Practical Laser System Research Articles

All-fiberized amorphous carbon nitride (a-CNx) based passive Q-switcher

This research presents the first-ever demonstration of a passively Q-switched fiber laser in the 1.5 μm wavelength region using amorphous carbon nitride (a-CNx) as a saturable absorber (SA) in an all-fiber configuration. The a-CNx thin film was prepared using plasma-enhanced chemical vapor deposition. The laser operation showed remarkable stability, achieving a repetition rate of 57.99 kHz and a pulse duration of 1.36 μs at maximum input pump power. The peak power and pulse energy reached 100.56 mW and 136.77 nJ, respectively. The utilization of a-CNx as an SA offers potential benefits such as a simpler and more flexible cavity design, making it an attractive candidate for various applications, including material processing and medical laser equipment. This work contributes to the advancement of fiber lasers and expands the possibilities for utilizing carbon nitride-based SAs in practical laser systems.

Compact 15 mJ Fiber–Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper

We report on a millijoule-level fiber–solid hybrid hundred-picosecond laser system with a stable performance and compact structure. The laser system is based on a master oscillator power amplifier structure containing an all-fiber master oscillator, a quasi-continuous-wave side-pumped Nd:YAG regenerative amplifier, and a double-pass amplifier. By using the filtering effect of fiber Bragg grating and the dispersion characteristics of single-mode fiber stretcher, the spectrum broadening caused by self-phase modulation effect is effectively suppressed. Thus, the gain linewidth of the Yb-doped fiber seed source and Nd:YAG laser amplifiers is accurately matched. The reason for thermally induced depolarization in the solid-state laser amplifier is theoretically analyzed, and a more flexible depolarization compensation structure is adopted in amplifier experiment. Furthermore, the pulse energy of 14.58 mJ and pulse width of 228 ps is achieved at 500 Hz repetition rate. The central wavelength is 1064.1 nm with a 3 dB bandwidth of 0.47 nm. The beam quality factors in the horizontal and vertical directions are 1.49 and 1.51, respectively. This laser system has a simple and compact structure and has a power stability of 1.9%. The high pulse energy and beam quality of this hundred-picosecond laser are confirmed by latter theoretical simulation of copper laser ablation. It is a very practical laser system for material processing and laser-induced damage.

Plasmonic Saturable Absorbers

Plasmonic materials are able to spatially confine light to a nanometer scale far smaller than the diffraction limit, resulting in drastically enhanced electrical field strength and stimulating applications in nonlinear optics, energy, and biomedicine. Resonant excitation of plasmonic nanostructures by ultrafast pulses results in a subpicosecond‐scale transient photobleaching that originates from the thermalization and cooling of hot carriers, which manifests strong optical nonlinearity that is leveraged for optical modulation and switching. Herein, recent advances in the use of noble and non‐noble metal‐based plasmonic materials as saturable absorbers (SAs) in both solid‐state and fiber lasers for the generation of Q‐switched and mode‐locked pulses are discussed. Starting with a brief introduction on the operation mechanisms of pulsed lasers and photophysics of plasmonics, a discussion on the nonlinear optical (NLO) properties as well as the recent developments of pulsed lasers driven by these plasmonic SAs is focused upon. The pros and cons of using different plasmonic materials for SAs in terms of their material properties and linear/NLO responses are further analyzed. Along with a short summary of the current progress of plasmonic SAs, highlight future challenges in the development of plasmonic SAs for practical laser systems are finally highlighted.

Flip-Chip Wafer-Fused OP-VECSELs Emitting 3.65 W at the 1.55-μm Waveband

Optically pumped vertical external cavity surface emitting lasers (VECSELs) based on flip-chip gain mirrors emitting at the 1.55 μm wavelength range are reported. The gain mirrors employ wafer-fused InAlGaAs/InP quantum well heterostructures and GaAs/AlAs distributed Bragg reflectors fixed on a diamond heat-sink substrate in a flip-chip geometry, incorporated in a V-cavity configuration. A maximum output power of 3.65 W was achieved for a heatsink temperature of 11°C and employing a 2.2% output coupler. The laser exhibited circular beam profiles for the full emission power range. This demonstration represents more than five-fold increase of the output power compared to the state-of-the-art flip-chip VECSELs previously reported at the 1.55 μm wavelength range. It opens new perspectives for developing practical VECSEL-based laser systems operating at a wavelength range widely used in many applications.

Transient temperature effects on the optical power wavelength shift of a high-power laser system

The effect of junction temperature change on the wavelength shift and optical power output of high-power laser systems utilizing a fiber laser design is investigated using a mathematical model. Several laser diode array configurations, along with two different cooling fluids, are used to analyze the laser performance by numerically calculating the optical power out of the fiber laser for each case. The modeling approach presented here captures the dependence of optical power output on laser diode junction temperature by way of the optical power interactions between the laser diodes and fiber gain media components. Through the presented research, the beginning of a Multiphysics modeling approach has been created for better understanding the thermal requirements for a robust and practical high-power laser system. Future research will build off this and focus on the temperature dependent aspects of individual components (laser diodes, fiber gain media, and combining optics) in high power laser systems. In conjunction, high fidelity modeling of the listed components will provide a clear picture of the aggregate system-level thermal requirements. Gains in system-level thermal knowledge are instrumental for progress in high power laser technology and application.

Synchronization Maintenance of Synchronized Time-Lens Source in the Presence of Repetition Rate Drift of the Mode-Locked Laser for Coherent Raman Scattering Microscopy

Synchronized time-lens source can be synchronized to mode-locked lasers and has emerged as a new solution for two-color synchronized laser source. It has found widespread applications in coherent Raman scattering microscopy. Relative timing jitter between the synchronized time-lens source and the mode-locked laser is the key parameter for estimating the synchronization performance. Recently, it has been demonstrated theoretically that if the repetition rate of the mode-locked laser fluctuates around a mean value, then the relative timing jitter is only a fraction of the intrinsic timing jitter of the mode-locked laser. However, in some practical mode-locked laser systems, the repetition rate may also suffer from directional drift. Whether the time-lens source can be synchronized to such mode-locked lasers remains unknown. Here, we establish a physical model to account for both the directional repetition rate drift and the stochastic repetition rate fluctuation of the mode-locked laser, and demonstrate detailed numerical simulation results of the relative timing jitter in such synchronized systems. Our results indicate that various forms of repetition rate drift lead to an increase in the relative timing jitter compared with the drift-free case. However, the relative timing jitter is still smaller than the stochastic repetition rate fluctuation of the mode-locked laser when there is no drift, under practical experimental conditions. This suggests that the synchronized time-lens source is still a robust solution for two-color synchronized laser source.

Description of the NIF Laser

The possibility of imploding small capsules to produce mini-fusion explosions was explored soon after the first thermonuclear explosions in the early 1950s. Various technologies have been pursued to achieve the focused power and energy required for laboratory-scale fusion. Each technology has its own challenges. For example, electron and ion beams can deliver the large amounts of energy but must contend with Coulomb repulsion forces that make focusing these beams a daunting challenge. The demonstration of the first laser in 1960 provided a new option. Energy from laser beams can be focused and deposited within a small volume; the challenge became whether a practical laser system can be constructed that delivers the power and energy required while meeting all other demands for achieving a high-density, symmetric implosion. The National Ignition Facility (NIF) is the laser designed and built to meet the challenges for study of high-energy-density physics and inertial confinement fusion (ICF) implosions. This paper describes the architecture, systems, and subsystems of NIF. It describes how they partner with each other to meet these new, complex demands and describes how laser science and technology were woven together to bring NIF into reality.

Investigation of phase effects of coherent beam combining for large-aperture ultrashort ultrahigh intensity laser systems.

Large-aperture ultrashort ultrahigh intensity laser systems are able to achieve unprecedented super-high peak power. However, output power from a single laser channel is not high enough for some important applications and it is difficult to improve output power from a single laser channel significantly in the near future. Coherent beam combining is a promising method which combines many laser channels to obtain much higher peak power than a single channel. In this work, phase effects of coherent beam combining for large-aperture ultrashort laser systems are investigated theoretically. A series of numerical simulations are presented to obtain the requirements of spatial phase for specific goals and the changing trends of requirements for different pulse durations and number of channels. The influence of wavefront distortion on coherent beam combining is also discussed. Some advice is proposed for improving the performance of combining. In total, this work could help to design a practical large-aperture ultrashort ultrahigh intensity laser system in the future.

Adiabatic soliton laser.

The key to generating stable optical pulses is mastery of nonlinear light dynamics in laser resonators. Modern techniques to control the buildup of laser pulses are based on nonlinear science and include classical solitons, dissipative solitons, parabolic pulses (similaritons) and various modifications and blending of these methods. Fiber lasers offer remarkable opportunities to apply one-dimensional nonlinear science models for the design and optimization of very practical laser systems. Here, we propose a new concept of a laser based on the adiabatic amplification of a soliton pulse in the cavity-the adiabatic soliton laser. The adiabatic change of the soliton parameters during evolution in the resonator relaxes the restriction on the pulse energy inherent in traditional soliton lasers. Theoretical analysis is confirmed by extensive numerical modeling.

High-power diode-pumped ytterbium-doped fiber laser at 1150 nm

A model of steady-state rate equations including amplified spontaneous emission for long-wavelength ytterbi­um-doped fiber laser is set up, which provides design principle for a practical laser system. We demonstrate a diode-pumped all-fiber Yb-doped fiber laser at 1150 nm with an output power of 33.6 W, the optical efficiency is 60%.

An efficient 14.5 W diamond Raman laser at high pulse repetition rate with first (1240 nm) and second (1485 nm) Stokes output

Efficient operation of a pulsed diamond Raman laser at output powers up to 14.5 W is reported. An external-cavity Raman resonator 2.5 cm long employing a low-loss CVD diamond 9.5 mm long generated first (1240 nm) and second (1485 nm) Stokes output powers of up to 3 W and 13 W respectively when pumped with a 30 W q-switched Nd:YVO4 laser with approximately 22 ns pulses at a 36.5 kHz pulse repetition frequency. We show that this approach to efficient wavelength conversion is promising for a range of practical pulsed laser systems with high average power at first and cascaded Stokes orders in the near-infrared and including eye-safe laser output.

Modeling of end-pumped Yb:YAG thin-disk lasers with nonuniform temperature distribution

A plane wave model with nonuniform temperature distribution in the thin-disk crystal is developed to describe the dynamic behavior of an end-pumped Yb:YAG thin-disk laser. A set of couple-rate equations and 2D stationary heat-conduction equations are derived. The stable temperature distribution in the disk crystal is calculated using a numerical iterative method. The analytic expression is capable of dealing with more practical laser systems than previous works on this subject as it allows for nonuniform temperature distribution in the disk crystal. Based on these results, we examined laser output intensity as a function of pump intensity, dopant concentration, resonator coupler reflectivity, crystal thickness and temperature of cooling liquid.

High-Speed Wavelength-Swept Laser Source With High-Linearity Sweep for Optical Coherence Tomography

This paper reports on a high-speed, wide-range, wavelength-swept laser operating at 1310 nm with highly linear sweep performance for optical coherence tomography (OCT) applications. The simple and robust laser comprises a pigtailed semiconductor optical amplifier and a wavelength-scanning filter in an extended fiber cavity configuration. The tunable filter consists of a diffraction grating and polygon mirror scanner. A photodiode is used to generate a start trigger signal synchronized to start of each frequency sweep. Intracavity prism beam expanders are aligned to provide constant laser linewidth and linear frequency sweep. The laser exhibits a peak power of 15 mW. The tuning range of the laser is 117 nm maximum, and 100 nm full-width at half-maximum at a scanning frequency of 20 kHz. OCT signal in time-constant fast Fourier transform mode is compared with OCT signal using conventional wavelength rescaled processing. Coherence length was measured to be 5 mm. This approach realizes a practical and robust laser system suitable for deployment in swept-source-based OCT real-time imaging systems.

Analytical solutions for the electromagnetic fields of flattened and annular Gaussian laser modes I Small F-number laser focusing

Many laser interaction models assume that incident focused laser fields are Gaussian and use either the approximate TEM00 series model or the exact integral Gaussian angular-spectrum solution. Many practical laser systems, however, produce flat-top transverse intensity profiles, and indeed, such profiles are often desired. Here, an exact, integral solution is derived for all of the vector components having a general flattened Gaussian profile using the angular-spectrum method. This solution includes the pure and annular Gaussian modes as special cases. The resulting integrals are solved for tight focusing conditions exactly by making use of a Fourier-Gegenbauer expansion. This technique follows closely that of Sepke and Umstadter [Opt. Lett.31, 1447 (2006)] but, by redefining the expansion coefficients, the simplicity of the model is greatly enhanced and the computation time reduced by roughly a factor of 2 beyond the 2 orders of magnitude improvement obtained previously. This series solution is stable at all points and converges after S∼20w0 terms, where w0 is the 1/e waist normalized to the laser wavelength.

Effects of focusing-beam field variations on the stability of laser oscillators

Semiclassical models for the dynamical behavior of single-longitudinal-mode, homogeneously broadened, unidirectional ring-laser oscillators have long been available. However, in most of these models the beam's cross section is assumed to be independent of longitudinal position in the laser, while in practical laser systems it is common for the diameter to vary with position. A model for lasers with focusing-beam field variations is developed here. Linear stability analysis shows that higher values of excitation are required to reach the type 1 instability threshold for lasers with longitudinal variations of the beam diameter than for the uniform plane-wave laser model.

Q-switched operation of end-pumped Yb:YAG lasers with non-uniform temperature distribution

Equations on quasi-three-level end-pumped Q-switched Yb:YAG lasers are collected. The presented model is capable of dealing with more practical laser systems than previously published works on this subject as it allows for non-uniform temperature distribution in an end-pumped laser rod. Moreover, thermal analysis for an axially heated cylinder with thermally convective boundary at the periphery is performed to obtain temperature distribution along laser rod. To illustrate the utility of the collected equations, a diode end-pumped Q-switched Yb:YAG laser is examined. The results are compared with the former study. Output pulse energy reductions of about 21.7% and 14.5% under certain cooling conditions for single-pass pumping and double-pass pumping geometries are predicted, respectively.

Dental hard tissue modification and removal using sealed transverse excited atmospheric-pressure lasers operating at lambda=9.6 and 10.6 microm.

Pulsed CO(2) lasers have been shown to be effective for both removal and modification of dental hard tissue for the treatment of dental caries. In this study, sealed transverse excited atmospheric pressure (TEA) laser systems optimally tuned to the highly absorbed 9.6 microm wavelength were investigated for application on dental hard tissue. Conventional TEA lasers produce an initial high energy spike at the beginning of the laser pulse of submicrosecond duration followed by a long tail of about 1-4 micros. The pulse duration is well matched to the 1-2 micros thermal relaxation time of the deposited laser energy at 9.6 microm and effectively heats the enamel to the temperatures required for surface modification at absorbed fluences of less than 0.5 J/cm(2). Thus, the heat deposition in the tooth and the corresponding risk of pulpal necrosis from excessive heat accumulation is minimized. At higher fluences, the high peak power of the laser pulse rapidly initiates a plasma that markedly reduces the ablation rate and efficiency, severely limiting applicability for hard tissue ablation. By lengthening the laser pulse to reduce the energy distributed in the initial high energy spike, the plasma threshold can be raised sufficiently to increase the ablation rate by an order of magnitude. This results in a practical and efficient CO(2) laser system for caries ablation and surface modification.

Concentration effects on the IR-luminescent channels for Er- and Ho-doped LiYF 4 crystals

In this paper we report the ideal concentrations for the main infrared laseof channels r the Er 3+- and Ho 3+-doped LiYF 4 (YLF) crystals, under Xe lamp pumping. The number of photons per luminescent channel was also obtained for both ions. It was determined that 10–20% of Erbium at.wt is the ideal concentration for laser action at 2.74 μm, as well as the 1.7% Er-doped YLF crystal is the best one for lasing at 0.85, 1.23, and 1.72 μm under flashlamp pumping. The present method is a good approach in order to indicate the ideal concentration for an optimized four-level laser system. For the transitions at 1.62 μm (Er:YLF) and at 2.07 μm (Ho:YLF) it was observed that the luminescence intensities are maximized in the concentration range (25–35)% for Er ions and in the range (10–15)% for Ho ions in the YLF crystals. However, these concentration values are much higher than the ones used in a practical three-level laser system.

Lisk-Broom: A laser concept for clearing space debris

So-called “space junk” forced a change of plan for a recent Shuttle mission. However, ground-based lasers with atmospheric-turbulence-compensating beam directors represent a singularly effective method of de-orbiting space junk, because they use cheap Earth-based power, and because they lend themselves to rapid retargeting. Plasma physics and lasertarget interaction theory dictate the laser parameters for a particular mission. We will discuss a practical laser system and beam director with 20-kW average power at 0.5-µm wavelength that is capable of clearing most low-Earth-orbit objects with mass less than 100 kg in about 4 years. This is a special application of the Laser Impulse Space Propulsion (LISP) concept, by which objects are propelled in space by the ablation jet produced on their surface by a remote laser.

Field uniformity in a Chang electrode system

The effect of manufacturing and positioning errors and of system artefacts, such as supports, on the field uniformity in a Chang profile electrode system isstudied using a computational model. The Chang profile electrode is widely used in transverse discharge lasers. Computed field distributions on the electrode surface for a wide range of system errors and artefacts are presented; these permit the specification of tolerances and sensitivity to support structures to be determined for a practical laser system using Chang electrodes.