Short-pulse laser device

Details Short-pulse laser device Short-pulse laser device

1999-03-05

2002-02-05

Davie, James W. (Department: 2881)

Coherent light generators

Particular temperature control

Heat sink

C372S066000, C372S072000

Reexamination Certificate

active

06345061

ABSTRACT:

The invention relates to a passively mode-locked short pulse laser arrangement comprising a laser resonator to which a pump beam is supplied, a laser crystal, in particular a titanium-sapphire-(Ti:S-)laser crystal, and laser mirrors, the laser crystal, which is subjected to a thermal load on account of the beam focussing, being mounted on a cooling body provided for the removal of heat, which cooling body includes a bore for the passage of the laser beam.
Such laser arrangements are used for scientific purposes, on the one hand, and can be used in material processing, on the other hand, particularly if fine structures are to be produced.
In the mode-locked state, a laser emits laser pulses instead of a continuous laser light (continuous wave (cw) operation), by storing energy and emitting it thereafter in pulse-like manner. The duration of the periods of these pulses will generally correspond to the round trip time of the pulses in the laser resonator, and, e.g., with a length of the linear resonator of 2 m, pulses with a frequency of approximately 75 MHz will be generated; here, the laser light pulse passes the laser resonator in both directions, which in the instant example will correspond to a length of 4 m. For mode-locking, a loss is periodically introduced (with the resonator round trip frequency)—e.g. by deflecting or blocking the laser beam—so that the laser begins to pulse. This results in a peak power of the pulses that is substantially higher (amounting to 100 kW to 200 kW, e.g.) than the output power of the laser in cw operation (which is 150 mW to 300 mW, e.g.).
Basically, it can be differentiated between two types of mode-locking.
In active mode-locking, a periodic loss is introduced by means of an active element, a modulator, which is supplied with energy from the outside via a driver, e.g. by the modulator periodically deflecting the laser beam from its direction of propagation. Thus, the laser is forced to perform its laser activity in those time intervals in which there is a lower loss, whereas the laser can store energy in those time intervals in which there are high losses.
In passive mode-locking, the effect of an optical non-linearity in the resonator is utilized, i.e. an optically non-linear element is arranged in the path of the laser beam, and this non-linear element changes its optical properties, such as the transmission or reflectivity, proportionally to the intensity of the laser beam. As such a non-linear element, the laser crystal itself may, e.g., be used which forms a so-called saturatable absorber in which the loss will become the lower the higher the intensity of the impacting laser light. By a fluctuation in the laser power, a pulse is generated which “sees” a substantially lower loss than does the laser in cw operation (cf. also U.S. Pat. No. 5,079,772 A). The laser body (solid state laser) consists of a non-linear material whose optical “thickness” varies with the field intensity distribution of the laser radiation. The non-linear index of refraction, e.g., is a function of the square of the field intensity, i.e. the laser beam whose field intensity distribution may be considered to be like a Gaussian curve, effectively “sees” an element with an optical thickness that varies over its cross-section in the case of a laser crystal having plane-parallel faces. In this manner, a focussing lense results from a plane-parallel non-linearity.
This optical Kerr effect may be utilized for mode-locking in two manners (so-called “Kerr-lens mode-locking”): In the case of the so-called “soft aperture” (cf. Spence et al., Optics Letters, Jan. 1, 1991, Vol. 16, p. 42-44), the pump beam (in Ti:S lasers the energy is supplied by means of green laser, such as, e.g., argon laser) is very much focussed in the laser crystal so that the resonator beam produced by the Ti—S laser (approximately 800 nm, infrared) may then take up the greatest part of the pump energy, i.e. may have the highest gain, if it has the smallest diameter. Thus, the higher the intensity, or the field strength, respectively, of the pulse, the more the laser pulse will be focussed and the greater its gain at any passage through the laser crystal, whereby its intensity is increased again. This positive feedback leads to a stable mode-locking.
In the case of the so-called “hard aperture” (cf. e.g. U.S. Pat. No. 5,079,772 A) the effect is utilized that an aperture restricts the resonator beam at a site where it has a larger diameter at that time when the intensity (field strength) is lower, and has a smaller diameter at that time when the intensity is higher and the resonator beam thus is focussed in the laser crystal.
Other passive mode-locking techniques, e.g. semiconductor-saturable absorbers, are also known, cf. e.g. R. Fluck et al., “Broadband saturable absorber for 10-fs pulse generation”, Optics Letters, May 15, 1996, Vol. 21, No. 10, pp. 743-745.
To generate extremely short and thus high intensity pulses (in the femtosecond range) it is necessary to control the group dispersion in the resonator. Since pulses which are extremely short in the time range have a broad spectrum in the frequency range, there occurs the undesirable effect that in the laser crystal, the different frequency components “see” a different index of refraction and thus a different optical length of the laser crystal and thus are differently delayed when passing through the laser crystal. Thus, the pulses are lengthened again. To counteract this, the beam can be partitioned in terms of frequency by arranging optical prisms; the different frequency components will travel paths of different lengths, and in a further prism the beam is collimated (directed in parallel) again. As a consequence, the different frequency components will be delayed just reversely as in the laser crystal, whereby the dispersion introduced in the laser crystal is compensated again (cf. U.S. Pat. No. 5,079,772 A).
According to a further suggestion (e.g. Stingl et al., “Generation of 11-fs pulses from a Ti:sapphire laser without the use of prisms”, Optics Letters, Feb. 1, 1994, Vol. 19, No. 3, pp. 204-206), special laser mirrors are used which are assembled of many (>40) layers, the different components of the wave lengths penetrating to different depths in the mirror before being reflected. Accordingly, the different components of the wave lengths of the laser beam are delayed in the mirror for different periods of time; the short-wave components are reflected at the surface, whereas the long-wave components are reflected at a deeper location in the mirror and thus experience a delay as compared to the short-wave components. The advantage of the last-mentioned method is a better dispersion compensation, whereby extremely short pulses can be produced directly from a resonator.
Irrespective of the dispersion compensation technique used in detail, it is also important for the dispersion control, for producing extremely short laser pulses (in the order of 10 fs and therebelow), to keep low the material dispersion—primarily in the laser crystal, and for this it is suitable to use a thin, i.e. short, laser crystal (that is a laser crystal having a short path length). For reasons of compensation, the laser crystal should have a high dotation (e.g., already within 2 mm, it absorbs more than 70%). To keep the pumping threshold as low as possible and to thus ensure an efficient conversion of pumping power into laser output power, the pumping beam and the resonator beam should be focussed as much as possible. The greatly reduced dimensions of the pumped volume of the laser crystal will thus lead to an increased thermal load.
Thus, it is an object of the invention to provide a laser arrangement of the initially defined type in which an improved heat removal is provided for the laser crystal so that an increased thermal load on the laser crystal—at comparatively small dimensions of the same—and consequently, an increase in the output power will be rendered possible.
The inventive laser device of the initialy defined type thus is characterised in that a crys

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