Unified design of wavelength-independent deep-etched fused-silica gratings

We present a unified design of wavelength-independent deep-etched fused-silica gratings as polarizing beam splitters and polarization-independent two-port beam splitters by using the simplified modal method. By defining unified grating parameters as the ratio of incident wavelength to grating period and the ratio of groove depth to grating period, unified grating structures are found to be approximately wavelength-independent, which is based on the modal view of the accumulated phase difference of two excited propagating grating modes. Diffraction efficiencies given by the rigorous coupled-wave analysis (RCWA) verified this unified design at the wavelength of 1064 nm. It should be noted that this unified design of wavelength-independent gratings is an analytic result, which is impossible to be derived with the well-known numerical RCWA. Modal method is powerful and presents a clear physical picture for us to obtain this unified design. Therefore, this unified design can be used as a general guideline for designing deep-etched fused-silica gratings as beam splitters for practical applications from ultraviolet to near infrared.