The mid-infrared (mid-IR) spectral range is typically defined from 3 to 20 µm wavelength and has become an important regime for a plethora of industrial or medical purposes, due to the presence of strong absorption lines of many important molecules. This spectral range has been extensively studied during the last two decades, particularly driven by the progress in quantum cascade lasers (QCLs). As next step, the photonic integration of QCLs with low-loss passive waveguides is highly interesting, as it would enable the development of advanced systems with unique features such as high compactness, robustness, reliability or low operating powers. In this regard, and since QCLs are mainly based on III-V group materials, InGaAs/InP-based platforms are particularly interesting for a monolithic integration. Moreover, the wide transparency window of InP and InGaAs semiconductors and the current technological capability to achieve a low effective background doping make these materials very promising for developing low-loss optical waveguides in the mid-IR. For instance, a recent work has demonstrated low propagation losses in the mid-IR based on air-cladded InGaAs-on-InP waveguides operating in TE polarization [1].
Because their phase can be compensated by a grating-based stretcher compressor and further controlled by RF injection, quantum cascade laser based optical frequency comb generation allowed the generation of pulse as short as 630fs after compression, value confirmed using an upconversion technique with a sub-picosecond time resolution. Another possibility is the direct generation of optical solitons using ring quantum cascade lasers in which, by using a very low lateral loss waveguide, the symmetric counter-propagating modes undergo a spontaneous symmetry breaking and generate solitons.
Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table-top comb is available. I will discuss direct THz comb generation from both homogeneous and heterogeneous quantum cascade lasers. Octave spanning emission spectra and comb operation on bandwidth larger than 1 THz are reported for heterogeneous cascades. I will also report on a series of new structures with homogeneous cascade design that feature a very low threshold current density (< 100 A/cm2), a bandwidth of roughly 1 THz centered a 3 THz and an extremely wide bandwidth (>1.8 THz) when driven in the NDR region. This extremely broadband emission in the NDR is studied as well with NEGF simulation and is based on an interplay between strong photon assisted transport due to the highly diagonal transition and domain formation.These structures are also showing RF injection locking with extremely reduced microwave powers. We will discuss locking experiments as well as a method to finely control the repetition rate of the laser based on controlled optical feedback. Time resolved spectral measurements aimed to clarify the physics of field domains in the NDR will be also presented.
The ultrafast gain recovery dynamics [1] observed in quantum cascade lasers (QCLs) fundamentally restricts the formation of intracavity pulses. On the other hand, this picosecond gain response makes the QCL uniquely suited for microwave modulation of its pump current. Here, we leverage on this property and generate short optical pulses (~ 30 ps) with up to Watt level peak power. Lasing on a single longitudinal mode is achieved via optical injection seeding. We characterize the generated optical pulses in both frequency and time domain using a spectrometer in combination with an optical sampling method. The obtained results are interpreted in the framework of laser rate equations.
In this work, we demonstrate control over the time-domain state quantum cascade laser output state using microwave modulation. We demonstrate narrow, pulse-like features with a full-with at half-maximum of 558 fs when isolated, which corresponds to the expected Fourier-transform limited pulse-width.
We demonstrate the manipulation of the quantum cascade laser output state using microwave-injection. In the spectral domain, the optical bandwidth can be doubled, whereas in the time-domain, we observe narrow, approximately 1 ps wide features.
We present strong radio-frequency current modulation close to their repetition frequency as a means to control the emitted state of quantum cascade laser frequency combs. In particular, more than doubling of the spectral bandwidth compared to free-running can be achieved throughout the dynamical range of the device. By changing the modulation frequency, the spectral bandwidth and center-frequency can be tuned and by fast switching between modulation frequencies we can multiplex spectral regions with negligible overlap from the same device. In the time-domain, we are able to transition from quasi-continuous to long-pulse output by injecting at high power.
In this work, we control the quantum cascade laser output state using microwave modulation. We demonstrate doubling of the spectral bandwidth as well as the generation of very narrow, approximately 1 ps wide features.
In this work, we demonstrate the generation of 630 fs, 4.5 W pulses from a mid-infrared quantum cascade laser by gain modulation induced spectral broadening and external pulse compression. Such sources open new pathways for broadband supercontinuum generation in the mid-infrared.
Quantum cascade lasers (QCLs) offer an attractive platform for mid-infrared frequency combs, due to their compact, on chip geometry, emitting powers as high as 1 W at room-temperature [1–3]. A defining feature is their quasi-continuous intensity output, which is predominantly frequency modulated. To force the QCL into amplitude-modulated operation, active modelocking has been used to achieve pulses on the picosecond-scale [4] and external compression has yielded pulses close to the Fourier Limit at 630 fs [5]. For shorter length scales beyond the Fourier-limit, superoscillations, which are caused by the interference of the different Fourier components, offer great potential for use in ultrafast spectroscopic techniques [6], [7].
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