A demultiplexing scheme based on semiconductor optical amplifier (SOA) and optical filter for optical time division multiplexing differential quadrature phase shift keying (OTDM-DQPSK) system is proposed and investigated experimentally. With only a common half baudrate electrical clock modulated 33% duty cycle return-to-zero (RZ-33) optical clock signal as pump, this scheme is cost-effective, energy-efficient, and integration-potential. A proof-of-concept experiment is carried out for the demultiplexing of a 2 \times 40-GBd OTDM-DQPSK signal. Error-free performance is demonstrated, and the average power penalty for both channels is about 3 dB.

A quality-of-service (QoS) aware scheme, called precautionary performance monitoring, is proposed to solve the optical impairments and congestion control in coherent optical orthogonal frequency division multiplexed (CO-OFDM) networks. The centralized path computation element (PCE) extensions based on the QoS level are applied to optical performance monitoring in this letter.

The intensity and position of the coupling points in high birefringence (Hi-Bi) fibers can be detected effectively using distributed polarization coupling (DPC) detection. The detection sensitivity can decrease due to mechanical vibration disturbance and environment noise. Thus, a method based on empirical mode decomposition is proposed to detect weak mode coupling points. The simulation and experimental results illustrate that the proposed method can suppress the noise effectively and improve sensitivity significantly. The method can identify coupling points as weak as -60 dB embedded in noise automatically and effectively. The algorithm is applicable for DPC, and the experimental sensitivity is improved by 10 dB.

The nonlinear effects that limit the performance of the multi-frequency probe (MFP) based coherent optical time domain reflectometry (C-OTDR) are investigated. Based on theoretical analysis and experimental results, compared with conventional C-OTDR, when the probe pulse has power gradient within the pulse width, self-phase modulation (SPM) and cross-phase modulation (XPM) are strengthened in the new C-OTDR scheme. The generation of four-wave mixing (FWM) is dependent on SPM and XPM, and with modulation frequency of phase modulator higher than 40 MHz, the stimulated Brillouin scattering (SBS) threshold can be enhanced by more than 5 dB, which benefits the maximum dynamic range of the MFP C-OTDR.

Some analysis of the transient response of the Yb-doped fiber amplifier are performed by solving a set of time-dependent rate and power transfer equations based on finite-difference method. Meanwhile, the variation of time to reach the steady state for upper level population distribution, the forward and backward amplified spontaneous emissions (ASEs) and stored energy on the system parameters including pump power, fiber length, Yb-doped concentration, and core area are numerically simulated, respectively. The results show that, by optimizing pump pulse width, stored energy can reach or even exceed the steady state value of continuous wave (CW) pump. By increasing Yb-doped concentration and core area, stored energy is increased, the ASE is suppressed and the ASE built-up time is postponed. In addition, the experimental results show the validity of the theoretical ASE built-up time. The obtained results can provide important guiding for the optimization of pump pulse width and fiber parameters.

Laser multilayer deposition of Rene88DT superalloy on DD3 single-crystal substrate is conducted. The influences of the crystal orientation of the substrate and the profile of the solid/liquid interface of the molten pool on the deposited microstructure are investigated. A unique strategy is proposed by adjusting the angle between the substrate surface and the substrate crystal orientation. This approach prevents the formation of the turned dendrite, thus obtaining a fully directional microstructure in the deposits.

Nd-doped phosphate glass is the dominant amplifier material used in solid state high average power laser systems. Surface imperfection and subsurface damage (SSD) of the glass, resulting from the optical fabrication process, limit the increment of laser system energy output. Thus, it is important to enhance the surface damage threshold of Nd-doped phosphate glass surface. The in°uence of abrasive size, polishing powder, grinding mode, and chemical treatment on the laser-induced damage threshold (LIDT) of Nd-doped phosphate glass surface is investigated. Results show that the LIDT is affected little by different polishing powders and grinding modes. The LIDT correlates with the abrasive size, which produced different depths of SSD. A suitable acid etching treatment can remove the imperfection and the SSD for improving the LIDT of Nd-doped phosphate glass surface. The combination of several effective techniques and methods, which are low-cost and practical, should be useful to enhance the LIDT of Nd-doped phosphate glass surface.

A quantum cascade (QC) laser-based spectrometer is developed to measure trace gases in air. The proposed spectrometer is tested for N2O, and the results presented in this letter. This system takes advantage of recent technology in QC lasers by utilizing intra-pulse scan spectroscopy, which allows high sensitive measurement. Without calibration gases, the gas concentration can be calculated with scan integration and the corresponding values from the HITRAN04 database. By analyzing the Allan variance, a detection limit of 2 ppb is obtained. Continuous measurement of N2O sampled from ambient air shows the applicability of the proposed system for the field measurements of gases of environmental concern.

A pulsed laser system is realized with graphene employed as a Q-switch. The graphene is exfoliated from its solution using an optical deposition and the optical tweezer effect. A fiber ferrule that already has the graphene deposited on it is inserted into an erbium-ytterbium laser (EYL) system with linear cavity configuration. We successfully demonstrate a pulsed EYL with a pulse duration of approximately 5.9 \mu s and a repetition rate of 20.0 kHz.

We report on the lasing characteristics of a two-color InAs/InP quantum dots (QDs) laser at a low temperature. Two lasing peaks with a tunable gap are simultaneously observed. At a low temperature of 80 K, a tunable range greater than a 20-nm wavelength is demonstrated by varying the injection current from 30 to 500 mA. Under a special condition, we even observe three lasing peaks, which are in contrast to those observed at room temperature. The temperature coefficient of the lasing wavelength was obtained for the two colors in the 80-280 K temperature range, which is lower than that of the reference quantum well (QW) laser working in the same wavelength region.

Amorphous Si waveguides with gradient refractive index cladding structure are proposed and fabricated using plasma-enhanced chemical vapor deposition method. Compared with 6 dB/cm for ridge waveguide without gradient cladding, the propagation loss of the gradient cladding waveguides is less than 1 dB/cm with both TE and TM polarizations.

The transverse electro-optic (EO) modulation system is built based on cubic boron nitride (cBN) single crystals unintentionally doped and synthesized at a high pressure and high temperature. The photoelectric output of the system includes two parts that can be measured respectively and the value of elements in the linear EO tensor of the cBN crystal can be obtained. This method does not need to measure the absolute light intensity. All of the surfaces of the tiny cBN crystals whose hardness is next to the hardest diamonds are {111} planes. The rectangular parallelepiped cBN samples are obtained by cleaving along {110} planes and subsequently grinding and polishing f112g planes of the tiny octahedral cBN flakes. Three identical non-zero elements of the EO tensor of the cBN crystal are measured via two sample configurations, and the measured results are very close, about 3.68 and 3.95 pm/V, respectively, which are larger than the linear EO coefficients of the general III-V compounds.

A high-quality Cr3+:CdWO4 single crystal at a size of approximately \Phi 25 \times 80 mm is grown using the Bridgman method with CdO, WO3, and Cr2O3 as raw materials and their molar ratio of 100:100:0.5. The temperature gradient of solid-liquid interface at growth is approximately 50 oC/cm and the growth rate is 0.05 mm/h. The X-ray diffraction (XRD), absorption, excitation, and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated. Two strong broad optical absorption bands of about 472 and 708 nm are observed, and they are associated with the transitions 4A2->4T1 and 4A2->4T2. The weak 4T2->2E transition (the R-line) at 632 nm is also observed. The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4, 776.8, and 3 427.6 cm-1, respectively, according to the absorption spectra and crystal-splitting theory. A broadband fluorescence at about 1 000 nm due to 4T2->4A2 transition is produced by exciting the samples at 675 nm. After being annealed in an O2 atmosphere, the crystals become more transparent, while the effective light absorption of Cr3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.

To resolve the problem of missed evanescent waves in a beam focusing system, a hyperlens-based beam focusing device is proposed in this letter. This device can convert the evanescent waves into propagating waves, and then a super-resolution spot is formed at the center of the hyperlens. The working principle of the device is presented, and the way in which the material and structural parameters of the hyperlens affect the resolution and transmission is analyzed in detail. A multibeam focusing device is optimally designed, and the simulated results verify that a nanoscale spot with a diameter of 15.6 nm (corresponding to \lambda 0/24, where \lambda 0 is the working wavelength in vacuum) is achieved, which is far less than the diffraction limited resolution with a value of 625 nm (1.7\lambda 0). The device is expected to find numerous applications in optical data storage and nano{photolithography, among others.

A scheme for the photonic generation of frequency-tunable millimeter wave and terahertz wave signals based on a highly flat optical frequency comb is proposed and demonstrated experimentally. The frequency comb is generated using two cascaded phase modulators (PMs) and an electro-absorption modulator (EAM). The frequency comb covers a 440-GHz frequency range, with 40-GHz comb spacing and less than 2-dB amplitude variation. By filtering out two of the comb lines with 50 dB out of the band suppression ratio, high frequency-purity and low phase noise millimeter wave or terahertz wave signals are successfully generated, with frequencies ranging from 40 to 440 GHz.

We show that controlled-controlled-NOT (CCN) operation-based entanglement purification protocol can be further improved. CCN protocol requires Bell state measurements after performing the CCN operations. In the original CCN protocol, the measured states are assumed to be destroyed. However, if controlled-NOT gates are used to perform such Bell state measurements, in some unsuccessful situations of the CCN protocol, one can further purify the two mixed entangled states which are to be measured. In this way, the total efficiency of the CCN protocol is further increased.

Road pavement reflectance is usually assumed to be invariant in short periods of time in some quantitative remote sensing applications. To examine its variability, reflectance sequences of concrete and asphalt pavement are measured in field for half a day in visible and near-infarecd (VNIR) spectral range using dual-beam method. As much as 20.7% and 3.52% of relative changes are found in asphalt and concrete reflectance data at 550 nm, and all VNIR bands demonstrate similar variations found to correlate with both illumination geometry and the relative portion of diffuse irradiance. In this letter, this effect is interpreted from a mathematic view. Further studies are needed to model the dynamics of reflectance physically.

A novel method for online correction of light intensity fluctuation in a practical tunable diode laser absorption spectroscopy (TDLAS) system with wavelength modulation is presented. The proposed method is developed according to the linear relation between peaks at multiple frequencies of sine modulation in the power spectral density of the demodulated second-harmonic (2f) signal and the incident light intensity. Those peaks are demonstrated experimentally and explained as residual power at the first-harmonic and third-harmonic frequencies after 2f demodulation of the residual amplitude modulation signal due to the limited integrating time constant of the lock-in-amplifier. This method can achieve real-time correction of light intensity fluctuations with only little calculation. It can work well in a very large range of light intensity and has great potential applications in the wavelength modulation spectroscopy system.

The continuous monitoring of H2S gas concentration is a common problem in natural gas desulfurization process technology. Tunable diode laser absorption spectroscopy (TDLAS) is a preferred technology for continuous monitoring of gas in industrial sites, because of its high selectivity, high sensitivity, and fast response. We discuss the technical solutions of on-line monitoring of H2S in natural gas desulfurization process technology based on TDLAS, and study the security design of monitoring system in inflammable and explosive areas. We also design a weak photocurrent signal converting circuit and perform experiments on transmission characteristics of different distances. The signal-to-noise ratio (SNR) of laser absorption spectrum does not decrease after the 1 500-m transmission. The detection limit is 300 ppb. The system can be operated stably and reliably, and satisfies the need for continuous monitoring of the H2S in natural gas desulfurization process.

Two detection techniques of broadband terahertz (THz) time-domain spectroscopy-THz air-biased coherent detection (THz-ABCD; from 0.3 to 14 THz) and electro-optical (EO) detection (from 0.3 to 7 THz) - are both performed at several different relative humidity levels. The THz power exponentially decays with the increase in relative humidity. The dynamic range of the main pulse in the time domain linearly decreases as the relative humidity increases from 0% to 40%, and linear fittings show that the slopes are -0.017 and -0.019 for THz-ABCD and EO detection, respectively. Because of the multiple reflections caused by the crystal in the common EO detection, THz-ABCD has better spectral resolution (17 GHz) than that of EO detection (170 GHz). The spectrum of water vapor absorption measured by THz-ABCD is also compared with that measured by the Fourier transform infrared spectroscopy (FTIR).

A new approach is developed to measure the dynamic characteristics of metal sheet under laser shock, including deformation velocity, strain, and strain rate. The detecting laser beam is partially shaded by the target deformation induced by the laser action. A photodiode transforms the received beam intensity real time into an electrical signal which could record the process of the target deformation. The functional relation between the electrical signal and the deformation of the metal sheet is derived. The deformation curve of a thin aluminum and the velocity curve of its deformation are also obtained during the experiment. The results indicate that the average velocity of the elastic deformation of the target can reach 2.999￡103 m/s in the central area. This new method provides an approach in the study of the effect of strain rate on deformation.

We propose the use of a power pulse shape of the widely known optical soliton, corresponding to the hyperbolic secant square function, for both conventional atmospheric optical communication systems and, especially, for new full-optical wireless communications. We analyze the performance of the proposed pulse in terms of peak-to-average optical power ratio (PAOPR) and bit error rate (BER). During the analysis, we compare the proposed pulse shape against conventional rectangular and Gaussian pulse shapes with reduced duty cycle. Results show the noticeable superiority of the proposed pulse for atmospheric optical links.