AM and high-harmonic FM laser mode locking

We demonstrate a new technique of active mode locking that combines amplitude-modulated (AM) mode locking at the cavity fundamental repetition rate with frequency-modulated (FM) mode locking at a high harmonic. This method combines the advantages of pulse shortening by high-harmonic mode locking while preserving the higher peak powers available at the fundamental repetition rate. We demonstrate this technique using a Nd:YAG laser that is simultaneously AM mode locked at 80 MHz and FM mode locked at the 22nd harmonic (1.76 GHz). Pulses as short as 16 ps with a peak power of 6.25 kW were measured.     

太赫兹量子级联激光器波导数值模拟

采用传输矩阵法(TMM)计算比较了太赫兹量子级联激光器两种波导限制方式(单面等离子波导和双面金属波导)的特点.结果表明双面金属波导的限制因子较单面等离子波导的限制因子有明显的提高,具有良好的模式限制作用,可以有效地降低波导层的厚度.     

RF linewidth in passively mode locked quantum well lasers

Investigation of the stability of pulse repetition rate emitted with semiconductor lasers in the regime of passive mode locking has been conducted. It was shown experimentally that the decrease of an overlap integral of the quantum-sized active layer with the waveguide mode and the increase of the time of the carrier capture on the emitting level resulted in narrowing of the radio-frequency linewidth.     

Simulation of double quantum well GalnNAs laser diodes

The simulation of double quantum well (QW) GalnNAs ridge-waveguide (RW) lasers is performed over a wide range of cavity lengths and operating temperatures using a comprehensive in-house 2D laser simulator that takes into account all of the major device physics, including current spreading, capture escape processes, drift diffusion in the QW, 2D optical modes and fully resolved lasing spectra. The gain data used by the simulator were fitted to experimental gain spectra measured by the segmented contact method. The gain model includes the band-anticrossing model for the conduction band and a 4 x 4 kldrp model for the valence band. Using a carrier density-dependent and temperature-dependent linewidth broadening parameter, a good fit with experiment over a temperature range of 300-350 K was obtained. A Shockley-Read-Hall (SRH) lifetime of 0.5 ns and an Auger recombination coefficient of 1 x 10^-28 cm⁶/ s, were extracted from the calibration of the laser simulator to experimental device characteristics of broad-area (BA) devices. Using the same set of parameters for BA devices, except for a reduced SRH lifetime of 0.45 ns underneath the etch, 2D simulation results were found to agree well with the measured RW laser operating characteristics. The impact of the various recombination processes in the RW laser at threshold has also been identified using the calibrated laser simulator.     

Effect of transient susceptibility on femtosecond pulse propagation in semiconductor quantum well waveguide

Ultra fast pulse evolution in a semiconductor quantum well structure (QWS) is theoretically analyzed. The polarization induced in the medium due to an incident Gaussian electromagnetic beam has been obtained using the semiconductor Bloch equations. The non-linear Schrödinger equation is used to analyze the effect of the induced polarization on the pulse. An interesting manifestation of the intensity dependence of the refractive index in non-linear media occurs through self-phase modulation (SPM), a phenomenon that leads to spectral broadening of the optical pulses. In doing the miniaturization of the device, we use semiconductor nanostructures in which the non-linearity is very large as compared to their bulk counterparts. Consequently, the phenomenon of SPM becomes significant at lower length scale leading to the limitations of the device. Numerical analysis was performed for a 150 fs Ti:sapphire laser radiation propagating along the transverse plane of a GaAs/AlGaAs QWS with realistic material parameters reveals asymmetric spectral broadening of the pulse due to SPM. The results agree qualitatively well with those available in the literature.     

Temperature stability of photoluminescence in heterostructures with InGaAs/GaAs quantum well and Mn-delta-doped acceptor layer in GaAs barrier

The temperature dependence of the electro- and photoluminescence of heterostructures with InGaAs/GaAs quantum well and a closely spaced manganese delta (δ)-doped layer in the GaAs barrier was investigated. It is found that the proposed heterostructures exhibit increased temperature stability and decreased temperature quenching as compared to the control structures containing no δ-doped layers. An increase in the operating temperature is explained by the appearance of an additional potential barrier for electrons due to the δ-doped acceptor layer formation in the near-surface barrier.     

Manifestation of active medium astigmatism at transverse mode locking in a diode end-pumped stable resonator laser

Transverse mode locking in a diode end-pumped Nd:YAG laser with up to approximately 140 cm resonator length was investigated. It was found that for each resonator degeneracy, there are two degenerate lengths where the fundamental mode is very different from the Gaussian mode. Fundamental mode intensity patterns for these lengths expand in directions perpendicular to each other. Experimental results are in a good agreement with numerical calculations, taking into account active medium (AM) astigmatism and inhomogeneous gain. Optical powers of astigmatic AM can be found directly from measurements of degenerate lengths without using numerical modeling.     

The vertical beam quality of GaInP/AlGaInP strained multiple quantum well laser

Abstract

The waveguided mode of the visible GaInP/AlGaInP compressive strained multiple quantum well laser is calculated by using transfer matrix method. On the basis of the nonparaxial vectorial moment theory of light beam propagation, the vertical (perpendicular to junction plane) optical beam quality factor M ²? of the waveguided mode is shown to be smaller than unity. This result may be useful for the design of semiconductor lasers and analysis of nonparaxial beam propagating characteristics.     

Ion-exchanged glass waveguides with low birefringence for a broad range of waveguide widths

Optical communications networks require integrated photonic components with negligible polarization dependence, which typically means that the waveguides must feature very low birefringence. Recent studies have shown that waveguides with low birefringence can be obtained, e.g., by use of silica-on-silicon waveguides or buried ion-exchanged glass waveguides. However, many integrated photonic circuits consist of waveguides with varying widths. Therefore low birefringence is consequently required for waveguides having different widths. This is a difficult task for most waveguide fabrication technologies. We present experimental results on waveguide birefringence for buried silver-sodium ion-exchanged glass waveguides. We show that the waveguide birefringence of the order of 10(-6) for waveguide mask opening widths ranging from 2 to 10 microm can be obtained by postprocessing the sample through annealing at an elevated temperature. The measured values are in agreement with the values calculated with our modeling software for ion-exchanged glass waveguides. This unique feature of ion-exchanged waveguides may be of significant importance in a wide variety of integrated photonic circuits requiring polarization-independent operation.     

Saturable Bragg reflector-based continuous-wave mode locking of Yb:KGd(WO 4) 2 laser

We demonstrate the saturable Bragg reflector (SBR)-based stable self-starting continuous-wave mode locking of a Yb:KGd(WO₄)₂ laser. With a double quantum well SBR structure the shortest pulses observed had durations of 169 fs, average powers of 18 mW and an oscillating wavelength centred on 1028 nm.     

Light propagation in a planar dielectric waveguide with a gyrotropic layer

The nature of the magneto-optic Kerr effect in a planar dielectric waveguide geometry has been investigated by calculation of the Jones matrix for a planar waveguide structure with a gyrotropic magnetic material as one wall. The intensity of the component of the field that is in the polarization state orthogonal to the input was calculated as a function of length of the gyrotropic material and input polarization state. The degree of polarization rotation depends on the relative orientation of the magnetization in the magnetic material and the direction of propagation. It is found that there exists an optimal waveguide length and input polarization at which the output signal is maximized and that a significant enhancement in polarization rotation is available with respect to free-space reflection. These results indicate that a magnetic-film-bounded planar waveguide can be used for device applications such as magnetic field sensors or magneto-optic modulators.     

Anisotropic photoluminescence characteristics of Al0.08Ga929292As single quantum well laser structure

A nominal well width (20 nm) of Al_0.08Ga_0.92As quantum well structure has been fabricated by molecular beam epitaxy technique with the aim of obtaining a lasing device. The temperature evolution of quantum well photoluminescence was studied in the range 10–300 K which shows excitons being trapped at the interfacial defects below 100 K. The linear polarization effects in the photoluminescence have been studied for the incident and collected light propagating parallel to the plane of the well layer. In a very careful study, the luminescence was found to be fully polarized for the incident electric vector parallel to well layers, while it showed depolarized behaviour for the incident electric vector perpendicular to the well layers. The earlier conclusions based on photoluminescence excitation and absorption studies of heavy- and light-hole emissions are supported. The 20 nm quantum well structure has been corroborated using scanning tunnelling microscopy.     

Mode locking in coherently driven laser systems

A new concept of mode locking for a three-level gain medium with two coherent fields is proposed. The strong field applied on the coupling transition produces broadening of the lasing transition. The broadened emission spectrum allows one to generate pulses with durations far beyond limits inherent to a conventional mode-locking technique.     

The surface mode of a dielectric waveguide with metal substrate

The conditions of excitation and the waveguide characteristics are determined in an analytical form for the TM₀ mode of a planar waveguide with metal substrate. This mode has a surface character, in contrast to other (bulk) modes excited in the system. The propagation and damping constants and the energy flux density distributions are compared for the TM₀ and TE₀ modes.     

Tunable wakefield waveguide structure with the possibility of mode selection

The possibility to control the frequency spectrum of Cherenkov radiation in a microwave wakefield dielectric waveguide with the aid of an external ferroelectric layer has been studied. Using the proposed multilayer waveguide structure in combination with a special configuration of control electrodes, it is possible both to adjust the wake field frequency spectrum and to attenuate the modes corresponding to beam-deflecting (defocusing) fields in the waveguide.     

Design of Stark-tuned far-infrared laser with circular hollow waveguide

A novel design of a Stark-tuned far-infrared laser using a circular hollow dielectric waveguide is proposed, and its characteristics are studied. Supplementary electrodes are inserted inside the circular hollow dielectric tube to suppress charge accumulation while keeping field uniformity. In what is believed to be a new design, the mode property is found to be improved, and the angular dependency of the attenuation loss according to the beam polarization is estimated to be much smaller than that of the conventional rectangular hybrid waveguide design. In this new design, DeltaM=0 far-infrared (FIR) transition as well as DeltaM=+/-1 transition can be observed, and the power enhancement for the DeltaM=0 FIR transition is expected.     

Influence of segregation in quantum well structures

The potential associated with quantum well (QW) structures is usually assumed to be a step function; this implies a compositional abruptness at each interface. But abrupt interfaces do not occur in practice, especially if one of the atoms segregates during growth. This leads to asymmetries in the QW potentials which could radically affect device performance.     

Photon storage with nanosecond switching in coupled quantum well nanostructures

Photon storage with nanosecond switching was implemented with indirect excitons in coupled quantum well nanostructures. The storage and release of photons was controlled by the gate voltage pulses. The nanosecond write and readout times were much shorter than the storage time, which reached microseconds. Furthermore, the presented control of excitons on a time scale much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps.     

Broadband wavelength conversion in hydrogenated amorphous silicon waveguide with silicon nitride layer

Broadband wavelength conversion based on degenerate four-wave mixing is theoretically investigated in a hydrogenated amorphous silicon (a-Si:H) waveguide with silicon nitride inter-cladding layer (a-Si:HN). We have found that enhancement of the non-linear effect of a-Si:H waveguide nitride intermediate layer facilitates broadband wavelength conversion. Conversion bandwidth of 490 nm and conversion efficiency of 11.4 dB were achieved in a numerical simulation of a 4 mm-long a-Si:HN waveguide under 1.55 μm continuous wave pumping. This broadband continuous-wave wavelength converter has potential applications in photonic networks, a type of readily manufactured low-cost highly integrated optical circuits.     

An active lasing region with a quantum well and a quantum dot array

A combined active lasing region of the new type, containing an In_0.2Ga_0.8As quantum well (QW) and a single-layer array of InAs quantum dots (QDs) located outside the QW, was studied. In this system, the QW accumulates the injected charge carriers and the QD array serves as a radiator. The energy levels of electrons and holes in a QD were calculated. It is shown that the QDs can be filled by the resonance tunneling of holes from the QW to an unoccupied QD. The electron energy level in an unoccupied QD is markedly higher than that in the QW, but occupation of the QD by a hole leads to a resonance of the electron levels. Theoretical conclusions agree with the results of observations on a prototype laser with a combined active region.