Optical Power Efficiency Research Articles

Organic Light-Emitting Transistors with Simultaneous Enhancement of Optical Power and External Quantum Efficiency via Conjugated Polar Polymer Interlayers.

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, an efficient injection of charges into the emissive layer is required. The introduction of solution-processed conjugated polyelectrolyte (CPE) films at the emissive layer/electrode interface represents a promising strategy to improve the electron injection process by dipole formation. However, their use in optoelectronic devices also involves some limitations because of the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs because the conjugated backbones of CPPs are functionalized with polar nonionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene-containing phosphonate groups underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET-emissive layer and, accordingly, a more than 2-fold increased light power and a 5 times higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices, allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs.

Optical Wireless Communication Channel Measurements and Models

Optical wireless communications (OWCs) refer to wireless communication technologies which utilize optical carriers in infrared, visible light, or ultraviolet bands of electromagnetic spectrum. For the sake of an OWC link design and performance evaluation, a comprehensive understanding and an accurate prediction of link behavior are indispensable. Therefore, accurate and efficient channel models are crucial for the OWC link design. This paper first provides a brief history of OWCs. It also considers OWC channel scenarios and their utilization trade-off in terms of optical carrier, range, mobility, and power efficiency. Furthermore, the main optical channel characteristics that affect the OWC link performance are investigated. A comprehensive overview of the most important OWCs channel measurement campaigns and channel models, primarily for wireless infrared communications and visible light communications, are presented. OWCs channel models are further compared in terms of computation speed, complexity, and accuracy. The survey considers indoor, outdoor, underground, and underwater communication environments. Finally, future research directions in OWCs channel measurements and models are addressed.

A High Efficiency Optical Power Splitter in a Y-Branch Photonic Crystal for DWDM Optical Communication Systems

AbstractIn this paper a high efficiency optical power splitter based on two-dimensional photonic crystals is proposed. The photonic crystal cavity is assumed to be constructed by TiO

Thermo-optical and lasing characteristics of Cr^2+-doped CdSe single crystal as tunable coherent source in the mid-infrared

We report on a comprehensive characterization of Cr2+-doped CdSe single crystal as an efficient active material for tunable laser applications in the mid-infrared spectral region. Optical gain, thermo-optical behavior, power efficiency, scalability and wavelength tunability have been thoroughly investigated. Using an antireflection-coated crystal pumped by a Tm-fiber laser at at 1.94 μm, 1-W CW output power and 50% slope efficiency at 2.65 μm emission wavelength have been obtained in a diffraction-limited output beam. An output peak power of 2.5 W has been achieved without significant beam distortion in a quasi-CW regime. Exploiting an intra-cavity diffraction grating in a Littrow configuration, a maximum tuning range of 900 nm from 2.22 to 3.12 μm, limited by the finite bandwidth of resonator components, has been demonstrated with an uncoated crystal.

Low-Coherence light source design for ESPI in-plane displacement measurements

The ESPI method for surface deformation measurements requires the use of a light source with high coherence length to accommodate the optical path length differences present in the apparatus. Such high-coherence lasers, however, are typically large, delicate and costly. Laser diodes, on the other hand, are compact, mechanically robust and inexpensive, but unfortunately they have short coherence length. The present work aims to enable the use of a laser diode as an illumination source by equalizing the path lengths within an ESPI interferometer. This is done by using a reflection type diffraction grating to compensate for the path length differences. The high optical power efficiency of such diffraction gratings allows the use of much lower optical power than in previous interferometer designs using transmission gratings. The proposed concept was experimentally investigated by doing in-plane ESPI measurements using a high-coherence single longitudinal mode (SLM) laser, a laser diode and then a laser diode with path length optimization. The results demonstrated the limitations of using an uncompensated laser diode. They then showed the effectiveness of adding a reflection type diffraction grating to equalize the interferometer path lengths. This addition enabled the laser diode to produce high measurement quality across the entire field of view, rivaling although not quite equaling the performance of a high-coherence SLM laser source.

Theoretical and experimental investigations of efficient light coupling with spatially varied all dielectric striped waveguides

Integrated photonic systems require efficient, compact, and broadband solutions for strong light coupling into and out of optical waveguides. The present work investigates an efficient optical power transferring the problem between optical waveguides having different widths of in/out terminals. We propose a considerably practical and feasible concept to implement and design an optical coupler by introducing gradually index modulation to the coupler section. The index profile of the coupler section is modulated with a Gaussian function by the help of striped waveguides. The effective medium theory is used to replace the original spatially varying index profile with dielectric stripes of a finite length/width having a constant effective refractive index. 2D and 3D finite-difference time-domain analyzes are utilized to investigate the sampling effect of the designed optical coupler and to determine the parameters that play a crucial role in enhancing the optical power transfer performance. Comparing the coupling performance of conventional benchmark adiabatic and butt couplers with the designed striped waveguide coupler, the corresponding coupling efficiency increases from approximately 30% to 95% over a wide frequency interval. In addition, to realize the realistic optical coupler appropriate to integrated photonic applications, the proposed structure is numerically designed on a silicon-on-insulator wafer. The implemented SOI platform based optical coupler operates in the telecom wavelength regime (λ = 1.55 μm), and the dimensions of the striped coupler are kept as 9.77 μm (along the transverse to propagation direction) and 7.69 μm (along the propagation direction) where the unit distance is fixed to be 465 nm. Finally, to demonstrate the operating design principle, the microwave experiments are conducted and the spot size conversion ratio as high as 7.1:1 is measured, whereas a coupling efficiency over 60% in the frequency range of 5.0–16.0 GHz has been also demonstrated.

Optical Power Efficiency Versus Breakdown Voltage of Avalanche-Mode Silicon LEDs in CMOS

We report on the dependency of the optical power efficiency $\eta $ on the breakdown voltage ${V}_{\text {BR}}$ of avalanche-mode (AM) light-emitting diodes (LEDs) in silicon. Lateral p+-n-n+ LEDs have been designed in a 65-nm bulk CMOS technology, where ${V}_{\text {BR}}$ is varied between 2 and 9 V. This tunes both the magnitude and the spatial distribution of the reverse electric field, which governs AM electroluminescence. Experiments show that a maximum $\eta $ of $\sim 1.7 \times 10^{-6}$ is obtained for ${V}_{\text {BR}} \sim 6$ V. For ${V}_{\text {BR}} V, non-local avalanche results in a lower $\eta $ , while for ${V}_{\text {BR}} > 6$ V, a gradual reduction in $\eta $ with increasing ${V}_{\text {BR}}$ is obtained. This trend is compared with two recently proposed opto-electronic models. A maximum in $\eta $ at relatively low voltages is attractive for monolithic opto-electronic integration in silicon

Maximum Optical Power Efficiency in SDM-Based Optical Communication Systems

We demonstrate the existence of an optimal spectral efficiency that maximizes the power efficiency and/or capacity of a power limited erbium-doped fiber amplifier (EDFA)-based space division multiplexing transmission system. Optimal spectral efficiency observed experimentally is in good agreement with a theoretical estimate.

Heavy Metal Free Nanocrystals with Near Infrared Emission Applying in Luminescent Solar Concentrator

Low reabsorption, near infrared (NIR) emission, and heavy metal free are key trends for the nanocrystals (NCs) based luminescent solar concentrator (LSC). In this research, heavy metal free AgInS2/ZnS (AIS/ZnS) NCs have been synthesized and utilized by LSC technique with NIR emission for the first time. The AIS/ZnS demonstrated a tetragonal crystal phase and an NIR emission at 918 nm when the ratio of [Ag]:[In] = 1: 0.5 for metal elements in precursors. The quantum efficiency was also improved to 60.3% as the absolute quantum yield (QY) after inorganic surface passivation. Moreover, AIS/ZnS LSCs have been fabricated by in situ polymerization. Benefiting from low reabsorption by “large effective Stokes shift” and high emission efficiencies of the AIS/ZnS NCs, the optical power efficiency (OPE) of large area AIS/ZnS LSC reached to 3.94%, and interestingly, the LSC also revealed high color transmittance in the visible range, which could maintain high quality of the transmitted natural light with color rendering properties at above 97 for Ra, and correlated color temperature of around 4500 K. These NCs and related LSCs will play an important role in smart building integrated photovoltaics.

Space Codes for MIMO Optical Wireless Communications: Error Performance Criterion and Code Construction

In this paper, we consider a multiple-input-multiple-output optical wireless communication (MIMO-OWC) system in the presence of log-normal fading. In this scenario, a general criterion for the design of full-diversity space code (FDSC) with the maximum likelihood detector is developed. This criterion reveals that in a high signal-to-noise ratio regime, MIMO-OWC offers both large-scale diversity gain, governing the exponential decay of the error curve, and small-scale diversity gain, producing traditional power-law decay. Particularly for a two by two MIMO-OWC system with unipolar pulse amplitude modulation, a closed-form solution to the design problem of a linear FDSC optimizing both diversity gains is attained by taking advantage of the available properties on the successive terms of Farey sequences in number theory as well as by developing new properties on the disjoint intervals formed by the Farey sequence terms to attack the continuous and discrete variables mixed max–min design problem. In fact, this specific design not only proves that a repetition code is the optimal linear FDSC optimizing both the diversity gains but also uncovers a significant difference between MIMO radio frequency communications and MIMO-OWC that space dimension alone is sufficient for a full large-scale diversity achievement. Computer simulations demonstrate that FDSC substantially outperforms uncoded spatial multiplexing with the same total optical power and spectral efficiency, and the latter provides only the small-scale diversity gain.

Reference-free holographic diversity interferometry via iterative measurements for high accuracy phase detection.

To obtain a phase distribution without the use of an optical path besides an object beam, a reference-free holographic diversity interferometry (RF-HDI) has been proposed. Although the RF-HDI can generate an internal reference beam from the object beam, the method has a problem of measurement accuracy due to insufficient power of the internal reference beam. To solve the problem, we newly propose a RF-HDI via iterative measurements. Our method improves the measurement accuracy by utilizing iterative measurements and feedback of each obtained phase image to the measurement system. In the experiment, the phase image, which has a random pattern, can be measured as an object beam with a higher accuracy than in the conventional RF-HDI. To support this result, we also evaluated the wavefront accuracy and optical power efficiency of an internal reference beam in this method. As a result, we verified that our method enables us to generate an internal reference beam that has the wavefront of a near single plane wave and a higher power efficiency than the conventional RF-HDI. In addition, our method can be applied to measurement for the modal content in an optical fiber, atmosphere turbulence, etc., where it is difficult to prepare an external reference beam with a high coherency.

Bandlimited Optical Intensity Modulation Under Average and Peak Power Constraints

Bandlimited optical intensity channels, arising in applications such as visible light communications, require that all signals satisfy a bandwidth constraint as well as average, peak, and non-negativity amplitude constraints. In this paper, a 2-D signal space for bandlimited optical intensity channels is presented. A novel feature of this model is that the non-negativity and peak constraints are relaxed, and the signal space parameterizes the probability of the negative or peak amplitude excursions in the output. Although the intensity channel only supports non-negative amplitudes, the impact of clipping on system performance is shown to be negligible if the likelihood of the negative excursion is small. A tractable approximation using finite series is used to accurately compute the probability of clipping under average and peak optical power constraints. The optical power and spectral efficiencies using hexagonal lattice constellations are computed. Schemes designed in this paper have higher average and peak optical power gains than $M$ -ary pulse amplitude modulation (PAM) using previously established techniques for spectral efficiencies greater than 2.5 and 3.5 bits/s/Hz, respectively. For high spectral efficiency, e.g., greater than 6 bits/s/Hz, the proposed scheme attains a more than 2-dB average and peak optical power gain over 16-PAM using the previous approaches.

Polarity Information Coded Flip-OFDM for Intensity Modulated Systems

A polarity-information-coded flip orthogonal frequency division multiplexing is proposed for intensity modulation/direct detection optical communications in this letter. In the proposed scheme, the modulated signals in the frequency domain are not constrained to have Hermitian symmetry. The real and imaginary parts of the time-domain complex signals are separated, and the polarities of the real and imaginary parts are jointly encoded and modulated. The transmit strategy and the receive algorithm of the proposed scheme are analyzed in detail. The major advantage of the proposed scheme is that its spectral and optical power efficiencies are higher than existing schemes, which is validated in simulation.

Fading-free transmission of 124-Gb/s PDM-DMT signal over 100-km SSMF using digital carrier regeneration.

The coherent reception of intensity modulated signal has been recently widely investigated, in which the signal is recovered by the envelop detection. High linewidth tolerance is achieved with such scheme. However, strong optical carrier exists during the transmission, which degrades the optical power efficiency. In this paper, an efficient modulation scheme for discrete multi-tone (DMT) signal is proposed based on the Mach-Zehnder modulator (MZM). Different from the traditional intensity modulation, the proposed method employs both intensity and phase domain. Thus, the optical carrier power can be greatly reduced by adjusting the bias of MZM around the null point. By employing coherent detection and digital carrier regeneration (DCR), the carrier suppressed DMT signal can be recovered using envelop detection. No carrier frequency or phase estimation is required. Numerical investigations are made to demonstrate the feasibility, in which significant improvements are found for the proposed DCR method, showing great tolerance against laser linewidth and carrier power reduction. Finally, a 124-Gb/s transmission of polarization-division multiplexed DMT (PDM-DMT) signal is demonstrated over 100-km SSMF, with only -8 dB optical carrier to signal power ratio (CSPR).

Study on the similarity of photonic crystal ring resonator cavity modes and whispering-gallery-like modes in order to design more efficient optical power dividers

In this study, we discuss the wave pattern characteristics of confined cavity modes inside photonic crystal ring resonators (PCRRs). Despite different physical origins, these cavity modes are analogous to the whispering-gallery (WG-like) modes. Because of the absence of perfectly circular symmetry in our PC cavity, the WG-like modes are not degenerate, but they form a close doublet in which the field pattern of each of these doublets repeats itself by $$180^{\circ }/m$$180?/m where "m" is the azimuthal index number of WG-like modes. The cavity modes are named according to their symmetric field pattern, and WG-like ones are named by their azimuthal mode numbers "m." Based on the study of these similarities, we propose a $$1\times 2$$1×2 and a $$1\times 4$$1×4 T-junction- like power dividers. Through study of the similar properties of these cavity modes with WG-like modes, we have enhanced the output power of both dividers to 49.6 and 24.5 %, respectively. The permittivity of the hexagonal photonic crystal rod is $$\varepsilon _{h}= 12.04$$?h=12.04 surrounded by air as the background medium. For our PCRR, the dominant resonating mode is the cavity mode with hexapole field pattern or equivalently WG-like mode with azimuthal mode number of $$m\,=\,6$$m=6. The normalized frequencies for this mode and its doublet are $$a/\lambda = 0.348066$$a/?=0.348066 and 0.348301, respectively. The photonic band structure, PC waveguide guided mode and electric field patterns of the confined cavity modes inside the PCRR are calculated using the PWE method, and the transmission spectra are calculated by means of 2D-FDTD simulation method.

Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots

Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture. Here, we use quantum dots of ternary I-III-VI2 semiconductors to realize the first large-area quantum dot-luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum. By incorporating CuInSexS2-x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows. Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%. Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.

1-D and 2-D Nanocontacts for Reliable and Efficient Terahertz Photomixers

New types of continuous-wave (CW) terahertz (THz) photomixers were fabricated using 1-D and 2-D nanocontacts on low-temperature-grown (LTG) GaAs as well as nitrogen-ion-implanted $({\rm N}^{+}{\rm i})$ GaAs. The 1-D and 2-D nanocontacts were formed by silver nanowires and graphene sheets, respectively. A silver nanowire (Ag-NW) with a physical diameter of $\varnothing \hbox{60 nm}$ and $\varnothing \hbox{120 nm}$ can handle currents of $>\! \hbox{10}$ and $>\! \hbox{30 mA}$ , respectively, without electromigration, thus enabling reliable high photocurrents. The small diameter of such a nanowire electrode in 1-D photomixers reduces the device capacitance by a factor of more than ten compared with photomixers with conventional interdigital fingers. Thus, these nanowire properties can be used for the fabrication of photomixers with cut-off frequencies $>\hbox{2 THz}$ and with extremely reliable operation. A multilayer graphene (MLG) of 6–8 layers shows unique properties with high short-circuit current capabilities of $>\hbox{9 mA/(10}~\mu \hbox{m width})$ and reliable high photocurrents. Furthermore, the MLG absorbs only 2.3% of white light and is ideal as a transparent nanocontact. The use of these graphene electrodes in 2-D photomixers allows an efficient optical power illumination for high photocurrent generation and shows the potential for reliable CW THz emission. Initial THz measurements confirmed the high possibilities of these new nanocontact photomixers.

An adaptive pulse width modulation technique for projectors based on blue laser

An adaptive pulse width modulation (APWM) technique for increasing the optical power efficiency and brightness of projection systems using blue laser is proposed. The proposed approach adaptively drives the current pulse width-modulated signals of blue laser to increase the lifetime and decrease the junction temperature of laser. This APWM technique stabilizes the junction temperature of laser diodes according to the driving environment and the forward current capacity. Through the proposed method, the brightness of blue laser is improved by about 37% compared to the conventional current driving method in a projection system.

Effect of the Quantum-Well Shape on the Performance of InGaN-Based Light-Emitting Diodes Emitting in the 400–500-nm Range

The electrical and optical properties of InGaN–GaN light-emitting diodes (LEDs) emitting in the 400–500-nm range and having a v-shaped quantum well (VSQW) and a u-shaped quantum well (USQW) are numerically investigated using APSYS simulation program. The simulation results showed that the devices containing VSQW have superior performance in terms of optical power and internal quantum efficiency droop compared to those with USQW. The optical power of the LEDs containing USQW increases gradually and reaches a maximum at 460 nm; however, the optical power of the LEDs with VSQW improves gradually, and the maximum is obtained in a window from 420 to 436 nm as a result of radiative recombination enhancement. The simulation results suggest that the higher performance of the VSQW is due to piezoelectric field reduction and an enhancement of electron and hole wave functions overlap.

Size dependent efficiency of photophoretic swimmers.

We investigate experimentally the efficiency of self-propelled photophoretic swimmers based on metal-coated polymer particles of different sizes. The metal hemisphere absorbs the incident laser power and converts its energy into heat, which dissipates into the environment. A phoretic surface flow arises from the temperature gradient along the particle surface and drives the particle parallel to its symmetry axis. Scaling the particle size from micro to nanometers, the efficiency of converting optical power into motion is expected to rise with the reciprocal size for ideal swimmers. However, due to the finite size of the metal cap, the efficiency of a real swimmer reveals a maximum depending sensitively on the details of the metal cap shape. We compare the experimental results to numerical simulations.