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A cavity ring-down sensor having an optical path of mirrors and an interface proximate to a mirror with a normal incidence optical fiber coupling. The interface may be a block of transmissive materials having different indices of refraction for receiving a light beam of normal incidence from an opti

A cavity ring-down sensor having an optical path of mirrors and an interface proximate to a mirror with a normal incidence optical fiber coupling. The interface may be a block of transmissive materials having different indices of refraction for receiving a light beam of normal incidence from an optical fiber and directing the beam into the optical path. The sensor may be used applications where the optical fiber needs to be straight into the sensor without unusual bending because of space limitations. One application of this sensor may be for ground water testing via a narrow well casing.

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What is claimed is: 1. A fluid sensor comprising: a plurality of mirrors forming a closed light path; a refraction block having a first surface proximate to the plurality of mirrors; and a light channel attached to a second surface of the refraction block at a normal of the second surface; wherein

What is claimed is: 1. A fluid sensor comprising: a plurality of mirrors forming a closed light path; a refraction block having a first surface proximate to the plurality of mirrors; and a light channel attached to a second surface of the refraction block at a normal of the second surface; wherein the light channel is associated with a structure sufficient for holding the refraction block and the plurality of mirrors in suspension within a sampling environment. 2. The sensor of claim 1, wherein the light channel extends approximately straight from the second surface of the refraction block when the refraction block and the plurality of mirrors are held in suspension. 3. The sensor of claim 2, wherein a light path proceeds through the light channel in a direction approximately perpendicular relative to the second surface of the refraction block, through the refraction block and in the closed light path. 4. The sensor of claim 3, wherein the light channel comprises an optical fiber. 5. The sensor of claim 3, wherein: the refraction block comprises: a first portion having a first index of refraction; and a second portion adhered to the first portion; and the second portion has a second index of refraction. 6. The sensor of claim 3, further comprising: a detector for detecting light of a ring-down from the closed light path; and wherein an amplitude and/or period of a ring-down in the closed light path is for providing information about a fluid in the closed light path. 7. The sensor of claim 6, wherein light for the closed light path is infrared. 8. The sensor of claim 3, further comprising: a enclosure for holding the plurality of mirrors and refraction block; and wherein: the enclosure has at least one opening; the at least one opening is covered with a membrane for permitting gas but not liquid to pass through to the plurality of mirrors; the enclosure is for placing the plurality of mirrors in a well casing; and the well casing has a screen at a certain level for permitting fluid to enter the well casing and be proximate to the membrane of the enclosure. 9. The sensor of claim 8, wherein the plurality of mirrors is situated in a ring laser gyroscope cavity. 10. A cavity ring-down system comprising: a cavity having a ring-like light path; a refractive interface optically coupled to the cavity; an optical fiber having a first end attached to the first surface of the interface; and wherein the refractive interface is for directing a light beam from approximately perpendicular to a first surface of the interface into the ring-like light path; the optical fiber is for providing the light beam to the first surface of the interface; and the optical fiber is associated with a structure for suspending the cavity and the refractive interface within a sample environment. 11. The system of claim 10, wherein the longitudinal dimension of the optical fiber is approximately perpendicular to the first surface of the interface. 12. The system of claim 10, wherein: the optical fiber is approximately straight between a point for holding the structure and the refractive interface and approximately perpendicular to the first surface of the interface, while the refraction block and the cavity are held in suspension. 13. The system of claim 12, wherein: the interface comprises: a first block having the first surface of the interface and having a second surface; and a second block having a first surface adjacent to the second surface of the first block and having a second surface adjacent to the cavity; the first block has a first index of refraction; and the second block has a second index of refraction. 14. The system of claim 13, wherein the ring-like light path comprises a plurality of mirrors. 15. A method for providing a cavity ring-down system comprising: providing a ring-down optical path formed by a plurality of mirrors; providing an interface having a first surface adjacent to the ring-down optical path and proximate to the plurality of mirrors; making the interface with two layers of material having different indices of refraction; and attaching a light channel to a second surface of the interface to provide a light beam into the interface at a direction perpendicular to the second surface of the interface and through the interface into the ring-down optical path wherein the two layers are mounted against each other with their common face at an angle relative to the direction perpendicular to the second surface. 16. The method of claim 15, further comprising: designing the layers with dimensions so that the interface can guide an incoming light beam, perpendicular to the second surface of the interface, through the interface into the ring-down optical path. 17. The method of claim 16, wherein the light channel comprises an optical fiber. 18. The method of claim 17, further comprising: providing an enclosure for the ring-down optical path; and connecting a supporting line to hold the enclosure and interface; and wherein the optical fiber is approximately straight from the interface; and the enclosure comprises a portion having an opening covered with a membrane which permits entry of a gas and prevents entry of a liquid into the enclosures.