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 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 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
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
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
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.