Infrared sensor for soil or water and method of operation thereof

1. A method of operating an infrared (IR) sensor, comprising:

generating IR light from an IR light source;

receiving in an optical fiber the IR light from the IR light source, wherein a selective ion-exchange (IX) medium is associated with the optical fiber and the IR light generates an evanescent field about the optical fiber as the IR light propagates therethrough, the selective IX medium configured to transport an ion species in a subject liquid about the optical fiber; and

receiving in an IR light detector the IR light from the optical fiber, wherein the ion species affects the evanescent field and thereby a characteristic of the IR light.

2. The method as recited in claim 1 wherein the selective IX medium is selected from the group consisting of:

an IX resin coating,

a permselective membrane, and

an ion-exchange film.

3. The method as recited in claim 2 wherein the IR detector is selected from the group consisting of:

a thermal detector, and

a photonic detector.

4. The method as recited in claim 2 , wherein at least the optical fiber is surrounded by an outer case.

5. The method as recited in claim 4 , wherein at least one filter mesh is located within the outer case and surrounds the sensing element.

6. The IR sensor as recited in claim 2 , wherein a wavelength-selective optical filter interposes the optical fiber and the IR detector.