Continuous-wave power transmission and thermal lensing of a saturable absorber subject to excited-state absorption

Rate-equation analysis has been used in an investigation of the role of saturation and excited-state absorption in the power transmission characteristics and thermal lensing of an absorber. Use of an iterative approach gives explicit analytical results for power transmission and thermal focal length in the presence of excited-state absorption. Sample calculations indicate that pump absorption can increase or decrease with increasing incident pump power, depending on the relative strength of the excited-state absorption cross section with respect to the ground-state absorption cross section. In the case of thermal lensing, results further indicate that saturation and excited-state absorption act as two competing effects, the former reducing the strength of the thermal lens and the latter causing the opposite effect. The analytical formulas derived in this analysis should prove useful to experimentalists in determination of ground-state and excited-state absorption cross sections from experimental power transmission and lensing data.     

Dy(2+):CaF(2) saturable-absorber Q switch for the ruby laser

Passive Q switching of a flash-lamp-pumped ruby laser has been demonstrated with a Dy(2+):CaF(2) saturable absorber at room temperature. Typically a single Q-switched pulse of 4.5 mJ and 94 ns was observed. At 694 nm, the ground-state absorption cross section and the excited-state absorption cross section of the Dy(2+):CaF(2) absorber were found to be 1.2 × 10(-18) cm(2) and 0.9 × 10(-18) cm(2) respectively, with a four-level slow-relaxing saturable-absorber model.     

Optical absorption and nonlinear transmission of tetrahedral V (d) in yttrium aluminum garnet

The saturation of the optical absorption in V³⁺ : YAG crystal is investigated. The absorption cross section of tetrahedral V³⁺ at 1.08 μm is estimated to be 8.2±2.5x10^-18 cm². Q-switching and passive mode-locking for a number of solid state lasers with wavelengths at 747 nm, 780 nm, 1.06 μm and 1.34 μm have been obtained with a V³⁺ :YAG saturable absorber.     

Nanocrystals of PbSe core, PbSe/PbS, and PbSe/PbSxS(1-x) core/shell as saturable absorbers in passively Q-switched near-infrared lasers

The saturable optical absorption properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbScore/shell and PbSe/PbSexS(1-x) core/alloyed-shell NCs, were examined at lambda = 1.54 microm. Saturation intensities of approximately 100 MW/cm2 were obtained. The NCs act as passive Q switches in near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of 40-55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carried out according to a model that includes ground-state as well as excited-state absorption. For pulses approximately 10 ns long, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption cross section of 10-16-10-15 cm2, an excited-state absorption cross section of sigma(es) is congruent to 10(-16) cm2, and an effective lifetime of tau(eff) is congruent to 5 x 10(-12) s.     

Analytical model for optimizing the parameters of an external passive Q-switch in a fiber laser

An analytical model is developed for optimizing two key parameters of an external passive Q-switch in a fiber laser from the criterion of the minimum average mode area inside the saturable absorber. One parameter is the optimum focal position that is analytically derived to be a function of the thickness and initial transmission of the saturable absorber. The other parameter is the optimum magnification of the reimaging optics that is analytically derived to be in terms of the numerical aperture and core radius of the laser fiber as well as the thickness and initial transmission of the saturable absorber. To demonstrate the utilization of the present model, an experiment on the subject of the passively Q-switched fiber laser is performed and optimized.     

Optimization of the pulse-width of diode-pumped passively Q-switched mode-locked c-cut Nd:GdVO4 laser with a GaAs saturable absorber

By considering the single-photon absorption and two-photon absorption processes in the GaAs saturable absorber, the coupled rate equations for a diode-pumped passively Q-switched and mode-locked (QML) laser with GaAs coupler under Gaussian approximation are given. These rate equations are solved numerically. The key parameters of an optimally coupled passively QML laser with the shortest pulse-width envelope are determined. These key parameters include the parameters of the gain medium, the saturable absorber, and the resonator, which can minimize the pulse-width of a singly Q-switched envelope. Sample calculations for a diode-pumped passively Q-switched mode-locked c-cut Nd:GdVO(4) laser with a GaAs coupler are presented to demonstrate that the shortest pulse-width envelope can be obtained by selecting the optimal small-signal transmission of the saturable absorber and the reflectivity of the output mirror.     

Passively Q-Switched Diode-Pumped Continuous-Wave Nd:YAG-Cr(4+):YAG Laser with High Peak Power and High Pulse Energy

A high peak power and high pulse energy passively Q-switched diode-pumped cw Nd:YAG laser at 1.064-mum wavelength has been demonstrated with Cr(4+):YAG crystal as the saturable absorber. The average output power of 7-12 W and pulse duration of 100-250 ns was obtained with kilohertz repetition rates. The highest peak power and pulse energy obtained were 30 kW and 3.4 mJ, respectively. All the output resulted from the TEM(00) mode with M(2) < 1.1. The thermal lensing effect of the saturable absorber was investigated, demonstrating that it played an important role in optimization of the output.     

Fabrication, characterization and mode locking application of single-walled carbon nanotube/polymer composite saturable absorbers

We present the fabrication of a high optical quality single-walled carbon nanotubes (SWNTs) polyvinyl alcohol (PVA) composite film. The composites demonstrate strong saturable absorption at ～1.5 μm, the spectral range for optical communications. By measuring the nonlinear transmission of a sub-picosecond pump pulse through the film, we were able to deduce a saturation fluence of ～13.9 μJ/cm² and a modulation depth ～16.9% (in absorption) at a high pulse fluence ～200 μJ/cm². Transient saturable absorption is investigated by measuring the transmitted autocorrelation traces at various incident power levels. Observed side-peak suppression indicates a fast recovery time on the scale of ～1 ps for our saturable absorber devices. Furthermore, we use these SWNT-PVA composite saturable absorbers as mode-lockers in an Er³⁺ fiber ring laser and achieve ～560 fs pulse generation with good jitter performance and long term stability. The laser performance is also associated with the parameters of our SWNT based saturable absorber.     

Saturable-absorber-based spatial filtering of high-power laser beams

A saturable-absorber-based technique for spatial filtering of high-average-power laser beams is described. For a focused, radially symmetric beam having its highest intensity at the center, this saturable absorber behaves like a soft aperture with gradually increasing attenuation toward the beam edges, thus selectively transmitting the low divergence components that are confined close to the central axis of the propagating laser beam. This technique has been successfully used to reduce the divergence of a high-power, high-repetition-rate, tunable, narrowband, pulsed dye laser. Our results demonstrate how a judicious choice of operating parameters allows spatial filtering to be achieved with the introduction of a minimum absorption loss of the laser beam in the saturable absorber. Following a time-dependent analysis of a rate equation model describing the propagation and interaction of the laser beam with the saturable absorber, we have also obtained theoretical estimates for the extent of spatial filtering. Our theoretical estimates have been found to be in good agreement with our experimental observations.     

Effect of post-epitaxial-growth processing on the nonlinear response of saturable absorber

The nonlinear response of an ion-irradiated saturable Bragg reflector, further modified by thermal annealing, has been studied. We demonstrate that the absorption recovery time and the effective saturation fluence dependent on the pulse duration can be tailored over a wide range after epitaxial growth by the proper combination of ion irradiation and subsequent annealing. Thermal treatment is also an efficient means for the reduction of the unbleachable losses and the stabilization of the absorber parameters.     

Optimization of doubly Q-switched lasers with both an acousto-optic modulator and a GaAs saturable absorber

A doubly Q-switched laser with both an acousto-optic (AO) modulator and a GaAs saturable absorber can obtain a more symmetric and shorter pulse with high pulse peak power, which has been experimentally proved. The key parameters of an optimally coupled doubly Q-switched laser with both an AO modulator and a GaAs saturable absorber are determined, and a group of general curves are generated for what we believe is the first time, when the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs are combined, and the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density as well as the influence of the AO Q-switch are considered. These key parameters include the optimal normalized coupling parameter, the optimal normalized GaAs saturable absorber parameters, and the normalized parameters of the AO Q-switch, which can maximize the output energy. Meanwhile, the corresponding normalized energy, the normalized peak power, and the normalized pulse width are given. The curves clearly show the dependence of the optimal key parameters on the parameters of the gain medium, the GaAs saturable absorber, the AO Q-switch, and the resonator. Sample calculations for a diode-pumped Nd3+:YVO4 laser with both an AO modulator and a GaAs saturable absorber are presented to demonstrate the use of the curves and the relevant formulas.     

A theoretical assessment of a thermal technique to measure acoustic power radiated by ultrasound transducers

The parameters affecting the temperature rise in an insonified absorber are studied computationally. Finite-element and analytical solutions are obtained for the transient energy equation in a cylindrical absorber. When the ultrasound beam radius is less than the radius of the absorber, the temperature field is seen to be considerably more complex than when the absorber cross section is uniformly heated. Circumstances in which power predictions based upon uniform heating would result in appreciable error are identified. The rise time required to achieve equilibrium is studied as a function of operational parameters, including absorber geometry and thermal properties as well as ultrasound beamwidth and frequency. The rise time is seen to increase approximately as the square of the absorber length, while optimized temperature rise increases linearly with absorber length, demonstrating a tradeoff in ultrasound power determination via equilibrium temperature measurements: longer lengths produce higher sensitivity, but also longer times before measurements can be made. A transient technique that may bypass this tradeoff is suggested     

Efficient continuous-wave radiatively cooled cr(4+):forsterite lasers at room temperature

Results of a detailed experimental investigation aimed at reducing the thermal loading problem in a cw Cr(4+):forsterite laser at elevated temperatures are presented. From a Cr(4+):forsterite crystal with a differential absorption coefficient of 0.57 cm(-1), as much as 900 mW of cw output power has been obtained at 1.26 mum and at a crystal boundary temperature of 15 degrees C with an absorbed pump power of only 4.5 W at 1.06 mum. No chopping of the pump beam was necessary. An efficient radiative cooling technique was further employed to cool the laser and no subsequent power fading was observed. To the author's knowledge, the measured absorbed power slope efficiency of 29.5% represents the highest cw power performance reported to date from a Cr(4+):forsterite laser pumped by a Nd:YAG laser around room temperature. The role of the low differential absorption coefficient in the reduction of thermal loading is further elucidated by presenting comparative cw power performance data with a second Cr(4+):forsterite crystal having a differential absorption coefficient of 1.78 cm(-1) in the temperature range between 12 and 35 degrees C. Finally, some interesting multipulse effects of the laser observed in the millisecond regime during quasi-cw operation at 50% duty cycle are described.     

Resonance phonon scattering on stable F 2 + centers in LiF crystals irradiated in the stressed state

The joint action of gamma irradiation and mechanical stress on the optical absorption spectra and thermal conductivity of LiF crystals has been investigated both theoretically and experimentally. Optimum irradiation doses and stress magnitudes for the stabilization and the efficient accumulation of F ₂ ⁺ centers in LiF are determined. The intensity of the absorption band due to F ₂ ⁺ centers is reduced by less than a factor of two in measurements a year apart. The expression for the cross section of resonance phonon scattering on F ₂ ⁺ centers is obtained, and the parameters of the model of the center are determined. The temperature dependence of the thermal conductivity is calculated, and a bend in the (T) curve revealed atT3–5 K is explained.     

Glass doped with PbS quantum dots for passive Q switching of a 1.54- microm laser

By using a PbS-doped phosphate glass as an intracavity saturable absorber, we achieved passive Q switching of a 1.54-mum Er:Yb:glass laser. We obtained Q-switched pulses with 110-ns duration and 0.2-mJ energy. The saturation intensity at 1.54 mum was estimated to be ~60 kW/cm(2).     

Measurements and modeling of the thermal properties of a calorimeter having a sapphire absorber

The response of a magnetic calorimeter with a sapphire crystal serving as an X-ray absorber has been studied as a function of temperature. Several different Au films were used to connect thermally the magnetic sensor to the absorber. The amplitude and time dependence of the signal resulting from the absorption of an X-ray were fit using an idealized model for the calorimeter. The values of the various parameters resulting from a fit of the data are internally consistent and provide a physical understanding of the processes determining the performance of the calorimeter. The fraction of the energy of the X-ray that is captured by the film without having first been down-converted to thermal phonons in the sapphire is found to depend on both the area and the thickness of the film. The rate at which the energy is transferred between thermal phonons in the sapphire and the electrons in the film is determined by the electron/phonon interaction in the gold. Also, an additional heat capacity was observed to be present in the sapphire, which, for want of a better means of characterization, is ascribed to the tunneling systems. The magnitude of this additional heat capacity and its thermal coupling to the lattice has been studied.     

Quasi-Resonant Nonlinear Absorption for Optical Power Limiting: solgel-Processed Er(3+)-Doped Multicomponent Silica Glass

We demonstrate optical power limiting by what we believe to be a new mechanism of nonlinear absorption, which involves a quasi-resonant ground-state absorption that is either phonon assisted or assisted by the presence of defect sites (tail absorption). Such a mechanism provides high transmittance at low intensity yet optical limiting under cw conditions. The sample used was a novel solgel-processed Er(3+)-doped multicomponent silica glass. In this system the nonlinear absorption process is achieved because the resonant excited-state ((4)I(13/2) ? (4)S(3/2)) absorption cross section is larger than the quasi-resonant ground-state ((4)I(15/2) ? (4)I(9/2)) absorption cross section.     

The influence of the thermal and mechanical properties of optical materials in thin film form on their damage resistance to pulsed lasers

The influence of the thermal and mechanical properties of thin films on the damage threshold for pulsed-laser-induced damage is studied using the impurity-dominated breakdown model. This model consists of an inclusion embedded in a thin film. The cross section for the absorption of radiation by the inclusion is calculated using Mie scattering theory and found to be proportional to the geometric cross section in some cases. The geometric cross section is thus used when applicable for simplification.In a previous work the dependence of the damage threshold on the thermal and optical properties of the materials in the system was shown to be a strong function of the size of the imaginary part of the index of refraction of the inclusion. This led to a set of scaling laws that depended on the properties of the film, the inclusion and the incident laser pulse. The present work is an extension of that specifically including the thin film aspects.The thin film aspects of the problem are included by using a cylindrical impurity embedded within the film. Justification for this geometry is presented. Experimental data are also presented which support some of the major predictions of this model.The primary conclusion relating to the mechanical properties of the materials is that a thorough understanding of the absorption mechanism and thermal diffusion is required before the effect of mechanical properties can be realized.     

Features of mode locking in laser with quantum well in broad waveguide layer

Passive mode locking (PML) regimes in two-section lasers with quantum wells in broad waveguides operating at λ = 1.06 μm have been studied. The room temperature spectrum of the saturable absorber section retains exciton peak at the absorption edge, which decreases in amplitude shifts toward longer wavelengths by 18 meV when the reverse bias voltage is varied from 0 to 14 V. The PML regime is observed at relatively large voltages (above 9 V) that are necessary to compensate for the effect of band broadening in the gain section by the Stark shift of absorption in the reversely biased section. The PML regime exists, beginning with threshold values of the pumping current, and is characterized by a narrow RF signal with a 20-kHz linewidth. As the reverse bias voltage on the absorber section is increased, a low-frequency amplitude modulation superimposes on the mode-locked laser radiation.     

Rank Prize Funds Minisymposium on Non-classical Light for Communication

Abstract

We studied both experimentally and theoretically the transmission and polarization characteristics of a Cr⁴⁺ : YAG saturable absorber crystal as a function of the intensity and polarization state of an incident laser beam. We found that a birefringent absorption behaviour is induced and we show that the experimental results are well described by a full numerical model which includes excited-state absorption as well as a ground-state cross-absorption term. By using pump-probe measurements we show that a finite cross-saturation of the absorption is achieved along the crystal axes orthogonal to the polarization of the optical radiation. No induced refractive index birefringence is observed.