https://hal.archives-ouvertes.fr/hal-00140068/document

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

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, we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally. This is the first step before studying the complex problem of thermal lensing (part III). 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. Part IV will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in part V, we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.

We present what we believe to be the first terawatt diode-pumped laser employing single-crystalline Yb:CaF2 as the amplifying medium. A maximum pulse energy of 420 mJ at a repetition rate of 1 Hz was achieved by seeding with a stretched femtosecond pulse 2 ns in duration, preamplified to 40 mJ. After recompression, a pulse energy of 197 mJ and a duration of 192 fs were obtained, corresponding to a peak power of 1 TW. Furthermore, nanosecond pulses containing an energy of up to 905 mJ were generated without optical damage.

Terawatt diode-pumped Yb:CaF2 laser

An optimized Yb: doped fiber mode-locked oscillator and fiber amplifier system for seeding Nd: or Yb: doped regenerative amplifiers. The pulses are generated in the Yb: or Nd: doped fiber mode-locked oscillator, and may undergo spectral narrowing or broadening, wavelength converting, temporal pulse compression or stretching, pulse attenuation and/or lowering the repetition rate of the pulse train. The conditioned pulses are subsequently coupled into an Yb: or Nd: fiber amplifier. The amplified pulses are stretched before amplification in the regenerative amplifier that is based on an Nd: or Yb: doped solid-state laser material, and then recompressed for output.

Utilization of Yb: and Nd: mode-locked oscillators in solid-state short pulse laser systems

The first demonstration of laser action in a Yb3+:Sr3Y(BO3)3 crystal is reported. This new material exhibits particularly broad absorption and emission bandwidths (6 and 60 nm, respectively) and hence is of particular interest for diode-pumped femtosecond and tunable laser sources. Efficient operation from diode pumping (1.4 W of cw laser power obtained with 3.6 W of diode pump power and as much as 3.4 W of peak power from high-power quasi-cw pumping) together with smooth tunability (over 50 nm FWHM) is reported. For the comparison, efficiency and thermal limitations of Yb:glass and Yb:Ca4GdO(BO3)3 have been investigated under the same conditions.

Spectroscopy and efficient laser action from diode pumping of a new broadly tunable crystal: Yb 3+ :Sr 3 Y(BO 3 ) 3

Summary form only given. Ytterbium-doped laser materials have recently received a great deal of interest. The low pump saturation intensity of Yb:glass at 975 nm (/spl sim/4 kW/cm/sup 2/) can benefit from diode pumping and the fluorescence bandwidth is broad enough to support subpicosecond pulses. By using a regenerative amplifier, the challenge of efficiently extracting energy from a low emission cross-section material can be met. Such advantages make it possible for Yb:glass lasers to be compact and cost-efficient, and thus, great candidates for applications such as surgery and micro-machining. We have demonstrated, for the first time, millijoule, subpicosecond pulses from a directly diode-pumped laser system. The gain medium was a 400-/spl mu/m thick, Kigre QX phosphate glass doped with 15% wt. Yb/sub 2/O/sub 3/, shaped in a keystone slab geometry. The initial cavity was composed of 2 spherical mirrors and 2 plane mirrors: a regular X cavity. Cylindric optics were then used to improve the mode matching between the pump and the lasing beams and to generate a TEM/sub 00/-like output beam. By replacing spherical optics with cylindrical optics, we improved the tangential M/sup 2/ value of the output beam of the bare cavity from 114 to 1.7, while maintaining the sagittal M/sup 2/ value at 1.3. The free running output power of the regenerative amplifier shows more obvious thermal effects than does that of the bare cavity at higher average pump powers because the combination of thermal birefringence and polarization sensitive optics, such as thin film polarizers, results in higher cavity losses.

Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier

We demonstrated a Yb:glass laser directly side-pumped by two 20-W diode bars with negligible thermal effects. The experimental setup has two cw 20-W, l-cm InGaAs laser-diode bars from Thomson-CSF, France, which were combined together by using a polarizer and a halfwave plate.

Directly diode-pumped Yb:glass laser with 2.8-W cw output power

An Yb:glass regenerative amplifier directly side pumped by four 20-W diodes is demonstrated. By use of a novel pumping scheme and introduction of cylindrical optics into the cavity, a free-running average output power as great as 4 W with a TEM(00) -like mode was achieved from the bare cavity, with a 0.56 pump duty cycle. When the regenerative amplifier injected, 1-mJ 200-fs FWHM pulses were obtained following compression by use of 2-ms pump pulses and up to a 150-Hz repetition rate.

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Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier

Directly diode-pumped Yb:glass laser with 2.8-W cw output power

Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier