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    Area of Science:

    • Terahertz Spectroscopy and Sensing
    • Quantum Cascade Laser (QCL) Technology
    • Optical Interferometry

    Background:

    • Terahertz (THz) frequency range offers unique material probing capabilities.
    • Quantum cascade lasers (QCLs) are compact and efficient THz sources.
    • Laser feedback interferometry (LFI) can enhance detection sensitivity.

    Purpose of the Study:

    • To investigate the detection sensitivity of an LFI scheme utilizing a THz QCL.
    • To quantify the minimum optical feedback power required for voltage variation detection.
    • To determine the noise equivalent power (NEP) of the developed system.

    Main Methods:

    • Implementation of a laser feedback interferometry setup with a THz QCL.
    • Measurement of laser voltage variations induced by optical feedback.
    • Characterization of system sensitivity by varying optical reinjection power.
    • Determination of noise equivalent power (NEP) through noise analysis.

    Main Results:

    • High detection sensitivity achieved, resolving voltage variations with reinjected powers as low as ∼-125 dB.
    • Measured noise equivalent power (NEP) of approximately 1.4 pW/√Hz.
    • Estimated effective NEP coupled to the QCL active region is as low as ∼1 fW/√Hz after accounting for losses.

    Conclusions:

    • The THz QCL-based LFI scheme exhibits exceptional sensitivity for detecting small optical perturbations.
    • The system's performance is suitable for applications requiring ultra-sensitive measurements in the THz domain.
    • Further optimization of optical coupling can enhance the effective sensitivity of the QCL active region.