Pump Profile Research Articles

Cavity optimization of optically pumped broad-area microcavity lasers

We demonstrate a cavity optimization method for efficient optical pumping of a semiconductor microcavity. An enhanced pumping efficiency makes it possible to pump broad-area microcavities for, e.g., high-power lasers or parallel optical processing of information. This is especially important in the near-infrared where a pump not absorbed into the active material is absorbed into the substrate and converted into heat. We apply the method to optical pumping of a 80μm diameter laser with a spatially uniform pump profile. This method could also prove useful for the design of vertical external cavity surface emitting lasers.

Measurement of orientation and alignment moment relaxation by polarization spectroscopy: theory and experiment.

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump-probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Sigma+ -X 2Pi (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Sigma+ -X 2Pi (0,0) band with He as the collision partner. The decay of the experimental PS pump-probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions.

Study of raman amplification properties in triangular photonic crystal fibers

The Raman properties of triangular photonic crystal fibers (PCFs) are analyzed in order to design a fiber for Raman amplification with enhanced performances. By casting the Raman intensity propagation equations, the Raman effective area and the Raman gain coefficient are introduced - two meaningful parameters that take into account the overlap between the pump and signal profiles. The behavior of these two parameters is examined in silica PCFs as a function of the geometrical characteristics of the triangular lattice. The numerical results show that a proper design of the hole diameter and the spacing between air holes can minimize the Raman effective area and maximize the Raman gain coefficient. The paper then focuses on PCFs with a germania-doped core. It is found that, for a given PCF cross section and dimension of the doped region, the Raman gain coefficient increases linearly with germania concentration. Moreover, by enlarging the doped region, it is discovered that a PCF with a germania-doped area internally tangent to the first ring of air holes has a maximum Raman gain coefficient. Finally, the calculated values of the Raman gain coefficient are compared with those of other highly nonlinear fibers presented in the literature, showing that a well-designed triangular PCF can significantly improve Raman gain performance.

Spontaneous transverse patterns in a microchip laser with a frequency-degenerate resonator.

We experimentally observe the formation of high-order transverse patterns in a microchip laser with a high degree of frequency degeneracy. With a doughnut pump profile the spontaneous transverse patterns are well localized in the Lissajous trajectories. The observed transverse patterns are reconstructed with a high degree of accuracy by use of the coherent state of quantum theory. The accurate reconstruction suggests that laser resonators can be designed to obtain a more thorough understanding of the quantum-classical connection.

Patterns in a quasiconfocal optical parametric oscillator

The formation of transverse patterns in a triply resonant optical parametric oscillator is studied both numerically and analytically for a spherical cavity close to confocality. While the pump profile is Gaussian, the signal and idler intensities may be made of many rings, either stationary or time dependent. The mode selection and the time dependence are understood with the help of the linear stability analysis. It might explain observations reported for a quasiconfocal cavity with a KTP crystal.

Thermal modeling of solid state lasers with super-Gaussian pumping profiles

The pump beam for an end-pumped solid state laser can likely be fit by a super-Gaussian beam. We normalize the super-Gaussian beam for different orders (m). Numerical temperature solutions are obtained for super-Gaussian beams using the finite element method. We find that these numerical temperature solutions can be fit by a Gaussian function. This is important as all of the subsequent thermal radial and tangential stresses, refractive index changes, optical path differences (OPDs), birefringences and depolarization losses can be solved analytically and they follow Gaussian profiles. Different m have different thermal effects as well as different overlap efficiencies. We analyze the effects of different m on different pump powers, different pump beam sizes, and different rod radii. This analysis could provide valuable data for correcting phase distortions of high-power operation.

Observation of transverse patterns in an isotropic microchip laser

An isotropic microchip laser is used to study the characteristics of high-order wave functions in a two-dimensional (2D) quantum harmonic oscillator based on the identical functional forms. With a doughnut pump profile, the spontaneous transverse modes are found to, generally, be elliptic and hyperbolic transverse modes. Theoretical analyses reveal that the elliptic transverse modes are analogous to the coherent states of a 2D harmonic oscillator; the formation of hyperbolic transverse modes is a spontaneous mode locking between two identical Hermite-Gaussian modes.

LOW-PROFILE INLET FOR CENTRIFUGAL BLOOD PUMP

Anatomical compatibility of centrifugal pumps used as VADs can be challenging due to the intrinsic right-angle orientation of inflow and outflow ports. Consequently a novel low-profile inflow was developed. The design goals were to create a 90° bend in the flow path while minimizing swirl and preserving uniformity of the velocity profile. Several candidate elbow- and scroll designs were evaluated by computational fluid dynamics (CFD) analysis, leading to a symmetric scrolled configuration. A sequence of over ten CFD iterations resulted in two generations of prototypes that were studied by particle-trace visualization. The chosen optimal design was adapted to a third-generation magnetically-levitated centrifugal pump (HeartQuestTM VAD, MedQuest® Products). The inlet features a 12 mm dia. inlet, which expands through a rectangular diffuser, enters into a bifurcated volute, and finally exits to an annular shaped outlet (16mm ID, 20mm OD). The inner diameter of the annulus provides a smooth transition to the central spindle hub of the pump, and the outer diameter mates with the eye of the impeller housing. The cross sectional area of the scroll was optimized to provide uniform flow without separation, excessive jet formation, or secondary flow. The flow path was analyzed over a range from 2.5 Ipm to 12 Ipm, demonstrating pressure drop from appx. 1.5 mmHg to 20 mmHg, resp. The resulting design adds minimal height to the external pump profile, and enables variable angulation of inlet to outlet ports. Initial in-vivo experience has demonstrated favorable anatomic compatibility.

Optical target and spiral patterns in a single-mirror feedback scheme

Spatio-temporal structures with a regular time dependence are observed in a nonlinear optical system consisting of a sodium-vapour cell and a single feedback mirror. Target and spiral patterns appear spontaneously in the intensity of the transmitted light field as a result of a self-organisation process. In contrast to the results obtained in most other pattern-forming systems, the radial motion of the waves is directed towards the centre. The emergence of oscillatory structures is traced back to a Hopf bifurcation at a finite wave number. The preference for a radial drift motion is due to a nonlinear guiding effect which results from phase gradients created by the inhomogeneous pump profile. The direction of the drift can be reversed by externally applied phase gradients. The system is a striking example of a case in which the radial variation of the pump profile has a decisive influence on pattern formation.

Observation of laser transverse modes analogous to a SU(2) wave packet of a quantum harmonic oscillator

We report the generation of a new type of laser transverse mode that is analogous to a SU(2) elliptical wave packet of a quantum harmonic oscillator. Experimental results show that using a doughnut pump profile to excite an isotropic microchip laser in a spherical cavity can generate the elliptical transverse modes. The formation of elliptical transverse modes is found to be a spontaneous locking process of Hermite-Gaussian modes within the same family. The chaotic relaxation oscillation caused by the interaction of two nearly degenerate elliptical modes is also observed.

Spontaneous transverse pattern formation in a microchip laser excited by a doughnut pump profile

A transition from a pure Laguerre–Gaussian (LG) mode to a pattern of optical vortex lattices in a large-Fresnel-number microchip laser is experimentally demonstrated by controlling the cavity Q-factor. The cooperative frequency locking of nearly degenerate modes is found to be a primary process for the generation of the optical vortex lattices in a class-B laser. When the cavity Q-factor is high enough, a LG-like mode and a structure of optical vortex lattices are found to coexist. Competition between coexisting transverse patterns of different symmetry gives rise to chaotic fluctuations.

Parametric amplification by spatial soliton induced transverse amplitude profile cloning

Transverse profile cloning (TPC) of the amplitudes of a probe beam and of a generated four-wave mixing (FWM) beam by a strong spatial-soliton-like pump is obtained in an all-optical waveguiding configuration. The cloning arises from self-referential parametric amplification (PA) and creates a multiwave spatial soliton on propagation. We show that the PA is far higher than would be expected for plane-wave (PW) pump and probe fields. This is a result of the pump-induced focusing towards the axis of the highly amplified regions of the probe and FWM transverse profiles, which derive from the region of the pump profile that is near saturation. In this region, the phase mismatch is reduced to almost zero due to nonlinear effects.

Yb:YAG power oscillator with high brightness and linear polarization

A diode-pumped Yb:YAG laser with a novel end-pumped zigzag slab architecture has been developed. This architecture provides uniform transverse pump profiles, conduction cooling of the laser crystal, mechanical robustness, and ready scalability to higher powers. At room temperature the laser emits 415 W of cw power with 30% optical conversion efficiency. An image-inverting stable resonator permits a high-brightness output of 252 W with linear polarization and an average M(2) beam quality of 1.45. Q-switched pulse energies of as much as 20 mJ and average Q<i/>-switched powers of as much as 150 W were obtained while M(2) was maintained at <1.5.

Experimental observation of traveling waves in the transverse section of a laser

The selection of a transverse traveling wave by an inhomogeneous pump profile has been experimentally observed in a class B laser structure. The laser structure consisted of a wide-aperture edge-emitting laser diode operating in pulsed mode to avoid thermal guiding effects. The injection current's profile was modified from the usual top-hat configuration to a Lorentzian-like profile by the inclusion of a 10-mum p-type expitaxial spreading layer. Spatial dependance of the far field on the near field was observed. The same behavior is also demonstrated numerically by use of Maxwell-Bloch equations for semiconductor lasers.

Analysis of the generation of amplitude-squeezed light with Gaussian-beam degenerate optical parametric amplifiers

We investigate the generation of amplitude-squeezed states with degenerate optical parametric amplifiers that are pumped by focused Gaussian beams. We present a model that facilitates the calculation of the squeezing level for an experimentally realistic configuration in which there is a Gaussian input signal beam that has the same confocal parameter and waist location as the Gaussian pump beam, with no restriction on the interaction length-to-confocal parameter ratio. We show that the 3-dB squeezing limit that was thought to be imposed by the Gaussian pump profile can be exceeded in the (previously uninvestigated) tight-focusing regime. We find the maximum possible amplitude squeezing in this regime to be 4.65 dB. However, it is possible to increase the squeezing level further by spatially filtering the tails of the output signal beam, resulting in squeezing levels in excess of 10 dB.

New optimization criteria for slit-apertured and gain-apertured KLM all-solid-state lasers

The aim of this paper is to discuss the limitations of the conventional mode-locking parameter δ, and to introduce new Kerr-lens mode-locking (KLM) criteria that we believe to be more reliable indicators of the preference for mode-locked operation in real-world diode-pumped lasers. The new criteria take into account both the losses at an intra-cavity slit and the gain-aperturing by the pump profile in the laser medium. The criteria are compared to the conventional δ criterion and applied to both 4-element and 3-element cavities. The interplay of gain aperture and slit aperture KLM is investigated for 4-element systems.

Global rate equation description of a laser

The global integro-differential rate equations describing a multimode laser are analyzed. Expressions for the relaxation oscillation frequencies and their damping rates in the single-mode and two-mode regimes are obtained without specifying either the cavity geometry or the longitudinal pump profile. On the same level of generality, we prove the existence of universal relations relating the peaks of the power spectra in the two-mode regime. For a Fabry-Perot with arbitrary longitudinal pump profile, series expansions of all the physical functions are derived in powers of the pump moments. These moments are averages of the pump profile over cavity modes at linear combinations of the lasing frequencies and their harmonics. These results apply to end-pumped and/or partially filled lasers. For a single mode Fabry-Perot laser, we prove that the contribution to the steady state intensity from the lasing mode varies from 75% close to the lasing threshold to zero at high intensity. The remainder comes from the harmonics of the lasing mode. Analyzing the steady state single mode intensity equation in terms of the pump gratings, we prove that close to the lasing threshold only the space average of the pump and its grating oscillating at twice the lasing wave number do not vanish. This provides a hint towards the justification of the usual modal rate equations which retain only these two functions in the dynamical evolution of a laser. For a Fabry-Perot with constant pump profile, an exact expression for the upper boundary of the stable single mode regime is derived. In that two-mode regime, we prove that there is a critical value of the pump at which the ratio of the two relaxation oscillation frequencies is 2, leading to an internal resonance.

The effect of north-south gyre width on the inertial recirculation

The effect of the gyre north-south width on the inertial recirculation has been studied using a non-eddy resolving QG model. It is found that for the traditional sinusoidal Ekman pumping profile with a fixed maximum interior transport and fixed viscosity, a smaller north-south width will produce a stronger inertial recirculation, unless the north-south width of the gyre becomes comparable with that of the inertial recirculation. This is because the intensity of the inertial recirculation is mainly determined by the local advection of the potential vorticity anomaly in the vicinity of midlatitude. In a case of small north-south gyre width, the local speed of the northern part of the western boundary current will be increased by the northward shift of the latitude with the maximum interior Sverdrup transport. This tends to enhance the advection of potential vorticity anomaly, namely, nonlinearity within the northwestern corner and in turn a strong Q-anomaly in the midlatitude jet. As a positive feedback, the resulting recirculation can dramatically increase the speed of the western boundary current, and in turn a stronger potential vorticity advection which further intensify the recirculation. As a result, in a double-gyre circulation that has a subpolar gyre of a smaller north-south width, and that has the same Sverdrup flows in both gyres, the recirculation can be significantly stronger in the subpolar gyre than in the subtropical gyre.

Transverse modes in oxide confined VCSELs: Influence of pump profile, spatial hole burning, and thermal effects

We present experimental studies on the transverse mode emission behaviour of oxide-confined Vertical Cavity Surface Emitting Lasers (VCSELs). VCSELs with aperture diameters of 6um and 11um exhibit a wide variety of emission patterns from low order Hermite- Gaussian modes to high order Laguerre-Gaussian modes. We obtain detailed information about the spatial gain distribution by recording spontaneous emission intensity profiles during lasing operation. We conclude from these profiles, that the spatial carrier distribution is primarily governed by the in uence of pump induced current spreading and is only secondarily in uenced by further effects such as spatial hole burning, and thermal gradients in the laser. The combination of these mechanisms causes a strong tendency towards the emission of high order transverse modes.

Quantum noise initiation and macroscopic fluctuations in optical parametric oscillators

The signals in an optical parametric oscillator (OPO) build up from quantum noise. In pulsed OPO’s this can lead to fluctuations in such macroscopic signal properties as rise time, pulse energy, frequency spectrum, and transverse profile. The strength of these fluctuations is investigated by use of simulation models that include quantum noise, multiple longitudinal modes, dispersion, birefringence, arbitrary output coupling, and transverse pump profile. The results are compared with results obtained with classical deterministic models to find out how well such models can estimate expectation values of signal observables.