Impact of external carrier noise on the linewidth enhancement factor of a quantum dot distributed feedback laser

This paper demonstrates that the linewidth enhancement factor of quantum dot lasers is influenced by the external carrier transport issued from different external current sources. A model combining the rate equation and semi-classical carrier noise is used to investigate the different mechanisms leading to the above phenomenon in the context of a quantum dot distributed feedback laser. Meanwhile, the linewidth enhancement factor extracted from the optical phase modulation method shows dramatic differences when the quantum dot laser is driven by different noise-level pumps. Furthermore, the influence of external carrier noise on the frequency noise in the vicinity of the laser's threshold current directly affects the magnitude of the linewidth enhancement factor. Simulations also investigate how the external carrier transport impacts the frequency noise and the spectral linewidth of the QD laser. Overall, we believe that these results are of paramount importance for the development of on-chip integrated ultra-low noise oscillators producing light at or below the shot-noise level.

Optical noise characteristics of injection-locked epitaxial quantum dot lasers on silicon

This work theoretically investigates the relative intensity noise (RIN) and spectral linewidth characteristics of epitaxial quantum dot (QD) lasers on silicon subject to optical injection. The results show that the RIN of QD lasers can be reduced by optical injection, hence a reduction of 10 dB is achieved which leads to a RIN as low as -167.5 dB/Hz in the stable injection-locked area. Furthermore, the spectral linewidth of the QD laser can be greatly improved through the optical injection locked scheme. It is reduced from 556.5 kHz to 9 kHz with injection ratio of -60 dB and can be further reduced down to 1.5 Hz with injection ratio of 0 dB. This work provides an effective method for designing low intensity noise and ultra-narrow linewidth QD laser sources for photonics integrated circuits on silicon.

Linewidth narrowing in self-injection-locked on-chip lasers

Stable laser emission with narrow linewidth is of critical importance in many applications, including coherent communications, LIDAR, and remote sensing. In this work, the physics underlying spectral narrowing of self-injection-locked on-chip lasers to Hz-level lasing linewidth is investigated using a composite-cavity structure. Heterogeneously integrated III-V/SiN lasers operating with quantum-dot and quantum-well active regions are analyzed with a focus on the effects of carrier quantum confinement. The intrinsic differences are associated with gain saturation and carrier-induced refractive index, which are directly connected with 0- and 2-dimensional carrier densities of states. Results from parametric studies are presented for tradeoffs involved with tailoring the linewidth, output power, and injection current for different device configurations. Though both quantum-well and quantum-dot devices show similar linewidth-narrowing capabilities, the former emits at a higher optical power in the self-injection-locked state, while the latter is more energy-efficient. Lastly, a multi-objective optimization analysis is provided to optimize the operation and design parameters. For the quantum-well laser, minimizing the number of quantum-well layers is found to decrease the threshold current without significantly reducing the output power. For the quantum-dot laser, increasing the quantum-dot layers or density in each layer increases the output power without significantly increasing the threshold current. These findings serve to guide more detailed parametric studies to produce timely results for engineering design.

Mid-infrared hyperchaos of interband cascade lasers

Chaos in nonlinear dynamical systems is featured with irregular appearance and with high sensitivity to initial conditions. Near-infrared light chaos based on semiconductor lasers has been extensively studied and has enabled various applications. Here, we report a fully-developed hyperchaos in the mid-infrared regime, which is produced from interband cascade lasers subject to the external optical feedback. Lyapunov spectrum analysis demonstrates that the chaos exhibits three positive Lyapunov exponents. Particularly, the chaotic signal covers a broad frequency range up to the GHz level, which is two to three orders of magnitude broader than existed mid-infrared chaos solutions. The interband cascade lasers produce either periodic oscillations or low-frequency fluctuations before bifurcating to hyperchaos. This hyperchaos source is valuable for developing long-reach secure optical communication links and remote chaotic Lidar systems, taking advantage of the high-transmission windows of the atmosphere in the mid-infrared regime.

Uncovering recent progress in nanostructured light-emitters for information and communication technologies

Semiconductor nanostructures with low dimensionality like quantum dots and quantum dashes are one of the best attractive and heuristic solutions for achieving high performance photonic devices. When one or more spatial dimensions of the nanocrystal approach the de Broglie wavelength, nanoscale size effects create a spatial quantization of carriers leading to a complete discretization of energy levels along with additional quantum phenomena like entangled-photon generation or squeezed states of light among others. This article reviews our recent findings and prospects on nanostructure based light emitters where active region is made with quantum-dot and quantum-dash nanostructures. Many applications ranging from silicon-based integrated technologies to quantum information systems rely on the utilization of such laser sources. Here, we link the material and fundamental properties with the device physics. For this purpose, spectral linewidth, polarization anisotropy, optical nonlinearities as well as microwave, dynamic and nonlinear properties are closely examined. The paper focuses on photonic devices grown on native substrates (InP and GaAs) as well as those heterogeneously and epitaxially grown on silicon substrate. This research pipelines the most exciting recent innovation developed around light emitters using nanostructures as gain media and highlights the importance of nanotechnologies on industry and society especially for shaping the future information and communication society.

Private communication with quantum cascade laser photonic chaos

Mid-infrared free-space optical communication has a large potential for high speed communication due to its immunity to electromagnetic interference. However, data security against eavesdroppers is among the obstacles for private free-space communication. Here, we show that two uni-directionally coupled quantum cascade lasers operating in the chaotic regime and the synchronization between them allow for the extraction of the information that has been camouflaged in the chaotic emission. This building block represents a key tool to implement a high degree of privacy directly on the physical layer. We realize a proof-of-concept communication at a wavelength of 5.7 μm with a message encryption at a bit rate of 0.5 Mbit/s. Our demonstration of private free-space communication between a transmitter and receiver opens strategies for physical encryption and decryption of a digital message.

1.3-µm passively mode-locked quantum dot lasers epitaxially grown on silicon: gain properties and optical feedback stabilization

This work reports on an investigation of the optical feedback in an InAs/InGaAs passively mode-locked quantum dot (QD) laser epitaxially grown on silicon. Under the stably-resonant optical feedback condition, experiments demonstrate that the radio-frequency linewidth is narrowed whatever the bias voltage applied on the saturable absorber (SA) is; on the other hand, the effective linewidth enhancement factor of the device increases with the reverse bias voltage on the SA, hence it is observed that such an increase influences the mode-locking dynamic and the stability of device under optical feedback. This work gives insights for stabilizing epitaxial QD mode-locked lasers on silicon which is meaningful for their applications in future large-scale silicon electronic and photonic applications requiring low power consumption as well as for high-speed photonic analog-to-digital conversion, intrachip/interchip optical clock distribution and recovery.

Effect of p-doping on the intensity noise of epitaxial quantum dot lasers on silicon

This work experimentally investigates the impact of p-doping on the relative intensity noise (RIN) properties and subsequently on the modulation properties of semiconductor quantum dot (QD) lasers epitaxially grown on silicon. Owing to the low threading dislocation density and the p-modulation doped GaAs barrier layer in the active region, the RIN level is found very stable with temperature with a minimum value of -150dB/Hz. The dynamical features extracted from the RIN spectra show that p-doping between zero and 20 holes/dot strongly modifies the modulation properties and gain nonlinearities through increased internal losses in the active region and thereby hinders the maximum achievable bandwidth. Overall, this Letter is important for designing future high-speed and low-noise QD devices integrated in future photonic integrated circuits.

High coherence collapse of a hybrid III–V/Si semiconductor laser with a large quality factor

The high-speed dynamics of a hybrid distributed feedback semiconductor laser heterogeneously integrated onto silicon is experimentally investigated in the presence of external optical feedback. The laser fabrication relies on a proper modal engineering in which light is generated in the III–V material and stored in the low-loss silicon region in order to substantially enhance the quality factor of the cavity resonator. In this work, the hybrid laser is found to be insensitive to parasitic reflections leading to a 10 Gbps floor-free transmission with a power penalty no greater than 1.5 dB at room temperature. As a conclusion, owing to the large quality factor, a high coherence collapse level is unveiled in such laser indicating its vast potential to serve as an alternative solution for the development of isolator-free applications in future photonics integrated circuits. A qualitative interpretation is also provided by linking the standard feedback equations to the quality factor of the resonator.

Spectral linewidth reduction of quantum cascade lasers by strong optical feedback

In this work, we propose to employ strong optical feedback to narrow the spectral linewidth of quantum cascade lasers without using any phase control. Rate equation analysis demonstrates that optical feedback beyond a certain level always reduces the laser linewidth for any feedback phase. It is also found that the linewidth becomes less sensitive to the feedback phase for higher feedback strength. Simulations show that optical feedback with a feedback ratio of −10 dB can suppress the laser linewidth by about two orders of magnitude. This is in contrast to near-infrared laser diodes, which can be easily destabilized by strong feedback.

Frequency comb dynamics of a 1.3µm hybrid-silicon quantum dot semiconductor laser with optical injection: erratum

In Opt. Lett.45, 5755 (2019)OPLEDP0146-959210.1364/OL.44.005755, a factor is missing in the result of Eq. (1). Thus, the width of the comb spectrum $ \Delta \nu $Δν becomes $ \Delta \nu = 2{\sqrt 3} \Gamma {\alpha _e} $Δν=23Γα_e.

Frequency comb dynamics of a 1.3 μm hybrid-silicon quantum dot semiconductor laser with optical injection

This work reports on the influence of bias voltage applied on a saturable absorber (SA) on a subthreshold linewidth enhancement factor (LEF) in hybrid-silicon quantum dot optical frequency comb lasers. Results show that the reverse bias voltage on SA contributes to enlarge the LEF and improve the comb dynamics. Optical injection is also found to be able to improve the comb spectrum in terms of 3 dB bandwidth and its flatness. Such novel findings are promising for the development of high-speed dense wavelength-division multiplexing photonic integrated circuits in optical interconnects and datacom applications.

Introduction to the Issue on Physics and Applications of Laser Dynamics (IS-PALD 2017)

In this paper, we introduce the Optics Express feature issue of the 7th International Symposium on Physics and Applications of Laser Dynamics (IS-PALD). This issue consists of expanded papers related to oral and poster presentations. Selected papers represent the best of IS-PALD 2017.

Experimental investigation of broad area quantum cascade lasers under external feedback

Mode creation and temporal response of broad-area quantum cascade lasers (BA-QCL) placed within an external feedback cavity are described in this publication. The critical feedback parameter becomes the mirror angle relative to the BA-QCL facet. With judicious angle choices, a plethora of curious modes can be created, each with their particular threshold and slope efficiency. These range from a nearly single far-field intensity peak to highly multimode emission similar to their diode counterparts. Dynamics are strongly dominated by transverse mode competition ranging for less than 20MHz to greater than 100MHz. When the mirror is parallel to the facet, higher frequency external cavity oscillations become undamped.

Frequency noise suppression of optical injection-locked quantum cascade lasers

This work theoretically investigates the frequency noise (FN) characteristics of quantum cascade lasers subject to the optical injection through a set of coupled rate equations with Langevin noise sources. It is shown that the low-frequency FN is completely suppressed by the optical injection, and the suppression bandwidth increases with the increasing injection ratio. The optimal FN peak suppression ratio at an injection ratio of 10 dB reaches 2.9 dB. In addition, it is found that the optical injection at positive frequency detunings close to the locking boundary invokes an additional peak in the FN spectrum, which can be higher than the carrier noise-induced one of free-running lasers. This peak amplitude strongly depends on the value of the linewidth broadening factor. Unlike injection-locked interband lasers, the FN peak does not necessarily exhibit a resonance.

Rate equation modeling of the frequency noise and the intrinsic spectral linewidth in quantum cascade lasers

This work theoretically investigates the frequency noise (FN) characteristics of quantum cascade lasers (QCLs) through a three-level rate equation model, which takes into account both the carrier noise and the spontaneous emission noise through the Langevin approach. It is found that the power spectral density of the FN exhibits a broad peak due to the carrier noise induced carrier variation in the upper laser level, which is enhanced by the stimulated emission process. The peak amplitude is strongly dependent on the gain stage number and the linewidth broadening factor. In addition, an analytical formula of the intrinsic spectral linewidth of QCLs is derived based on the FN analysis. It is demonstrated that the laser linewidth can be narrowed by reducing the gain coefficient and/or accelerating the carrier scattering rates of the upper and the lower laser levels.

Multimode optical feedback dynamics in InAs/GaAs quantum dot lasers emitting exclusively on ground or excited states: transition from short- to long-delay regimes

The optical feedback dynamics of two multimode InAs/GaAs quantum dot lasers emitting exclusively on sole ground or excited lasing states is investigated. The transition from long- to short-delay regimes is analyzed, while the boundaries associated to the birth of periodic and chaotic oscillations are unveiled to be a function of the external cavity length. The results show that depending on the initial lasing state, different routes to chaos are observed. These results are of importance for the development of isolator-free transmitters in short-reach networks.

Comparison of optical feedback dynamics of InAs/GaAs quantum-dot lasers emitting solely on ground or excited states

We experimentally compare the dynamics of InAs/GaAs quantum dot lasers under optical feedback emitting exclusively on ground states (GSs) or excited states (ESs). By varying the feedback parameters and putting focus either on their short or long cavity regions, various periodic and chaotic oscillatory states are found. The GS laser is shown to be more resistant to feedback, benefiting from its strong relaxation oscillation damping. In contrast, the ES laser can easily be driven into complex dynamics. While the GS laser is of importance for the development of isolator-free transmitters, the ES laser is essential for applications taking advantages of chaos.

Beam shaping in high-power broad-area quantum cascade lasers using optical feedback

Broad-area quantum cascade lasers with high output powers are highly desirable sources for various applications including infrared countermeasures. However, such structures suffer from strongly deteriorated beam quality due to multimode behavior, diffraction of light and self-focusing. Quantum cascade lasers presenting high performances in terms of power and heat-load dissipation are reported and their response to a nonlinear control based on optical feedback is studied. Applying optical feedback enables to efficiently tailor its near-field beam profile. The different cavity modes are sequentially excited by shifting the feedback mirror angle. Further control of the near-field profile is demonstrated using spatial filtering. The impact of an inhomogeneous gain as well as the influence of the cavity width are investigated. Compared to existing technologies, that are complex and costly, beam shaping with optical feedback is a more flexible solution to obtain high-quality mid-infrared sources.

Contribution of off-resonant states to the phase noise of quantum dot lasers

The phase noise of quantum dot lasers is investigated theoretically by coupling the Langevin noise sources into the rate equations. The off-resonant populations in the excited state and in the carrier reservoir contribute to the phase noise of ground-state emission lasers through the phase-amplitude coupling effect. This effect arises from the optical-noise induced carrier fluctuations in the off-resonant states. In addition, the phase noise has low sensitivity to the carrier scattering rates.

Chaotic light at mid-infrared wavelength

The onset of nonlinear dynamics and chaos is evidenced in a mid-infrared distributed feedback quantum cascade laser both in the temporal and frequency domains. As opposed to the commonly observed route to chaos in semiconductor lasers, which involves undamping of the laser relaxation oscillations, quantum cascade lasers first exhibit regular self-pulsation at the external cavity frequency before entering into a chaotic low-frequency fluctuation regime. The bifurcation sequence, similar to that already observed in class A gas lasers under optical feedback, results from the fast carrier relaxation dynamics occurring in quantum cascade lasers, as confirmed by numerical simulations. Such chaotic behavior can impact various practical applications including spectroscopy, which requires stable single-mode operation. It also allows the development of novel mid-infrared high-power chaotic light sources, thus enabling secure free-space high bit-rate optical communications based on chaos synchronization.

Optically injected InAs/GaAs quantum dot laser for tunable photonic microwave generation

We present an experimental investigation on the period-one dynamics of an optically injected InAs/GaAs quantum dot laser as a photonic microwave source. It is shown that the microwave frequency of the quantum dot laser's period-one oscillation is continuously tunable through the adjustment of the frequency detuning. The microwave power is enhanced by increasing the injection strength providing that the operation is away from the Hopf bifurcation, whereas the second-harmonic distortion of the electrical signal is well reduced by increasing the detuning frequency. Both strong optical injection and high detuning frequency are favorable for obtaining a single sideband optical signal. In addition, particular period-one oscillation points of low sensitivity to the frequency detuning are found close to the Hopf bifurcation line.

Frequency-dependent linewidth enhancement factor of optical injection-locked quantum dot/dash lasers

Combining theoretical and experimental studies show that optical injection strongly changes the behavior of the linewidth enhancement factor (α(H)-factor) and the FM-to-AM indices ratio (FAIR) in quantum dash/dot semiconductor lasers. In contrast to solitary lasers, both the α(H)-factor and the FAIR at low-frequency modulation are reduced by optical injection. At high modulation frequency, however, the phase-amplitude coupling characteristics are little influenced by optical injection.

Self-referenced technique for monitoring and analysing the non-linear dynamics of semiconductor lasers

We propose in this paper a self-referenced method based on asynchronous sampling to monitor the waveform of periodic and quasi-periodic signals, with a low number of samples, typically 2¹⁴ or lower. It provides a high-resolution representation of the signal under test, representative of the analog intensity signal under test. Additionally, the proposed approach is robust to the timing jitter of the signal, as experimentally demonstrated. Such features enable the accurate display of periodic and quasi-periodic signals. The method is applied to the characterization of laser dynamics, such as time series and phase portrait of periodic nonlinear regimes in optically injected lasers.

Introduction to the issue on Physics and Applications of Laser Dynamics (IS-PALD 2013)

We introduce the Optics Express special issue of the 3rd symposium on Physics and Applications of Laser Dynamics (IS-PALD). This issue consists of expanded papers related to oral and poster presentations. Selected papers represent the best of IS-PALD 2013.

Linewidth enhancement factor in semiconductor lasers subject to various external optical feedback conditions

The linewidth enhancement factor α of a semiconductor laser under the influences of optical feedback with different feedback strengths, external cavity lengths, and feedback phases are studied both experimentally and theoretically. The value of α is determined from the minimum of the Hopf bifurcation curve when the laser is subject to both optical feedback and optical injection. In the experiment, a pellicle beamsplitter mounted on a PZT stage placed on a linear translation stage is used as the reflector, where the external cavity length can be adjusted continuously from the long cavity regime to the short cavity regime with phase accuracy. With a moderate feedback strength, α is found to increase as the feedback strength increases. Moreover, while α is insensitive to the feedback phase in the long cavity regime, it can be tuned continuously in the short cavity regime when varying the phase. A normalized variation range of 21.59% is obtained experimentally at an external cavity length of 1.5 cm, which can be further enhanced by shortening the external cavity. To the best of our knowledge, this is the first detailed study of α from the long to the short cavity regime in a semiconductor laser subject to optical feedback. More particularly, the continuous tuning of α under phase variation is demonstrated the first time.

Two-color multi-section quantum dot distributed feedback laser

A dual-wavelength emission source is realized by asymmetrically pumping a two-section quantum-dot distributed feedback laser. It is found that under asymmetric bias conditions, the powers between the ground-state and excited-state modes of the two-section device can be equalized, which is mainly attributed to the unique carrier dynamics of the quantum-dot gain medium. As a result, a two-color emission with an 8-THz frequency difference is realized that has potential as a compact THz source. It is also shown that the combination of significant inhomogeneous broadening and excited-state coupled mode operation allows the manipulation of the quantum-dot states through external optical stabilization.

Near-field distribution of optical transmission of periodic subwavelength holes in a metal film

Optical transmission of a two-dimensional array of subwavelength holes in a metal film has been numerically studied using a differential method. Transmission spectra have been calculated showing a significant increase of the transmission in certain spectral ranges corresponding to the excitation of the surface polariton Bloch waves on a metal surface with a periodic hole structure. Under the enhanced transmission conditions, the near-field distribution of the transmitted light reveals an intensity enhancement greater than 2 orders of magnitude in localized ( approximately 40 nm) spots resulting from the interference of the surface polaritons Bragg scattered by the holes in an array.