Embedded fiber-optic Fabry-Perot ultrasound sensor

A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.     

An economical piezoelectric phase modulator for fiber optic sensors

A homemade piezoelectric phase modulator for interfero-metric fiber optic sensors was fabricated using piezoelectric buzzers as strain elements. Six piezoelectric elements were embedded between the two halves of a bakelite cylinder split along its axis and secured tightly together again to form a cylinder. Single-mode optical fiber was then wound around the cylinder to complete the unit. Up to a frequency of 500 Hz, the phase shift produced by the modulator is linearly proportional to the amplitude of the applied voltage. The sensitivity of the phase modulator is about 3.6 rad/V and has a dynamic range of 1,000 rad, which is sufficient for most phase modulation purposes.     

Electrical properties of sol-gel processed PLZT thin films

Sol-gel processed lanthanum-modified lead zirconate titanate (PLZT) thin films consisting of two different perovskite phase contents were fabricated on indium tin oxide coated Corning 7059 glass substrates with two different heating schedules: direct insertion at 650° C for 30 min and at 500° C for 2h. Optical transmittance spectra, polarization versus electric field curves, relative dielectric constant versus frequency and capacitance versus d.c. bias voltage curves of the samples were investigated. The samples showed a good transparency of over 70% and interference oscillation. A thin film consisting of mainly perovskite phase showed a slim loop hysteresis in the polarization versus electric field curve and in the capacitance versus d.c. bias voltage curve, indicating the presence of ferroelectric domains, but a film consisting of mainly pyrochlore phase did not. The dielectric constant and loss factor of the thin film consisting of mainly perovskite phase were about 90 and about 0.2, respectively, at relatively low frequency and showed dispersion of the dipolar polarization of permanent dipole moment in the ferroelectric perovskite phase in the frequency range between 10 kHz and 1 MHz.     

Effects of frequencies of AC modulation voltage on piezoelectric-induced images using atomic force microscopy

Lead zirconate titanate (PZT) thin film is prepared by sol-gel method on Pt/Ti electrode/SiO₂/Si wafer. Local poling is performed on the PZT film using an atomic force microscope (AFM). The topography and piezoelectric-induced (PEI) images on the polarized PZT film are recorded using AFM at piezo-responsive mode, operated with an AC voltage at varying frequencies. The best PEI image was obtained at the frequency around 300 kHz. It is explained that the change of piezoelectric vibrations and input noise signals with the frequency of AC modulation voltage affects the intensity of PEI images.     

Binary phase spatial modulation using photoinduced anisotropy in amorphous As(2)S(3) thin film

We present a method for binary phase spatial modulation that uses photoinduced anisotropy in a chalcogenide amorphous As(2)S(3) thin film and its application to binary phase-only filters in a VanderLugt optical correlator. The time-dependent light-transmission properties of the photoilluminated As(2)S(3) thin film are analytically examined by use of third-order nonlinear polarization theory. Experimental results on optical correlation are discussed.     

Preparation and characterization of preferred oriented PZT films on amorphous substrates

The (001) preferred orientation of Nb-doped Pb(Zr_0.52Ti_0.48) O₃ (PZT) thin films was successfully realized on amorphous glass substrate with LaNiO₃(LNO) as electrode by rf-sputtering method. It was found that the LNO film greatly promotes formation of the PZT film with pervoskite phase on amorphous substrate and the preferred orientation of the PZT film depends strongly on the process of preparation. The experimental results show that the dielectric constant and loss of the PZT films with the (001) preferred orientation are 1308 and 0.042, respectively, at 1 kHz, 0.05 V. The remanent polarization (P_r), saturation polarization (P_s) and coercive field (E_c) are 34.5, 43 C/cm² and 105 kV/cm, respectively. The PZT films also show a 33 kV/cm internal bias field due to its (001) preferred orientation. The piezoelectric coefficient d₃₃ of the PZT film without the poled treatment is about 15 pC/N due to its (001) preferred orientation. The effect of the foreign stress on the piezoelectric voltage response of the PZT/LNO/glass was investigated. The results make us consider using the PZT film as an artificial skin to realize the self-diagnosis of amorphous materials under the action of stress.     

High sensitivity optical fiber current sensor based on polarization diversity and a Faraday rotation mirror cavity

A novel high sensitivity optical fiber current sensor (OFCS) based on polarization diversity and a Faraday rotation mirror cavity is proposed and demonstrated. Comparing with single-channel detection in a conventional OFCS, a signal power gain of 6?dB and a signal-to-noise ratio improvement of over 30?dB have been achieved in the new scheme. The cavity amplifies magnetic field-induced nonreciprocal phase modulation, while the Faraday rotation mirrors suppress the reciprocal birefringence. A linear response is obtained for current amplitude as low as several mA at an AC frequency of 1?kHz.     

Ultrathin Planar Cavity Metasurfaces

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a‐Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two‐level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization‐independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.     

弹光调制器谐振特性研究及其谐振频率自跟踪

弹光调制器是一种由各向同性的弹光晶体和压电晶体组成的高品质因数热机电耦合器件,广泛用于偏振测量、光谱测量等诸多领域。但是在高压谐振状态下,其谐振频率会随着温度变化出现漂移,导致弹光调制器的相位调制幅值不稳定以及驱动效率降低。针对该问题,首先对弹光调制器谐振频率特性进行分析,建立了弹光调制器及其高压谐振驱动电路的复合谐振网络模型,提出了利用谐振网络的幅频特性进行频率跟踪的实现方法,并设计了基于现场可编程门阵列(field programmable gate array,FPGA)的控制测试系统,实现了谐振频率跟踪以及调制幅度的测量。通过测试验证了该方案可有效进行谐振频率跟踪,提高了弹光调制器的稳定性以及驱动效率,测试时长大于90 min,相位调制幅度的标准偏差为0.83% rad。

     

串并联型电阻分压器相角偏差的自校验方法

设计了一种串并联结构的同轴型电阻分压器,基于该分压器提出一种新的相角偏差量值溯源方法。该方法主要通过分压器相角偏差与电阻阻值之间的特殊比例关系实现,通过测量两结构尺寸完全相同的分压器相角偏差的差值实现自校验;设计完成了分压比为100:1,电阻阻值为100 Ω的电阻分压器,通过新的相角偏差溯源方法对该分压器相角偏差在400 Hz到100 kHz频率范围内进行自校验,并对自校验结果进行了不确定度评估,在100 kHz时分压器相角偏差标准不确定度优于30 μrad。     

Optimized optical design of thin-film transistor arrays for high transmittance and excellent chromaticity

Transmittance and chromaticity are essential requirements for optical performance of thin-film transistor (TFT) arrays. However, it is still a challenge to get high transmittance and excellent chromaticity at the same time. In this paper, optimized optical design by using antireflection film theory and optical phase modulation is demonstrated in low temperature poly-silicon (LTPS) TFT arrays. To realize high transmittance, the refractive index difference of adjacent films is modified by using silicon oxynitride (SiO_xN_y) with adjustable refractive index. To realize excellent chromaticity, the thicknesses of multilayer films are precisely regulated for antireflection of certain wavelength light. The results show that the transmittance and chromaticity have been improved by about 6% and 18‰, respectively, at the same time, which is a big step forward for high optical performance of TFT arrays. The device characteristics of the TFT arrays with the optimal design, such as threshold voltage and electron mobility, are comparable to those of conventional TFT arrays. The optimized optical design results in enhanced power-conversion efficiencies and perfects the multilayer film design on the basic theory, which has great practicability to be applied in TFT arrays.     

Secure holographic memory by double-random polarization encryption

A novel optical encryption based on polarization is proposed and applied to a holographic memory system. Original binary data are described as two orthogonal linear polarization states. These input polarization states can be modulated by use of two polarization-modulation masks located at the input and the Fourier planes. Each modulation mask can convert an input polarization state into a random polarization state. Once encrypted, the polarization state is recorded as a hologram. For the decryption, the hologram can generate a vector phase-conjugate beam. When the same polarization-modulation masks are used, the vector phase-conjugate readout can cancel the polarization modulation at each mask, and the original polarization state can be recovered. The encryption of the proposed method is evaluated numerically. We also present experimental results by demonstrating holographic recording in a bacteriorhodopsin film.     

Intensity, polarization, and phase information in optical disk systems

Digital information in optical data storage systems can be encoded in the intensity, in the polarization state, or in the phase of a carrier laser beam. Intensity modulation is achieved at the surface of the storage medium either through destructive interference from surface-relief features (e.g., CD or DVD pits) or through reflectivity variations (e.g., alteration of optical constants of phase-change media). Magneto-optical materials make use of the polar magneto-optical Kerr effect to produce polarization modulations of the focused beam reflected from the storage medium. Both surface-relief structures and material-property variations can create, at the exit pupil of the objective lens of the optical pickup, a phase modulation (this, in addition to any intensity or polarization modulation or both). Current optical data storage systems do not make use of this phase information, whose recovery could potentially increase the strength of the readout signal. We show how all three mechanisms can be exploited in a scanning optical microscope to reconstruct the recorded (or embedded) data patterns on various types of optical disk.     

Influencing factors on the pyroelectric properties of Pb(Zr,Ti)O3 thin film for uncooled infrared detector

Zr-rich PZT thin films were synthesized by metallorganic decomposition and their dielectric and pyroelectric properties were investigated with different ratios of zirconium/titanium and poling condition. All the films became effectively (1 1 1) textured and well crystallized at the annealing temperature of 700 °C. With increasing Zr content, coercive field increased and voltage dependent capacitance curve appeared asymmetrical, indicating the presence of antiferroelectric phase, PbZrO₃, in film composition. The pyroelectric coefficient in the practically applicable temperature ranges of 20–60 °C was found to be maximum for the thin film with 0.85 mol of zirconium in PZT. Further increase in zirconium content led to severe deterioration in pyroelectric properties. The values of pyroelectric coefficient and figures of merit were greatly influenced by poling direction and temperature. The result was explained in terms of electric phase and state of polarization in film.     

Differential phase decoder in a polarized optical heterodyne interferometer

A differential-phase decoder (DPD) together with a polarization common-path optical heterodyne interferometer is set up. Based on this interferometric configuration and a novel balanced-detector scheme, the performance of the quantum-noise-limited differential-phase decoder is demonstrated and analyzed. The minimum-detectable differential phase is on the order of 10(-7) rad/sqrt Hz when a 2.5 mW He-Ne laser is used. Verified experimentally, the DPD is immune to the common-phase noise induced by an electro-optic phase modulator or by thermal disturbance within the interferometer. This signifies that the minimum-detectable differential phase can become 10(-8) rad/sqrt Hz if a 300 mW continuous wave laser is employed instead.     

Structure and electrical properties of (111)-oriented Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 thin film for ultra-high-frequency transducer applications

Ternary lead magnesium niobate-lead zirconate titanate system 0.4Pb(Mg(1/3)Nb(2/3))O(3)-0.25PbZrO(3)-0.35PbTiO(3) (40PMN-25PZ-35PT) thin film with a thickness of 1.5 μm was grown on Pt(111)/Ti/SiO(2)/Si substrate via chemical solution deposition. X-ray diffraction and transmission electron microscopy results suggested the film obtained was highly (111)-oriented. The remanent polarization and coercive electric field of the film were found to be 25.5 μC/cm(2) and 51 kV/cm, respectively. In addition, at 1 kHz, the dielectric constant was measured to be 1960 and the dielectric loss 0.036. The film was observed to undergo a diffuse ferroelectric-to-paraelectric phase transition at around 209°C. The leakage current appeared to depend on the voltage polarity. If the Au electrode was biased positively, the leakage current was dominated by the Schottky emission mechanism. When the Pt electrode was biased positively, the conduction current curve showed an ohmic behavior at a low electric field and space-charge-limited current characteristics at a high electric field.     

Optical rectification effect in nanocarbon films

When a nanocarbon film obtained by plasmachemical deposition is fixed between two parallel electrodes and exposed to the pulsed radiation of a Q-switched neodymium laser, a pulsed electric voltage appears between the electrodes, with the pulse shape repeating the laser pulse envelope. It is shown that the amplitude and polarity of the pulsed voltage strongly depend on the angle of incidence and polarization of laser beam and on the spatial orientation of a carbon film with electrodes relative to the laser beam. The observed phenomenon exhibits all features characteristic of the optical rectification effect. For the optimum spatial orientation of a film, the factor of conversion of the laser pulse power into electric voltage amounted to 500 mV/MW, which is many times greater than the values observed in the case of optical rectification in well-known dielectric nonlinear optical crystals.     

Diffractive optical elements designed for highly precise far-field generation in the presence of artifacts typical for pixelated spatial light modulators

Diffractive optical elements (DOEs) realized by spatial light modulators (SLMs) often have features that distinguish them from most conventional, static DOEs: strong coupling between phase and amplitude modulation, a modulation versus steering parameter characteristic that may not be precisely known (and may vary with, e.g., temperature), and deadspace effects and interpixel cross talk. For an optimal function of the DOE, e.g. as a multiple-beam splitter, the DOE design must account for these artifacts. We present an iterative design method in which the optimal setting of each SLM pixel is carefully chosen by considering the SLM artifacts and the design targets. For instance, the deadspace-interpixel effects are modeled by dividing the pixel to be optimized, and its nearest neighbors, into a number of subareas, each with its unique response and far-field contribution. Besides the customary intensity control, the design targets can also include phase control of the optical field in one or more of the beams in the beam splitter. We show how this can be used to cancel a strong unwanted zeroth-order beam, which results from using a slightly incorrect modulation characteristic for the SLM, by purposely sending a beam in the same direction but with the opposite phase. All the designs have been implemented on the 256 x 256 central pixels of a reflective liquid crystal on silicon SLM with a selected input polarization state and a direction of transmission axis of the output polarizer such that for the available different pixel settings a phase modulation of ~2pi rad could be obtained, accompanied by an intensity modulation depth as high as >95%.     

A Frequency Synthesizer for 10/2? kHz

A sinusoidal voltage source of frequency nominally 10/2? kHz (? = 104 rad/s) is derived from a 5-MHz standard. Digital counters are used to generate the required frequency, which differs from 10/2? kHz by less than 2 parts in 108, and is as stable as the standard. Using a crystal filter and low-distortion amplifiers, an output of 10 V rms with total harmonic distortion less than 1 part in 103 is obtained.     

Polarization mode dispersion measurement based on continuous polarization modulation

A novel technique is proposed for measurement of the group delay (GD) and the differential group delay (DGD) of optical material, components, and fiber. This new method is based on continuous polarization modulation of the stimulus optical field as opposed to sequential polarization state switching used in the traditional phase shift method. A new complete derivation of the phase shift method, based on the modified Jones calculus of elementary matrices, is presented. The derivation reveals that the phase shift measurement actually depends on all eight elementary parameters that represent DGD and optical frequency derivatives of polarization-dependent loss (PDL). Thus, the new expression for the phase shift includes the combined effect of PDL and DGD. The proposed method is evaluated by measuring a section of polarization-maintaining fiber and a 50 km length of single-mode fiber over a wavelength range from 1530 to 1610 nm. Measurements of DGD in a long single-mode fiber are shown to be highly insensitive to environmentally induced GD drift.