Theoretical and experimental study of optical gain, refractive index change, and linewidth enhancement factor of p-doped quantum-dot lasers

A theoretical and experimental study of the optical gain, refractive index change, and linewidth enhancement factor (LEF) of a p-doped quantum-dot (QD) laser is reported. These parameters are measured by injecting an external pump, which induces cross-gain and cross-phase modulation. A comprehensive theoretical model for the optical gain and refractive index change of InAs QD lasers is introduced with the quasi-equilibrium approximation of carrier distribution. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. We match the experimental data with the theoretical results when the thermal effect is isolated by an additional pulsed current measurement. We also calculate theoretically the optical gain, refractive index change, and LEF of an undoped QD laser of the same structure except the absence of p-type doping. We show that the differential gain and LEF of the p-doped QD laser are improved compared with those of the undoped QD laser due to the reduced transparency carrier density.     

Characteristics measurement of gain and refractive index of traveling-wave semiconductor optical amplifier

A novel method to measure the gain and refractive index characteristics of traveling-wave semiconductor optical amplifier（TMA） is presented. In-out fiber ends of TWA are used to construct an external cavity resonator to produce big ripple on amplified spontaneous emission（ASE） spectrum. By this means,Hakki-Paoli method is adepted to obtain the gain spectra of TWA over a wide spectral range. From measured longitudinal mode spacing and peak wavelength shift due to increased bias current, we further calculate the effective refractive index and the refractive index change. Special feature of refractive index change above lasing threshold is revealed and explained.     

Comparison of linewidth enhancement factor between p-doped and undoped quantum-dot lasers

The optical linewidth enhancement factor (LEF) of a p-doped quantum-dot (QD) laser is measured below threshold and compared with a theoretical calculation. The optical gain, refractive index, and LEF are well matched with our theoretical model when the thermal effect is isolated by an additional pulse current measurement of the LEF. We also theoretically calculate the LEF of an undoped QD Fabry-Pe/spl acute/rot (FP) laser assuming that the structure of the undoped FP QD laser is the same as that of the p-doped QD FP laser except the p-type doping. The changes in modal gain and refractive index due to the respective QD ground and excited states are calculated. Based on the theoretical results, we show that the LEF of the p-doped QD laser is smaller than that of the undoped QD laser due to the reduced transparency carrier density.     

Continuously tunable organic solid-state DFB laser utilizing molecular reorientation in molecular glasses

We report a facile way for continuously tuning the lasing wavelength of an organic thin-film distributed feedback (DFB) laser after device fabrication by varying the effective refractive index seen by the one-dimension DFB laser structure. Varying the effective refractive indices of the organic gain medium and thus the effective refractive index of a one-dimension DFB laser structure after device fabrication is made possible with reorientation of molecules in a molecular glass at elevated temperatures. Distributions of molecular orientations can be fine controlled by annealing temperatures and times, permitting continuous tuning of optical properties and lasing wavelengths. Molecular reorientation can be conducted after devices are made, thus giving one the freedom to set or tune the lasing wavelength to meet a particular purpose with a common structure.     

Theory and experiment on the amplified spontaneous emission fromdistributed-feedback lasers

The amplified spontaneous emission (ASE) of a strained quantum-well distributed feedback (DFB) laser biased below laser threshold is used to extract the gain and refractive index spectra in a systematic manner. A modified Hakki-Paoli method is used to obtain the gain and differential gain spectra. The refractive index change due to carrier injection is obtained from the shift of the Fabry-Perot peaks in the ASE spectrum. The measured ASE spectrum, gain, refractive index change, and linewidth enhancement factor are then compared with our theoretical model for strained quantum-well lasers. Our model takes into account the realistic band structure and uses the material and quantum-well dimensions directly in the calculation of the electronic and optical properties. The theory agrees very well with the experiment     

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved.<>     

Optical gain and refractive index of a laser amplifier in the presence of pump light for cross-gain and cross-phase modulation

The wavelength dependent changes in optical gain and refractive index in a Fabry-Perot semiconductor optical amplifier are measured for various detunings of the pump wavelength from the quasi-Fermi level separation. The refractive index change is nearly constant over a very large wavelength range. We also present data for the linewidth enhancement factor due to optical injection. Estimates of the carrier densities and stimulated recombination rates are made using our optical gain model based on realistic band structure calculations for a strained quantum-well laser. Our results are very useful for ultra-broad-band wavelength conversion by cross-gain and cross-phase modulation (XGM, XPM).     

Effect of an excited-state optical transition on the linewidth enhancement factor of quantum dot lasers

An analytical expression is derived for the linewidth enhancement factor of a quantum-dot laser, which makes it possible to describe its dependence on optical loss and photon density in an explicit form. The model accounts for refractive index variations at the ground-state optical transition due to gain/absorption variations upon the first excited-state transition in quantum dots. It is shown that a decrease in optical loss, an increase in saturated gain, and an increase in the energy separation between the excited-state and ground-state transitions results in a decrease in the α factor both at and above the lasing threshold.     

温度变化对光纤光栅外腔半导体激光器激射波长的影响

在考虑了光纤光栅的位相后，从光纤光栅外腔半导体激光器(FGESL)所满足的阈值条件出发，从理论上研究了温度的变化对FGESL激射波长的影响。数值模拟的结果表明：由于温度变化造成半导体介质和光纤的折射率发生变化从而导致FGESL的纵模发生移动，因此FGESL的激射波长随着温度的升高将存在向长波长方向发生移动的趋势。对于短外腔，FGESL的纵模间距较大，由于半导体介质折射率隧温度的变化程度大于光纤折射率随温度的变化程度，因而存在模式跳跃现象；对于长外腔，由于FGESL的纵模间隔很小，因而不存在明显的模式跳跃现象。这些结果符合其他研究者的实验观测。     

Coherently tunable mid-infrared distributed feedback lasers

We propose utilization of quantum interference effects in quantum well structures to tune lasing wavelengths of mid-infrared distributed feedback lasers. The interference effects are generated via interaction of an intense laser field with an n-doped quantum well, causing coherent suppression or enhancement of refractive indexes of the conduction intersubband transitions. We show that these processes allow us to shift lasing wavelength to shorter or longer wavelengths by adjusting the intensity and frequency of the intense laser. This study is done for two types of lasers: 1) an electromagnetically induced distributed feedback intersubband laser formed by embedding a longitudinal corrugation of several periods of the quantum well structure within a waveguide structure and 2) a phase-shifted distributed feedback laser where the quantum well is inserted in the middle of an index grating, forming an active phase shift region. In the former the intense laser field is responsible for generation of optical feedback while shifting the coherently induced stop-band. In the latter, however, this field changes the optical length of the phase shift region, tuning the lasing mode within the stop-band. We show that the amount of the wavelength shift, which can reach 17 nm, is controlled by the intensity of the intense laser. The sign of the tuning process (red or blue shift), however, is decided by the frequency of this field, after proper choice of the corrugation periods. We investigate the optical feedback mechanisms in such coherently tunable lasers and discuss how they are related to an electromagnetically induced transparency process that happens in the conduction intersubband transitions.     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

Optimization of Thermo‐Optic Parameters for Temperature‐Insensitive LPWG Refractometers

In this paper, we report numerically calculated results of testing a temperature‐insensitive refractive sensor based on a planar‐type long‐period waveguide grating (LPWG). The LPWG consists of properly chosen polymer materials with an optimized thermo‐optic coefficient for the core layer in a four‐layer waveguide structure. The resonant wavelength shift below the spectral resolution of the conventional optical spectrum analyzer is obtained accurately over a temperature change of ±7.5°C even without any temperature control. The refractive index sensitivity of the proposed grating scheme is about 0.004 per resonant wavelength shift of 0.1 nm for an optimized thermo‐optic coefficient.     

Chaotic self-pulsation and cross-modulation in awavelength-selective external-cavity laser diode

Chaotic self-pulsation in a single wavelength external-cavity laser diode is observed. It is shown that the self-pulsation is caused by interdependencies between the optical output power and the compound cavity losses through the refractive index of the laser diode material. Refractive index changes result in a detuning between the externally selected wavelength and the weak internal-mode structure of the anti-reflection coated laser diode. This detuning is directly related to the compound cavity losses. On the one hand, a change in optical output power results in a change of the refractive index via the carrier density. On the other hand, it results in a change of refractive index via temperature changes. Compared to the carrier induced refractive index change, the temperature induced refractive index change is opposite in sign, a factor of ~10² smaller and slower. The switch-on and switch-off time of the self-pulsation is governed by the carrier life time. The repetition rate of the self-pulsation is governed by the thermal time constant and is in the megahertz region. Cross-modulation resulting from the thermal induced refractive index change is demonstrated. In a two-wavelength double external-cavity laser diode, optical power at one wavelength effects the optical power at the other wavelength. This cross-modulation is shown to be related to previous experiments on a laser neural network. A novel technique is introduced to measure the thermal impedance of a laser diode that is based on the cross-modulation     

Nondegenerate optical Kerr effect in semiconductors

We calculate the nondegenerate bound electronic nonlinear refractive index n₂(ω₁;ω₂) (i.e., an index change at frequency ω₁ due to the presence of a beam at frequency ω₂) in semiconductors. We calculate this nonlinearity and its dispersion using a Kramers-Kronig transformation on the calculated nondegenerate nonlinear absorption spectrum due to two-photon absorption, electronic Raman and optical Stark effects. The calculated n₂ values and their dispersion are compared to new experimental values for ZnSe and ZnS obtained using a 2-color Z-scan     

Single-Photon Response and Spectroscopy of a Photonic Molecule Based on Diamond Microrings

The optical properties of a photonic molecule consisting of three diamond microring cavities are theoretically investigated. The probability of single-photon excitation (optical response) of a photonic molecule by a weak laser field in the steady-state mode with regard to the dissipative effects is calculated using the model analogous to the tight binding approximation. It is shown that the spectrum can be fine tuned by depositing additional layers onto the photonic-molecule surface. The dependences of the wavelength of the mode corresponding to the zero-phonon optical transition in the NV center on the thickness of these layers and refractive index of their material are established. The NV center localized in the electromagnetic field antinode effectively interacts with the photonic molecule eigenmode, which can be observed as anticrossing points in the dependences of the optical response of the system on exciting laser and transition frequencies.     

Effect of excited-state transitions on the thresholdcharacteristics of a quantum dot laser

The general relationship between the gain and spontaneous emission spectra of a quantum dot (QD) laser is shown to hold for an arbitrary number of radiative transitions and an arbitrary QD-size distribution. The effect of microscopic parameters (the degeneracy factor and the overlap integral for a transition) on the gain is discussed. We calculate the threshold current density and lasing wavelength as a function of losses. The conditions for a smooth or step-like change in the lasing wavelength are described. We have simulated the threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in the GaAs matrix and obtained; a small overlap integral for transitions in the QDs and a large spontaneous radiative lifetime. These are shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition for short (several hundreds of micrometers) cavity lengths     

Theoretical study on enhanced differential gain and extremelyreduced linewidth enhancement factor in quantum-well lasers

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor (α factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the α factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the α factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the α factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning     

Asymmetric quantum well broadband thyristor laser

A broadband thyristor laser based on InGaAs/GaAs asymmetric quantum well (AQW) is fabricated by metal organic chemical vapor deposition (MOCVD). The 3-μm-wide Fabry-Perot (FP) ridge-waveguide laser shows an S-shape I-V characteristic and exhibits a flat-topped broadband optical spectrum coverage of ~27 nm (Δ_-10 dB) at a center wavelength of~1090 nm with a total output power of 137 mW under pulsed operation. The AQW structure was carefully designed to establish multiple energy states within, in order to broaden the gain spectrum. An obvious blue shift emission, which is not generally acquired in QW laser diodes, is observed in the broadening process of the optical spectrum as the injection current increases. This blue shift spectrum broadening is considered to result from the prominent band-filling effect enhanced by the multiple energy states of the AQW structure, as well as the optical feedback effect contributed by the thyristor laser structure.     

Structural and the optical dispersion parameters of nano-CdTe thin film/flexible substrate

CdTe thin films of different thicknesses were deposited on polymer substrates for flexible optical devices applications. X-ray diffractogram of different thicknesses for CdTe films are measured and their patterns exhibit polycrystalline nature with a preferential orientation along the (111) plane. The optical constants of CdTe films were calculated based on the measured transmittance spectral data using Swanepoel's method in the wavelength range 400–2500 nm. The refractive index n and absorption index k were calculated and the refractive index exhibits a normal dispersion. The refractive index dispersion data followed the Wemple–DiDomenico model based on single oscillator. The oscillator dispersion parameters and the refractive index no. at zero photon energy were determined. The possible optical transition in these films is found to be allowed direct transition with energy gap increase from 1.46 to 1.60 eV with the increase in the film thickness. CdTe/flexible substrates are good candidates in optoelectronic devices     

掺镱光纤激光器的多波长振荡特性

利用Sagnac梳状滤波器,研究了环形腔掺镱光纤激光器(YDFL)在室温和77 K时的多波长振荡行为。结果表明,在室温和77 K时,掺镱光纤(YDF)均呈现明显的非均匀加宽效应,使得掺镱光纤激光器的起振波长数随抽运光功率的提高而增多。但在室温下,因均匀加宽效应强,掺镱光纤增益的非均匀加宽并不能补偿其内禀不平坦的增益谱,造成波长缺失现象,且当波长间隔减小至0.8 nm时,均匀加宽效应引起的波长竞争造成了多波长振荡的稳定性下降。而在77 K时,掺镱光纤非均匀加宽效应明显增强,在0.8 nm的波长间隔下,无波长缺失现象,将起振波长数增加到32个,且多波长振荡稳定。