Bragg grating Fabry-Perot interferometer with variable finesse

Abstract

We demonstrate the performance of a fiber Fabry-Perot ~FFP! interferometer and show how the finesse of the interferometer can be adjusted by changing the operating wavelength. These sensors are extraordinarily sensitive and might have uses for high-density wavelength division multiplexing and high-resolution fiber sensor applications. We cannot resolve the transmission peaks using an optical spectrum analyzer and consequently use a tunable laser diode to study them. Fiber Bragg gratings act as the mirrors for the FFP interferometer and consist of an optical fiber with a periodic change in the index of refraction. The reflectivity spectrum depends on the grating profile. The period of the refractive index profile and therefore the reflectivity spectrum can be changed with stress and temperature. Consequently Bragg gratings are used as sensors for biosensing, civil engineering, and industrial monitoring. All-fiber lasers can be constructed by combining two Bragg gratings as mirrors at the ends of a gain medium. Tunable fiber lasers have been reported where the output wavelength is adjusted by varying the period of the Bragg gratings. In these reports, care was taken to insure that the lasers operate in a single mode. In addition, the mirrors were fabricated onto a single fiber to create a more stable cavity. There have also been reports of a passive FFP interferometer—the fiber laser without the gain medium. Early experiments utilized Bragg gratings with relatively narrow spectral widths and finesse values ranging from 67 to 1278 to 5000. Later studies examined chirped Bragg gratings with much wider spectral widths. Experimental results showed relatively low values of finesse while computational results showed that extremely high finesse values and wide spectral widths could be achieved with chirped Bragg gratings. Again, the Bragg gratings were written onto the same fiber in order to eliminate connectors and to achieve more stable operation.