Decrease In Optical Power Research Articles

Mutual influence of intermodal and fundamental four-wave mixing of 1.0 µm and 1.5 µm nanosecond pulses in a dual-wavelength fiber laser.

We introduce the experimental and theoretical investigation of the mutual influence of two four-wave mixing (FWM) processes during the propagation of high-power nanosecond pulses at 1030 nm and 1562 nm wavelengths in the few-mode optical fiber. Both fundamental mode FWM (FM FWM) of laser pulses at these wavelengths, and intermodal FWM (IM FWM) of 1030 nm pulses, have common anti-Stokes components at 1003 nm wavelength. This leads to the mutual influence of FWM effects. As a result, the decrease of optical power of the fundamental mode is observed at both 1030 nm and 1562 nm wavelengths. The method for the suppression of such an adverse effect is proposed. It is based on the attenuation of the 1030 nm higher LP11 mode at the few-mode optical fiber input.

Optical power degradation mechanisms in 271 nm AlGaN-based deep ultraviolet light-emitting diodes.

The degradation of AlGaN-based UVC LEDs under constant temperature and constant current stress for up to 500 hrs was analyzed in this work. During each degradation stage, the two-dimensional (2D) thermal distributions, I-V curves, optical powers, combining with focused ion beam and scanning electron microscope (FIB/SEM), were thoroughly tested and analyzed the properties and failure mechanisms of UVC LEDs. The results show that: 1) the opto-electrical characteristics measured before/during stress indicate that the increased leakage current and the generation of stress-induced defects increase the non-radiative recombination in the early stress stage, resulting in a decrease in optical power; 2) the increase of temperature caused by the deterioration of the Cr/Al layer of p-metal after 48 hrs of stress aggravates the optical power in UVC LEDs. The 2D thermal distribution in conjunction with FIB/SEM provide a fast and visual way to precisely locate and analyze the failure mechanisms of UVC LEDs.

Proton-irradiation-induced degradation in GaN-based UV LEDs: Role of unintentionally doped carbon

Degradation and defect evolution in GaN-based UV LEDs under 3 MeV proton irradiation were throughly investigated in this work. Combined with the yellow luminescence band at ∼2.2 eV in photoluminescence spectra with the energy level of Ev + 0.16 eV extracted by deep-level transient spectroscopy measurement, an intrinsic CN-related defect with an abnormally decreased concentration after irradiation was identified. Based on energy levels, the spatial configuration of defects, and their correlations, several possible origins and evolution processes of the defects are systematically discussed. A defect evolution model under proton irradiation was established: C atoms departed from the CN-related defect aligned along the dislocation in the active region stimulated by the displacement damage effect, leaving behind a nitrogen vacancy (VN) aligned along the dislocation, accompanied by the generation of a non-interacting carbon interstitial (Ci). Both the increased VN and Ci were found to contribute jointly to the decrease in optical power and the increase in leakage current. This indicates that carbon—a common unintentional dopant in GaN-based optical devices—can be an important factor in the degradation of GaN-based UV LEDs under proton irradiation, and should be further noted in radiation resistance applications.

Analysis of Current Transport Layer Localized Resistivity Increase After High Stress on InGaN LEDs

This article reports on the effects of high current stress on InGaN-based light-emitting diodes (LEDs.) The analyzed devices are high power blue LEDs with an emitting area of 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> subjected to a constant current stress at increasing current until the catastrophic device failure. By means of optical, electrical and photoluminescence analysis we hypothesize that the optical power decrease is related to a degradation in the current transport layers of the devices. The effect has been modelized as a gradual and localized increase in the current transport layer resistance; the model has been validated by means of SPICE electrical simulations. The results clearly indicate that, to extend the operating range of high-power devices, it is crucial to control the current crowding effect, which is here considered to play a significant role in the degradation in the current transport layer.

Long-Term Temperature-Dependent Degradation of 175 W Chip-on-Board LED Modules

We report on the degradation dynamics and mechanisms of the commercially available chip-on-board (COB) high-power light-emitting diode (LED) modules with an electrical power of 175 W. Due to the associated thermal load, the temperature dependence of the aging processes is additionally analyzed within the scope of this work. The aging tests were performed for a period of 6000 h at four different case temperatures between 55 °C and 120 °C. The results of the accelerated stress tests indicate a temperature-activated aging process, which severely limits the lifetime of the modules. In addition, the following key findings can be reported: 1) a significant decrease in optical power occurs within 6000 h of operation; 2) depending on the stress test condition the accompanying color shifts exceed a limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {u}\,'{v}\,'={0}.{007}$ </tex-math></inline-formula> ; and 3) the limiting degradation mechanism can be attributed to the package of the device and can be accelerated with temperature, current, and chemicals. Reported findings can be manifested by additional optical material inspections, allowing to use the results for optimizations of future module generations.

Characterization and analysis of a POF sensor embedded in different materials: Towards wearable systems for stiffness estimation

This paper presents the development, characterization and analysis of an intensity variation-based polymer optical fiber (POF) force sensor using cyclic transparent optical polymer (CYTOP) fiber through side-coupling light source embedded in different materials. Tests with application of vertical force on the sensor with different supports were performed to analyze the sensor responses when embedded in different materials. In addition, vertical force tests along the fiber were performed to analyze the sensor feasibility of detecting the location of applied force. Results of the force characterization on different materials show that the POF-CYTOP force sensor response is related to elastic modulus of the materials. Using the support I, more flexible (elastic modulus of 18.6MPa), the sensor responses presented a high influence of the light coupling, resulting in an increase of the optical power. In contrast, the responses using the support II (elastic modulus of 1.6GPa), presented a decrease of optical power variation with only a minor increase in the optical power when small forces are applied. This is due to the higher strain in the POF, resulting in optical power decrease due to radiation losses and stress-optical effect. Tests with application of the force along the fiber show that the sensor had different sensitivities for impacts located at different distances of the lateral section, leading to the possibility of estimating the impact location using a multiplexed sensor array. The differences in the sensor’s responses over the different supports show the feasibility of using the proposed sensor system for the stiffness assessment in users, where wearable optical fiber sensors can be employed on the real-time estimation of soft tissues, which finds important applications in rehabilitation as well as wearable robotics control and design.

Optical limiting and reverse-saturable absorption in glycerol

The input–output power response of the glycerol solution with different glycerin concentrations is studied. It is found that there is an optical power limiting activity for glycerol. The transmission of the optical power is found to decrease with increasing the concentration. The decrease in optical power follows the well-known Beer–Lambert law at lower powers. However, at higher powers we observe an optical limiting property with saturated output powers. The saturated output power of the solution for various concentrations is estimated and is found to decrease with an increase in the concentration of the solution. The limiting mechanism is found to be reverse-saturable absorption and is discussed on the basis of the five level energy diagram of the molecular system.

Degradation of green InGaN-based LEDs during the tests at the constant current

The article presents the results of investigation of the degradation of electrical and optical characteristics of green InGaN-based LEDs under tests at constant direct current for 5000 hours. It is determined that LEDs which were subjected to a 4-hours exposure of high-density pulsed current degrade slowly than LEDs not exposed to this exposure. The external quantum efficiency (EQE) of the short-wave components of the radiation spectrum of the LEDs when testing reduced by a greater amount than the EQE of the long-wave components of the spectrum. For LEDs that have a current-voltage characteristic with a non-ideality factor of m≈2, the rate of decrease of optical power during testing is lower than for LEDs with m> 2.

Optical Fiber Polarizer With Fe–C Film for Corrosion Monitoring

The detection of early corrosion is a key issue to mitigate the damage induced by metal corrosion. This paper presents a corrosion sensor for steel based on an iron film-coated optical fiber polarizer. The polarizer is fabricated by sputtering a Fe-C film on a side-polished single mode fiber. The new fabricated polarizer shows highly birefringence, indicated by the difference of the transmission between the TE and TM modes, as well as the extinction ratio (ER). A series of simulations has been performed based on two models. The influences of the parameters of the sensor fabrication, the surrounding media, and film composition on the ER ratio and leakage are studied by the simulations. The accelerated corrosion experiments by immersing the polarizer into NaCl solution show that the polarizers become weakly birefringent after exposure. The results show that the ER and measured optical power decrease gradually during the corrosion process. The proposed polarizer can be used for monitoring the early stage of steel corrosion by building the relationship between the corrosion status and the polarization characteristics.

Defect-Related Degradation of AlGaN-Based UV-B LEDs

This paper describes an extensive analysis of the degradation of (InAlGa)N-based UV-B light-emitting diodes (LEDs) submitted to constant current stress. This paper is based on combined electrical characterization, spectral analysis of the emission, deep-level transient spectroscopy (DLTS) and photocurrent (PC) spectroscopy. The results of this analysis demonstrate that: 1) UV-B LEDs show a gradual degradation when submitted to constant current stress; the decrease in optical power is stronger for low measuring current levels, indicating that degradation is related to the increase in Shockley–Read–Hall (SRH) recombination; 2) the current–voltage characteristics measured before/during stress show an increase in the current below the turn-on voltage, that is ascribed to the increase in trap-assisted tunneling (TAT) components; and 3) DLTS analysis and PC spectroscopy measurements were carried out to identify the properties of the defects responsible for the degradation of the optical and electrical characteristics. The results indicate that stress induces or activates defects centered around 2.5 eV below the conduction band edge. These defects, close to midgap, can explain both the increased SRH recombination and the increase in TAT components detected after stress. Moreover, the DLTS measurements allowed to identify the signature of Mg-related acceptor traps.

Ultrahigh responsivity of optically active, semiconducting asymmetric nano-channel diodes

We present our research on the fabrication and optical characterization of novel semiconducting asymmetric nano-channel diodes (ANCDs). We focus on optical properties of ANCDs and demonstrate that they can be operated as very sensitive, single-photon-level, visible-light photodetectors. Our test devices consisted of 1.2-μm-long, ∼200- to 300-nm-wide channels that were etched in an InGaAs/InAlAs quantum-well hetero structure with a twodimensional electron gas layer. The ANCD I-V curves were collected by measuring the transport current both in the dark and under 800-nm-wavelength, continuous-wave-light laser illumination. In all of our devices, the impact of the light illumination was very clear, and there was a substantial photocurrent, even for incident optical power as low as 1 nW. The magnitude of the optical responsivity in ANCDs with the conducting nano-channel increased linearly with a decrease in optical power over many orders of magnitude, reaching a value of almost 10,000 A/W at 1-nW excitation.

Ageing of InGaN-based LEDs: Effects on internal quantum efficiency and role of defects

This paper describes the degradation of InGaN-based LEDs submitted to constant current stress; based on combined electroluminescence, photoluminescence and deep-level transient spectroscopy we show that: (i) when submitted to constant current stress, LEDs can show a measurable decrease in the optical power, which is more prominent in the low current regime; (ii) the decrease in optical power is strongly correlated to the increase in the Shockley–Read–Hall recombination coefficient A, as estimated by differential lifetime measurements; (iii) stress induces the increase in the concentration of a trap level, with activation energy between 0.6 and 0.7eV, which is supposed to be located next to/within the active region. The results suggest that the optical degradation can be ascribed to the increase in non-radiative recombination, rather than to a decrease in carrier injection efficiency.

GaN基蓝绿光LED电应力老化分析

The behavior of the related reliability of InGaN-based blue and green LED chips were investigated with 20,40,60 mA constant current stress up to 424 h at room temperature. Under 60mA constant current stress,the green LED chip showed more prominent optical power degradation at low measuring current levels than high measuring current levels. While,the blue LED chip showed the consistent optical power decrease at different measuring current levels. It should be pointed out that,at high measured current levels( 20 mA),blue and green LED chips expressed similar optical degradation( 18%) after the aging tests. Meanwhile,the aging stress does not significant affect the forward-bias electrical characteristics of blue and green LED chips,which indicated the similar degradation process in some aging tests. It is believed that the induced defects in blue LED by electrical stress mainly contribute to the nonradiative recombination centers,while in green LED chip the defects contribute to the localized leakage paths rather than the nonradiative recombination centers.Based on the mechanism analysis of blue and green LED,the design of GaN based epitaxial structure was optimized,and the thickness of quantum barrier was reduced to weaken the internal electrical field in the active layer to improve the reliability of LED.

Analysis of the physical processes responsible for the degradation of deep-ultraviolet light emitting diodes

This paper reports an extensive analysis of the degradation of deep-ultraviolet light-emitting diodes submitted to dc stress test. The study was carried out by means of combined electrical and optical characterization techniques. Results described in the paper indicate that the following: (i) stress can induce a significant decrease in the optical power emitted by the devices; (ii) optical power decrease is more prominent at low measuring current levels, thus suggesting that degradation is related to an increase in the concentration of defects; (iii) stress induces a significant increase in the green-yellow parasitic emission of the devices; and (iv) stress causes a localized increase in the apparent charge distribution profiles in the active region of the devices. Experimental evidence collected within this work suggest that degradation is due to an increase in the defectiveness in the active layer of the devices, with subsequent worsening of their radiative efficiency.

Defect-related degradation of Deep-UV-LEDs

With this paper we describe an extensive analysis of the electro-optical degradation of deep ultraviolet Light-Emitting Diodes emitting at 310 nm, submitted to accelerated stress test. The results obtained within this study indicate that: (i) high current stress can induce a significant decrease in the optical power emitted by the LEDs; (ii) optical power decrease is more prominent at low measuring current levels, indicating that degradation is related to an increase in the concentration of defects, with subsequent decrease in the radiative efficiency of the active layer; (iii) stress can induce a significant increase in the operating voltage of the LEDs, due to the increase in the resistivity of the ohmic contacts or p-type semiconductor; and (iv) after stress, LEDs show a significantly increased green parasitic emission. This result suggest that stress induced an increase in the defectiveness of the active layer. Information on the location of the degraded region is achieved by the analysis of the Apparent Charge Distribution curves obtained by capacitance–voltage measurements.

A Review on the Reliability of GaN-Based LEDs

We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications.

A model for the thermal degradation of metal/(p-GaN) interface in GaN-based light emitting diodes

This paper analyzes the thermal degradation of GaN-based light emitting diodes with hydrogen-rich passivation layer by combined capacitance and current measurements. The decrease of optical power arising during thermal treatment at T=250°C is well related to a decrease in the high-frequency capacitance and to the generation of a further peak in the conductance/frequency curves. Deep level transient spectroscopy measurements excluded the role of further deep levels introduced/generated in the p-n junction region. Transmission line method analysis showed that stress induces nonlinearity of the characteristics of the contacts, related to the increase of Ohmic contact resistivity. All these modifications are explained using a small-signal model in which a parasitic impedance arises as a consequence of stress in the portion of metal/(p-GaN) interface exposed to diffusion of hydrogen from the passivation layer. Therefore, degradation is shown to be related to the worsening of the properties of the metal/semiconductor interface at the p side of the diode, due to the interaction between the hydrogen-rich passivation layer and the acceptor dopant. This interaction is well confined to the immediate proximity of the interface, leaving the activated acceptor concentration in the bulk almost unaffected.

VISUAL EFFETCS OF HUMAN EYE WITH LASIK AND ARTIFICAL LENS IMPLANTATION

The behavior of the human crystalline lens during accommodation is analytically studied. The lens is modeled as a closed axisymmetrical membrane shell of varying thickness enclosing an incompressible liquid. To simulate zonular tension associated with lenticular accommodation, an axisymmetrical radial force or displacement is imposed around the shell equator. Two second-order, simultaneous, nonlinear governing differential equations are derived. Numerical results, obtained from the investigation of human lens profiles of three independently published MRI images and a drawing of a microphotograph, demonstrate that when zonular traction within the physiological force range of the ciliary muscle is exerted, both central lens thickness and central optical power increase. Qualitatively, these increases are independent of lens shape. However, the magnitude of these changes is dependent on the initial profile of the lens and is enhanced by the “natural” variation in capsular thickness. Only when a pulling force significantly exceeds the force capacity of the ciliary muscle does the lens flatten and its central thickness and optical power decrease.

Comparative Analysis of λ≈9µm GaAs/AlGaAs Quantum Cascade Lasers with Different Injector Doping

An experimental and theoretical comparative analysis of the output characteristics of λ ≈ 9m GaAs/Al0.45Ga0.55As quantum cascade lasers based on single and double phonon resonance depopulation mechanisms were presented. The layer structures were grown with solid source molecular beam epitaxy and consist of 48 or 36 active stages embedded in a symmetrical plasmon enhanced waveguide. From the wafers, ridge waveguide lasers were fabricated by optical lithography and dry etching. The theoretical model is based on a fully non-equilibrium Schrödinger- Poisson self-consistent analysis of the coupled scattering rate and single-temperature energy balance equations, taking all relevant electron-LO phonon, electron-electron and electron-ionised impurity scattering processes into account. Single phonon resonance devices exhibit clear current saturation, simultaneously with a decrease of the optical power. In the moderate doping regime, a quasi-linear dependence of both the threshold and saturation current densities on injector doping, were measured, in a very good agreement with theoretical predictions. Double phonon resonance lasers exhibit ‘saturation’ mechanism evident from their decrease in optical power, but without pronounced current saturation. Previously reported saturation of the ‘maximal’ current under higher injector doping in single phonon resonance lasers, is also observed in the double phonon resonance structure for injector sheet doping above 8x1011cm-2.

Influence of short-term low current dc aging on the electrical and optical properties of InGaN blue light-emitting diodes

This work describes an experiment on degradation mechanisms of InGaN light-emitting diode (LED) test structures which do not fulfill the requirements of longlife products. We present a combined capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), electroluminescence (EL), and cathodoluminescence (CL) study of short-term instabilities of InGaN∕GaN LEDs submitted to low current aging tests at room temperature. In the early stages of the aging tests, the EL and CL characterizations showed an optical power decrease, more prominent at low current levels. The C-V profiles indicated that the stress induced an apparent charge increase, well related to the deep level changes detected by DLTS and to the optical power decrease. It is supposed that the main cause of the degradation is the generation of nonradiative paths, due to the generation/propagation of defects activated by carrier transport.