We present a step-swept light source with high linearity and tunable frequency step based on the lightwave synthesized frequency sweeper (LSFS). Different from the traditional internal modulation, this light source can effectively avoid complex frequency-calibration and deduce the spectrum stitching error by using the external modulation and a time delayed spectrum stitching technique. Experimentally, a k-space swept with > -0.99997 swept linearity is achieved. The swept range are 5.135 nm with 10 GHz swept step and 3.902 nm with 7.5 GHz swept step. The spectrum stitching-error is less than 2.92% and 0.65% respectively.
A multiple frequency-swept source based on a recirculating frequency shifter loop (RFSL) is established. Three distributed feedback (DFB) lasers are used as a seed source and sweep in the RFSL synchronously. The swept spectra of the separate DFB lasers are precisely controlled and stitch together without overlap. The key significance of this technique is that the swept range increases as the number of the multiple seed wavelengths increase. Experimentally, the swept range of our system can broaden to 4.5 nm/3.9 nm with an 8.6 GHz/7.5 GHz swept step. The swept rate is 200 kHz. The output of the source is sent into a Mach-Zehnder interferometer and the interference signal is detected to measure the length difference of interferometer arms.
In recent years, an approach of photonic true-time delay (TTD) unit which employs a linear chirped fiber grating (LCFG) and a tunable multi-wavelength laser (TMWL) for phased array antennas (PAAs) has been reported. In this paper, several key parameters of the LCFG and TMWL are analyzed. The beam-pointing resolution is effected by the parameters including the group delay slope of LCFG, the tuning accuracy of the wavelength spacing of TMWL. The relationship between the beam-pointing angle and the wavelength spacing of TMWL is an inverse trigonometric function. So linear output of the beam-pointing angle could be obtained by tuning the wavelength spacing as sine function. Furthermore, the relationship of the beam-pointing resolution and the beam-pointing angle with different tuning accuracies of the wavelength spacing of TMWL and different group delay slopes of LCFG are also analyzed.