Changshui Fang

Fabrication of 4 x 4 tapered MMI coupler with large cross section

MMI coupler with large cross section has low coupling loss between the device and fiber. However, large chip area is required. Recently proposed N/spl times/N tapered MMI coupler shows a substantial reduction in device geometry. No such kind of devices with N>2 has yet been realized up to now. The authors have demonstrated a 4/spl times/4 parabolically tapered MMI coupler with large cross section that can match the SM fiber in silicon-on-insulator (SOI) technology. The device exhibits a minimum uniformity of 0.36 dB and excess loss of 3.7 dB. It represents a key component for realization of MMI-based silicon integrated optical circuit technology.

Polymorphism and metastable growth of DKDP

Abstract Single crystal K ( H 1− x D x ) 2 PO 4 , DKDP or deuterated pottassium dihydrogen phosphate in the low temperature tetragonal form is desired as single crystals for electro-optic applications. Its growth by slow cooling of aqueous solutions is limited by the fact that the temperature interval for cooling is usually limited by the upper temperature set by the tetragonal-monoclinic transition. The dependence of the transition temperature on deuterium concentration in solution has been studied. Careful studies showed that at moderate conditions of supersaturation the metastable tetragonal form persists and can be crystallized in the monoclinic stability field. These results were used to allow growth of good quality crystals over an extended temperature interval including the monoclinic stability region.

Electro-optic and electromechanical properties of poled polymer thin films

A simple interferometric technique has been developed to sequentially measure the piezoelectric and electro-optic (EO) coefficient of the poled polymer films. This technique, similar to the Michelson interferometer, is based on compensating the change of the optical path due to the piezoelectric and electric-optic effect of the poled polymer films by opposite piezoelectric effect of a quartz crystal. Our technique has advantages of simplicity, high sensitivity, and improved accuracies over the previous ones. The experimental values of the EO coefficients agree with the values predicted by the two-level model.

Signal bandwidth of general n/spl times/n multimode interference couplers

Analytical expression of signal bandwidth of general straight and tapered N/spl times/N multimode interference (MMI) couplers is presented. The signal bandwidth is characterized as a function of mode relative energy, mode propagation delay time, and mode pulse broadening in the multimode section of MMI coupler. The model is used to evaluate the signal bandwidth of specific couplers. Results indicate that the signal bandwidth decreases seriously with the increase of channel number and channel guide space. Compared with the straight MMI coupler, the tapered MMI coupler has an improved signal bandwidth.

Optical properties of NAEC-PMMA nonlinear polymeric thin film

Novel guest nonlinear optical (NLO) chromophore molecules (4′-nitrobenzene)-3-azo-9-ethylcarbazole (NAEC) were doped in poly (methyl methacrylate) (PMMA) host with a concentration of approximately 15% by weight. For a useful macroscopic electro-optic (EO) effect, these NLO molecules NAEC were arranged in a noncentrosymmetric structure in the host polymer by corona-onset poling at elevated temperature (COPET). For applying NAEC-PMMA polymer in optical devices such as EO switch, its optical properties have been investigated. The UV/Visible absorption spectra for the unpoled and poled polymer film were determined. The refractive index of the film was also determined from measurements of the coupling angles with the reflective intensity at 632.8 nm wavelength. Using the simple reflection technique, the EO coefficient γ33 value was measured as 60 pm/V at 632.8 nm wavelength. The second-order nonlinear coefficient d33 was characterized by the second-harmonic-generation (SHG) experimental setup and the calculated d33 value reached 18.4 pm/V at 1064 nm wavelength. The relation between the second-order nonlinear coefficients d33 and d13 for the poled polymer film was also discussed in detail and the ratio d33/d13 value was obtained as 3.3.

Synthesis and characterization of thiazolylazo chromophore for nonlinear optical applications

Bathochromic thiazolylazo second-order nonlinear optical chromophore, having 2-hydroxylethyl group in a molecule, has been prepared. The temperature decomposition of the chromophore was measured by differential scanning calorimetry (DSC). The second-order polarizabilities at zero energy and the dispersion of second-order polarizabilities were measured by solvatochromic method. The other parameters of the chromophore were also measured.

Synthesis and characterization of nonlinear optical chromophores containing α-cyan with thermal stability

Abstract Nonlinear optical donor–acceptor chromophores containing α-cyan were synthesized. The decomposition temperature was determined by differential scanning calorimetry, and the absorption spectra were measured. The molecular first hyperpolarizabilities were measured by solvatochromic method, and the dispersion of linear electro-optic effects was firstly measured according to the experimental results of UV spectrum and dielectric constants for three chromophore molecules. And an additional merit of these chromophores has a noticeably high thermal stability and good solvability.

Thermally stable chromophores for nonlinear optical applications

Three thermal organic second-order nonlinear optical chromophores were synthesized. The decomposition temperature was determined by DSC, and the absorption spectra was measured. The second-order polarizabilities at zero energy and the dispersion of second-order polarizabilities were measured by solvatochromic method.

Dielectric relaxation of poled polymer DCNP/PMMA

We have prepared the polymer thin films of a 3-(1,1-dicyanothenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DChTP)/poly (methyl methacrylate) (PMMA) guest-host system by spin coating. In order to investigate their temporal and temperature stability, we have measured their dielectric relaxation spectra including the frequency dependence of the real and imaginary parts of dielectric constants. The investigated frequency ranged from 50 Hz to 10 MHz. The measured temperature range above the glass transition temperature T-g (95 degrees C) of the DCNP/PMMA system was from 95 degrees C to 1250C. By using the Adam-Gibbs model, the temperature dependence of the characteristic time tau above T-g was fitted, and the values of the characteristic times tau below T-g were estimated. The lifetimes of the polymer were evaluated by the Kohlrausch-Williams Watts (KWW) empirical decay model

Formation of scatter particles in KDP (DKDP) crystals

Abstract Scatter particles in potassium dihydrogen phosphate crystal and deuterated potassium dihydrogen phosphate crystals have been observed and described by using transmission electron microscopy. The components were analyzed by electron probe microanalyzer. The formation mechanism of the scatter particles is proposed and discussed.