The progress of nanotechnologies has triggered the emergence of many photonic artificial structures: photonic crystals, metamaterials, plasmonic resonators. Recently the intriguing class of PT-symmetric devices, referring to Parity-Time symmetry  has attracted much attention. The characteristic feature of PT-symmetry is that the structures' refractive index profile is complex-valued due to the presence of alternating gain and loss regions in the system. Apart from fundamental research motivations, the tremendous interest in these artificial systems is strongly driven by the practical outcomes expected to foster a new generation of tunable, reconfigurable and non-reciprocal devices.
The principle of gain-loss modulation lying in the heart of PT-symmetry optics enables a range of innovative solutions in the field of integrated optics at 1.5μm [2-7]. By using PT-symmetric coupled waveguides and Bragg reflectors as fundamental building blocks, it is possible to build a wide variety of functional optical devices. The PT-symmetry principle provides an alternative way for the realization of active devices that could become functional in a new platform for integrated optics. For instance one major bottleneck of the III-V/Si hybrid integration approach is that each type of active devices (laser, modulator, etc) requires a specific composition of III-V semiconductor alloy, involving a variety of (re)growth challenges. The advantage of the PT-symmetry solution is that the fabrication of all these devices can be done with a single stack of III-V semiconductor alloys that greatly simplifies the technological process.
The aim of the current contribution is to provide a survey of the most promising applications of PT-symmetry in photonics with a particular emphases on the transition from theoretical concepts to experimental devices. The intention is to draw attention to the risks and issues related to the practical implementation that are most often overlooked in the basic theoretical models. An analysis of solutions to circumvent or overcome these issues to achieve a proper devices operation will be presented. Preliminary results on the experimental realization of PT symmetric structures using III-V's technology will be communicated.
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 J. Čtyroký, V. Kuzmiak, and S. Eyderman, “Waveguide structures with antisymmetric gain/loss profile,” Opt. Express 18, 21585-21593 (2010).
 A. Lupu, H. Benisty, A. Degiron, “Switching using PT symmetry in plasmonic systems: positive role of the losses,” Opt. Express 21, 21651-21668 (2013).
 S. Phang, A. Vukovic, H. Susanto, T. M. Benson, and Ph. Sewell, “Ultrafast optical switching using parity-time symmetric Bragg gratings. J. Opt. Soc. Am. B 30, 2984 (2013).
 H. Benisty, A. Lupu, A. Degiron, “Transverse periodic PT symmetry for modal demultiplexing in optical waveguides,” Phys. Rev. A 91, 053825 (2015).
 S. Phang, A. Vukovic, S. C. Creagh, P. D. Sewell, G. Gradoni, T. M. Benson, T. M. “Localized Single Frequency Lasing States in a Finite Parity-Time Symmetric Resonator Chain,” Scientific Reports, 6, 20499 (2016).
 A. Lupu, H. Benisty, A. Lavrinenko, “Tailoring spectral properties of binary PT-symmetric gratings by using duty cycle methods,” JSTQE 22, 35-41 (2016).
Chip-scale frequency comb sources are key elements for a variety of applications, comprising massively parallel optical communications and high-precision optical metrology. In this talk, we give an overview on our recent progress in the area of integrated optical comb generators and of the associated applications. Our experiments cover modulator-based comb sources, injection locking of gain-switched laser diodes, quantum-dash mode-locked lasers, as well as Kerr comb sources based on cavity solitons. We evaluate and compare the performance of these devices as optical sources for massively parallel wavelength division multiplexing at multi-terabit/s data rates, and we report on comb-based approaches for high-precision distance metrology.
Optical frequency combs have great potential for ultra-high bit rate telecommunications e.g. optical orthogonal
frequency-division multiplexing superchannels. For frequency comb generation, monolithic Quantum Dash
semiconductor mode-locked lasers are very attractive candidates owing to their broadband optical spectrum, inherent
intrinsic low noise and compactness. The active region is based on InAs nanostructures grown on InP for operation in
the 1.55 μm window. Owing to enhanced nonlinear effects, a single gain section generates short pulses in the modelocking
regime without resorting to an absorber section. An optical bandwidth over 1.3 THz yielding over 100 channels,
10 GHz spaced, is reported. Mode-locking properties are analyzed in the frequency domain using the concept of supermodes.
An Allan deviation down to ~ 10<sup>-9</sup> is reported for these passively mode-locked lasers. The low timing jitter, longterm
stability and high channel count of these QD based combs are of great potential for Tb/s data transmission with
only one single FP type laser source.
Monolithic semiconductor passively mode locked lasers (MLL) are very attractive components for many applications
including high bit rate telecommunications, microwave photonics and instrumentation. Owing to the three dimensional
confinement of the charge carriers, quantum dot based mode-locked lasers have been the subject of intense investigations
because of their improved performance compared to conventional material systems. Indeed, the inhomogeneous gain
broadening and the ultrafast absorption recovery dynamics are an asset for short pulse generation. Moreover, the weak
coupling of amplified spontaneous emission with the guided modes plus low loss waveguide leads to low timing jitter.
Our work concentrates on InAs quantum dash nanostructures grown on InP substrate, intended for applications in the
1.55 μm telecom window. InAs/InP quantum dash based lasers, in particular, have demonstrated efficient mode locking
in single section Fabry-Perot configurations. The flat optical spectrum of about 12 nm, combined with the narrow RF
beat note linewidth of about 10 kHz make them a promising technology for optical frequency comb generation.
Coherence between spectral modes was assessed by means of spectral phase measurements. The parabolic spectral phase
profile indicates that short pulses can be obtained provided the intracavity dispersion can be compensated by inserting a
single mode fiber.
The effect of controlled optical feedback has been investigated for InAs/InP laser structures operating in the 1.55
μm fiber window. Mode locked lasers in particular show extremely small phase noise when subjected to optical
feedback, implying a very low timing jitter which is of interest for many applications.
Monolithic semiconductor mode-locked lasers (MLLs) are rising considerable interest for such diverse applications as
very high speed optical time division multiplexing sources (40-160 GHz), all-optical signal processing, and low noise
sampling for signal monitoring of optical networks.
In a large number of these applications, MLLs may be subjected to optical feedback generated by unwanted reflections
in optical systems which may greatly degrade laser performance. A number of experimental studies have been performed
to evaluate the sensitivity of MLLs to optical feedback showing an increase of phase noise [1-5]. Quantum-dash (Qdash)
based Fabry Perot lasers have been shown to exhibit an improved tolerance to feedback .
In this work, optical feedback tolerance is investigated for a monolithic quantum-dash-based passive mode-locked laser
emitting at 1.58 μm. The two-section device generates ~5 ps pulses at a repetition rate of 17 GHz. The onset of the
coherence collapse (CC) regime is experimentally determined by measuring the broadening of the longitudinal modes in
the optical spectrum. Depending on bias condition, the CC regime is reached for values of feedback ranging from -35 dB
to -29 dB at which emitted pulses were slighly broadened. The radio-frequency (RF) linewidth was simultaneously
assessed and a drastic reduction of the RF linewidth with increasing feedback strength is evidenced. This indicates a
reduction of the phase noise, thus implying a low "high frequency" timing jitter. We in particular observed an RF
linewidth narrowing down to a value of less than 1 kHz under optical feedback.
This paper presents recent progress in the field of semiconductor lasers based on self-assembled quantum dots grown
either on GaAs or InP substrates.
Quantum dot (QD) based lasers are attracting a lot of interest owing to their remarkable optoelectronic properties that
result from the three dimensional carrier confinement. They are indeed expected to exhibit much improved performance
than that of quantum well devices. Extremely low threshold currents as well as high temperature stability have readily
been demonstrated in the InAs/GaAs material system.
The unique properties of quantum dot based active layers such as broad optical gain spectrum, high saturation output
power, ultrafast gain dynamics and low loss are also very attractive for the realization of mode-locked lasers.
Recent results in the field of directly modulated InAs/GaAs lasers emitting in the 1.3 μm window are discussed. We
report in particular on temperature independent linewidth enhancement factor (or Henry factor α<sub>H</sub>) up to 85°C. This is a
key parameter which determines many laser dynamic properties. Optical feedback insensitive operation of specifically
band-gap engineered devices, compatible with high bit rate isolator-less transmission is also reported at 1.55 μm.
Monolithic mode locked lasers based on InAs/InP quantum dashes have been investigated for 1.55 μm applications. Subpicosecond
pulse generation at very high repetition rates (> 100 GHz) is reported for self-pulsating one-section Fabry
Perot devices. Specific applications based on these compact pulse generators including high bit rate clock recovery are
We present an extension of an early work on external optical feedback in semiconductor lasers. A more general
formalism has been developed, which takes into consideration the anisotropy properties of an external cavity. The
expressions are derived for description of the feedback phenomena in a system composed of a laser diode and a Fabry-Perot cavity which is optically birefringent. We show that the emission behavior of such a system can be strongly
affected by the polarization states of the feedback waves, and that therefore multiple solutions become possible for
stabilization of a composite mode. Particular attention is paid to the angle-dependent phase condition. Examples are
given for a laser emitting at the wavelength of 1.54 μm and for an external cavity made of a quartz crystal.
The quantum dots have added great benefits to the photonic activity, among them the decoupling between the lattice parameter of the substrate and the dot has opened the way to enlarge the spectral windows which can be accessible on different substrates. For example on a GaAs substrate a long wavelength laser emission of 1.46 μm
has been demonstrated at room temperature. The specific properties like: large material gain, large spectral
bandwidth, high speed carrier dynamics, have improved device performances. The minimum threshold current
densities of laser devices, the large spectral bandwidth of semiconductor optical amplifiers and the very high
repetition rate and very short pulse width on mode locked lasers are other benefits.
We show that the spontaneous-emission behavior of an emitter-embedded periodic photonic band-gap (PBG) stack, described in general on the basis of the approximation of dipole emission, can be considerably modified if the macroscopic features of the emitter are considered. We have developed a model which takes into account the optical thickness of the emitter in the usual formalism. The extended model is presented in comparison with the classical dipole-emission model. Some numerical results are given and discussed, by using GaAs-embedded SiO2 / TiO2-coated quarter-wave stacks as a specific configuration. Our model provides quantitative arguments for optimization of spontaneous-emission power in terms of radiation frequency and emitter localization. It can be directly applied to optimum design of more complex systems, such as multi-emitter-embedded periodic stacks and any other passive structure.
We show that cascading two finite one-dimensional periodic photonic band-gap structures may result, due to the multiple confinement of the electromagnetic field, in a significant modification of the modal distribution of the electromagnetic density of modes (DOM) around a band edge. As a result of the combination of the band-gap effects and the band-edge effects, the enhancement and the optimum localization of the DOM can be simultaneously obtained. The general approach is presented. The basic rules and the main steps for the design of such a structure are described. Some numerical results are given and discussed, by using alternating Ga0.7Al0.3As/AlAs layer quarter-wave and mixed quarter-wave/half-wave stacks. The principle of the present work, although described within the limits of linear and dispersionless materials, may be directly applied to nonlinear parametric processes.