Electrically tunable true time delay line based on a chirped fiber Bragg grating

In this work an electrically tunable true time delay line (TTD), useful for phased array antennas, is experimentally demonstrated. The TTD line operates at a single optical wavelength and it is based on a linearly chirped fiber Bragg grating (LCFBG). Continuous TTD is achieved by changing the temperature of the LCFBG. In order to reduce the range of temperatures required for supply high time delay variations, the grating was bonded onto a metallic support and the effect of the thermal apparent strain was considered. The time delay response of the system has been measured with an optical signal modulated with Double Side Band technique at a frequency of 2GHz. The experimental results demonstrate a minimum time delay of 3.1ps employing a temperature controller with a stability of +/- 0.1 degrees C and a maximum time delay of 96ps over a temperature range of 16-40 degrees C. The time delay-temperature characteristic offers a slope of 3.8ps/C with a mean deviation from the linearity of 3.5ps. Such a delay line, by operating at a single optical wavelength and by using a simple actuating system, offers many advantages being low cost, compact and reliable. The main limitation to the TTD resolution is the amplitude ripple of the LCFBG and a performance enhancement can be obtained with specifically tailored LCFBG.