Photonic true-time delay (TTD) beamforming has been considered as a promising technique for future wide-band phased array antenna systems. A TTD beamforming system based on a linear chirped fiber grating (LCFG) requires a tunable multi-wavelength laser source, in which the wavelengths are tuned simultaneously with equally increased or decreased wavelength spacing. We present a novel approach of a TTD laser source employing a DQPSK modulator and a highly nonlinear fiber (HNLF). The wavelength tuning is achieved by tuning the frequency of the radio frequency signal generator that drives the DQPSK modulator. Up to 41 lasing wavelengths with 0.08 nm spacing, 29 lasing wavelengths with 0.06 nm spacing and 31 lasing wavelengths with 0.12 nm spacing in 3 dB bandwidth were obtained. In theory, the multi-wavelengths can be simultaneously tuned from 10 MHz to 20 GHz in our experiments based on the fact that the frequency range of the RF signal source is from 10 MHz to 20 GHz and the bandwidth of DQPSK modulator is up to 22 GHz. The tuning precision of the RF signal generator is up to 1Hz and phase noise is about -104dBc/Hz. Therefore, the wavelengths can be tuned precisely and stably.
We proposed and demonstrated a linearly frequency-swept multi-wavelength laser source for optical coherence tomography (OCT) eliminating the need of wavenumber space resampling in the postprocessing progress. The source consists of a multi-wavelength fiber laser source (MFS) and an optical sweeping loop. In this novel laser source, an equally spaced multi-wavelength laser is swept simultaneously by a certain step each time in the frequency domain in the optical sweeping loop. The sweeping step is determined by radio frequency (RF) signal which can be precisely controlled. Thus the sweeping behavior strictly maintains a linear relationship between time and frequency. We experimentally achieved linear time-frequency sweeping at a sweeping rate of 400 kHz with our laser source.