Laser Design Parameters Research Articles

Efficient TEM00-mode solar laser using four Nd:YAG rods/four off-axis parabolic mirrors pumping approach

A four-rod/four-TEM00-mode beam off-axis parabolic mirror solar pumping concept is proposed. Four off-axis parabolic mirrors with 10 m2 total collection area were used as the primary solar concentrators to pump four 3.1-mm diameter, 84-mm length neodymium-doped yttrium aluminum garnet (Nd:YAG) rods within four 2V-shaped pump cavities through four secondary fused-silica aspheric concentrators and four rectangular fused-silica light guides, ensuring a good absorbed pump power distribution within each rod and avoiding the serious thermal lensing and thermal stress issues associated with classical single large rod solar lasers. The laser design parameters were optimized using ZEMAX© and LASer Cavity Analysis and Design (LASCAD©) analysis software to maximize the TEM00-mode laser power. 155.29-W TEM00-mode total laser power was numerically calculated, corresponding to 15.5 W / m2 solar laser collection efficiency and 1.72% solar-to-TEM00-mode laser conversion efficiency, respectively. This result represents an improvement of nearly 2 and 1.24 times, in solar laser collection efficiency and solar-to-TEM00-mode laser conversion efficiency, respectively, as compared with the previous experimental records of the TEM00-mode solar laser pumped through the parabolic mirror primary concentrator. It provides also a 1.14 and 1.19 times improvement, as compared to the previous numerical record of the TEM00-mode solar laser pumped by the Fresnel lens primary solar concentrator.

Ce:Nd:YAG Solar Laser with 4.5% Solar-to-Laser Conversion Efficiency

The efficiency potential of a small-size solar-pumped laser is studied here. The solar laser head was composed of a fused silica aspheric lens and a conical pump cavity, which coupled and redistributed the concentrated solar radiation from the focal zone of a parabolic mirror with an effective collection area of 0.293 m2 to end-side pump a Ce (0.1 at%):Nd (1.1 at%):YAG rod of 2.5 mm diameter and 25 mm length. Optimum solar laser design parameters were found through Zemax© non-sequential ray-tracing and LASCAD™ analysis. The utilization of the Ce:Nd:YAG medium with small diameter pumped by a small-scale solar concentrator was essential to significantly enhance the end-side pump solar laser efficiency and thermal performance. For 249 W incoming solar power at an irradiance of 850 W/m2, 11.2 W multimode solar laser power was measured, corresponding to the record solar-to-laser power conversion efficiency of 4.50%, being, to the best of our knowledge, 1.22 times higher than the previous record. Moreover, the highest solar laser collection efficiency of 38.22 W/m2 and slope efficiency of 6.8% were obtained, which are 1.18 and 1.02 times, respectively, higher than the previous records. The lowest threshold solar power of a Ce:Nd:YAG solar-pumped laser is also reported here.

Renovating electrical power-to-TEM00 mode laser power conversion efficiency with four-lamp/four-rod pumping scheme

One of the major drawbacks in lamp-pumped laser development resides in its low electrical input-to-TEM00 mode laser conversion efficiency. Here is proposed a four-lamp/four-rod solid-state laser pumping concept. Four Nd:YAG rods were pumped by four 1.0 kW input power Krypton CW arc lamps within a quadruple ellipse pump cavity, ensuring a good absorbed pump power distribution within each rod. The laser design parameters were optimized in Zemax® and LASCAD® software to maximize the TEM00 mode laser power. 35.80 W TEM00 mode total laser power was numerically calculated, corresponding to 0.90% electrical input-to-TEM00 mode laser conversion efficiency and 2.25 times improvement over that of the state-of-the-art lamp-pumped laser. For the extraction of a single TEM00 mode laser beam, a novel laser cavity was presented. 25.94 W laser beam brightness figure of merit was numerically calculated, corresponding to a 4-fold increase over that from each of the four TEM00 mode laser beams.

Stable TEM00-mode Nd:YAG solar laser operation by a twisted fused silica light-guide

To improve the output beam stability of a TEM00-mode solar-pumped laser, a twisted fused silica light-guide was used to achieve uniform pumping along a 3mm diameter and 50mm length Nd:YAG rod. The concentrated solar power at the focal spot of a primary parabolic mirror with 1.18m2 effective collection area was efficiently coupled to the entrance aperture of a 2D-CPC/2V-shaped pump cavity, within which the thin laser rod was pumped. Optimum solar laser design parameters were found through ZEMAX© non-sequential ray-tracing and LASCAD© laser cavity analysis codes. 2.3W continuous-wave TEM00-mode 1064nm laser power was measured, corresponding to 1.96W/m2 collection efficiency and 2.2W laser beam brightness figure of merit. Excellent TEM00-mode laser beam profile at M2≤1.05 and very good output power stability of less than 1.6% were achieved. Heliostat orientation error dependent laser power variation was considerably less than previous solar laser pumping schemes.

Opaque bubble layer incidence in Femtosecond laser-assisted LASIK: comparison among different flap design parameters.

The purpose of this study was to evaluate the incidence of opaque bubble layer (OBL) in femtosecond laser-assisted in situ keratomileusis (LASIK) flaps created with the support of Visumax Carl Zeiss femtosecond laser, planned with different flap diameters (7.90, 8.0, and 8.20mm) and the same laser energy and power settings. Incidence of intraoperative OBL in flaps of consecutive 108 patients (216 eyes) subjected to bilateral femtosecond-assisted LASIK was considered. Flap creation was performed with the same laser design parameters (spot distance and energy offset) and different presetting diameters of 7.90mm (72 eyes, group 1), 8mm (72 eyes, group 2), and 8.20mm (72 eyes, group 3). The incidence of OBL was considered and its extension was reported measuring involvement of different four corneal flap quadrants in which was theoretically divided the entire flap area; based on these data, OBL presence was classified as none (no evidence of OBL), minimal (minimal presence in not more that one quadrants corneal flap), mild (OBL presence in almost two or three quadrants without tendency to invade central cornea), and moderate (OBL presence in almost three quadrants with tendency to invade central cornea). In group 1, the incidence of OBL was of 23.6% (17 eyes) with a mild/moderate presence; in group 2, incidence was 20.8% (15 eyes) with mild presence. Group 3 presented a reduced OBL incidence (4.1%, 3 eye) with a minimal presence. No statistically significant difference was found between group 1 and 2 (p=0.8414).We found statistically significant differences between group 1 and group 3 (p=0.0012) and between groups 2 and 3 (p=0.0044). A significant reduction and extension of OBL incidence were evident when LASIK flap settings diameter was increased, and flap edge was closer to the contact glass border; this is probably consequent to a more effective gas dispersion outside of corneal flap.

Optimization of Yb:YAG thin-disk-laser design parameters considering the pumping-light back-reflection

Abstract A new approach for optimization of thin-disk laser design parameters is introduced in this paper. The back-reflected pumping power (BRP) is considered as one of the design parameters; for this reason optimization is conducted based on minimum pumping light back-reflection and acceptable optical efficiency. The effects of laser parameters on the laser efficiency and back-reflected pumping power are evaluated using analytical and numerical models. The agreement between the numerical results and experimental data verifies the validity of the numerical method. Based on the simulation results, the design procedure of Yb:YAG laser with maximum efficiency and minimum BRP is explained. Using the presented method for a typical Yb:YAG thin-disk-laser, it is possible to decrease the BRP from 300 W to 40 W while increasing laser efficiency from 49% to 54% for a disk of 190 μm thickness and 14% doping concentration. In this condition the BRP is just 40 W which is acceptable from view point of damage problems. The study reveals a pattern of results which shows that the detrimental back-reflection of pumping power could be controlled only by modifying active medium parameters without any change in beam-shaping or pumping-system configurations.

Performance investigation on clipping and RIN induced degradation for a single- and two-tone IM-DD SCM optical link

Subcarrier multiplexing (SCM) is preferred multiplexing approach for its simplicity and cost effectiveness in optical broadband transmission. In this work four basic SCM based intensity modulation direct detection (IM-DD) systems models have been devised to investigate laser source induced noise mechanisms. These system models reveal unique spectral behavioural characteristics associated to harmonic distortion (HD), inter-modulation distortion (IMD), clipping and random intensity noise (RIN) mechanisms. Response of clipping and RIN mechanisms to changes in system operational and laser design parameters show a direct dependence of clipping spillover on modulation current and an insensitivity of the RIN spectrum to changes in carrier amplitude. High bias currents are shown to reduce negative influence of clipping and RIN peak power while shifting RIN peak above subcarrier operating range of frequencies but increases RIN spectrum width. Low laser quantum efficiency is observed to favor clipping and RIN induced distortion while RIN peak position is insensitive to laser quantum efficiency. With active layer volume there will exists a tradeoff between clipping spillover and RIN peak power on one hand and RIN peak position and RIN spectral width on the other hand. High laser carrier lifetime will favor clipping spillover reduction. All the results so obtained can be exploited for identifying a suitable laser design in a suitable system model for achieving better system performance for the SCM based IM-DD systems.

Design of n-type silicon-based quantum cascade lasers for terahertz light emission

The design of terahertz quantum cascade lasers based on electronic intersubband transitions in Ge∕SiGe quantum wells is investigated. A detailed theoretical model of the conduction-band lineup of these heterostructures is first presented and used to show that large quantum confinement in the L valleys can be obtained with properly selected layer compositions and thicknesses. Computation of the key laser design parameters is then discussed, including the important role played by the L-valley ellipsoidal constant-energy surfaces. Finally, the main design issues specific to this material system and its potential for high-performance operation are illustrated by means of two exemplary structures, designed for emission near 50 and 25μm.

Two-dimensional Bragg grating lasers defined by electron-beam lithography

Two-dimensional Bragg grating (2DBG) lasers with two quarter-wave slip line defects have been designed and fabricated by electron-beam lithography and reactive ion etching. Unlike conventional two-dimensional photonic crystal defect lasers, which use a large refractive index perturbation to confine light in a plane, the 2DBG structures described here selectively control the longitudinal and transverse wave vector components using a weak index perturbation. Two line defects perpendicular to each other are introduced in the 2DBG to define the optical resonance condition in the longitudinal and transverse directions. In this article, we describe the lithography process used to pattern these devices. The 2DBG lasers were defined using polymethylmethacrylate resist exposed in a Leica Microsystems EBPG 5000+ electron-beam writer at 100kV. A proximity correction code was used to obtain a uniform pattern distribution over a large area, and a dosage matrix was used to optimize the laser design parameters. Measurements of electrically pumped 2DBG lasers showed modal selection in both the longitudinal and transverse directions due to proper design of the grating and defects, making them promising candidates for single-mode, high power, high efficiency, large-area lasers.

Atmospheric sodium monitor for Laser Guide Star Adaptive Optics

Laser Guide Star Adaptive Optics (AO) makes use of a laser to excite the mesospheric sodium (Na) layer and thereby creates an artificial star that can be used as reference for the AO wavefront sensor. The intensity of the created star depends strongly on the amount of Na present in the atmosphere. Experiments to measure the column density and details of the excitation and scattering properties of sodium atoms in the corresponding atmospheric layer are thus very important to refine the design parameters of laser and assess the power requirement. This article reviews the current knowledge on the mesospheric sodium derived from both astronomical and atmospherical studies. It then presents possible methods to study the sodium layer characteristics relevant for adaptive optics laser guide star systems.

Cavity length dependence of high-speed 1.55-μm multiple-quantum-well laser diode characteristics

Spontaneous emission spectra below threshold were measured from the side wall of InGaAs QW laser diodes to extract laser design parameters such as cavity length dependence of gain, linewidth enhancement factor, and serial resistance. The threshold current varies according to the change of cavity length, and thus, the lasing peak shifts and the serial resistance changes. It is interesting that the linewidth enhancement factor, however, is not deteriorated by shortening cavity length. The short cavity length would rather improve the linewidth enhancement factor mainly by shifting the lasing peak to smaller wavelength side, where the linewidth enhancement factor is inherently low.

Pulsed white light dye laser system: a theoretical study

A new configuration of a pulsed white light dye laser system is proposed. Design and detailed calculation of the laser cavity are presented. A special coated prism is used inside the cavity to guide the tricolor light. The output beam is expected to be a colinear balanced white light. Numerical values of laser design parameters are also given as an example.

Computer simulation of high-bit-rate optical fiber transmission using single-frequency lasers

Chirp-induced dispersion penalties in high-bit-rate optical fiber transmission are assessed using numerical integration of laser rate equations and a Fourier transform fiber dispersion routine. The roles of the imposed modulation waveform and laser design parameters are evaluated from computer generated eye diagrams and simple analytical observations. Consistent with experiment, we find device dependent optimum laser extinction ratios. In addition, we address the delicate balance between nonlinear chirp-induced dispersion penalties and the speed limitations imposed by linear current filtering on both the laser transmitter and the receiver. These considerations become increasingly important at higher bit rates such as 8 Gbit/s.

Kinetics of picosecond pulse generation in semiconductor lasers with bimolecular recombination at high current injection

The kinetics of generating ultrashort light pulses by gain switching unbiased semiconductor lasers emitting relaxation oscillations is theoretically modeled and described using phase portraits. Biomolecular recombination processes and realistic injection current pulse shapes are incorporated in the model. Approximate analytical solutions of the rate equations are derived for high current injection. Laser pulse widths, pulse peak power, electrical to optical pulse delay times, and time difference to subsequent relaxation oscillations are computed. Their dependence on injection current to threshold current density ratio ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J/J_{t}</tex> ) and on material and laser design parameters is explicitly derived and is in good agreement with experiment. In particular the remarkable observation that the laser pulse width is broadly independent of the injection current rise and fall time can thus be understood.

Thermal characteristics of GaAs laser junctions under high power pulsed conditions

The temperature rise at the junction of a GaAs laser has been calculated for a variety of pulse currents and base temperatures. The problem is first solved analytically by an approximation assuming constant thermal parameters, and then by a machine calculation taking into account the temperature variation of the thermal conductivity and the heat capacity. These solutions are compared and discussed in terms of practical laser design parameters. The effects of bulk resistivity and contact resistance are included.