Maximum Modulation Bandwidth Research Articles

25 Gb/s NRZ transmission at 85°C using a high-speed 940 nm AlGaAs oxide-confined VCSEL grown on a Ge substrate.

In this Letter, we present a comprehensive analysis of the high-speed performance of 940 nm oxide-confined AlGaAs vertical-cavity surface-emitting lasers (VCSELs) grown on Ge substrates. Our demonstration reveals a pronounced superiority of Ge-based VCSELs in terms of thermal stability. The presented Ge-VCSEL has a maximum modulation bandwidth of 16.1 GHz and successfully realizes a 25 Gb/s NRZ transmission at 85 ∘C. The experimental results underscore the significance and potential of Ge-VCSELs for applications requiring robust performance in high-temperature environments, laying the cornerstone for the future development of VCSEL devices.

Enhanced Modulation Bandwidth by Integrating 2D Semiconductor and Quantum Dots for Visible Light Communication

Light‐emitting devices present a tremendous potential for visible light communication (VLC) due to their dual functionality as both communication and lighting devices. Herein this study, the significant enhancement in VLC modulation bandwidth by integrating two‐dimensional (2D) semiconductor and quantum dots (QDs) emitter is reported. Generally, the modulation bandwidth of CdSe‐based QDs is limited to only less than 25 MHz; however, with the proposed hybrid emitter, a maximum modulation bandwidth of 130 MHz for CdSe/ZnS QDs emitter is able to be achieved. The WSe2 monolayer is integrated into an Au–nanorod–decorated CdSe/ZnS QDs emitter to achieve high modulation performance. The modulation bandwidth of the hybrid QD–Au–WSe2 emitter (130 MHz) is found to be higher than those of the pristine QDs and QD–WSe2 heterostructure without Au nanorods (79 and 91 MHz, respectively). A significant increase is observed in the transition rate of QDs excitons when they are integrated with Au nanorods and WSe2 monolayer, which is substantiated by a reduction in average carrier lifetime from time‐resolved photoluminescence analysis. This approach and the findings open an opportunity to apply 2D semiconductors into the next‐generation miniature VLC devices, for high‐speed optical communications.

Dynamic modulation and impedance characteristics of a terahertz quantum cascade laser

The modulation and impedance properties of a terahertz (THz) quantum cascade laser (QCL) are investigated theoretically based on a three-level rate equation model. The effect of different device parameters, namely, facet reflectivity, injection efficiency, spontaneous emission factor, and operating current, on the modulation and impedance characteristics of the QCL is analyzed in detail. The device shows a maximum modulation bandwidth (f3dB) of 21 GHz and an intrinsic impedance of 3.8 mΩ when operating under the designed conditions. The modulation bandwidth and impedance increase with the increase in facet reflectivity and injection efficiency. However, the modulation bandwidth increases but the impedance decreases with the increase in operating current. The spontaneous emission factor has no effect on both the modulation bandwidth and impedance. The theoretical model will aid in the design of THz QCLs requiring a large modulation bandwidth and the external circuit design to match the standard 50 Ω source for reducing reflections and improving the coupling efficiency.

850 nm VCSEL with sub quantum well and p-type [formula omitted]-doping in the active layers for improved high-speed and high-temperature performance

Thermal rollover is one of the fundamental limitations that affect the output power and modulation bandwidth of high-speed laser in monolithic laser integrated circuits (ICs). Herein, an 850 nm vertical cavity surface emitting laser (VCSEL) with p-doping and sub quantum well (sub QW) centers active layer is proposed and investigated. The p-doping and sub QW insert in the offset position of each barrier, which are designed for reducing the thermal rollover and increasing the maximum intrinsic modulation bandwidth of the laser. Numerical simulations show that adding the p-doping and sub QW in active region could reduce the threshold currents (Ith), improve the slope efficiency (SE) and delay the thermal rollover in VCSEL. Comparing with previous VCSEL, our proposed VCSEL has excellent high temperature output performance, and the maximum simulation 3-dB bandwidth is 43.9 GHz at 25 °C and 33.1 GHz at 85 °C, respectively. Furthermore, temperature-dependent time-resolved photoluminescence (PL) and PL excitation reveal this novel QW has strong radiative recombination rate, which is suitable for designing active region of high-speed and high power lasers.

Extrinsic Parasitics and Design Considerations on Modulation Bandwidth of 850-nm Vertical Cavity Surface Emitting Lasers

The extrinsic parasitics generally limit the modulation bandwidth of vertical cavity surface emitting lasers (VCSELs) for optical data links and interconnects. In this article, the extrinsic parasitics of 850-nm VCSELs are investigated by three different designs: oxide-only (design A), oxide-implant (design B), and oxide-implant with benzocyclobutene (BCB) (design C). The static and dynamic characteristics of all the designs with~7- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> aperture are measured and compared. The maximum modulation bandwidth of design A is only about 12 GHz mainly limited by the large parasitic capacitances. With a proper proton implantation, a thick nonconducting layer is created inside the device with design B, which dramatically reduces the parasitic capacitances, and thus the bandwidth is increased to about 18 GHz. Based on design B, a 4- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -thick BCB layer with low dielectric constant is employed underneath the bond pad of design C to further reduce the pad capacitance, leading to the highest bandwidth about 24 GHz and the highest modulation current efficiency factor (MCEF) of 10.8 GHz/mA <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . The microwave reflection coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{11}}$ </tex-math></inline-formula> ) parameters of the three designs are measured and compared. Results indicate that design C has the lowest parasitic capacitances and the highest modulation bandwidth, which is preferred for high-speed VCSELs to obtain good static and dynamic performance.

InGaN Micro‐LED Array Enabled Advanced Underwater Wireless Optical Communication and Underwater Charging

AbstractIn this paper, the underwater applications of micro‐LED (light‐emitting diode) in light emission, optical detection, and solar cell are demonstrated. Based on a high‐bandwidth and low‐power micro‐LED array, duplex underwater wireless optical communication (UWOC) and underwater charging are realized. Using the non‐return‐to‐zero on‐off‐keying modulation scheme, the maximum modulation bandwidth and data rate of a micro‐LED transmitter in the 2.3 m duplex UWOC system are 251.3 MHz and 660 Mbps, respectively. In addition, the micro‐LED‐based photodetector can achieve maximum data rates of 52.5 and 60 Mbps at 0 and −5 V in the same UWOC system, respectively. Furthermore, while achieving the communication function, the micro‐LED is used to realize the collection of external light energy, which is successfully employed to drive a 660 nm laser diode, proving the photovoltaic power generation of micro‐LED. This experiment has made a breakthrough in the duplex UWOC system based on an InGaN micro‐LED array, and verifies significant potential of micro‐LED application in the field of duplex UWOC and underwater charging.

Power, Bandwidth, and Efficiency of Single VCSELs and Small VCSEL Arrays

Single-emitter vertical-cavity surface-emitting lasers (VCSELs) and multiple-emitter VCSEL arrays designed for emission at 980 nm, and with large oxide aperture diameters ( φ ) from ∼10.5 to 33.5 μm and with φ of ∼7.5 μm that are configured in three-emitter and seven-emitter electrically parallel triangular and hexagonal arrays, respectively, have record bandwidths and optical output powers for VCSELs with such large cumulative emitting areas. We demonstrate room temperature (RT) maximum small-signal modulation bandwidths ( f 3dBmax) of 26.6 to 18.6 GHz with corresponding continuous wave (CW) optical output powers ( L ) of 16.2 and 47 mW for VCSELs with φ ∼13.5 and 33.5 μm, respectively. For parallel 980 nm triple and septuple arrays with φ ∼7.5 μm for each VCSEL, we demonstrate RT f 3dBmax of 25.5 and 24.8 GHz with corresponding CW L of 22.7 and 50.1 mW, respectively. We perform a comparative analysis of the RT results, which consist of standard static light output power–current–voltage ( LIV ) characteristics to determine the typical VCSEL figures-of-merit including wall plug efficiency (WPE), LI slope efficiency, spectral emission versus current ( I ), and small-signal frequency response curves. We examine the tradeoffs in optical output power, small-signal modulation bandwidth, and WPE for the various VCSEL designs, and find that while the combination of f 3dBmax and L are record values for single VCSELs and VCSEL arrays, these parameters fall off as the cumulative VCSEL array area increases to emit higher overall optical power.

The effects of sound coder carrier rate and modulation bandwidth on voice pitch perception in cochlear implant users

We employed the dual filter-bank “STEP” coder to separately control the spectral and temporal modulation resolution of analysis channels. Previously we compared vowel pitch ranking and gender classification with eight subjects using enhanced modulation at F0—including across-channel synchronised modulation to the ACE coder. There was no significant improvement using modulation enhanced coding versus ACE across subjects. In a follow-up experiment we looked at the effect of stimulation rate on voice pitch perception. Since there are large inter-subject differences in overall temporal pitch acuity we hypothesised that some subjects’ performance may be more greatly influenced by carrier rate than others, or that some subjects may find sound quality satisfactory with lower carrier rates than those in their clinical processors. We used a version of STEP with a very short temporal envelope analysis window of 2 ms which allows a very low latency real-time processing implementation and large maximum modulation bandwidth. Subjects were tested using carrier rates of 1000, 500 and 250 pps/ch with modulation bandwidths controlled via low-pass filtering. Pilot data indicated that the new low-latency coder provides very good sound quality compared to ACE using 1000 pps/ch or 500 pps/ch. Also the modulation bandwidth could be tuned at different carrier rates to optimize voice pitch perception based on temporal cues. This opens the potential for lower stimulation rates to be used in CI coding while maintaining optimal temporal resolution.We employed the dual filter-bank “STEP” coder to separately control the spectral and temporal modulation resolution of analysis channels. Previously we compared vowel pitch ranking and gender classification with eight subjects using enhanced modulation at F0—including across-channel synchronised modulation to the ACE coder. There was no significant improvement using modulation enhanced coding versus ACE across subjects. In a follow-up experiment we looked at the effect of stimulation rate on voice pitch perception. Since there are large inter-subject differences in overall temporal pitch acuity we hypothesised that some subjects’ performance may be more greatly influenced by carrier rate than others, or that some subjects may find sound quality satisfactory with lower carrier rates than those in their clinical processors. We used a version of STEP with a very short temporal envelope analysis window of 2 ms which allows a very low latency real-time processing implementation and large maximum modulation ban...

GeSn Nanobeam Light-Emitting Diode as a GHz-Modulated Light Source

Designs and theoretical analysis are presented for a room temperature resonant-cavity-enhanced GeSn LED whose emission peaks at the 2 $\mu$ m wavelength. The Ge/GeSn/Ge PIN hetero-diode of length 1 $\mu$ m is embedded in a rib-type Ge-on-Si nanobeam having either 24 or 36 air holes. The maximum LED modulation bandwidth $f_{3\text{dB}}$ is proportional to the Purcell factor and is inversely proportional to $\tau _{s\,p0}$ the GeSn bulk spontaneous emission lifetime. For an emission linewidth of 200 nm and $\tau _{s\,p0}$ of 10 ns, an $f_{3\text{dB}}$ of 1.6 GHz is predicted.

Heat dissipation effect on modulation bandwidth of high-speed 850-nm VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of high-speed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the −3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth.

Temperature-Insensitive High-Speed Directly Modulated 1.55-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mu \text{m}$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Quantum Dot Lasers

Modulation properties and temperature stability of short cavity ridge waveguide lasers based on high-quality InAs quantum dots exhibiting a total modal gain of ~90 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are reported. The 338-μm-long lasers show a threshold current of 20 mA at room temperature and an output powers of up to 36 mW. A maximum small signal modulation bandwidth of 15 GHz was obtained at 14 °C, which degrades to 13 GHz at 60 °C and 8 GHz at 80 °C. Digital modulation at 25 Gb/s between 15 °C and 50 °C was obtained with clear open eyes under constant drive conditions (dc and ac). The maximum data rates of 32 and 35 Gb/s were obtained for 338- and 230-μm-long lasers, respectively, at 14 °C.

GaN-based LEDs for light communication

Rapid improvement in the efficiency of GaN-based LEDs not only speed up its applications for general illumination, but offer the possibilities for data transmission. This review is to provide an overview of current progresses of GaN-based LEDs for light communications. The modulation bandwidth of GaN-based LEDs has been first improved by optimizing the LED epilayer structures and the modulation bandwidth of 73 MHz was achieved at the driving current density of 40 A/cm2 by changing the multi-quantum well structures. After that, in order to increase the current density tolerance, different parallel flip-chip micro-LED arrays were fabricated. With a high injected current density of ~7900 A/cm2, a maximum modulation bandwidth of ~227 MHz was obtained with optical power greater than 30 mW. Besides the increase of carrier concentrations, the radiative recombination coefficient B was also enhanced by modifying the photon surrounding environment based on some novel nanostructures such as resonant cavity, surface plasmon, and photonic crystals. The optical 3 dB modulation bandwidth of GaN-based nanostructure LEDs with Ag nanoparticles was enhanced by 2 times compared with GaN-based nanostructure LEDs without Ag nanoparticles. Our results demonstrate that using the QW-SP coupling can effectively help to enhance the carrier spontaneous emission rate and also increase the modulation bandwidth for LEDs, especially for LEDs with high intrinsic IQE. In addition, we discuss the progress of the faster color conversion stimulated by GaN-based LEDs.

High-Speed Oxide Confined 850-nm VCSELs Operating Error-Free at 40 Gb/s up to 85&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$^{\circ}{\rm C}$&lt;/tex&gt;&lt;/formula&gt;

We present the temperature dependence of the performance characteristics of our latest generation high-speed oxide confined 850-nm vertical cavity surface-emitting lasers (VCSELs). Using a 7-μm oxide aperture diameter VCSEL, we demonstrate a maximum modulation bandwidth of ~ 27 GHz at room temperature and ~ 21 GHz at 85°C. With a new high-speed optical receiver, these bandwidths enable error-free data transmission (defined as a bit-error-rate <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> ) at bit-rates up to 47 Gb/s at room temperature, and up to 40 Gb/s at 85°C.

Theoretical investigation of injection-locked high modulation bandwidth quantum cascade lasers

In this study, we report for the first time to our knowledge theoretical investigation of modulation responses of injection-locked mid-infrared quantum cascade lasers (QCLs) at wavelengths of 4.6 μm and 9 μm, respectively. It is shown through a three-level rate equations model that the direct intensity modulation of QCLs gives the maximum modulation bandwidths of ~7 GHz at 4.6 μm and ~20 GHz at 9 μm. By applying the injection locking scheme, we find that the modulation bandwidths of up to ~30 GHz and ~70 GHz can be achieved for QCLs at 4.6 μm and 9 μm, respectively, with an injection ratio of 5 dB. The result also shows that an ultrawide modulation bandwidth of more than 200 GHz is possible with a 10 dB injection ratio for QCLs at 9 μm. An important characteristic of injection-locked QCLs is the nonexistence of unstable locking region in the locking map, in contrast to their diode laser counterparts. We attribute this to the ultra-short upper laser state lifetimes of QCLs.

Impact of Losses in the Bragg Section on the Dynamics of Detuned Loaded DBR Lasers

The dynamics of a distributed Bragg reflector laser with optical losses in the Bragg section is studied in detail. It is found that the modulation response depends not only on the detuning of the lasing wavelength from the Bragg reflectivity peak but also on the magnitude of the waveguide losses in the Bragg section. Depending on the losses, the damping of the relaxation peak can either increase or decrease when the laser is detuned on the long wavelength flank of the Bragg peak. Hence, in order to achieve maximum modulation bandwidth of the laser, the laser needs not only to have the correct detuning but also an optimized waveguide loss in the Bragg section. The physical reason for this dependence is discussed in terms of a modified rate equation model.

Fabrication, performance and parasitic parameter extraction of 850 nm high-speed vertical-cavity lasers

High-speed, oxide-confined, polyimide-planarized 850 nm vertical-cavity surface emitting lasers (VCSELs) with different aperture sizes were fabricated and characterized. Comprehensive small signal measurements and analysis were conducted. The VCSELs exhibit intrinsic, parasitic and thermal maximum bandwidth limitations of 42.3 GHz, 21.5 GHz and 17.6 GHz, respectively. Devices with a 10 µm oxide aperture exhibited a maximum modulation bandwidth of 15.3 GHz, limited by thermal effects, and a modulation current efficiency factor (MCEF) of 9.2 GHz mA−1/2. A VCSEL equivalent circuit model which incorporates the frequency dependence of the polyimide dielectric permittivity and loss is presented. A genetic algorithm (GA) was utilized to extract the parasitic circuit elements from measured microwave reflection coefficients (S11) over a frequency range of 50 MHz to 20 GHz for different device sizes at different bias currents. Good agreement between extracted and calculated parasitic circuit element values was obtained. Several modifications to the device's fabrication steps and structure are suggested to further improve the device's output power and modulation bandwidth.

Gain Compression and Above-Threshold Linewidth Enhancement Factor in 1.3-$\mu\hbox{m}$ InAs–GaAs Quantum-Dot Lasers

Quantum-dot (QD) lasers exhibit many useful properties such as low threshold current, temperature and feedback insensitivity, chirpless behavior, and low linewidth enhancement factor (alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> -factor). Although many breakthroughs have been demonstrated, the maximum modulation bandwidth remains limited in QD devices, and a strong damping of the modulation response is usually observed pointing out the role of gain compression. This paper investigates the influence of the gain compression in a 1.3-mum InAs-GaAs QD laser and its consequences on the above-threshold alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> -factor. A model is used to explain the dependence of the alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> -factor with the injected current and is compared with AM/FM experiments. Finally, it is shown that the higher the maximum gain, the lower the effects of gain compression and the lower the alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> -factor. This analysis can be useful for designing chirpless QD lasers with improved modulation bandwidth as well as for isolator-free transmission under direct modulation.

Analysis of small-signal intensity modulation of semiconductor lasers taking account of gain suppression

This paper demonstrates theoretical characterization of intensity modulation of semiconductor lasers (SL’s). The study is based on a small-signal model to solve the laser rate equations taking into account suppression of optical gain. Analytical forms of the small-signal modulation response and modulation bandwidth are derived. Influences of the bias current, modulation index and modulation frequency as well as gain suppression on modulation characteristics are examined. Computer simulation of the model is applied to 1.55-µm InGaAsP lasers. The results show that when the SL is biased far-above threshold, the increase of gain suppression increases both the modulation response and its peak frequency. The modulation bandwidth also increases but the laser damping rate decreases. Quantitative description of the relationships of both modulation bandwidth vs. relaxation frequency and maximum modulation bandwidth vs. nonlinear gain coefficient are presented.

Enabling Low-Distortion Varactors for Adaptive Transmitters

A varactor diode configuration is presented for the implementation of low-distortion tunable matching networks, phase shifters, and filters. The configuration is based on varactors with an "exponential" C(V) relation in antiseries configuration with specific control voltage harmonic loading conditions. The resulting network combines a high capacitance tuning range with relatively low control voltages and excellent linearity. The linearity is independent of the applied control voltage and does not degrade at low tone spacing, which is ideal for the realization of adaptive transmitters and modulators. A specific N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> x-2 varactor doping profile is required for exact third-order intermodulation (IM3) cancellation. The remaining distortion is dominated by the much smaller fifth-order nonlinearities. The required impedance levels of the harmonic terminations are studied in terms of the maximum modulation bandwidth for which the proposed configuration remains linear. The doping profile optimization for quality factor, tuning range, and breakdown voltage is discussed. For experimental verification, the proposed varactor configuration has been implemented using a silicon-on-glass technology. The measurements demonstrate IM3 cancellation and indicate a superior linearity for modulated signals up to 10-MHz bandwidth.

The impact of p-doping on the static and dynamic properties of 1.5μm quantum dash lasers on InP

p -type modulation doping in the range of 0–100 acceptors per quantum dash (QDash) has been carried out to investigate the impact on QDash lasers on (100) InP. The differential gain was found to increase more than 50% for doping concentrations of 50 acceptors per QDash for constant cavity length lasers. However, this benefit is overcompensated by enhanced gain compression and enlarged thermal heating due to high internal losses in highly p-doped devices. The maximum modulation bandwidth of 8GHz in continuous wave operation at room temperature is, therefore, obtained for a moderate p-doping level of 10 holes per QDash.