Daniel Benedikovic

Subwavelength index engineered surface grating coupler with sub-decibel efficiency for 220-nm silicon-on-insulator waveguides.

Surface grating couplers are fundamental components in chip-based photonic devices to couple light between photonic integrated circuits and optical fibers. In this work, we report on a grating coupler with sub-decibel experimental coupling efficiency using a single etch process in a standard 220-nm silicon-on-insulator (SOI) platform. We specifically demonstrate a subwavelength metamaterial refractive index engineered nanostructure with backside metal reflector, with the measured peak fiber-chip coupling efficiency of -0.69 dB (85.3%) and 3 dB bandwidth of 60 nm. This is the highest coupling efficiency hitherto experimentally achieved for a surface grating coupler implemented in 220-nm SOI platform.

High-directionality fiber-chip grating coupler with interleaved trenches and subwavelength index-matching structure.

We present the first experimental demonstration of a new fiber-chip grating coupler concept that exploits the blazing effect by interleaving the standard full (220 nm) and shallow etch (70 nm) trenches in a 220 nm thick silicon layer. The high directionality is obtained by controlling the separation between the deep and shallow trenches to achieve constructive interference in the upward direction and destructive interference toward the silicon substrate. Utilizing this concept, the grating directionality can be maximized independent of the bottom oxide thickness. The coupler also includes a subwavelength-engineered index-matching region, designed to reduce the reflectivity at the interface between the injection waveguide and the grating. We report a measured fiber-chip coupling efficiency of -1.3  dB, the highest coupling efficiency achieved to date for a surface grating coupler in a 220 nm silicon-on-insulator platform fabricated in a conventional dual-etch process without high-index overlays or bottom mirrors.

Single-etch subwavelength engineered fiber-chip grating couplers for 1.3 µm datacom wavelength band

We report, for the first time, on the design and experimental demonstration of fiber-chip surface grating couplers based on subwavelength grating engineered nanostructure operating in the low fiber chromatic dispersion window (around 1.3 μm wavelengths), which is of great interest for short-reach data communication applications. Our coupler designs meet the minimum feature size requirements of large-volume deep-ultraviolet stepper lithography processes. The fiber-chip couplers are implemented in a standard 220-nm-thick silicon-on-insulator (SOI) platform and are fabricated by using a single etch process. Several types of couplers are presented, specifically the uniform, the apodized, and the focusing designs. The measured peak coupling efficiency is -2.5 dB (56%) near the central wavelength of 1.3 μm. In addition, by utilizing the technique of the backside substrate metallization underneath the grating couplers, the coupling efficiency of up to -0.5 dB (89%) is predicted by Finite Difference Time Domain (FDTD) calculations.

Germanium-on-silicon mid-infrared grating couplers with low-reflectivity inverse taper excitation

A broad transparency range of its constituent materials and compatibility with standard fabrication processes make germanium-on-silicon (Ge-on-Si) an excellent platform for the realization of mid-infrared photonic circuits. However, the comparatively large Ge waveguide thickness and its moderate refractive index contrast with the Si substrate hinder the implementation of efficient fiber-chip grating couplers. We report for the first time, to the best of our knowledge, a single-etch Ge-on-Si grating coupler with an inversely tapered access stage, operating at a 3.8 μm wavelength. Optimized grating excitation yields a coupling efficiency of -11  dB (7.9%), the highest value reported for a mid-infrared Ge-on-Si grating coupler, with reflectivity below -15  dB (3.2%). The large periodicity of our higher-order grating design substantially relaxes the fabrication constraints. We also demonstrate that a focusing geometry allows a 10-fold reduction in inverse taper length, from 500 to 50 μm.

Influence of nonlinear effects in WDM system with non-equidistant channel spacing using different types of high-order PSK and QAM modulation formats

The objective of this paper is to investigate nonlinear effects in Wavelength-Division Multiplex (WDM) systems in the case when different types of high-order M-PSK and M-QAM modulation formats for various structures of channel spacing are used optical signals. In general, the degradation mechanisms are caused by transmitted optical signals and their impact on each optical channel in WDM can be very different. Therefore, it is suitable to investigate possibilities for channel arrangement from the point of view of equidistant and non-equidistant channel spacing, respectively, what would lead to the suppression of nonlinear effects. In this article we investigate new types of high-order modulation formats that have ability for increasing of spectral efficiency and total improvement of performance of the transmission WDM system. The attention is put on two classes of channel spacing in WDM system, equidistant channel spacing (Δf = 100, 50, 25 and 12.5 GHz) and non-equidistant channel spacing (Δf ≠ const.), respectively. For investigation of signal propagation the numerical model is created. The model is based on mathematical method Symmetrical-Split Step Fourier Method (S-SSFM), which is utilized for solving the coupled nonlinear Schrödingers equations (CNLSE) describing the transmission of signals in multichannel systems. The results of the created numerical model are analyzed, compared to each other and interpreted in a way that leads to the determination of suitable high-order modulation formats and we try to propose the optimal arrangement of optical channels in WDM system. The key issue is to suppress the impact of nonlinearities on modulated signals for each channel with respect to the employed types of digital modulation formats, various system parameters, different types of optical fibers and localization of reference channel in the wavelength area.

Numerical representation of optical sources for coherent lightwave systems

The numerical model of CW-DFB optical source for fiber-optic transmission systems employing coherent detection technique is presented. The developed model is based on numerical solution of quantum-mechanical coupled rate equations. The numerical model is suitable for optical transmission systems for purpose of generation of optical carrier wave on the transmitter side as well as it can be used as local laser for coherent detection on the receiver side. The main attention of proposed optical source model is focused on signal, time-dependent characteristics to determine spectral properties of presented laser, which fully satisfies current requirements on coherent optical transmission systems. The efficiency of numerical model is also discussed. We have showed that by employing the re-sampling technique, both computational effort and accuracy of presented laser model are markedly enhanced.

Numerical model for DGD estimation in optical transmission system

In this paper we investigate the nonlinear and polarization effects in optical transmission systems and its influence on transmitted pulses. The main attention is focused on fundamental description of refractive index in nonlinear birefringent environment. In general, optical fiber is nonlinear transmission medium. The mutual interaction between transmission medium and optical intensity induces changes in the refractive index resulting to nonlinear effects that can be polarization-dependent in presence of birefringence. The global effect has a significant impact on pulse propagation. The aim of this paper is to present a numerical model, which will be suitable for estimation of DGD (Differential Group Delay) parameter in nonlinear birefringence medium. The DGD is crucial for evaluation of the impact of PMD (Polarization Mode Dispersion) in high-bit-rates fiber-optic system. Our approach opens the novel opportunity for DGD estimation based on numerical model of optical pulse propagation in nonlinear birefringence medium. Numerical model is based on solving NLSE (Nonlinear Schrodinger Equation) through the SSFM (Split-Step Fourier Method). The model is compatible for various input parameters, different kinds of optical fibers and also for different types of modulation formats. The obtained results show the DGD for different system parameters (such as input power and wavelength) and for different fiber polarization characteristics (birefringence and mode coupling). The total pulse broadening is also calculated and illustrates how all degradation effects influence the performance of fiber-optic transmission system.

Modeling of WDM transmission system with high-order phase modulation formats

The objective of this paper is focused on numerical modeling of WDM (Wavelength-Division Multiplex) transmission systems that employ new classes of high-order PSK (Phase-Shift Keying) modulation formats. This paper provides investigation of signals propagation with corresponding degradation mechanisms in physical layer, when high-order phase modulation formats are used for their transmission. The impacts of linear and nonlinear effects on modulated signal in multichannel system are studied. For this purpose, it was created numerical model by solving CNLSE (Coupled Nonlinear Schrödingers Equations) through SSFM (Split-Step Fourier Method). The fundamental characteristics of optical fibers, employed modulation format and WDM system parameters are significant for our investigation, because they play important role for behavior of modulated signals. Our results are analyzed and interpreted by the way of finding out the suitable and optimal system settings for transmission of phase modulated signals.

Numerical investigation of noise chracteristics of telecommunication laser sources for various modulation formats

We present numerical investigation of noise properties of lasers for telecommunication purposes with emphasis on widely used distributed feedback (DFB) lasers and its influence on multi-level modulation formats. DFB lasers can be used in optical transmitters with internal and external modulation, as well as in optical receivers employing coherent detection, where they act as local oscillator. The main noise factors influencing signal characteristics of semiconductor-based lasers are intensity and phase noise. These random impairments cause degradation of fundamental output laser characteristics such as power and phase fluctuation, which are directly related to the optical signal-to-noise ratio and the laser linewidth. In case of implementation of new modulation formats into the transmission system, these stochastic processes are of main importance and significantly impact the transmission properties of modulated signals and total performance of fiber-optic transmission systems. Throughout this paper, the noise influence on the multi-level modulation formats is evaluated by effective signal-to-noise ratio (SNR) algorithms in combination with bit-error ratio (BER) formulas for appropriate type of modulation format. The obtained results have shown that the noise of DFB laser is serious restriction for multi-level modulation formats and can be improved by higher power levels, yielding to higher SNR, as required for better values of BER.

Integrated waveguide PIN photodiodes exploiting lateral Si/Ge/Si heterojunction

Germanium photodetectors are considered to be mature components in the silicon photonics device library. They are critical for applications in sensing, communications, or optical interconnects. In this work, we report on design, fabrication, and experimental demonstration of an integrated waveguide PIN photodiode architecture that calls upon lateral double Silicon/Germanium/Silicon (Si/Ge/Si) heterojunctions. This photodiode configuration takes advantage of the compatibility with contact process steps of silicon modulators, yielding reduced fabrication complexity for transmitters and offering high-performance optical characteristics, viable for high-speed and efficient operation near 1.55 μm wavelengths. More specifically, we experimentally obtained at a reverse voltage of 1V a dark current lower than 10 nA, a responsivity higher than 1.1 A/W, and a 3 dB opto-electrical cut-off frequency over 50 GHz. The combined benefits of decreased process complexity and high-performance device operation pave the way towards attractive integration strategies to deploy cost-effective photonic transceivers on silicon-on-insulator substrates.