This research explores the optical properties of a large group of silicon/silica nanofibrous thin films. A picosecond pulse laser was employed for indirect deposition of ablated silicon on glass substrates. Prominent parameters such as laser power, repetition rate, pulse duration and scanning speed were changed to vary the structural and compositional properties of synthesized nanofibrous thin films. Transmission and specular reflection measurements along with material characterization techniques, Raman and FTIR, were employed for better interpretation of the results. By and large, an increase in the values for repetition rate and scanning speed produced a corresponding increase in optical data intensity, while an increase in power and pulse duration produced a drop in the same data. The results show that degree of oxidation and inherent porous structure are driving the light interaction in thin samples, as indicated by the changes in intensity or spectrum shape. Observation of these trends enabled us to apply tunable fabrication procedures to obtain desired groups of nanofibrous thin films. Electron and optical microscopy as well as background knowledge certify tangled nano-wired morphology in most cases. Structures with highly desirable usages, such as in sensing technology, can be optimized by their porosity, density and thickness.

In the current passive optical networks (PONs), we address the challenge of dealing with asymmetric channel conditions and asymmetric data rates in order to meet the traffic demand of optical network unit (ONU) to ONU communications. A joint network-coding (NC) and power domain non-orthogonal multiple access (NOMA) approach (NC-NOMA) is discussed. The novelty of this NC scheme lies in the way the inter-ONUs communicates with an optical line terminal (OLT) at different modulation format. Specifically, in the uplink communications, at each ONU side, multiple data streams from different ONUs are superimposed into one single symbol. Meanwhile, NOMA is considered by adjusting the optical transmission power of ONUs such that the OLT can decode the signals from different ONUs based on successive interference cancellation. At the OLT side, the adding zeros approach at the specific positions of the shorter bit sequence is introduced. In the downlink communications, the ONUs obtain their own data by decoding the received messages. By applying NC-NOMA, the ONU with better channel condition only needs to demodulate the fictitious symbols with a low order modulation format instead of demodulating the symbols with a high order modulation format. The simulation results show the NC-NOMA with hierarchical adding zeros approach outperforms the NC-NOMA with traditional zero padding approach in terms of having lower average end-to-end BER of ONUs with no additional complexity.

Extracting the information of a random surface based on its speckle field is an important part of non-destructive measurement. To the best of our knowledge, few have emphasised the correspondence between the topography of a random surface and its speckle field. In this paper, Finite Difference Time Domain (FDTD) Solutions software is used to calculate the speckle field at the position of the maximum height of a random surface, called the interface speckle field. An interface speckle field is also extracted experimentally. The results are consistent with the theoretical calculations. The correspondence between the physical quantities of the interface speckle field and the topography of the random surface and the influence of various factors on the correspondence are discussed in detail. Studies indicate that when the lateral correlation length of random surfaces or the wavelength of incident light increases and the roughness or the refractive index of the random surfaces decreases, the correspondence improves. The topography of a random surface is almost identical to the phase distribution of its interface speckle field when its refractive index is small or its roughness is on the order of nanometres.

The single-wall carbon nanotubes (SWCNTs) with the specific chirality (6,5) and (7,5) were sorted out by the density gradient ultracentrifugation (DGU) method. 2D photoluminescence excitation-emission map and the Raman spectrum confirmed the successful chirality-selection. The nonlinear optical performance of the chirality-selected SWCNTs was investigated by an open-aperture Z-scan setup. By employing the prepared SWCNTs saturable absorber (SWCNTs-SA), the minimum pulse duration of 115 ns with a pulse repetition rate of 550 kHz was obtained in the passively Q-switched (PQS) laser operation.

In this paper, we propose a high-resistance (Hi-R) liquid crystal (LC) micro-lens arrays (MLAs) with a low operating voltage and a controllable focal length for integral imaging (II) 3D display. The focal lengths of the Hi-R LC MLAs can be effectively controlled by applying different operating voltages and driving frequencies. The experimental results indicate that the focal lengths can be adjusted from 4.27 mm to 2.88 mm when the operating voltage applied to the Hi-R LC MLAs increases from 1.8 Vrms to 2.8 Vrms at the driving frequency of 130 kHz. Compared with the II 3D display by using ITO circular-hole electrodes-based or Al circular-hole electrodes-based LC MLAs without Hi-R layer, the II 3D display based on the Hi-R LC MLAs with Al circular-hole electrodes can reveal clear 3D scenes at the corresponding center depth plane (CDP) when a low operating voltage is applied to the Hi-R LC MLAs. Furthermore, the generated elemental images with different depths can be reconstructed at different CDPs by electrically controllable focal lengths of the Hi-R LC MLAs, and these 3D scenes can be realized the switchable display in the same II 3D display system, which can effectively present the optical performance of the II 3D display.

In the digital holographic microscopy, the phase aberration is a key problem limiting the quantitative phase measurement, especially for the microstructure whose phase distribution is dense with few and discontinuous background region. A numerical phase aberration compensation method with global curve fitting preprocessing and automatic extraction of the background region for microstructure testing is proposed. The global curve fitting is implemented to compensate most of the tilt, quadratic and partial high order aberrations and make the profile of the measured object significantly distinguishable. Then the background region is automatically extracted by directly segmenting the converted grayscale phase image. At last, the Zernike polynomial fitting is performed on the extracted background region, and the obtained coefficients are used to calculate the phase aberration of the whole phase image. Simulation and experimental results demonstrated that the proposed method is able to accurately extract background region and compensate the phase aberration for the complicated dense microstructure.

Electro-optic crystals are extensively utilized to manipulate polarized light. In this paper, a general and accurate three-dimensional polarization ray tracing calculus for characterizing the evolution of polarization state in electro-optic crystals is reported for the first time. The analytical formulas of the parameters describing the propagation properties of the light passing through an electro-optical crystal with an arbitrary incident direction are derived. The partial interference between the divergent light beams is considered in the proposed calculus. Both experiments and calculations on an electro-optic modulator based on a lithium niobate crystal over the azimuth angle of 0∘-360°and the incidence angle of 0∘-3°are presented as an example. The experimental results show very good agreement with the theoretical results.

In this paper, we present a Gaussian to tophat beam shaper (GTBS) based on an all-dielectric metasurface lens for sub terahertz (sub-THz) applications. In order to calculate the required phase profile of the GTBS, we use an analytical procedure based on the geometrical transformation technique. The calculated phase profile is then realized by a silicon (Si) metasurface lens consisting of rectangular-shaped pillars of subwavelength dimensions. Because of large solution domain relative to the operation wavelength, we combined the beam envelope and the finite element methods to simulate the structure with a high precision. By designing an anti-reflection metasurface made up of periodically arranged Si pillars of subwavelength dimensions at the back surface of the beam shaper, we reduced the reflection from the lens surface considerably. The proposed idea is firstly investigated in a two-dimensional (2D) structure composed of an array grooves in a Si substrate and then is extended to three dimensions (3D) by considering a 2D array of subwavelength pillars or holes. The 3D structure is shown to be polarization insensitive. The compactness, ease of fabrication, low transmission loss, polarization independence, and straightforward design of the proposed metasurface beam shaper lens make it a promising choice for the implementation of sub-THz quasi-optical elements especially for standoff imaging systems.

In this work, we show that unequal-arm two-beam interferometers, for example, single-pass Mach–Zehnder interferometer or double-pass Michelson interferometer, illuminated with coherent point source (spherical wave front) or coherent line source (cylindrical wave front), unambiguously determine the sign of the phase shift. Theoretical consideration shows that a pattern with finite diameters of interference fringes is asymmetric with respect to the sign of the optical path difference (OPD) or phase shift (PS). In a short arm of the interferometer, the light passing through the layer of medium, characterized by positive OPD (positive PS), leads to the decrease of the diameter of interference fringe. The layer of the medium with negative OPD increases the diameter of inference fringe. When placing an object in the long arm of the interferometer, the contrary situation is observed. For variants with a coherent linear source, the above results relate to the distances between strait fringes of the same order. Experiment demonstrates that the positive optical path length of a conventional material can be compensated in air by a layer of metamaterial with a negative optical path length located at a distance significantly longer than the wavelength of light.

A modified Henon chaos mapping-based Tone Reservation (TR) method is presented to simultaneously improve the performance and security of orthogonal frequency division multiplexing passive optical network (OFDM-PON) system, which did not need any redundant sideband information. With the aid of high sensitivity of the low bits in the decimal part of chaos, the modified Henon chaos is constructed iteratively by extracting low bits of the previous chaotic value, which enhances the complexity and randomness of chaotic sequence values. And, by utilizing this chaos, the permutation vector can be conducted and is employed to scramble the distribution of reserved tones on the frequency domain and OFDM frames respectively, which can simultaneously achieve the peak-to-average power ratio (PAPR) reduction and data encryption. The feasibility of this scheme has been experimentally proven in 10-Gb/s OFDM-PON system through 20km fiber (SMF) transmission. Experimental results manifest that, compared to the common OFDM-PON system, the proposed scheme can achieve the ∼3dB PAPR reduction and ∼1dB BER improvement.

Transitions to 5G are accompanied by rapid increase in data traffics, creating exciting opportunities for competitive edges in 5G spectral efficiency (SE) solutions. We present the first reported vertical cavity surface emitting laser (VCSEL)-based heterodyne technique in context of four-level pulse amplitude modulation (PAM-4) over 15 km SMF fibre and 12 m radio frequency (RF) wireless link. Two low-cost temperature stabilized VCSELs with an extinction ratio (ER) of 4.6 dB and aggregated bit rate of 7 Gbps OOK data to meet the high data demands in wireless connectivity are heterodyned to dynamically generate low phase noise 37.9 GHz RF for 30 m wireless transmission link. PAM-4 modulation technique is adopted to maximize the RF carrier SE in transmission of 14 Gbps data over 12 m wireless link, doubling the network bit rate. It is the first time a VCSEL-based photonically generated RF carrier system is reported with a doubled network bitrate upgrade functionality in the contest of PAM-4. A receiver sensitivity of -23.19 dBm is attained on the 7 Gbps OOK data with a transmission penalty of 2.62 dB over a 30 m link. However, a 14 Gbps PAM-4 modulation incurred a 6.17 dB penalty over the 12 m wireless transmission link. Heterodyned VCSEL technology offer dynamic RF carrier flexibility while PAM-4 upgrades the RF carrier SE without replacing the network optics.

Speckle deconvolution is a forcefully effective technique for imaging through scattering layers. However, this technique is not suitable for imaging through motional scattering layers, since the motion of the scattering layer after the point-spread-function (PSF) measurement makes the recovered image become blurred. To recover the image, the motion of the scattering layer is analyzed in four dimensions: rotation around the optical axis, translation along the optical axis, rotation around the cross-axis, and translation along the cross-axis. As a result, the speckles output from the scattering layer are distorted in four ways: speckle rotation, speckle scaling, speckle stretch, and speckle shift. According to these speckle distortions, the object image can be sharply recovered again by numerically reshaping the premeasured PSF. Moreover, the motion of the scattering layer can be roughly determined during the PSF reshaping process. This method effectively breaks the limitation of the deconvolution technique and has its meanings.

Future optical integrated circuits can be created by overcoming the diffraction limit of light that imposes a lower bound on the size limit of optical devices and plasmonics can meet the desired requirements of producing very compact optical devices. A plasmonic based compact and broadband polarizer, capable of producing single polarized ray concurrently around the two communication windows of 1.31μm and 1.55μm, based on hexagonal lattice photonic crystal fiber is proposed. The results show that the confinement loss of x- and y-polarized modes are 1014.05 dB/cm and 0.048 dB/cm, respectively, at the wavelength of 1.31μm meaning that x-polarized mode is 20812 times lossier than y-polarized mode. Similarly, the achieved loss ratio between two orthogonal polarized modes is 6439 at the wavelength of 1.55μm. In addition, the filter can also maintain a broad bandwidth of over 800 nm by keeping crosstalk greater than 20 dB for the device length of 100μm. The bandwidth and maximum crosstalk value can be extended by loosing compactness of the device. The simultaneous filtering performance around two communication windows with broad bandwidth and compact in length makes the proposed filter a suitable candidate for optical communication and sensing system.

Polarization holography offers the possibility to register not only the intensity and phase of light, but also its polarization. When the interfering beams are with orthogonal linear polarizations, in addition to the photoinduced linear birefringence in the media, chiral structures are formed. They strongly influence the properties of the recorded polarization diffraction gratings. In the present work, we study photoinduced chirality in thin films, based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt], shortly PAZO. Nanocomposite samples are also studied, prepared from the same azopolymer, doped with TiO2 nanoparticles with size 21 nm. Exciting light from a He-Cd laser (λ = 442 nm) has been used, with input ellipticity varying from –1 (left circularly polarized light) to +1 (right circularly polarized light). The dependences of the output ellipticity and the angle of self-rotation of the polarization azimuth on the input ellipticity are presented. This allows us to estimate the effective value of the photoinduced circular birefringence and to evaluate the diffraction efficiency of a polarization holographic grating recorded with two plane waves with orthogonal linear polarizations.

In this work, a new type of emitting direction tunable slotted laser array for Lidar applications is proposed. By designing the structure of periodic slots, the laser array can achieve tunable emitting direction in a wide range with low vertical divergence angle. The laser array can be directly used as Lidar emitting chip without complex beam reshaping, coupling or mechanically moving parts, which makes it has the advantages of good stability, system simplification and low cost compared with other Lidars. Experimentally, an 11-channel laser array of C band with periodic slots at the output end is fabricated based on AlGaInAs/InP material system. As the slot period changes from 8μm to 18μm, the inclined angles of emission from 66.6° to 20.25° with all the vertical divergence angles less than 3.5° are obtained, and the minimum vertical divergence angle is even 1.3°. Typical optical power is 13.5 mW under 600 mA continuous injection.

Visible light communications (VLC) has been regarded as a promising technology for high-speed indoor wireless accessing since it can offer both lighting and network. However, the spectral efficiency of the VLC system based on orthogonal frequency division multiplexing (OFDM) is always smaller than RF-OFDM because light-emitting diodes (LED) require real-value signals. Pilots occupy the spectrum in proportion for channel estimation(CE) to improve communication quality. Based on this consideration, we firstly present the idea of introducing deep learning technology into the CE scheme in the VLC system and propose a CE scheme based on Deep Neural Networks(DNN) perform as well as conventional CE schemes with fewer pilots. The result of experiments validates the feasibility of DNN-based CE.

The uncertainty principle sets a bound on our ability to predict the measurement outcome of two incompatible observables precisely. The entropic uncertainty lower bound can be improved by considering an additional particle as the quantum memory B which has correlation with the measured particle A. Here, we consider the quantum memory-assisted entropic uncertainty for the case in which the quantum memory and the measured particle are topological qubits. In our scenario the topological quantum memory B interacts with environment. We will study Ohmic-like fermionic and bosonic environments. We also investigate the effect of the fermionic and bosonic environments on the lower bounds of the amount of the key which can be extracted per state by Alice and Bob for quantum key distribution protocols.

We proposed an efficient and universal sensitivity amplification method for most kinds of sensors based on fiber in-line two-beam Fabry–Perot interferometers (FPIs) by using the femtosecond laser exposure. As an example of two beam FPI, a liquid-pressure sensor based on a micro-bubble at the end of the single mode fiber (SMF) was fabricated. To improve the sensitivity, a third refection mirror in SMF near the bubble at a certain distance was fabricated by using femtosecond laser exposure. As a result, the Vernier effect is generated, which improves the pressure sensitivity from −0.414 nm/MPa to −3.973 nm/MPa. Besides, the synchronous measurement of liquid-pressure and temperature can be achieved by monitoring the interference peak and the Vernier envelope at the same time.

We propose a system to generate optomechanically induced transparency (OMIT) by considering two atoms in a cavity. The emission spectrum has been manipulated through dipole–dipole interaction between the two atoms enclosed in an optomechanical cavity [Mirza (2016)]. We consider the same setup and investigated the characteristics of the output probe field. One mirror of the cavity is coherently driven through pump and probe optical fields whereas the other mirror of the cavity has a mechanical oscillator via radiation pressure. We report the interaction of two atoms with the optical cavity which exhibits the OMIT in the output probe field. Based on our theoretical results, we find that the OMIT of the probe field can be manipulated via optomechanical interaction (gmc), the interaction of atoms with a cavity (gac) and the dipole–dipole interaction (J). Interestingly, we find double OMIT at two different frequencies in the presence of three parameters such as gmc, gac and J. In addition, the medium exhibits slow and fast light at two different frequencies.

In this study, we realize an integrated generation of optical Single SideBand with Carrier (SSB+C) signal for microwave photonics applications. It has been achieved using a Micro Ring Modulator (MRM) combined with a cavity based wavelength selective filter. MRM, when applied with an RF input, results in a Double SideBand with Carrier (DSB+C) signal where one sideband is suppressed by applying this signal to the filter. The filter has been created using a single resonance-split self-coupled cavity with an extremely high shape factor of 0.69. The sideband suppression ratio between DSB+C and SSB+C ranges from 16 dB to 55 dB for a frequency range of 4 GHz to 20 GHz. Tunable suppression ratio of 21 dB has been achieved at a fixed frequency of 15 GHz. Dynamic range performance of the generated signal has been evaluated at a noise floor of -156 dBm. The dynamic range remains stable in the range of 1 GHz - 5 GHz at ∼ 80 dB.Hz2∕3.

Cu12Sb4S13 nanocrystals were successfully grown via thermal implantation and applied in a solid-state pulsed laser for the first time. By applying Cu12Sb4S13 as an saturable absorber, a stable passively Q-switched Tm,La:CaF 2 laser was obtained at 2 μm. Under an absorbed pumped power of 3.79 W, an average output power of 445 mW was generated with a maximumrepetition rate of 26.85 kHz and pulse width of 1.24 μs, corresponding to a peak power of 11.75 W and pulse energy of 15.29 μJ. The results demonstrate the potential of Cu12Sb4S13 nanocrystals as economical, practical, and flexible band-gap saturable absorbers.

Several schemes for the electrical pumping of dielectric loaded surface plasmon polariton waveguides are analyzed. Two schemes that are designed to move the electrical contact away from the plasmon mode are compared to a Schottky contact scheme which has the electrical contact at the same metal/semiconductor interface which supports the surface plasmon mode. One scheme involves transporting carriers from lateral contacts via a thin semiconductor pathway. The other scheme involves burying the electrical contact inside the metal surface and having the surface plasmon mode skip over the break in the metal/semiconductor interface. The Schottky contact scheme is shown to have the best confinement of the surface plasmon mode on the gain medium. However, the other two alternative schemes can be engineered to require significantly less gain to overcome metal losses. Current densities required in the non Schottky contact schemes are found to be compatible with current semiconductor laser electrical contacts.

Two new angled half-beam transformation systems (BTSs) for the reduction of the beam-divergence problem in laser-diode arrays with large smiles are investigated. Both simulations and experiments show that the angle of divergence of laser-diode arrays with the parabola-shaped smile and the S-shaped smile can be reduced by more than 37% and 23% respectively, when two angled half-BTSs are used in front of the laser-diode array. Furthermore, a fiber-coupling module, with a 400μm core-fiber and two angled half-BTSs, is fabricated. The optical/optical coupling efficiency increases to more than 85%, which is an improvement by more than 4% over laser diodes that use a full-BTS.

The Berreman analysis for plane wave propagation through an anisotropic metamaterial is modified to derive the transfer function matrix for propagation in such a structure. The eigenvalues of the Berreman matrix, which determine the transfer function, depend on the anisotropy. Using the transfer function, propagation of TM and TE polarized beams in hyperbolic metamaterials is analyzed theoretically, and numerically based on the developed theory. For TM polarization, self-focusing of Gaussian beams in a hyperbolic metamaterial can be analytically explained using the q-parameter approach. Furthermore, the transmission coefficient for plane wave propagation through an anisotropic metamaterial slab is extended to a transmission transfer function, which is then used to analyze the propagation of beams through hyperbolic metamaterials and the ensuing spatial shifts. For TM polarized beams, negative refraction is verified. The main objective of this paper is to provide simple analytical tools for understanding self-focusing and negative refraction in anisotropic hyperbolic metamaterials as an alternative to, and check the validity of, rigorous numerical computations. Propagation of the longitudinally (z-) polarized optical field profile is derived. The transfer function approach should be useful for the analysis of arbitrary beam profiles through composite metamaterials.

The uniformity of temperature distribution of an end-pumped laser can be improved simply and effectively by introducing the correct amount of spherical aberration into the pump system. An iterative model of optical-thermal coupling is proposed to calculate the temperature distribution of the end-pumped laser amplifier. The results showed that, the thermal effect of the laser gain medium in the coupling system composed of spherical lenses decreased significantly compared with the coupling system composed of aspherical lenses. This is proved by a neodymium-doped yttrium orthovanadate (Nd:YVO4) amplification experimental system, which was end-pumped by a fiber-coupled laser diode (LD) with the optimized wavelength of 878 nm. A comparative study of the two pump systems showed that the decrease in the gain medium thermal wavefront peak-to-valley (PV), the thermal lens effect, and an improvement in beam quality are still observed in the pump amplification experiment with spherical aberration. At last, the relationship between the spherical aberration and the output brightness of the amplifier was established, which provides guidance for further optimization of the spherical aberration of coupling pump systems and improving the beam quality.

Light field particle image velocimetry (LF-PIV) based on a single light field camera is a novel technique for the measurement of volumetric velocity field via a single optical perspective. In LF-PIV, the volumetric resolution of the light field camera and the measurement volume location greatly affect the reconstruction location accuracy of the tracer particles, and hence the measurement accuracy of volumetric velocity field. This work presents a quantitative characterization of the volumetric resolution of the light field camera by the use of back projection method (BPM) based on geometrical optics. Through the quantitative characterization of the volumetric resolution in the object space, the specific high-resolution zones for the measurement volume at different optical parameters of the light field camera are determined and the light field camera configuration is optimized With the optimized configuration of the light field camera, the reconstruction of a single particle and multiple particles are performed numerically. The influence of the volumetric resolution of the light field camera on the reconstruction resolution and location accuracy of the tracer particle are further investigated. A proper particle concentration for practical LF-PIV experiments is determined. Results indicate that the volumetric resolution of the light field camera determines the reconstruction resolution of the tracer particle, and the reconstruction location accuracy is relatively high in the high-volumetric resolution zones of the light field camera. The optimization of the measurement volume location is useful for the improvement of the reconstruction location accuracy of the tracer particle.

Terahertz (THz) radiation suffers severe signal loss due to the high absorptivity of water vapor abundant in normal atmosphere, greatly limiting its potential in noninvasive material characterization. We propose a novel THz signal processing method that enables effective extraction of hidden sample information leading to accurate thickness determination. The thicknesses of multiple silicon wafers are measured using only a single THz pulse obtained in ambient atmosphere without any prior sample information. The results verify our proposed approach to achieve accurate and precise characterization of materials in a realistic environment.

The gold pulse compression gratings (PCG) used in the Bidirectional Grating Compressor (BGC) suffers a nano- and femtosecond dual pulses irradiation. In the case that the ns pre-pulse comes 10 nanoseconds early before the fs pulse, experiments were conducted to investigate damage behaviors under the dual pulse irradiation. 1-on-1 laser induced damage threshold (LIDT) of the ns-fs dual pulses and fs single pulse were tested and compared. Scanning Electron Microscope (SEM) was employed to capture damage onset and exam the damage macro- and micro-morphologies. The mono-shot method, comparison between the damage zone and spatial distribution of laser pulses, was employed to analyze the difference of the damage onset fluences for the two cases thoroughly. It was found that there is no degradation of laser damage resistance due to the ns pre-pulse. Instead, a slightly increase (∼10%) of fs LIDT caused by the ns pre-pulse was found.

We present the Simplified Lissajous Ellipse Fitting (SLEF) method for the calculation of the random phase step and the phase distribution from two phase-shifted interferograms. We consider interferograms with spatial and temporal dependency of background intensities, amplitude modulations and noise. Given these problems, the use of the Gabor Filters Bank (GFB) allows us to filter–out the noise, normalize the amplitude and eliminate the background. The normalized patterns permit to implement the SLEF algorithm, which is based on reducing the number of estimated coefficients of the ellipse equation, from five terms to only two. Our method consists of three stages. First, we preprocess the interferograms with GFB methodology in order to normalize the fringe patterns. Second, we calculate the phase step by using the proposed SLEF technique and third, we estimate the phase distribution using a two–steps formula. For the calculation of the phase step, we present two alternatives: the use of the Least Squares (LS) method to approximate the values of the coefficients and, in order to improve the LS estimation, a robust estimation based on the Leclerc’s potential. The SLEF method’s performance is evaluated through synthetic and experimental data to demonstrate its feasibility.

The self-focusing and pulse sharpening of an intense short pulse laser in n-type indium antimonide, a semiconductor with non-parabolic energy band, is studied. The nonlinearity arises through the dependence of electron effective mass on quiver velocity and gives effective permittivity a saturating function of intensity. A laser beam with Gaussian transverse intensity profile undergoes periodic self-focusing. The faster self-focusing of the peak of the pulse causes significant sharpening of the pulse in course of propagation. For 10.6μm CO2 laser of picosecond pulse length and intensity ∼1 – 2 GW/cm2, significant self- focusing and pulse length shortening occurs over a length of 0.6 mm.

We present a two-dimensional (2D) optical phased array (OPA) with 8 × 8 elements in which light is electro-optically controlled by silicon phase modulators. The randomly distributed initial phases are calibrated through an interference technique to achieve a beam divergence of 0.92∘ × 0.32∘ and a scanning range of 8.9∘ × 2.2∘. A bandwidth of 324 MHz and beam steering at a point-to-point speed of 20 MHz was realized on the fabricated chip. Systematic analysis revealed that the 2D electro-optical OPA is promising and feasible for many applications ranging from optical communication to light detection and ranging (LiDAR).

Underwater wireless optical communication (UWOC) is an emerging technology for discovering, exploiting, and protecting various underwater resources. Due to the diverse water clarity conditions, light signals are expected to propagate in a fashion dominated by the line-of-sight (LOS) to non-line-of-sight (NLOS) regime. To fully benefit from the high capacity underwater internet that UWOC could offer, especially in the presence of turbid water, a system that obviates the stringent requirements for pointing, acquisition, and tracking (PAT) is required. Herein, we demonstrated a robust NLOS UWOC link fully relieving the requirement on PAT. Based on a system design consisting of an ultraviolet (UV) laser for enhanced light scattering and a high sensitivity photomultiplier tube (PMT), we established an NLOS link with a data rate of 85 Mbit/s and a transmission distance of 30 cm using on-off keying (OOK) in emulated highly turbid harbor water. Further, a data rate of 72 Mbit/s is still achieved when the alignment is totally lost, i.e., the pointing directions of the transmitter and receiver are parallel. A longer transmission distance up to 40 m is also envisaged. Our findings will pave the way for a practical, short-reach, NLOS UWOC link in realistic oceanic scenarios.

We report an experimental study on the ultrafast dynamics of coherent longitudinal acoustic phonons (CLAPs) in GaAs single crystals with different orientations by using femtosecond pump-probe technique. Under an above-bandgap pumping at 400 nm, the CLAP wave is generated on the surface of a GaAs crystal with [100] orientation, and then it propagates into the crystal and is detected by the transient reflection of the probe beam at 800 nm. As the pumping intensity is increased, the amplitude and lifetime of generated CLAPs in GaAs crystals are increased and decreased respectively, while the phonons oscillation frequency remains a constant of 44.3±0.2 GHz. Furthermore, the CLAPs frequency is varied in GaAs crystals with different orientations which is determined by the elastic anisotropy. In addition, from the initially opposite oscillation phase of [100] and [111] oriented GaAs, tensile or compressive lattice stresses can be easily determined. Our results can facilitate the understanding of ultrafast phonon dynamics in GaAs single crystals.

Circular dichroism (CD) is widely used in bio-sensing, pharmaceuticals, and molecular chemistry. Symmetry breaking is an important method of achieving CD signals. However, the relationship between the magnitude of CD and symmetry breaking remains unclear. In this study, we introduce nanorods into symmetric nanostructures comprising vertical Q-shaped nanostructure arrays (VQNAs) to prove the relativity of the symmetry breaking. The numerical simulations results show that the CD spectra of VQNAs present four modes in the visible spectrum. The coupling between nanorods differs under left- and right-handed circular polarized illuminations, leading to the production of the CD. The CD magnitude is not monotonic when the parameters of VQNAs change, and it is the largest for the structure at an appropriate scale. Besides, VQNAs have stable optical properties. Results can help to understand more deeply the CD mechanism produced by symmetry breaking, and design chiral plasmonic nanostructures.

Visible light communication (VLC) emerges as a promising candidate in vehicle-to-infrastructure (V2I) networks. For VLC downlink between street lamps and a mobile vehicle, received signals tend to experience a time-varying multi-source and multi-path channel condition. This will affect the effectiveness of channel equalization when assuming constant channel characteristics within each data packet. In this paper, in order to track the time-varying mobile VLC channel effectively, we propose a channel estimation method for V2I downlink based on decision feedback with orthogonal frequency division multiplexing. Considering different system parameters, extensive numerical experiments are carried out. Results show that compared with the conventional scheme using only training sequences, adopting decision feedback can achieve higher channel estimation accuracy, thus improving link performance significantly. For vehicle velocity at 15 m/s, bit error rate (BER) < 10 −4 can be achieved at ∼199 Mbps if transmit power per lamp is > 30 W. Moreover, the proposed method is almost unaffected by the change of vehicle moving state, with stable BER achieved.

In this study, Cr doped hexagonal CuGaO2 (CGO) nanoplates (NPs) are fabricated successfully. It is worth noting that an emission band located at approximately 700 nm has been found in the photoluminescence (PL) spectrum of CuGa1−xCrxO2 NPs, which can be ascribed to the deep levels emission caused by the Cr 3d and 4s states. Moreover, six resonance peaks also appear in the PL spectrum, which prove that resonance is formed in CGO NPs cavity. The resonant process and resonance mode were investigated in experiment and theory. As the consequence, mode spacing is approximately 8.5 nm and the mode numbers are 60, 59, 58, 57, 56 and 55 corresponding to the peaks of 691 nm, 699 nm, 707 nm, 715 nm, 723 nm and 731 nm in the PL spectrum. The finite difference time domain (FDTD) solution is also used to study resonance mechanism of CGO NPs. These results reveal that the mode calculations and FDTD simulations well agree with the experimental data. The present work further improves the potential applications of CuGa1−xCrxO2 NPs in the field of optoelectronic devices.

Progress in swept source laser technology and graphics processing unit (GPU) have led to the realization of real time four-dimensional optical coherence tomography (4D-OCT). Various rendering algorithms have been introduced for volumetric OCT leading to enhanced spatial comprehension. These algorithms still cannot provide sufficient visual realism. Shadowing is one of the advanced rendering techniques used to provide realistic spatial comprehension for objects in space. We developed and implemented a shadow extension for ray casting for real-time 4D-OCT, using both a swept source (SS)-OCT and spectral domain (SD)-OCT. Our technique builds upon previously developed ray casting techniques, adding an additional shadowing computation. The shadow extension for ray casting yields greater three-dimensionality of objects via self-shadowing, and also improves depth perception of objects. We imaged a beveled needle hovering over a flat surface and found that the positioning of a beveled needle can be accurately determined in relation to the environment. The shadowing algorithm was implemented using texture memory of the GPU, which realized video rendering rate. The complete processing pipeline requires 47 ms with a volume size of 1024 ×256× 100, with a 100 kHz line rate SS-OCT. This new algorithm may enhance interactive surgical guidance during ophthalmic microsurgery.

We propose a solid-core Bragg few-mode fiber with alternating layers of high/low refractive indices. The refractive index of the low-index layer in the cladding is the same as that of the fiber core. The influences of filling ratio, periodic structure width and core size on neff, Δneff and Aeff are investigated. Bending loss of each guided mode and differential mode delay (DMD) between adjacent spatial modes are also studied at 1550 nm. With appropriate geometrical parameters, the minimum effective refractive index difference (minΔneff) between adjacent modes is 2×10−3 at 1550 nm. Wavelength-dependent performance is analyzed subsequently ranging from 1180 nm to 1630 nm. Numerical simulations indicate that such fiber could support 7 linearly polarized (LP) modes and have good bending insensibility performance. These results illustrate that the proposed fiber could be a promising candidate for mode division multiplexing to enhance optical transmission capacity.

In this paper, we investigate the performance of an Underwater Wireless Optical Communication (UWOC) system employing on-off keying modulation at a data-rate of 500 Mbps over a link-range of 30 m. Transmit/receive diversity schemes, namely Multiple-Input to Single-Output (MISO), Single-Input to Multiple-Output (SIMO) and Multiple-Input to Multiple-Output (MIMO) techniques with and without RS-coding have been employed to mitigate the effects of weak oceanic turbulence and beam attenuation. The novel closed-form analytical bit error rate (BER) expressions of single-input to single-output (SISO), SIMO, MISO and MIMO links for un-coded and RS-coded cases have been computed using the hyperbolic tangent distribution and validated with Monte-Carlo simulation results. The obtained BER results show that the use of (63,51) RS-coded 4 × 5 MIMO UWOC system offers at-least 35 dB of transmit power gain compared with the un-coded SISO UWOC system at a BER of 10−5. Emerging technologies like the fifth-generation (5G) networks and the internet of underwater things (IoUT) will have a high impact on UWOC as these systems require a high degree of information integrity, high data rates and energy efficiency when employed in conjunction with data transfer between underwater vehicles and objects. The proposed RS coded MIMO UWOC system offers high reliability and power efficiency and it has the potential to be gainfully employed in IoUT applications.

In this study, a single-frequency 1064-nm Yb fiber laser tandem-pumped at 1018 nm is demonstrated for the first time. The single-frequency operation of 1064 nm laser is achieved by means of a unidirectional ring cavity together with an unpumped active fiber served as a saturable absorber (SA). A maximum single longitudinal mode (SLM) output power of 231 mW was obtained under the 1018-nm pump power of 1.4 W. The signal-to-noise ratio, spectral linewidth, and relative intensity noise were 72 dB, 1.4 kHz, and –141 dB/Hz, respectively. The spectral linewidth of 1064-nm single-frequency output under 1018-nm pumping was narrowed significantly in comparison with that under 976-nm pumping. Additionally, the laser performance under single- and multi-longitudinal-mode (MLM) 1018-nm pump light was investigated. The result revealed that the SLM pump helped improve the behavior of noise.

As one of the basic components of integrated optics, optical waveguide with the characteristics of low loss and high integration has received more and more attention recently. In this paper we proposed a new optical waveguide structure with high transmission rate based on Lithium Niobate integrated two-dimensional (2D) photonic crystal. The structural modeling and band gap analysis to four kinds of two-dimensional photonic crystals with lithium niobate are carried out by using the plane wave expansion (PWE) method. Compared to the other three structures, the triangular lattice structure of the photonic crystal has a wider infrared band electromagnetic band gap. The dielectric column structure (a=697 nm and r/a=0.228) has a complete TE photonic band gap (PBG) of 1285 nm to 1768 nm, while the air holes column structure ( a=628 nm and r/a=0.351) has a complete TM PBG of 1372 to 1807 nm. The finite difference time domain (FDTD) method is used to simulate the transmission characteristics of the two structures with line defect. By adjusting and optimizing the line defect structure, the transmission rate of the curved waveguide of the two structures is increased by more than 90%, which is of great significance to the study of all-optical integrated networks.

We present a simple yet effective method to realize holographic three-dimensional (3D) display by optimizing arrangement of holograms. After a 3D object is divided into a set of layers in axial direction, these layers are calculated into corresponding sub-holograms by Fraunhofer diffraction. The hologram uploaded on SLM consists of sub-holograms side-by-side. Both simulations and experiments are carried out to verify the feasibility of the proposed method. The difference between Fresnel diffraction and Fraunhofer diffraction in the proposed method is analyzed. Aiming at the problems exposed in the experiments, reconstructed results are discussed and further improved by optimizing arrangement of sub-holograms. The process of 3D modeling is simple and the computational complexity is accordingly reduced. This method may provide a promising solution in future holographic 3D realization.

We report on the fabrication of dual-line waveguides with low propagation losses and single-mode guidance in Pr:CaF2 crystals by applying femtosecond laser inscription. In contrast to the typical highly elliptic modes previously reported in dual-line waveguides, the guiding mode obtained in our work is highly-localized with a near-circular profile at wavelength of 633 nm. The investigations on confocal micro-photoluminescence and micro-Raman imaging enable us to illustrate the effect of different femtosecond laser filamentations on the localized refractive index change in Pr:CaF2 crystals. In company with the mode simulation by finite element method, we further reconstruct the refractive index distribution of the fabricated waveguides. Our results indicate that highly-localized refractive-index increment caused by compressive stress between the lower parts of the filaments is the main mechanism for waveguiding, whereas the damage-induced refractive-index reductions at filaments and in the region between the upper parts of the filaments are responsible for the strong light confinement in both horizontal and vertical directions.

The periodic oscillation of the optical transmittance in the nematic liquid crystals (5CB) doped with azobenzene compound (DR1) between the crossed polarizers has been demonstrated and investigated by monitoring the signal intensity variation of the transmitted light in different conditions of the light irradiation and environment temperature. It is found that the oscillation of the transmittance lasts more than 4 periods before the nematic-isotropic phase transition when the dye doped liquid crystal sample is heated from 20 to 40 °C. The oscillation is attributed to the continuous decrease of the birefringence of the dye doped liquid crystals with the temperature increasing. The oscillation curve shifts forward significantly when the sample is exposed to the pump light irradiation, which indicates that the pump light irradiation induces an additional decrease of the birefringence. A more accurate and comprehensive way for estimating the magnitude of the birefringence of the anisotropic materials is proposed based on the transmittance oscillation phenomenon. The switchable properties and the potential applications of the transmittance oscillation are discussed.

This study aimed to achieve the simultaneous measurement of a mass concentration of a glucose solution and temperature. We proposed an asymmetric dual-taper sensor approach, based on a cascade structure of three-core fiber, which can effectively overcome cross-sensitivity. With this approach, our experiment successfully fabricated a sensor with a distance of 3 mm between the double tapers: One taper was fabricated at the three-core fiber (TCF) near the TCF-multimode fiber junction and the other taper was fabricated at the TCF-MMF junction. Afterwards, the length of the sensor increased by only 1.31 mm, which indicated that the sensor was not easy to break. The interference contrasts of the two interference resonance dips exceeded 16 dB. Considering the feasibility of double-parameter sensing and the improvement of sensitivity, an asymmetric double-taper was designed. The temperature responses to the proposed sensor were tested in the range from 25 °C to 75 °C, and responses of the mass concentration of glucose solution were tested in the range from 1% to 15%. Experimental results showed that the two interference peaks of an asymmetric dual-taper robust structure had different sensitivities to changes in the mass concentration of the glucose solution and to temperature. Moreover, these two parameters can be simultaneously demodulated by use of matrix method.

A high sensitivity wide range surface plasmon resonance (SPR) refractive index sensor based on photonic crystal fiber (PCF) has been proposed in this paper. The finite element method (FEM) is used to calculate and analyte the structure of PCF. The optical properties of the PCF are investigated by adjusting the refractive index of liquid analyte. By optimization, the fiber sensor achieves a high linearity refractive index sensitivity of 0.9979 and 1931.03 nm / RIU over a refractive index range from 1.350 to 1.460 and achieves a short PCF length of 1 mm. It is allowed the sensor to be small enough to be integrated into inspection equipment,and expected to gain important applications in the fields of biological monitoring, environmental monitoring, and chemical production.

This paper evaluates the performance of an energy harvesting based asymmetric radio frequency (RF)/ free space optical (FSO) system with transmit antenna selection (TAS) and received diversity techniques. The source is equipped with multiple transmit antenna and employs TAS to send information to the destination via the relay node. At the same time, the energy-constrained source harvests energy from the relay node through the receive diversity techniques such maximum ratio combining (MRC) and selection combining (SC). Based on this, relay node functions as wireless power transmitting node as well as data receiving node. The RF/FSO channels are subjected to Rayleigh/Malaga (M) distributions with non-zero boresight (ZB) pointing error on the FSO link. Thus, the closed-form expression for the concerned system outage probability is then provided. Through on this expression, Gaussian-Chebyshev quadrature method is employed to obtained the average bit error rate of the under studied system. The accuracy of the derived expressions is validated through Montel Carlo simulations. At high SNR, the results demonstrate an error floor due to zero diversity order. In addition, the results show that the atmospheric turbulence and NB pointing error significantly affect the system performance with the MRC offers better performance compared to SC.

We present an efficient method for quantum estimation of gravitational acceleration in a modulated optomechanical system. We use the squeezed mechanical mode, which could potentially enhance the optomechanical interaction between cavity field and mechanical oscillation into the strong-coupling regime. We show that the strong optomechanical coupling can significantly improve the measurement sensitivity of gravitational acceleration. The proposed scheme can provide a platform for further investigations on precision measurement of gravitational acceleration.

We elaborate an elliptic averaging procedure to estimate astigmatism and defocus of optical transfer functions that allows to reduce the problem to a simpler one of estimating defocus only. Based on approximate closed-form formulas for the zeros of defocus optical transfer functions, we reparametrise the frequency domain and derive a class of nonlinear transformations that turn elliptic null-curves of astigmatic optical transfer functions into circles. Our results make astigmatic images amenable to restoration.

Multiplexing measurement has two well-known drawbacks—photon noise inapplicability and limited denoising capability—making it easily replaceable by a high-performance detector. Using a self-multiplexing with small-order coding matrix, we report a noise-type-free denoising measurement to address the limitations of photon noise and the denoising capability of the detector. Simulations and experiments show that multiplexing could break the limitations caused by detector capability, photon noise, and low light making it an unreplaceable unique solution in some applications.

We present the response of the electron to a weak XUV pulse in the presence of an intense IR 800 nm laser field. Simulations are performed on an aligned molecular ion H2+ by numerically solving the corresponding time-dependent Schrödinger equation. It is found that high order harmonic generation (HHG) spectra exhibit signature of harmonic humps, which is shown to be sensitive to the XUV pulse polarization and frequency, and the time delay between the two pulses. Harmonic humps correspond to the perturbative XUV absorption. The enhancement of harmonic humps in IR fields allows to control electron dynamics by XUV-IR combined fields in HHG processes.

Introducing a reflective deformable mirror into an existing optical system for aberration correction requires a folded optical path and relay optics, increasing of the bulk and cost. This paper proposes a low-cost deformable lens (DL) that can be easily integrated into the existing system for correction of low-order aberrations. It is formed by a transparent medium layer sandwiched between a thick glass plate and a piezo-actuated thin glass plate with a transparent area in the middle. The thin glass plate presses the underlying transparent medium to produce optical path difference for correcting the aberrations of the transmitted light. The fabricated DL prototype accurately reproduced the first 9 Zernike mode shapes. Furthermore, the DL was integrated into a customized microscopy imaging system to correct the aberrations, significantly improving the quality of microscopic images.

There are several methods to focus light behind a scattering medium, but very few use fluorescence light as feedback and can be used without access to the distal side of the scatterer. Among all the wave-front shaping techniques, retrieving the transmission matrix of a scattering material is the only one that allows for focusing on multiple spots after a single set of measurements. Here we propose a method to retrieve the transmission matrix of a weak scatterer using fluorescence light as feedback without access to the distal side. The advantage of this method is that it allows focusing on the whole field of view after a single matrix measurement without access to the distal side of the sample, making it suitable for reflection epi-illumination geometry microscopy.

Metrology based on self-mixing interferometry (SMI) covers a wide range of metrical application such as measurement of distance displacement, vibration, velocity, flow and size of Brownian particles. In general, two SMI parameters namely, optical feedback factor C and linewidth enhancement factor (LEF) α are significant in the above-mentioned measurements. Particularly, the joint estimation of C and α is the key to real-time displacement self-mixing (SM) sensor with sub-wavelength resolution. The application of the approach to the immediate estimation of C and α proposed by Kim et al. (2018) is limited to the moderate and strong feedback regime. The paper presents an approach to the immediate estimation of C for the weak feedback regime, to which the approach by Kim et al. (2018) is impossible to be applied, thus allowing the immediate joint estimation method to be applied to all the regimes. Because the proposed method is incomparably simpler than the previous ones and very efficient in hardware, it will be of great assistance to developing a low-cost real-time SM displacement sensor with sub-wavelength resolution in which complex computation such as optimization are impossible. In addition, the paper discusses the effect of C and α on the accuracy of displacement reconstruction.

Removing particles from the device is a challenging in the micro-field. The traditional method is easy to often result in device damage. Therefore, developing an alternative method to resolve the shortcomings is necessary for micro-contamination removal. In this study, a non-contact method is presented, which is based on the shock wave for removal of particles from substrate surface. A series of experiments of particle removal and simulation mode of shock wave characteristics was performed to investigate the removal mechanisms, particle removal modes, and the influence of relevant parameters (laser energy, particle properties, and gap distance d) on particle removal. By analyzing the experimental and simulation results, the particle removal mechanism is dominated by the shock wave ejection mechanism, which is owned to its strong pressure. The particle removal velocity is closely related to the gap distance d and the laser energy, and the maximum velocity was approximately 0.0138Mach for gap distance d is ∼1.45 mm at the energy of 9.32 mJ. Moreover, it is considered that the detachment will be obtained by the combination of lifting, rolling and sliding modes. Our basic research contributes to a better understanding of the particle removal.

A coherent front-end for X-band multichannel chirped radar based on the phase-synchronous optoelectronic oscillator (OEO) with low phase noise is proposed and experimentally demonstrated. In the experiment, the pulses with an instantaneous frequency decreasing from 9.96 to 9.94-GHz are transmitting and receiving. The corresponding phase noise is about –132.1 dBc/Hz at 10-kHz offset frequency. Thanks to the direct digital synthesizer (DDS), the repetition rate and occupied bandwidth can be flexible tuned. For the simulative three-channel system, the signal-to-noise ratio (SNR) of the received signal was enhanced as 4.73-dB relative to that before pulses accumulation. The proposed method can be potentially applied in the multi-channel radar.

This paper presents new detection threshold methods to improve the On-Off Keying (OOK) receiver scheme. In the paper, three definitions are discussed considering low-mobility, fast-mobility, and non-mobility scenarios: Integration Method (IM), Artificial Neural Network (ANN) Method-1 and ANN Method-2. For non-mobility scenario, we use IM which has interconnected structure since the receiver uses a test signal to determine the threshold value. In low-mobility case, the ANN Method-1 is very successful compared to ideal system which is completely knows the threshold value. According to simulation results, ANN Method-1 significantly improves Bit Error Rate (BER) performance at a 2.25 m distance. Therefore, the communication distance can be increased from 2.25 m to 2.52 m at a BER of 10−3. Moreover, we think that the received optical power can suddenly change depend to dimming level for simulation and practical environments. The ANN Method-1 cannot detect the threshold value when the percent deviation of threshold level is higher than 100%. In order to solve this problem, ANN Method-2 is proposed in the paper. From simulation and practical, it is shown that ANN method-2 is successfully detects the threshold value from received signal for 200% or more deviation. The proposed methods are designed on Field Programmable Gate Arrays (FPGA) board to observe real-time results. From simulation and practical results, it is shown that BER performance of ANN Method-2 is very close to BER performance of ideal receiver scheme.

In this paper, we demonstrate a novel dual-branch multi-layer perceptron (DBMLP) based post-equalizer(PE). Since the two branches of the DBMLP can compensate linear and nonlinear distortion in the received signal, respectively, the complexity of the neural network is reduced and the performance of the post-equalizer is improved. Compared with multi-layer perceptron (MLP) based PE the space complexity of DBMLP based PE reduces 59.5%, and the bit error rate (BER) at 3.1 Gbps reduces 45.5%. The reduction of spatial complexity helps the application of post-equalizer based on neural network algorithms in practical communication systems. Meanwhile, increased BER performance helps underwater visible light communication (UVLC) systems reach higher rates. We achieved 3.2Gbps in the 64 quadrature amplitude modulation (64QAM)-carrierless amplitude and phased modulation (CAP) modulated UVLC system using DBMLP based PE. To the best of our knowledge, this is the highest record of data rate in the field of one single chip blue LED based UVLC system.

We propose a simple and effective method to generate an ultra-long and high uniform optical needle through tightly focusing a radially polarized beam with a pure-phase filter modulation. An optical needle with strong longitudinally polarized field is obtained through creating, shifting and merging multifocal spots on the optical axis. Genetic algorithm is introduced to improve the optimizing efficiency and provide a universal approach for solving multifocal positions simultaneously. The axial full-width at half-maximum (FWHM) along the optical axis has been extended to 20.3λ. More attractively, the 18λ long central part of the needle exhibits a uniformity of 96.7%. The maximum side lobes in axial and radial directions are 2% and 23% respectively. We further explored the maximum uniform length can be achieved in the needle under various amounts of focal spots to be merged, which may provide a reference for designing a uniform optical needle with customized length. The proposed methods offer simple and effective design processes for ultra-long and high uniform optical needles, which may find wide applications in particle acceleration and optical trapping.

An ultra-wideband measurement for arbitrary optical magnitude response is proposed and experimentally demonstrated by using optical carrier-suppressed double sideband modulation and fixed low-frequency detection. Compared with previous optical single sideband modulation-based methods, the proposed scheme doubles the measurement range. Moreover, the response can be detected at a fixed low frequency by incorporating microwave photonic frequency down-conversion. The proposed approach has the merit of measuring arbitrary response avoiding high-speed and wideband detectors. To verify the concept, simulation and experiment are carried out. The magnitude responses of two fiber Bragg gratings, which have bandwidths of 19.8 GHz and 67 GHz, are characterized with a resolution of 10 MHz by using a 43.5 GHz microwave signal source.