Fast Gain Dynamics Research Articles

Anti-correlation phenomena in quantum cascade laser frequency combs

In quantum cascade laser frequency combs, the intensity distribution of the optical spectrum can be split into two well-separated lobes of longitudinal modes that, even when far apart, have a common phase relation and preserve equal frequency separation. The temporal dynamics of two lasers emitting at 4.4 and 8.1 µm operating in this bilobed regime are here investigated. The laser intensity shows a peculiar temporal behavior associated with the spectral features whereby, every half a round-trip, the total emitted power switches from one lobe to the other, with a perfect temporal anti-correlation. The anti-correlation between the lobes is also observed in the intensity noise figure of the emission. This coherent phenomenon arises from gain nonlinearities induced by spatial hole burning and the extremely fast gain dynamics typical of quantum cascade lasers.

Laser Frequency Combs with Fast Gain Recovery: Physics and Applications

AbstractThe dynamics of semiconductor lasers with fast gain is generally considered to suppress amplitude modulation, resulting in a frequency‐modulated output. Since their first demonstration of frequency comb operation, quantum cascade lasers—ideal representatives of this class of lasers with fast gain dynamics—were indeed shown to emit frequency‐modulated combs. The understanding of these frequency comb regimes has progressively evolved, being refined from simpler arguments to more advanced theories, considering the role of Kerr nonlinearity and linewidth enhancement factor. By now, the traditional frequency‐modulated combs are rather well understood in quantum cascade lasers, but new types of frequency combs have emerged, reopening interesting physics questions in these lasers. Recent advancements on two open frontiers will be discussed: harmonic frequency combs, characterized by mode skipping of many free spectral ranges, and spatiotemporal structures with solitonic features, recently reported in unidirectional ring cavities. These new types of frequency combs open technological and fundamental perspectives, sprouting new interest in quantum cascade lasers.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection (Laser Photonics Rev. 15(12)/2021)

Quantum Cascade lasers In article number 2100242, Barbara Schneider and co-workers present strong microwave injection in quantum cascade laser frequency combs as a means for controlling the lasing state. Varying modulation frequency and power enables us to broaden and shift the spectral output as well as modify the phase profile. Moreover, the fast gain dynamics enable rapid switching between separate spectral regions with just a single device.

Femtosecond pulses from a mid-infrared quantum cascade laser.

The quantum cascade laser has evolved to be a compact, powerful source of coherent mid-infrared light; however, its fast gain dynamics strongly restricts the formation of ultrashort pulses. As such, the shortest pulses reported so far were limited to a few picoseconds with some hundreds of milliwatts of peak power, strongly narrowing their applicability for time-resolved and nonlinear experiments. Here we demonstrate an approach capable of producing near-transform-limited subpicosecond pulses with several watts of peak power. Starting from a frequency-modulated phase-locked state, ultrashort high-peak-power pulses are generated via spectral filtering, gain modulation-induced spectral broadening and external pulse compression. We assess their temporal nature by means of a novel asynchronous sampling method, coherent beat note interferometry and interferometric autocorrelation. These results open new pathways for nonlinear physics in the mid-infrared.

Generalized Haus master equation model for mode-locked class-B lasers.

Using an asymptotic technique, we develop a generalized version of the class-B Haus partial differential equation mode-locking model that accounts for both the slow gain response to the averaged value of the field intensity and the fast gain dynamics on the scale comparable to the pulse duration. We show that unlike the conventional class-B Haus mode-locked model, our model is able to describe not only Q-switched instability of the fundamental mode-locked regime but also the leading edge instability leading to harmonic mode-locked regimes with the increase of the pump power.

High-Power Sub-300-Femtosecond Quantum Dot Semiconductor Disk Lasers

Self-assembled quantum dots (QDs) as active media for ultrafast semiconductor disk laser offer large gain bandwidths, fast gain dynamics, and high temperature stability. We report on the shortest pulses and the highest pulse peak power from an optically pumped vertical external cavity surface emitting laser (VECSEL) based on QDs and optimized for passive modelocking at 1035 nm using a semiconductor saturable absorber mirror. We demonstrate 216-fs pulses with an average output power of 269 mW at a pulse repetition rate of 2.77 GHz and 396 W peak power. At a lower pulse repetition rate of 1.67 GHz, we achieve 193-fs pulses with 112 mW of average output power. We remark a higher optical-to-optical pump efficiency compared to our previous QW VECSELs in the sub-300-fs regime. This is further confirmed by a comparative analysis of the saturation recovery which reveals longer carrier lifetimes for the QD compared to QW VECSELs.

Bringing High-Performance GaInNAsSb/GaAs SOAs to True Data Applications

We experimentally demonstrate the high-speed data processing capabilities of a GaInNAsSb semiconductor optical amplifier operating at $1.55~\mu \text{m}$ . The investigated structure exhibits good thermal characteristics and fast gain dynamics with 10%–90% recovery time of 55 ps. Successful wavelength conversion of 10-Gb/s signals is reported. A maximum power penalty of <2.4 dB for return to zero formatting and of 1.9 dB for nonreturn to zero is demonstrated.

Mode-locked semiconductor disk lasers with weakly saturated absorbers

The effect of absorber saturation on pulse shaping was studied both numerically and experimentally in a semiconductor disk laser. It was found that steady-state mode locking can be achieved even with very weak saturation of the absorber when the pulse energy is well below the saturation energy of absorption. The study discloses for the first time the direct impact of absorber modulation depth on the pulse width. Numerical analysis confirms that partial bleaching of absorption is adequate for formation of stable mode locking in SDLs. The fast gain dynamics of the semiconductor medium approach the dynamics of a slow gain medium at low pulse energies and high repetition rates with a pulse period much shorter than the gain recovery time. The presented results are of practical importance for multigigahertz repetition rate lasers, indicating that sufficient pulse shaping can occur when the saturable absorber is not fully bleached.

80 Gb/s wavelength conversion using a quantum-dot semiconductor optical amplifier and optical filtering

Wavelength conversion of 40 Gb/s and 80 Gb/s return-to-zero on-off-keying signals using a quantum-dot semiconductor optical amplifier in combination with a delay interferometer as subsequent filter is demonstrated. The performance of the 80 Gb/s wavelength converter measured in terms of the bit-error ratio demonstrated here is the highest reported up to now for quantum-dot semiconductor optical amplifiers. The typical fast gain dynamics manifests itself in open eye diagrams of the converted signal. The slow phase dynamics of the carrier reservoir however induces severe patterning and requires compensation. Adaptation of the free-spectral range of the delay interferometer is necessary in order to mitigate these phase effects and to achieve error-free wavelength conversion.

SIGNAL PROCESSING IN PHOTONIC CRYSTALS AND NANOSTRUCTURES

Optical devices employing photonic crystals and novel nanostructure materials may exhibit useful properties for applications to all-optical signal processing. In this work we analyze as a first example four-wave mixing of polarized beams in photonic crystal fibers. We show that by properly tuning the pump wavelength and the linear dispersion properties of the fiber one may obtain broadband parametric amplification and frequency conversion. Next we consider the in-line periodic amplification of short optical pulses by means of quantum-dot semiconductor optical amplifiers. We show by numerical simulations that pattern-free amplification of a 40 Gbit/s soliton signal at 1300 nm is possible without any inter-symbol interference or nonlinear pulse distortion caused by the fast gain dynamics.

External cavity multiwavelength semiconductor mode-locked lasers gain dynamics

The gain dynamics of a semiconductor optical amplifier (SOA) were measured using pump-probe techniques for the amplification of 750 fs pulses, 6.5 ps pulses and multiwavelength pulses, obtained from an external cavity semiconductor mode-locked laser. Furthermore, the intracavity gain dynamics of an external cavity semiconductor mode-locked laser was measured under multiwavelength operation. The experimental results show how the inherent chirp in pulses from external cavity semiconductor mode-locked lasers result in a slow gain depletion without significant fast gain dynamics. This mitigates gain competition between wavelength channels and nonlinearities in the gain media (SOA), enabling the multiwavelength operation of external cavity semiconductor mode-locked lasers. Numerical simulations support the experimental results.

Investigation of semiconductor optical amplifier integrated with DBR laser for high saturation power and fast gain dynamics

A detailed theoretical investigation of an integration of a semiconductor optical amplifier (SOA) and distributed Bragg reflector (DBR) laser is presented. The dependence of the device performance on those key design parameters such as the lasing wavelength, light injection direction (co- and contra-propagation), lasing power of the DBR laser, and the biasing condition of the SOA is examined systematically by means of a comprehensive time-domain traveling-wave model. As this integrated structure is particularly designed for high saturation power and fast gain dynamics, these characteristics are simulated and compared with the results from the conventional structures. Depending on different requirements, superior performance on either saturation power or noise figure without compromise on the optical gain can be achieved by different integration configurations (i.e., by different light injection directions). For the structure with the light injection from the output end of the SOA (namely, the integrated SOA-laser structure), the fast gain dynamics is found through simulation, which helps to reduce the large-signal waveform distortion in the amplification of narrow pulses.

Clock Recovery by a Fiber Ring Laser Employing a Linear Optical Amplifier

We report on subharmonic optical clock recovery (OCR) at 10 GHz from 160-Gb/s optical time-division multiplexed signals. The OCR circuit is based on a passively mode-locked principle in a fiber ring laser that utilizes fast gain dynamics of a linear semiconductor optical amplifier. To decrease the jitter amount in the clock pulse considerably, postelectrical signal processing is performed. The recovered clock is a 1.8-ps 10-GHz pulse train with 0.37 pulsewidth-bandwidth product.

Monolithic colliding pulse mode-locked semiconductor lasers

In this paper we investigate the temporal and spectral properties of monolithic colliding pulse mode-locked (CPM) quantum-well lasers. A large-signal model for the CPM laser is derived, which explains the mode-locking behaviour of both passively and hybridly mode-locked CPM lasers. The pulse-shaping and wavelength chirp of the CPM pulses are found to be the result of a complicated interplay between the gain and saturable absorber dynamics. The experimentally measured pulse width dependence upon variation of gain current and applied reverse bias on the saturable absorber is explained by fast gain dynamics. The experimentally observed broadening of the optical power spectrum with gain current is found mainly to be due to the self-phase modulation in the gain sections.