Effect Of Optical Loss Research Articles

Enhancement of Optomechanical Squeezing of Light Using the Optical Coherent Feedback.

A coherent feedback scheme is used to enhance the degree of squeezing of the output field in a cavity optomechanical system. In the feedback loop, a beam splitter (BS) plays the roles of both a feedback controller and an input-output port. To realize effective enhancement, the output quadrature should take the same form as the input quadrature, and the system should operate at the deamplification situation in the meantime. This can be realized by choosing an appropriate frequency-dependent phase angle for the generalized quadrature. Additionally, both the transmissivity of the BS and the phase factor induced by time delays in the loop affect optical squeezing. For the fixed frequency, the optimal values of transmissivity and phase factor can be used to achieve the enhanced optical squeezing. The effect of optical losses on squeezing is also discussed. Optical squeezing is degraded by the introduced vacuum noise owing to the inefficient transmission in the loop. We show that the enhancement of squeezing is achievable with the parameters of the current experiments.

Device physics of homojunction perovskite solar cells: a design omitting all the charge transport layers with efficiency exceeding 26.3%

Perovskite solar cells (PSCs) omitting all the charge transport layers with p–n homojunction structure are considered a promising alternative for commercialization owing to their low fabrication cost and simplified structure. Deep understanding of the device physics of these all-free p–n homojunction structured PSCs is of paramount importance. Here, a thorough investigation of all-free perovskite–perovskite p–n homojunction structured PSCs is performed by using a photoelectrical coupling model. Four different configurations including a standard n–i–p cell, electron transport layer-free cell, hole transport layer-free cell, and all-free cell are compared to identify the limiting performance factors, and the results indicate that no extra built-in electric field in the perovskite layer and severe surface recombination occurring at the perovskite interface are the two main factors limiting the power conversion efficiency (PCE) of all-free p–n homojunction structured PSCs. Based on doping engineering, a highly efficient all-free p–n homojunction structure is designed, which consists of an asymmetric p–n junction with both a front surface field layer and a back surface field layer. The effects of optical loss, thickness of the emitter, doping concentration for both the emitter and base, and diffusion length on the performance of p–n homojunction structured PSCs are optimized. After optimization, the PCE of the all-free p–n homojunction structured PSCs reaches 26.33%, which is slightly higher than that of a standard n–i–p heterojunction cell (26.22%). This work demonstrates that all-free p–n homojunction structured PSCs are a promising alternative to standard n–i–p heterojunction structured PSCs for realizing high efficiency, which may pave the way toward commercialization of PSCs in the future.

Total-internal-reflection-based optical rotation quasi-phase-matching technique realized in a biaxial electro-optic chiral material to generate highly efficient second harmonic

This work demonstrates a total-internal-reflection-based optical rotation quasi-phase-matching technique availing polarization rotation under the influence of an external electric field in a rectangular-shaped potassium niobate crystal coated with a thin layer of gallium phosphide. This thin film is utilized to regulate the phase shifts caused by the propagation of p - and s -polarized light at the slab-film interface at each total internal reflection bounce point. The peak conversion efficiency of 3.3%/W/cm, obtained from the computer-aided simulation, reflects the combined field effect of optical rotation quasi-phase-matching and fractional quasi-phase-matching techniques. The effect of optical losses and the nonlinear law of reflection have also been included to simulate the generation of a second-harmonic wavelength of 2150 nm.

Strongly resonant silicon slot metasurfaces with symmetry-protected bound states in the continuum.

In this work, a novel all-dielectric metasurface made of arrayed circular slots etched in a silicon layer is proposed and theoretically investigated. The structure is designed to support both Mie-type multipolar resonances and symmetry-protected bound states in the continuum (BIC). Specifically, the metasurface consists of interrupted circular slots, following the paradigm of complementary split-ring resonators. This configuration allows both silicon-on-glass and free-standing metasurfaces and the arc length of the split-rings provides an extra tuning parameter. The nature of both BIC and non-BIC resonances supported by the metasurface is investigated by employing the Cartesian multipole decomposition technique. Thanks to the non-radiating nature of the quasi-BIC resonance, extremely high Q-factor responses are calculated, both by fitting the simulated transmittance spectra to an extended Fano model and by an eigenfrequency analysis. Furthermore, the effect of optical losses in silicon on quenching the achievable Q-factor values is discussed. The metasurface features a simple bulk geometry and sub-wavelength dimensions. This novel device, its high Q-factors, and strong energy confinement open new avenues of research on light-matter interactions in view of new applications in non-linear devices, biological sensors, and optical communications.

An Optimal Design for Decreasing Dispersion in Photonic Crystal Fiber

Fiber optic transmission systems are friendly data transmission systems environmentally due to their low losses, high bandwidth and acceptable reliability. Therefore, they are often used for communication infrastructures. However, the effects of optical loss and dispersion can cause problems in fiber optic communications. In this paper, we mitigate these effects by using photonic crystals. The goal is to minimise the dispersion of the fibers by changing the photonic crystal fiber parameters. Specifically, we change the number, radius, and shape of the holes to minimise dispersion. A new photonic crystal fiber (PCF) is proposed with hexagonal grids of air holes and 11 layers in this structure. An optimal holes number, radius and distance between holes are used to obtain the least dispersion. Next, we use several elliptical and stellar holes to reduce dispersion. The germanium impurity is exploited in the PCF core as a defect. The addition of germanium impurity to the core causes the doping atoms reflect a stronger optical signal with the same attenuated input signal properties. The simulation results show that dispersion value is zero in three points at wavelengths between 1.48 and 1.55 μm. In this wavelength range, the dispersion value was obtained between − 0.3 and 0.6 ps/(nm km).

Compound-induced transparency in three-cavity coupled structure* *Project supported by the National Natural Science Foundation of China (Grant No. 61575014).

We propose a three-cavity coupled cavity optomechanical (COM) structure with tunable system parameters and theoretically investigate the probe-light transmission rate. Numerical calculation of the system’s spectra demonstrates distinctive compound-induced transparency (CIT) characteristics, including multiple transparency windows and sideband dips, which can be explained by a coupling between optomechanically-induced transparency (OMIT) and electromagnetically-induced transparency. The effects of optical loss (gain) in the cavity, number and topology of active cavity, tunneling ratio, and pump laser power on the CIT spectrum are evaluated and analyzed. Moreover, the optical group delay of CIT is highly controllable and fast–slow light inter-transition can be achieved. The proposed structure makes possible the advantageous tuning freedom and provides a potential platform for controlling light propagation and fast–slow light switching.

Temperature tunable Anderson localization for graphene surface plasmons

In this paper, we propose a one-dimensional disordered plasmonic structure composed of a graphene single layer placed on a random grating composed of InAs. The propagation of a plasmonic wave through this structure is investigated numerically. By calculation of normalized localization length for systems with different disorder strengths, it is determined whether or not the system is in the localized regime. For some frequencies, depending on the disorder level, Anderson localization occurs for plasmonic waves propagating through the graphene layer. Furthermore, the effect of optical loss on the localization length is studied. By calculating the localization length at different temperatures, it is observed that Anderson localization of graphene plasmons is temperature dependent and can be controlled by changing the temperature. In the transmission spectrum for each random realization, there are some resonance peaks that are blueshifted with increasing the temperature. Finally, the effects of Fermi energy level of the graphene layer and width of air gaps on the individual transmission resonances are examined.

Enhanced Modal Coupling Loss in a Two-Mode SOI Waveguide of Broken Parity-Time Symmetry

Effect of optical loss, presented in a superstructure grating with a complex refractive index profile, on the mode coupling is studied in a two-mode silicon-on-insulator waveguide. In this scheme, both our theory and experimental results indicate that the optical loss not only provides optical absorption of a targeted output mode and thus the corresponding modal coupling loss, but also induces a broken parity-time symmetry phase that strongly prohibits power coupling from the incident mode to the output mode. As a result, the latter can give rise to a much-enhanced modal coupling loss of the targeted output mode. Under a reasonable device length about 37 μ m, the enhancement factor can be as high as ∼20 dB in simulation and ∼17 dB in experiment. This paper may offer a different way to exploit the role of optical loss in boosting modal coupling loss that can directly translate to power extinction ratio. It thus may find potential applications in novel intensity modulator designs.

Optimized conditions for the improvement of thin film CdS/CdTe solar cells

Efficient thin film CdS/CdTe solar cell performance requires optimum parameters of each layer of this cell and of the barrier structure. Moreover, the effect of optical losses, recombination losses at front and back surface of CdTe and recombination losses in the space-charge region (SCR) must be considered in order to really analyze the role of these parameters on the performance of these cells. This work is focused on studying theoretically the effect of the thickness of the front contact (ITO), thickness of the window layer (CdS), thickness of the absorber layer (CdTe), width of the space-charge region and electron lifetime on the efficiency of CdS/CdTe solar cells. The reflection losses from interfaces and absorption losses in ITO and CdS, front and rear surface recombination losses of CdTe as well as recombination losses in SCR have been studied. It has been observed that the short-circuit current strongly depends on the thickness of ITO, thickness of CdS, thickness CdTe and electron lifetime. The concentration of uncompensated impurities (Na−Nd) in CdTe, which determines the width of SCR, plays a key role in the generation of photocurrent. The recombination losses in the SCR decrease rapidly with increasing the carrier lifetime in this region and can be ignored at lifetime of 10−7s. The reflectivity from the back contact introduces a small influence in increasing the short-current density particularly at thick absorber layer (5–8μm). Under the conditions of Na−Nd~1016cm−3, τn=10−6s, dCdTe=8μm, dITO=100nm, and dCdS=100nm, the recombination and optical losses record their minimum ratio of 27%. Most of these losses (24%) are due to the optical losses. The efficiency of CdS/CdTe under these parameters is about 18.2% which is exactly matching with the recent experimental studies. Moreover, an ultrathin CdTe (=1μm) is sufficient to introduce high efficiency of 16.4%.

Enhancing the performance of thin film CdS/PbS photovoltaic solar cells

This work investigates dependence of the short-circuit current density, open-circuit voltage, fill factor and efficiency of a thin film CdS/PbS solar cell on thickness of transparent conductive oxide (TCO) layer, thickness of window layer (CdS), concentration of uncompensated acceptors (width of space-charge region), carrier lifetime in PbS and the reflectivity from metallic back contact. The effect of optical losses, front and rear recombination losses as well as the recombination losses on space-charge region are also considered in this study. As a result, by thinning the front contact layer indium tin oxide from 400 to 100 nm and window layer (CdS) from 200 to 100 nm it is possible to reduce the optical losses from 32 to 20%. The effect of electron lifetime on the internal and external quantum efficiency can be neglected at high width of the space-charge region. The maximum current density of 18.4 mA/cm2 is achieved at wide space-charge region (concentration of uncompensated acceptors = 1015 cm−3) and the longest lifetime (τn = 10−6 s) where the optical and recombination losses are about 55%. The maximum efficiency of 5.17%, maximum open-circuit voltage of 417 mV and approximately fixed fill factor of 74% are yielded at optimum conditions such as: electron lifetime = 10−6 s; concentration of uncompensated acceptors = 1016 cm−3; thickness of TCO = 100 nm; thickness of CdS = 100 nm; velocity of surface and rear recombination = 107 cm/s and thickness of absorber layer = 3 μm. When the reflectance from the back contact is 100%, the cell parameters improve and the cell efficiency records a value of 6.1% under the above conditions.

Ef fect of optical losses on the transmission performance of a radio-over-fiber distributed antenna system

The effects of optical losses on a directly-modulated radio-over-fiber (RoF) system used for distributed antenna networks are determined. The results show that with a properly designed bidirectional amplifier, the RoF link can tolerate over 20 and 16 dB of optical losses for down- and up-links, respectively. Simulation results are also consistent with the experimental data. These findings can contribute to the design of RoF distributed antenna systems with different topologies.

Optical loss effect on Sagnac sensitivity of circle-coupled resonator structure

We propose a configuration of integrated waveguide structure consisting of resonators coupled to a circle waveguide. Sagnac effect in this structure and loss effect as well as impact of through coupling coefficient on system sensitivity were analyzed and discussed. It showed that system sensitivity gets higher with more passive ring resonators in lossless structure, while considering loss the sensitivity can only be improved by adding more resonators when the propagation loss is lower than the threshold, which depends on structure size, through coefficient and light wavelength. As the propagating loss of waveguide gets lower, this configuration has larger potential to realize highly integrated optical gyroscope for rotation sensing.

Wideband antenna array beam steering with free-space optical true-time delay engine

A 6–12 GHz Vivaldi wideband antenna array made of four sub-arrays (two elements per sub-array) was built to steer the beam with a White cell optical true-time delay (OTTD) engine. The OTTD engine was simulated to produce delays with a time delay increment of 25 ps. The beam is then steered combining the simulated OTTD delays and the measured mutual coupling of a Vivaldi array to a maximum scan angle of ±50°. The results demonstrate that the effect of optical losses and mutual coupling effects of the combined beam steering system is not negligible and must be carefully accounted for when implementing frequency-independent TTD beam steering systems.

Detailed balance analysis of solar thermophotovoltaic systems made up of single junction photovoltaic cells and broadband thermal emitters

This paper presents a detailed balance analysis of a solar thermophotovoltaic system comprising an optical concentrator, a cut-off broad band absorber and emitter, and single junction photovoltaic cells working at the radiative limit with an integrated back-side reflector in a configuration in which the cells enclose the emitter to form an optical cavity. The analysis includes the effect of multiple variables on the system performance (efficiency and electrical power density), such as the concentration factor, the emitter-to-absorber area ratio, the absorber and emitter cut-off energies, the semiconductor band-gap energy and the voltage of the cells. Furthermore, the effect of optical losses within the cavity such as those attributed to a back-side reflector with reflectivity lower than one or to a semi-open optical cavity is also included. One of our main conclusions is that for a planar system configuration (the emitter, the cells and the absorber have the same area) the combination of low concentration and a spectrally selective absorber provides the highest system efficiencies. The efficiency limit of this kind of systems is 45.3%, which exceeds the Shockley–Queisser limit of 40.8% (obtained for a single junction solar cell, directly illuminated by the sun, working under maximum concentration and with an optimized band-gap). This system also has the great benefit of requiring a very low concentration factor of 4.4 suns.

Effect of Optical Loss and Antenna Separation in 2$\,\times\,$2 MIMO Fiber-Radio Systems

We study the impact of fiber-optic loss due to realistic cable deployment and optical connector loss variation and distance between the transmitter antennas in a radio-over-fiber transmission system with 2 × 2 MIMO. We evaluate an upper bound on the system capacity based on measured values of the error vector magnitude and the condition number of the channel matrix. For this, assuming a Rayleigh fading environment, we derive conditional probability density functions of eigenvalues of a 2 × 2 Wishart matrix. We compare the obtained results with another approach, which is based on IEEE 802.11 relations between the allowed relative constellation error and the achievable data rate for an individual stream. Trends predicted by both models agree very well. Our analysis shows that (i) about 1 meter spatial separation of fiber-fed transmitter antennas is optimum and results in the maximized system performance and (ii) the maximum tolerable optical power imbalance in two fiber optic links is about 6 dB.

Time-Domain Analysis of Resonator Array Buffers

We present design and time-domain simulations of microresonator array optical delay lines. The proposed buffer consists of an n times m microresonator array unidirectionally coupled via a straight bus waveguide. We show that the use of a two-dimensional arrangement of resonators can compensate for the high order of dispersion and more specifically the third-order dispersion. Numerical calculations confirm that pulses can be stored in microresonator arrays without appreciable distortion. Finally, we briefly describe the detrimental effects of optical losses and technological errors.

Quantum Theory Analysis of Loss Effects in Guided Nonlinear-Optic Twin-Photon Generation

Integrated nonlinear-optic (NLO) devices for generation of twin-photon beams are a key device for quantum information techniques. This paper presents a quantum theory analysis on the effect of optical losses in twin-photon generation. The analysis uses a multi-mode-pair description, and is based upon unitary transformations of amplitude operators and state vectors for describing the NLO and loss phenomena in infinitesimal sections in the interaction range. Full description of the quantum states of the twin-photon beams affected by the losses is obtained. Mathematical expressions useful for prediction of the performances degradation are deduced

Engineering the nonlinear phase shift with multistage autoregressive moving-average optical filters

We propose and demonstrate the application of concepts from digital filter design in order to optimize artificial optical resonant structures to produce a nearly ideal nonlinear phase shift response. Multistage autoregressive moving average (ARMA) optical filters (ring-resonator-based Mach-Zehnder interferometer lattices) are designed and studied. The filter group delay is used as a measure instead of finesse or quality factor to study the nonlinear sensitivity for multiple resonances. The nonlinearity of a four-stage ARMA filter is 17 times higher than that of the intrinsic material of the same group delay. We demonstrate that the nonlinear sensitivity can be increased within constant bandwidth by allocating more in-band phase or by using higher-order filter structures and that the nonlinear sensitivity enhancement improves with increasing group delay. We also investigate methods to precompensate the nonlinear response to reduce the occurrence of optical bistabilities. The effect of optical loss, including linear absorption and two-photon absorption, is discussed in postanalysis. In addition, we discuss how the improvement in nonlinear response scales with respect to various filter parameters.

Characteristics of optical bandpass filters employing series-cascaded double-ring resonators

The filtering characteristics of a series-cascaded double-ring optical resonator (SDRR) are investigated. The roles of coupling coefficients and the effects of optical loss are analyzed. The relationships of the coupling coefficients with the characteristics of the SDRR filter are expressed with simple analytical formulas. With the derived approximate formulas, it is found that the bandwidth ratio of the SDRR filter is mainly dependent on the shape factor defined in this paper. The performance of the SDRR filter, especially the bandwidth ratio, is improved significantly in comparison with that of the single-ring-resonator filter. The analytical results also indicate that the optical loss in the microrings has a strong influence on the characteristics of the SDRR filter.

Effect of optical losses on optical and magneto-optical properties of one-dimensional magnetophotonic crystals for use in optical isolator devices

One-dimensional magnetophotonic crystal (1-D MPC) film composed of thin rare-earth iron garnet, SiO 2 and Ta 2O 5 films simultaneously exhibits very high optical transmittance and Faraday rotation. We investigated the structure of 1-D MPC, which by theoretical calculation has an optical isolation performance favorable from an industrial perspective. Furthermore, we considered, theoretically, whether it could be used as an optical isolator if a film had absorption. As a result, the structure of 1-D MPC film with satisfactory optical isolation has an industrially favorable structure by using a high refractive index material such as Si. It was also found that if the optical absorption (extinction coefficient κ) of a film was less than 0.000001, it did not affect the characteristics. This makes it very attractive for actual optical isolator fabrications.