Áron Szabó

Group-II acceptors in wurtzite AlN: A screened hybrid density functional study

We systematically studied the group-II acceptors in wurtzite AlN by screened hybrid density functional calculations. We show that the shallowest isolated group-II substitutional defect is Mg, while codoping of Mg and O may yield even shallower acceptor level.

Photonic bandgap fibers with resonant structures for tailoring the dispersion.

Numerical simulations on different kinds of realistic photonic bandgap fibers exhibiting reversed dispersion slope for the propagating fundamental mode are reported. We show that reversed or flat dispersion functions in a wide wavelength range using hollow-core, air-silica photonic bandgap fibers and solid core Bragg fibers with step-index profile can be obtained by introducing resonant structures in the fiber cladding. We evaluate the dispersion and confinement loss profiles of these fibers from the Helmholtz eigenvalue equation and the calculated fiber properties are used to investigate the propagation of chirped femtosecond pulses through serially connected hollow core fiber compressors.

Investigation of an All-Optical Black-Box PPLN-PPLN BPSK Phase Regenerator

We investigate the phase squeezing performance of an all-optical black-box binary phase-shift keyed phase regenerator constructed from periodically-poled lithium-niobate (PPLN) waveguides. Phase regeneration is performed with different levels of phase noise and constant amplitude noise added to the phase modulated input. We experimentally show that, for moderate phase noise, unwanted phase-to-amplitude noise conversion can be minimized without degradation of the phase squeezing performance, by controlling the signal-to-pump power ratio at the input to the PPLN-based phase-sensitive regenerator.

Defects at nitrogen site in electron-irradiated AlN

In high resistance AlN irradiated with 2 MeV electrons, an electron paramagnetic resonance (EPR) spectrum, labeled EI-1, with an electron spin S=1/2 and a clear hyperfine (hf) structure was observed. The hf structure was shown to be due the interaction between the electron spin and the nuclear spins of four A27 nuclei with the hf splitting varying between ∼6.0 and ∼7.2 mT. Comparing the hf data obtained from EPR and ab initio supercell calculations we suggest the EI-1 defect to be the best candidate for the neutral nitrogen vacancy in AlN.

2.1 kW single mode continuous wave monolithic fiber laser

A robust, alignment-free monolithic 2.1 kW single-mode continuous wave fiber laser, operating at 1083 nm is demonstrated. The laser is pumped with commercial fiber pigtailed multimode diodes through all-fiber pump-signal power combiners in a MOPA architecture. The oscillator was formed with high reflector and output coupler fiber Bragg gratings written in 11/200 μm (mode field/cladding diameter) single-mode fiber. The gain medium was a 19m OFS commercial 11/200 μm double clad Yb-doped fiber (DCY). Pump light was coupled to the oscillator using two 11/200 μm pump-signal power combiners (PSC). A total of 20 commercially available 58W pump diodes at 915 nm were used to generate 800W of signal, as measured before the amplifier. The Raman power after the oscillator was more than 60 dB below the signal power. The amplifier was built using 13 m of 14/200 µm DCY and two (18+1)x1 PSC combiners with more than 95% pump and signal light transmission. A total of 2 kW of power was used to bi-directionally pump the amplifier. The output was measured after 3 m 14/200 μm fiber, and 10 m 100/360 μm delivery cable. Total signal output power was 2.1 kW, corresponding to an amplifier slope efficiency of 77%. The Raman power is more than 30 dB below the signal power. At maximum power, no modal instabilities, thermal effects, nor power rollover were observed. With higher power pumps, it is predicted that a power level of 2.6 kW can be achieved with the Raman level below 20 dB.

Numerical Comparison of WDM Interchannel Crosstalk in FOPA- and PPLN-Based PSAs

We propose a model and numerical simulations to calculate crosstalk (XT) between multiple wavelength-division-multiplexing (WDM) channels in copier + phase sensitive amplifier (PSA) chains based on periodically poled lithium niobate (PPLN). The calculated XT is compared with XT generated in copier + PSA chains based on highly nonlinear fiber (HNLF). For ordinary device parameters, the results show that for an increasing channel number, the growth of interchannel XT in PPLN is less rapid compared with HNLF, as a consequence of the finite quasi-phase matching band of PPLN. This ensures lower XT in PPLN-based chains for 18 channels, whereas HNLF chains show better XT performance for 4 channels.

Multi-channel phase squeezing in a PPLN-PPLN PSA

We investigate phase squeezing of up to four signal channels in a non-degenerate phase-sensitive amplifier constructed from periodically-poled-lithium-niobate waveguides and observe that additional channels have little impact the phase squeezing performance.

Phase-sensitive amplification in a single bi-directional PPLN waveguide.

We investigate phase-sensitive amplification (PSA) and phase regeneration of a binary phase-shift keying (BPSK) signal using a single periodically poled lithium niobate (PPLN) waveguide. The PPLN is operated bi-directionally in order to simultaneously achieve phase correlated signals and phase-sensitive (PS) operation. We use injection-locking for carrier phase recovery and a lead zirconate titanate (PZT) fiber stretcher to correct path length deviations in the in-line phase regenerator. We observe a trade-off between high PS gain provided by high pumping power and stability of the device.

Numerical Study on the Saturable Absorber Parameter Selection in an Erbium Fiber Ring Oscillator

Based on a non-distributed numerical model including fiber birefringence and orthogonal mode coupling, we investigate the effects of saturable absorber (SA) parameters on the mode-locking of a realistic Erbium fiber ring laser. We compute the pulse evolution in the oscillator where the pulse properties are also affected by the intracavity polarization controller (PC). Simultaneous effect of the SA and PC parameters on the properties of the generated vector solitons are studied, and their stability against intracavity polarization perturbation at different SA parameter ranges is discussed.

Computationally efficient method for Fourier transform of highly chirped pulses for laser and parametric amplifier modeling

We developed an improved approach to calculate the Fourier transform of signals with arbitrary large quadratic phase which can be efficiently implemented in numerical simulations utilizing Fast Fourier transform. The proposed algorithm significantly reduces the computational cost of Fourier transform of a highly chirped and stretched pulse by splitting it into two separate transforms of almost transform limited pulses, thereby reducing the required grid size roughly by a factor of the pulse stretching. The application of our improved Fourier transform algorithm in the split-step method for numerical modeling of CPA and OPCPA shows excellent agreement with standard algorithms.