A high finesse optical sensor cell for analyzing a gaseous sample using a light source, comprising: a housing defining a linear optical cavity, having an optical axis extending between a first open end portion and a second open end portion of the housing, the housing comprising: a first set of contact geometry located adjacent to a first end of the optical cavity, and a second set of contact geometry located adjacent to a second end of the optical cavity, wherein at least the first set of contact geometry is radially symmetrical about the optical axis, and wherein the first set of contact geometry defines a planar contact surface that is perpendicular to the optical axis of the optical cavity, the first and second ends of the optical cavity being located within the first and second open end portions of the housing, respectively, and an inlet port and an outlet port for conveying the gaseous sample into and out of the optical cavity; a first highly reflective mirror assembly comprising: a first HR mirror having a concave reflective surface; and a first end cover being configured to cover the open end of the first open end portion and to maintain the concave reflective surface of the first HR mirror in contact with the first set of contact geometry, the first end cover having an opening for receiving an entrant beam of light coincident with the optical axis, a second HR mirror assembly comprising: a second HR mirror having a reflective surface; and a second end cover being configured to cover the open end of the second open end portion and to maintain the reflective surface of the second HR mirror in contact with the second set of contact geometry, the second end cover having an opening for emitting the beam of light to a detector coincident with the optical axis, wherein the center of curvature of the concave reflective surface of the first HR mirror lies on the optical axis of the optical cavity. a housing defining a linear optical cavity, having an optical axis extending between a first open end portion and a second open end portion of the housing, the housing comprising: a first set of contact geometry located adjacent to a first end of the optical cavity, and a second set of contact geometry located adjacent to a second end of the optical cavity, wherein at least the first set of contact geometry is radially symmetrical about the optical axis, and wherein the first set of contact geometry defines a planar contact surface that is perpendicular to the optical axis of the optical cavity, the first and second ends of the optical cavity being located within the first and second open end portions of the housing, respectively, and an inlet port and an outlet port for conveying the gaseous sample into and out of the optical cavity; a first set of contact geometry located adjacent to a first end of the optical cavity, and a second set of contact geometry located adjacent to a second end of the optical cavity, wherein at least the first set of contact geometry is radially symmetrical about the optical axis, and wherein the first set of contact geometry defines a planar contact surface that is perpendicular to the optical axis of the optical cavity, the first and second ends of the optical cavity being located within the first and second open end portions of the housing, respectively, and an inlet port and an outlet port for conveying the gaseous sample into and out of the optical cavity; a first highly reflective mirror assembly comprising: a first HR mirror having a concave reflective surface; and a first end cover being configured to cover the open end of the first open end portion and to maintain the concave reflective surface of the first HR mirror in contact with the first set of contact geometry, the first end cover having an opening for receiving an entrant beam of light coincident with the optical axis, a first HR mirror having a concave reflective surface; and a first end cover being configured to cover the open end of the first open end portion and to maintain the concave reflective surface of the first HR mirror in contact with the first set of contact geometry, the first end cover having an opening for receiving an entrant beam of light coincident with the optical axis, a second HR mirror assembly comprising: a second HR mirror having a reflective surface; and a second end cover being configured to cover the open end of the second open end portion and to maintain the reflective surface of the second HR mirror in contact with the second set of contact geometry, the second end cover having an opening for emitting the beam of light to a detector coincident with the optical axis, a second HR mirror having a reflective surface; and a second end cover being configured to cover the open end of the second open end portion and to maintain the reflective surface of the second HR mirror in contact with the second set of contact geometry, the second end cover having an opening for emitting the beam of light to a detector coincident with the optical axis, wherein the center of curvature of the concave reflective surface of the first HR mirror lies on the optical axis of the optical cavity.