Active resonators based on semiconductor gain media encompass a large optical nonlinearity that arises from gain saturation and enables bright soliton generation. The ability to operate these resonators below the lasing threshold as tunable passive devices –– filters, modulators, phase shifters –– opens up an untapped potential of seamlessly integrated reconfigurable devices for both generation of multimode mid-infrared (4 – 12 μm) light and its manipulation.
Terahertz Quantum Cascade lasers are very versatile sources of terahertz radiation. Frequency comb operation, surface emitting arrays, external cavity tuning have been demonstrated. For all these implementations broadband gain is strongly demanded. The intersubband gain mechanism allows to the design of different wavelength active region and their integration in the same waveguide. We have developed active regions consisting of up to four different intersubband designs. To enable a common operation not only the gain curve needs to be aligned over all sections but also the alignment electric field and subsequently the operating current. Fabry-Perot devices fabricated from the four-section active region show lasing over more than one octave. Ring resonators show also broadband laser operation and comb formation. Broadband operation is a large advantage of random lasers which we turn into useful devices by an optical machine learning approach. This allows the control of the emission wavelength beyond discrete cavity modes.
Optical frequency combs (OFCs) stand as the cornerstone of modern optics, with
applications ranging from fundamental science to sensing and spectroscopy. Generation of
short optical soliton pulses in passive media such as optical fibers and microresonators has
been an established technique for stable OFC formation with a broad optical spectrum –
however these platforms are driven by an external optical signal and often rely on
additional bulky elements that increase the complexity of the system.
Here, we aim to overcome these difficulties by direct OFC generation in mid-infrared
semiconductor lasers, such as quantum and interband cascade lasers. After a general
introduction to such combs and their nonlinear dynamics, the soliton concept from
microresonator Kerr combs will be generalized to active media that are electrically-driven
and a new type of solitons in free-running semiconductor laser integrated on a chip will be
demonstrated.
Monolithic ring Quantum Cascade Lasers (QCLs) have recently emerged as a new platform for frequency comb generation in the mid-infrared with immediate applications in molecular gas spectroscopy and photonic generation of stable coherent sub-THz tones. In this talk I will show that depending on the way they are driven, ring QCLs can act as carrier generators, integrated intensity modulators, tunable filters, and on-chip optical amplifiers. The natural predisposition of these components to photonic integration opens a route to compact mid-infrared WDM transceivers for free space optical links and miniaturized 2D IR spectrometers.
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. Here we discuss the role of a Bloch gain induced giant Kerr nonlinearity in Fabry-Pérot and ring cavity QCLs, paving the way towards electrically pumped Kerr combs.
Quantum cascade lasers and other semiconductor laser types constitute an attractive integrated platform for spectroscopic applications, as they emit self-starting Frequency Combs (FCs), unlike traditionally-used mode-locked lasers. Here, we explain self-starting FCs due to nonlinear effects arising from the laser gain itself, with particular attention on the coupling of the amplitude and phase of light, quantified by the Linewidth Enhancement Factor (LEF). We study both cavity geometries, Fabry-Perot and ring, reporting the conditions for stable comb formation and different methods of optimizing their performance. In analogy with Kerr microresonators, ring lasers show the formation of temporal localized soliton-like structures, indicating towards an untapped potential for discovering new states of light.
Ring resonators are interesting alternative cavity solutions to the commonly used ridge type waveguide for THz Quantum Cascade lasers. They either support a standing wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. We have fabricated two devices structures. The first one is episide-up with bonding pads. The measurements show a complex behavior of comb-formation most probably influenced by spatial hole burning. The second structure is a pure ring mounted episode down on Si-substrate. This structure shows a totally different comb formation as well as much reduced threshold currents.
Frequency combs are ideal candidates to build chip integrated spectrometers without moving parts. I will give an overview on comb generation in the mid-infrared using interband cascade lasers.
We demonstrate that ICLs naturally show the same frequency comb characteristics as QCL frequency combs and shed light into previous experiments. To generate the comb, we utilize the intrinsic fast time dynamics of the laser gain medium to enable phase-locking via four-wave mixing that is due to anti-phase oscillations of the population inversion. The observed comb state, the frequency modulated state, is fundamentally different to traditional mode-locking, where short pulses are generated.
As a main characterization technique, we use the linear RF phase measurement technique shifted wave interference Fourier transform spectroscopy (SWIFTS). A detailed comparison between SWIFTS and the intensity autocorrelation for the pulse shape characterization will be presented. A more intuitive picture of the synchronization states in frequency combs is provided by the analogy to coupled clocks, which reveals a illustrative understanding of how these lasers can be switched to the pulsed regime. Using this knowledge we demonstrate the active mode-locked mid-infrared ICLs with picosecond pulse emisson.
A key feature of ICLs is that the very same layer structure can also be used as sensitive photodetectors. The fact that ICLs utilize fast carrier transport via intersubband scattering is a great advantage for the high frequency response of the on-chip photodetectors. Combined with the low power requirements of the laser this makes ICL technology an ideal platform to realize future miniaturized dual-comb spectrometers for hand-held and battery driven devices.
[1] B. Schwarz et al. Optica 6, 890 (2019)
[2] J. Hillbrand et al. Optica 6, 1334 (2019)
[3] H. Lotfi et al. APL 109, 151111 (2016)
Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. This novel type of combs is gaining vast attention from researchers due to their self-starting nature and compactness, making them an ideal platform for further development of spectroscopic applications. Their spontaneous formation was explained through an interplay of phenomenological nonlinearity and dispersion in the laser active region, although the actual physical processes remained unclear until now. Here we show that Bloch gain – a phenomenon described by Bloch and Zener in the 1930s – plays an essential role in their formation. We demonstrate that a Bloch gain contribution is present in any quantum cascade laser and becomes particularly dominant under saturation.
Bloch gain in QCLs with ultrafast gain recovery induces a giant Kerr nonlinearity, which is two orders of magnitude larger than the bulk values. The resonant Kerr nonlinearity provides coherent coupling between the amplitude and the phase of the laser field, which serves as a locking mechanism for frequency comb operation. We show that in Fabry-Pérot QCLs this results in frequency-modulated combs with a linear frequency chirp. In ring cavity QCLs, the Bloch gain is able to induce a single-mode instability by tuning the laser in the phase turbulence regime. This can lead to the formation of locked spatial patterns that are related to dissipative Kerr solitons, paving the way towards electrically pumped Kerr combs.
The recent generalised theory of frequency comb generation in externally pumped cavities with and without population inversion suggested an intimate link between quantum cascade lasers (QCLs) and Kerr resonators. In this talk we overview recent experimental developments in chip-scale ring cavity QCLs with and without output coupling ports, that allow operation in self-pumped and externally pumped configurations, and their ability to support cavity solitons.
Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics. Here, an overview on the generation electrically pumped optical frequency combs on integrated platforms using semiconductor lasers.
This includes self-starting generation of frequency modulated combs in quantum cascade laser in the 8um and interband cascade lasers in the 3-4um wavelength region, respectively. Furthermore, we will discuss how to integrate efficient high-speed modulators in these devices in order to facilitate the generation of picosecond pulses.
It is a well-established truth that spatial hole burning (SHB) in a standing-wave cavity is an essential single-mode instability mechanism for multimode operation of quantum cascade lasers (QCLs). We discovered recently that another instability mechanism–phase turbulence–is capable of triggering an onset of previously unseen types of frequency combs in traveling-wave ring cavity QCLs in absence of SHB. This new regime of laser operation reveals a connection with Kerr combs and paves the way to manipulation and engineering of comb states in QCLs.
Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts. ICLs are perfect for the realization of miniaturized spectrometers, due to their low power consumption and the possibility to build sensitive on-chip detectors using the same epilayer material. Here, we present an overview of our current work on ICL frequency combs. This includes the generation of self-starting frequency combs utilizing their gain nonlinearity, resulting in a dominant frequency modulation similarly to quantum cascade lasers frequency combs. Furthermore, we demonstrate the generation of 3ps pulses with a peak to average power ratio of over 40 via active mode-locking.
Optical frequency combs are coherent sources that emit a series of evenly spaced lines. In the mid-infrared, comb based spectroscopy is of particular interest and, without the need of any movable parts, will potentially lead to a breakthrough in miniaturization. Interband cascade lasers, with their low power consumption and inherent detection functionality, are an ideal candidate for practical implementations.
Here, we present the generation of low-dissipation optical frequency comb utilizing interband cascade lasers. Other than one might have expected, the long lifetime of the interband transition does not automatically lead to slow gain dynamics that would favor in-phase mode-locking. We discuss why ICLs should be considered as fast gain media and why passive mode-locking is difficult or even impossible to be achieved. We applying shifted-wave interference Fourier transform spectroscopy to show that ICL frequency combs naturally favor repulsive intermode beat synchronization with the same chirped FM character recently found in QCL combs. Furthermore, we show first evidence of multiple normal modes of the intermodal beats in frequency combs and picosecond pulse generation from ICLs.
Following the goals of single-chip integrated dual comb spectrometers, we report on recent results on mid-infrared frequency combs. We demonstrate frequency comb operation with a bi-functional quantum cascade material, which allows the integration of lasers and detectors on one chip. With this device, we hold the power and efficiency record of QCL frequency combs. In the second part, we will present first evidence of frequency comb generation using mode-locked interband cascade lasers. With the demonstration of picosecond pulse generation in the mid-infrared, we open a new path towards battery driven sensitive high-resolution spectrometers miniaturized to chip-scale dimensions.
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