High power laser based electro-optic sensor systems are generally bulky. Currently, several of these systems have to be located near exit apertures on airborne platforms due to lack of high damage resistant flexible optical conduits offering high mode fidelity and transmission efficiency. In several cases, system functionality has to be scaled down in a space-constrained environment. Coherent Technologies, Inc. has developed flexible, efficient, metallic rectangular hollow waveguides for use with high-energy laser systems such that they can be re-located to a convenient, safe, and protective location. These ribbon-like waveguides provide >96%/m transmission efficiency with near diffraction-limited performance and high mode fidelity besides transporting high-energy optical radiation. In this paper, damage threshold measurement carried out using aluminum based rectangular hollow waveguides is reported. Measurements at 2 micrometers and 10.6 micrometers wavelengths in 1-m long waveguides with aperture heights of 100 to 250 micrometers have been performed. Damage thresholds greater than 1 GW/cm2 has been measured at 2 micrometers wavelength. Being thin and flexible, these waveguides are low cost, easy to fabricate, and are amenable for integration into fuselage of an airplane. It is anticipated that distributed aperture coherent ladar systems and high power optical directed energy on space/airborne platforms would benefit from this technology.