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Common mode frequency instability in internally phase-locked terahertz quantum cascade lasers
M C Wanke1, A D Grine, C T Fuller
1Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185, USA. mcwanke@sandia.gov
Optics Express
|November 24, 2011
Summary
Researchers phase locked difference frequencies (DFs) in a quantum cascade laser (QCL) using integrated diode mixer feedback. While locked DFs showed narrow linewidths, common mode frequency stability remained limited, similar to free-running QCLs.
Area of Science:
- Terahertz (THz) Photonics
- Quantum Cascade Lasers (QCLs)
- Optical Frequency Stabilization
Background:
- Quantum cascade lasers (QCLs) are crucial sources for THz applications.
- Controlling the frequency and stability of QCLs is essential for high-precision spectroscopy.
- Phase locking longitudinal modes offers a pathway to enhanced frequency control.
Purpose of the Study:
- To investigate the phase locking of difference frequencies (DFs) between Fabry-Perot (F-P) modes in a 2.8 THz QCL.
- To assess the effectiveness of feedback from an integrated diode mixer for frequency stabilization.
- To evaluate the common mode frequency stability of the phase-locked QCL.
Main Methods:
- Integration of a diode mixer into a 2.8 THz QCL for feedback.
- Utilizing the feedback loop to phase lock the difference frequencies (DFs) among F-P longitudinal modes.
- Characterization of locked DF signal linewidth and drift.
- Mixing measurements between two QCLs and between a QCL and a molecular gas laser to assess common mode stability.
Main Results:
- Approximately 40% of the DF power was successfully phase locked.
- The phase-locked DF signal exhibited a linewidth of ≤ 1 Hz and negligible drift over ~30 minutes.
- Common mode frequency stability was found to be comparable to that of a free-running QCL.
Conclusions:
- Integrated diode mixer feedback can achieve phase locking of QCL difference frequencies with narrow linewidths.
- The achieved phase locking significantly improves the stability of individual DF signals.
- Common mode frequency stability remains a challenge, indicating limitations in the current feedback approach for absolute frequency referencing.
