Methods of dispersion improvement in a fiber-fed spectrograph scheme

Different ways to increase dispersion and spectral resolution of a fiber-fed spectrograph working in NIR range are considered. Optical schemes with composite diffractive units are investigated in details. The first discussed option is use of a pair of volume-phase transmission gratings and the second one is use of combination of a reflective grating and a prism. For each of them a certain optical scheme designed and modeled and comparison of key properties is provided.