QDash Lasers Research Articles

QDash-Based Directly Modulated Lasers for Next-Generation Access Network

In this letter, we report the strong effect of p-doping on dynamic performances of QDash lasers. Therefore, we demonstrate 10 Gb/s transmission in standard single mode fiber up to 65 km with a dynamic extinction ratio using directly modulated lasers based on this material and commercial Fabry-Pérot etalon filter.

Chirped InAs/InP quantum-dash laser with enhanced broad spectrum of stimulated emission

We report on the demonstration of 50 nm (full-width at half-maximum) broadband stimulated emission from a chirped AlGaInAs barrier thickness multi-stack InAs/InP quantum dash (Qdash) laser. The 2 μm wide uncoated Fabry-Perot (FP) ridge-waveguide laser exhibits a total power of 0.18 W, corresponding to an average spectral power density of 3.5 mW/nm, under pulsed current conditions. Intentional extended inhomogeneity across the Qdash stacks have been attributed to the enhancement of broadband emission.

Terahertz-bandwidth coherence measurements of a quantum dash laser in passive and active mode-locking operation

This research carries out coherence measurements of a 42.7 GHz quantum dash (QDash) semiconductor laser when passively, electrically, and optically mode-locked. Coherence of the spectral lines from the mode-locked laser is determined by examining the radio frequency beat-tone linewidth as the mode spacing is increased up to 1.1 THz. Electric-field measurements of the QDash laser are also presented, from which a comparison between experimental results and accepted theory for coherence in passively mode-locked lasers has been performed.

Spectral Analysis of Quantum-Dash Lasers: Effect of Inhomogeneous Broadening of the Active-Gain Region

The effect of the active region inhomogeneity on the spectral characteristics of InAs/InP quantum-dash (Qdash) lasers is examined theoretically by solving the coupled set of carrier-photon rate equations. The inhomogeneity due to dash size or composition fluctuation is included in the model by considering dispersive energy states and characterized by a Gaussian envelope. In addition, the technique incorporates multilongitudinal photon modes and homogeneous broadening of the optical gain. The results predict a red shift in the central lasing wavelength of Qdash lasers on increasing the inhomogeneous broadening either explicitly or implicitly, which supports various experimental observations. The threshold current density and the lasing bandwidth are also found to increase.

Mode coherence measurements across a 15 THz spectral bandwidth of a passively mode-locked quantum dash laser

The mode coherence of adjacent and non-adjacent spectral modes of a passively mode locked quantum dash (QDash) semiconductor laser are deduced through radio frequency beat-tone linewidth measurements. A wavelength conversion scheme that uses degenerate four wave mixing in a semiconductor optical amplifier is proposed which considerably extends the mode spacing beyond the limit imposed by conventional fast-photodetection and electrical spectrum analysis of around 100 GHz. Using this scheme, the mode coherence of the QDash laser was measured out to the thirty-first harmonic, or a mode separation of 1.5 THz.

Effect of the number of stacking layers on the characteristics of quantum-dash lasers

A theoretical model is evaluated to investigate the characteristics of InAs/InP quantum dash (Qdash) lasers as a function of the stack number. The model is based on multimode carrier-photon rate equations and accounts for both inhomogeneous and homogeneous broadenings of the optical gain. The numerical results show a non monotonic increase in the threshold current density and a red shift in the lasing wavelength on increasing the stack number, which agrees well with reported experimental results. This observation may partly be attributed to an increase of inhomogeneity in the active region.

Modeling the lasing spectra of InAs/InP Quantum dash lasers

We report a theoretical model for InAs/InP quantum-dash (Qdash) lasers incorporating a coupled set of rate equations taking into account the inhomogeneous broadening due to Qdash size fluctuation, the homogeneous broadening due to optical gain of a single Qdash, and the longitudinal cavity modes. The role of cavity length on the Qdash lasing characteristics, particularly the redshift in the peak lasing wavelength, is analyzed and compared with the experimental results by attributing it to the active region inhomogeneity.

Small-signal parameters of quantum dash lasers with multiple coupled energy states

The small-signal parameters of quantum dash (QDash) lasers with multiple coupled energy states have been derived using a rate equation model. Analytical expressions for the small-signal differential gain, resonance frequency, and recombination lifetime have been derived. The linewidth enhancement factor of QDash lasers with multiple coupled energy states is studied using our derived model. With the help of our model, we find that introducing p-type doping in the active region of the QDash layer does not enhance the small-signal resonant frequency, but reduces the small-signal recombination lifetime and the linewidth enhancement factor. Also, we find that increasing the n-type doping concentration decreases slightly the small-signal recombination lifetime and the resonant frequency, and increases slightly the linewidth enhancement factor. Our analysis reveals that an undoped QDash laser can be designed to operate at the second excited state energy and to yield high modulation bandwidth and low linewidth enhancement factor.

Cavity design and characteristics of monolithic long-wavelength InAs/InP quantum dash passively mode-locked lasers

By extending the net-gain modulation phasor approach to account for the discrete distribution of the gain and saturable absorber sections in the cavity, a convenient model is derived and experimentally verified for the cavity design of two-section passively mode-locked quantum dash (QDash) lasers. The new set of equations can be used to predict functional device layouts using the measured modal gain and loss characteristics as input. It is shown to be a valuable tool for realizing the cavity design of monolithic long-wavelength InAs/InP QDash passively mode-locked lasers.

High Bandwidth Operation of Directly Modulated Laser Based on Quantum-Dash InAs–InP Material at 1.55 $\mu$m

The modulation bandwidth has been identified as a specific limitation of quantum-dot or quantum-dash (QDash) lasers for direct modulation application. Solutions using tunnel injection and p-doping have already been demonstrated to increase the modulation bandwidth above 10 GHz, but with complex tunnel injection design and p-doping induced high internal losses. We show in this letter that the use of optimized QDashes and waveguide structure is sufficient to reach such high bandwidth at 1.55 mum. The device is validated by a large signal modulation demonstration at 10 Gb/s.

The impact of p-doping on the static and dynamic properties of 1.5μm quantum dash lasers on InP

p -type modulation doping in the range of 0–100 acceptors per quantum dash (QDash) has been carried out to investigate the impact on QDash lasers on (100) InP. The differential gain was found to increase more than 50% for doping concentrations of 50 acceptors per QDash for constant cavity length lasers. However, this benefit is overcompensated by enhanced gain compression and enlarged thermal heating due to high internal losses in highly p-doped devices. The maximum modulation bandwidth of 8GHz in continuous wave operation at room temperature is, therefore, obtained for a moderate p-doping level of 10 holes per QDash.

Quantum Dash Intermixing

We investigate the intermixing effect in InAs/InAlGaAs quantum-dash-in-well structures grown on InP substrate. Both impurity-free vacancy disordering (IFVD) via dielectric cap annealing, and impurity-induced disordering (IID) using nitrogen ion-implantation techniques have been employed to spatially control the group-III intermixing in the quantum-dash (Qdash) system. Differential bandgap shifts of up to 80 nm and 112 nm have been observed from the IFVD and IID processes, respectively. Compared to the control (nonintermixed) lasers, the light-current characteristics for the 125 nm wavelength shifted Qdash lasers are not significantly changed, suggesting that the quality of the intermixed material is well preserved. The intermixed lasers exhibit a narrower linewidth as compared to the as-grown laser due to the improved dash homogeneity. The integrity of the material is retained after intermixing, suggesting the potential application for the planar integration of multiple active/passive Qdash-based devices on a single InP chip.

Dynamic properties of 1.5 µm quantum dash lasers on (100) InP

The static and dynamic properties of state-of-the-art InP-based QDash lasers around 1.55 µm are reported. Demonstrated are ridge waveguide lasers with a total output power of 37 mW and a total slope efficiency of 0.40 W/A that show a 3 dB modulation bandwidth of 8.0 GHz and a modulation efficiency of 0.76 GHz/√mA in continuous-wave operation.

InP-based quantum dash lasers for wide gain bandwidth applications

Self-assembled quantum dash (QDash) lasers based on InP with an emission wavelength of about 1.5 μm were grown by molecular beam epitaxy with solid-state sources. Two different approaches were investigated to further extend the broad gain bandwidth of QDash structures. One approach is based on layer thickness variations in a stacked QDash structure with thick barriers to avoid vertical strain coupling. In the alternative structure, the barrier thickness is reduced to allow strain coupling which modifies automatically the QDash size from layer to layer. With both approaches, the gain bandwidth could be significantly increased in comparison to structures with a single QDash layer. However, lasers based on active regions with reduced barrier widths show additionally a significant improvement of the device performance. Ridge waveguide lasers based on this new material show continuous wave (cw) operation up to 75 °C. At 20 °C, a threshold current of 65 mA was obtained for 2 mm long devices with 2.5 μm wide ridges.

1.54 µm singlemode InP-based Q-dash lasers

Singlemode lasers based on AlGaInAs/InAs/InP quantum-dash layers were for the first time fabricated with emission wavelengths in the 1.55 /spl mu/m telecommunication band. A low thermal shift of the gain maximum in quantum-dash structures allows a wide operation temperature range for distributed feedback lasers.