Large Signal Modulation Characteristics Research Articles

Large-signal modulation characteristics of a GaN-based micro-LED for Gbps visible-light communication

The large-signal modulation characteristics of a GaN-based micro-LED have been studied for Gbps visible-light communication. With an increasing signal modulation depth the modulation bandwidth decreases, which matches up with the increase in the sum of the signal rise time and fall time. By simulating the band diagram and the carrier recombination rate of the micro-LED under large-signal modulation, carrier recombination and the carrier sweep-out effect are analyzed and found to be the dominant mechanisms behind the variation of modulation bandwidth. These results give further insight into improving the modulation bandwidth for high-speed visible-light communication.

Large Signal Modulation Characteristics in the Transition Regime for Two-State Lasing Quantum Dot Lasers**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0402302, and the National Natural Science Foundation of China under Grant No 91433206.

Large-signal modulation capability, as an important performance indicator, is directly related to the high-speed optical communication technology involved. We experimentally and theoretically investigate the large-signal modulation characteristics of the simultaneous ground-state (GS) and the excited-state (ES) lasing in InAs/GaAs quantum dot laser diodes. The large-signal modulation capability of total light intensity in the transition regime from GS lasing to two-state lasing is unchanged as the bias-current increases. However, GS and ES large-signal eye diagrams show obvious variations during the transition. Relaxation oscillations and large-signal eye diagrams for GS, ES, and total light intensities are numerically simulated and analyzed in detail by using a rate-equation model. The findings show that a complementary relationship between the light intensities for GS and ES lasing exists in both the transition regime and the two-state lasing regime, leading to a much smaller overshooting power and a shorter settling time for the total light intensity. Therefore, the eye diagrams of GS or ES lasing are diffuse whereas those of total light intensity are constant as the bias-current increases in the transition regime.

20-Gb/s Modulation of Silicon-Integrated Short-Wavelength Hybrid-Cavity VCSELs

We investigate the dynamics of silicon-integrated 850-nm-wavelength hybrid-cavity vertical-cavity surface-emitting lasers (VCSELs). The VCSELs consist of a GaAs-based half-VCSEL attached to a dielectric distributed Bragg reflector on a silicon substrate using ultra-thin divinylsiloxane-bis-benzocyclobutene adhesive bonding. A 5- $\mu \text{m}$ oxide aperture diameter VCSEL, with a small signal modulation bandwidth of 11 GHz, supports data transmission at bit rates up to 20 Gb/s. The modulation bandwidth and the large signal modulation characteristics are found to be impaired by the high thermal impedance.

Dynamic characteristics of two-state lasing quantum dot lasers under large signal modulation

Large signal modulation characteristics of the simultaneous ground-state (GS) and excited-state (ES) lasing quantum dot lasers are theoretically investigated. Relaxation oscillations of ‘0 → 1’ and ‘1 → 0’ in the GS lasing region (Region I), the transition region from GS lasing to two-state lasing (Region II) and the two-state lasing region (Region III) are compared and analyzed. It is found that the overshooting power and settling time in both Regions I and III decrease as the bias current increases. However, there exist abnormal behaviors of the overshooting power and settling time in Region II owing to the occurrence of ES lasing, which lead to fuzzy eye diagrams of the GS and ES lasing. Moreover, the ES lasing in Region III possesses much better eye diagrams because of its shorter settling time and smaller overshooting power over the GS lasing in Region I.

1.3 [micro sign]m short-cavity VCSELs enabling error-free transmission at 25 Gbit/s over 25 km fibre link

The large-signal modulation characteristics of 1.3 µm vertical-cavity surface-emitting lasers (VCSELs) with buried tunnel junction and reduced cavity length are presented. A short overview of the VCSEL design and static device characteristics is given. Employing these devices, the error-free data transmission at 25 Gbit/s over up to 25 km of singlemode fibre is reported for the first time. The impact of transient laser chirp and fibre dispersion on the received optical power and the bit error rate is discussed.

High-Speed Modulation of Index-Guided Implant-Confined Vertical-Cavity Surface-Emitting Lasers

An etched photonic crystal (PhC) or holey wedge structure induces index confinement into 850-nm implant-confined vertical-cavity surface-emitting lasers (VCSELs) to engineer the spatial overlap between the optical mode and laser gain for improved high-speed operation and reduced relative intensity noise. Large-signal operation of 12.5 Gb/s is achieved with a single transverse-mode PhC VCSEL and 15 Gb/s with a single transverse-mode holey VCSEL. An excessive current diffusion effect is found when the difference between the electrical and optical diameter is large (>4 mum), which limits the large-signal modulation of single-mode VCSELs. The design rules for optimal single transverse-mode high-speed PhC and holey VCSELs are extracted from a parametric study of their large-signal modulation characteristics.

Large-signal simulation of semiconductor lasers on device level: numerical aspects of the harmonic balance method

Large-signal modulation characteristics are key specifications in analog and digital applications of optoelectronic devices. In this work we present a novel approach for the modeling of these characteristics for semiconductor lasers on device level. A multi-tone harmonic balance method for a physics-based electro-thermal-optical model is utilized. The numerical aspects of the framework are described and illustrated for a realistic VCSEL example.

Dynamics of quantum dot photonic crystal lasers

Quantum dot photonic crystal membrane lasers were fabricated and the large-signal modulation characteristics were studied. The authors find that the modulation characteristics of quantum dot lasers can be significantly improved using cavities with large spontaneous emission coupling factor. Their experiments show, and simulations confirm, that the modulation rate is limited by the rate of carrier capture into the dots to around 30GHz in their present system.

Influence of the optical feedback on the large signal modulation characteristics of the external cavity semiconductor laser

After taking into account the multiple reflections of light in external cavity, the influence of the optical feedback on the large signal modulation characteristics of the external cavity semiconductor laser (ECSL) has been theoretically investigated. The numerical simulations show that, with the increase of the modulation index, the peak photon number of the ECSL tends to higher level on the whole. When the optical feedback is strong or weak, the peak photon number of the ECSL shows single period. However, the peak photon number exhibits bifurcation or chaos for different modulation index when the optical feedback is intermediate.

Enhanced modulation bandwidth of surface-emitting laser with external optical feedback

We propose a monolithic external-cavity vertical-cavity surface emitting laser (VCSEL) for achieving ultra-high speed direct modulation beyond the limit of a relaxation oscillation frequency. We present the modeling of small-signal and large signal modulation characteristics of VCSELs with optical feedback. It is predicted that a 3 dB-modulation bandwidth can be increased by 50% with optimizing an external cavity length. Also, better eye opening can be expected in 40Gbps direct modulation.