Time-domain Traveling-wave Model Research Articles

Design of distributed feedback lasers grown on Si using time-domain traveling wave model combined with mode refractive index method

This paper proposes an optimization method combining the time-domain traveling wave model and the mode refractive index method, employed for characterizing both the transverse and longitudinal modes of quantum dot distributed feedback (DFB) lasers grown on Si. We use this method to optimize the overall performance of the Si-based DFB laser, and determine the material and chip structural parameters, including the ridge width, etching depth, grating thickness and grating position as optimization parameters. Here, the optimal DFB laser operating under fundamental transverse and single longitudinal mode is obtained. Its threshold current is as low as 5 mA, the slope efficiency is as high as 0.77 mW mA−1, and the side mode suppression ratio is up to 48 dB. When the injection current is 150 mA, the output power exceeds 100 mW. The corresponding ridge width, etching depth and grating thickness are 2 μm, 1.3 μm and 20 nm, respectively. The distance from the grating to the active region is 200 nm. Therefore, the novel method presented in this paper offers an effective scheme for the design of DFB lasers grown on Si with excellent performance.

Narrow-linewidth distributed feedback moiré-grating laser for high-speed optical communications.

A time-domain traveling wave model is described to simulate a novel distributed feedback moiré-grating (DFM) laser, which can suppress the longitudinal spatial hole burning (LSHB) effect and improve the spectral linewidth for high-speed optical communications. The longitudinal photon distribution, linewidth, and dynamic modulation characteristics of the DFM lasers are calculated and compared with the state-of-the-art distributed feedback lasers. We found that the DFM laser can also improve the light-current slope efficiency and modulation bandwidth by introducing multiple $\unicode{x03C0} $π-phase shifts at the moiré envelope index changes while effectively suppressing the LSHB.

Design of 1.3- $\mu \text{m}$ High-Performance Directly Modulated Lasers Based on High-Order Slotted Surface Gratings

1.3-μm high-speed directly modulated lasers (DMLs) based on high-order slotted surface gratings are proposed. The time-domain travelling-wave model combined with the 2-D scattering matrix method is employed for the analysis of the proposed DMLs. By optimization an intrinsic direct-modulation bandwidth up to 33 GHz has been predicted for a 250-μm-long laser with a threshold current of about 4 mA and a slope efficiency of about 0.4 mW/mA. Such lasers are regrowth free and have strong potential to realize low-cost high-reliability DMLs.

Polarization dependence of patterning effects in quantum well semiconductor optical amplifier-based wavelength conversion

In this paper, polarization dependence of patterning effects in quantum well semiconductor optical amplifier-based wavelength conversion is experimentally and theoretically investigated. The carrier and photon density rate equations are numerically solved by using the time-domain traveling wave model. The material gain calculation, including the strain effect in the active layer, is based on the k·p method. By comparing experimental and computational results, it is demonstrated that the polarization of the injection signal has a significant influence on the gain recovery time of quantum well semiconductor optical amplifier. Under the cross-polarized signals injection, the output signals suffer the weakest and strongest patterning effects both for unstrained and tensile strained quantum well semiconductor optical amplifiers.

Modeling of Single-Section Quantum Dot Mode-Locked Lasers: Impact of Group Velocity Dispersion and Self Phase Modulation

A short pulse train with pulsewidth was generated in a quantum dot mode-locked laser (QD MLL). Due to the short dispersion length, it is required to include group-velocity dispersion (GVD) in modeling pulse train generation and evolution from QD MLLs. On the other hand, Kerr effect is also required to consider due to high peak power density in the laser cavity, and its induced self-phase modulation (SPM) also contributes to the pulse evolution. In this paper, a time domain traveling wave model, including the effect of GVD and SPM, combined with rate equations, is established to model the pulse evolution in a single-section QD MLL. It is shown that the pulse evolution calculated by this model is in reasonable agreement with the experiments. The contribution to the pulse evolution by the GVD and SPM impact is discussed.

Widely Tunable Semiconductor Laser Based on Digital Concatenated Grating With Multiple Phase Shifts

In this paper, we proposed a widely tunable distributed Bragg reflector (DBR) laser based on digital concatenated grating with multiple phase shifts (MPS-DCG). The MPS-DCG can provide a comblike reflectivity spectrum with flat envelop covering over 100-nm wavelength range. Static characteristics of the MPS-DCG DBR laser, including the wavelength tuning range, output power, side-mode suppression ratio (SMSR), and L-I curves, are simulated using a time-domain traveling-wave model combined with a digital filter method. Tuning range of approximately 90 nm is obtained with good SMSR. The L-I curves show that the laser threshold current is around 30 mA.

Theoretical Analysis of Tunable Three-Section Slotted Fabry–Perot Lasers Based on Time-Domain Traveling-Wave Model

In this paper, we present a simple time-domain traveling-wave dynamic model for the simulation of wavelength tunable three-section slotted Fabry-Perot (3s-SFP) semiconductor lasers. The longitudinal spatial hole burning, the nonlinear gain compression, and the refractive index changes with carrier density are included in this model. The slot structure is characterized by using the boundary conditions of the optical fields propagating through the slot facets. For the first time, the tuning mechanism of the 3s-SFP laser is studied in detail by using the proposed model. Characteristics of the 3s-SFP laser including the lasing wavelength, the side-mode suppression ratio, the output spectrum, and the wavelength switching dynamics are simulated. The potential of increasing the tuning range of the 3s-SFP laser is also discussed by optimizing design parameters of the device. Virtual digital infinite-impulse response bandpass filters with different central wavelengths are developed to examine the mode power of different channels during wavelength switching events. Switching times of approximately 2-5 ns are demonstrated theoretically. Good agreements have been found between the simulation results and the experimental measurements.

Characterization of one-dimensional lasing in semiconductor disordered nanopowders

In this paper, we present characteristics of one-dimensional ZnO-based random lasers by using a time-domain traveling-wave model. The results reveal that by increasing pump intensity, at first the number of laser modes is increased, and then it gets saturated. Also, output intensity shows that by increasing pump pulse width at a fixed pump intensity, laser modes are made to appear; at first, by raising pump pulse width, more laser modes appear, and then the intensity of the output spectrum is increased. Additionally, we show that by nonuniform pumping, a single-mode laser can be achieved. Through investigation of the nanopowder's average size, we show that the output spectrum has its highest intensity and the sharpest laser modes for average nanopowder sizes less than 100 nm. Moreover, the results show that by decreasing pump area, lasing threshold is inclined.

Dynamic Analysis of High-Order Laterally Coupled DFB Lasers Using Time-Domain Traveling-Wave Model

This paper describes the application of a time-domain modeling approach for a laterally coupled distributed feedback (LC-DFB) semiconductor laser for the first time. We numerically study the effect of the radiation modes on LC-DFB laser properties. We integrate the Streifer's coefficients, which represent the effects of radiation and evanescent modes into the time-domain coupled-wave equations. High-order corrugated gratings with λ/4 phase-shift are analyzed, where the degree of longitudinal spatial hole burning can be effectively reduced by means of fine tuning of the grating duty cycle. Additionally, we show a remarkably enhanced side-mode suppression ratio (SMSR). For example, for the third-order gratings with a 50% duty cycle, an SMSR as high as 45 dB can be predicted.

Two-Section Quantum-Dot Mode-Locked Lasers Under Optical Feedback: Pulse Broadening and Harmonic Operation

In this paper, we numerically investigate the role of gain and absorber dynamics on the operation of monolithic two-section mode-locked quantum-dot lasers under optical feedback. The analysis is carried out by means of a time-domain traveling wave model for propagation in the gain and absorbing sections. The obtained results indicate that in devices with slow dynamics, pulse duration tends to increase significantly with feedback. On the contrary, devices with fast dynamics exhibit an operation that depends primarily on the external cavity length.

Time-Domain Large Signal Investigation on Dynamic Responses of the GDCC Quarterly Wavelength Shifted Distributed Feedback Semiconductor Laser

A numerical investigation on the dynamic large-signal analysis using a time-domain traveling wave model of quarter wave-shifted distributed feedback semiconductor lasers diode with a Gaussian distribution of the coupling coefficient (GDCC) is presented. It is found that the single-mode behavior and the more hole-burning effect corrections of quarter wave-shifted distributed feedback laser with large coupling coefficient can be improved significantly by this new proposed light source.

Joint Experimental and Theoretical Investigations of Two-State Mode Locking in a Strongly Chirped Reverse-Biased Monolithic Quantum Dot Laser

The influence of different biasing conditions of a passively mode-locked strongly chirped two-section quantum-dot laser on its ground state (GS) and excited state (ES) emission is investigated both experimentally and theoretically. Numerical simulations performed with a modified time-domain travelling-wave model are in qualitative agreement with experimental results and allow to identify the origin of the experimentally observed mode-locking regimes: the complex time-dependent coupling between the ES and GS absorption bleaching in the saturable absorber section. This coupling, together with the difference in group velocities of ES and GS pulses, is found to be responsible for low-frequency inter-modulation signatures as observed experimentally and verified in the simulations.

Time-Domain Traveling Wave Model of Quantum Dot DFB Lasers

We present a time-domain traveling wave model for the simulation of InAs/GaAs quantum dot DFB lasers. The optical susceptibility of the quantum dot (QD) active material is represented through a series of numerical filters that account for the interplay between homogeneous and inhomogeneous broadening of the gain spectrum of the self-assembled QDs. We also consider separate rate equations for electrons and holes for representing the different dynamics of carriers in the conduction and the valence band. We present examples of simulation results focusing in particular on the dynamic properties of the laser (modulation bandwidth, nonreturn-to-zero direct modulation, chirp, α-parameter, etc.) with the purpose of comparing these simulation results with the experimental ones found in the literature.

Time-Domain Travelling-Wave Model for Quantum Dot Passively Mode-Locked Lasers

We present a time-domain travelling-wave model for the simulation of passive mode-locking in quantum dot (QD) lasers; accurate expressions for the time varying QD optical susceptibility and the QD spontaneous emission noise source are introduced in the 1-D wave equations and numerically described using a set of infinite-impulse response filters. The inhomogeneous broadening of the density of states of the whole QD ensemble as well as the homogeneous linewidth of each QD interband transition are properly taken into account in the model. Population dynamics in the QD, quantum well, and barrier states under both forward and reverse bias conditions are modeled via proper sets of multi-population rate equations coupled with the field propagation equations. The model is first applied to the study of gain and absorption recovery in a QD semiconductor optical amplifier under both forward and reverse bias conditions. Simulations of passive mode-locking in a two-section QD laser are then performed as a function of the bias parameters. Gain and absorption dynamics leading to the generation of mode-locking pulses is described. The onset of a trailing-edge instability at low currents is observed and fully explained in the framework of the described model.

Investigation of semiconductor optical amplifier integrated with DBR laser for high saturation power and fast gain dynamics

A detailed theoretical investigation of an integration of a semiconductor optical amplifier (SOA) and distributed Bragg reflector (DBR) laser is presented. The dependence of the device performance on those key design parameters such as the lasing wavelength, light injection direction (co- and contra-propagation), lasing power of the DBR laser, and the biasing condition of the SOA is examined systematically by means of a comprehensive time-domain traveling-wave model. As this integrated structure is particularly designed for high saturation power and fast gain dynamics, these characteristics are simulated and compared with the results from the conventional structures. Depending on different requirements, superior performance on either saturation power or noise figure without compromise on the optical gain can be achieved by different integration configurations (i.e., by different light injection directions). For the structure with the light injection from the output end of the SOA (namely, the integrated SOA-laser structure), the fast gain dynamics is found through simulation, which helps to reduce the large-signal waveform distortion in the amplification of narrow pulses.

High-power single-mode ZnO thin-film random lasers

The possibilities to realize high-power single-mode ultraviolet lasing in highly disordered zinc oxide (ZnO) thin films are investigated. An effective one-dimensional time-domain traveling-wave model is developed to simulate random laser action in ZnO thin-film waveguides with ridge structure. Spectral and spatial redistributions of lasing modes, due to lasing mode localization and repulsion, are shown in highly disordered media. Hence, by controlling the photon density distribution, selective excitation of lasing modes can be achieved. A coupled-cavity ZnO thin-film random laser is also proposed to achieve high-power single-mode operation under nonuniform pumping. Preliminary experimental results have verified the possibility to realize high-power single-mode emission by the use of the proposed coupled-cavity design.

Performance simulation and design optimization gain-clamped semiconductor optical amplifiers based on distributed Bragg reflectors

Performance of the gain-clamped semiconductor optical amplifier (GC-SOA) based on distributed Bragg reflector (DBR) structures is investigated by a comprehensive broad-band time-domain traveling wave model. Critical factors, e.g., the material gain profile, the waveguide grating structures, the longitudinal variation of the optical field, and the carrier density as well as the broad-band spontaneous noise emission are considered in the model. The effects of operation parameters (e.g., the electrical bias current and the input optical power) and design parameters (e.g., the normalized coupling coefficient /spl kappa/L and the lasing wavelength position /spl lambda//sub L/) on device performance are examined in detail. Based on the results of the performance simulation, guidelines for design optimization are discussed and, in particular, unbalanced grating configurations combined with nonuniform current injection schemes are proposed to reduce the noise figure without much sacrifice on other device performance.

Dynamic simulation of clock recovery with self-pulsating three-section distributed-feedback lasers

A numerical investigation on the dynamic behavior of self-pulsating three-section distributed feedback (3S-DFB) lasers has been carried out in great detail using a time-domain travelling-wave model. For the first time, the system parameters of the recovered clock, such as the conditions for clock recovery and the jitter of the recovered clock, have been calculated, also considering a realistic data signal of a 2/sup 13/ - 1-bit sequence affected by chirp, jitter, and limited extinction ratio. This analysis has confirmed the great potential of these 3S-DFB devices.

An improved time-domain traveling-wave model for vertical-cavity surface-emitting lasers

The use of the time-dependent transfer matrix and wide-angle beam propagation method is proposed to improve the computational speed and accuracy of a recently developed time-domain traveling-wave model of vertical-cavity surface-emitting lasers. With suitable utilization of the transfer matrix, significant reduction of the total execution time of the traveling-wave model can be obtained. In addition, the use of the wide-angle beam propagation method can minimize the calculation error of the traveling-wave model due to the diffraction of light from the small aperture of the laser cavity.

Dynamic analysis of erbium-doped optically pumped waveguide lasers using a time-domain travelling wave model

A simple and straightforward dynamic model of Er3+-doped optically pumped waveguide lasers using a time-domain algorithm is developed. The theoretical model is based on (a) time-dependent rate equations of a quasi-two-level system for the population densities and (b) time-dependent travelling wave equations for the pump and signal power which are solved simultaneously in the time domain. In addition, the longitudinal distribution of spontaneous emission noise is also taken into consideration. Using the fast Fourier transform, the spectrum profile of the time-domain signal power is calculated. In this paper, the transient responses of population densities, pump and signal power are investigated for Er3+-doped Ti:LiNbO3 waveguide lasers. To demonstrate the sophistication of this time-domain model, the influence of the cross-coupled effect of the travelling waves due to the optical feedback of an etched grating is also studied. Dynamic response of the waveguide laser with and without a grating are compared.