Abstract: A fiber assembly (60) for capnography or oxygraphy employing an housing (61), a collimator (64), a retroreflector (67) and a single mode optical fiber (63). Housing (61) including a respiratory gas detection chamber (62). Collimator (64) is rigidly disposed within or detachably attached to housing (61), and retroreflector (67) is rigidly disposed within or detachably attached to housing (61). Collimator (64) and retroreflector (67) are optically aligned within housing (61) across respiratory gas detection chamber (62). Optical fiber (63) is optically aligned with collimator (64) within or external to the housing (61). In operation, optical fiber (63) emits a gas sensing light beam through collimator (64) across respiratory gas detection chamber (62) to retroreflector (67), and optical fiber (63) receives a gas detection light beam reflected from retroreflector (67) across respiratory gas detection chamber (62) through collimator (64) to optical fiber (63).