Gain-switched Semiconductor Laser Research Articles

Electrically smoothing gain-switched optical pulses from a semiconductor laser diode.

The typical optical pulse from a gain-switched semiconductor laser diode (LD) usually consists of a first-spike (FS) component and a quasi-steady-state (QSS) lasing component. For the stability and accuracy in some specific applications of sensing and detection, it is necessary to achieve a smooth QSS component without the FS component (regarded as spike noise). This Letter reports a technique to smooth the optical pulse shape from gain-switched LDs via stepped electric pulse, which can eliminate or suppress the FS component effectively, without any postprocessing. Rate-equation calculations well reproduced the major features of the experimental results and revealed that the pre-pump of the stepped electrical pulse plays a crucial role by adjusting the accumulated carrier density to be close to threshold before lasing in the LD, which suppresses the FS generation during the main pump injects and allows LD transit more rapidly into the QSS mode. The stepped electrical pulse pump provides a feasible and convenient method to smooth the optical pulse shape of gain-switched semiconductor LDs for various applications.

Experimental demonstration of the time-dependent source side-channel and countermeasures in polarization-based BB84 QKD systems

High-speed is one development tendency of the practical and commercial quantum key distribution (QKD) systems, and the gain-switched semiconductor laser (GSSL) and Sagnac interferometer have been popular components for high-speed commercial polarization-based BB84 QKD systems thanks to their stability, compactness and low cost. However, due to the finite extinction ratio (ER) of the GSSL, the time-dependent source side-channel, which is theoretically presented in [Phys. Rev. A 106(6): 062618 (2022)10.1103/PhysRevA.106.062618], also exists in these QKD systems. In this article, we experimentally investigate this side-channel in the polarization-based BB84 QKD system. The results show an obvious correlation in the output polarization states between the weak leakage light and its adjacent pulses, which will cause information to leak. More importantly, we have proposed several countermeasures, retested the side channel, and developed a mathematical framework to quantitatively assess the impact of the time-dependent side channel and these countermeasures on the secure key rate. Finally, we offer a trade-off analysis that considers defense effectiveness, pulse impact, and cost for various countermeasures in QKD. This provides a practical recommendation for improving the security of QKD systems.

Low-cost dual-comb spectrometer for CO2 monitoring based on gain-switched semiconductor lasers

Dual-comb spectroscopy (DCS) is an emerging technique that has attracted the attention of the research community due to its fast data acquisition and high-spectral resolution properties. Despite its versatility and mechanical stability, the complexity and cost of high performance DCS are still experimentally challenging. We propose a simple and compact DCS system that significantly reduces system costs by incorporating three low-cost key components: direct digital synthesizers and step recovery diodes in the signal generator to operate the lasers under gain-switching regime, and a software-defined radio as a digitizer. The viability of this configuration is demonstrated by performing DCS measurements of carbon dioxide (CO2), with a standard deviation of residual error of only ∼ 0.01. These results will pave the way to the commercial availability of compact, high-resolution DCS systems.

Phase Shifts in Gain-Switched Semiconductor Laser Subharmonic Pulse Trains

Phase Shifts in Gain-Switched Semiconductor Laser Subharmonic Pulse Trains

Extreme pulses in gain-switched semiconductor lasers.

Semiconductor lasers subjected to strong current modulation produce gain-switched optical pulse trains. These lasers can also produce pulse trains at sub-harmonic repetition rates relative to the driving current modulation. We experimentally observe, and numerically model, that these pulse trains can be interrupted by single-cycle extreme pulses whose characteristics and statistics are similar to rogue waves. Modeling indicates that drops in the circulating optical power in the optical cavity precede the appearance of extreme pulses. At the single photon level, the stochastic source terms in the optical field equation dominate the circulating optical power.

Divergence of the Variance of the Optical Phase in Gain-Switched Semiconductor Lasers Described by Stochastic Rate Equations

We report a theoretical study of the phase diffusion in a gain-switched single-mode semiconductor laser. We use stochastic rate equations for the electric field to analyze the phase statistics of the gain-switched laser. Their use avoids the instabilities obtained with rate equations for the photon number and optical phase when the photon number is small. However, we show that a new problem appears when the field equations are integrated: the variance of the optical phase becomes divergent. This divergence cannot be observed with the numerical integration of the commonly used equations for the photon number and optical phase because of the previous instabilities. The divergence of the phase variance means that this quantity does not reach a fixed value as the integration time step is decreased. We find that the phase variance increases as the integration time step decreases, with no sign of saturation behavior even for tiny steps. We explain the divergence by making the analogy of our problem with two-dimensional Brownian motion. The fact that the divergence appears is not surprising because in 1940 Paul L\`evy demonstrated that the variance of the polar angle in two-dimensional Brownian motion is a divergent quantity. Our results show that stochastic rate equations for the photon number and phase are not appropriate for describing the phase statistics when the photon number is small. Simulation of the stochastic rate equations for the electric field are consistent with L\`evy's results but gives unphysical results since an infinite value is obtained for a quantity that can be measured.

Microresonator-based spectral translation of a gain-switched semiconductor laser comb.

Gain-switched semiconductor laser technology provides a simple and low-cost method to generate optical frequency combs. However, the spectral coverage of these compact comb sources has been limited to the near-infrared range. Here, we combine a gain-switched laser comb with a continuous-wave translation laser within a periodically poled lithium niobate microresonator and demonstrate efficient and broadband sum-frequency conversion, spectrally translating the near-infrared comb to the visible domain. The broadband nature of the nonlinear conversion arises from a chirping of the domain inversion grating period along the microresonator circumference. We also validate the coherence of the visible-wavelength comb teeth which underlines the general applicability of this spectral translation approach.

High-density and broad band optical frequency combs generated by pseudo-random phase modulation of optically injected gain-switched semiconductor lasers

We have demonstrated optical frequency combs (OFCs) featuring low and continuously tunable line spacing in the kilohertz range. The OFCs are generated by pulsed gain-switching of optically injected commercial semiconductor lasers emitting at around 1.55 μm. After generation, the OFCs are efficiently densified by pseudo-random-binary-sequence phase modulation. The case of an OFC generated by gain-switching at a repetition rate of 1 GHz has been numerically and experimentally analyzed and the conditions for optimal densification have been found. In this case, the comb has been densified by a factor of 2047 and exhibits a 90 GHz flat and broad spectrum consisting of more than 184,000 spectral lines, separated 488 kHz. Although this densification factor is not the largest that can be achieved with the method, it is more than 100 times larger than previously reported factors for gain-switching OFCs. These results open prospects for the use of potentially integrable gain-switching OFCs in applications requiring sub-megahertz line spacing.

Dynamics of 2nd quantized state laser oscillation in gain-switched quantum-well semiconductor lasers

The dynamics of second-quantized-state laser oscillation were investigated for semiconductor laser diodes with quantum-well structures inside. We found that the second-quantized state often dominates laser oscillation instead of the first-quantized state under intensive pulse excitation, while the DC bias superposition tends to suppress the second-quantized-state oscillation. The operation characteristics were studied in detail through experimental studies and numerical calculations.

Random-modulated pulse lidar using a gain-switched semiconductor laser with a delayed self-homodyne interferometer.

We propose the generation of random-modulated pulses using a gain-switched semiconductor laser with a delayed self-homodyne interferometer (DSHI) for lidar applications. By emitting non-repetitive random-modulated pulses, ambiguity in ranging and interference in detection can be mitigated. When gain-switched, the wavelength of the laser fluctuates abruptly at the beginning of the pulse and then drops until it stabilizes toward its continuous-wave (CW) state. By beating the two pulses with instantaneous frequency detuning from the DSHI, pulses consisting of random and down-chirped modulations can be generated without any complex code generation and modulation. In this study, we investigate the waveforms and spectra of the random-modulated pulses generated under various homodyne delay lengths, switching currents, and pulsewidths. We characterize their signal-to-noise ratio (SNR), precision, and cross-correlation between consecutive pulses to evaluate their performance in lidar applications. For a good SNR of over 12 dB, the generated pulses have an optimal precision of approximately 1 mm in ranging, which is substantially better than the chaos-modulated pulses generated based on laser feedback dynamics. By establishing a random-modulated pulse lidar based on the proposed gain-switched homodyne scheme, we successfully demonstrate 3D imaging and profiling with good precision.

Spontaneous emission rate and phase diffusion in gain-switched laser diodes

Quantum random number generation (QRNG) has become a topic of growing interest in recent years due to important applications in cryptography and simulations. Interferometric detection of phase diffusion in gain-switched single-mode semiconductor lasers is one of the main generation techniques. In this paper, we study experimentally and theoretically the phase diffusion in gain-switched discrete mode laser diodes. We derive a stochastic rate equations model for the laser electric field that avoids numerical instabilities that appear when simulating amplitude and phase equations. Measurements are performed in order to extract the parameters of our semiconductor laser. Spontaneous emission rate coupled into the lasing mode is measured as a function of the carrier number for bias currents below threshold. A quadratic dependence is obtained that permits us to evaluate the validity of the linear approximations that have been used to describe laser phase diffusion in QRNG. The good agreement between experiments and theory permits us to give a realistic quantitative description of the dynamical evolution of the phase statistics in this type of QRNGs.

Gain-switched dual frequency comb at 2 µm.

This article shows a dual frequency comb in the 2 µm wavelength region using mutually injection locked gain-switched semiconductor lasers. Strained InGaAs multi-quantum-well discrete mode lasers and gain switching were used to generate two optical frequency combs with repetition rates of 2 GHz and 2.0001 GHz respectively, centred at 2.002 µm. Each optical comb spanned approximately 100 GHz. Through mutual injection locking to an edge comb line common in both combs, a phase locked dual frequency comb was demonstrated with 44 beating tones unique to single comb line pair interactions. This scheme allows for the comb information to be compressed into a 5 MHz detection bandwidth and captured with millisecond acquisition times, which could be of benefit to a number of sensing applications.

Progress in Gain-Switched Semiconductor Lasers for Quantum Communication

Progress in Gain-Switched Semiconductor Lasers for Quantum Communication

Phase diffusion in gain-switched semiconductor lasers for quantum random number generation.

In this paper, we report a theoretical and experimental study of the phase diffusion in a gain-switched single-mode semiconductor laser. Our modelling includes nonlinear dependencies of the carrier recombination rate and the semiconductor material gain on the carrier number. Also a comparison with the results obtained with linear models is performed. We focus on the below threshold operation, that is, the regime where most of the phase diffusion occurs in quantum random number generators based on the above mentioned system. Using the extracted parameters for our laser and simple analytical expressions for the laser linewidth, we show that logarithmic material gain and cubic carrier recombination dependence on the carrier number must be considered for a good agreement between experiments and theory. In this way we show that consideration of these nonlinearities is essential for obtaining a good quantitative description of the phase diffusion when using rate equations modelling.

Statistics of the Optical Phase of a Gain-Switched Semiconductor Laser for Fast Quantum Randomness Generation

The statistics of the optical phase of the light emitted by a semiconductor laser diode when subject to periodic modulation of the applied bias current are theoretically analyzed. Numerical simulations of the stochastic rate equations describing the previous system are performed to describe the temporal dependence of the phase statistics. These simulations are performed by considering two cases corresponding to random and deterministic initial conditions. In contrast to the Gaussian character of the phase that has been assumed in previous works, we show that the phase is not distributed as a Gaussian during the initial stages of evolution. We characterize the time it takes the phase to become Gaussian by calculating the dynamical evolution of the kurtosis coefficient of the phase. We show that, under the typical gain-switching with square-wave modulation used for quantum random number generation, quantity is in the ns time scale; that corresponds to the time it takes the system to lose the memory of the distribution of the initial conditions. We compare the standard deviation of the phase obtained with random and deterministic initial conditions to show that their differences become more important as the modulation speed is increased.

Influence of Chirp, Jitter, and Relaxation Oscillations on Probabilistic Properties of Laser Pulse Interference

Interference of laser pulses is an essential ingredient of quantum randomness; therefore, probabilistic properties of laser pulses gains new relevance. Here, we consider in detail the combined influence of the three effects – chirp, jitter and relaxation oscillations – on the probability density function of the interference of pulses from a gain-switched semiconductor laser. We develop a rigorous model based on laser rate equations and demonstrate that only consideration of all these three effects together allows describing the interference statistics properly. We supplement our theoretical calculations with corresponding measurements at various pump currents. Experimental results demonstrate perfect agreement with predictions of the model and are well reproduced by Monte-Carlo simulations.

Dual-Comb Spectrometer Based on Gain-Switched Semiconductor Lasers and a Low-Cost Software-Defined Radio

Dual-comb spectroscopy has become a topic of growing interest in recent years due to the advantages it offers in terms of frequency resolution, accuracy, acquisition speed, and signal-to-noise ratio, with respect to other existing spectroscopic techniques. In addition, its characteristic of mapping the optical frequencies into radio-frequency ranges opens up the possibility of using non-demanding digitizers. In this paper, we show that a low-cost software defined radio platform can be used as a receiver to obtain such signals accurately using a dual-comb spectrometer based on gain-switched semiconductor lasers. We compare its performance with that of a real-time digital oscilloscope, finding similar results for both digitizers. We measure an absorption line of a H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> N cell and obtain that for an integration time of 1 s, the deviation obtained between the experimental data and the Voigt profile fitted to these data is around 0.97% using the low-cost digitizer while it is around 0.84% when using the high-end digitizer. The use of both technologies, semiconductor lasers and low-cost software defined radio platforms, can pave the way towards the development of cost-efficient dual-comb spectrometers.

Gain-switched semiconductor lasers with pulsed excitation and optical injection for dual-comb spectroscopy.

In this work we demonstrate the capability of two gain-switched optically injected semiconductor lasers to perform high-resolution dual-comb spectroscopy. The use of low duty cycle pulse trains to gain switch the lasers, combined with optical injection, allows us to obtain flat-topped optical frequency combs with 350 optical lines (within 10 dB) spaced by 100 MHz. These frequency combs significantly improve the spectral resolution reported so far on dual-comb spectroscopy with gain-switched laser diodes. We evaluate the performance of our system by measuring the transmission profile of an absorption line of H13CN at the C-band, analyzing the attainable signal-to-noise ratio for a range of averaging times.

Linewidth Sharpening in Optical Frequency Combs via a Gain Switched Semiconductor Laser With External Optical Feedback

In this article, we propose a novel scheme for an optical frequency comb (OFC) generator which implements a gain switched (GS) semiconductor laser (SL) in the presence of external optical feedback (EOF). Numerical results demonstrate that with appropriate choices of the basic system parameters (i.e., the feedback coupling fraction, the laser bias current and the external cavity length), the linewidths in the OFCs generated by the GS SL with EOF can be significantly reduced below that of the free running continuous wave (CW) SL. Such a significant linewidth reduction can be described by a simple analytic expression presented in this article which is found to be in good agreement with the numerical results.

Generations of chaos-modulated pulses based on a gain-switched semiconductor laser subject to delay-synchronized optical feedback for pulsed chaos lidar applications.

We generate and analyze chaos-modulated pulses based on a gain-switched semiconductor laser subject to delay-synchronized optical feedback for pulsed chaos lidar applications. Benefited by the aperiodic and uncorrelated chaos waveforms, chaos lidar possesses the advantages of no range ambiguity and immunity to interference and jamming. To improve the detection range while in compliance with the eye-safe regulation, generating chaos-modulated pulses with higher peak power rather than chaos in its CW form is desired. While using an acousto-optic modulator to time-gate the CW chaos into pulses could be lossy and energy inefficient, in this paper, we study the generation of chaos-modulated pulses using a gain-switched laser subject to delay-synchronized optical feedback. Under different feedback strengths and modulation currents of gain-switching, we investigate the quality of the chaos-modulated pulses generated by analyzing their ratio of chaos oscillations, peak sidelobe levels (PSLs), and cross-correlation peaks under different mismatching conditions between the pulse repetition interval (PRI) and the feedback time delay τ. With proper feedback strengths and modulation currents, we find that synchronizing the gain-switching modulation with the delayed feedback (PRI = τ) is essential in generating the chaos-modulated pulses suitable for the pulsed chaos lidar applications. When mismatching occurs, we identify sequences of dynamical periods including stable, periodic, and chaos oscillations evolved within a pulse.