Linewidth and tuning characteristics of terahertz quantum cascade lasers

Area of Science:

• Optics and Photonics
• Quantum Electronics
• Spectroscopy

Background:

• Quantum cascade lasers (QCLs) are semiconductor sources emitting in the terahertz (THz) range.
• Precise characterization of THz QCL spectral properties is crucial for applications.
• Existing linewidth measurement techniques for THz lasers can be limited in precision.

Purpose of the Study:

• To accurately measure the spectral linewidth of continuous-wave (CW) quantum cascade lasers operating at terahertz frequencies.
• To investigate the frequency tuning characteristics of THz QCLs as a function of injection current.

Main Methods:

• Heterodyne measurement technique involving the free-running THz QCL and two far-infrared gas lasers.
• Detection of beat notes using a Gallium Arsenide (GaAs) diode mixer.
• Analysis of beat notes with a microwave spectrum analyzer for precise frequency measurements.

Main Results:

• Instantaneous spectral linewidths of the measured THz QCLs were determined to be less than -30 kHz.
• Precise frequency measurements were achieved through the heterodyne beat note detection.
• Frequency tuning characteristics of the QCLs were mapped by varying the injection current.

Conclusions:

• The heterodyne method provides a highly precise technique for measuring THz QCL linewidths.
• The measured linewidths are significantly narrow, indicating high spectral purity.
• Understanding frequency tuning behavior is essential for controlling THz QCL output in applications.