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dc.contributor.author
Singleton, Matthew
dc.contributor.supervisor
Faist, Jérôme
dc.contributor.supervisor
Burghoff, David
dc.contributor.supervisor
Khurgin, Jacob
dc.date.accessioned
2021-12-30T13:24:26Z
dc.date.available
2021-12-30T11:38:22Z
dc.date.available
2021-12-30T13:24:26Z
dc.date.issued
2021
dc.identifier.uri
http://hdl.handle.net/20.500.11850/522464
dc.identifier.doi
10.3929/ethz-b-000522464
dc.description.abstract
Quantum cascade lasers (QCLs) are bright, coherent sources, which operate at mid-infrared and terahertz wavelengths. The 2012 discovery that they also lock to form broadband combs has enabled applications such as dual comb spectroscopy, in which high signal to noise broadband spectra can be taken on millisecond timescales, impossible with the older globar technology. The locking is passive, requiring no external stimulus or separate absorber. Before the start of this thesis, it was already well understood that the comb forms by combination of spatial hole burning and four-wave mixing, and that the emission should tend to a constant intensity with time rather than a pulse train owing to the strong damping in the active region. Questions remained though on the exact nature of the state, experimental evidence lacking in the mid-infrared. This is a rich dynamical system, comprising parts with different characteristic timescales working together to form a stable comb emission. Understanding experimentally how the emission evolves with time serves as both a useful, granular comparison for simulation, and as a direct characteristic for applications which seek to take advantage of the higher peak brightness and lower electrical dissipation of a pulse driven laser. We use a technique which makes use of a boxcar integrator to track the spectral evolution from <10 ns to 1 us. We find significant broadening over these timescales (<1 us) compared to when driven with a constant bias, which could find application in similar space to quasi-CW driving of DFB lasers. Together, the spectral amplitude and phase determine whether the comb in the time domain is a pulse train, frequency modulated (FM, broadband continuous wave), or something else. For the QCL, models had predicted a so-called pseudo-random type of FM, whereby within each identical period, the laser would exhibit a complicated chirp pattern. For such an emission, standard pulse characterisation techniques are unsuitable. We use a coherent technique called SWIFTS (Shifted Wave Fourier Transform Spectroscopy), which works by spectrally resolving the the beats between neighbouring mode pairs, thereby directly measuring their phase differences. Applied to a broadband mid-IR comb, we found what is now understood to be quite a general behaviour in QCLs and other laser systems: a periodic linear chirp, laser sweeps its bandwidth from its longest wavelength to its shortest. Moreover, we confirmed 3 key properties of the comb: that the state was reproducible, stable in time, and tunes smoothly with current. Armed with knowledge of the phase and stability properties, the laser output can be converted to a stable pulse train by suitably delaying the different frequency components. Since the phase of the linear chirp takes on a simple form - broadly speaking a parabola - means for such lasers we do not need to use the more sophisticated/less commercially attractive spatial light modulator for example, which is difficult to come by affordably in the mid-IR. Instead, we opt for a grating based stretcher-compressor, a staple of the ultrafast community, which can be constructed relatively cheaply using off the shelf optical components. With this, we were able to modify the emission from ~CW with 134 ps period, to a train of pulses of width around 12 ps, and increase the peak intensity to average ratio from 4 to 40. For the future, this implies efforts can be concentrated on generating broader, flatter spectra, e.g. through bandstructure engineering or microwave injection, which in turn are expected to yield a more parabolic phase to better fit the compressor's transfer function. Ring QCLs are expected to behave very differently from Fabry-Perot ones, since spatial hole burning is absent when operating unidirectionally. Indeed, comb spectra in rings tend to be structurally different, and indeed much narrower, despite carrying comparable levels of optical power. We studied different ring QCLs, using a combination of dual comb spectroscopy and SWIFTS to measure their phase. In a first experiment, we find unidirectional lasing, and a quiet comb state with significant amplitude modulation. In a follow-up experiment, we find a laser emitting a sech2 spectrum, a signature of soliton emission, atop a CW background. Its measurement indeed confirms that this is a pulse, and with a grating filter, we separate it from the background. In this way, from a DC pumped QCL, are able to extract 3.7 ps pulses.
en_US
dc.format
application/pdf
en_US
dc.language.iso
en
en_US
dc.publisher
ETH Zurich
en_US
dc.rights.uri
http://rightsstatements.org/page/InC-NC/1.0/
dc.subject
QCL
en_US
dc.subject
frequency comb
en_US
dc.subject
Quantum cascade laser (QCL)
en_US
dc.subject
mid infrared
en_US
dc.subject
Mid-infrared photonics
en_US
dc.subject
mid-infrared laser
en_US
dc.subject
spectroscopy
en_US
dc.subject
semiconductor lasers
en_US
dc.title
Combs in Quantum Cascade Lasers: Linear Chirp and Pulse Compression
en_US
dc.type
Doctoral Thesis
dc.rights.license
In Copyright - Non-Commercial Use Permitted
dc.date.published
2021-12-30
ethz.size
260 p.
en_US
ethz.code.ddc
DDC - DDC::5 - Science::530 - Physics
en_US
ethz.code.ddc
DDC - DDC::6 - Technology, medicine and applied sciences::600 - Technology (applied sciences)
en_US
ethz.code.ddc
DDC - DDC::6 - Technology, medicine and applied sciences::621.3 - Electric engineering
en_US
ethz.identifier.diss
27976
en_US
ethz.publication.place
Zurich
en_US
ethz.publication.status
published
en_US
ethz.leitzahl
ETH Zürich::00002 - ETH Zürich::00012 - Lehre und Forschung::00007 - Departemente::02010 - Dep. Physik / Dep. of Physics::02510 - Institut für Quantenelektronik / Institute for Quantum Electronics::03759 - Faist, Jérôme / Faist, Jérôme
en_US
ethz.leitzahl
ETH Zürich::00002 - ETH Zürich::00012 - Lehre und Forschung::00007 - Departemente::02010 - Dep. Physik / Dep. of Physics::02510 - Institut für Quantenelektronik / Institute for Quantum Electronics::03759 - Faist, Jérôme / Faist, Jérôme
ethz.leitzahl.certified
ETH Zürich::00002 - ETH Zürich::00012 - Lehre und Forschung::00007 - Departemente::02010 - Dep. Physik / Dep. of Physics::02510 - Institut für Quantenelektronik / Institute for Quantum Electronics::03759 - Faist, Jérôme / Faist, Jérôme
en_US
ethz.date.deposited
2021-12-30T11:38:30Z
ethz.source
FORM
ethz.eth
yes
en_US
ethz.availability
Open access
en_US
ethz.rosetta.installDate
2021-12-30T13:24:33Z
ethz.rosetta.lastUpdated
2022-03-29T17:08:18Z
ethz.rosetta.versionExported
true
ethz.COinS
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