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Phase locking of THz QC-VECSELs to a microwave reference
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  • Christopher A. Curwen ,
  • Jonathan Kawamura ,
  • Darren J. Hayton ,
  • Sadhvikas J. Addamane ,
  • John L. Reno ,
  • Benjamin Williams ,
  • Boris Karasik
Christopher A. Curwen
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Jonathan Kawamura
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Darren J. Hayton
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Sadhvikas J. Addamane
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John L. Reno
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Benjamin Williams
University of California Los Angeles

Corresponding Author:[email protected]

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Boris Karasik
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High resolution frequency study and phase-locking have been performed on terahertz quantum-cascade vertical-external-cavity surface-emitting-lasers at 2.5 THz and 3.4 THz. A subharmonic diode mixer is used to downconvert the THz signal to a gigahertz intermediate frequency (IF). Feedback from reflections at the mixer are observed to have a strong influence on the free-running QC-VECSEL frequency stability as a result of efficient coupling to free-space compared to more typical ridge waveguide lasers. Instabilities in feedback result in free-running linewidths of tens of MHz, and at times >100 MHz. The QC-VECSEL IF signal is phase locked to a 100 MHz reference using the bias on the device as a means of error correction. Between 90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied RF reference, and amplitude fluctuations on the order of 1-10% are observed, depending on the bias point of the QC-VECSEL. The bandwidth of the locking loop is ~1 MHz. Many noise peaks in the IF signal corresponding to mechanical resonances in the 10 Hz-10 kHz range are observed. These peaks are generally -30 to -60 dB below the main tone, and are below the phase noise level of the multiplied RF reference which ultimately limits the phase noise of the locked QC-VECSEL.
Sep 2023Published in IEEE Transactions on Terahertz Science and Technology volume 13 issue 5 on pages 448-453. 10.1109/TTHZ.2023.3280451