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Unlike a conventional white light source that emits a continuous and broad spectrum of light, the dual wavelength white light emitting diode (LED) generates white light by mixing blue and yellow lights, so there are two distinct peaks in its intensity spectrum. Prior works had shown that the spectral property of the dual wavelength white LED can affect the vertical scanning interferometry negatively if the spectral effects are not compensated. In this paper, we shall examine this issue by modeling the spectral property and variation of the dual wavelength white LED, followed by investigating its effects on the interference signal of vertical scanning interferometry. Instead of compensating the spectral effects of the dual wavelength white LED, we harness its spectral property to improve the performance of a phase-based height reconstruction algorithm in vertical scanning interferometry.

Second harmonic generation (SHG) has been applied to reduce background signals in near-field optical imaging, but this technique is usually limited to samples with strong second-order nonlinear susceptibilities. To overcome this limitation, in this paper, we present a versatile background-free SHG configuration, where it utilizes the second-order nonlinear susceptibility of the probe which essentially functions as a near-field polarizer capable of filtering out the background signal component. In the theoretical analysis, we first model the probe-sample optical interactions at both the fundamental frequency and the second harmonic frequency by using the coupled dipole method. The theoretical model reveals that the proposed versatile background-free SHG configuration requires two conditions. The first condition is that the incident optical field must be s-polarized. The second condition is that the probe must be made of crystals from symmetry class 222, symmetry class 622, symmetry class 422, symmetry class 42m, symmetry class 43m or symmetry class 23. To demonstrate the effectiveness of the proposed versatile background-free SHG configuration, a probe made of deuterated potassium dideuterium phosphate (DKDP) crystal from symmetry class 42m is analyzed numerically. It is shown that when imaging samples with negligible second-order nonlinear susceptibilities, the proposed background-free SHG configuration improves the imaging contrast by more than one-order of magnitude as compared to all other imaging configurations. Moreover, we also investigate the dependence of its performance on other parameters, such as the probe-sample distance, the relative size between probe and sample, and the tilt angle of probe crystal. It is believed that the proposed configuration could be widely used to achieve high contrast near-field optical imaging.

Laguerre-Gaussian beams have become increasingly important in areas like optical manipulation and quantum communications. A good understanding of the accuracy and sensitivity of their displacement measurements are required for these applications. In this Letter, we present a method to analyze and compare the performance of these measurements, and an estimation technique is proposed to improve their measurement accuracies. The effectiveness of our methods is verified by simulation studies.

Optical coherence tomography (OCT) reconstruction by using frequency measurements in the wavelength domain is presented in this paper. The method directly recovers the axial scan by formulating the frequency domain OCT (FD-OCT) into an algebraic reconstruction problem. In this way, the need for interpolation is removed. Then by solving the problem with ℓ1 optimization, the computational load is significantly reduced. It is demonstrated by experiment and simulation that the proposed method can achieve high resolution and longer imaging depth compared to the FD-OCT method.

This paper presents a new method to reduce the topographical artifacts in scanning near-field optical microscopy (SNOM) images. The method uses the harmonics caused intrinsically by the nonlinearity in the oscillation of the SNOM probe even when the probe is working in a normal condition without extra excitation. Using these intrinsic harmonics, the gradient of the received SNOM signal with respect to the probe motion is obtained. Then, taking advantage of a SNOM capable of simultaneously obtaining both the topographical and optical signals, topographical artifacts are calculated from the product of the gradient and the topographical signal, and then removed from the received SNOM signal. The effectiveness of the proposed method is demonstrated experimentally.

A near-field ellipsometry method is presented for nano-scale thin film characterization. The technique fuses the topographic and ellipso-metric optical measurements that are simultaneously obtained by a scanning near-field optical microscopy (SNOM). It is shown that the proposed near-field ellipsometry is able to attain nano-scale lateral resolution and correct artifacts in characterization. The effectiveness of the proposed method is verified by simulation and experimental studies.

A novel lattice structure for the design of two-port optical interleavers is presented. With the proposed lattice structure, the exact linear-phase characteristics of the two outputs of interleavers are guaranteed. The desired spectral characteristics are achieved with appropriate choices of design parameters through an optimization algorithm. It is shown that the linear-phase characteristics of the proposed interleaver are retained even in the presence of design parameter variations. The simulation results verified the effectiveness of the proposed structure and design scheme.

We present an adaptive autofocusing scheme. In this scheme, the focus measure is updated with focus tuning. To achieve this, we construct the focus measure by using image moments and develop an adaptive focus-tuning strategy to estimate the measure in closed loop. It is shown that the adaptive updating of the focus measure enables us to overcome the dependence of autofocusing on the image contents. Such an adaptive closed-loop focusing operation also effectively suppresses both the effect of the noise in optical imaging and the effect of time delay due to image processing time. Therefore a high accuracy of autofocusing is guaranteed. The effectiveness of the proposed scheme is demonstrated by simulations and experiments.