Increasing Pump Current Research Articles

Broadband chaos of an interband cascade laser with a 6-GHz bandwidth.

Near-infrared semiconductor lasers subject to optical feedback usually produce chaos with a broad bandwidth of a few GHz. However, the reported mid-infrared interband cascade lasers (ICLs) only show chaos with a limited bandwidth below 1 GHz. Here we show that an ICL with optical feedback is able to generate broadband chaos as well. The mid-infrared chaos exhibits a remarkable bandwidth of about 6 GHz, which is comparable to that of the near-infrared counterpart. In addition, the spectral coverage in the electrical domain reaches as high as 17.7 GHz. It is found that the chaos bandwidth generally broadens with increasing feedback ratio and/or increasing pump current of the laser, while it is insensitive to the feedback length.

Feedback-induced locking in semiconductor lasers with strong amplitude-phase coupling

The influence of optical feedback on semiconductor lasers has been a widely studied field of research due to fundamental interests as well as the optimization of optical data transmission and computing. Recent publications have shown that it is possible to induce a periodic pulsed like output in quantum-dot and quantum-well laser diodes utilizing the locking of the external cavity modes and the relaxation oscillation frequency. We present an in-depth analysis of this effect. We choose submonolayer quantum dots as a gain system, as these provide a relatively strong amplitude-phase coupling, which has proven to be very beneficial for these locking effects to occur. Introducing a new theoretical model we can correctly reproduce the essential features of the gain system and validate them by comparison to our experimental results. From this starting point we can further explore how the staircase behavior of the oscillation frequency with increasing pump current can be influenced by changing various laser parameters. The staircase behavior is induced by a reordering of the Hopf bifurcations giving birth to the regular pulsed-like oscillations.

Intensity Noise and Pulse Oscillations of an InAs/GaAs Quantum Dot Laser on Germanium

This paper investigates the intensity noise and pulse oscillation characteristics of an InAs/GaAs quantum dot laser epitaxially grown on germanium. We show that the relative intensity noise of the free-running laser generally decreases with increasing pump current, and the minimum value reaches down to about $-$ 126 dB/Hz. The intensity noise is hardly affected by the optical feedback, unless there is a resonance or pulse oscillation in the noise spectrum. The laser pumped at a high current is more sensitive to the optical feedback. Interestingly, it is found that the free-running Ge-based quantum dot laser generates self-sustained pulse oscillations with one period or two periods upon the pump current, without incorporating saturable absorbers. This behavior is valuable for both self-generation of photonic microwaves and for understanding nonlinear dynamics of semiconductor lasers.

Linewidth broadening factor of an interband cascade laser

This work reports the linewidth broadening factor (LBF) of a continuous-wave interband cascade laser operated both below and above threshold. Using the Hakki-Paoli method, the below-threshold LBFs around the gain peak are determined to be in the range of 1.1–1.4, where the thermal effect is carefully removed. Employing the self-mixing interferometry technique, the above-threshold LBFs are measured to be around 2.2, which do not increase with increasing pump current.

Ultranarrow-waveguide AlGaAs/GaAs/InGaAs lasers

We have designed, fabricated and studied ultranarrow-waveguide heterostructure lasers emitting in the spectral range 1000 – 1100 nm. The lasers have been characterised by current – voltage, light – current, far-field intensity distribution and internal optical loss measurements. The ultranarrow-waveguide lasers have been shown to have a threshold current density of ∼75 A cm−2, internal quantum efficiency near 100 % and internal optical loss near the lasing threshold under 1 cm−1, which corresponds to the level of standard heterostructures. We have demonstrated the possibility of obtaining up to 5 W of output power in continuous mode and up to 30 W in pulsed mode, with a beam convergence (FWHM) of 17.8°. The slope of the internal optical loss as a function of pump current for the ultranarrow-waveguide lasers can be markedly lower than that in lasers with a standard design, but internal quantum efficiency drops to 40 % with increasing pump current. The use of barrier layers in ultranarrow-waveguide lasers makes it possible to substantially reduce the drop in internal quantum efficiency.

Increase in the internal optical loss with increasing pump current and the output power of quantum well lasers

The operating characteristics of semiconductor quantum-well lasers, calculated with consideration for an increase in the internal optical loss in the waveguide region with increasing pump current, are presented. The condition of global electroneutrality in the structure is used. This condition consists in that the total charge of electrons in the 2D active region (quantum well) and bulk waveguide region (optical confinement layer) is equal to the total charge of holes in these two regions. Good agreement between the calculated and experimentally determined light–current characteristics is obtained.

On the problem of internal optical loss and current leakage in laser heterostructures based on AlGaInAs/InP solid solutions

Semiconductor lasers based on MOCVD-grown AlGaInAs/InP separate-confinement heterostructures are studied. It is shown that raising only the energy-gap width of AlGaInAs-waveguides without the introduction of additional barriers results in more pronounced current leakage into the cladding layers. It is found that the introduction of additional barrier layers at the waveguide–cladding-layer interface blocks current leakage into the cladding layers, but results in an increase in the internal optical loss with increasing pump current. It is experimentally demonstrated that the introduction of blocking layers makes it possible to obtain maximum values of the internal quantum efficiency of stimulated emission (92%) and continuouswave output optical power (3.2 W) in semiconductor lasers in the eye-safe wavelength range (1400–1600 nm).

Understanding Ground-State Quenching in Quantum-Dot Lasers

Quantum-dot lasers can exhibit simultaneous ground- and excited-state lasing. With increasing pump current, a quenching of the ground-state lasing intensity is sometimes observed. The causes for this are investigated, and its dependence on temperature, gain, and electron–hole asymmetry is studied via an analytical approach. A numerical model based on the semiconductor Bloch equations with a set of rate equations for electrons and holes is used for validation. We also investigate the influence of doping and different cavity lengths on the two-state lasing dynamics. We find that ground-state quenching is more common in p-doped, short cavity devices with low gain.

High-temperature luminescence in an n-GaSb/n-InGaAsSb/p-AlGaAsSb light-emitting heterostructure with a high potential barrier

The electroluminescent properties of an n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with a high potential barrier in the conduction band (large conduction-band offset) at the n-GaSb/n-InGaAsSb type-II heterointerface (ΔE c = 0.79 eV) are studied. Two bands with peaks at 0.28 and 0.64 eV at 300 K, associated with radiative recombination in n-InGaAsSb and n-GaSb, respectively, are observed in the electroluminescence (EL) spectrum. In the entire temperature range under study, T = 290–480 K, additional electron-hole pairs are formed in the n-InGaAsSb active region by impact ionization with hot electrons heated as a result of the conduction-band offset. These pairs contribute to radiative recombination, which leads to a nonlinear increase in the EL intensity and output optical power with increasing pump current. A superlinear increase in the emission power of the long-wavelength band is observed upon heating in the temperature range T = 290–345 K, and a linear increase is observed at T > 345 K. This work for the first time reports an increase in the emission power of a light-emitting diode structure with increasing temperature. It is shown that this rise is caused by a decrease in the threshold energy of the impact ionization due to narrowing of the band gap of the active region.

Zero-lag synchronization and bubbling in delay-coupled lasers

We show experimentally that two semiconductor lasers mutually coupled via a passive relay fiber loop exhibit chaos synchronization at zero lag, and study how this synchronized regime is lost as the lasers' pump currents are increased. We characterize the synchronization properties of the system with high temporal resolution in two different chaotic regimes, namely, low-frequency fluctuations and coherence collapse, identifying significant differences between them. In particular, a marked decrease in synchronization quality develops as the lasers enter the coherence collapse regime. Our high-resolution measurements allow us to establish that synchronization loss is associated with bubbling events, the frequency of which increases with increasing pump current.

Near-threshold spectral modes and coherence of a semiconductor laser and a diode-pumped neodymium laser

The spectral, coherence, and polarization characteristics of a semiconductor injection laser and a diode-pumped Nd laser operating on second harmonic have been thoroughly traced for the first time during the transition via the generation threshold and on the passage from one mode to another with increasing pump current. It is shown that the behavior of the intensity, coherence, and noise near the threshold is indicative of the possible analogy between laser generation and phase transitions of the first or second order.

Theory of photon-number squeezing in a heterojunction LED by the nonlinear backward pump process

We present a theoretical framework for sub-Poissonian-photon-state generation based on a microscopic backward-pump (BP) process in a semiconductor heterojunction light-emitting diode (LED). The model, which takes into account the BP and nonradiative recombination processes as well as the dynamical response of the external circuit, is applicable to practical LED's. It is shown that the nonlinear dependence of the BP rate on the carrier number in the active region brings about the photon-number squeezing below the full-shot-noise level, without recourse to a high-impedance constant-current source. We also point out that the bandwidth of the squeezing by the nonlinear BP process is wider than that limited by the recombination lifetime. In addition, we discuss the origin of the nonlinearity of the BP rate based on a microscopic model for the injected carriers in the active region. The nonlinearity is suggested to be due to the rises of the electron temperature and of energetic position of the quasi-Fermi level, with increasing pump current.

High single-mode power observed from a coupled-resonator vertical-cavity laser diode

We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits single fundamental-mode operation. The output powers are as high as 6.1 mW with side mode suppression ratios greater than 30 dB. The sizes of the implant and oxide current apertures are shown to be important for demonstrating the required selectivity for the fundamental lasing mode. With a fixed bias current on the implant cavity and increasing oxide cavity current, mode switching from single-mode operation to multimode operation and back to single-mode operation was observed. The intensities of the fundamental and first transverse modes were calculated by solving a set of multimode rate equations. The calculation indicates that the observed mode switching can be identified with changes in the optical length of the oxide cavity with increasing pump current. The observed mode dynamics are unique to coupled-resonator vertical-cavity lasers.

A coherent laser based on a two-well structure

The theory of steady-state single-mode oscillations in a coherent cascade laser that incorporates two quantum wells is developed. The power and frequency of oscillations are determined in relation to the coherent-pump current and the structure parameters. It is shown that the conditions for self-tuning also exist in a two-well structure, which ensures an efficient lasing and a linear increase in power with increasing pump current.

Polarization selective symmetry breaking in the near-fields of vertical cavity surface emitting lasers

We study symmetries in multi-transverse-mode near-fields of circular vertical cavity surface emitting lasers (VCSELs). The 0 ◦ polarized component of the near-field always has circular or high-order rotational symmetry, indicating that all significant optical properties of the laser are isotropic for light of that polarization. In contrast, the simultaneously present 90 ◦ polarized part of the same near-field is always symmetrical only upon reflection. This is evidenced by sequences of near-field images of increasing complexity for increasing pump current. The presence of a preferred direction of the symmetry axis is evidence for a symmetry-breaking anisotropy. We attribute this anisotropy to birefringence, which is induced into any electrically pumped VCSEL by the applied vertical electrical field via the linear electro-optic effect. Thus, the optical index becomes a function of the transverse component of the k-vector of light inside the cavity, which corresponds to an angular dependent index. As the functional dependence on the emission angle is of different strength for orthogonal polarizations, the circular symmetry of the laser is broken only for 90 ◦ polarized light, while the effect on the 0 ◦ polarization is too small to affect the near-field. The highly symmetrical near-field of the unaffected polarization shows remarkable similarities to another physical system of circular symmetry, the static patterns in cellular flames. Our analysis of the symmetry properties of the near-fields has implications for the design of VCSELs as well as for future modelling activities.

Influence of the backward-pump process on photon-number squeezing in a constant-current-driven heterojunction LED: Transition from thermionic emission to diffusion limits

Physical mechanisms which limit the squeezing bandwidth in a heterojunction light-emitting diode (LED) have been extensively studied both theoretically and experimentally. It is proven that our experimental results of pump-current dependence of the squeezing bandwidth in the constant-current-driven heterojunction LED at room temperature cannot be explained by previous theoretical predictions. We present a theoretical framework, including the effects of a microscopic backward-pump (BP) process, generally applicable to a heterojunction LED. Parameters describing the relative significance of the BP process are determined by the measurements of current-versus-voltage characteristic and differential resistance of the LED, independent of the noise measurements. As a consequence, the experimental results can be explained by our model in a unified manner over a whole range of injection current, and it is clarified that the pump situation of the LED moves continuously from thermionic emission to diffusion limits with increasing pump current.

Pump current modulated charge pump PLL

The performance of a charge pump PLL (CP-PLL) is strongly influenced by the pump current magnitude in the pump ON interval. An examination is presented of the effect of linearly increasing pump current on the transient response of a CP-PLL. It has been observed that the process of pump current ‘modulation’ may be used to enhance the transient behaviour of the CP-PLL.

Sub-Poissonian photon-states generated by light-emitting-diodes: Coulomb blockade of pump events and Stark-effect blockade of emission events

We review our experimental result on the generation of sub-Poissonian photon fluxes from high-speed light emitting diodes (LEDs) driven by high-impedance constant-current sources and present our new scheme on the basis of Stark-effect blockade of photon emissions. The experiments demonstrate a large noise-suppression of spontaneous photon stream, 3.1 dB below the standard quantum limit level at 80 K and increasing bandwidth (3–16 MHz) for the noise-suppression with increasing pump current (50 μA-5 mA) at room temperature. The latter has been clearly interpreted in terms of the distinctive contribution of collective Coulomb-blockade for pump events and of recombination dynamics to the bandwidth. The basic physics behind the proposed scheme is illustrated, theoretically demonstrating, as a consequence of the coexistence of the Stark-effectand Coulomb-blockade, 4.5 dB-noise-suppression to below the standard quantum limit level in a tailor-made diode driven by a constant voltage source.

Influence of excess carriers on the permittivity of GaAs at the frequency of radiative transitions in injection lasers

A change in the resonance frequencies of the Fabry-Perot resonator of a GaAs laser diode with increasing pump current was detected and investigated. The value of ∂∊/∂N (∊ is the permittivity and N is the density of free electrons) was 2.6×10 cm−20 cm3 when the current density was 15 kA/cm2, temperature was 300°K, and wavelength was about 901 nm.