Photonic Broadband Characterisation of a 415 GHz Filter

We report on the photonic-based broadband characterisation of a 415 GHz band-pass filter (BPF) in the frequency range between 0.3 to 0.8 THz covering several rectangular wave guide bands. BPFs are widely used to reduce out-of-band noise and spurs on the intermediate frequency (IF), e.g. in communication and THz imaging. However, traditional electronic-based characterization methods cannot cover the required bandwidths. Our laser engine is based on a fixed cw laser and a mode-hop-free, widely tuneable external cavity diode lasers optically phased-locked to an erbium-fibre frequency comb at 1550 nm (DFC Core 200+) [1], [2]. The two continuous wave lasers provide a phase-coherent, tuneable difference frequency of up to 10 THz with tuning rates of up to 1 THz/s (Fig. 1a). The difference frequency is converted to terahertz and detected in a free-space spectroscopy setup using photoconductive antennas (PCA) (Fig. 1b). The frequency is linearly swept, and the receiver signal is averaged before applying the Hilbert transform to retrieve amplitude and phase of the transmission. The photonic measurement (Fig1c, blue) is taken at ~0.2 THz/s with ~1.3 MHz frequency resolution in a total measurement time of 200 s. The WM570 band is indicated for comparison. Our sample device under test (DUT) consists of a WM570 filter with centre frequency of 415 GHz and a 3 dB bandwidth of 16.9 GHz (fractional bandwidth of 4.1%) (Fig1c). The filter consists of a symmetrical arrangement of 15 cavities shown in the inset.