Increase In Optical Loss Research Articles

Application of advances in the development and production of optical fiber in the development and design of fiber optic devices.

The dimensions of devices based on optical fiber are limited by the sensitivity of the optical fiber to macrobending, which leads to an increase in optical losses at small bending radii. New types of fibers with low macrobending losses make it possible to significantly tighten the dimensions. However, there may be a risk of fiber failure at bending points during the life of the product due to mechanical fatigue. In the article, based on the use of a static fatigue model of silica glass, a simple expression is obtained to estimate the probability of fiber failure under long-term bending stress depending on the length of the bent section, bend radius and humidity, taking into account the result of a proof-test carried out by the fiber manufacturer. The possibility of increasing the mechanical reliability of silica fiber by applying a hermetical carbon coating is discussed. The use of the obtained expressions in design makes it possible to reduce the dimensions of fiber optic products without the risk of reducing their reliability.

Dual‐Wavelength Controllable and Exciplex Emission for Laser Concealment

AbstractAs an advanced signal source, laser is very suitable for various photonic devices but implementing tunability of wavelength is more challenging. Herein, the study designs and synthesizes boron difluoride derivative tBuBF2, systematically studying the impact of assembly behavior on laser performance. The laser test results indicate that the emission wavelength of tBuBF2 microcrystals can be controlled by varying the length of the microcrystals, with the shorter wavelength of 0–1 vibronic lasing emission at 444 nm and the longer wavelength of 0–2 lasing emission at 468 nm. Mechanistic studies reveal that laser color change comes from an increase in optical propagation losses due to self‐absorption at the 0–1 vibronic band. Furthermore, CBP@tBuBF2 microcrystals constructed by doping with 4,4′‐bis(9‐carbazolyl)‐1,10‐biphenyl (CBP) as the guest molecule, which primarily exhibits 580 nm exciplex yellow emission while retaining weak blue emission. Moreover, it is demonstrated that the laser emission only occurred within the 0–2 vibronic band in the blue region owing to the increased self‐absorption phenomenon with 0–1 band, thus achieving controllable dual‐wavelength and exciplex emission for laser concealment. This work expands novel regulation strategy available for laser behavior control by enhancing the self‐absorption process, describing structure‐property relationships that will guide the design of advanced laser materials.

Radiographic description phase composition of lithium niobate crystals with anomal growth of domains during soft proton exchange

The formation of periodically polarized waveguide structures in lithium niobate crystals for nonlinear optical transformations of laser radiation in the telecommunications wavelength range is an important applied problem. One of the options for forming waveguides with low switching fields can be waveguides obtained by soft proton exchange. In this work, using X-ray diffraction analysis, it is shown that anomalously low polarization switching thresholds occur in layers with so-called intermediate κ-phases when varying the time of soft proton exchange, and when varying the concentration of lithium benzoate, a transition from the metastable β-phase to the intermediate κ-phase is revealed. The results obtained support the hypothesis that the proton concentration gradient leads to a decrease in the threshold field for polarization switching. However, high-concentration metastable and intermediate crystalline phases lead to an increase in optical losses and temperature instability of waveguides.

Investigation of the anatase-to-rutile transition for TiO2 sol-gel coatings with refractive index up to 2.7

This work describes the elaboration of rutile titanium dioxide films with high refractive indices and low scattering losses by sol-gel process and controlled crystallization. The evolutions of the optical properties and crystalline structure of sol-gel processed titania coatings on fused silica were investigated for different thermal budgets of the annealing post-treatment using ellipsometry, spectrophotometry, X-ray diffraction and electronic microscopy. It reveals that anatase and rutile coatings with refractive indices of 2.51 and 2.73 can be prepared with associated optical loss of 0.5 % and 1 %, respectively, which are an excellent compromise for applications in integrated photonics. These evolutions are associated to the thermally induced mass transfer and phase transitions occurring during thermal annealing that involves first the nucleation growth and sintering of anatase polyoriented nanocrystals, followed by the transformation into rutile polyoriented nanocrystals. Concomitantly, rutile crystals with (110) faces parallel to the surface consume surrounding anatase and rutile nanocrystals by diffusive sintering to yield micron-size rutile monocrystalline and monooriented platelets patchwork, exhibiting refractive index of 2.73 and 1.2 % optical loss. The formation of these platelets is governed by surface energies and is responsible for the increase in optical loss.

Optical Loss in Microdisk Lasers With Dense Quantum Dot Arrays

We discuss the origin of optical losses in microdisk lasers with a dense array of InGaAs quantum dots in the active region. In particular, we study the effect of microlaser diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {D}$ </tex-math></inline-formula> variation from 15 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200~\mu \text{m}$ </tex-math></inline-formula> on optical losses of different nature. A strong dependence of the lasing wavelength on the diameter is observed: the blue-shift with decreasing disk size implies an increase in optical losses, although in the case of an ideal cylinder, a noticeable optical loss should appear only at diameters comparable to the wavelength of light. A comparison of the spectral characteristics of microlasers with those of broad-area stripe lasers, for which optical loss can be easily found, gives a tool to evaluate optical loss in microdisk lasers, which was found to be unexpectedly high. It changes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\propto }\mathbf {D}^{\mathbf {-1}}$ </tex-math></inline-formula> from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> cm−1 in the smallest microlasers to ~ 5 cm−1 in the largest ones. Several possible physical mechanisms of the appearance of optical losses in microlasers are considered, such as radiative loss due to the curvature of the cylindrical cavity, free carrier absorption, light scattering due to roughness of the side walls, and absorption of light in the near-surface region. The latter type of optical loss was found to be the dominant one and can explain the experimental results once the absorbing layer with a thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> was suggested. Using the Gaussian approximation for Using the Gaussian approximation for the gain spectrum, the wavelength-loss relationship was simulated and a good agreement with the experimental dependence was found. The variation of the experimental results on optical loss for nominally identical microlasers was attributed to the variation of the scattering loss. The same reason can explain the scatter of the slope efficiency, which varies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~0.03$ </tex-math></inline-formula> to 0.25 W/A being governed by the ratio of the scattering loss to the surface absorption loss.

Radiation robustness of laser ceramics and single crystal for microchip laser remote analysis

Laser pulse energy and pulse build-up time were measured during gamma irradiation at a dose rate of 150 Gy h−1 to evaluate the radiation-induced effects and robustness on a laser remote inspection system using ceramics and single crystal microchip laser. Results showed a time-dependent decrease in pulse energy and an increase in pulse build-up time during irradiation. Both effects were larger for ceramics than for single crystals due to the amount of generated optical loss. The behaviors of pulse energy and build-up time with the increase in optical loss in the gain medium were simulated using rate equation calculations. The radiation effect on build-up time was larger than that on pulse energy for both specimens. Therefore, build-up time measurement is highly sensitive and effective for the evaluation of the radiation effect generated in laser medium, especially for radiation robust specimens or at low radiation dose rate.

Internal optical loss and internal quantum efficiency of a high-power GaAs laser operating in the CW mode

We continue the comparative studies of high-power AlGaAs/InGaAs/GaAs semiconductor lasers with different waveguide designs. In this work, measurements were carried out for a continuous-wave mode of operation: light–current and current–voltage characteristics, threshold characteristics and their temperature dependences and crystal temperature depending on the pump current. The data obtained made it possible to reconstruct the dependences of internal optical losses and internal quantum efficiency using calculations. It was shown that the main factor affecting the power characteristics in the continuous mode of operation is the temperature stability of the laser and its relationship with internal optical losses and internal quantum efficiency. Thus, lasers based on a heterostructure with a doped GaAs waveguide with characteristic temperatures T 0 and T 1 of 120 K and 170 K, respectively, demonstrated an increase in internal optical losses to 2 cm−1 and a drop in the quantum efficiency to 90%, which led to saturation of the output power by high pump currents. Lasers based on the heterostructure with an AlGaAs waveguide, which are characterized by higher temperature stability (T 0 = 161 K and T 1 = 280 K), had lower internal optical losses and a higher quantum efficiency. This made it possible to obtain optical power 1 W higher than that for lasers with GaAs waveguides.

Revealing optical loss from modal frequency degeneracy in a long optical cavity.

Optical loss plays a significant role in optical experiments involving optical cavities such as recycling cavities and filter cavities in laser interferometer gravitational-wave detectors. For those cavities, modal frequency degeneracy, where the fundamental and a higher order mode resonate inside the cavity simultaneously, is a potential mechanism which may bring extra optical loss to the cavity thus degrade detection sensitivity. In this paper, we report observation of modal frequency degeneracy in a large-scale suspended Fabry-Pérot cavity. The cavity g-factor is tuned by a CO2 laser heating one test mass, and the cavity finesse is obtained from a ring-down measurement of the transmitted light. We demonstrate that the modal frequency degeneracy can cause a reduction of the cavity finesse by up to ∼30%, corresponding to a ∼2-fold increase in total optical loss. To minimize optical loss in gravitational-wave detectors, the effect of modal frequency degeneracy needs to be taken into account in the design and operation of the detector.

Chemical Reaction and Ion Bombardment Effects of Plasma Radicals on Optoelectrical Properties of SnO2 Thin Films via Atomic Layer Deposition.

In this study, the effect of radical intensity on the deposition mechanism, optical, and electrical properties of tin oxide (SnO2) thin films is investigated. The SnO2 thin films are prepared by plasma-enhanced atomic layer deposition with different plasma power from 1000 to 3000 W. The experimental results show that plasma contains different amount of argon radicals (Ar*) and oxygen radicals (O*) with the increased power. The three deposition mechanisms are indicated by the variation of Ar* and O* intensities evidenced by optical emission spectroscopy. The adequate intensities of Ar* and O* are obtained by the power of 1500 W, inducing the highest oxygen vacancies (OV) ratio, the narrowest band gap, and the densest film structure. The refractive index and optical loss increase with the plasma power, possibly owing to the increased film density. According to the Hall effect measurement results, the improved plasma power from 1000 to 1500 W enhances the carrier concentration due to the enlargement of OV ratio, while the plasma powers higher than 1500 W further cause the removal of OV and the significant bombardment from Ar*, leading to the increase of resistivity.

Vertical cavity surface emitting laser of 1.55 μm spectral range, manufactured by molecular beam epitaxy and wafer fusion technique

The heterostructure design for 1.55 μm range VCSELs is proposed and realized. The wafer fusion technique was used to form the final heterostructure. The growth of AlGaAs/GaAs distributed Bragg reflectors (DBRs) on GaAs substrate and the optical cavity with an active region on InP substrate as well as a tunnel junction (TJ) regrowth was performed by molecular beam epitaxy (MBE). A key feature of the proposed design is the use of n++-InGaAs/p++-InGaAs/p++-InAlGaAs TJ, which allows, due to the effective removal of oxide from the InGaAs surface, to use MBE for re-growth of the TJ surface relief. Despite of the presence in heterostructure a narrow-gap InGaAs layers, a noticeable increase in internal optical loss in lasers can be avoided due to the short-wavelength shift of the edge of interband light absorption in ++-InGaAs layers (Burshtein-Moss effect). Fabricated VCSELs demonstrate single-mode operation with a threshold current less than 2 mA and a slope efficiency of ~ 0.46 W/A, which are comparable with characteristics of VCSELs with n++/p++-InAlGaAs TJ with a similar level of mirror losses.

Performance deterioration of GaN-based laser diode by V-pits in the upper waveguide layer

Abstract The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In0.017Ga0.983N/u-GaN multiple UWG or u-In0.017Ga0.983N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm2, and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In0.017Ga0.983N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

Lasing mode regulation in zinc oxide microcavity

As a short-wave optoelectronic device, ultraviolet (UV) nano laser has a broad prospect in the fields of electronics, information, communication, environmental monitoring and biomedical testing. Zinc oxide (ZnO) microcavities show significant advantages in the realization of UV laser with low threshold and high quality factor. Resonance mode regulation and single-mode realization are the key problems for the practical application in ZnO microcavity UV laser. Based on the basic theory of microcavity mode, this paper reviews the progress of lasing mode regulation of ZnO microcavity according to different regulation mechanisms. Firstly, the important progress of traditional static regulated methods, such as cavity structure control and vernier effect, in the lasing mode effective regulation of ZnO microcavity is discussed. Secondly, the dynamic regulation of lasing mode in ZnO microcavity is realized by modulating effective refractive index through piezoelectric or electro-optic physical effects. Finally, the latest progress of surface plasmon effect in the realization of micro and nano cavity laser regulation is summarized. Many traditional methods, with the construction of distributed Bragg reflector (DBR) or distributed feedback (DFB) structure, cavity size control, vernier effect and so on, have been used to realize the lasing mode effective regulation and single-mode-lasing output of ZnO microcavity. However, these traditional methods are only suitable for the pre-designed microcavity structure, and they are highly dependent on the complex micro processing technology. Reducing the size of the cavity will inhibit the acquisition of optical gain, resulting in the increase of optical loss and laser threshold. In addition, these mode regulations are not reversible. In order to solve these problems, many scholars have taken various effective measures. The introduction of surface plasmon, a new physical effect, can make the optical field of ZnO microcavity get better confinement, so as to improve the microcavity gain and reduce the lasing threshold. Based on piezoelectric effect or electro-optic effect, the effective refractive index of ZnO can be regulated, and the dynamic regulation of ZnO microcavity mode can be realized by introducing external stress or external electric field. In addition, the combination of surface plasmon and ZnO nanocavity not only breaks the diffraction limit in size, but also regulates the laser mode in intensity, threshold and mode volume, which provides more theoretical basis and reference value for the future pursuit of nano laser with lower threshold, stronger intensity and smaller mode volume. These methods are not only suitable for ZnO UV laser, but also provide reference for other materials and other band microcavity laser.

Вертикально-излучающий лазер спектрального диапазона 1.55 &amp;mu;m с туннельным переходом на основе слоев n-=SUP=-++-=/SUP=--InGaAs/p-=SUP=-++-=/SUP=--InGaAs/p-=SUP=-++-=/SUP=--InAlGaAs

The design of the n++-InGaAs/р++-InGaAs/р++-InAlGaAs tunnel junction (TJ) for 1.55 μm range vertical-cavity surface-emitting lasers (VCSELs), developed by wafer fusion technique of InAlGaAsP/InP optical cavity with AlGaAs/GaAs distributed Bragg reflectors is proposed and realized. The presence of oxidation-resistant InGaAs layers allows the use of molecular-beam epitaxy at all stages of the heterostructure fabrication, including for regrowth of the TJ surface relief. In the case of using the n++-InGaAs/р++-InGaAs/р++-InAlGaAs TJ, a noticeable increase in the internal optical losses compared to the n++/р++-InAlGaAs TJ design was not obtained. The increase in internal optical loss in lasers can be avoided due to Burshtein-Moss effect in n++-InGaAs layers and thickness minimization of р++-InGaAs layer. As a result, the characteristics of fabricated lasers are comparable with characteristics of VCSELs with n++/p++-InAlGaAs TJ with a similar level of mirror losses.

Measurements of internal optical loss inside an operating laser diode

An experimental technique for measuring internal optical loss in high-power edge-emitting semiconductor lasers is demonstrated. The technique is based on coupling a probe beam into the waveguide of a pulse-pumped laser diode. It allows measuring free-carrier absorption (FCA) in a laser heterostructure at different temperatures and at pump current levels up to 30 kA/cm2. Measurement results are presented for two laser heterostructure designs, which vary in the waveguide doping level and material. For both heterostructures, the pump current increase induces a significant rise in FCA and a corresponding increase in internal optical loss, from 0.4–0.7 cm−1 at the threshold current to 2–2.5 cm−1 at 15 kA/cm2. At higher temperatures, the dependence is even stronger and the internal optical loss rises to 6 cm−1 (65 °C, 27 kA/cm2). The gradient of the FCA current dependence is lower for the laser heterostructure with a doped GaAs waveguide, while the heterostructure with an undoped AlGaAs waveguide displays a larger increase in FCA but better internal quantum efficiency at high currents. These results show that the proposed experimental method has significant potential.

Degradation of III–V Quantum Dot Lasers Grown Directly on Silicon Substrates

Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 μ m are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions.

Strain-Related Degradation of GaN-Based Blue Laser Diodes

Degradation of GaN-based laser diodes (LDs) is studied using Electroluminescence (EL) and Transmission Electron Microscopy (TEM). The slope efficiency decrease is observed in addition to threshold current increase. Further studies on the optical loss and microstructures of the LDs indicate that the reduction of the slope efficiency is attributed to the increase of optical loss (from 14.2 cm−1 to 24.2 cm−1) and decrease of injection current efficiency induced by the defect generation around the waveguide and cladding layers. Injection current induced blue-shift of the spontaneous EL peak reduced after >3000 hrs operation, which is ascribed to the compensation of Quantum-Confined Stark Effect (QCSE) caused by partial strain relaxation near the active region.

Influence of Output Optical Losses on the Dynamic Characteristics of 1.55-μm Wafer-Fused Vertical-Cavity Surface-Emitting Lasers

The results of studying the dynamic characteristics of 1.55-μm single-mode vertical-cavity surface-emitting lasers (VCSELs) formed by the fusion of wafers of high-quality Bragg reflectors and an active region based on thin highly strained InGaAs/InAlGaAs quantum wells are presented. It is found that the proposed design of the active region and optical microcavity of the laser make it possible in principle to attain a high level of differential laser gain in the temperature range of 20°C–85°C, but weak electron localization leads to an increase in gain compression at elevated temperatures. Due to this fact, the VCSEL modulation bandwidth at 20°C can be increased from 9.2 to 11.5 GHz due to an increase in output optical losses, while the modulation bandwidth at 85°C does not exceed 8.5 GHz, depends weakly on the output optical losses, and is mainly limited by the optical-gain saturation.

Plastic fiber optic sensor for continuous liquid level monitoring

The study demonstrates the development of a novel low-cost plastic optical fiber (POF) based sensor for liquid level measurement. The sensor works on the principle of reduction in scattering-based optical losses (in contrast to the increase in optical losses for evanescent wave-based sensing) in a straight decladded fiber with an increase in the liquid level due to refractive index changes in the medium surrounding the fiber. A compact optical setup is developed in-house consisting of two U-bent fiber probes coupled to a single LED on one end and two photodetectors on the other ends to measure the optical intensity changes. One of the U-bent probes is decladded over the length of the fiber to act as a test probe to measure the liquid level, while the other probe acts as a reference to compensate for the light intensity fluctuations due to light source instability and ambient conditions. The difference in voltage responses of the two photodetectors gives a measure for liquid level. We have investigated the fiber optic level sensor response to rising and falling liquid levels over 55 cm in presence of aqueous liquids of different refractive index values (1.33 to 1.38) and 95% ethanol as well as DI water at various temperatures from 16 °C up to 70 °C. A level sensitivity of 1.4 and 3.3 mV/mm for water level changes was obtained for the liquid level below and above 45 cm respectively. In addition, the fiber optic level sensor shows a stable and reproducible response over several cycles of 30 min duration at different liquid levels. The results demonstrate that this fiber optic level sensor is not only easy-to-make, cost-effective and robust but also offers sensitive, stable and reproducible instantaneous level measurements.

Analysis of the Internal Optical Losses of the Vertical-Cavity Surface-Emitting Laser of the Spectral Range of 1.55 µm Formed by a Plate Sintering Technique

The results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.

Анализ внутренних оптических потерь вертикально-излучающего лазера спектрального диапазона 1.55 &amp;mu;m, сформированного методом спекания пластин

AbstractThe results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm^–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm^–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.