This work theoretically explores the impact of external carrier noise from pump source on the optical noise characteristics of Quantum Dot (QD) lasers. The investigation includes simulations of the effects of a normal pump with a Gaussian carrier distribution and a quiet pump with a sub-Poisson carrier distribution on the spectral linewidth and Relative Intensity Noise (RIN). The results reveal that the spectral linewidth and RIN are significantly reduced when using a quiet pump compared to a normal pump across bias currents ranging from 1.5 to ten times the threshold currents, attributed to the lower carrier noise level of the quiet pump. At six times the threshold current, the spectral linewidth of the quiet pump decreases to 339.8 kHz, approximately half of that observed with normal pump, while the RIN value improves from -142.4 dB/Hz to -169.5 dB/Hz. Moreover, due to the larger external carrier noise of the normal pump at higher currents, this disparity in spectral linewidth and RIN between normal and quiet pump states becomes more pronounced, with the QD laser under normal pump exhibiting a broadening phenomenon that does not occur with the quiet pump. At ten times the threshold current, the spectral linewidth under normal pump broadens to 536.0 kHz, while under quiet pump, the spectral linewidth continues to decrease to 130.8 kHz, and the RIN value decreases from -145.5 dB/Hz to -172.4 dB/Hz. This work thus paves the way for the application of QD lasers in next-generation photonic integrated circuits by effectively reducing both the spectral linewidth and RIN of these optical sources through a straightforward and manageable strategy.
We experimentally demonstrate that InAs/GaAs quantum dot (QD) lasers exhibit high reflection insensitivity in the wide temperature range of 293 K to 353 K, which is due to the low and thermal stable linewidth enhancement factor. This work shows the potential of QD lasers as uncooled and isolator-free on-chip laser sources for next-generation photonic integrated circuits.
Quantum dot lasers directly grown on silicon are excellent candidates to achieve energy and cost-efficient optical transceivers thanks to their outstanding properties such as high temperature stability, low threshold lasing operation, and high feedback tolerance. In order to reach even better performance, p-type doping is used to eliminate gain saturation, gain broadening due to hole thermalization and to further reduce the linewidth enhancement factor. Optical transceivers with low relative intensity noise are also highly desired to carry broadband data with low bit-error rate. Indeed, the intensity noise stemming from intrinsic optical phase and frequency fluctuations caused by spontaneous emission and carrier noise degrades the signal-to-noise ratio and the bit-error rate hence setting a limit of a highspeed communication system. This paper constitutes a comprehensive study of the intensity noise properties of epitaxial quantum dot lasers on silicon. Results show minimal values between - 140 dB/Hz and - 150 dB/Hz for doping level between 0 and 20 holes/dot in the active region. In particular, the intensity noise is insensitive to temperature for p-doped QD laser. Modulation properties such as damping, carrier lifetime, and K-factor are also extracted from the noise characteristics and analyzed with respect to the doping level. We also provide numerical insights based on an excitonic model illustrating the effects of the Shockley-Read-Hall recombination on the intensity noise features. These new findings are meaningful for designing high speed and low noise quantum dot devices to be integrated in future photonic integrated circuits.
Photonics integrated circuits on silicon are considered as a key technology for data centers and high-performance computers. Owing to the ultimate carrier confinement and reduced sensitivity to crystalline defects, semiconductor quantum dot lasers directly grown on silicon exhibit remarkable properties such as low threshold current, high temperature stability and robust tolerance to external reflections. This latter property is particularly important for achieving large-scale integrated circuits whereby unintentional back-reflections produced by the various passive/active optoelectronic components can hinder the stability of the lasers. In this context, it is known that quantum dot lasers are more resistant to optical feedback than quantum well ones thanks to the low linewidth enhancement factor, the large damping, and the possible absence of upper lasing states. In this work, we theoretically investigate the reflection sensitivity of quantum dot lasers directly grown on silicon by studying the peculiar role of the epitaxial defects, which induce nonradiative recombination through the Shockley-Read-Hall process. By using the Lang and Kobayashi model, we analyze the nonlinear properties of such quantum dot lasers through the bifurcation diagrams and with respect to the nonradiative lifetime. In particular, we show that the increase of the Shockley-Read-Hall recombination shrinks the chaotic region and shifts the first Hopf bifurcation to higher feedback values. We believe that these results can be useful for designing novel feedback resistant lasers for future photonics integrated circuits operating without optical isolator.
This work reports on the optical feedback dynamics of InAs/GaAs QD lasers epitaxially grown on silicon operating in both the short and long delay regimes. Both undoped and p-doped QD lasers are considered. Whatever the external cavity length, no chaotic oscillations are observed on both samples as a result of the small α-factor observed in the silicon QD lasers. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In addition, the transition from the short to long delay regimes can be finely covered by varying the external cavity length from 5 cm to 50 cm, and the boundaries associated to the appearance of the periodic oscillation are identified. In the short-cavity region, boundaries show some residual undulations resulting from interferences between internal and external cavity modes; whereas in the long-delay regime, the feedback ratio delimiting the boundaries keeps decreasing, until it progressively becomes rather in- dependent of the external cavity length. Overall, our results showed that the p-doped device clearly exhibits a much higher tolerance to the different external feedback conditions than the undoped one, seeing that its periodic oscillation boundaries are barely impossible to retrieve at the maximum feedback strength of -7 dB. These results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short- and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.
Direct epitaxial growth of III-V lasers on silicon provides the most economically favorable means of photonic integration but has traditionally been hindered by poor material quality. Relative to commercialized heterogeneous integration schemes, epitaxial growth reduces complexity and increases scalability by moving to 300 mm wafer diameters. The challenges associated with the crystalline mismatch between III-Vs and Si can be overcome through optimized buffer layers including thermal cyclic annealing and metamorphic layers, which we have utilized to achieve dislocation densities < 7×106 cm-2. By combining low defect densities with defect-tolerant quantum dot active regions, native substrate performance levels can be achieved. Narrow ridge devices with threshold current densities as low as ~130 A/cm2 have been demonstrated with virtually degradation free operation at 35°C over 11,000 h of continuous aging at twice the initial threshold current density (extrapolated time-to-failure >10,000,000 h). At 60°C, lasers with extrapolated time-to-failure >50,000 h have been demonstrated for >4,000 h of continuous aging. Lasers have also been investigated for their performance under optical feedback and showed no evidence of coherence collapse at back-reflection levels of 100% (minus 10% tap for measurement) due to the ultralow linewidth enhancement factor (αH < 0.2) and high damping of the optimized quantum dot active region.
A common way of extracting the chirp parameter (i.e., the α-factor) of semiconductor lasers is usually performed by extracting the net modal gain and the wavelength from the amplified spontaneous emission (ASE) spectrum. Although this method is straightforward, it remains sensitive to the thermal effects hence leading to a clear underestimation of the α-factor. In this work, we investigate the chirp parameter of InAs/GaAs quantum dot (QD) lasers epitaxially grown on silicon with a measurement technique evaluating the gain and wavelength changes of the suppressed side modes by optical injection locking. Given that the method is thermally insensitive, the presented results confirm our initial measurements conducted with the ASE i.e. the α-factor of the QD lasers directly grown on silicon is as low as 0.15 hence resulting from the low threading dislocation density and high material gain of the active region. These conclusions make such lasers very promising for future integrated photonics where narrow linewidth, feedback resistant and low-chirp on-chip transmitters are required.
The integration of optical functions on a microelectronic chip brings many innovative perspectives, along with the possibility to enhance the performances of photonic integrated circuits (PIC). Owing to the delta-like density of states, quantum dot lasers (QD) directly grown on silicon are very promising for achieving low-cost transmitters with high thermal stability and large insensitivity to optical reflections. This paper investigates the dynamical and nonlinear properties of silicon based QD lasers through the prism of the linewidth broadening factor (i.e. the so-called α-factor) and the optical feedback dynamics. Results demonstrate that InAs/GaAs p-doped QD lasers epitaxially grown on silicon exhibit very low α-factors, which directly transform into an ultra-large resistance against optical feedback. As opposed to what is observed in heterogeneously integrated quantum well (QW) lasers, no chaotic state occurs owing to the high level of QD size uniformity resulting in a near zero α-factor. Considering these results, this study suggests that QD lasers made with direct epitaxial growth is a powerful solution for integration into silicon CMOS technology, which requires both high thermal stability and feedback resistant lasers.
Quantum cascade lasers (QCLs) are optical sources exploiting radiative intersubband transitions within the conduction band of semiconductor heterostructures.1 The opportunity given by the broad span of wavelengths that QCLs can achieve, from mid-infrared to terahertz, leads to a wide number of applications such as absorption spectroscopy, optical countermeasures and free-space communications requiring stable single-mode operation with a narrow linewidth and high output power.2 One of the parameters of paramount importance for studying the high-speed and nonlinear dynamical properties of QCLs is the linewidth enhancement factor (LEF). The LEF quantifies the coupling between the gain and the refractive index of the QCL or, in a similar manner, the coupling between the phase and the amplitude of the electrical field.3 Prior work focused on experimental studies of the LEF for pump currents above threshold but without exceeding 12% of the threshold current at 283K4 and 56% of the threshold current at 82K.5 In this work, we use the Hakki-Paoli method6 to retrieve the LEF for current biases below threshold. We complement our findings using the self-mixing interferometry technique5 to obtain LEFs for current biases up to more than 100% of the threshold current. These insets are meaningful to understand the behavior of QCLs, which exhibit a strongly temperature sensitive chaotic bubble when subject to external optical feedback.7
In this work, we theoretically investigate the relative intensity noise (RIN) properties of quantum dot (QD) lasers through a rate equation model including the Langevin noises and the contribution from the off resonance energy levels. It is shown that the carrier noise significantly enhances the RIN which can be further reduced by properly controlling the energy separation between the first excited and the ground states. In addition, simulations also unveil that the RIN of QD lasers is rather temperature independent which is of prime importance for the development of power efficient light sources. Overall, these results indicate that QD lasers are excellent candidates for the realization of ultra-low noise oscillators hence being advantageous for fiber optics communication networks, short reach optical interconnects and integrated photonics systems.
In this paper, we investigate the temperature dependence of spectral linewidth of InAs/InP quantum dot distributed feedback lasers. In comparison with their quantum well counterparts, results show that quantum dot lasers have spectral linewidths rather insensitive to the temperature with minimum values below 200 kHz in the range of 283K to 303K. The experimental results are also well confirmed by numerical simulations. Overall, this work shows that quantum dot lasers are excellent candidates for various applications such as coherent communication systems, high-resolution spectroscopy, high purity photonic microwave generation and on-chip atomic clocks.
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