In this work, a narrow-linewidth double-mode quantum cascade laser (QCL) is demonstrated. The linewidth (full width at half maximum) of the DFB (distributed feedback) QCL has been narrowed from 45 MHz to 500 kHz by the laser frequency locking. An AOFS (acousto-optic frequency shifter) has been used to obtain a double-mode operation. The spectral distance between the both (zero and first order diffracted) beams can be adjusted by the acoustic wave frequency from 65 to 95 MHz by controlling the tuning voltage of the RF driver.
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