Near-field amplitude and phase measurements using heterodyne optical feedback on solid-state lasers

Heterodyne optical feedback on a solid-state laser is experimentally investigated as an efficient tool to characterize coherently near-field evanescent waves. A well-known topography of evanescent field is obtained via a total internal reflection of the light beam emitted by a class B Yb:Er glass laser. A subwavelength size optical fiber tip is scanned to locally probe the resulting evanescent wave in the near field. After a frequency shifting using a pair of acousto-optic modulators, the collected light is optically reinjected to excite the relaxation oscillations of the laser. The resulting dynamical response simultaneously allows very sensitive measurements of the amplitude and the phase of the evanescent wave. Extension of these preliminary results to near-field optical microscopy is suggested and discussed.     

Simultaneous measurement of amplitude and phase in surface second-harmonic generation

The intensity and the phase in surface second-harmonic generation were simultaneously measured by use of an ac optical balanced homodyne detection system. In this system the surface second-harmonic wave was superimposed upon a local oscillator wave generated by a barium borate nonlinear optical crystal and then detected as an interference signal. Extremely high sensitivity of 3 aW (6 photons/s) and high precision were achieved by use of a lock-in amplifier, in which an interference signal of fundamental waves was used as a reference signal. Simultaneous measurement of the intensity and the phase in surface second-harmonic waves generated from native-oxidized Si(111) surfaces is demonstrated.     

Optimal tunability of waveguides based on silicon photonic crystals infiltrated with liquid crystals

In this work we study the optimization of the tunability range in waveguides based on two-dimensional silicon photonic crystal infiltrated with liquid crystal. The analyzed structure consists of a two-dimensional silicon photonic crystal with a triangular lattice of circular holes where a line of scatterers in the direction Γ–K has been replaced by a line of circular holes with different radius infiltrated by E7 liquid crystal. To this end, we use the plane-wave expansion method considering anisotropy and modelling supercells to account for the lattice defects that define the waveguide. Finally we study the field distributions of the guided modes in order to analyze their symmetries and confinement.     

A continuous-wave ultrasound system for displacement amplitude and phase measurement

A noninvasive, continuous-wave ultrasonic technique was developed to measure the displacement amplitude and phase of mechanical structures. The measurement system was based on a method developed by Rogers and Hastings ["Noninvasive vibration measurement system and method for measuring amplitude of vibration of tissue in an object being investigated," U.S. Patent No. 4,819,643 (1989)] and expanded to include phase measurement. A low-frequency sound source was used to generate harmonic vibrations in a target of interest. The target was simultaneously insonified by a low-power, continuous-wave ultrasonic source. Reflected ultrasound was phase modulated by the target motion and detected with a separate ultrasonic transducer. The target displacement amplitude was obtained directly from the received ultrasound frequency spectrum by comparing the carrier and sideband amplitudes. Phase information was obtained by demodulating the received signal using a double-balanced mixer and low-pass filter. A theoretical model for the ultrasonic receiver field is also presented. This model coupled existing models for focused piston radiators and for pulse-echo ultrasonic fields. Experimental measurements of the resulting receiver fields compared favorably with theoretical predictions.     

Low-loss amorphous Si waveguides with gradient refractive index cladding structure

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

Spectral amplitude and phase measurement of ultrafast pulses using all-optical differential tomography

We demonstrate a simple, all-optical, fiber-based method for characterizing the spectral amplitude and phase of ultrafast pulses using a differential tomographic measurement realized via four-wave mixing. The technique is applied to subpicosecond pulses in the C-band of the telecommunication spectrum. Characterization of amplified pulses and propagation through dispersive media is demonstrated and compared with autocorrelation measurements and calculated predictions. We show how our approach can be extended to larger bandwidths in similar systems, extending tomographic reconstruction of coherent fields to nearly an octave of bandwidth while maintaining a robust, waveguide-based geometry.     

正交振幅正关联正交位相反关联光束的贝尔态直接测量

提出了一种新的EPR光束关联度直接测量的方法.这种方法可以不用本地光直接测量具有正交振幅正关联、正交位相反关联特性的EPR光束的关联度.然后提出了在这种测量方法基础之上的量子密集编码实验方案.     

The measurement of amplitude and phase characteristics of time-varying optical inhomogeneities in transparent media

The solution of the phase problem in optics is considered as applied to the problems of studying time-varying amplitude and phase characteristics of a medium with the use of the spectral modulation method, in particular, for ultrashort times. The analysis is carried out by way of transilluminating the medium or the object under study with a probing optical signal with a known structure. The information required is extracted by directly recording intensity distributions for the spectrum of the probing signal transmitted through the medium and for the spectrum of the signal transmitted through the medium and subjected to additional modulation formed in a special way. The modulation should provide, to some extent, a visualization of the phase information. Two varyings of the analysis are considered. The first varying is related to the action of the medium under study on probing radiation in the form of its temporal modulation. The second varying is associated with the study of media whose action on radiation leads to redistribution of radiation in time and is described by convolution.     

Si_3N_4波导上的费涅尔透镜

本文提出一种新型的费涅尔波导透镜.它不仅具有衍射受限的聚焦特性,而且有与理想费涅尔波导透镜几乎一样的效率,并使工艺简化.在氮化硅波导上进行的工艺实验表明,实验结果与理论分析具有较好的一致性.     

近场X射线位相衬度成像及位相恢复算法

理论分析了X射线衍射位相成像和近场位相恢复算法。定义了一个最佳成像距离，给出了基于最佳成像距离和特征空间频率的衍射场光强分布新的表达形式。基于模拟位相物体给出了数值模拟结果。由此得出了探测距离和多色辐照对相衬图像和位相恢复结果的影响。本文给出的结果将对同轴x射线相衬成像实验具有一定的指导作用。     

电压调控多种金属膜波导特性研究

采用自由空间耦合的方法,激发双面金属包覆波导中的超高阶导模,通过在双面金属包覆介质波导的两个金属膜上加不同的电压,改变导波层铌酸锂的厚度和折射率,得到不同参数下的衰减全反射谱,从而验证超高阶导模的偏振不灵敏性及对导波层厚度和折射率十分灵敏的特殊性质,实验结果与理论相吻合。比较相同条件下反射率的变化,得到铝包覆波导的灵敏度要高于金包覆波导。所得结果对集成光电子器件的研制有一定的参考价值。     

Dispersion tailoring and soliton propagation in silicon waveguides

The dispersive properties of silicon-on-insulator (SOI) waveguides are studied by using the effective-index method. Extensive calculations indicate that an SOI waveguide can be designed to have its zero-dispersion wavelength near 1.5 microm with reasonable device dimensions. Numerical simulations show that soliton-like pulse propagation is achievable in such a waveguide in the spectral region at approximately 1.55 microm. The concept of path-averaged solitons is used to minimize the impact of linear loss and two-photon absorption.     

Soliton fission and supercontinuum generation in silicon waveguides

We show through numerical simulations that silicon waveguides can be used to create a supercontinuum extending over 400 nm by launching femtosecond pulses as higher-order solitons. The physical process behind continuum generation is related to soliton fission, self-phase modulation, and generation of Cherenkov radiation. In contrast with optical fibers, stimulated Raman scattering plays little role. As low-energy (approximately 1 pJ) pulses and short waveguides (<1 cm) are sufficient for continuum generation, the proposed scheme should prove useful for practical applications.     

Surface emitting characteristics of silicon waveguides

This paper concerns the surface emitting characteristics of silicon waveguides in the millimeter-wave frequency band. The waveguides used in the experiment are rectangular slabs of high resistivity silicon (30,000 ohm-cm). A series of periodic perturbations on the waveguide surface provide a radiating antenna. A rectangular grating with a period of =1.8 mm, a height of 0.35 mm, and a duty cycle of 0.46 was sawn into the top surface of a silicon waveguide with a width of 3 mm and a height of 1.41 mm. Experiments were performed to measure the attenuation, dispersion and the radiation characteristics of the waveguides. The test setup was used to monitor the frequency, radiation angle, and the radiated power. Measurements are made over a band of frequencies around the second Bragg frequency. The detector was scanned from 88–95 GHz and changes were observed in the attenuation constant, dispersion relation and the far field radiation pattern. From these results we were able to verify the grating theory.Supported in part by the Army Research Office.     

Spontaneous Raman scattering in ultrasmall silicon waveguides

We report spontaneous Raman scattering at 1550 nm in ultrasmall silicon-on-insulator (SOI) strip waveguides of 0.098-microm2 cross-sectional area. The submicrometer-scale dimensions provide tight optical confinement and, hence, highly efficient Raman scattering with milliwatt-level cw pump powers. The prospect of Raman amplification in such a deeply scaled-down waveguide device in the presence of various loss mechanisms, particularly free-carrier loss that arises from two-photon absorption, is discussed, and the feasibility of high-gain SOI-based fully integrated optical amplifiers is shown.     

Direct measurement of fringe amplitude and phase using a heterodyne interferometer operating in broadband light

We describe a simple heterodyne method for measuring fringe amplitude and phase difference using an interferometer operating in broadband light. A demonstration system is presented using a Michelson-type interferometer with a bandwidth of greater than 300 nm.     

Single-mode condition for silicon rib waveguides with trapezoidal cross-section

Single-mode condition for silicon rib waveguides with trapezoidal cross-section was obtained using a numerical method based on imaginary-distance beam propagation method with non-uniform discretization. Both quasi-transverse-electric and quasi-transverse-magnetic modes were investigated. Simulated single-mode condition is given by a modified equation. Comparison with reported results shows that the Marcatili’s method is in a better agreement with our results.     

Single polarization transmission in pedestal-supported silicon waveguides

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.     

FWM in silicon nanocrystal-based sandwiched slot waveguides

The observation of four-wave mixing in a 6 mm long sandwiched slot waveguide filled with Si-nc/SiO₂ is reported for optical powers usually employed in telecommunication systems. A −47 dB conversion efficiency is measured in fabricated waveguides for input signal powers around 12 dBm on chip and a waveguide length of 6 mm. Furthermore, the calculated non-linear coefficient is found to be n₂ = 2.67 × 10⁻¹⁷ m²/W. It is also expected that, by using longer waveguides, it would be possible to achieve wavelength conversion.