Fresnel Reflection Effects Research Articles

Parabolic microlensed optical fiber for coupling efficiency improvement in single mode fiber

The efficiency of optical coupling between an optical fiber and other components be it a light source, a photodetector or another fiber, often depends on the performance of the focusing components. In optoelectronics, microlenses are generally incorporated at the end of optical fibers to ensure optimal coupling. These microlenses are primarily fabricated with a spherical profile easier to achieve, with a determined radius and at low production costs. However, these microlenses exhibit a relatively large waist due to intrinsic spherical aberrations making it difficult to couple light into single mode fibers. This paper presents the results of a study of a microlens having a parabolic profile that has been made of polydimethylsiloxane (PDMS) at a single mode optical fiber (SMF 9/125) end terminal. The contribution of the parabolic profile as compared to spherical shaped one is analyzed. Estimates at the wavelengths of primary importance, λ =1.310 µm and λ =1.550 µm, have shown a decrease in the spot radius diagram in the focal plane by 3, from an root mean square (RMS) value of 0.623 µm in the spherical case to less than 0.229 µm in the parabolic case. The measured optical coupling has improved to 98.5% under optimal conditions (without taking into account the bulk absorption and the effects of Fresnel reflection). For different studied microlens curvatures’ radii, the obtained waist values vary from 1.00 to 4.90 µm with working distances from 5.80 to 48.80 µm, respectively.

Performance Improvement of GaN-Based Light-Emitting Diodes With a Microhole Array, 45° Sidewalls, and a SiO2 Nanoparticle/Microsphere Passivation Layer

The characteristics of GaN-based light-emitting diodes (LEDs) with a hybrid structure incorporating a microhole array, 45 sidewalls, and an appropriate SiO2 nanoparticle (NP)/microsphere (MSs) passivation layer are studied and reported. The use of a SiO2 NP/MSs passivation layer causes a remarkable reduction in reverse-biased leakage current. The employment of this hybrid structure leads to substantial enhancements in optical properties without any degradation in electrical performance. In addition, a lower content of SiO2 NP in the mixed SiO2 NP/MSs solution leads to enhanced optical behavior due to the improved transmittance. Experimentally, as compared with a conventional LED (Device A), the studied Device E shows 50.6%, 50.9%, 48.4%, and 49.9% enhancements in light output power, luminous flux, luminous efficacy, and wall-plug efficiency, respectively. These advantages are mainly attributed to the increased scattering probability and the opportunity to find photon escape cones as well as the reduced total internal reflection and Fresnel reflection effects. Therefore, the studied hybrid structure provides a promise for high-performance GaN-based LED applications.

Grating Coupler With Reduced Back Reflection Using λ/4 Offset at Its Grating Sub-Teeth

We demonstrate a grating coupler with reduced back reflection on silicon-on-insulator (SOI) platform. Each grating tooth of the grating coupler is uniformly divided into multiple sub-teeth with λ /4 offset between two adjacent sub-teeth, reducing the Fresnel reflection from interfaces between core and cladding materials in transmission direction. The back reflection can be theoretically suppressed from −12 dB to below −30 dB for a focusing grating coupler working at transverse magnetic (TM) polarization. Experimental results show that the effect of Fresnel reflection on transmission spectrum of the proposed grating coupler is significantly reduced compared to conventional grating coupler. Furthermore, a microring resonator using the proposed focusing grating coupler to couple light into and out of waveguide is fabricated. The measured spectra show that fluctuations on transmission spectra of the ring resonator are effectively suppressed, especially for the drop-port, a well-shaped resonance peak is clearly observed, making the resonance wavelength easily distinguishable, which is highly required in sensing applications using wavelength interrogation.

THEORETICAL ANALYSIS OF SINGLE-FIBER TRANSMISSION OVER WDM-PON SYSTEMS

In this paper, a rigorous theoretical analysis on optical beat interferometric (OBI) noises caused by the Rayleigh backscattering (RB) and Fresnel reflection (FR) effects on the performance of a single-fiber single-wavelength WDM-PON (wavelength division multiplexing-passive optical network) transmission system has been presented. Two categories of PON are evaluated: a conventional and a long-reach WDM-PON system. The conventional WDM-PON took place in an access network for a typical transmission distance of 20 km without any optical amplification at the remote node (RN). In this case, the gain of the optimal reflective optical network unit (ONU) will depend on the relative RB power and also the FRs magnitudes and locations along the optical fiber channel/transmission link. However, in the cases of the long-reach WDM-PON deployed at a transmission distance of 100 km or further, the optical amplifiers are indispensable to enhance the optical power budget and to prevent the effect of fiber nonlinearities that may occur at high power injection levels. Although the presence of an optical amplifier (OA) in a long-reach WDM-PON will actually give some additional gain to the whole system, as a result it also produces the worse RB and FR effects to the system itself. A crosstalk to signal (C/S) ratio is used as a key performance indicator of whether the transmission system is in good working order or not. Therefore, this study may provide insight and relevant information in terms of transmission optimization for WDM-PON systems and developments in the future. To educate on the current technologies and developments surrounding conventional and long-reach WDM-PON, a brief overview is provided in the background part of the introduction.

A parametric reflectance approximation for rendering Japanese lacquerware and Maki-e

This paper proposes a reflectance distribution model that is able to express complex reflections from materials such as Japanese lacquerware and Maki-e to produce highly realistic computer graphics. Our method improves the Ward model for materials with anisotropic reflection, and uses the approximate coefficients calculated from measured reflectance distribution data to reconstruct the actual reflectance. Our research derives a Gaussian distribution summation method and a modified Fresnel reflectance approximation function from measured reflectance data. Our Fresnel reflectance approximation function adds parameters to Schlick's approximation function to control the whole curve and extinction component, and is thus closer to the actual Fresnel reflectance. Based on these functions, a decision process is developed for the approximation coefficients, and this makes it possible to easily reconstruct the actual reflectance. The reconstructed reflectance distributions obtained using these coefficients are compared with measured reflectance data using the L2 norm of the difference. In addition, reconstructed Fresnel reflection effects are compared with those from actual sample materials. Using these coefficient values, this paper shows a sample of simulated Maki-e images under two different lighting environments.

Optimizing textured structures possessing both optical gradient and diffraction properties to increase the extraction efficiency of light-emitting diodes

The external quantum efficiency of a light-emitting diode (LED) is strictly limited by total internal reflection and Fresnel reflection effects. In this study, we sought to optimize light extraction by monitoring the shape effects of four kinds of periodic textured structures (nanorod, inverted rod, pyramid, inverted pyramid) on the surface of gallium nitride (GaN)-based LEDs. We employed the three-dimensional rigorous coupled waves approach to calculate the direct emissions at different incident angles on the various textured structures, and then determined an optimized structure that would improve the extraction efficiency of LEDs. The optical gradient of the inverted pyramid structure could decrease not only the Fresnel reflection at incident angles less than the critical angle but also the total internal reflection at incident angles greater than the critical angle. Many inverted pyramid structures at the GaN–air interface, with various sizes and periods, provided enhancement factors of greater than 150%.

Cross-Seeding Schemes for WDM-Based Next-Generation Optical Access Networks

Rayleigh backscattering (RB) and Fresnel reflection (FR) effects can cause severe degradation on the performance of a WDM-PON system utilizing loop-back schemes that provide colorless upstream transmitters. We proposed cross-seeding schemes to overcome these problems and also to improve the utilization efficiency of the seeding lights. The operation principles of two types of cross-seeding schemes: mutual seeding and shared seeding are discussed in detail for different applications. A generalized formulation of the crosstalk from the RB and FR effects is derived for comparing the performance between a conventional WDM-PON and a cross-seeding WDM-PON. It is found that the cross seeding schemes can be most beneficial for the system with low loop-back gain or low reflection at the distribution fiber or drop fiber. This is especially advantageous for the shared seeding scheme in which the seeding light can be transmitted along a separate fiber with high power or by optical amplification such that the gain of the loop-back transmitter can be kept small. Application examples are used to demonstrate the merits and feasibility of the cross-seeding schemes.

Growth of Hill gratings in photosensitive fibres: effects of Fresnel reflection

The Letter presents the results of an experimental investigation into the effect of removing the Fresnel reflection from the far end of an optical fibre in which a Hill grating is growing. The results represent further experimental evidence that a two photon local model accurately describes the early stages of Hill grating growth.

Light-Collecting Properties of a Perfect Circular Optical Fiber*

The meaning and the limitations of the numerical aperture of a perfect circular, cylindrical fiber are discussed. The light-collecting properties of the fiber due to skew rays collected beyond the meridional or nominal numerical aperture are shown. An effective numerical aperture which completely describes the light collection capacity of the fiber can be defined. It is shown that appreciably more light can be accepted by a fiber when all skew rays are considered than would be expected in a meridional approximation. The angular distribution of the emitted light has also been calculated for a typical case, and the effects of Fresnel reflection at the faces of the fiber are considered.