Gain Compression Factor Research Articles

Parametric study of the transient period characteristics of distributed feedback laser diodes

Subject of study. The operating response characteristics for a distributed feedback laser model has presented. Aim of study. A numerical optimization of model parameters is used to reduce the laser transient period by analyzing the effects of the laser injection current, the temperature variation, the dc-bias level, and the gain compression factor on transient period characteristics. Method. The transient period value decreases when the current ratio (i.e., injection current/threshold current) is increased. Main results. Significant reduction is observed in the relaxation oscillation period and the laser turn-on time delay as the effect of increasing the injection current and/or the biasing current. Practical significance. However, varying temperatures resulted in distributed feedback operating in the off-mode region due to increasing the transient period value. Meanwhile, the relaxation oscillation period value is reduced significantly with a faster stabilization period due to the damped sinusoidal oscillations as a result of increasing the ε value.

Gain-Switched Short Pulse Generation from 1.55 µm InAs/InP/(113)B Quantum Dot Laser Modeled Using Multi-Population Rate Equations

The gain-switching properties of an InAs-InP (113)B quantum dot laser based on multi-population rate equations are examined theoretically in detail to generate shorter pulses with the application of a Gaussian pulse beam to the laser excited state. The numerical results demonstrated that as the homogeneous and the inhomogeneous broadening increase, the differential gain, the gain compression factor, and the threshold current of the excited state decrease while the threshold current of the ground state increases. It was also observed that the contribution of the excited state to gain-switched output pulses depend on not only the value of the inhomogeneous broadening but also the magnitude of the applied current. Additionally, it was found that without an optical beam, the output pulse has a long pulse width due to ground state emissions and the change in the parameters strongly affecting the output. However, under the optical beam, narrow pulses around 26 ps have high peak power owing to the excited state emission generated even at low currents and also the change in the laser parameters having a negligible effect. Finally, the gain-switching characteristics with and without a Gaussian pulse beam are shown to be similar for liner-gain and nonlinear-gain cases except that higher peak power and narrower output pulses are obtained for the linear-gain case.

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.

Modeling of a quantum dot gain chip in an external cavity laser configuration

External cavity semiconductor lasers with strong optical feedback already exist using a gain chip medium. Owing to their ultrafast carrier dynamics, strong output power, and high temperature reliability, quantum dots as a gain medium are now envisioned as a promising solution to replace the current quantum well technology. This paper presents a semi-analytical rate equation model which is used to describe a quantum dot gain chip capable of lasing only with a free space external cavity laser. It investigates the evolution of the dynamical properties such as the turn-on delay and the damping rate. It also confirms the model’s validity through the relative intensity noise and the frequency noise with respect to both material and device parameters like the linewidth enhancement factor, the gain compression factor, or the cavity length. Overall, this numerical investigation provides initial building blocks for future fabrication research and development of high performance devices including filters or gratings as wavelength-selective components.

Theoretical studies of the generation of picoseconds pulses with two-section blue-violet semiconductor lasers

We report the results of theoretical investigations of the generation of sub-10 ps pulses by a blue-violet InGaN two-section laser. The principle of pulse generation is active Q-switching. We study numerically the influence of the length of the switching section and the emission wavelength on the steady-state and dynamic behaviour. We investigate also the impact of the gain-compression factor on peak power, pulse energy and pulse width and compare the numerical results with semi-analytic expressions.

Dynamic and nonlinear properties of epitaxial quantum dot lasers on silicon for isolator-free integration

This work investigates the dynamic and nonlinear properties of quantum dot (QD) lasers directly grown on silicon with a view to isolator-free applications. Among them, the chirp parameter, also named the αH factor, is featured through a thermally insensitive method analyzing the residual side-mode dynamics under optical injection locking. The αH at threshold is found as low as 0.32. Then, the nonlinear gain is investigated from the gain compression factor viewpoint. The latter is found higher for epitaxial QD lasers on silicon than that in heterogeneously integrated quantum well (QW) devices on silicon. Despite that, the power dependence of the αH does not lead to a large increase of the chirp coefficient above the laser’s threshold at higher bias. This effect is confirmed from an analytical model and attributed to the strong lasing emission of the ground-state transition, which transforms into a critical feedback level as high as −6.5 dB, which is ∼19 dB higher than a comparable QW laser. Finally, the intensity noise analysis confirms that QD lasers are overdamped oscillators with damping frequencies as large as 33 GHz. Altogether, these features contribute to fundamentally enhancing the reflection insensitivity of the epitaxial QD lasers. This last feature is unveiled by the 10 Gbit/s error-free high-speed transmission experiments. Overall, we believe that this work is of paramount importance for future isolator-free photonics technologies and cost-efficient high-speed transmission systems.

Controlling the Q-Point in Distributed Feedback Lasers Using a Numerical Optimization Methodology

In this paper, a new methodology for controlling the Q-point in the distributed feedback (DFB) lasers is proposed. The method based on reducing the DFB transient period (TP) by optimizing laser’s model parameters numerically. The analysis has taken into account investigated the effects of the laser injection current (Iinj), the dc-bias level (Ibias), the temperature (T) variation, and the gain compression factor (ε). Results showed that by optimizing the value of Iinj, Ibias, T and ε; the Q-point could be controlled effectively. Where increasing the current ratio (i.e., Iinj/Ith) leads to reduce the TP value. In addition, by increasing Iinj and/or Ibias, the relaxation oscillation period (TRO) and the laser delay time (TDelay) are reduced significantly. From the other hand, the temperature varying may push the DFB laser to operate in an improper region through increasing the TP value; which may lead it to operate in the off-mode. Moreover, as ε is increased, the sinusoidal oscillations are dramatically damped results in a reduction in the TRO value and larger period of stabilized.

Effect of separate confinement hetero-structure layer on tunnel injection transistor laser-based transmitter for high-speed optical communication networks

Transistor Laser has already established as a promising candidate for high speed optical interconnects and optical telecommunication networks. The present work is focused on the performance of multiple quantum wells (MQW) based transistor laser by incorporating tunnel injection phenomenon and it’s performance by varying the width of the separate confinement heterostructure layer. The virtual state (VS) is assumed as a path for the movement of injected charge carriers from bulk to nanostructures. We have estimated the various characteristics such as base threshold current, light power output, and optical modulation response by considering various physical parameters such as diffusion time, gain compression factor, optical confinement factor, photon lifetime, tunneling time and diffusion length, as a function of separate confinement hetero-structure (SCH) layer thickness. It is found that with increasing thickness of the SCH layer, base threshold current reduces, the light output power increases, but the optical modulation response remains almost constant, which ensures the potentiality of the proposed structure for high speed opto-electronic transmitter with low power consumption.

Enhanced gain saturation model of non‐linear semiconductor optical amplifiers

This study proposes an enhanced gain saturation model of non-linear semiconductor optical amplifiers (SOAs) by incorporating material-dependent gain compression factor. The rate equations are utilised with the extra gain compression term for Indium-Gallium-Arsenide material-based SOA to account for the steep relaxation oscillations behaviour of non-linear SOAs. The proposed gain saturation model is verified with experimental results that showed very good agreements with a mean square error of 0.094.

Gain Switching of Monolithic 1.3 μm InAs/GaAs Quantum Dot Lasers on Silicon

We report the first demonstration of gain-switched optical pulses generated by continuous-wave 1.3 μm InAs/GaAs quantum dot (QD) broad-area lasers directly grown on silicon. The shortest observed pulses have typical durations between 175 and 200 ps with peak output powers of up to 66 mW. By varying the drive current pulsewidth and amplitude systematically, we find that the peak optical power is maximized through sufficiently long high-amplitude drive pulses, whereas shorter drive pulses with high amplitudes yield the narrowest achievable pulses. A three-level rate equation travelling-wave model is used for the simulation of our results in order to gain a first insight into the underlying physics and the laser parameters responsible for the observed behavior. The simulations indicate that a limited gain from the InAs QDs and a very high gain compression factor are the main factors contributing to the increased pulsewidth. As the optical spectra of the tested broad-area QD laser give a clear evidence of multitransverse-mode operation, the laser's dynamic response could be additionally limited by transversal variations of the gain, carrier density, and photon density over the 50 μm wide laser waveguide.

The Influence of Gain Compression Factor on Dynamical Properties of Single Level InAs/GaAs Quantum Dot Lasers

In this paper, by representing a single level rate equation model for InAs/GaAs quantum dot lasers and computations by fourth-order Runge–Kutta method some characteristics of the output laser are considered. The change of photon number in time and current and also the output power versus current with different gain compression factors are investigated for lasing from ground state (GS). Afterwards, the response function of small signal modulation for ground state in constant current but different gain compressions is surveyed. At last, we find an optimum value for gain compression factor in lasing from GS.

Design Methodology for Reducing RIN Level in DFB Lasers

The relative intensity noise (RIN) characteristics in distributed feedback (DFB) lasers are analyzed theoretically by proposing a new methodology. In addition to temperature variation (T), the effect of other model parameters such as injection current (Iinj), active layer volume (V), spontaneous emission (βsp) and gain compression (ε) factors on RIN characteristics is investigated. The numerical simulations shows, the peak RIN level can be reduced to around –150 dB/Hz, while relaxation oscillation frequency (ROF) is shifted towards 5.6 GHz. In addition, the RIN level is increased with temperature by the rate of 0.2 dB/ºC and ROF is reduced by the rate of 0.018 GHz/ºC. Results show, the low RIN level can be obtained by selecting model parameters reasonably.

Effect of gain compression above and below threshold on the chirp characteristics of 1.55 µm distributed feedback laser

We have observed and quantified the adiabatic and transient chirp in a directly modulated laser. The wavelength excursion for both chirp terms is well characterized by the phase rate equation model that describes the chirp behavior. In this study, the effect of gain compression and linewidth enhancement factor, below and above threshold, on the chirp characteristics is investigated by simulation. We have observed the trade-off between the two chirp terms i.e., transient part of the chirp is reduced by the strong damping introduced by gain compression, while as the adiabatic part increases with gain compression. We have observed that above threshold the α-factor increases with bias current, which is attributed to the enhancement of gain compression coefficient. It is shown that the higher the maximum gain, the lower the effects of gain compression and lower the α-factor. Finally, the effects of gain compression on the transmission characteristics are investigated.

Dynamic characteristics of 410 nm semipolar (202¯1¯) III-nitride laser diodes with a modulation bandwidth of over 5 GHz

The dynamic characteristics of III-nitride multi-quantum well laser diodes (LDs) emitting at 410 nm were investigated. LDs were grown on semipolar (202¯1¯) bulk GaN substrates and fabricated into devices with cavity lengths ranging from 900 nm to 1800 nm. A 3-dB bandwidth of 5 GHz and 5 Gbit/s direct modulation with on-off keying were demonstrated, which were limited by the bandwidth of the photodetector used for the measurements. The differential gain of the LDs was determined to be 2.5 ± 0.5 × 10−16 cm2 by comparing the slope efficiency for different cavity lengths. Analysis of the frequency response showed that the K-factor, the gain compression factor, and the intrinsic maximum bandwidth were 0.33 ns, 7.4 × 10−17 cm3, and 27 GHz, respectively.

The Effect of Homogenous and Inhomogeneous Broadening and Gain Compression Factor on Dynamical Characteristics and Modulation of Tunneling Injection InGaAs/GaAs Quantum Dot Lasers

The Effect of Homogenous and Inhomogeneous Broadening and Gain Compression Factor on Dynamical Characteristics and Modulation of Tunneling Injection InGaAs/GaAs Quantum Dot Lasers

Impact of gain compression factor on modulation characteristics of InGaAs/GaAs self-assembled quantum dot lasers

This paper investigates the influence of gain compression factor on the modulation response of InGaAs/GaAs self-assembled quantum dot laser based on rate equations. For different gain compression factors the output power-current characteristics, light emissions of quantum dot laser have been simulated and effect of gain compression factor changes on quantum dot laser is illustrated. Also, small and large-signal response of quantum dot lasers is studied and the impact of the gain compression factor is presented. It explains that increase of gain compression factor, decreases small-signal modulation characteristics, nevertheless, improves large-signal response of quantum dot lasers. It helps to generate better laser signal quality, higher eye and smaller jitter. The large-signal behavior of a laser diode determines its capability for digital data transfer. The modulation speed of quantum dot lasers is of specific importance if such lasers are considered for optical communication systems.

Gain Compression and Linewidth Enhancement Factor in Mid-IR Quantum Cascade Lasers

We have observed and quantified the adiabatic and transient chirp in a directly modulated quantum cascade laser (QCL). Those wavelength tuning effects are well-characterized in diode lasers, and the rate equation model that successfully describe the diode laser behavior also provides an excellent fit for the QCL data. In this study, we have extracted the linewidth enhancement factor ( $\alpha _{H}$ ) from the transient chirp and the gain compression factor from the adiabatic chirp. We postulate that the extraction of the $\alpha _{H}$ from the transient chirp is valid for the QCL case, despite additional tuning effects in QCLs (e.g., voltage tuning) that are negligible in diode lasers. Also in the QCL the adiabatic chirp coefficient strongly increases with laser output power but still stays an order of magnitude below typical values known from diode lasers. We hypotesize possibility of the adiabatic chirp to be connected to the χ(3) nonlinearity (responsible for four-wave mixing that was recently attributed to the FM self-locking in QCLs), but the exact origin remains to be experimentally confirmed in future work.

Small-signal modulation characteristics of a polariton laser

Use of large bandgap materials together with electrical injection makes the polariton laser an attractive low-power coherent light source for medical and biomedical applications or short distance plastic fiber communication at short wavelengths (violet and ultra-violet), where a conventional laser is difficult to realize. The dynamic properties of a polariton laser have not been investigated experimentally. We have measured, for the first time, the small signal modulation characteristics of a GaN-based electrically pumped polariton laser operating at room temperature. A maximum −3 dB modulation bandwidth of 1.18 GHz is measured. The experimental results have been analyzed with a theoretical model based on the Boltzmann kinetic equations and the agreement is very good. We have also investigated frequency chirping during such modulation. Gain compression phenomenon in a polariton laser is interpreted and a value is obtained for the gain compression factor.

Frequency modulation response due to the intensity modulation of fiber-grating Fabry–Perot lasers

A comprehensive study on the small-signal frequency modulation (FM) characteristics of a fiber-grating Fabry–Perot (FGFP) laser is numerically conducted. Fiber Bragg grating (FBG) is used as a wavelength selective element to control the properties of the laser output by controlling the external optical feedback (OFB) level. In addition to the external OFB level, the effect of other parameters such as temperature, injection current, cavity volume, nonlinear gain compression factor, linewidth enhancement factor, and fiber-grating (FG) parameters on FM characteristics of the laser are investigated. The study is performed by modifying a set of rate equations that are solved by considering the effects of external OFB and ambient temperature (T) variations. The temperature dependence (TD) of FM characteristics is calculated according to TD of laser parameters instead of using well-known Pankove relationship. Results show that the optimum external fiber length (Lext) is 3.1 cm and the optimum range of working temperature for FGFP laser is within ± 2 °C from the FBG reference temperature (To). Also, it is shown that antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the FM response. The FM spectra peak amplitude is less than 5 dB with 5 mW output power. Good temperature stability is also obtained.

High Performance InAs/${\rm In}_{0.53}{\rm Ga}_{0.23}{\rm Al}_{0.24}{\rm As}$/InP Quantum Dot 1.55 $\mu{\rm m}$ Tunnel Injection Laser

The characteristics of 1.55 μm InAs self-organized quantum-dot lasers, grown on (001) InP substrates by molecular beam epitaxy, have been investigated. Modulation doping of the dots with holes and tunnel injection of electrons have been incorporated in the design of the active (gain) region of the laser heterostructure. Large values of To=227 K (5°C ≤ T ≤ 45°C) and 100 K were derived from temperature dependent measurements of the light-current characteristics. The modal gain per dot layer is 14.5 cm-1 and the differential gain derived from both light-current and small-signal modulation measurements is ~ 0.8×10-15 cm2. The maximum measured -3 dB small-signal modulation bandwidth is 14.4 GHz and the gain compression factor is 5.4×10-17 cm2. The lasers are characterized by a chirp of 0.6 Å for a modulation frequency of 10 GHz and a near zero α-parameter at the peak of the laser emission. These characteristics are amongst the best from any 1.55 μm edge-emitting semiconductor laser.