We discuss and present preliminary experimental evidence of three novel add-on functionalities that can make Optical Feedback Imaging a truly small footprint and label-free bioimaging technology. The first is single-pixel compressed sensing. Here we report on scanless optical feedback imaging in free space by spatially modulated illumination of the target. The second is chemical sensitivity. Here, we report the identification of several pigments by selective spectral discrimination at three different wavelengths. The third functionality is the integration of OFI in silicon photonic chips. Here we identify the building blocks necessary to implement a scanless imaging system in an integrated photonic chip and show evidence of laser modulation through optical feedback provided by the emitted radiation after passing through a silicon passive integrated waveguide.
So far, dissipative temporal solitons in a laser with a saturable absorber have been studied mainly in the context either of Haus's master equation or of the cubic-quintic complex Ginzburg-Landau equation.
We present here a study based on an equation which includes saturation of the amplifier via a cubic approximation and saturation of the absorber at all orders. The equation describes well a system where both gain and absorption are fast, the laser is close to threshold, the unsaturated absorption is small, and the saturation intensity of the amplifier is much larger than that of the absorber. The model is appropriate for fast semiconductor lasers, such as quantum cascade lasers, since it encompasses the relevant phase-amplitude coupling via the linewidth enhancement factors of the gain and absorption media. Our study shows the crucial role played by these factors in the transition from cw emission to various types of pulsed emission, including dissipative temporal solitons.
We compare the spontaneous formation of Optical Frequency Combs (OFCs) in ring and Fabry-Perot (FP) Quantum Cascade Lasers (QCLs), adopting a reduced model derived from the Effective Semiconductor MaxwellBloch Equations near threshold and in the hypothesis of fast medium polarization and carriers. This model consists of a single spatiotemporal equation for an order parameter represented by the electric field. While in the ring configuration it reduces to a single complex Ginzburg-Landau equation (CGLE), in agreement with recent studies, in the FP configuration a non local integral term accounts for the coupling between forward and backward fields and the corresponding equation is equivalent to two coupled CGLEs. We first demonstrate how this approach allows us to reproduce the main features of OFCs in the two considered configurations. Then, we leverage its reduced mathematical complexity and universality to gain more physical insight into not fully understood phenomena, such as the self-starting emergence of harmonic OFCs.
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.
We address the dynamics of a III-V/SiN external cavity hybrid laser with a frequency-selective mirror. Simulations through a set of time-delayed algebraic equations accounting for the narrowband SiN mirror reflectivity demonstrate that CW instability occurs when relaxation oscillations become resonant with the beating between longitudinal modes. In the CW unstable regime, we observe the emergence of frequency combs due to four wave mixing, characterized by a modulation of amplitude and frequency. We then characterize the resulting combs in terms of achievable bandwidth. These results are also confirmed through a more accurate time-domain traveling wave model.
We study the dynamics of a multimode VCSEL with an elliptical oxide aperture for datacom applications. We simulate the laser dynamics through a set of coupled rate equations for the modal components of the electric field and the carrier density, accounting for coherent mode mixing and spatial hole burning. Our simulations show what are the relevant frequency detuning configurations to control in order to improve noise performance. Simulations with NRZ PRBS performed in order to explore the applications of these devices in short-reach data transmission show potentially reachable transmission speeds of 65 Gbit/s.
Bright pulses of light are unstable states in free-running semiconductor lasers. Stable bright solitons require an optical bistability---as predicted by mean-field theories such as the Complex Ginzburg Landau Equation (CGLE) or the Lugiato-Lefever Equation (LLE). However, this restriction is relaxed when two lasers are coupled to one another. Here, we identify a new state of light in a pair of semiconductor ring lasers with fast gain dynamics. Two racetrack (RT) quantum cascade lasers (QCLs) when coupled along their straight sections spontaneously produce a frequency comb over the hybridized modes of the coupled cavity. Waveform reconstruction measurements reveal the hybridized comb manifests itself as a pair of bright and dark pulses circulating the coupled cavity simultaneously. In addition, split-step integration of a pair of mutually forced CGLEs faithfully reproduces our experimental measurements, providing some insight on the formation of such states.
In this manuscript, we employ a time-domain traveling-wave model with a coupled-mode theory to characterize the dynamic behavior of a mid-Infrared (MIR) Quantum Cascade Laser (QCL) in the Distributed-Feedback (DFB) configuration. Our investigation underscores the crucial influence of the linewidth enhancement factor (LEF) and spatial hole burning (SHB) on the single-mode behavior of DFB QCLs. Disregarding these factors leads to an overestimation of the range of pump currents granting single-mode emission and results in an inaccurate simulation of the multimodal dynamics of DFB QCLs. The numerical simulations presented in this work closely align with experimental observations, specifically focusing on a DFB QCL operating at a wavelength of 9.34 μm.
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.
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.
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.
We use a prototypical model based on the Complex Ginzburg Landau Equation to study the dynamics of a multimode ring quantum cascade laser. We predict the existence and stability of different classes of localized structures in the system. In presence of coherent injection, we report the formation dissipative Temporal Solitons (TSs) which manifest multi-stability and coexistence with a stable CW solution. We show how these features allow for external manipulation of the spectral content of the optical frequency combs associated with TSs with a big impact on applications in the field of e.g. high precision spectroscopy and wireless communications.
We generalized the well-known Lugiato-Lefever Equation to unify the description of combs and localized structures formation in nonlinear optical systems such as Kerr micro-resonators (passive systems) and Quantum Cascade Lasers (QCL) (active systems). In particular this model was applied to the study of pattern formation in a unidirectional ring QCL driven by a coherent injected field. We showed the existence of Dissipative Solitons (DS) and Turing rolls associated to standard and harmonic Optical Frequency Combs (OFC) in the system. We also provided a proof of principle demonstration of the possibility to deterministically control the spectral properties of these OFC by switching-on one or more DS with suitable addressing pulses.
These results considerably increase the theoretical insight in chip-scale combs sources in the mid-infrared region of the electromagnetic spectrum for timely applications in the field of e.g. high resolution and/or time resolved molecular spectroscopy, long range and high bit rate wireless communications.
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.
We study the stability of a hybrid laser source consisting of a III-V reflective semiconductor optical amplifier (RSOA) edge-coupled to a silicon photonic mirror, based on two coupled high-Q microring resonators, providing a narrow band effective reflectivity. We simulate the laser dynamics through a model of time-delayed algebraic equations accounting for the frequency-selective mirror reflectivity, demonstrating single-mode emission, self pulsing, and turbulent regimes. Further, we identify the regions of higher CW operation in terms of bias current and laser detuning with respect to the reflectivity peak. Finally, we test the CW laser stability with respect to optical feedback, mimicking the effect of spurious back-reflections from the passive parts of the circuit, and demonstrate ultra-stable CW operation for a sizeable range of detuning.
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.
We propose a generalization of the well-known Lugiato-Lefever Equation to unify the description of combs- and structures- forming nonlinear optical systems. This approach rigorously connects for the first time passive systems such as Kerr micro-resonators and active systems such as Quantum Cascade Lasers (QCLs) which were so far treated separately. The model effectively describes a unidirectional ring QCL driven by a coherent signal where we show the existence of temporal solitons and Turing rolls, previously identified only in Kerr micro-resonators, considerably increasing the theoretical insight and the technological potential of chip-scale comb sources.
We developed a Time Domain Traveling Wave model to properly study the dynamics of a hybrid lasers realized by coupling a III-V Reflective Semiconductor Optical Amplifier with a Silicon Photonics mirror providing a narrow effective reflectivity (<10GHz). In free running operation mode, we show that for realistic values of the Henry factor stable single mode emission only occurs around the maximum of the reflectivity slope. Very interestingly for applications, in presence of optical-feedback, we access a regime of ultra-stability with respect to unwanted reflections or to self-oscillations triggered by a photon-photon resonance phenomenon.
We study the coherent multi-mode phenomena in single section Quantum Dot (QD) lasers using a time domain traveling wave approach.
In the conventional Fabry-Perot configuration and close to the lasing threshold, we report a continuous wave solution (CW) instability consisting in several longitudinal modes turned on by the strong carrier grating due to the standing wave pattern. In this regime we found intervals of the bias current where the FP-QD laser spontaneously emits Optical Frequency Combs (OFC) as narrow, equally spaced, spectral lines with locked phases.
Interestingly, in the unidirectional ring configuration, where carrier grating due to standing wave pattern cannot take place, our simulations show the occurrence (at high pump current) of a new type of self-pulsing phenomenon leading to sub-picosecond pulses with THz repetition rate, multiple of the ring free spectral range. The linear stability analysis of the CW solution of the ring laser is in good agreement with the numerical simulation and it allows to establish an analogy between the observed self-pulsing regime and the well known Risken-Nummedal-Graham-Haken instability consisting in the amplification of the Rabi frequency of the system. Systematic simulations also indicate that, contrary to what happens for self-generation of OFC in FP laser, THz self-pulsing is robust and controllable over a wide range of bias currents, device lengths and degree of inhomogeneous gain broadening. Our results on FP lasers well agree with recent experimental evidences.
We experimentally study the emission dynamics of a monolithic multi-section semiconductor laser based on InAs/InGaAs quantum dot (QD) material in the regime of passive mode-locked (ML) operation obtained via saturable losses in the absorber (reversed biased). When the active section is biased above the lasing threshold we observe emission of a regular train of optical pulses at 1250nm with characteristic repetition rate of 6 GHz. By sweeping back the pump current below lasing threshold, we verify that the ML solution coexists with the zero intensity ("off") solution, even in absence of any external optical injection.1 These evidences are very promising for the observation of temporal localized structures in compact, monolithic semiconductor photonics devices. Experimental results are validated by numerical simulations performed using a multi-section delayed differential equation (DDE) model to compute the evolution of the electrical field, coupled with the rate-equations that describe the carrier dynamics in the QD active and absorber media.
Optical Frequency Combs (OFC) generated by semiconductor lasers at optical communication wavelengths are promising laser sources for high capacity optical interconnects exploiting WDM techniques; very often they are integrated with Silicon Photonic integrated circuits to realize compact and low-cost transmitters. Quantum Dot (QD) or Quantum Dash (QDash) single section Fabry-Perot lasers have turned to be a good candidate for this application because they can generate a comb of self-locked optical lines using just one laser diode operating in CW and no saturable absorber section. In this talk we review the state-of-art of these devices and their applications, evidencing also the analogies with single section Quantum Cascade Lasers, that, as for QD and QDash lasers, generates optical combs in the mid-IR or THz range.
We will focus on the understanding of the physical effects that can explain the self-locking of the lasing lines and we will compare the self-locking mechanism in Quantum Dot and Quantum Well lasers. We will then present the numerical simulation tool we have developed to simulate the self-locking in Quantum Dot Fabry-Perot lasers. Our model is based on a Time-Domain Traveling-Wave (TDTW) approach that properly accounts for coherent radiation-matter interaction in the semiconductor active medium and includes the carrier grating generated by the optical standing wave pattern in the laser cavity. We show that the latter is the fundamental physical effect at the origin of the multi-wavelength spectrum appearing just above the laser threshold, but it is not enough for forcing the self-locking of the optical lines. The self-mode-locking regime associated with the emission of OFC is achieved for higher bias currents and it ascribed to nonlinear phase sensitive effects as Four Wave Mixing (FWM). To quantify the locking of the lines we have calculated some indicators that are obtained by the post processing of the calculated optical electric field of the laser output. These indicators are the RF spectrum at the beat note, the optical linewidth of the lasing lines and the Relative Intensity Noise (RIN) spectrum for both the total power and the power of each line. Varying the CW injected current above threshold we have observed three different regimes: in the first one, at low current, the laser is dominated by multi-wavelength emission with rather wide RF beat note and high low frequency RIN, this regime corresponds to an unlocked regime. In the optical spectrum we observe an optical line and side bands due to FWM components. In the second regime, at much higher current, the RF beat note is extremely narrow and the low frequency RIN of each line reduces significantly; in the optical spectrum the lines narrow and the side-bands disappear. This is a self-locked regime. In an intermediate current range, we have a transition regime where the state (locked or unlocked) depends on the initial conditions.
Our results explain in detail the behavior observed experimentally by different research groups and in different QD and Quantum Dash (QDash) devices.
Optical frequency combs generated by self mode-locking of single-section quantum dot based semiconductor lasers are ideal sources for applications in high capacity optical interconnects or high precision dual comb spectroscopy. We investigate a 1mm long InAs/InGaAs quantum dot semiconductor laser both experimentally and by simulations using a time-domain traveling-wave model. We observe that by increasing the injection current, the laser output exhibits an unlocked multi-mode behavior above the lasing threshold up to a certain current were the modes lock due to an internal non-linear effect in the active laser medium. This phase locking is experimentally and numerically observed by RF beat note line width analysis as well as by integrated relative intensity noise analysis. Both of these properties are significantly reduced above this locking threshold. The lowest experimentally measured RF line width amounts to 20 kHz, while for lower currents prior to the threshold the line width can be as high as hundreds of MHz. Our simulations confirm this threshold behavior and the simulated spectra are in good qualitative and quantitative agreement.
We report a systematic analysis of the phenomenon of self-generation of optical frequency combs in single section Fabry-Perot Quantum Dot lasers using a Time Domain Travelling Wave model. We show that the carriers grating due to the standing wave pattern (spatial hole burning) peculiar of Quantum Dots laser and the Four Wave Mixing are the key ingredients to explain spontaneous Optical Frequency Combs in these devices. Our results well agree with recent experimental evidences reported in semiconductor lasers based on Quantum Dots and Quantum Dashes active material and pave the way to the development of a simulation tool for the design of these comb laser sources for innovative applications in the field of high-data rate optical communications.
We study the dynamics of a Quantum Cascade Laser subject to strong optical feedback in the framework of the
Lang-Kobayashi model. In particular, we demonstrate that the continuous wave instability may lead to coherent
multimode oscillations that indicate spontaneous phase-locking among external cavity modes. We recently
predicted that this unique behavior is linked to the absence of relaxation oscillations in unipolar semiconductor
lasers, which are characterized by a fast carriers recombination time (class-A lasers). These theoretical evidences
may help understanding the mechanisms possibly leading to spontaneous mode-locking and pulse generation in
QCLs.
We demonstrate superresolution in position tracking sensing based on feedback interferometry in quantum cascade lasers
(QCLs). QCLs with optical feedback make highly compact sensors since they work as mixer oscillator and detector of
infrared radiation. Additionally, QCL continuous-wave emission remains stable at steady state in strong feedback
regimes, permitting to gain control on the nonlinearity of the QCL active medium. Here, nonlinear frequency mixing in a
QCL-based common-path interferometer is exploited to unveil object’s position with nanometer-scale resolution, far
beyond the intrinsic limit of half-wavelength. Experimental results are in excellent agreement with simulations based on
Lang-Kobayashi model encompassing multiple-target dynamics.
We demonstrate a common-path optical interferometer based on a quantum-cascade laser (QCL), in which the QCL acts
both as source and detector of the infrared radiation. The collinear arms of the interferometer are terminated by a plastic
surface (acting as the beam splitter) and by a metallic one (acting as the mirror). The different reflectivity of the surfaces
allows for high contrast feedback-interferometry fringes exhibited on the laser-emitted power and revealed by voltage
compliance measurement at the QCL terminals. The displacement of each surface can be identified and measured with
sub wavelength resolution. The experimental results are in excellent agreement with the numerical simulations based on
the Lang-Kobayashi model for multiple cavities. Applications to microfluidics and resonant chemical detection can be
envisaged.
We consider a multi-transverse mode Vertical Cavity Surface Emitting Laser (VCSEL) subject to optical feedback.
When the field profile can be described in terms of few Gauss-Laguerre modes we show that the self-mixing
interferometric signal exhibits features peculiar of the spatial distribution and/or polarization state of the re-injected
field. Based on these results we provide both theoretically and experimentally the proof-of-principle of
an operational scheme for a sensor that can be used to simultaneously measure target translations along the
optical axis and target rotations in the orthogonal plane.
The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased
the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the
laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and
time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material
and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the
removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal
rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical
feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties
(AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber
amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with
the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a
resolution of 0.41 μm by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector
system.
In the plane wave approximation, we study spatio-temporal dynamics of a semiconductor class B laser driven
by a coherent injected field in a Fabry-Perot configuration. Below the lasing threshold, we manage to reduce
the dynamics to a single evolution equation for the carrier density, to analytically compute the stationary field
configurations and to predict their stability. The numerical simulations, performed by implementing an efficient
and accurate split-step code, perfectly agree with the analytical results.
We consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit. We introduce a general stability analysis to evidence modulational-instabilities leading to the destabilization of a homogeneous field profile, eventually causing the formation of 3D structures. Further on, for accessible parametric domains, we show in simulations the phenomenon of total radiation confinement leading to the formation of 3D localized bright structures. Such structures are a direct generalization of 2D Cavity Solitons, recently observed in broad-area VCSELs, but they are confined also in the propagation dimension. At difference from freely propagating light bullets, here the self-organization proceeds from the resonator feedback/dissipation, combined with diffraction and nonlinearity.
We show that such cavity light bullets can be independently excited and erased by appropriate pulses. They can be addressed to form arrays in the transverse field profile as well as serial trains in the longitudinal direction of the resonator thus combining serial and parallel encoding in the same device. Once created, they endlessly travel the cavity roundtrip.
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