Due to the high peak-to-average power ratio (PAPR) and the very close sub-carrier spacing for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system, the sub-carriers walking-away resulted by link dispersion greatly affects the nonlinear damage of the optical fiber. We firstly study the nonlinear damage and system performance of CO-OFDM system without/with dispersion compensating optical fiber (DCF) under the conditions of different distributions of dispersion and different residual dispersion. For the single channel 40 Gb/s CO-OFDM system, the Q factor of DCF link is 5.1 dB higher than that of completely compensated DCF link. For DCF link, when the residual dispersion is added from 0 to 1200 ps/nm, the maximum Q factor is improved by 4 dB, and the nonlinear threshold is increased by 4 dBm. When the residual dispersion is 1200 ps/nm, the performance of DCF link system is almost the same as the system without DCF.

The cladding light in double clad-fiber laser will not only affect the laser beam quality, but also damage the optical devices. The reasons of the generation of cladding light in the passive double-clad fiber are analyzed, and the influence of these reasons on the cladding light generation is studied experimentally by the method of power and optical field distribution detection. The results indicate that, low-quality splices and mode field mismatch between the passive double-clad fibers would raise the coupling between fundamental mode and cladding modes, while the fiber combiner and isolator would lead to the leakage of light from the core to the cladding, which results in the generation of cladding light. Notes of restraining the generation and reducing the influence of cladding light are discussed.

To reduce the polarization beat length wavelength sensitivity in birefringence holey fiber so as to produce a stable broadband fiber-optic polarizer, a holey fiber cladding structure design with a rectangular array distribution of air holes in cladding is presented. Asymmetric structures with different birefringence trends are introduced into the cladding. The influence of different structural parameters on the polarization beat length stability is calculated and analyzed by the full vector beam propagation method, including the material dispersion of fused silica. The results show that the fiber cladding structure reduces the sensitivity of the polarization beat length to the wavelength. In the wavelength of 1.26~1.60 [μm], the relative change in polarization beat length is less than ± 4% and the broadband stability requirements for producing optical wave plate are met. Moreover, the optimized geometrical cladding structure has a large error tolerance, further reducing the difficulty of fabrication.

Internal corrosion of oil and gas pipeline is one of the main factors leading to pipeline failure. Based on principle of the fiber Bragg grating strain sensor as well as the multiplexing technology, an on-line monitoring system based on optical fiber grating sensors is studied. The corrosion degree of the inner pipeline can be determined by the surface stress changes, and array sensor network using 7 channels of FBG sensor is set up in the monitoring system, through temperature compensation grating to realize temperature compensation. Experiments show that the system pressure measuring accuracy is high, with the system error less than 2.23%. The feasibility and stability of the system are validated by comparing the measurement results with the actual pressure from the scene of the production, meeting the requirements of internal corrosion online monitoring.

With the development of fiber-to-the-home (FTTH) network, bend-insensitive fiber (BIF) has attracted more and more attention. We focus on bending losses of the trench-assisted bend-insensitive optical fiber. For the trench-assisted bend-insensitive fiber, the refractive index difference of trench, the width of trench and the distance between the core and trench are three important parameters which could influence the bending loss. Combining the three parameters, we investigate the impact of fiber radius and refractive index of the coating layer on the bending losses. It is shown that: bending losses can be effectively reduced by changing the radius of the fiber; with refractive index of the coating layer increasing, the bending losses of the fiber could be effectively reduced for certain bending radius. The conclusion is of significance for designing and manufacturing of bend-insensitive fibers.

The aim of this paper is to develop a digital holographic imaging (DHI) system, capable of capturing three-dimensional (3D) images of aquatic species, with the characteristics of in situ, non-interference, dynamic and 3D detection. The DHI system records the in-line holographic image with a scientific grade CCD, while a camera lens is used to magnify the holographic image. This optical field is retrieved by numerical algorithms, which enable the reconstruction of the field at different distances relative to the detector from a single hologram. Conclusion is drawn that digital holographic sensing and imaging is an appropriate and highly effective method for aquatic microorganism study.

Diffraction based overlay (DBO) metrology has fully demonstrated its remarkable advantages in terms of high resolution, high precision and low tool-induced shift (TIS) and so on . DBO technologies have been gradually replaced the traditional image based overlay (IBO) technologies and have also been developed to address the overlay metrology challenges for 22 nm technology node and beyond. In comparison with IBO technologies, the biggest problems that DBO faces now are high mark costs and long measurement time. IBO only uses single pad for two-dimensional (2D) overlay errors measurement, while DBO should adopt one-dimensional gratings for measurement with x, y directions. Furthermore, at least two marks are needed for one direction measurement. We introduce 2D periodic structure based DBO technologies. The DBO physical model is established through rigorous coupling wave analysis (RCWA) program. The feasibility of this method is proved by means of analysis of the overlay measurement sensitivity and main measurement errors. The application of 2D DBO measurement technology can help to reduce the measurement time and mark cost by half. The DBO measurement cost will thus be dramatically lowered, and the measurement efficiency will be promoted.

This paper focuses on the application of digital gradient sensing (DGS) method to the determination of stress intensity factors (SIFs) for cracks in polymethyl methacrylate (PMMA). First，the full-field displacement fields of the test sample are recordedusing digital image correlation (DIC), and then the angular deflections of light rays propagating through the material are obtained based on the elasto-optical effect exhibited by transparent materials subjected to non-uniform state of stress. Under plane stress conditions, the deflections of light rays can be related to two orthogonal in-plane stress gradients. In this way, the stress gradients near the crack on PMMA plates are quantified, and then cracktip SIFs are extracted from DGS Measurements using the method of nonlinear least-squares proposed. With the obtained SIFs, crack tip location is determined simultaneously. For validation, the SIF determined is compared with theoretical results, confirming the effectiveness and accuracy of the proposed technique. Therefore, it DGS method is a practical and effective tool for SIF measurement.

A novel design scheme for high speed [1.31 μm]TM mode lasers based on InGaAsP (well)/InGaAlAs (barrier) strain-compensated multiple quantum wells (MQWs) is proposed. Calculation on luminescence property with five different MQWs (10 nm, 1% tensile-strained In0.49Ga0.51As0.79P0.21 well with 12 nm, 0.5% compress-strained InGaAlAs barriers of Eg =1.309, 1.232, 1.177, 1.136, 1.040 eV, respectively) and simulation on laser diodes based on these MQWs are presented. The results of these studies indicate that moderately small barrier height can not only provide effective confinement for charge carriers, but also make carrier distribution in the MQWs more uniform, thus acquiring advantageous luminescence property and device performance. Our investigation gives guidance to the design and fabrication of [1.31 μm] TM mode lasers of low threshold current, high characteristic temperature, and wide modulation bandwidth.

In the study of thermal distortion of resonator mirrors under high-power laser irradiation, the high reflective coatings are usually ignored. The thermal distortion distributions of resonator mirrors with high reflectivity fixed in different ways are analyzed by finite element analysis software ANSYS under the specific laser irradiation (absorbed power of 250 W, outer radius of 4 cm, inner radius of 2 cm, co-axial annular beam, 30 s). The effects of two different high reflective coatings and the field distribution of standing wave inside the coatings are considered, and the results are further compared and analyzed with the usual approximate method that the high reflective dielectric coatings are ignored. The results show that, the difference of thermal deformations among the three situations, i.e., considering two different high reflective coatings and ignoring the high reflective coatings, is relatively small when the resonator mirror is clamped radially. However, the difference becomes obvious when the resonator mirror is clamped axially and the center of the mirror can be convex, concave and flat. If the resonator mirror is fixed by three cones, though the center of the mirror can be relatively flat, there is still obvious difference among the three situations in the irradiation zone of the annular laser. Consequently, the contribution of the standing wave inside the high reflective coatings to the thermal distortion of resonator mirrors cannot be ignored in some applications.

Due to quality defects on the surface of stainless and carbon steel laminated plates prepared conventionally, laser cladding is introduced for preparing metal laminated plates. By adjusting two main process parameters of the laser power and scanning speed, their influences on the metallographic structure of laminated platesby laser cladding are investigated. Then the parameters are optimized according to metallographic structure analysis and tensile test of stainless-carbon steel laminated plates. The results show that: with the decrease of the energy density of the laser, stainless steel laminated plates interface is arranged in three microstructure morphology, which have wave heights of 0.01～0.03 mm, 0.08～0.10 mm and 0.11～0.14 mm, and the layer thickness increases gradually. Meanwhile, the yield strength increases as the interface wave height increases. When the wave height of the interface of stainless steel laminated plates is 0.11~0.14 mm, thehigh yield strength of 410 MPa is obtained. Stainless-carbon steel laminated plates prepared by laser cladding have strong metallurgy bonding quality and good microstructure properties. The tensile strength and elongation indicators have met the requirements of the standard of stainless-carbon steel laminated plates. This method is feasible, and provides theoretical and experimental basis for the wide application of laser cladding preparation of laminated plates.

The laser radar cross section (LRCS) of complex target is a very important parameter in laser acquisition and stealth all the time. The theoretical method of calculation of LRCS is comparatively mature,but in the overall combination design of target modeling, target surface coating materials and LRCS calculation integration, there are still some problems remaining to be solved.Aiming at this problem, we introduce anapplication integration developing method of complex target′s (coating material) calculation of LRCS based on Unigraphics modelling software, model of bidirectional reflectance distribution function (BRDF) and graphical electromagnetic computing (GRECO). The program′s application integration design is effective, proved by calculation examples based on this method, besides, this method has high precision and accuracy in computing.

CoCrAlYTa-Al2O3-ZrB2 composite coating is deposited on the Co base alloy surface by laser remelting. The microstructure, phase and micro-hardness are studied using scanning electron microscope (SEM) and X-ray diffractometer (XRD)．The high temperature oxidation test is conducted. It is found that the defects of as-sprayed coating like lamellar stacking microstructure and pores are eliminated by laser remelting, and the remelted coating possesses a denser microstructure. The micro-hardness of the remelted coating at the cross section is promoted from an average of 899.47 HV to 929.484 HV. The properties of oxidation resistance are significantly improved. The increased mass of the remelted coating is 1/6 of that of the clad coating, and the oxide film of the remelted coating is homogeneous.

The analytical expression of temperature field around inclusion is obtained by establishing the model of temperature field due to the absorption of the single impurity defect in the KDP crystal using Green′s function method based on the theoretical solution of the point source heat transfer model. The influence of pulsed laser parameters on the temperature fieldis analyzed and the model of multi-inclusions absorption in the certain area of KDP crystal is obtained based on the single impurity defect absorption theory. The results show that the temperature variation caused by the inclusion absorption is closely related to the parameters of the pulsed laser. When the distance among the inclusions is less than the thermal diffusivity length in the duration of laser pulses, it will lead to the cumulative effect of temperature field. If the density of the inclusions is large enough, the temperature around the inclusion group will rise rapidly, which will enlarge the possibility of laser induced damage for crystals.

The effect of localized surface plasmon resonance of Ag nanoparticles on the diffraction efficiency of holographic polymer dispersed liquid crystal grating (H-PDLC) is reported. The surface plasmon resonance characteristic of spherical Ag nanoparticles within the PDLC material is analyzed. According to Mie theory, the extinction spectra of spherical Ag nanoparticles surrounded by PDLC material are simulated and then the resonance wavelength is found in the extinction spectra, which is very similar to the resonance wavelength value measured by spectrometer. In addition, the electric field distribution inside and outside the spherical Ag nanoparticles is simulated according to quasi-static approximation method. It is testified that Ag nanoparticles exhibit surface plasmon resonance in the PDLC material during holographic recording process, thus the phase separation between liquid crystal and polymer is significantly improved and the grating structure is much smoother than the sample without Ag nanoparticles, which greatly increases the diffraction efficiency of H-PDLC grating.

The measurement of electron beam parameters is very important for a linear induction accelerator (LIA). The measurement equipments must meet the demands of high time resolution, high sensitivity and high stability. The ultra-high-speedperformance is especially the most important one.One kind of 3-frame framing camera with ultra- high-speed and high performance has been developed recently. The optical principle of the framing camera is based on splitting the focused imaging light beam in the field of image space. It features good imaging quality and high light energyefficiency. The gated image intensifier with high speed shutter, the scientific CCD camera and the high speed controller based on large scale field programmable gate array are the main portions. The shutter time and the frame interval time can be set independently. The effective image size is 25 mm in diameter. The highest shutter time is about 3 ns. The spatial resolution is better than 35 lp/mm inimage space.

Solar blind ultraviolet (UV) detection system has attracted widespread attention in both civil and military fields because of its unique advantages in detection, especially its incomparable superiority in missile warning field. The principle and advantages of UV warning system and the key technology of UV detection system are analyzed. The design indices of solar blind UV optical system are determined according to the application requirement of UV warning system. Based on the analysis of selection method of initial structure, the optical power is assigned according to the aberration theory. Then the optical design software Zemax is used to optimize the initial structure and evaluate the image quality. A solar blind UV optical system with the focal length of 150 mm, relative aperture of 1∶3 and field of view of ±4° is finally designed. The result shows that the encircled energy is higher than 90% in a pixel size (25 mm) of intensified charge-coupled device (ICCD), and the modulation transfer function (MTF) is higher than 0.7 at the Nyquist frequency (20 lp/mm). All of the data satisfy the design requirement. The system is highly valuable because of its characteristics of excellent imaging quality, compact configuration and light weight.

To meet the requirement of uniform illumination, a new automatic optimization design method for LED extended source is proposed. According to point source approximation theory, a reflector for uniform illumination is established. With chips-on-board (COB) type LED source replacing point source, our approach is used to achieve uniform illumination on the target plane with optimized illumination function defined by the “scheme” language in TracePro. A Lambert emission of LED with diameter of 20 mm is used as simulation source. The result shows that our optimization method works well with uniformity (the ratio of average illumination to the maximum illumination) of 85% or more and optical coupling efficiency of 96% or more at the target plane 300 mm away from the source. This design method is easy and useful, with a wide range of applications.

Due to the difficulties in eliminating manufacturing and assembly errors, different degrees of nonlinear distortions occur in the optical system of digital video cameras, which seriously affects the accuracy of real-time image stitching. Through the comparison of a traditional distortion model of digital camera and the Brown model, we propose to utilize the simplified Brown model as the distortion model of digital video cameras. At the meantime, least squares adjustment is applied to solve the nonlinear distortion parameters based on single-photo space resection. Finally, three types of digital video cameras are calibrated by three- dimensional control field. Experimental results show that the calibration accuracy of a video camera via the proposed distortion model is distinctively higher than the classic distortionmodel, which is less than 0.5 pixel. Moreover, the calibrated parameters are stable and faithful, which effectively amends the distortion and helps to recover the relative geometric relationship inside an image.

A straight waveguide optical voltage sensor for measurement of power frequency voltage based on Y-cut LiNbO3 waveguide is proposed. Using the Pockels effect of LiNbO3 crystal and adjusting the optical bias to [π2], linear result can be achieved. Making use of the waveguide′s distinguishing feature that optical fiber can couple with waveguide directly, all optical devices are connected with optical fiber so that the optical path is simplified. Both the sensor′s principles and the output/input characteristics are analyzed. The power frequency voltage in the range of 100~3000 V is measured by using this sensor. The temperature dependence is researched preliminarily, and in the range of -40 ℃~60 ℃, transformer ratio is 5.29%. The measurement results show that this kind of voltage sensor is suitable for measurements of high frequency voltage.

Selective area epitaxy is applied to grow GaN {11-22} semipolar plane template, followed by InGaN/GaN multiple quantum wells （MQWs） growth. The results indicate that the GaN template is composed of the {11-22} side facet and planer c plane, and the MQWs show dual-color emission. Local cathodoluminescence reveals that the 390 nm emission peak originates from the MQWs on semipolar plane, while the 400 nm emission peak results from the MQWs on c plane. The large red-shift in emission wavelength for c plane MQWs compared with that of {11-22} semipolar plane MQWs is due to the indium enrichment originating from additional source supply due to the surface migration effect and lateral vapor-phase diffusion during selective area epitaxy. Another important reason is the reduced polarization effect InGaN/GaN MQWs on semipolar plane. At the same time, the growth rate of the semipolar plane is lower than that of the polar c plane under the same conditions.

Finite element method is used to simulate the process of the pressing in compression molding of glass lens array. Five-element general Maxwell model and Tool-Narayanaswamy shift function are incorporated into the finite element method to describe the viscoelastic property and the heat flow characteristic of glass at high temperature, respectively. The filling ratios of mold cavity of glass lens array in different locations and the effect of temperature and molding velocity on cavity filling ratios in lens array are emphatically analyzed. The simulation result shows that cavity filling ratios are lower at the edge of the mold than that in the middle of the mold, and the higher temperature and molding speed will lead to higher cavity filling ratios.

A novel optical switching based on polymerdispersed liquid crystal (PDLC) is presented. Utilizing the electro-optic properties of PDLC, the optical switching functionis realized.The PDLC films are prepared by photo-polymerization induced phase separation (PIPS),and the detailed process including the material selection, process route and preparation flow is described. The electro-optic properties of PDLC are tested, compared and analyzed,and a better preparation method is obtained. A small volume driving scheme of optical switching is proposed and realized, and a stable method to extract the electrode is given. The experimental results show that the circular optical switching diameter is 10 mm, the threshold voltage is 4.316 V, the saturation voltage is 13.938 V, the turn-on response time is 2.6 ms, the turn-off response time is 96.7 ms, and the contrast is 11.46.The optical switch has the miniature and low-voltage-driven features.

Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degradation of beam quality and currently limits the power scaling of diffraction-limited high power fiber laser. The investigation progress on MI in high power fiber amplifiers is introduced in detail. The characteristics of MI are described and the ways to mitigateMI and increase the threshold power are summarized. The development trends of study on MI in high power fiber amplifier are briefly discussed.

Spatial light modulator (SLM) is a kind of optical modulating elements for the optical field distribution. It is widely used in optical information processing, beam transformation, output display and many other application fields. With the wide applications of high-resolution SLMs, the microscopic imaging resolution and contrast of phase and amplitude samples are enhanced greatly, which can not only achieve traditional phase microscopic techniques，but also achieve new type microscopic imaging with a more complicated way and flexibility to modulate phase. In microscopy SLM can not only be used to control the sample illumination, but also act as spatial Fourier filters in the imaging path. Some of these flexible applications are reviewed in this article.

Stratospheric airship works in stratosphere about 20 km above ground. It can cruise or belocated in the air, and has become a good platform for reconnaissance, surveillance or track. Photoelectric system is an important equipment on the stratospheric airship. The advantage of remote sensing on stratospheric platform is introduced, especially on detectable distance. The components and principle are introduced according to the typical existing photoelectric systems. Furthermore, the key technique of photoelectric system for stratospheric airship is analyzed, and the design method is explained. This paper can be referenced for the design of such systems in the future.

In recent years, the US Navy has conducted development work on three major types of lasers for potential use on US Navy surface ships, including fiber solid state lasers (SSLs) , slab SSLs, and free electron lasers (FELs). The structure, development and testing of some kinds of shipborne laser weapons are introduced. These laser weapon systems include Laser Weapon System (LaWS), Tactical Laser System (TLS), Maritime Laser Demonstration (MLD) and the Free Electron Laser System (FELS), all of which are based upon those three types of lasers mentioned above. Some critical technological challenges in the development are analyzed.

We review and discuss the methods of efficient frequency convension of broadband Nd:glass laser and its significance and difficulties. Firstly, we present the existing methods such as angular spectral dispersion, chirp-adapted phase-matching, multicrystal quadrature, and retracing point phase-matching in detail. Also we discuss their properties and limitations. On this basis, we propose a simple and efficient broadband frequency tripling scheme by mixing a narrowband pulse and a broadband one in the sum-frequency process. We study the conversion bandwidth numerically and discuss the physical cause of its high conversion bandwidth. The application prospect of this scheme on high power laser device is discussed as well.

Phase microscopy, especially quantitative phase microscopy (QPM), as a non-invasive and non-destructive tool for imaging phase objects, plays an important role for the structure analysis, identification and dynamic behavior analysis of the biological cells. According to the optical path characteristics of phase microscopy imaging recorded, the typical QPM techniques in both cases of on-axis and off-axis interference are analyzed comparatively, and their several important phase retrieval methods are introduced. In addition, the latest research progress of three-dimensional (3D) QPM is given simply. At last, the trend of QPM is predicted.

Based on the electromagnetic response characteristics of split ring resonators (SRRs) and the rigorous electromagnetic field theory, the enhancement effect of terahertz (THz) wave and the resonance characteristics in the metamaterial consisting of split ring resonators are studied. The spatial distributions of the electric field, the electric energy density and the power flow in split ring resonators are mainly simulated. The variation of the electric energy density with different terahertz frequencies is also investigated. Furthermore, the effect of the parameters of the split ring resonators on the enhancement effect of terahertz wave and the resonance characteristics in the metamaterial are analyzed. It can be shown from our simulation results that the enhancement effect of terahertz wave occurs obviously in the gap of the split ring resonators. Not only the electric field near the opening is obviously stronger than that in other regions, especially in the gap of the split ring resonators, but also the maximum of the electric energy density appears in the opening of the SRRs, and the electric field is obviously stronger near the resonance frequency of the SRRs. Moreover, the resonance frequency and the electric energy density vary obviously with the parameters of the SRRs.