Kun-Huang Chen

Measurement of the pretilt angle and the cell gap of nematic liquid crystal cells by heterodyne interferometry

This work proposes a simple method, based on the crystal rotation technique and heterodyne interferometry, to simultaneously determine the pretilt angle and cell gap of nematic liquid crystal cells. When heterodyne light passes through a nematic liquid crystal cell, the phase retardation given by the characteristic parameters of the cell can be measured accurately by heterodyne interferometry. This phase retardation relates to the pretilt angle, cell gap, and angle of incidence on the cell. By using the measured phase retardations at two incident angles, the pretilt angle and cell gap of the nematic liquid crystal cell can be estimated by numerical analysis. This method is feasible, requiring only two incident angles and prior knowledge of two characteristic parameters--extraordinary and ordinary refractive indices of the liquid crystal. It is characterized by the advantages of simplicity of installation, ease of operation, high stability, high accuracy, and high resolution.

Multiport optical circulator by using polarizing beam splitter cubes as spatial walk-off polarizers

Optical circulators are necessary passive devices applied in optical communication systems. In the design of optical circulators, the implementation of the function of spatial walk-off polarizers is a key technique that significantly influences the performance and cost of a device. This paper proposes a design of a multiport optical circulator by using polarizing beam splitter cubes as spatial walk-off polarizers. To show the feasibility of the design, a prototype of a six-port optical circulator was fabricated. The insertion losses are 0.94-1.49 dB, the isolations are 25-51 dB, and return losses are 27.72 dB.

Measurement of small displacements with polarization properties of internal reflection and heterodyne interferometry

This study proposes a method for measuring small displace- ments. To this purpose, a circularly polarized heterodyne light beam, reflected from a mirror, impinges into a semispherical prism in a radial direction and is reflected at the flat base of the prism passing then through a properly oriented analyzer for interference. The phase differ- ence between s- and p-polarized light results are sensitive to the imping- ing angle when it is equal to the internal reflection polarization angle. If the mirror is displaced, then it causes a small variation of the impinging angle and phase changes. The phase difference obtained allows evalu- ating the mirror displacement through heterodyne interferometry. The feasibility of this method was demonstrated, and the displacement mea- surement resolution is 46 nm.

Heterodyne moiré surface profilometry

In this study, a novel moiré fringe analysis technique is proposed for measuring the surface profile of an object. After applying a relative displacement between two gratings at a constant velocity, every pixel of CMOS camera can capture a heterodyne moiré signal. The precise phase distribution of the moiré fringes can be extracted using a one-dimensional fast Fourier transform (FFT) analysis on every pixel, simultaneously filtering the harmonic noise of the moiré fringes. Finally, the surface profile of the tested objected can be generated by substituting the phase distribution into the relevant equation. The findings demonstrate the feasibility of this measuring method, and the measurement error was approximately 4.3 μm. The proposed method exhibits the merits of the Talbot effect, projection moiré method, FFT analysis, and heterodyne interferometry.

Employing dual-saturable-absorber-based filter for stable and tunable erbium-doped fiber ring laser in single-frequency

In this demonstration, a stable and wavelength-tunable erbium-doped fiber (EDF) ring laser using dual-saturable-absorber-based (DSAB) filter inside loop cavity is proposed and experimentally investigated. The proposed DSAB filter not only can filter the side-mode in single-frequency output, but also can obtain the flattened output power spectrum within 1 dB variation in the effectively range of 1529 to 1563 nm. In addition, the output stabilities of wavelength and power are also measured experimentally and discussed.

Optical method for measuring optical rotation angle and refractive index of chiral solution.

Based on the phenomena of Brewsters angle and the principles of common-path heterodyne interferometry, we present an optical method for measuring the optical rotation angle and the refractive index of a chiral solution simultaneously in one optical configuration. A heterodyne light beam and a circularly polarized heterodyne light beam are separately guided to project onto the interface of a semicircle glass and a chiral solution. One of the beams is transmitted through the solution, and the other is reflected near Brewsters angle at the interface. Then the two beams pass through polarization components respectively for interference. The phase differences of the two interference signals used to determine the rotation angle and the refractive index become very high with the proper azimuth angles of some polarization components, hence achieving an accurate rotational angle and a refractive index. The feasibility of the measuring method was demonstrated by our experimental results. This method should bear the merits of high accuracy, short sample medium length, and simpler operational endeavor.

Comparison of Kretschmann-Raether configuration angular and thickness regimes with phase-difference shift for measuring changes in refractive index

We derive the phase difference equation between the p- and s-polarizations of reflection light based on the Kretschmann-Raether configuration. This phase difference equation is used to examine the relationship of the incident angle and metal film thickness versus the phase differences under a small refractive index variation. For a fixed incident angle, the phase difference has a higher measurement sensitiv- ity than the reflectivity change. At the critical angle, there is a higher sensitivity when the metal film thickness is smaller than the skin depth. The surface plasmon resonant SPR angle dominates when the metal film thickness is greater than the skin depth. The phase measuring sen- sitivity at the SPR angle is higher than that at the critical angle by 1 order.

Design of Crystal-Type Multiport Optical Quasi-Circulator

An alternative macrooptic design of crystal-type multiport optical quasi-circulator is proposed. A prototype of six-port optical quasi-circulator was fabricated and tested with insertion loss of 1.97 dB, and isolation of 39.6 dB. The advantages of this design include high performance, being polarization-independent , and resolved polarization-mode dispersion. In addition, the number of ports can be easily expanded.

Simple erbium-doped dual-ring fiber laser configuration for stable and tunable dual-wavelength output

We propose and experimentally investigate a stable and tunable dual-wavelength erbium-doped fiber (EDF) dual-ring laser scheme. Here, two tunable bandpass filters (TBFs) are used inside the dual-ring gain cavity to generate dual-wavelength lasing. And, due to the gain competition of the EDF gain cavity, the mode-spacing (Δλs) of lasing dual-wavelength will be limited at different operating wavelengths. Hence, the minimum and maximum mode-spacing in the proposed EDF laser scheme are 0.75 and 15.35 nm in C-band range. Besides, the output performances of proposed fiber laser have also been studied and analyzed.

Simplified Design of Multiport Optical Circulator With Parallel Connection of Mirror-Image Arranged Spatial- and Polarization-Modules

This letter proposes a simplified design of multiport optical circulator with parallel connection of mirror-image arranged spatial- and polarization-modules. To show the feasibility of the design, a prototype of six-port optical circulator was fabricated. The insertion losses are 0.51-1.05 dB, the isolations are 26.20-45.13 dB, and return losses are 27.72 dB. This design should bear merits of a simple and symmetric structure that is low cost, easily fabricated, polarization-independent, and polarization-mode dispersion resolved, and exhibits high performance.