Volume 27, Issue 2, May 2015
Index of content:
- Technical Articles
27(2015); http://dx.doi.org/10.2351/1.4904834View Description Hide Description
On the basis of the comparison of diffraction efficiencies calculated by the simplified modal method to exact results predicted from rigorous coupled-wave analysis, the accuracy of modal method for subwavelength triangular-groove gratings is evaluated. It is revealed that a larger error exists in smaller grating period and lower groove depth. To enhance the validity of simplified modal method, we consider the reflection loss of propagating grating modes by using the optical thin film theory. Then, we design a highly efficient transmission two-port beam splitter grating with subwavelength triangular-grooves by using the enhanced simplified modal method. The physical mechanism of the designed grating splitter with a high transmitted performance over a broad spectrum and a wide angular spectrum can be explained by using the modal method with consideration of accumulated average phase difference of two excited propagating grating modes. Obviously, the designed splitter with triangular-grooves has higher transmitted diffraction efficiency and more broadband property than that with general rectangular ones. The broadband highly efficient transmission characteristic can be attributed to gradient effective index from air-grating interface to grating-substrate interface.
27(2015); http://dx.doi.org/10.2351/1.4906079View Description Hide Description
By using 800-nm femtosecond laser irradiation and chemical selective etching with hydrofluoric acid, microchannels are fabricated in silicon carbide. The diameter of the microchannel is about 1.5 μm. The morphology of the channel is characterized by using scanning electronic microscopy equipped with an energy dispersive X-ray spectroscopy. The formation mechanism of silicon carbide channels is attributed to the formation of laser-induced structural change zones in silicon carbide and the reaction of the laser-induced structural change zones with hydrofluoric acid. In addition, the influences of the laser average power and scanning velocity on the position of the microchannel are discussed.
27(2015); http://dx.doi.org/10.2351/1.4906127View Description Hide Description
Due to its high electric and thermal conductivity, copper has a wide range of applications. Many of those require an efficient and reliable welding process most preferably performed with laser beams. The low absorptivity of copper at a wavelength of 1 μm together with its high heat conductivity however make remote laser welding of copper a challenging task. With the currently commercially available lasers, welds with penetration depths of several millimeters in copper can only be achieved at comparatively low welding speeds (v < 10 m/min), which, however, leads to numerous defects such as melt ejections and pores. In this paper, we discuss the approach of laser power modulation to stabilize the welding of copper. It is demonstrated that the modulation of the laser power with the appropriate parameters significantly improves the weld quality. The influence of the average power, the modulation amplitude, welding speed, focal diameter, and modulation frequency on the welding quality is discussed.