to 650 nm light is obtained. Three to six wavelengths can be output simultaneously.

Dynamics of two-component vector dark solitons are investigated by the variational approach in the defocusing nonlinear media, and effects of the weak nonlocality on the soliton propagation and interaction are analyzed. The nonlocality degree determines the intensity distribution of the dark solitons in the stationary states, enhances the intensity transfer between two vector solitons, and affects the propagation and interaction. The numerical results confirm the theoretical findings.

The thin mirrors are widely used in active optical system. In this letter, magnetic medium assistant polishing (MMAP) technology and device are discussed for thin mirrors optical finishing. The principle of MMAP is introduced, the magnetic tool for polishing is designed, and the removal function of magnetic polishing tool is studied. On the basis study of the material removal function especially removal property in the edge region, the tool-path is optimized, and the dwell time distribution of computer-controlled MMAP is researched. The glass thin mirror is polished by MMAP technology. The diameter of the work-piece equals S22201 150 mm and the thickness is 5 mm. The initial surface error is 0.19\lambda (root mean square (RMS)) , and after two steps fabrication, the final surface error reaches 0.02\lambda. The experimental rsults verify that the technology is effective for thin mirrors finishing.

Toroidal surface and biconic surface are employed increasingly, however their profile cannot be null tested easily for they are non-rotationally symmetrical. Null testing method with cylinder compensator is proposed to solve this problem. The theory of this method is revealed. The errors of this method are present. Three typical testing optical systems with cylinder compensator are demonstrated at last. The design results and total error indicate that this method is feasible.

A structure which consists of photoresist film sandwiched by Ag nano-particle and metal film is proposed to modify localized hotspot both in transversal and longitudinal direction. It shows that there is strong plasmonic coupling between Ag nano-particle and metallic surface, which helps to reduce the width and elongate the depth of the plasmonic hotspot localized inside photoresist film. And that fringes and side lobes around hotspots can be effective attenuated by bottom-side illumination. Influences of illumination, particles inter-space, and polarization are also studied. The method opens avenue for the potential applications such as lithography, optical storage, etc.

We present high power terahertz quantum laser at about 3 THz based on bound-to-continuum active region design. At 10 K, corrected by the collection efficiency, the maximum peak power of 137 mW is obtained in pulsed mode. What’s more, we firstly introduce monolithically integrated THz quantum cascade laser (QCL) array and the maximum peak power increased to 218 mW after correction. In total, the array shows better performance than single device, implying cheerful prospect.

Nonlinear correlation between attenuation and absorption due to the presence of scattering is the main reason for inaccurate spectroscopic quantitative investigations. The polarization subtraction methods are applied to reduce the scattering in order to linearise attenuation to absorption. Monte Carlo simulation shows that the polarized light offers better performance than unpolarized light at giving the most accurate estimate of the concentration ratio of absorbers using the modified Lambert-Beer law. Our results demonstrate that spectrophotometry with polarized technique offers the potential to be a simple and costeffective system.

We develop a phenomenological model to investigate dynamics of ionization-induced injection. In the "bubble" regime of laser wakefield acceleration (LWFA), it is found that there is an upper limit for laser intensity of ionization-induced injection. In the plane perpendicular to the laser polarization, when the laser pulse is linearly polarized, ionization-induced injected electrons exhibit a filamented structure and semi-coherent betatron oscillation.

A closed-form bit-error rate (BER) expression is derived for free-space optical (FSO) communication systems with circle polarization shift keying and spatial diversity receivers in the gamma-gamma (GG) distribution fading channel. This model can predict the performance without the need of lengthy simulation runs. The performance can be analyzed by some system parameters such as atmospheric conditions, link length, communication wavelength, receiver aperture size, and number of spatial diversity receivers. Numerical results demonstrate the influence of the above parameters on the FSO systems and show quantitatively the differences in behavior among various different parameters.

In the letter the polarization properties of quasi-homogenous (QH) beam propagating in Kolmogorov and non-Kolmogorov turbulence are studied. The results show that the polarization properties of QH beam undergoes three stages during the propagation in turbulence: in the “near field”, the degree of polarization (Dop) and the state of polarization (Sop) fluctuate with source parameters and transverse position; after that the beam come to the “middle field” where its properties are affected by source parameters and turbulence perturbation; in the final “far field”, the values come to constants which dependent only on source parameters.