Peter P. Vasil'ev

Fast phenomena in semiconductor lasers

Although diode lasers are almost ideal sources for ultrahigh-speed data communication systems, system performance remains critically dependent on the quality of the optical pulses that they generate. Uniquely among lasers, the output power can be modulated directly by modulating the diode current. However, this leads to relaxation oscillations and a roll-off at high frequencies that is superimposed on the frequency response of the drive circuit. This is limited by parasitics and maximum modulation frequencies range from 1 to 70?GHz, depending on the type of laser and its packaging. The higher values are obtained with short cavity lengths and tight optical confinement, the highest frequencies being achieved in vertical-cavity devices. Modulation bandwidth is usually limited by circuit parasitics, device heating and the maximum power-handling capability of the laser facets. The generation of picosecond and femtosecond pulses demands special techniques and three - gain switching, Q-switching and mode locking - are discussed in detail, with their relative advantages and disadvantages compared. Very short pulses inherently contain a significant spread of wavelengths and their generation requires the optical gain in the laser medium to extend across that wavelength range. While diode lasers satisfy this criterion better than many other types, the effect of gain non-linearities and carrier-transport effects prevent the Fourier-transform limit from being achieved in practice. As a result, external pulse-compression techniques, which exploit the detailed temporal and spectral properties of the laser pulses, such as frequency chirping, self-phase modulation and group velocity dispersion, are becoming more important, and diode lasers are increasingly challenged as primary sources by compact, efficient, diode-pumped solid-state lasers. The paper summarizes the levels of maximum pulse power and minimum duration that have been achieved using the various techniques.

Femtosecond superradiant emission in inorganic semiconductors

A review of an experimental study of superradiance in semiconductor inorganic structures is presented. It is demonstrated that unique properties of superradiant emission are determined by unusual properties of electrons and holes, namely, the formation of BCS-like state in a system of collectively paired electrons and holes. This can adequately explain all features of superradiance, including its femtosecond pulse duration, record peak power, optical spectrum, spatial and temporal coherency and macroscopically large fluctuations. The effect of non-equilibrium condensation of electrons and holes in the phase domain at room temperature is experimentally demonstrated. The critical temperature of condensation in a strongly degenerate system of electrons and holes is finally theoretically estimated.

Phase-conjugation Broad Area Twin-contact Semiconductor Laser

Experimental demonstration of lasing in a broad area twin-contact semiconductor laser which operates as a phase-conjugation (PC) mirror in an external cavity configuration is reported. This allows “self-aligned” and self-pumped spatially nondegenerate four-wave mixing to be achieved without the need for external optical signals. The external cavity laser system is very insensitive to tilt misalignments of the external mirror in the PC regime and exhibits very good mechanical stability. The resonant frequency of the external cavity lies in the GHz range which corresponds to a subnanosecond time response of phase conjugation processes in the semiconductor laser.

Mode locking in monolithic two-section InGaN blue-violet semiconductor lasers

Passive mode-locked pulses with repetition frequencies in the range 40 to 90 GHz were observed in blue-violet GaN-based quantum-well lasers without external cavities. The lasers had two-section geometry with built-in saturable absorber section. The individual pulses had durations as short as 3–5 ps at peak powers of around 320 mW.

Superradiant emission from a tapered quantum-dot semiconductor diode emitter

Superradiant emission pulses from a quantum-dot tapered device are generated on demand at repetition rates of up to 5 MHz. The pulses have durations as short as 320 fs at a wavelength of 1270 nm.

Superradiant Emission in Semiconductor Diode Laser Structures

This paper reviews recent advances in superradiant (SR) emission in semiconductors at room temperature, a process which has been shown to enable the generation on demand of high power picosecond or subpicosecond pulses across a range of different wavelengths. The different characteristic features of SR emission from semiconductor devices with bulk, quantum-well, and quantum-dot active regions are outlined, and particular emphasis is placed on comparing the characteristic features of SR with those of lasing. Finally, potential applications of SR pulses are discussed.

Graded-index polymer multimode waveguides for 100 Gb/s board-level data transmission

We report enhanced graded-index multimode polymer waveguides with >70GHz×m for MMF launch and >200GHz×m for restricted launch, indicating the capability of on-board waveguide transmission of >100 Gb/s. Simulations using the measured refractive index profile agree well with the experiments.

Long-range order in a high-density electron-hole system at room temperature during superradiant phase transition

We have experimentally investigated the formation of off-diagonal long-range order and non-equilibrium BEC-like condensation in GaAs/AlGaAs heterostructures during superradiant (SR) emission at room temperature. The conclusive evidence of the establishment of phase coherence over a macroscopic range during the superradiant quantum phase transition is reported. The first-order spatial correlation function of the e-h system is determined by evaluating interference patterns of SR emission using Youngs double slit.

Experimental study of delay-time statistics of superradiant pulses generated from semiconductor structures

We study the delay-time statistics of femtosecond pulses generated from GaAs/AlGaAs multiple contact heterostructures under the superradiant (SR) emission generation. The turn-on time of superradiant pulses was measured by a single-shot streak camera with a few picoseconds temporal resolution using low-jitter reference pulses. The fluctuations of the delay times of SR pulses were found to be by more than one order of magnitude larger as compared to standard lasing and a few times larger than for SR emission from other types of media.

Dicke superradiance in GaN quantum wells

We numerically modeled the spatio-temporal dynamics of Dicke superradiance in GaN/InGaN heterostructure quantum wells in a ridge waveguide cavity. Model predictions envisage ultrashort pulses of intensities superior to what can be obtained in mode-locked lasers.