Dissipative nonlinear structures in optical fiber systems III : dissipative solitons via nonlinear mapping

In the third and final talk on dissipative structures in fiber applications, we discuss mathematical techniques that can be used to characterize modern laser systems that consist of several discrete elements. In particular, we use a nonlinear mapping technique to evaluate high power laser systems where significant changes in the pulse evolution per cavity round trip is observed. We demonstrate that dissipative soliton solutions might be effectively described using this Poincaré mapping approach.

Short-circuit and ground fault analyses and location in VSC-based DC network cables

The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

Widely tunable gain-switched operation of external cavity grating-coupled surface emitting laser

Widely tunable gain switching of a grating-coupled surface-emitting laser (GCSEL) has been demonstrated in a simple external cavity configuration for the first time. Pulse duration in range of 40-100ps and wavelength tuning over 100nm have been achieved. High power, tail-free optical pulses have been observed at 980nm.

A study of the relationship between exit, voice, loyalty and neglect and commitment in India

Drawing on exit, voice, loyalty and neglect (EVLN) literature, this study examines direct and interactive associations between organizational-level commitment and team-level commitment and the use of EVLN by managers in India. The study is based on a survey of 200 managers and supervisors from seven Indian firms. The findings on the use of voice are consistent with the past research in Western countries, but challenge the prevailing assumption about the use of voice in high power distance societies. The results also indicate that team-level commitment moderates the association between organizational-level commitment and the use of EVLN.

Fiber optical parametric amplifier performance in a 1-Tb/s DWDM communication system

We have reduced signal-signal four-wave mixing crosstalk in a fiber optical parametric amplifier (OPA) by using a short nonlinear fiber for the gain medium and a high-power pump. This allowed us to obtain less than 1 dB penalty for amplification of 26 dense wavelength-division multiplexed (WDM) channels modulated at 43.7Gb/s return to zero-differential phase-shift keying, with the OPA placed between transmitter and receiver. We then used the same OPA in several different roles for a long-haul transmission system. We did not insert the OPA within the loop, but investigated this role indirectly by using equivalent results for small numbers of loop recirculations. We found that standard erbium-doped fiber amplifiers currently hold an advantage over this OPA, which becomes negligible for long distances. This paper shows that at this time OPAs can handle amplification of WDM traffic in excess of 1 Tb/s with little degradation. It also indicates that with further improvements, fiber OPAs could be a contender for wideband amplification in future optical communication networks.

In-fiber linear polarizer based on UV-inscribed 45° tilted grating in polarization maintaining fiber

We report an in-fiber linear polarizer structured by UV-inscribing a 45° tilted fiber grating (TFG) into polarization maintaining (PM) fiber along its principal axis. The polarization extinction ratio (PER) achieved by a 48 mm long 45° TFG has reached 46 dB at 1550 nm and the overall PER is >40 dB over a 50 nm wavelength range. Such 45° TFG based polarizers have many advantages over conventional products, including low loss, low cost, simple fabrication process, and no physical modification to the fiber, thus offering high stability and capable of handling high power.

Dissipative nonlinear structures in optical fiber systems III:dissipative solitons via nonlinear mapping

In the third and final talk on dissipative structures in fiber applications, we discuss mathematical techniques that can be used to characterize modern laser systems that consist of several discrete elements. In particular, we use a nonlinear mapping technique to evaluate high power laser systems where significant changes in the pulse evolution per cavity round trip is observed. We demonstrate that dissipative soliton solutions might be effectively described using this Poincaré mapping approach.

Theory of parabolic pulse generation in tapered fiber

We examine similarities and differences between high-power parabolic pulse generation in an active medium and in tapered fiber with decreasing normal dispersion. Using a realistic tapered fiber design, we demonstrate the possibility of parabolic pulse generation without an external pump and determine the limitations of this approach. © 2007 Optical Society of America.

Optimization of erbium-doped actively Q-switched fiber laser implemented in symmetric configuration

We report the results of an experimental study aimed at improving the performance of actively Q-switched fiber lasers. Unlike generic design schemes employing photonic crystal fibers, largemodal diameter fibers or double-clad fibers, we demonstrate a high-power, actively Q-switched laser based on standard com- munication erbium doped fibers with peak irradiance beyond the state-of-the-art at 3.1 GW/cm2 . The laser had 2.2 kW peak power, 15.5 ns pulse duration and 36.8 µJ pulse energy. We have also investigated the dynamics of pulse generation and have success- fully suppressed pulse instabilities caused by backscattered laser emission reaching the pump laser diodes.

Bessel beams from semiconductor light sources

We report on recent progress in the generation of non-diffracting (Bessel) beams from semiconductor light sources including both edge-emitting and surface-emitting semiconductor lasers as well as light-emitting diodes (LEDs). Bessel beams at the power level of Watts with central lobe diameters of a few to tens of micrometers were achieved from compact and highly efficient lasers. The practicality of reducing the central lobe size of the Bessel beam generated with high-power broad-stripe semiconductor lasers and LEDs to a level unachievable by means of traditional focusing has been demonstrated. We also discuss an approach to exceed the limit of power density for the focusing of radiation with high beam propagation parameter M2. Finally, we consider the potential of the semiconductor lasers for applications in optical trapping/tweezing and the perspectives to replace their gas and solid-state laser counterparts for a range of implementations in optical manipulation towards lab-on-chip configurations. © 2014 Elsevier Ltd.

Efficient yellow-green light generation at 561 nm by frequency-doubling of a QD-FBG laser diode in a PPLN waveguide

A compact high-power yellow-green continuous wave (CW) laser source based on second-harmonic generation (SHG) in a 5% MgO doped periodically poled congruent lithium niobate (PPLN) waveguide crystal pumped by a quantum-dot fiber Bragg grating (QD-FBG) laser diode is demonstrated. A frequency-doubled power of 90.11 mW at the wavelength of 560.68 nm with a conversion efficiency of 52.4% is reported. To the best of our knowledge, this represents the highest output power and conversion efficiency achieved to date in this spectral region from a diode-pumped PPLN waveguide crystal, which could prove extremely valuable for the deployment of such a source in a wide range of biomedical applications.

Stable dual-wavelength oscillation of an erbium-doped fiber ring laser at room temperature

We propose a simple Er-doped fiber laser configuration for achieving stable dual-wavelength oscillation at room temperature, in which a high birefringence fiber Bragg grating was used as the wavelength-selective component. Stable dual-wavelength oscillation at room temperature with a wavelength spacing of 0.23nm and mutually orthogonal polarisation states was achieved by utilising the polarisation hole burning effect. An amplitude variation of less than 0.7dB over 80s period was obtained for both wavelengths.

Recent advances in the field of vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs) have proved to be versatile lasers which allow for various emission schemes which on the one hand include remarkably high-power multi-mode or single-frequency continuouswave operation, and on the other hand two-color as well as mode-locked emission. Particularly, the combination of semiconductor gain medium and external cavity provides a unique access to high-brightness output, a high beam quality and wavelength flexibility. Moreover, the exploitation of intra-cavity frequency conversion further extends the achievable radiation wavelength, spanning a spectral range from the UV to the THz. In this work, recent advances in the field of VECSELs are summarized and the demonstration of self-mode-locking (SML) VECSELs with sub-ps pulses is highlighted. Thereby, we present studies which were not only performed for a quantum-well-based VECSEL, but also for a quantum-dot VECSEL.

Modeling and optimization of cascaded erbium and holmium doped fluoride fiber lasers

Numerical modeling of cascade erbium-doped and holmium-doped fluoride fiber lasers is presented. Fiber lengths were optimized for cascade lasers that had fixed or free-running wavelengths using all known spectroscopic parameters. The performance of the cascade laser was tested against dopant concentration, energy transfer process, heat generation, output coupling, and pump schemes. The results suggest that the slope efficiencies and thresholds for both transitions increase with increasing Ho3+ or Er3+ concentration with the slope efficiency stabilizing after 1 mol% rare earth doping. The heat generation in the Ho3+-based system is lower compared to the Er 3+-based system at low dopant concentration as a result of the lower rates of multiphonon relaxation. Decreasing the output coupling for the upper (∼3 μm) transition decreases the threshold of the lower transition and the upper transition benefits from decreasing the output coupling for the lower transition for both cascade systems. The highest slope efficiency was achieved under counter-propagating pump conditions. Saturation of the output power occurs at comparatively higher pump power with dilute Er3+ doping compared with heavier doping. Overall, we show that the cascade Ho3+ -doped fluoride laser is the best candidate for high power output because of its higher slope efficiency and lower temperature excursion of the core and no saturation of the output. © 2013 IEEE.