Gas sensing based on modulation spectroscopy requires sinusoidal modulation of the laser sources. This work proposes a modulation scheme for quantum cascade lasers, using the period-one (P1) oscillations. The P1 oscillations are introduced by the tilted optical feedback. Although the optical linewidth of the laser is around 15.0 MHz, the beat-note electrical linewidth of the modulation is less than 2.0 kHz, which suggests that the optical sidebands induced by the P1 oscillations are highly coherent with the main optical mode. In addition, the modulation frequency can be simply tunned by adjusting the feedback length, and the modulation depth of the optical signal is in the range of 1.0 % to 3.0 %. In contrast to the direct modulation scheme and the external modulation scheme, the proposed P1 modulation method does not require any radio-frequency electronics.
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