Indium arsenide (InAs) is a direct bandgap semiconductor (E-g = 0.36eV) and is well suited for non-linear optic (NLO) applications. The NLO applications require high linear optical transmission for below-bandgap energy photons in the material. Due to this stringent criterion, there is a need for understanding the fundamental optical absorption mechanisms in the material as well as correlating them with the presence of native defects and impurities introduced during the crystal growth. In this paper, we will present the experimentally measured and theoretically analyzed infrared absorption in an undoped and Te-doped n-type InAs in the wavelength region of 4-20 mu m for horizontal Bridgman grown crystals. Transmission measurements at 300 K were performed on samples of undoped and Te-doped n-type InAs with carrier concentration in the range of 2.6 x 10(16)-1.7 x 10(17)cm(-3). The dependences of various absorption mechanisms as a function of wavelength have been presented. Free carrier absorption by electrons is found to be the dominant absorption mechanism at longer wavelengths (lambda > 9 mu m) for all the samples. At shorter wavelengths (lambda < 9 mu m), for samples with carrier concentration close to the intrinsic carrier concentration (n(i) = 1 x 10(15)cm(-3)), the inter-valence band transitions by free holes contributes significantly to the total absorption. Unique fitting parameters have been deduced to theoretically explain the infrared absorption mechanism in InAs for the above-mentioned carrier concentration range. (C) 2007 Elsevier B.V. All rights reserved.