Linewidth enhancement factor

Top PDF Linewidth enhancement factor:

Extensive study of the linewidth enhancement factor of a distributed feedback quantum cascade laser at ultra-low temperature

Extensive study of the linewidth enhancement factor of a distributed feedback quantum cascade laser at ultra-low temperature

unveiled more than 30 years ago in the case of distributed feedback laser diodes. 9 In interband lasers, the dynamical properties of the device are governed by several parameters of the lasers such as the carrier-to-photon lifetime, 10 the differential gain of the materials 11 and the linewidth enhancement factor (LEF). 12 In this paper, we focus on the influence of the latter and study its evolution when varying the pump current. The LEF can also be found in the literature under the designation linewidth broadening factor or α-factor, since it is responsible for the linewidth broadening in the optical domain, resulting in a linewidth enhanced by a factor (1 + α 2 ) compared to the Shawlow-Townes limit. In laser diodes, experiments with several techniques, such as direct measurement of the subthreshold optical spectrum as the injected current is varied, 13 radio-frequency measurements, 14 analysis
En savoir plus

11 En savoir plus

Low linewidth enhancement factor and high optical feedback resistance of p-doped silicon based quantum dot lasers

Low linewidth enhancement factor and high optical feedback resistance of p-doped silicon based quantum dot lasers

The integration of optical functions on a microelectronic chip brings many innovative perspectives, along with the possibility to enhance the performances of photonic integrated circuits (PIC) [1]. Owing to the delta-like density of states, quantum dot (QD) lasers epitaxially grown onto silicon are very promising for achieving low-cost transmitters with high thermal stability and large insensitivity to optical reflections [1]. In this work, we investigate the linewidth enhancement factor (LEF) of a p-doped QD laser as well as its sensitivity to optical feedback. The p-doping is used to improve the thermal stability which occurs in QD lasers due to thermal broadening of carriers. Our results unveil that the p-doped QD laser exhibits a very low LEF which transforms into a high degree of resistance against optical feedback without exhibiting a route to coherence collapse as opposed to what is observed in heterogeneously integrated quantum well (QW) lasers [2]. As no on-chip optical isolators with sufficient isolation ratio and low loss have been reported so far, these results are decisive for the realization of chirp-free and Peltier-free silicon-based transmission systems without optical isolation.
En savoir plus

3 En savoir plus

Semiconductor quantum dot lasers epitaxially grown on silicon with low linewidth enhancement factor

Semiconductor quantum dot lasers epitaxially grown on silicon with low linewidth enhancement factor

down to 6.7 mA. 9 In this paper, we go a step beyond by inves- tigating the behavior of the linewidth enhancement factor (a H -factor) of silicon based QD lasers. The a H -factor is known as one of the most important parameters of semiconductor lasers, hence driving, for instance, the spectral linewidth and the sensitivity to optical injection or optical feedback. 10 At the system level, a large a H also results in a frequency chirp-

6 En savoir plus

Influence of the polarization anisotropy on the linewidth enhancement factor and reflection sensitivity of 1.55- μm InP-based InAs quantum dash lasers

Influence of the polarization anisotropy on the linewidth enhancement factor and reflection sensitivity of 1.55- μm InP-based InAs quantum dash lasers

InAs/InP QDash polarization on the linewidth enhancement factor (LEF) and the impact of this anisotropy on the reflection sensitivity to external reflections. The LEF is known to be a vital parameter in semi- conductor laser physics, which influences many characteristics and performance such as the spectral linewidth and the frequency chirp under direct modulation and on the top of that the dynamical response to external optical feedback. 13 A semiconductor laser’s sensi- tivity to parasitic reflections is directly tied to its LEF, and hence, the smaller the LEF, the better the stability as recently demonstrated with epitaxial QDot lasers grown on silicon. 6,14 The last point is of particu- lar importance for isolator-free photonic integrated circuits which require expensive and bulky optical isolators, hence leading to a large increase in the device footprint for on-chip integration.
En savoir plus

7 En savoir plus

Frequency-dependent linewidth enhancement factor of optical injection-locked quantum dot/dash lasers

Frequency-dependent linewidth enhancement factor of optical injection-locked quantum dot/dash lasers

theoretical expression is derived to describe this effect, beyond the classical model. At low modulation frequencies (less than several GHz), optical injection can reduce the α H -factor value, and removes the large value of the FAIR. In addition, the α H -factor exhibits a peak in the modulation response at low injection ratio or at positive frequency detuning. However, the phase-amplitude coupling behavior at high modulation frequency (more than 10 GHz) is little changed by optical injection. These combined theoretical and experimental results show that optical injection offers a promising way to manipulate the linewidth enhancement factor of nanostructured lasers. Future work will involve additional experiments on other types of nanostructure lasers with different compositions and/or material substrates, spectral linewidth investigation of the modulation-frequency dependence of the αH-factor as well as comprehensive numerical studies to understand the physical origin of the peak in the αH- factor response.
En savoir plus

11 En savoir plus

Linewidth enhancement factor and optical injection in a hybrid-silicon quantum dot comb laser

Linewidth enhancement factor and optical injection in a hybrid-silicon quantum dot comb laser

Linewidth enhancement factor and optical injection in a hybrid-silicon quantum dot comb laser B. Dong 1 , A. Sawadogo 1 , J. Duan 1 , H. Huang 1 , G. Kurczveil 2 , D. Liang 2 , and F. Grillot 1,3 1) LTCI, Télécom ParisTech, 46 Rue Barrault, 75013 Paris, France 2)

3 En savoir plus

Video enhancement with convex optimization methods

Video enhancement with convex optimization methods

a compressed dataset. However, in practice,the learned model needs to be related to the compression ratio used. In a previous work [12], we presented a new theoretical im- age enhancement framework that handles multiple compressed representations while in [13], we show an extension to multi source compressed video and propose new applications. The framework has the particularity to explicitly model a generic compression stage. A first single source HEVC application of the framework in [14], where the SR potential on Intra frames is discussed. The present work further analyzes the framework behavior with single source HEVC encoding. In particular, predicted frames are now included in the SR framework, and a more thorough experimental evaluation is provided. The method’s behavior with respect to one of the best performing SS-SR approaches is discussed in multiple scenarios, showing the high adaptation capability of the proposed framework.
En savoir plus

6 En savoir plus

Bayesian factor analysis

Bayesian factor analysis

The marginal density of A posterior to observing (S,N) will in general differ from the marginal prior density of A , and may do so in such a way as to permit iden[r]

34 En savoir plus

Resolution enhancement in medical ultrasound imaging

Resolution enhancement in medical ultrasound imaging

b University of Toulouse, IRIT UMR CNRS 5505, Toulouse, France Abstract. Image resolution enhancement is a problem of considerable interest in all medical imaging modalities. Unlike general purpose imaging or video processing, for a very long time, medical image resolution enhance- ment has been based on optimization of the imaging devices. Although some recent works purport to deal with image postprocessing, much remains to be done regarding medical image enhancement via postprocessing, especially in ultrasound imaging. We face a resolution improvement issue in the case of medical ultrasound imaging. We propose to investigate this problem using multidimensional autoregressive (AR) models. Noting that the estimation of the envelope of an ultrasound radio frequency (RF) signal is very similar to the estimation of classical Fourier-based power spectrum estimation, we theoretically show that a domain change and a multi- dimensional AR model can be used to achieve super-resolution in ultrasound imaging provided the order is estimated correctly. Here, this is done by means of a technique that simultaneously estimates the order and the parameters of a multidimensional model using relevant regression matrix factorization. Doing so, the pro- posed method specifically fits ultrasound imaging and provides an estimated envelope. Moreover, an expression that links the theoretical image resolution to both the image acquisition features (such as the point spread func- tion) and a postprocessing feature (the AR model) order is derived. The overall contribution of this work is three- fold. First, it allows for automatic resolution improvement. Through a simple model and without any specific manual algorithmic parameter tuning, as is used in common methods, the proposed technique simply and exclu- sively uses the ultrasound RF signal as input and provides the improved B-mode as output. Second, it allows for the a priori prediction of the improvement in resolution via the knowledge of the parametric model order before actual processing. Finally, to achieve the previous goal, while classical parametric methods would first estimate the model order and then the model parameters, our approach estimates the model parameters and the order simultaneously. The effectiveness of the methodology is validated using two-dimensional synthetic and in vivo data. We show that, compared to other techniques, our method provides better results from a qualitative and a quantitative viewpoint.
En savoir plus

13 En savoir plus

Dynamic Factor Models

Dynamic Factor Models

criterion to the sample variance matrix of these residuals yields a consistent estimate of the number of dynamic factors, q. 2.6 Forecasting with Large Dynamic Factor Models One of the most important uses of dynamic factor models is forecasting. Both small scale (i.e. n small) and large scale dynamic factor models have been used to this end. Very often, the forecasted variable is published with a delay: for instance, euro-area GDP is published six weeks after the end of the corresponding quarter. The long delay in data release implies that different types of forecasts can be considered. GDP predictions for quarter Q, made prior to that quarter, in Q 1,Q 2, . . . for instance, are considered to be “true” out of sample forecasts. But estimates for quarter Q can also be made during that same quarter using high frequency data released within quarter Q. These are called “nowcasts”. Finally, since GDP for quarter Q is not known until a few weeks into quarter Q +1, forecasters keep estimating it during this time interval. These estimates are considered to be “backcasts”. As we have seen, dynamic factor models are very flexible and can easily handle all these predictions. Forecasts using large dimensional factor models were first introduced in the literature by Stock and Watson (2002a). The method, also denoted diffusion index forecasts, consists of estimating the factors f t by principal component analysis, and
En savoir plus

42 En savoir plus

Enhancement of mixing by rodlike polymers

Enhancement of mixing by rodlike polymers

III. MIXING ENHANCEMENT In Figure 1 we compare the temporal evolution of the scalar field with and without polymers starting from the initial condition B. In the absence of polymers, molecular diffusion simply blurs the borders between the white and black stripes, and even after a long time the scalar field remains essentially unmixed. Conversely, over a compa- rable time interval the chaotic flow induced by the rodlike polymers mixes the scalar field efficiently.

7 En savoir plus

Method Enhancement by Scenario Based Techniques

Method Enhancement by Scenario Based Techniques

1 Introduction The aim of analysis methods is to define the specification of a future system. In the new generation of such analysis methods ([1], [2], [3]) scenario-based approaches have been introduced to bridge the gap between the user view and the functional view of the future system and therefore ensure that the future system will meet the requirements of its users. In the CREWS 1 project, four different scenario-based approaches have been developed with the aim of supporting requirements acquisition from real world scenes [4] and from natural language scenario descriptions [5], [6] and requirements validation though scenario walkthrough [7] and scenario animation [8]. The hypothesis of the project is that each of the approaches might be useful in specific project situations which are not well tackled by existing analysis methods and therefore, that it is worth looking for the integration of such approaches in current methods. This shall lead to an enhancement of the existing methods with scenario- based techniques.
En savoir plus

19 En savoir plus

3.3 µm interband-cascade resonant-cavity light-emitting diode with narrow spectral emission linewidth

3.3 µm interband-cascade resonant-cavity light-emitting diode with narrow spectral emission linewidth

11 the band gap temperature variation, whereas the series resistance remains essentially constant at ̴ 10 . It is interesting to note that the optical power increases with temperature. For a CW driving current of 36 mA, the output power increases from 60 µW to 90 µW at 20°C and 80°C, respectively. This effect is attributed to a better alignment of with the microcavity resonance [Sarcan F, Nordin M S, Erol A, and Vickers A J 2017 Superlattices and Microstructures, 102 27-34 ] as well as maintaining its high quantum efficiency at elevated temperatures. The intensity of the IC- RCLED emission increased with the temperature in the studied range by a factor of ̴ 1.35, while maintaining the same FWHM of ̴ 20 nm. The measured emission linewidth is about 20 times smaller as compared to the conventional IC-LED in the whole studied temperature range. Another advantage of the IC-RCLED is a better temperature stability of the emission wavelength. Indeed, as it is shown on the inset of the Figure , the emission peak linearly increases with a rate of ̴ 0.32 nm/°C, which is considerably lower than the temperature shift those measured for IC-LED [28] and very similarly than other Sb-based VCSELs [13] [10]. Contrary to the IC-LED where the tunability of about 1.37 nm/°C [28] is related
En savoir plus

14 En savoir plus

Simple method to measure laser linewidth using intensity noise spectrum based on Rayleigh Backscattering effect

Simple method to measure laser linewidth using intensity noise spectrum based on Rayleigh Backscattering effect

Fig. 1 Sketch of the experimental setup. Fig. 2 The experimental results for 2 types of laser: The RIN spectrum at the RB emulator output (a-1, a-2), the linewidths inferred from the RIN spectrum (b-1, b-2) and the linewidths measured by the delayed self heterodyne technique (c-1, c-2). Fig. 2 shows experimental results for two types of laser: A DFB laser with a linewidth of 2.4 MHz (upper curves) and a tunable external-cavity laser (Tunics TM ) with a linewidth of 120 kHz (lower curves). The RIN spectrum obtained at the RB emulator output (figure1) are shown in figures 2.a-1 and 2.a-2. The laser lines are inferred by restituting the negative side and dividing the frequency scale by two, from the RIN spectrum, given in figures 2.b-1 and 2.b-2. These linewidths are then compared to those obtained in figures 2.c-1 and 2.c-2, thanks to a commercial delayed-self-heterodyne linewidth test set (Advantest Q73321). The results are in good agreements and show that our all-fibered technique is well suited to study linewidth from 100 kHz to several 100 MHz.
En savoir plus

2 En savoir plus

Resolution enhancement in medical ultrasound imaging

Resolution enhancement in medical ultrasound imaging

b University of Toulouse, IRIT UMR CNRS 5505, Toulouse, France Abstract. Image resolution enhancement is a problem of considerable interest in all medical imaging modalities. Unlike general purpose imaging or video processing, for a very long time, medical image resolution enhance- ment has been based on optimization of the imaging devices. Although some recent works purport to deal with image postprocessing, much remains to be done regarding medical image enhancement via postprocessing, especially in ultrasound imaging. We face a resolution improvement issue in the case of medical ultrasound imaging. We propose to investigate this problem using multidimensional autoregressive (AR) models. Noting that the estimation of the envelope of an ultrasound radio frequency (RF) signal is very similar to the estimation of classical Fourier-based power spectrum estimation, we theoretically show that a domain change and a multi- dimensional AR model can be used to achieve super-resolution in ultrasound imaging provided the order is estimated correctly. Here, this is done by means of a technique that simultaneously estimates the order and the parameters of a multidimensional model using relevant regression matrix factorization. Doing so, the pro- posed method specifically fits ultrasound imaging and provides an estimated envelope. Moreover, an expression that links the theoretical image resolution to both the image acquisition features (such as the point spread func- tion) and a postprocessing feature (the AR model) order is derived. The overall contribution of this work is three- fold. First, it allows for automatic resolution improvement. Through a simple model and without any specific manual algorithmic parameter tuning, as is used in common methods, the proposed technique simply and exclu- sively uses the ultrasound RF signal as input and provides the improved B-mode as output. Second, it allows for the a priori prediction of the improvement in resolution via the knowledge of the parametric model order before actual processing. Finally, to achieve the previous goal, while classical parametric methods would first estimate the model order and then the model parameters, our approach estimates the model parameters and the order simultaneously. The effectiveness of the methodology is validated using two-dimensional synthetic and in vivo data. We show that, compared to other techniques, our method provides better results from a qualitative and a quantitative viewpoint.
En savoir plus

14 En savoir plus

Production factor returns : the role of factor utilisation

Production factor returns : the role of factor utilisation

Our empirical analysis merges two firm-level annual datasets constructed by the Banque de France: FiBEn and a survey on factor utilisation degrees (FUDS, hereafter). FiBEn is a large database built on fiscal documents, including balance sheets and profit-and-loss statements. It features all French firms with sales exceeding 750,000€ per year, or with a credit outstanding higher than 380,000€. Every year, these accounting data are available for about 200,000 firms. In 2004, FiBEn was covering 80 % of the firms with 20 to 500 employees, and 98 % of those employing more than 500 employees. This database allows calculating firm-level value added (Q), capital (K) and employment (L) volumes:
En savoir plus

21 En savoir plus

Nanostructured Thin Films for Fluorescence Enhancement

Nanostructured Thin Films for Fluorescence Enhancement

Figure 6. Global and local (”hot spot”) fluorescence enhancement factor versus silver deposition. The proportion of active molecules is deduce from silver coverage measured on TEM images The observed fluorescence enhancement on figure 5 is associated with the global enhancement of the dye layer. However, it is well known that the the local electromagnetic field is strongly enhanced in localized area, called ”hot spot”. 8 In particular, fluorescence is quenched when the dye molecules are deposited directly on the metallic nanoparticles in favour of the surface enhanced RAMAN scattering. Hence, the total fluorescence intensity is the result of only a part of the dye molecules situated in ”active sites”.
En savoir plus

8 En savoir plus

Hit the road: assisted migration as population enhancement?

Hit the road: assisted migration as population enhancement?

Hit the road: assisted migration as population enhancement? Danielle Frechette 1, *, Normand Bergeron 1 & Mélanie Dionne 2 (1) INRS, Centre Eau Terre Environnement, Québec, Canada; (2) Ministère des Forêts, de la Faune et des Parcs, Québec, Canada 1. WHAT IS ASSISTED MIGRATION (A.M)?

1 En savoir plus

Wavelet noise reduction : application to speech enhancement

Wavelet noise reduction : application to speech enhancement

4. Proposed approach The wavelet thresholding technique is a simple method to remove noise from corrupted signal. As pointed out previously, it was not successfully applied in speech enhancement. These difficulties are simultaneously associated to the speech signal complexity and to the nature of the noise. To improve the wavelet thresholding enhancement, we then propose:

2 En savoir plus

Vision Enhancement in Homogeneous and Heterogeneous Fog

Vision Enhancement in Homogeneous and Heterogeneous Fog

Fig. 7. Principle of a Fog Vision Enhancement System (FVES): the restored image is displayed to the driver by means of a HUD which allows the driver to better see potential obstacles and thus decreases his reaction time. With uniform fog, enhancement with free-space segmenta- tion (FSS) and with no-black-pixel constraint combined with planar scene assumption (NBPC+PA) gives the best results. A second group of algorithms with similar performance for uniform fog images contains: dark channel prior (DCP), no- black-pixel constraint (NBPC) and multiscale retinex (MSR). These last three algorithms are less efficient than the first two due to the difficulty to restore the correct average intensity on the road part of the image. NBPC+PA brings the performance of NBPC at long range distances without contrast distortions on the road part of the image thanks to the combination with the planar assumption.
En savoir plus

13 En savoir plus

Show all 1476 documents...