Oleksiy V. Shulika

Femtosecond parabolic pulse shaping in normally dispersive optical fibers.

Formation of parabolic pulses at femtosecond time scale by means of passive nonlinear reshaping in normally dispersive optical fibers is analyzed. Two approaches are examined and compared: the parabolic waveform formation in transient propagation regime and parabolic waveform formation in the steady-state propagation regime. It is found that both approaches could produce parabolic pulses as short as few hundred femtoseconds applying commercially available fibers, specially designed all-normal dispersion photonic crystal fiber and modern femtosecond lasers for pumping. The ranges of parameters providing parabolic pulse formation at the femtosecond time scale are found depending on the initial pulse duration, chirp and energy. Applicability of different fibers for femtosecond pulse shaping is analyzed. Recommendation for shortest parabolic pulse formation is made based on the analysis presented.

Supercontinuum generation at 800 nm in all-normal dispersion photonic crystal fiber

We have numerically investigated the supercontinuum generation and pulse compression in a specially designed all-normal dispersion photonic crystal fiber with a flat-top dispersion curve, pumped by typical pulses from state of the art Ti:Sapphire lasers at 800 nm. The optimal combination of pump pulse parameters for a given fiber was found, which provides a wide octave-spanning spectrum with superb spectral flatness (a drop in spectral intensity of ~1.7 dB). With regard to the pulse compression for these spectra, multiple-cycle pulses (~8 fs) can be obtained with the use of a simple quadratic compressor and nearly single-cycle pulses (3.3 fs) can be obtained with the application of full phase compensation. The impact of pump pulse wavelength-shifting relative to the top of the dispersion curve on the generated SC and pulse compression was also investigated. The optimal pump pulse wavelength range was found to be 750 nm < λp < 850 nm, where the distortions of pulse shape are quite small (< -3.3 dB). The influences of realistic fiber fabrication errors on the SC generation and pulse compression were investigated systematically. We propose that the spectral shape distortions generated by fiber fabrication errors can be significantly attenuated by properly manipulating the pump.

Formation of ultrashort triangular pulses in optical fibers

Specialty shape ultrashort optical pulses, and triangular pulses in particular, are of great interest in optical signal processing. Compact fiber-based techniques for producing the special pulse waveforms from Gaussian or secant pulses delivered by modern ultrafast lasers are in demand in telecommunications. Using the nonlinear Schrödinger equation in an extended form the transformation of ultrashort pulses in a fiber towards triangular shape is characterized by the misfit parameter under variety of incident pulse shapes, energies, and chirps. It is shown that short (1-2 m) conventional single mode fiber can be used for triangular pulse formation in the steady-state regime without any pre-chirping if femtosecond pulses are used for pumping. The pulses obtained are stable and demonstrate linear chirp. The ranges and combinations of the pulse parameters found here will serve as a guide for scheduling the experiments and implementation of various all-fiber schemes for optical signal processing.

Theoretical Study of Optical Transition Matrix Elements in InGaN/GaN SQW Subject to Indium Surface Segregation

We investigate the dependence of dipole matrix elements for InGaN/GaN single quantum well structures on the indium surface segregation (ISS). Obtained results show that the influence of the surface segregation on the dipole matrix element is not equal for all optical transition. This effect results from the joint action of the piezoelectric polarization and ISS that change selection rules. In addition, surface segregation at each interface of the quantum well has different impact on optical characteristics depending on the direction of the piezoelectric polarization. The effect of the surface segregation has been estimated applying the global sensitivity analysis in the frame of six-band approximation for the valence band and parabolic approximation for the conduction band.

Chirp compression with single chirped mirrors and its assembly

Chirped mirrors are widely used in ultrafast optics as broadband devices for dispersion compensation. Time domain analysis in contrast to frequency domain one allows the direct investigation of pulse profile changing during chirp compression. We model the chirped pulse compression with single chirped mirror and assembly of similar mirrors. Our analysis shows there is similar effect of chirp compensation in both cases. However, injurious group delay oscillations in the single mirror strongly decrease pulse quality in assembly of mirrors and can even damage pulse waveform. Angle of incidence and polarization also play crucial role and should be taken into account.

Quantum capture area in layered quantum well structures

Carrier capture in quantum well structures defines high-speed properties of the lasers and amplifiers based on them. We introduce general definition of the capture area in low-dimensional heterostructures based on intersubband coupling coefficient. Spatial dependencies of intersubband coupling coefficient, which governs in fact capture rate, suggest on insufficiency of classical definition of capture area and necessity of quantum-mechanical computation of this value. Special case of layered quantum-well structures is considered. Computational results show necessity to take into account dependence of capture area on the temperature and device operating point.

Characterization of all-normal-dispersion microstructured optical fiber via numerical simulation of passive nonlinear pulse reshaping and single-pulse flat-top supercontinuum

The supercontinuum (SC) generated by pumping in anomalous dispersion is sensitive to the input pulse fluctuations and pump laser’s shot noises and does not possess a single-pulse waveform, so the incident pulse becomes a noise-like train of spikes. A simple method of creating pulsed lasers with either pulse-maintaining ultrabroad SC or specially shaped pulse waveforms can be implemented using all-normal-dispersion microstructured optical fibers (ANDi-MOFs). An ANDi-MOF with a simple topology and dispersion profile maximum at 800 nm was designed using the effective index method. Its properties and suitability were characterized via numerical simulation of femtosecond parabolic pulse formation and generation of an octave-spanning pulse-maintaining SC using a generalized propagation equation. The designed ANDi-MOF is suitable for resolving both problems and allows some detuning of the pulse’s wavelength around 800 nm. However, a better choice for SC generation is pumping at or near the wavelength where the third-order dispersion becomes zero. This configuration benefits from the absence of pulse break-up under large pulse energies, which appears otherwise. The fiber can provide a low-cost method for developing supercontinuum sources and a solution to the problems of parabolic waveform formation to meet the needs of optical signal processing and pulse amplification and compression.

Sub-10-fs Pulses Produced From Compression of Supercontinuum Generated in All-Normal Dispersion Photonic Crystal Fiber

We have shown numerically that supercontinuum generated in ANDi PCF from a few nJ 100 fs pulses can be efficiently compressed with a simple quadratic compressor producing sub-10-fs pulses.

Formation of parabolic optical pulses in passive optical fibers

We have investigated a nonlinear pulse reshaping towards parabolic pulses in the passive normal dispersive optical fibers. We have found that pulses with parabolic intensity profile, parabolic spectrum and linear chirp can be obtained due to the passive nonlinear reshaping at the propagation distance exceeding a few dispersion lengths. These pulses preserve parabolic profile during subsequent pulse propagation in a fiber. We have examined the influence of initial pulse parameters and fiber parameters on the resulted pulse shape.

Chirped Multilayer Mirror Based on Silicon Nitride (Si 3 N 4 ) with Air-Gap Interlayers

Chirped multilayer mirror based on silicon nitride with air-gap interlayers is proposed and designed. The mirror provides high reflectivity and good dispersion properties in the range 400-1200 nm supporting the few-cycle pulses processing.