Small-signal Modulation Research Articles

Single-fundamental-mode cryogenic (3.6 K) 850-nm oxide-confined VCSEL

Cryogenic oxide-confined vertical-cavity surface-emitting laser (VCSEL) has promising application in cryogenic optical interconnect for cryogenic computing. In this paper, we demonstrate a cryogenic 850-nm oxide-confined VCSEL at around 4 K. The cryogenic VCSEL with an optical oxide aperture of 6.5 μm in diameter can operate in single fundamental mode with a side-mode suppression-ratio of 36 dB at 3.6 K, and the fiber-coupled output power reaches 1 mW at 5 mA. The small signal modulation measurements at 298 and 292 K show the fabricated VCSEL has the potential to achieve a high modulation bandwidth at cryogenic temperature.

Manipulation of lasing modes in a deformed octagonal microcavity laser

We have demonstrated a deformed octagonal microcavity semiconductor laser with manipulated lasing modes for bistable operation and direct modulation. There are two sets of degenerated four-bounced modes, S01 and S02, in the octagonal microcavity, and the degeneracy between them is broken by introducing a square hole into the center of the cavity. In the deformed octagonal microcavity laser, mode S01 dominates the lasing process during the current rising process. However, mode S02 also lases when the current decreases and interacts nonlinearly with S01, which is caused by the non-uniform distribution of the refractive index induced by the square-ring-shaped current injection. We observe a counterclockwise bistable hysteresis loop with continuous injection current ranging from 31 to 13 mA at 288 K. We also study the small-signal modulation response of the laser at high and low states with different injection currents. By utilizing the photon-photon resonance effect between modes S01 and S02, we effectively increased the 3-dB bandwidth of the laser from 12 GHz to 16.2 GHz.

Spin-vertical-cavity surface-emitting lasers with by-design-defined birefringence for high-speed modulation.

We propose a new, to the best of our knowledge, type of spin-vertical-cavity surface-emitting laser (VCSEL) with controlled by design birefringence. To this aim, we utilize the so-called columnar thin films (CTFs) in the VCSEL dielectric distributed Bragg mirror and/or in a second dielectric cavity. We design such CTF-VCSELs with pre-defined birefringence and calculate their polarization-resolved resonant longitudinal modes and the corresponding quantum-well confinement factors and threshold gains. Using the spin-flip VCSEL model, we show that such spin CTF-VCSELs can achieve small-signal modulation response with a 3 dB cutoff frequency of several hundreds of GHz.

Particle swarm optimization-assisted approach for the extraction of VCSEL model parameters.

We propose a direct particle swarm optimization (PSO) method for extracting the parameters of a physical model describing the behavior of vertical-cavity surface-emitting lasers (VCSELs), starting from the light-current (L-I) characteristics and the small signal modulation (S21) responses, at different currents and temperatures. With an optimal choice of hyperparameters of the algorithm, the method is able to predict parameters that accurately reproduce the behavior of the device. Its prediction capabilities are compared to those of two commonly used nonlinear optimizers (Interior Point and Levenberg-Marquardt), to benchmark its performances.

Small signal modulation of photonic crystal surface emitting lasers

We report the small-signal characterization of a PCSEL device, extracting damping factors and modulation efficiencies, and demonstrating -3 dB modulation bandwidths of up to 4.26 GHz. Based on modelling we show that, by reducing the device width and improving the active region design for high-speed modulation, direct modulation frequencies in excess of 50 GHz are achievable.

Theoretical study of small signal modulation behavior of Fabry-Perot germanium-on-silicon lasers

This work investigated the modulation responses of Fabry–Perot Ge-on-Si lasers by modeling and simulations. The 3 dB bandwidth dependence on the structure parameters such as poly-Si cladding thickness, Ge cavity width and thickness, and minority carrier lifetime were studied. A 3 dB bandwidth of 33.94 GHz at a biasing current of 270.5 mA is predicted after Ge laser structure optimization with a defect limited carrier lifetime of 1 ns. The eye diagrams simulated show a stable eye-opening window at 20 Gb·s−1 NRZ. The improvement to 10 ns minority carrier lifetime would reduce the threshold current to 6.85 mA, and increase the 3 dB bandwidth to 36.89 GHz.

Impact of cavity design on the modulation properties of a GaAs-based VCSEL

GaAs-based vertical-cavity surface-emitting lasers (VCSELs) are commonly used as light sources for optical interconnects in short-range systems. Here, we analyse the impact of the cavity Q factor and the number of quantum wells in the active region on the small-signal modulation response of a GaAs-based VCSEL. By tuning the reflectivity of the mirror it is possible to obtain flat response curves with a high −3 dB bandwidth. It is also beneficial to increase the number of quantum wells in the active region.

Liquid-Crystal Spin-VCSEL with Electro-Optically Controllable Birefringence

We suggest a new construction of spin-VCSEL with an embedded nematic liquid crystal (LC) in a second cavity. We design such a coupled-cavity LC-VCSEL and develop a procedure for calculating its LC-voltage dependent polarization resolved resonant longitudinal modes and their quantum-well confinement factors. Using these characteristics, we are able to slightly modify the spin-flip VCSEL model to include the voltage dependent birefringence and anisotropy. Then, we show that such an LC-VCSEL can reach small signal modulation response with a 3dB cut off frequency of several hundreds of GHz.

Impact of Carrier Transport and Capture on VCSEL Dynamics

Using a vertical-cavity surface-emitting laser (VCSEL) equivalent circuit model based on two carrier rate equations to include effects of carrier dynamics, we study the impact of carrier transport and capture on the small- and large-signal modulation response of high-speed VCSELs. The model also accounts for parasitics, current-induced self-heating, and gain compression. A variation of the effective capture time from 1 to 15 ps is found to have a large impact on the small-signal modulation response, with the 3 dB bandwidth decreasing from 40 to 15 GHz and the response transitioning from under-damped to over-damped. This is primarily due to the increasing low frequency parasitic-like roll-off with increasing effective capture time. A significant effect on the optical waveforms produced by the VCSEL under 56 Gbit/s on-off keying (OOK) non-return-to-zero (NRZ) and pulse-amplitude modulation 4 (PAM4) modulation is observed, with a short effective capture time leading to horizontal eye closure caused by timing jitter (TJ) and intersymbol interference (ISI) and a long effective capture time leading to vertical eye closure caused by long rise- and fall-times. However, for high modulation speed, a short effective capture time is needed and the photon lifetime should be set for clear eye opening. We also show the impact of the effective capture time on the output power vs current characteristics and map the dependence of internal temperature, carrier densities, carrier escape and leakage rates, and spontaneous recombination rates on current for different effective capture times.

Half-disk lasers with active region based on InGaAs/GaAs quantum well-dots

Half-disk lasers fabricated by cleaving initial full-disk lasers have an advantage of directional light outcoupling as well as increased output power and efficiency as compared to full-disk lasers of the same diameter. The continuous wave output power of a 200 µm diameter half-disk laser exceeds 70 mW. Quasi single-mode lasing with a high side-mode suppression ratio more than 20 dB is demonstrated for half-disk lasers of various diameters. A maximum 3 dB small signal modulation frequency of 4.9 GHz was measured for a 100 µm in diameter half-disk laser.

Analysis of the regimes of feedback effects in quantum dot laser

We investigated the optical feedback effects on the static and dynamic characteristics of 1.3 μm quantum-dot (QD) Fabry–Pérot laser under reflection from −40 dB up to −8 dB. The onset of coherence collapse is determined as −14 dB from the optical and electrical spectra. Although the degradation in small signal modulation is reported above this critical feedback level, transmission operation with available eye diagram under higher feedback is demonstrated. Under 10 Gb s−1 modulation, there is no obvious degradation in eye diagram regarding the eye shape and extinction ratio up to feedback ratio of −8 dB. The higher feedback tolerance of QD laser under large signal modulation is attributed to the impact of gain compression. This high-speed feedback-resistant operation also indicates that QD laser is a promising light source for isolator-free photonic integrated circuits.

Effect of Scattering Parameter on Small Signal analysis in Direct Modulated Dynamic Semiconductor Lasers

Effect of scattering parameter is that the effect which it is effected on the operating characteristics of direct modulated semiconductor lasers by small signal analysis for injection current density , where this parameter is effected on amount of quantum confinement carriers density where the part of this carriers density is lost in the active region. The scattering parameter leads to appear the nonlinear gain effects which they are reduced the response frequency of the direct modulation process. We use the rate equations of the single-mode semiconductor lasers and that they have characteristics are used to continuous-wave small-signal modulation. We can presented this effect and discussion it by the simple two-port device.

A Novel Frequency Double Millimeter Wave Signal Generation Scheme with Optical Carrier Suppression Based on a Single Polarization Modulator

In the work, a novel and simple double frequency millimeter wave (mm-wave) signal generation scheme with optical carrier suppression (OCS) enabled by a single polarization modulator is proposed. The scheme is a simple structure to achieve the mm-wave signal generation, and the core of which just depend on a polarization modulator and two linear polarizers, and electronic circuits is needless to control the drift of modulator bias points and no optical filters is needed to filter out undesirable harmonics. The polarization angle of the first linear polarizer (POL-1) is set to 45°, the polarization angle of the second linear polarizer (POL-2) is set to −45° to suppress even order optical sidebands, and the modulation index of polarization modulator (POLM) is adjusted to realize small signal modulation. Based on mathematical reasoning and computer simulation, the proposed scheme is well feasible. Simulation results show that using 20 GHz RF signal to drive the POLM, 40 GHz mm-wave signal with 47 dB optical sideband suppression ratio (OSSR) and 41.19 dB radio frequency sideband suppression ratio (RFSSR) was obtained.

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.

Small-signal modulation response and [formula omitted] bandwidth of reflective semiconductor optical amplifier based fiber cavity laser

In this paper we propose a small-signal model of a reflective semiconductor optical amplifier (RSOA) based fiber cavity laser (FCL), which can be used for determining the small-signal modulation response and the corresponding −3dB bandwidth. The model is encapsulated in a boundary value problem (BVP), which, through the boundary condition, accounts for the fiber cavity transfer function. Starting from the numerical solution of the BVP, we investigate the modulation response and the −3dB bandwidth of an RSOA-FCL. We show that, due to the round-trip group delay, the RSOA-FCL’s modulation response exhibits oscillatory behavior. The bandwidth of an RSOA-FCL is analyzed with respect to the RSOA’s bias current density and active region length, the distribution fiber length, and the spectral width of an optical band-pass filter. We find that, in case of significant fiber dispersion, the bandwidth of FCL is limited by the lower of the two bandwidths — RSOA’s or fiber’s, usually the latter one. In case of small or negligible fiber dispersion, the bandwidth of FCL can be improved by increasing the RSOA’s bias current or fiber length and/or selecting the optimal RSOA active region length, and is limited by the RSOA’s performance.

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).

Characterization of lasing regimes of 1.3 µm vertical-cavity surface-emitting lasers based on a short-period InGaAs/InGaAlAs superlattice

Lasing regimes of 1.3 µm vertical-cavity surface-emitting lasers (VCSELs) based on a short-period In0.57Ga0.43As/In0.53Ga0.27Al0.2As superlattice and two Al0.9Ga0.1As/GaAs distributed Bragg reflectors on a GaAs substrate were characterized in this study. VCSEL crystals were mounted on crystal holders with a radio frequency SMA connector and an optical fiber with ferrule connector (FC) for investigation. Current–voltage and output-power–current characteristics, optical and radio frequency spectra, and polarization stability of laser emission were experimentally investigated in the pump current range of 0–14 mA. The threshold current of the studied VCSELs was established to be 1.7 mA. The maximum output optical power at the output of the optical fiber of the VCSEL based on a short-period In0.57Ga0.43As/In0.53Ga0.27Al0.2As superlattice and the current-induced emission wavelength shift were demonstrated to be 1.34 mW and 0.5 nm/mA, respectively. The small-signal modulation frequency was in the GHz range. The investigated VCSELs can be used in the development of radio photonic devices for telecommunications. Further improvement of the static and dynamic properties of VCSELs is possible by optimizing the design of the short-period superlattice and buried tunnel junction.

Temperature stability of small-signal modulation response of WGM microlasers with InGaAs/GaAs quantum well-dots in the active region

In this paper, results are presented on III-V quantum well dot microring diode lasers tested at elevated temperatures. To the best of our knowledge, the first uncooled microdisk lasers with diameter of 40 μm with 3-dB bandwidth above 2 GHz at 55°C are demonstrated.

Vertical cavity surface emitting lasers of 1.3 μm spectral range based on the InGaAs/InGaAlAs superlattice

Vertical-cavity surface-emitting lasers of 1.3 μm spectral range with the active region based on the InGaAs/InGaAlAs superlattice were studied. VCSEL heterostructure was formed by a wafer-fusion of the heterostructure with an active region and two DBRs grown by molecular-beam epitaxy on InP and GaAs substrates respectively. Fabricated VCSELs have shown threshold current below 1.6 mA and frequency of small signal modulation near 9 GHz at 20°C.

Differential gain and gain compression of an overdamped interband cascade laser

This work investigates the differential gain and gain compression factor of an interband cascade laser (ICL), through the analysis of the small-signal modulation response. The differential gain of the ICL is extracted to be 7.9 × 10−16 cm2, which is comparable to that of typical quantum well lasers. On the other hand, the gain compression factor is determined to be 5.1 × 10−15 cm3, which is two orders of magnitude higher than the latter. In addition, we demonstrate that the ICL is overdamped due to the strong gain compression effect. It is found that the K factor governed by the damping factor and the resonance frequency is as high as 31.4 ns.