###### Abstract  
Femtosecond laser-excited generation of frequency-tunable microwave pulses, based on an unbalanced single-arm interferometer with frequency-to-time mapping, has been proposed and demonstrated with easy-to-obtain commercial devices. The optical wave-to-microwave frequency conversion, which involves continuous tuning in the range from 2.0 GHz to 19.7 GHz, was achieved based on simple spatial-optical group delay adjustment. Additionally, the pulse duration of the microwave waveform was measured to be 24 ns as the length of the linear dispersion optical fiber was fixed at 20 km. In addition, owing to the designs of the single-arm optical path and polarization-independent interference, the generated microwave pulse train had better stability in terms of frequency and electrical amplitude. Furthermore, a near-triangular-shaped microwave pulse at 4.5 GHz was experimentally obtained by the superposition of two generated sinusoidal signals, which verified the potential of this system to synthesize special microwave waveform pulses.  
frequency conversion; femtosecond laser; microwave photonics +
Footnote †: journal: Applied Sciences
have a fixed free-spectral range once fabricated [20; 21; 22; 23]. Therefore, the frequency of a microwave waveform could be tuned by only changing the length of the polarization-maintaining fiber or the dispersive fiber, which is incapable of fast-frequency tuning. To solve this problem, [PERSON] et al. reported a tunable microwave signal generator using a home-made group-delay modulator based on polarization switching, which cannot be widely promoted as it is not a commercial device [24; 25]. Additionally, [PERSON] et al. proposed a polarization-adjusted Lyot optical filter to achieve discrete frequency tuning of microwave pulses [26]. However, being limited due to polarization-based interference, these polarization-dependent optical-spectral shaping methods are sensitive to environmental fluctuations. Currently, convenient and continuous frequency tuning is still one of the bottlenecks in the development of optical-spectral shaping and frequency-to-time mapping technology.  
In this study, we proposed and experimentally demonstrated a femtosecond laser-excited frequency-tunable pulsed microwave waveform generator, based on an unbalanced single-arm interferometer with frequency-to-time mapping, which consisted of easy-to-obtain commercial devices. The key component in our scheme was a spatial-group-delay-based optical-spectral shaper, which had the ability to achieve continuous tuning of a free-spectral range while the total dispersion of the system was fixed. Therefore, different from previous works, the proposed scheme can achieve convenient and continuous frequency tuning of a generated microwave signal through simple spatial-delay-distance adjustment. Additionally, since the two coherent beams of the single-arm interferometer had the same path in the optical fiber, this system was polarization independent and environmentally insensitive, which is conducive to generating more stable microwave waveforms. Furthermore, a near-triangular-shaped microwave pulse was synthesized by two sinusoidal signals with appropriate ratios for frequency and amplitude in our experiments, which showed the potential of this system to generate special waveform pulses.