Sébastien Forget

On thermal effects in solid-state lasers: The case of ytterbium-doped materials

Abstract A review of theoretical and experimental studies of thermal effects in solid-state lasers is presented, with a special focus on diode-pumped ytterbium-doped materials. A large part of this review provides however general information applicable to any kind of solid-state laser. Our aim here is not to make a list of the techniques that have been used to minimize thermal effects, but instead to give an overview of the theoretical aspects underneath, and give a state-of-the-art of the tools at the disposal of the laser scientist to measure thermal effects. After a presentation of some general properties of Yb-doped materials ( Section 1 ), we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally ( Section 2 ). This is the first step before studying the complex problem of thermal lensing ( Section 3 ). We will focus on some newly discussed aspects, like the definition of the thermo-optic coefficient: we will highlight some misleading interpretations of thermal lensing experiments due to the use of the dn/dT parameter in a context where it is not relevant. Section 4 will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in Section 5 , we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.

Passively Q-switched diode-pumped Cr4+:YAG/Nd3+:GdVO4 monolithic microchip laser

Abstract The realization of high repetition rate passively Q-switched monolithic microlaser is a challenge since a decade. To achieve this goal, we report here on the first passively Q-switched diode-pumped microchip laser based on the association of a Nd:GdVO 4 crystal and a Cr 4+ :YAG saturable absorber. The monolithic design consists of 1xa0mm long 1% doped Nd:GdVO 4 optically contacted on a 0.4xa0mm long Cr 4+ :YAG leading to a plano–plano cavity. A repetition rate as high as 85xa0kHz is achieved. The average output power is approximately 400xa0mW for 2.2xa0W of absorbed pump power and the pulse length is 1.1xa0ns.

Picosecond laser source at 1 MHz with continuous tunability in the visible red band

Abstract We report the first demonstration to our knowledge of a continuously tunable picosecond laser operating around 1 MHz. The emission can be tuned from 640 to 685 nm and the repetition rate from 200 kHz to 1 MHz with a pulse duration of less than 200 ps. The system is based on a Nd:YVO 4 passively Q-switched microchip laser providing a few tens of nanojoules per pulse. Two cascaded stages of amplification are then used to increase the pulse energy to several microjoules. The frequency-doubled radiation is then used to pump a periodically poled niobate lithium (PPLN)-based optical parametric generator in an all-solid-state architecture. Twenty nanojoules of tunable signal radiation are obtained. We also demonstrated 300-ps pulses generation in the UV (355 nm) at 1 MHz.

Organic Lasers Resonators

In this chapter we present the main resonators used for organic solid-state lasers, with a special emphasis on those based on thin-films such as planar waveguides or vertical external surface-emitting cavities. The influence of the resonator type on the laser properties (threshold, efficiency slope, output power, beam quality) is reviewed, with a special emphasis on works published after the year 2000.

Continuously tunable visible compact laser source using optical parametric generation in microlaser-pumped periodically poled lithium niobate

We demonstrated a very compact laser system providing continuous tunability at high-repetition rate in the visible range by pumping an optical parametric generator (OPG) of periodically poled lithium niobate (PPLN) with a frequency-doubled microchip laser operating at 532 nm. The 660-nm pulses were 0.55 μJ (with 40% of slope efficiency) and 500 ps, and the threshold was only 1.3 μJ (75 MW.cm-2).

Organic Materials for Solid-State Lasers

In this chapter we describe organic materials used in laser systems. We define a classification for organic molecules, and give some details about the chemical structure and the optical properties of some of the main families widely used as gain materials or host matrices in organic lasers. The technological processes allowing organic molecule deposition in various forms are then reviewed, as well as the main experimental techniques used to measure crucial parameters such as optical gain.

Passively Q-Switched Diode-Pumped Cr4+:YAG/Nd3+:GdVO4 High Repetition Rate Monolithic Microchip Laser

we report on the first passively-Q-switched diode-pumped Nd:GdVO4/Cr:YAG microchip laser. The average power is 400 mW and the pulse length is 1.1 ns at a repetition rate as high as 85 kHz.

Towards Applications of Organic Solid-State Lasers

While the first decade of research on organic semiconductor lasers (and more generally organic solid-state lasers) aimed at understanding the physics of such emitters and demonstrating efficient laser devices, now numerous application-oriented projects are emerging. They exploit the typical properties of organic emitters (wide tunability, easy fabrication, low thresholds and low cost) and benefit from improvements in device lifetime, output powers, beam quality or wavelength agility. We start this chapter by a brief review of recent research works that are being developed to improve the performance of organic lasers in an application-oriented view: lowering the threshold, extending the wavelength coverage, the wavelength agility (or tunability), improving the conversion efficiency, the beam quality, and the device lifetime. We then report on three major applications for organic lasers: spectroscopy, chemical sensing and short-haul communications.

Novel Concepts for Organic Lasers

In this chapter, we attempt to analyze some novel research tracks in the field of organic lasers. The quest of the “organic laser diode”, or electrically-pumped organic semiconductor laser, is not properly speaking a novel research axis as it has been driving the scientific community since almost the beginning of the story of OSC development. However it remains unrealized at the time of writing this book, and it is still a major scientific challenge. Many works have contributed to a better understanding of the bottlenecks preventing electrical pumping of organic lasers, they are briefly reviewed here. In the meantime, new and alternative concepts came to birth. Optically-pumped organic lasers ceased to be regarded as intermediate steps leading to a future electrically-pumped device; instead they started to be regarded as interesting devices on their own, as far as cheap pump sources, such as laser diodes or even light-emitting diodes, could be used. A fundamental challenge which remains to be solved is the realization of a true CW laser with an organic solid-state medium. By “true CW” it is meant that the laser medium is not physically moving to mimic dye circulation. Triplets are central in this problem and they have to be “managed” in some way; some interesting solutions have been proposed which are also briefly reviewed. Finally, Interesting new research fields have recently emerged using organic gain media in devices that are not classical lasers anymore, but rather devices based on quasiparticles such as exciton-polaritons (mixing of photon states and excitons) or Surface Plasmons (and Surface Plasmon Polaritons). The latter are called spasers, not all of them are organic but these devices can clearly beneficiate from the high gain provided by organic materials; they offer great promises in the realization of sub-wavelength coherent sources.

Fundamentals of Organic Lasers

In this chapter the main characteristics and specificities of organic solid-state lasers are presented. We particularly highlight these aspects which are important for organic lasers and specific to them, and which are therefore not usually treated in classical textbooks on lasers. The objective of this chapter is to present a quite general, while not exhaustive, overview of the photophysics of organic compounds that are directly useful to understand the physics of organic lasers, as well as a theoretical framework suited to the description of these lasers in most practical situations.