Continuously variable optical delay line based on a chirped fiber Bragg grating

In this letter, a continuously variable optical delay line (ODL), useful for phased array antennas, is experimentally demonstrated. The ODL operates at a single optical wavelength and it is based on a linearly chirped fiber Bragg grating (LCFBG). Continuous true time delay is achieved by changing the temperature of the LCFBG. In order to limit the actuating temperature range, the grating was bonded to a metallic support and the effect of the thermal apparent strain was considered. The time-delay response of the system is measured with an optical signal modulated with double sideband technique at a frequency of 2 GHz. The experimental results demonstrate a time-delay resolution of 3.1 ps employing a temperature controller with a stability of +/- 0.1 degrees C-40 degrees C. The time-delay-temperature characteristic offers a slope of 3.8 ps/degrees C with a mean-square deviation from the linearity of 3.5 ps. Such a delay line, by operating at a single optical wavelength and by using a simple actuating system, offers many advantages being low cost, compactness, and reliability. The main limitation to the ODL resolution is due to the amplitude ripple of the LCFBG used and a performance enhancement can be obtained with specifically tailored LCFBG.