Improvement of an optical fiber sensor for the detection of low concentrations of solutes using the photothermal effect

• Fiber optic sensors can be used as refractometers.
• Refractive index changes are generated by a laser beam through a photothermal thermal lens effect.
• Concentration measurements at micromolar level are possible.
• The thermal lens effect can be monitored.

We report on the use of an optical fiber sensor for the determination of concentration of contaminants in solutions below the micromolar level based on the photothermal lens effect. A pump laser beam has been added to a conventional configuration widely used as refractometer, so that the device is improved to detect tiny changes of the refractive index of a sample related to the amount of heat generated following optical absorption. In this way, concentrations as low as 0.1 μM of methylene blue in distilled water were measured. A simple experiment is also performed to demonstrate the photothermal effect.