Low-phase-noise millimeter-wave generation toward high-speed communications using an externally optically injected gain-switched laser comb with a noise-like spectrum

Abstract

Low-phase noise, widely tunable millimeter-wave (MMW) sources are crucial for future 6G communications to alleviate spectrum congestion and support high-speed transmission. In this work, we propose a tunable, low-phase-noise MMW generation scheme based on a gain-switched (GS) laser frequency comb, achieved by combining electrical current modulation and external optical injection. Numerical simulations show that, under high modulation current, GS laser produces a broad, noise-like spectrum with poorly resolved comb lines, whereas external optical injection reestablishes distinct comb lines through injection locking, thereby extending the spectral range. In practice, the resulting comb-line linewidth can be narrowed and is primarily limited by the linewidth of the injected laser. This effectively improves the tuning range and phase noise of the MMW carriers. Experiments demonstrate MMW generation tunable from 7 to 140 GHz, achieving a phase noise of −98.45 dBc/Hz at 10 kHz offset at 140 GHz, which is 18.94 dB lower than that obtained by scaling the scheme's 7 GHz output to 140 GHz via 20× multiplication. To assess the impact of phase noise reduction on transmission performance, we employ a Kramers–Kronig receiver-based MMW that operates without a receiver local oscillator. Using a 120 GHz carrier generated by the proposed GS system, we demonstrate PS-32QAM transmission up to 45 Gbps with a bit error rate (BER) below the 20% SD-FEC threshold. Compared with conventional GS without optical injection, the proposed phase noise suppression approach achieves a 7.16 dB improvement in BER performance.