Frequency sweep operation of directly modulated optical negative feedback lasers is numerically and experimentally investigated for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) with a high signal-to-noise ratio (SNR), particularly over a long range. Low FM noise corresponding to a spectral linewidth of ∼2.0 kHz is sustained even with injection current modulation of an optical negative feedback laser through optical feedback from a Fabry-Perot etalon, and a beat note spectrum with a 30-dB SNR is achieved even when a 300-m delay fiber is used as a ranging sample. These results encourage an approach to provide directly modulated frequency-swept lasers for long-range FMCW LiDAR.

We investigated the FM noise characteristics of a distributed feedback laser under optical negative feedback from a Fabry–Perot optical filter using delayed self-homodyne coherent detection with a high-frequency range of 1 × 105–2 × 1010 Hz. The measured FM noise power spectral densities of the laser under the optical negative feedback strongly depend on the modulation responses of the laser. These results were reproduced in calculations based on a transfer function model. Our findings suggest that wide bandwidth semiconductor lasers are suitable for reducing FM noise in a wide frequency range and enhancing the feedback loop gain (FM noise reduction ratio) under a sufficient phase margin in optical negative feedback.

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