Small-signal Modulation Characteristics Research Articles

Small signal analysis of the modulation bandwidth of light-emitting diodes for visible light communication

We perform a theoretical investigation of the small-signal modulation characteristics of light-emitting diodes (LEDs) for visible light communication (VLC) applications. A comprehensive model for the small-signal frequency bandwidth is developed using a carrier rate equation with the ABC recombination model. Using the obtained general form of the 3-dB bandwidth (f3dB), we analyze the dependence of the bandwidth on various parameters of the LEDs. The internal quantum efficiency (IQE) is found to significantly influence the modulation bandwidth of LEDs. At a high current density, f3dB is approximately inversely proportional to IQE, suggesting the importance of a trade-off between high IQE and fast modulation speed. In addition, we analyze and discuss the effects of the radiative recombination coefficient, current density, and quantum-well thickness on f3dB, comparing the modeling results with previous experimental data. The presented model of the modulation bandwidth can be advantageous in understanding the modulation characteristics of LEDs for high-speed VLC.

Dynamic characteristics and noise modelling of directly modulated quantum well-dots microdisk lasers on silicon

The small-signal amplitude modulation, threshold, and spectral characteristics of microdisk lasers with InGaAs/GaAs quantum well-dots active region were studied jointly with the spectral and threshold parameters of edge-emitting lasers made from the same epitaxial heterostructure. Using the obtained material parameters, the relative intensity noise of the microdisk lasers was calculated as a function of the bias current and side-mode suppression ratio. It is shown that the integral noise is low enough for error-free optical data transmission with the maximum possible bitrate limited by the microdisk modulation bandwidth, if the bias current is above 1.7× threshold current (for side mode suppression ratio > 20 dB).

Effect of Collector Voltage on the Large and Small Signal Modulation Characteristics of 980 nm Transistor Laser

Theoretical analysis of Transistor Laser is carried out and the static and frequency responses for different collector voltages, under common emitter configuration are determined. The threshold current (Ith) is observed as 33mA and it increases linearly with reverse collector to base voltage (VCB). Meanwhile, the output optical power is found to decrease proportionately when VCB is increased. A maximum of 18.7GHz modulation bandwidth is observed when an input base current of 95 mA is applied at a fixed value of VCB (1V). The modulation bandwidth is found to decrease with increase in reverse VCB. The turn on delay increases with collector voltage. However, it decreases with increase in base current.

Correlation of Small-Signal Modulation with Digital Modulation Characteristics of High-Speed Semiconductor Lasers

This paper introduces correlation of the small-signal modulation characteristics with those of digital modulation of high-speed multiple-quantum-well semiconductor laser. The accuracy of applying the common small-signal approximation to calculate the modulation bandwidth is evaluated by comparing the obtained values with those calculated by the reliable technique of time-integration of the rate equations. Also, we present correlation between the maximum bit rate and the small-signal bandwidth at given bias current and modulation index. We show that the bandwidth frequencies predicted by the small-signal approximation are higher than those calculated by the time integration. The difference increases with the increase of the bias current reaching 8 GHz when the current exceeds 3.5 times the threshold current. We predict also the modulation conditions required to achieve modulation speed as high as 40 Gbps. The ratio of the maximum bit rate to the small-signal bandwidth exceeds unity and reaches 1.6 under deep modulation with modulation index equal to 0.5.

Small-Signal Equivalent Circuit Model of Photonic Crystal Fano Laser

We develop a new small-signal equivalent circuit model for photonic crystal Fano laser based on standard linearized equations. The results are compared with those obtained by the numerical simulation approach. This novel circuit model being a cost-effective fast tool is advantageous over solving the rate equations for designing and extending the photonic crystal Fano laser. Using this circuit model, we have investigated the laser current modulation via the conventional method and via modulating the Fano mirror. The results show that the bandwidths of the amplitude modulation of the laser through-port and the frequency modulation of its cross port are both in the range of THz. It is also shown that the larger the nanocavity detuning, the greater the relaxation oscillation frequency, increasing the possibility of widening the laser bandwidth, through which the modulation efficiency may decrease. Moreover, we show that the longer the laser active region, the narrower the modulation response bandwidth. The results show that the circuit model accurately explains the photonic crystal Fano laser small-signal modulation characteristics.

(Invited) High-Speed GaN-Based Micro-Scale Light-Emitting Diodes for Visible-Light Communication

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) light-fidelity (Li-Fi) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation due to their lower parasitic capacitance and ability for operation at high current densities where the carrier lifetime is significantly reduced. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate (inversely proportional to the carrier lifetime), the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths, especially at relatively low current densities. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (1010), and semipolar (2021) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. We demonstrate that the modulation bandwidths are well correlated with the wave function overlap in the active region by examining the orientation-dependence of the bandwidth. Record modulation bandwidths for GaN-based LEDs above 1 GHz are achieved for the nonpolar and semipolar orientations. We also show the modulation characteristics of an electrically injected nanowire-based LED, which achieves a modulation bandwidth of 1.2 GHz. Finally, we present a small-signal modeling method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection and discuss the results in the context of efficiency droop. Figure 1: Bandwidth vs. current density for the nonpolar LEDs (open black triangles), semipolar LED (open blue circles), and polar c-plane LED (open green squares) compared to other reported polar c-plane LEDs on sapphire or with a flip-chip design and compared to semipolar (1122) LEDs. The bandwidth characteristics for a nanowire-based LED are also included. Also shown are the crystal orientations used in the orientation-dependent bandwidth study. Figure 1

Impact of gain compression factor on modulation characteristics of InGaAs/GaAs self-assembled quantum dot lasers

This paper investigates the influence of gain compression factor on the modulation response of InGaAs/GaAs self-assembled quantum dot laser based on rate equations. For different gain compression factors the output power-current characteristics, light emissions of quantum dot laser have been simulated and effect of gain compression factor changes on quantum dot laser is illustrated. Also, small and large-signal response of quantum dot lasers is studied and the impact of the gain compression factor is presented. It explains that increase of gain compression factor, decreases small-signal modulation characteristics, nevertheless, improves large-signal response of quantum dot lasers. It helps to generate better laser signal quality, higher eye and smaller jitter. The large-signal behavior of a laser diode determines its capability for digital data transfer. The modulation speed of quantum dot lasers is of specific importance if such lasers are considered for optical communication systems.

Small-signal modulation characteristics of a polariton laser

Use of large bandgap materials together with electrical injection makes the polariton laser an attractive low-power coherent light source for medical and biomedical applications or short distance plastic fiber communication at short wavelengths (violet and ultra-violet), where a conventional laser is difficult to realize. The dynamic properties of a polariton laser have not been investigated experimentally. We have measured, for the first time, the small signal modulation characteristics of a GaN-based electrically pumped polariton laser operating at room temperature. A maximum −3 dB modulation bandwidth of 1.18 GHz is measured. The experimental results have been analyzed with a theoretical model based on the Boltzmann kinetic equations and the agreement is very good. We have also investigated frequency chirping during such modulation. Gain compression phenomenon in a polariton laser is interpreted and a value is obtained for the gain compression factor.

Small signal modulation characteristics of red-emitting (λ = 610 nm) III-nitride nanowire array lasers on (001) silicon

The small signal modulation characteristics of an InGaN/GaN nanowire array edge- emitting laser on (001) silicon are reported. The emission wavelength is 610 nm. Lattice matched InAlN cladding layers were incorporated in the laser heterostructure for better mode confinement. The suitability of the nanowire lasers for use in plastic fiber communication systems with direct modulation is demonstrated through their modulation bandwidth of f-3dB,max = 3.1 GHz, very low values of chirp (0.8 Å) and α-parameter, and large differential gain (3.1 × 10−17 cm2).

Frequency modulation response due to the intensity modulation of fiber-grating Fabry–Perot lasers

A comprehensive study on the small-signal frequency modulation (FM) characteristics of a fiber-grating Fabry–Perot (FGFP) laser is numerically conducted. Fiber Bragg grating (FBG) is used as a wavelength selective element to control the properties of the laser output by controlling the external optical feedback (OFB) level. In addition to the external OFB level, the effect of other parameters such as temperature, injection current, cavity volume, nonlinear gain compression factor, linewidth enhancement factor, and fiber-grating (FG) parameters on FM characteristics of the laser are investigated. The study is performed by modifying a set of rate equations that are solved by considering the effects of external OFB and ambient temperature (T) variations. The temperature dependence (TD) of FM characteristics is calculated according to TD of laser parameters instead of using well-known Pankove relationship. Results show that the optimum external fiber length (Lext) is 3.1 cm and the optimum range of working temperature for FGFP laser is within ± 2 °C from the FBG reference temperature (To). Also, it is shown that antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the FM response. The FM spectra peak amplitude is less than 5 dB with 5 mW output power. Good temperature stability is also obtained.

Fabrication and Characterization of Low-Threshold Polarization-Stable VCSELs for Cs-Based Miniaturized Atomic Clocks

Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of 894.6 nm have been fabricated for Cs-based atomic clock applications. For polarization control, monolithically integrated surface gratings are employed. Simulated and experimental results show that the longitudinal position of the surface grating has a significant influence on the threshold current. With a grating in the topmost in-phase layer, the threshold currents are reduced to 40% compared to earlier atomic clock grating VCSELs with inverted structure. The output polarization is parallel to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes of 25 dB even at substrate temperatures up to 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. Small-signal modulation characteristics of grating VCSELs are presented. The modulation bandwidth exceeds the required 5 GHz at a bias current of only 0.9 mA above threshold at room temperature. A revised chip design with smaller mesa size and thinner passivation layer has been implemented. The VCSELs show similar electrical parasitic bandwidths but require fewer fabrication steps and hence have reduced processing complexity. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factors of less than 0.4 ns corresponding to a maximum 3-dB bandwidth exceeding 25 GHz are obtained at 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C ambient temperature.

Characterization of small-signal intensity modulation of single-mode fiber grating Fabry-Perot laser source

A comprehensive study on the small-signal intensity modulation (IM) characteristics of a fiber grating Fabry-Perot (FGFP) laser is numerically investigated. The effect of external optical feedback (OFB), temperature, injection current, cavity volume, nonlinear gain compression factor, and fiber grating (FG) parameters on IM characteristics are presented. The temperature dependence (TD) of IM is calculated according to the TD of laser cavity parameters instead of using the well-known Parkove relationship. It has been shown that the optimum external fiber length (L ext) is 3.1 cm. The optimum range of working temperature for FGFP laser is between 23 to 27 °C. We also show that by increasing the laser injection current from 10 to 60 mA, the IM peak amplitude decreased from 6.3 to 0.2 dB and the relaxation-oscillation frequency (ROF) is shifted from 1.2 GHz towards higher frequency of 5.48 GHz. In addition, the AR coating reflectivity and gain compression factor have no significant effect on the IM. The study indicates that a stable operation and excellent modulation characteristic can be obtained after optimization process.

Improving the characteristics of the modulation response for fiber Bragg grating Fabry–Perot lasers by optimizing model parameters

A unified and comprehensive study on the small-signal intensity and frequency modulation characteristics of a fiber Bragg grating Fabry–Perot (FBG–FP) laser are numerically investigated. The effect of injection current, temperature, external optical feedback (OFB), nonlinear gain compression factor, fiber grating (FG) parameters and spontaneous emission factor on modulation response characteristics are presented. The rate equations of the laser model are presented in the form that the effect of temperature (T) and external optical feedback (OFB) are included. The temperature dependence (TD) of laser response is calculated according to the TD of laser cavity parameters instead of directly using the well-known Parkove equation. It is shown that the optimum external fiber length (Lext) is 3.1cm and the optimum range of working temperature for FGFP laser is within ±2°C from the FBG reference temperature (To). Also, the antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the modulation spectra. It is also show that modulation response is extremely sensitive to the OFB level, high injection current and gain compression factor. The study indicates clearly that good dynamic characteristic can be obtained by system parameters optimization.

Comparative Study of Mode Control in Vertical-Cavity Surface-Emitting Lasers With Photonic Crystal and Micropillar Etching

The dependence of spectral, power versus current and small-signal modulation characteristics versus the etch depth in two types of surface-etched vertical-cavity surface-emitting lasers (VCSELs) are experimentally and theoretically investigated. One type has a photonic crystal (PC) fabricated in the top distributed Bragg reflector (DBR), whereas the second type has a micropillar (MP) created by removing the DBR surrounding it. The aim of both fabrication designs is to improve the single-mode high-power output. Theoretical and experimental results are found to be in qualitative agreement. It is shown that mode-selective optical losses, introduced by the etched holes of the PC in the DBR, control the optical modes of the PC-VCSEL. Single-fundamental-mode radiation is observed for deeply etched PC- and MP-VCSELs. In contrast, improved modulation characteristics are found for shallowly etched devices. Higher-order single-mode generation with improved modulation characteristics is demonstrated for PC-VCSELs with an etch depth of 1.54 μm. PC-VCSELs demonstrate higher slope efficiency, lower threshold current, and series resistance compared with MP-VCSELs of the same etching depth.

Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

We numerically and experimentally investigate the high-speed small-signal cross-gain modulation (XGM) characteristics of a quantum-dot (QD) semiconductor optical amplifier (SOA). From a p-doped QD SOA operating at 1.3 mum, high-speed small-signal XGM responses up to 40 GHz are measured from low to high injection currents and improve at high injection currents. In the numerical model, we set up about six hundred coupled rate equations, where the carrier dynamics of QD electron and hole states are considered separately and the enhanced hole occupation due to p-type doping is included. The high-speed small-signal XGM spectra are calculated at various modulation frequencies and pump-probe detunings. We identify how the two separate XGM mechanisms of total carrier density depletion (TCDD) at low injection current and spectral hole burning (SHB) at high injection current affect the high-speed small-signal XGM behavior. From the measured and calculated results, we show that high-speed small-signal XGM responses of QD SOAs can be improved by injecting more carriers to the QD excited states, which enhances high-speed XGM induced by SHB rather than by TCDD.

Measurement of small-signal and large-signal responses of packaged laser modules at high temperature

In this paper, the pulsed injection method is extended to measure the chip temperature of various packaged laser modules, such as the DFB laser modules, the FP laser modules, and the EML laser modules. An optimal injection condition is obtained by investigating the dependence of the lasing wavelength on the width and period of the injection pulse in a relatively wide temperature range. The small-signal frequency responses and large-signal performances of packaged laser modules at different chip temperature are measured. The adiabatic small-signal modulation characteristics of packaged LD are first extracted. In the large-signal measurement, the effects of chip temperature, bias current and driving signal on the performances of the laser modules are discussed. It has been found that the large-signal performances of the EML modules depend on the different red-shift speeds of the DFB and EAM sections as chip temperature varying, and the optimal characteristics may be achieved at higher temperature.

A Critical Comparison of High-Speed VCSEL Characterization Techniques

This paper critically compares, for the first time, common microwave and optical procedures used for the high-speed characterization of vertical-cavity surface-emitting lasers (VCSELs). The intrinsic small-signal modulation characteristics of a VCSEL are measured, and the related rate equation parameters are extracted. Observed trends show excellent agreement with theory. The modulation characteristics of the VCSEL are determined by examining three different responses: relative intensity noise, S21 response, and high-resolution optical spectra. The various experimental techniques yielded consistent results. The relative strengths and weaknesses of each measurement are investigated

Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

The small-signal cross-gain modulation (XGM) characteristics of a p-doped quantum-dot (QD) semiconductor optical amplifier (SOA) are measured and compared with a theoretical model. The measured small-signal XGM responses show an average 3-dB bandwidth of 3.5 GHz and have no significant dependence on the pump-probe detuning wavelength. This indicates that the main XGM mechanism of the p-doped QD SOA is the total carrier density change rather than spectral hole burning. The measured small-signal conversion efficiency is compared with a theoretical model, which is derived from rate equations for QD SOAs. Based on our theoretical results, we find that the reduced intersubband (from the QD excited to ground state) carrier relaxation time due to p-type doping will decrease the small-signal 3-dB XGM bandwidth of QD SOAs

Direct Modulation Characteristics of Composite Resonator Vertical-Cavity Lasers

We report the small-signal modulation characteristics of a monolithic dual resonator vertical cavity surface emitting laser. The modulation response is described by a system of rate equations with two independent carrier populations and a single longitudinal optical mode. The independent optical overlaps and differential gains of the two active regions can each be adjusted to maximize the output response. We show that under certain conditions, the composite resonator may achieve a higher bandwidth than a single cavity laser with the same photon density. We find the relaxation oscillation frequency to depend mainly on the total photon density and not the individual currents in the two cavities. With appropriate current injection, the composite resonator laser achieves a maximum -3-dB bandwidth of 12.5 GHz and a maximum modulation current efficiency factor of approximately 5GHz/ma1/2

Small-signal modulation characteristics of p-doped 1.1- and 1.3-/spl mu/m quantum-dot lasers

We have investigated the small-signal modulation characteristics of 1.1and 1.3-μm p-doped quantum-dot lasers in order to evaluate the potential of acceptor doping. The maximum measured 3-dB bandwidth of the 1.1- and 1.3-μm lasers are 11 and 8 GHz, respectively, which are only marginally higher than those in the corresponding undoped devices.