Theoretical principles and design practices for edge filters and coatings

In this paper, edge filters including short pass (SP) filters and long pass (LP) filters are reviewed in terms of definition and features. The necessity of SP and particularly LP filters and their functions in an optical system are addressed in depth. Principles of defining the OD level in the blocking band are elaborated for different spectra ranges, particularly for long wavelength LP filters. For SP filters, the filter design, performance, and potential applications are discussed. An example is given on a DUV filter via reactive plasma ion assisted deposition of HfO2/SiO2 that suppresses solar background at wavelengths above 300 nm and transmits 260-290 nm radiation. For LP filters, the design principles of the blocking band for the LP filter using the substrate, absorption material in coating, and interference type reflective coating are discussed. Semiconductor materials and doping levels for different bandgap energies and cut-on wavelengths are proposed for blocking band solutions. Examples of design practices cover a broad spectra range including short wavelength infrared (SWIR), mid-wavelength infrared (MWIR), and long wavelength infrared (LWIR). Coating challenges, for example element segregations in the deposition of a compound semiconductor, are discussed. Finally, quality control and related issues are also addressed.