Mode-locked thin-disk laser

Details Mode-locked thin-disk laser Mode-locked thin-disk laser

2000-03-17

2004-12-21

Vannucci, James (Department: 2828)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S011000, C372S021000, C372S034000, C372S018000

Reexamination Certificate

active

06834064

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to lasers and, more particularly, to mode-locked thin-disk lasers (also called active-mirror lasers) and to methods for generating pulsed laser radiation. The invention also relates to an apparatus for emitting pulsed electromagnetic radiation.
BACKGROUND OF THE INVENTION
Conventional solid-state lasers comprise a rod made of a solid-state laser gain material. The laser rod typically has the geometry of a cylinder, its longitudinal dimension (length) being larger than its transverse dimension (diameter). The, laser gain material is optically excited (pumped) by light, e.g., from laser diodes, impinging in transverse direction upon the cylindrical surface or in longitudinal direction upon the end faces. The laser radiation is emitted in longitudinal direction and recirculated in a resonator cavity.
The laser rod must be cooled in order to avoid damage caused by heat absorbed from the pump light, especially in high-power lasers. In the conventional solid-state lasers described above, dissipated power is removed in transverse direction from the cylindrical surface of the laser rod, e.g., by a cooling liquid. Such transverse cooling leads to a transverse temperature gradient inside the laser rod, i.e., the temperature in the middle (on or near the axis) of the rod is significantly higher than on the surface of the rod. Due to the temperature dependence of the refractive index and to thermally induced mechanical stress, the refractive index of the gain material also varies in transverse direction and is generally higher in the middle of the rod. This results in “thermal lensing” or thermally induced birefringence which can cause a very detrimental degradation of the laser beam quality and efficiency losses.
In order to overcome the problem of thermal lensing, a concept called “thin-disk laser” or “active-mirror laser” has been proposed (cf. U.S. Pat. No. 5,553,088 by Brauch et al., “Laser Amplifying System”; A. Giesen et al., “Scalable Concept for Diode-Pumped High-Power Solid-State Lasers”, Appl. Phys. B 58, 365-372, 1994; T. Kasamatsu and H. Sekita, “Laser-diode-pumped Nd:YAG active-mirror laser”, Appl. Opt., Vol. 36, No. 9, 1879-1881, 1997; all incorporated herein by reference). The basic idea of this approach is a very thin laser-crystal disk, one surface of which is longitudinally pumped by laser diodes, whereas the other surface is mounted on a heat sink. If the thickness of the disk is smaller than the laser beam diameter, one obtains a nearly one-dimensional heat flow to the cooled surface. Therefore, a uniform pump intensity distribution can generate a temperature profile which is uniform in the transverse direction, which minimizes thermal-lensing effects. Nearly complete pump absorption can be achieved despite the small thickness of the disk by arranging multiple passes of the pump radiation through the disk, using appropriate pump optics. In continuous-wave (cw) operation, this concept has allowed to generate as much as 100 W output power in a diffraction-limited beam (M. Karszewski et al., “100 W TEM
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operation of Yb:YAG thin disk laser with high efficiency”, Conference on Advanced Solid-State Lasers, OSA Technical Digest Series (Optical Society of America, Washington D.C., 1998), p. 296, 1998), more than has been achieved with other laser concepts.
Lasers emitting short or ultrashort (in the sub-picosecond range) pulses are known in the art. A well-known technique for short or ultrashort pulse generation is mode locking. Mode locking is a coherent superposition of longitudinal laser-cavity modes. It is forced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity-roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator at a given time. The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronized to the cavity-roundtrip time. However, ultra-short-pulse generation relies on passive mode-locking techniques, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. Passive mode locking relies on a saturable absorber mechanism, which produces decreasing loss with increasing optical intensity. When the saturable-absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking is obtained.
Ultra-short passively mode-locked solid-state lasers often use Kerr-lens mode locking (KLM) (cf. U.S. Pat. No. 5,163,059 by Negus et al., “Mode-locked Laser Using Non-linear Self-focusing Element”, incorporated herein by reference). In KLM, self-focusing of the laser beam due to the Kerr effect combined with either a hard aperture or a “soft” gain aperture produces a self amplitude modulation. Passive mode locking can also be achieved with semiconductor saturable absorber mirrors (SESAMs) (cf. U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers”, Journal of Selected Topics in Quantum Electronics (JSTQE), Vol. 2, No. 3, 435-453, 1996, incorporated herein by reference). A SESAM is a nonlinear mirror inserted inside the laser cavity. Its reflectivity is higher at higher light intensities due to absorption bleaching obtained by using semiconductors as the nonlinear material. A SESAM typically consists of a bottom mirror, the saturable absorber structure and, optionally, an additional antireflection or reflecting coating on the top surface. To date, mainly continuous-wave (cw) or Q-swithced thin-disk lasers have been reported, although lasers emitting short or ultrashort pulses are important tools in a wide variety of applications in physics, chemistry, biology and medicine. German patent application No. 199 07 722 discloses a thin-disk laser mode locked by a Kerr-lens mode-locking mechanism, but points out that a SESAM would be unsuitable for a high-power thin-disk laser. There are mainly two problems which arise when one tries to insert a passive mode-locking device, especially a SESAM, into a thin-disk laser.
The first problem are Q-switching instabilities. An unwanted tendency for Q-switched mode locking (QML) is introduced by a saturable absorber in the laser cavity. This results from the fact that e.g. some increase of the intracavity pulse energy over the stationary value (caused maybe by a pump fluctuation) leads to stronger bleaching of the absorber and thus an increased net gain, which in effect causes an exponential growth of the pulse energy. This growth is suppressed if gain saturation limits the pulse energy in time. Solid-state lasers materials (and Yb:YAG in particular) have low laser cross-sections and thus weak gain saturation effects, so that Q-switching instabilities are often difficult to avoid. We have made a detailed investigation of this problem and found a number of counter-measures (C. Hönninger et al., “Q-switching stability limits of cw passive mode locking”, J. Opt. Soc. Am. B 16, 46, 1999), which we also applied to the mode-locked Yb:YAG laser as described in this document.
The second problem is possible damage of the saturable absorber. This can be caused either by over-heating, or by non-thermal effects at high optical intensities, particularly if Q-switching instabilities lead to the generation of high-energy pulses. These damage problems can be critical in passively mode-locked high-power lasers, but in this document we show that they can be solved for thin-disk Yb:YAG lasers and do not prevent scaling to very high average powers.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a thin-disk laser which can be passively mode locked. The laser shall have a good beam quality (e.g., emit the fundamental TEM
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mode), a high efficiency (e.g., 25% or more), and emit short pulses (in the picosecond range or shorter) with a high average power (e.g., 10 W and higher) and/or high pulse energy (e.g., 0.5 &

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