Photonic Filter True-Time Delay

Photonic microwave true-time delay eliminates the beam squint problem in broadband phased-array antenna. However the large insertion losses associated with the optical switching network that selects the appropriate delay line is a serious limitation. We have previously demonstrated a unique photonic filter that uses the optical carrier wavelength in the 1550 nm wavelength band to select the desired time delay [1]. In this paper, dual band (1550/1300 nm) operation of this Recirculating Photonic Filter True-Time-Delay is demonstrated. This dual-band operation offers flexibility in system design, enhanced antenna functionality and is expected to SPIE Vol. 3160. 0277-786X197/$1 0.00 27 lead to lower device and system cost. The reduction of the center wavelength from 1550 nm to 1300 nm results in a reduction of the free spectral range (FSR). Designing the waveguide grating at 1550 nm to have FSR larger than channel bandwidth one can easily obtain complete access to the channel bandwidth in both bands. Comparison of measured channel spacing, insertion loss and crosstalk of waveguide prism filter at 1550 nm and 1330 nm is shown in Tablel. An external cavity tunable laser, externally modulated using a lightwave network analyser from 2-12 GHz served as the input. Feedback delay lines consisted of optical fibers of different lengths connected between 4 symmetric input and output ports. The output from the chip was detected and fed to the network analyzer and the S21 phase was measured versus frequency to obtain time delay (Refer Figure la & ib). There is excellent agreement between the measured microwave time-delay and the estimated time-delay based on the fiber delay line length. Time delay measurements show low loss and low crosstalk in both 1550 and 1300 nm bands. This dual band property represents a significant advantage over alternative time delay techniques based on the Bragg fiber grating [2] which is inherently limited to narrowband operation.