Broadband mid-infrared optical circulator for quantum cascade laser frequency combs.

Area of Science:

• Optics and Photonics
• Laser Technology
• Spectroscopy

Background:

• Optical feedback (OF) can degrade the performance of sensitive laser systems.
• Quantum cascade lasers (QCLs) and their frequency combs (FCs) are crucial for mid-infrared applications.
• Existing circulators may not be optimized for broadband mid-infrared sources.

Purpose of the Study:

• To design and implement a free-space optical circulator for mid-infrared applications.
• To specifically address the needs of quantum cascade laser (QCL) frequency combs (FCs).
• To improve the isolation and reduce insertion loss for enhanced system performance.

Main Methods:

• The circulator is based on a novel phase-retarding mirror (PRM) design.
• Implementation in a free-space optical setup.
• Testing with single-mode QCLs and QCL-FCs, including operation at ~10 µm.
• Integration into an open-path dual-comb spectrometer's telescope transceiver.

Main Results:

• Achieved high isolation: 29.7 dB with a single-mode QCL and 25.2 dB with a QCL-FC.
• Demonstrated low insertion loss (<0.59 dB) for QCL-FCs at ~10 µm.
• Significantly attenuated unwanted optical feedback (OF).
• Improved sensitivity and stability of a dual-comb spectrometer by an order of magnitude.

Conclusions:

• The developed optical circulator effectively suppresses OF and enhances throughput for mid-infrared devices.
• It offers tunable performance for various mid-infrared sources, including FCs and tunable lasers.
• This technology significantly benefits remote sensing systems and other applications requiring stable mid-infrared light sources.