Ultrafast solid-state lasers

This article is intended to provide an introduction to ultrafast laser development for scientists new to the field and to provide a snapshot of the current state-of-the-art. In the first section, the main issues concerning ultrashort pulse generation are discussed and then the basic techniques of mode-locking are reviewed at a tutorial level. These include active mode-locking, passive mode-locking with real, resonant, saturable absorbers and passive mode-locking with the optical Ken effect. Emphasis is placed on practical ultrafast solid-state lasers for real-world applications.     

Effect of flashlamp diameter on luminescent coolants for a solid-state laser

Reducing the diameter of the flashlamp was found to increase the percent improvement in output of a solid-state laser caused by using luminescent coolants. However, this change in diameter also reduced the effectiveness of the flashlamp in producing long (150 s) laser pulses so that an overall decrease in laser output was observed. In the formation of short (40 s) laser pulses, on the other hand, larger absolute values were obtained using the smaller diameter flashlamp.     

Energy-scavenging amplifiers for miniature solid-state lasers

Longitudinally pumped miniature lasers are inefficient, partly because of their inefficient absorption of the pump light. Scavenging the unabsorbed pump light to pump an in-line amplifier can greatly enhance the efficiency of the system, with minimal added cost, size, or complexity.     

Diode-pumped ultra-short-pulse solid-state lasers

Many materials are good candidates for diode-pumped ultra-short-pulse lasers: several transition-metal-ion-doped crystals can or could support extremely short fs pulses. This goal, so far, has only been reached by Cr³⁺:LiSAF, but there are good chances for other crystals like Cr⁴⁺:YAG having its bandwidth within the third communication window, and the high-yield Cr²⁺:ZnSe with its impressive bandwidth in the near IR. Rare-earth-ion-doped media deliver only sub-ps pulses but allow unprecedented and scalable high average powers, like a SESAM mode-locked Yb:YAG thin-disk laser described recently. In all ranges of pulse durations there are fascinating applications ready for widespread employment as soon as compact, reliable and moderately priced ultra-short-pulse systems will be available for the non-laser-skilled user. The highest impact in the near future is attributed to microstructuring of materials and processing of biological samples, including dental enamel, by ps and sub-ps pulses, and optical coherence tomography needing pulses in the 10-fs regime at very modest average powers. Received: 29 June 2000 / Published online: 6 December 2000     

Scalable concept for diode-pumped high-power solid-state lasers

A new, scalable concept for diode-pumped high-power solid-state lasers is presented. The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink. This allows very high pump power densities without high temperature rises within the crystal. Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface. This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities. Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 4.4 W and a maximum slope efficiency of 68%. From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm³. Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs.     

Compact resonator designs for mode-locked solid-state lasers

Received: 16 February 1997/Revised version: 24 April 1997     

The efficiency of reflector systems for solid-state lasers

A technique for determining the efficiency of reflector systems is discussed. The efficiency and distribution of the light used for pumping in elliptical and circular cylindrical reflector systems are investigated experimentally. The experimental results are compared with calculations.In conclusion, the authors thank Yu. A. Anan'ev and O. N. Voron'ko for useful discussions and assistance.     

High-power single-frequency tunable solid-state lasers

Experimental results are presented on the achievement of single-frequency tunable lasing in ruby, Nd-glass, and Nd:YAG lasers with electrooptic Q switching of the cavity by the injection of an external signal. An optimization of the parameters is carried out for lasers on neodymium ions in yttrium aluminum garnet, lanthanum beryllate, chromium-doped gadolinium scandium gallium garnet, and lanthanum hexaaluminate with passive Q switching of the cavity by means of lithium fluoride shutters containing F ₂ ⁻ color centers. High-power single-frequency generation of giant pulses is achieved, with the output wavelength tunable over the half-width of the gain lines of the active media. Zh. Tekh. Fiz. 68, 74–79 (October 1998)     

Blue diode pumped solid-state lasers for digital projection

Lasers present many advantages over currently used light sources for projection applications. Compact as well as efficient displays can be realized with RGB laser systems. The extreme brightness and collimation of lasers enable very efficient light collection. For portable, battery-powered microprojectors or even integrated devices, where the efficiency becomes even more critical, 50 mW per color is enough for a luminous flux on the projection screen of 20 lm. While blue and red diode lasers in this power range are becoming widely available, the bottleneck for this application is still the lack of integrated green laser sources. We present here a blue-diode pumped Pr³⁺-doped LiYF laser emitting at 523 nm. By optimizing on many aspects of the crystal and resonator, we increased the laser output power up to 169.4 mW, which corresponds to a total power conversion efficiency of 7%. Moreover, lasing in red can be obtained with the same crystal with similar or even better output powers. This makes the Pr:YLF laser an ideal candidate for an RGB projection source together with blue InGaN diodes.     

New UV tunable solid-state lasers for lidar applications

We present the first lidar/DIAL system based on an Ultra Violet (UV) vibronic laser. A Nd:YAG-pumped Ce:LiSAF laser has been developed for this purpose, providing high pulse energy (5 mJ at 10 Hz), very high slope efficiency (33%), and a tuning range from 284 to 299 nm. Simultaneous measurements of SO₂ and O₃ concentration profiles are presented using this frequency-agile Ce:LiSAF-based lidar system.     

Nonelectrical method of pumping solid-state lasers

An alternative, nonelectrical method for obtaining a dense radiating plasma and the possibilities of using this method to pump solid-state lasers are investigated. The plasma was obtained experimentally by heating the working gas in a two-stage ballistic plasmatron. A new device — a vortex chamber — is proposed for transferring energy into the plasmatron-laser system. Zh. Tekh. Fiz. 68, 67–70 (September 1998)     

全固态激光中的光谱合成技术

对多种全固态激光中的光谱合成技术进行了探讨和研究,包括光纤激光、Yb:YAG板条激光和半导体激光。对于光纤激光,探讨了基于单个多层介质膜（MLD）光栅、一对MLD光栅、多个体布拉格光栅三种衍射光学元件的光谱合成技术中色散造成的光束质量退化问题,指出子束光谱线型的二阶矩全宽决定了光束质量的退化量,但所允许的光谱宽度又依赖于具体的技术选择途径。进而比较了三种光谱合成方案的优缺点。对于固体激光,实验演示了基于Yb:YAG晶体的板条激光实现光谱合成的原理可行性。通过设计一个基于MLD光栅的振荡器内的光谱合成装置,实现了7束子激光最高241 W的光谱合成输出,合成后光束质量β因子约4.1,表明大功率Yb:YAG板条激光具有通过光谱合束技术实现功率进一步提升的潜力。对于半导体激光,提出并设计了大模场外腔半导体激光+快轴光谱合成的技术。实验演示了9个1 mm宽LD芯片沿快轴方向的光谱合成,用β因子评价合成后的光束质量,在慢轴方向β≈6.3,在快轴方向β≈1.6,表明快轴光谱合成造成的光束质量退化是完全可控的。     

Polarization manipulated solid-state lasers with crystalline-orientations

The polarization states of 〈111〉-cut Yb:YAG crystal microchip lasers were investigated by pumped with the elliptically polarized pump beam from fiber-coupled laser-diode. The manipulated polarized lasers were achieved in laser-diode pumped Yb:YAG microchip laser by controlling the crystalline-orientations in 〈111〉-growth Yb:YAG crystal. Generally elliptically polarized lasers were obtained in laser-diode pumped Yb:YAG microchip lasers. However, crystalline-orientation manipulated linearly polarized laser was obtained when six different sites with different crystalline orientations were set to parallel to the major axis direction of the elliptically polarized pump beam. Six different sites in Yb:YAG crystal were separated with 30° and 90°, which were responsible for the linearly polarized laser oscillations. Circularly polarized lasers were observed when a Yb:YAG crystal was aligned to a special position between two sites responsible for linearly polarized laser oscillation. Effects of the polarization states of pump source on the laser polarization states of Yb:YAG microchip lasers and polarization direction of different polarized lasers with respect to Yb:YAG crystal rotation was addressed.     

Laser-related spectroscopy of ion-doped crystals for tunable solid-state lasers

In this work an overview of transition metal (TM) ion- and rare earth (RE) ion-doped crystals for application as tunable solid-state lasers will be given. Spectroscopic and laser results will be presented including recent research and advances in this field. Within this work tunability is defined as the possibility to achieve laser oscillation in the vibronic sideband of a transition. Tunable solid-state lasers are of interest for a wide field of applications, e.g. in scientific research, in medicine, for measurement and testing techniques, ultra short pulse generation, and communication. They can also be used as coherent light sources for second-harmonic generation, for optical parametric oscillators, and for sum- and difference-frequency generation. Tunable laser media based on 3d?3d transitions of transition-metal ions and 4f?5d transitions of rare-earth ions cover nowadays almost the whole spectral range between 270 nm and 4500 nm, see Fig. 1 [1-15]. In comparison to laser systems based on the 4f?4f transitions of trivalent rare-earth ions, tunable lasers based on 3d?3d and 4f?5d transitions are in general affected by a higher probability of excited-state absorption (ESA), a higher probability of non-radiative decay, and a higher saturation intensity leading to higher laser thresholds. Often laser oscillation cannot be obtained at all. These general topics will be considered in Sect. 1, where the basic aspects of tunable solid-state lasers are discussed: these are the preparational, the spectroscopic, and the laser aspect. In Sect. 2, the investigation of transition metal ion-doped crystals with respect to the realization of tunable laser oscillation is presented. The work is focused on transition-metal ions of the 3d row (Fe row) and divided into two subsections according to the octahedral and tetrahedral coordinations of the ion investigated. Each subsection is structured according to the electron configurations: 3d¹, 3d³, 3d⁴, and 3d⁸ for the octahedrally coordinated ions and 3d¹, 3d², and 3d⁴ for the tetrahedrally coordinated ions. Section 3 deals with interconfigurational transitions of divalent and trivalent rare-earth ions. Finally, in Sect. 4 the work is summarized. Received: 22 December 2000 / Published online: 30 March 2001     

Mode-locking of diode laser-pumped solid-state lasers

Mode-locked diode-pumped solid state lasers have become important sources for efficient and reliable short pulse generation. We review techniques for active mode-locking of diode-pumped lasers, highlighting techniques which have produced much shorter pulse durations than previous technologies and extended operating repetition rates to the several gigahertz regime. To achieve even shorter pulse durations a series of nonlinear passive mode-locking techniques have been developed. Self-starting additive pulse mode-locking, resonant passive mode-locking and self-mode-locking of diode-pumped solid-state lasers are described.     

High-power solid-state lasers with unstable resonators

The thermal lensing properties of stable and unstable resonators are compared and rules are given for the design of optimized unstable resonators containing a focusing rod. Experimental verification with a high-power Nd: YAG system proved that for unstable resonators the restricting relationship between beam quality and output power no longer holds. Careful resonator design enables high output power to be extracted with unstable resonators without destroying the output couplers.     

Broadband single-walled carbon nanotubes absorber for solid-state ultrafast lasers

The single-walled carbon nanotubes (SWCNT) absorber was fabricated by vertical evaporation method. Mode-locked picosecond Nd-doped solid-state lasers operating at 1.00 and 1.34 ??m and Tm-doped solid-state laser at 2 ??m with SWCNT absorbers were demonstrated, respectively. Watt-level average output powers of continuous wave mode locking were obtained. The operational bandwidth of 1 ??m is broader than other saturable absorbers fabricated by using other types of materials.