Radiation Spectrum Method Research Articles

Experimental research on the thermal throat of rocket based combined cycle combustor

Based on a previous one-dimensional theoretical analysis, direct-connection experiments of thermal throat generation and the factors that influence it in the Rocket Based Combined Cycle combustor are performed relative to the fuel injection position and equivalence ratio. The Mach 5.5 airflow experimental results indicate that the performance is improved by 3.0–8.3% by optimizing the fuel injection position, and at an airflow of Mach 4.0, the increase in the fuel equivalence ratio gradually shifts the generation of the thermal throat backward. When a thermal throat is generated at the end of the combustor, the thrust and specific impulse performance are optimal. The effectiveness of employing a thermal adjustment to improve engine performance is validated by the experimental studies. The radiation spectrum method is employed to measure the temperature along the flow field of the thermal throat. The temperature and pressure measurements along the flow path indicate that the Mach number variation trend calculated in the experiment is the same as that observed in the numerical simulation results in the thermal throat flow field. The generation process of the thermal throat is characterized by the acceleration of airflow from being subsonic to supersonic.

Hawking radiation of non-asymptotically flat rotating black holes

We study the Hawking radiation of non-asymptotically flat rotating linear dilaton black holes, which are the solutions to the $4D$ Einstein–Maxwell-dilaton–axion action by using the semi-classical radiation spectrum method. Using scalar perturbations, we show that both angular and radial equations produce exact analytical solutions. Thus, we obtain a precise radiation spectrum for the rotating linear dilaton black hole. The high-frequency regime does not yield the standard Hawking temperature of this black hole computed from the surface gravity. However, we show in detail that the specific low-frequency band of the radiation spectrum allows for the original Hawking temperature of the rotating linear dilaton black hole. The computations are also exhibited graphically.

Comparison of the behavior of a subwavelength diffractive lens in TE and TM polarization allowing some nonstandard functions

This Letter introduces and discusses a difference in the behavior of a cylindrical diffractive lens encoded with subwavelength structures illuminated with monochromatic coherent light in the cases of TE and TM polarization. The effective medium theory is used to model with new binary phase function the diffractive lens. A new algorithm combines the finite-difference time domain for the propagation in the near field and the radiation spectrum method for the propagation in the far field. We observe the existence in the TM polarization of a second spot at half the distance of the focal length not predictable by scalar theory.

Modeling of the angular tolerancing of an effective medium diffractive lens using combined finite difference time domain and radiation spectrum method algorithms

A new rigorous vector-based design and analysis approach of diffractive lenses is presented. It combines the use of two methods: the Finite-Difference Time-Domain for the study in the near field, and the Radiation Spectrum Method for the propagation in the far field. This approach is proposed to design and optimize effective medium cylindrical diffractive lenses for high efficiency structured light illumination systems. These lenses are realised with binary subwavelength features that cannot be designed using the standard scalar theory. Furthermore, because of their finite and high frequencies characteristics, such devices prevent the use of coupled wave theory. The proposed approach is presented to determine the angular tolerance in the cases of binary subwavelength cylindrical lenses by calculating the diffraction efficiency as a function of the incidence angle.

Extension of the radiation spectrum method for the reflection calculation at the end of a strongly guiding optical waveguide

In this work we extend the radiation spectrum method (RSM) with evanescent modes to use it for the calculation of the reflection coefficient at the end of a strongly guiding dielectric waveguide. The extension is made by considering the coupling between the radiation (or evanescent) modes at both sides of the optical discontinuity. To insure the convergence of the method, this extension is achieved analytically. Using this technique, we show that for small guide width, the generation of evanescent modes results in a complex reflection coefficient. The magnitude and the phase of the reflection coefficient are calculated and compared with the simple theory of the effective index as well as the finite difference time domain (FDTD) technique. The spectra of the transmitted and reflected fields are also obtained.

Optical modeling of waveguide photonic nanostructures using the radiation spectrum method (RSM) with evanescent modes

In this work, we study the effects of the evanescent modes in the simulation and modeling of optical integrated circuits based on photonic bandgap structures. We show that the contribution of these modes in the energy transfer in structures like the MOEM structures, can not be neglected. The radiation spectrum method, recently developed by the authors for the guided wave devices, is thus extended to account for the evanescent mode propagation. Applying this technique on an air-gap in a suspended waveguide a model of this gap is developed in terms of its parameters. This model is then integrated in an all optical simulator to predict the performance of photonic structures. Such technique enables to design and to optimize the photonic integrated circuits taking the evanescent modes effects into account.

A new solution to waveguide excitation suppressing the effects of the radiated field. Application to the Y-junction

When an integrated optical component is asymmetrically excited, a radiated field is generated. Most of the time, the energy carried by the radiated field is lost. However, with high-level integration structures, this field can couple back into another waveguide and then disturb the operating conditions of the whole component. An original waveguide geometry that rapidly removes this radiated field is thus proposed here. The study of a specific Y-junction shows that it is possible to reduce by a factor of at least 30 the length of the Y-junction's straight waveguide. Computer modelling was performed with the radiation spectrum method (RSM), a new beam propagation method, perfectly suited to dealing with the field coming from guided and radiation modes. It gives us a good physical understanding of the light propagation.

Rigorous modal analysis of multi-mode interference (MMI) structures by radiation spectrum method with multiple reflection

In this work, we present the analysis of MMI structures using a modal technique. This technique takes into account both the multiple reflection between the input and output interface junctions and the radiation modes in the input and output. The model is based on the radiation spectrum method (RSM) for the inclusion of the radiation modes. The results are compared to these obtained by BPM for weak guiding and with self-imaging (SI) analysis for strong guiding.

Towards a full vectorial and modal technique for the analysis of integrated optics structures: the Radiation Spectrum Method (RSM)

We present a modal method for the analysis of integrated optics structures. The presented technique, called Radiation Spectrum Method (RSM), is based on the analysis of the radiation mode spectrum generated at each waveguide discontinuity in the structure. The full wave capabilities of the RSM are demonstrated by its application for several non-paraxial tests as tilted waveguide and bent waveguide tests. We also demonstrate the method capability to correctly analyse components using abrupt index changes.

Intersecting waveguide modelization for any angle and guiding conditions with the radiation spectrum method

The radiation spectrum method (RSM) is an eigenmode beam propagation technique that allows to deal with integrated optical structures presenting both strong index differences and important direction changes. The principle of this method is exposed and we then discuss the originality of it compared to similar other beam propagation schemes. The RSM is used to investigate intersecting waveguides. The validity of our calculations is discussed by comparison with reference and experimental results.