Optical injection locking and optical-fiber data transmission by directly modulated wavelength tunable laser transmitters

Enhancement of the small- and large-signal modulation performance of wavelength tunable laser diode (TLD) transmitters under strong optical injection locking (OIL) is investigated numerically in back-to-back and optical-fiber transmission schemes. Our model is based on the spatiotemporal description of laser dynamics as due to the composite cavity design of TLDs, the usual rate equation formalism is not directly applicable. We demonstrate that TLD transmission strongly depends on wavelength tuning, which was investigated over a 21-nm range between 1529 and 1550 nm emission wavelengths. The best performance for both free-running (FR) and OIL TLDs is achieved at shorter wavelengths, 1529 nm for our device. Although in both cases this is due to larger differential material gain at shorter wavelengths, the underlying physics of the effect is completely different. For an FR TLD, it is the resonance oscillation frequency (ROF) that defines the best modulation speed, while for an OIL TLD, the achievable modulation speed depends on the cavity mode shift due to optical injection. Both the ROF and the cavity mode shift increase when the differential gain increases. However, the ROF is the device’s fixed parameter, while the cavity mode shift is defined by the OIL conditions, and thus, it can be optimized. The superior performance of the optical fiber digital data transmission with the OIL TLD is demonstrated at around 20-Gb/s modulation speed for standard fibers. This result is attributed to an enhanced modulation response and suppressed frequency chirping of the OIL TLD, and it is important for practical utilization of TLD transmitters

Overshoot mechanism in transient excitation of THz and Gunn oscillations in wide-bandgap semiconductors

A detailed study of high-field transient and direct-current (DC) transport in GaN-based Gunn diode oscillators is carried out using the commercial simulator Sentaurus Device. Applicability of drift-diffusion (DD) and hydrodynamic (HD) models to high-speed, highfrequency devices is discussed in depth, and the results of the simulations from these models are compared. It is shown, for a highly homogeneous device based on a short (2 μm) supercritically doped (1017 cm-3) GaN specimen, that the DD model is unable to correctly take into account some essential physical effects which determine the operation mode of the device. At the same time, the HD model is ideally suited to solve such problems due to its ability to incorporate non-local effects. We show that the velocity overshoot near the device contacts and space charge injection and extraction play a crucial role in defining the operation mode of highly homogeneous short diodes in both the transient regime and the voltagecontrolled oscillation regime. The transient conduction current responses are fundamentally different in the DD and HD models. The DD current simply repeats the velocity-field (v-F) characteristics, and the sample remains in a completely homogeneous state. In the HD model, the transient current pulse with a full width at half maximum of approximately 0.2 ps is increased about twofold due to the carrier injection (extraction) into (from) the active region and the velocity overshoot. The electron gas is characterized by highly inhomogeneous distributions of the carrier density, the electric field and the electron temperature. The simulation of the DC steady states of the diodes also shows very different results for the two models. The HD model shows the trapped stable anodic domain in the device, while the DD model completely retains all features of the v-F characteristics in a homogeneous gas. Simulation of the voltage-controlled oscillator shows that it operates in the accumulation layer mode generating microwave signals at 0.3 to 0.7 THz. In spite of the fact that the known criterion of a Gunn domain mode n0L > (n0L)0 was satisfied, no Gunn domains were observed. The explanation of this phenomenon is given. © 2012 Momox et al

Self-Consistent Simulation Model and Enhancement of Wavelength Tuning of InGaAsP/InP Multisection DBR Laser Diodes

A detailed investigation of various physical contributions to the refractive index change due to injection of free carriers and their effect on the wavelength tuning in a three-section InGaAsP/InP semiconductor tunable laser diode (TLD) is carried out using Crosslight PICS3D software. A comprehensive numerical model of a TLD is presented with the wavelength tuning based on a simultaneous consideration of optical and free carrier transport phenomena. The main difference of our study from most of the previous works is that the investigation is applied to the operating multisection active device rather than to a device with a predetermined refractive index change used as the input parameter. The only inputs in our model are the injection currents at the contacts of each section. We have demonstrated possibility of 30 nm discontinuous and 15 nm continuous tuning in the optimized TLD. We study the influence of design parameters, such as the coupling coefficient κ and Bragg section length L, on the tuning performance of the Bragg section of the TLD. The tuning performance of the TLD depends strongly on the mutual positions of the peak gain wavelength and the Bragg wavelength at the beginning of the tuning. The tuning range is strongly affected by the material composition of the Bragg grating. We found that wavelength tuning saturation may take place in typical devices in spite of the continuing change of the effective refractive index with the current injection. It is shown that with careful design optimization and selection of κ and L, the saturation effect can be completely eliminated and the tuning range of the TLD can be increased up to two to three times. © 1995-2012 IEEE

Modulation dynamic response of optical-injection-locked wavelength-tunable semiconductor laser diodes

The effect of optical injection locking (OIL) on modulation dynamics of tunable laser diodes (TLDs) is investigated. The stable locking boundary map of OIL TLD which depends on the lasing wavelength was calculated and the TLD response in different injection states was investigated. Dramatic change in the emission spectra and the relaxation oscillation frequency (ROF) and huge improvement of these characteristics under OIL is demonstrated. Effect of the OIL parameters (frequency detuning and power ratio) on the TLD dynamics was studied for different tuning wavelengths. We show that forward optical injection and positive frequency detuning and blue-wavelength tuning are preferable regimes for enhanced modulation performance of OIL TLDs. Large increase of the ROF (>20GHz) and the modulation bandwidth (>25GHz) of the OIL TLD are demonstrated. The novel feature of the modulation response of the OIL TLD compared with usual single-mode lasers emitting at a fixed wavelength is the frequency dispersion of the differential gain in TLDs which increases with wavelength tuning. It is shown that the differential gain increase with wavelength tuning and the optical injection effect on the cavity resonance frequency shift contribute to a substantial enhancement of the modulation response of the OIL TLD

Travelling‐wave modelling of the modulation dynamic performance of wavelength‐tunable laser diodes using the integrated VPI and PICS3D software

The modulation dynamic performance of a three-section bulk InGaAsP/InP tunable laser diode (TLD) operating at 1550-nm under direct intensity modulation during discontinuous tuning is investigated with the travelling-wave approach, using commercial software tools VPI and PICS3D. The authors demonstrate a strong effect of the gain spectra shape on the modulation response of the TLDs. Two models have been developed for simulation of the modulation response which incorporate real gain spectra of TLDs obtained either from experiment or ab-initio calculations: (i) the VPI + PICS3D integrated model, and (ii) the fitted parabolic shape gain model. The results obtained for both models are in good agreement. A significant ~3 times increase of the relaxation oscillation frequency and the corresponding modulation bandwidth was observed under blue wavelength tuning of the TLD over a 21-nm range from the initial 1550-nm lasing wavelength. The authors show that the main physical reason for this increase is a dispersion of the differential gain which increases about 4 to 5 times when the lasing wavelength decreases over the above tuning range. The reported enhancement of the modulation response of TLDs is important for their practical applications

Modulation dynamic response of optical-injection-locked wavelength-tunable semiconductor laser diodes

The effect of optical injection locking (OIL) on modulation dynamics of tunable laser diodes (TLDs) is investigated. The stable locking boundary map of OIL TLD which depends on the lasing wavelength was calculated and the TLD response in different injection states was investigated. Dramatic change in the emission spectra and the relaxation oscillation frequency (ROF) and huge improvement of these characteristics under OIL is demonstrated. Effect of the OIL parameters (frequency detuning and power ratio) on the TLD dynamics was studied for different tuning wavelengths. We show that forward optical injection and positive frequency detuning and blue-wavelength tuning are preferable regimes for enhanced modulation performance of OIL TLDs. Large increase of the ROF (>20GHz) and the modulation bandwidth (>25GHz) of the OIL TLD are demonstrated. The novel feature of the modulation response of the OIL TLD compared with usual single-mode lasers emitting at a fixed wavelength is the frequency dispersion of the differential gain in TLDs which increases with wavelength tuning. It is shown that the differential gain increase with wavelength tuning and the optical injection effect on the cavity resonance frequency shift contribute to a substantial enhancement of the modulation response of the OIL TLD