Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2000-07-18
2003-09-23
Davie, James (Department: 2828)
Coherent light generators
Particular temperature control
Heat sink
C372S034000
Reexamination Certificate
active
06625184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a cooling device comprising Peltier elements for a thermally highly loaded optical crystal, or laser crystal, respectively, from which laser beams, in particular laser pulses, are obtained, e.g. for the laser crystal of an optical amplifier or oscillator.
2. Description of Related Art
An effective cooling of optical crystals, or laser crystals, respectively, “crystals” in short hereinafter, in laser devices is of particular importance if the crystals, e.g. titanium-sapphire crystals (commonly termed Ti:S laser crystals) are subjected to high thermal loads during operation. This is, e.g., the case if in a passively mode-locked short-pulse laser arrangement (oscillator) the crystal is utilized as an optically non-linear element, and the pump beam and the resonator beam are focussed as highly as possible in the crystal; in doing so, the crystal should have small dimensions and, for compensation thereof, a high dotation so as to keep low the material dispersion, whereby the—specific—thermal load will rise, as has been explained in the earlier application WO-98/10 494-A not previously published. There it has also been explained that cooling to below 10° C. is a problem because of the humidity condensation occurring in that instance, wherein little drops condensed on the crystal may cause the crystal to be damaged rapidly or even to be destroyed.
What is of quite particular importance is, moreover, cooling of the crystal in case of an optical amplifier, as has already been mentioned in Optics Letters Vol. 22, No. 16, Aug. 15, 1997, pp. 1256-1258, “0.2-TW laser system at 1 kHz” by Backus et al. In such an optical post-amplification of oscillator pulses, e.g., also a Ti:S laser crystal is used in which the pulses from the oscillator having an energy of some nJ are amplified to an energy in the order of 1 mJ (i.e., by the factor 10
6
). To this end, the Ti:S amplifier crystal is “pumped” with green laser light which, e.g., has an average power of 10 to 20 W, which is a multiple of the pumping power at the laser pulse generation in the oscillator. Also by the fact that the optical amplifier is operated in pulses (the pulse frequency being, e.g., approximately 1 kHz), the pumping energy is concentrated to individual pulses which amplify the oscillator pulses. Due to the high powers occurring there, it is important to attain sufficient cooling for the crystal. Insufficient cooling of the crystal will not only result in a poor efficiency, similarly as with the oscillator, but also in an unfavorable beam profile, due to the “thermal lenses” effect which also is explained in the afore-mentioned article by Backus et al. If the crystal is heated, the temperature gradient thus occurring in its material will lead to a refraction index gradient which will unintentionally focus or defocus the laser beam during its passage—depending on the crystal material. Good cooling of the crystal will increase the thermic conductivity of the crystal material, and the temperature coefficient of the refraction index (which causes the “thermal lense” effect) becomes smaller at the low temperatures so that a beam profile approximately corresponding to the ideal Gaussian intensity profile (over the cross-section) will be attained; moreover, the degree of efficacy will be improved. According to the article by Backus et al., liquid nitrogen is used to cool the crystal, which does make it possible to attain extraordinarily low temperatures, by which, however, a practicable embodiment of the optical amplifier is prevented for many purposes of application, in particular for mobile uses.
A somewhat different optical amplifier has been described in the article “Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate” by S. Sartania et al., Optics letters Vol. 22, No. 20, Oct. 15, 1997, wherein general mention is made that a Peltier cooling device is used for cooling the amplifying crystal. Thus, the problem remains that with an intensive cooling not only condensation water will form on the crystal, but even ice, and that contaminations are present in the air which will deposit on the crystal; in operation, such ice formations and contaminations will lead to a—localized —destruction of the crystal surface by burning in.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome these problems and to provide a cooling device of the initially defined type with which, on the one hand, in spite of a simple construction that will render it particularly suitable for mobile applications, a good cooling in terms of a high degree of efficacy and an optimum beam profile will be achieved, and by which, on the other other hand, a long useful life of the laser crystal will be ensured by avoiding burning in of condensation water (ice), or impurities, respectively.
The inventive cooling device of the initially defined type is characterized in that the crystal, together with the Peltier elements provided for its cooling, is housed in an encasing container, that the interior of the container is evacuated and/or kept dry by means of a desiccating substance, and that the container comprises at least one Brewster window for the passage of the laser beams which is arranged under an angle relative to the optical axis which corresponds to the Brewster angle.
By providing an encasing container it becomes possible to evacuate the container interior or to keep it dry so that condensation water cannot deposit on the optical crystal, or laser crystal, respectively; moreover, defined clean surroundings (vacuum or pure, i.e. contamination-free, dry air) are possible for the crystal. Accordingly, long operating times can be achieved which is a great advantage also with a view to the expenditures required during the installation or during the precise adjustment of optical crystals, or laser crystals, respectively. Moreover, the present cooling device is characterized in that as a consequence of the use of the thermoelectric cooling elements, i.e. Peltier elements, in combination with the encasing container, a compact, simple, handy construction of the laser arrangement is made possible whereby, moreover, its use in vehicles, e.g. also in airplanes, is possible without any problems, since in contrast to cooling with liquid nitrogen, it is not gravity-dependent during its operation. The container may be provided with a tightly closable connection means for an evacuation as well as with tightly sealed electrical line passages for the power supply of the Peltier elements.
With a view to the high intensities occurring in the applications in question, so-called Brewster windows are provided on the container for the passage of the laser beams. In this manner, unintentional reflections can be prevented, i.e. without the broad-band antireflex coatings otherwise used therefor; because such dielectric coatings would not withstand the afore-mentioned high intensities (e.g. peak powers in the MW to GW range at beam diameters of <10 mm and at pulse durations in the 10 fs to ps range, starting from an average power of 10 mW up to the watt range; pump parameters: average power, a few W up to a few 10 W; pump energy, a few mJ; high repetition frequencies in the kHz range which will lead to peak powers in the kM to MW range).
It should be mentioned that with semi-conductor lasers it is known to use encapsulated modules, cf., eg., DE 33 07 933 C, DE 39 22 800 A, JP 1-122 183 A or EP 259 888 A, in which a laser diode element is present in combination with a Peltier element. However, there are no high laser powers and thus also merely low thermal loads on the laser diode elements, and the Peltier elements in fact are merely used for temperature stabilizing purposes. In the known semi-conductor lasers, this is important because in case of laser diodes, the laser wave length depends substantially on the temperature of the semi-conductor chip, and in many instances even its heating is required so as to obtain the correct wave length. Besides, in these known devices, an evac
Krausz Ferenc
Stingl Andreas
Davie James
Femtolasers Produktions GbmH
Ostrolenk Faber Gerb & Soffen, LLP
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