Universität zu Köln

Mahdizadeh, Sajjad (2023). Frequency Stabilization of 4.7 THz Quantum Cascade Lasers. PhD thesis, Universität zu Köln.

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Abstract

Heterodyne receivers for astronomy observing at 4.7 THz use Quantum Cascade Lasers (QCLs) as the local oscillator. QCLs are compact, powerful, and easy to use but prone to frequency instability from current noise, temperature fluctuations, and optical feedback, and they also lack absolute frequency reference. This work presents several solutions to this problem. A heterodyne laboratory receiver at 4.7 THz has been developed to host all the experiments. The receiver shows an uncorrected receiver noise temperature of around 5000K at an IF frequency of 5.1 GHz and a total power Allan stability time of around 500 seconds. With this receiver, measuring methanol's emission lines helped to frequency calibrate one of the QCLs. The first experiment uses a methanol absorption line for frequency discrimination and a Hot Electron Bolometer (HEB) as a total power detector. In an active control loop, the experiment locks the laser's frequency to the dip of the absorption line. This method applies a known modulation to the QCL's frequency, dominating the QCL's linewidth with an FWHM of 2.1 MHz. The second and the third experiments down-convert the QCL's frequency with a Superlattice Device (SLD) harmonic generator and mixer. A diode multiplier chain produces a 182.5 GHz signal to pump the SLD. The 26th harmonic is generated and mixed with the 4.7 THz QCL signal. The resulting IF signal at the SLD's output is 10 dB over the noise floor. The second experiment feeds the IF signal into a delay line frequency discriminator to produce the QCL frequency's error information. A power divider divides the amplified and filtered SLD's IF into two. Only one is delayed in 10 meters of coax cable, and homodyne mixing produces a DC voltage as a function of the QCL's frequency. The control electronics turn this into a correction current to the QCL. This method stabilized the QCL with more than 10 MHz of frequency deviations, to an FWHM of 780 kHz, for hours. The experiment even works at very low signal-to-noise conditions, such as 2 dB, and revealed that the optical feedback is the dominant QCL line broadening mechanism caused by the pulse tube refrigerator's forced motions. The third experiment uses a Phase Locked Loop (PLL). A phase detector compares QCL's phase with a reference, producing an error voltage. The loop filter transfers this to a correcting current, compensating the QCL's frequency disturbances. Phase locking is more challenging than frequency locking, and an exact understanding had to be developed on transfer functions and noise modeling. The experiment locked the QCL's phase, reducing the linewidth FWHM from 1.4 MHz to less than 7 kHz, stable for half an hour, and occasionally losing the lock for a few seconds. With the demonstrated works, it is possible to stabilize the frequency of 4.7 THz QCLs.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Mahdizadeh, Sajjad smahdiz1@smail.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-716223
Date:	2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Physics Technology (Applied sciences)
Uncontrolled Keywords:	Keywords Language Quantum Cascade Laser (QCL) English Frequency stabilization English Delay line frequency discriminator English Local oscillator English Superlattice sub-harmonic mixer English Phase locking English Frequency locking English Heterodyne receiver English SOFIA observatory English GREAT instrument English
Date of oral exam:	20 September 2023
Referee:	Name Academic Title Eckart, Andreas Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71622