A photonic approach for the generation of an amplitude- and phase-modulated microwave signal with tunable frequency and modulation bit-rate is proposed and demonstrated. Two coherent optical wavelengths are generated based on external modulation by biasing a Mach–Zehnder modulator (MZM) at the minimum transmission point to generate ±1-order optical sidebands while suppressing the optical carrier. The two sidebands are sent to a circulator and are then spectrally separated by a fiber Bragg grating notch filter. With one sideband being amplitude-modulated at another MZM and the other being phase-modulated at a phase modulator, a frequency-tunable amplitude- and phase-modulated microwave signal is generated by beating the two sidebands at a photodetector. The proposed technique is investigated theoretically and experimentally. As a result, a 20-GHz amplitude-modulated, 20-GHz phase-modulated, and 25-GHz amplitude- and phase-modulated microwave signals with tunable modulation bit-rate are experimentally generatedOCIS codes: 040.2840, 350.4010, 060.3735.

We propose an efficient method for characterizing the orbital angular momentum (OAM) of an optical vortex with a large topological charge (TC) through distinguishing the interference pattern of the non-uniformly-distributed multi-pinholes using fewer pinholes. This method overcomes the limit on large TC detection by multi-point interferometer and can be used to probe optical vortices with arbitrary sizes. In addition, it also has potential application in measuring light beam with OAM from astronomical sources.

We present a flexible, simple, and cost effective approach for generating high-quality multiple focal spots in the far field using composite spiral zone plates (SZPs), which serve as a synthesis of two SZPs with different topological charges. By changing the topological charges of the SZPs can obtain different types of multiple focal spots. The numerical solution, fabrication method, and experimental results are presented to prove the capabilities of this approach.

We propose a blind quadrature imbalance (QI) compensation algorithm based on the statistical properties of I and Q signals in a receiver. The algorithm estimates the QI parameters of a receiver by calculating the mean, variance, and correlation coefficient of I and Q components. Then, the estimated imbalance parameters are adopted to compensate for the QI in the receiver. Simulation results show that the Q factor is considerably optimized by the application of the QI compensation algorithm in an 80-Gb/s Pol-Mux coherent optical quadrature phase-shift keying (CO-QPSK) system. Compared with conventional algorithms, the proposed algorithm exhibits better performance when the phase deviation from QI exceeds +-15o.

Orthogonal frequency division multiplexing-passive optical network (OFDM-PON) and single-carrier frequency division multiplexing (SCFDM)-PON are promising solutions for future high-speed PON-based access. A polarization division multiplexing scheme with direct detection is proposed for OFDM-PON to effectively reduce bandwidth requirements for components. However, the scheme strictly requires spectrum overlapping of two orthogonal sidebands and the 4×4 multi-input-multi-output (MIMO) algorithm to eliminate complex cross-polarization interference. In this letter, we propose a polarization interleaving (PI) approach that significantly reduces bandwidth requirements for optical and electrical components while achieving a high-flexibility and low-complexity MIMO algorithm. Downstream single sideband PI-SCFDM transmission is experimentally demonstrated.

We report the fabrication of cascaded photonic crystal fiber (PCF) tapers in monolithic design. Flat broadband supercontinuum (SC) generation in cascaded PCF tapers pumped by sub-nanosecond pulses from a 1 064-nm microchip laser is demonstrated. The spectral width (20 dB) extends from 0.47 to 1.67 \mu m. In the optimal configuration, an ultraflat (3 dB) spectrum from 500 to 1 000 nm is achieved.

X-ray tomography of samples containing both weakly and strongly absorbing materials are necessary in material and biomedical imaging. Extending the validity of the phase-attenuation duality (PAD) method, the propagation-based phase-contrast computed tomography (PPCT) of a sample with hybrid compositions of both the light and dense components with 60 keV of synchrotron radiation is investigated. The experimental results show that the PAD-based PPCT is effective in imaging both the weakly and strongly absorbing components simultaneously. Compared with the direct PPCT technique, the PAD-based PPCT technique demonstrates its excellent capability in material discrimination and characterization. In addition, the PAD-based PPCT exhibits a striking performance on the image contrast enhancement and noise suppression. Therefore, this technique is useful for material and biomedical imaging applications, especially when the radiation dose involved imposes a serious constraint.

A multiwavelength Brillouin/erbium fiber laser (BEFL) with low threshold power is realized. A low threshold power of 3 mW and a wide tuning range of 18 nm can be achieved by controlling the reflected power in the nonlinear optical loop mirror (NOLM). Up to 24 lines with a wavelength spacing of 0.086 nm are generated at the Brillouin pump and at the 1 480-nm pump with 0.5 dBm (0.9 mW) and 25 mW of power, respectively.

This study presents an eye-safe, single-mode, nanosecond-pulsed, and all-fiber laser source with master-oscillator-power-amplifier configuration at 1 550 nm that is suitable for high-resolution three-dimensional (3D) imaging light detection and ranging (LIDAR) system. The output peak power of 7.6 kW is obtained at the 1.2-ns pulse width and 50-kHz repetition rate. The single-mode pulse laser output ensures the range precision and imaging results of the LIDAR system. The laser is used as a transmitter for the 3D imaging LIDAR system. The detailed characteristics of the LIDAR system and the results of the 3D imaging are presented.

We experimentally demonstrate a diode-pumped passively mode-locked femtosecond laser with Yb3+-doped yttrium lanthanum oxide ceramic. Mode-locking is achieved by using a semiconductor saturable absorber mirror, and intracavity dispersion is compensated by a pair of SF6 prisms. Laser pulses as short as 357 fs at a central wavelength of 1 075 nm are obtained. The maximum average output power is 670 mW under 4.5 W of pumping power with a slope efficiency of 20%. To the best of our knowledge, this is the shortest pulse generated from Yb-doped yttrium lanthanum oxide ceramic lasers with a sub-500 fs pulse duration.

Self-mixing interferometry (SMI) based on nanometer fringes and polarization flipping is realized. The interferometer comprises a single-mode He-Ne laser and a high-amplitude reflectivity feedback mirror. The nanometer fringes are obtained by tilting the external feedback mirror. The fringe density is 35 times higher than that derived with conventional two-beam interference, and each fringe corresponds to a \lambda/70 displacement in external cavity length. Moreover, polarization flipping occurs when the external feedback mirror moves in the opposite direction. Such movement can be used to easily distinguish displacement direction. Experimental results show an optical resolution of displacement measurement of 9.04 nm with a range of 100 μm. The proposed SMI presents promising application prospects in precisely measuring displacement and calibrating other micro-displacement sensors because of its optical wavelength traceability.

The murine model of hindlimb ischemia is extensively used in studies on the physiology and pathology of ischemia and angiogenesis. Traditional non-invasive evaluation methods, such as laser or ultrasound blood flow perfusion imaging and micro-CT angiography, are limited either by low resolution or toxic exogenous agents. Relying on intrinsic high optical absorption contrast, we conduct label-free imaging of subcutaneous blood vasculature in the hindlimb of murine models by photoacoustic microscopy (PAM). The angiogenesis induced by ischemia in the hindlimb is successfully observed at high resolution in vivo and non-invasively. PAM is a potentially powerful imaging method for studying ischemic diseases and resultant angiogenesis.

Intersubband linear and third-order nonlinear optical properties of conical quantum dots with infinite barrier potential are studied. The electronic structure of conical quantum dots through effective mass approximation is determined analytically. Linear, nonlinear, and total absorption coefficients, as well as the refractive indices of GaAs conical dots, are calculated. The effects of the size of the dots and of the incident electromagnetic field are investigated. Results show that the total absorption coefficient and the refractive index of the dots largely depend on the size of the dots and on the intensity and polarization of the incident electromagnetic field.

Applicability of the angular properties of scatter elements as a tool to achieve improved slow light performance with small group velocity dispersion and large bandwidth in photonic crystal waveguides is investigated. A polyatomic photonic crystal waveguide, including two scatter elements with different geometrical shapes in each primitive cell, is proposed to investigate the feasibility of our method. Numerical results show that a versatile control of the dispersion relation of slow light modes, with large normalized delay-bandwidth products ranging from 0.2085 to 0.3394, can be obtained using a unique geometrical parameter.

InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) are fabricated by a thermal selective wet-oxidation confinement technique. Post-oxidation annealing in a nitrogen environment at high temperatures is then conducted to improve the performance of the oxide-confined InGaAs/GaAs VCSELs. The optimum post-oxidation annealing conditions are determined by changing the furnace temperature and annealing time. Compared with a unannealed laser device, the light output power increases by about 12%. An aging test is carried out to examine the reliability of the annealed oxide-confined VCSEL device. The temperature dependence of the lasing wavelength of the annealed oxide-confined VCSELs is also investigated.

Multicolor manipulation of negative refraction in a three-level atomic system is theoretically investigated. Based on multicolor coherence, the negative refractive index can be obtained with reduced absorption. The refractive index can also be controlled by changing the sum of the phases of the sidebands in the trichromatic driving fields. By adjusting the sum phase, the refractive index can be varied between negative and positive in two different frequency bands. Furthermore, the frequency band corresponding to the negative refractive index is widened by increasing the intensity and the frequency difference of the trichromatic field.

This study shows that the principle of a recently proposed common-path laser interferometer containing a planar grating is nonexistent and apparently caused by a mathematical derivation error. Both p- and s-polarized beams in the proposed setup experience once the +1st-order diffraction and once the –1st-order diffraction by the grating. As a result, the phase of each beam remains unchanged and the interference fringes formed by the two beams are not expected to move when the grating is translated in the grating vector direction. We perform an experiment to confirm this prediction. Both our analysis and experimental observation cast doubt on the experimental results of the authors who proposed the interferometer.

The impact of lens aberrations becomes severe when the critical dimensions (CDs) shrink. The accurate measurement of both low- and high-order Zernike aberrations is important during a photolithographic process. Based on the multi-illumination settings and principal component analysis of aerial images, a novel in situ aberration measurement technique that can accurately measure all the Zernike aberrations, except for the sinusoidal 2-\theta and sinusoidal 4-\theta terms (under polar coordinates, and Z1 to Z4 are not considered) is proposed in this letter. The estimated maximum error of the Zernike aberrations ranges from 0.43 to 0.78 m\lambda when the amplitudes of the Zernike coefficients range from –20 to 20 m\lambda. The standard and root mean square errors are both in the range from 0.14 to 0.4 m\lambda.

We investigate the nonvolatile holographic storage characteristics of near-stoichiometric LiNbO3:Fe:Mn crystals with different Li2O contents. Experimental results indicate that the optimal value of Li2O content is about 49.6 mol%. Nonvolatile sensitivity S′ considerably improved to 0.15 cm/J because of the use of near-stoichiometric LiNbO3:Fe:Mn with 49.6 mol% Li2O.

We present an optical design for a fingerprint scanner that addresses the challenges involved in capturing the prints of rolling fingers. A rolling fingerprint scanner requires a high performance distortion free system with big object space numerical aperture (0.022) and larger capture size (40 \times 40 (mm)). We show how these requirements can be achieved with the approach of optical and computational hybrid distortion correction. In addition, dark background illumination is utilized to increase fingerprint contrast.

A high temperature sensor based on an ultra-abrupt tapered fiber Michelson interferometer fabricated by the fusion-splicing method is proposed. The sensor consists of a single abrupt taper and the cleaved surface is used as the reflection mirror. The thermal characteristic is investigated at 25 to 1 000 oC. The sensitivity of the sensor is observed to vary with the temperature, that is, 25 and 78 pm/ C at 25–300 and 300–1 000 oC, respectively. The Michelson interferometer sensors have the advantages of simple structure, cost effectiveness, compactness, and simple fabrication process.

We present a range-gating delayed detection super-resolution imaging lidar with high accuracy based on the signal intensities of three consecutive delay samples. The system combines the range and signal intensity information from multi-pulse detections to calculate the pulse peak position under the assumption of a Gaussian pulse shape. Experimental results indicate that the proposed algorithm effectively calculates pulse peak position and exhibits excellent accuracy with super-resolution. Accuracy analysis shows that accuracy is best improved by enhancing signal-to-noise ratio, strategically selecting samples, reducing pulse width, and appropriately choosing the delayed periods between samples.

A scheme of multi-wavelength pulse generator using optical frequency comb and arrayed waveguide grating (AWG) is proposed and experimentally demonstrated. A flattop optical frequency comb is shaped into multiple narrowband Gaussian spectra by using an AWG which contains a number of Gaussian channels, and then multi-wavelength optical pulses are achieved. In the experiment, six wavelength pulses with full width at half-maximum (FWHM) of 14.6 ps at 10 GHz are obtained, and two wavelength-interleaved pulse trains at 20 GHz and four wavelength-interleaved pulse trains at 40 GHz are demonstrated by using the multi-wavelength optical pulses. This scheme has flexibility because the pulse width, the repetition rate, and time-interval can be readily controlled.

We build a frequency resolved optical gating (FROG) setup based on the second harmonic generation (SHG) FROG to characterize the mid-infrared (MIR) few-cycle laser pulse in single shot basis. Considering the extremely wide bandwidth, we use 20-μm-thick BBO crystal as the nonlinear medium, and correct the spectral response with the frequency summing efficiency. Spatial splitting is adopted to avoid additional material dispersion. In combination with a 4f imaging, this configuration enables the setup to run in single shot. With the central wavelength of 1.8 μm, the measured pulse has a duration of 9.3 fs, which corresponds to about 1.5 cycles.

A WSi2/Si multilayer, with 300 bi-layers and a 2.18-nm d-spacing, is designed for X-ray monochromator application. The multilayer is deposited using direct current magnetron sputtering technology. The reflectivity of the 1st-order Bragg peak measured at E=8.05 keV is 38%, and the angular resolution (\Delta \theta/\theta) is less than 1.0%. Fitting results of the reflectivity curve indicate a layer thickness drift of 1.6%, mainly accounting for the broadening of the Bragg peaks. The layer morphology is further characterized by transmission electron microscopy, and a well-ordered multilayer structure with sharp interfaces is observed from the substrate to the surface. The material combination of WSi2/Si is a promising candidate for the fabrication of a high-resolution multilayer monochromator in the hard X-ray region.

A real-time method for measuring atmospheric parameters based on co-processor field-programmable gate array (FPGA) and main processor digital signal processing (DSP) is proposed for ground-based telescopes with adaptive optics (AO) systems. Coherence length, outer scale, average wind speed, and coherence time are estimated according to closed-loop data on the residual slopes and the corrected voltages of AO systems. This letter introduces the principle and architecture design of the proposed method, which is successfully applied in the 127-element AO system of the 1.8-m telescope of Yunnan Astronomical Observatory. The method enables real-time atmospheric observations with the same object and path of the AO system. This method is also applicable to extended objects.