Spacing and wavelength tunable frequency comb with a maintained spectral shape.

Optical frequency combs (OFCs) are becoming increasingly prevalent in applications such as optical communications, radio signal processing, and dual-comb spectroscopy. These applications often demand a broad, flat spectrum with flexible control over both the center wavelength and tone spacing, while maintaining a consistent spectral profile. However, the impact of spacing and wavelength tuning on the spectral shape remains relatively unexplored. Many existing OFC generators, such as cavity-based combs including micro-rings, mode-locked lasers, and cavity-enhanced electro-optic (EO) combs, offer limited tuning ranges and exhibit significant spectral variation during tuning. In this paper, we present a method for comb spacing and wavelength tuning with minimal distortion to the spectral profile. Specifically, we demonstrate spacing tuning over 25-32 GHz and center wavelength tuning across 1548-1568 nm, while still maintaining a Gaussian-like flat spectral shape after the pulse shaping and spectrum expansion stages in a nonlinear parametric comb. The method is based on our cavity-less OFC architecture combining single-pass electro-optic comb generation, fiber-based pulse shaping, and nonlinear parametric expansion. This approach yields a comb with high output power (>2 W), broad bandwidth (>90 nm), high optical signal-to-noise ratio (OSNR, >25 dB), and, importantly, a spectral profile unaffected by spacing and wavelength tuning. We demonstrate that this spectral stability can be achieved by primarily adjusting the power of the RF driving signal and the gain of optical amplifiers. Furthermore, we quantitatively analyze how spectral characteristics evolve with tuning, offering deeper insights into the design of high repetition rate, wideband OFCs with enhanced tunability. Our results open new possibilities for deploying frequency combs in a wide range of optical and radio-frequency systems.