Virtual Journal of Laser

A self-consistent numerical model has been developed for simulating lasing properties of a typical thin-disk laser in detail. The temperature-dependent form of the Boltzmann occupation factors, absorption and stimulated-emission cross sections, and thermal conductivity of the Yb:YAG crystal as a quasi-four-level atomic system have been utilized for obtaining various effective operating variables. A Monte Carlo ray-tracing-based code and 2-D finite-element analysis (FEA) with the ANSYS package have been employed to calculate the absorption power and temperature distribution inside the crystal, respectively. Rate equations have also been included in order to obtain other lasing properties. These equations predict that characteristics of the laser are affected by the Boltzmann occupation factors of the pump and the laser states simultaneously. Based on the results, optical pumping efficiency has been examined as a function of output coupler reflectivity, number of the pump beam passes, and temperature.

We analyze the impact of the population lens on electromagnetic (EM) field development in a $Q$ -switched unstable supergaussian cavity (QUSC) by a FFT based computational model. The results show distortions in a structure of the near field as well as of the far-field pattern and reveal detrimental impact on the capability of energy extraction from the gain material and the energy stability of pulses. The computational results were confirmed with the experimental analysis of a ruby laser with QUSC. The computational results are also in excellent agreement with the published work dealing with the Cr:LISAF QUSC cavity. Our investigations show that population lens should be taken into account when using chromium-doped laser media in a QUSC.

A widely tunable laser source based on double microrings is presented which exhibits tuning range up to 46 nm. Two microrings consisting of active GaInAsP/InP material are vertically coupled with a straight transparent waveguide using full wafer bonding techniques. Wafer bonding allows the fabrication of ultra short couplers where the critical coupling gap can be epitaxially controlled. The microrings are operated above threshold and act as laser sources forming a coupled cavity system. Their respective radii are chosen to be slightly different, greatly extending the coupled cavity free spectral range (FSR) providing sufficiently high mode suppression ratio ( $hbox{MSR}> 30~{hbox {dB}}$ ). The isolation of adjacent rings allows an individual electrical control and enables a wavelength tuning with pm accuracy. The tuning mechanism benefits from the Vernier effect resulting in a broad range of achievable wavelengths in a systematic and repeatable way.

In this paper, modulation crosstalk from 1310 nm upstream optical signals to 1490 nm downstream optical signals in an distributed feedback laser diode and monitor photodiode (DFB-LD and MPD) monolithically integrated optical transceiver has been studied numerically through the time-domain traveling wave (TDTW) model. Interaction between the $1310/1490$ nm wavelengths throughout the cavity originated from both cross gain modulation and cross phase modulation are the physical source of the crosstalk in the device, which were analyzed numerically in detail in the paper. A new structure is proposed to reduce the modulation crosstalk resulted from the interaction. Simulation shows that the modulation crosstalk can be drastically reduced by applying an adjustable current on the phase tuning section inserted between the DFB-LD and MPD, which verifies our design. Polarization and temperature dependence on the modulation crosstalk are also discussed.

Laser dynamics of a 10 GHz 0.55 ps asynchronously harmonic modelocked Er-doped fiber soliton laser are investigated both theoretically and experimentally. Theoretical analyses based on the master equation model solved by the variational method have indicated that all the pulse parameters of the laser output will exhibit complicated slow periodic variations in the asynchronous soliton modelocking (ASM) mode. New experimental methods based on analyzing directly the RF spectra of the ASM laser output have been developed to accurately determine the sinusoidal variation of the pulse timing and the pulse center wavelength for the first time. It is found that the pulse center wavelength variation can be as large as 1 nm half-peak-to-peak and the pulse timing variation can be as large as 3 ps. The consistency among all the experimental data and theoretical prediction is carefully examined and the results indicate that the ASM pulse dynamics observed experimentally are in good agreement with those obtained from the theoretical analyses.

We propose a secure optical communication system based on the principles of generalized and complete chaotic synchronization. A transmitter and a receiver both composed by two chaotic external-cavity semiconductor lasers are coupled in a master-slave configuration to provide generalized synchronization, while the master lasers in the transmitter and in the receiver are completely synchronized through the synchronization channel via an optical fiber. A message is added to the transmitter slave laser and sent to the receiver through the information channel to be compared with the output of the receiver slave laser. The system is robust to a small mismatch of the laser parameters or of the coupling between the master and slave lasers, unavoidable in a real system, and can even enable a good communication up to a 5 Gb/s transmission rate using the chaos masking encryption method, when the master laseres are coupled bidirectionally.

A design parameter subspace is explored to suggest epitaxial layer structures which maximize gain spectral density at a target wavelength for green $hbox{In}_{x}{hbox {Ga}}_{1-x}{hbox{N}}$ -based single quantum well active regions. The dependence of the fundamental optical transition energy on the thickness and composition of barriers and wells is discussed, and the sensitivity of gain spectral density to design parameters, including the choice of buffer layer material, is investigated.

Relaxation oscillations and gain switching of erbium-doped waveguide ring lasers (EDWRLs) are studied using numerical simulations based on time-dependent rate-propagation equations. The counter-directional wave suppression is analyzed for different waveguide ring cavity configurations and pumping schemes. It is shown that the counter-directional wave suppression in unidirectional EDWRLs undergoes relaxation oscillations synchronously with oscillating power. It is also shown that the suppression in the first spike is maximal, so the gain switching technique provides the most favorable conditions for unidirectional lasing. Furthermore, for the one-end-pumped gain-switched EDWRL, highly unidirectional operation is possible with no intracavity elements included. In this case the counter-directional wave suppression considerably exceeds its steady-state value. The gain-switched suppression caused by intracavity elements is close to the steady-state value.

Decay dynamics of the upper-laser energy level of $hbox{Nd:YVO}_{4}$ are re-evaluated. In order to reduce the effects of re-absorption, comparative measurements among crushed samples with different doping concentrations were conducted. The magnitude of re-absorption was estimated experimentally by comparing the relative intensity of the emitted fluorescence spectra at different wavelengths, and estimated theoretically by employing a simplified model. The temporal decay dynamics are found to be non-exponential and the associated rate parameters are presented. The room temperature intrinsic life time value of the $^{4}hbox{F}_{3/2}$ energy level is found to be significantly shorter than the value accepted today.

In this study, bismuth-doped fiber lasers operating at the wavelength of 1179 nm with an optical efficiency of up to 28% are realized. The fiber gain upon 1- $mu{hbox{m}}$ pumping declines, while the unsaturable absorption increases with increasing the small-signal absorption. We conclude that up-conversion and excited-state absorption are responsible for limiting the efficiency of such lasers.