Passively Q-switched solid-state laser with compressed pulse duration

Abstract

A passively Q-switched solid-state laser includes a resonator (1) with an active laser material (2) and a decoupling end mirror (6) for decoupling laser pulses that have a pulse duration of less than 1 ns from the resonator (1), an optical fiber (13), into which the laser pulses decoupled from the decoupling end mirror (6) are injected, and a chirped volume Bragg grating (17), at which the laser pulses are reflected after they have passed through the optical fiber (13) for shortening the pulse duration. The pulse duration after the reflection on the chirped volume Bragg grating (17) is less than 30 ps. The active laser material (2) is Nd:YAG and a saturable absorber (3) that is formed from Cr:YAG and has a transmission in the unsaturated state of less than 50% is also arranged in the resonator. The length (a) of the resonator (1) is from 1 mm to 10 mm and the laser pulses decoupled at the decoupling end mirror (6) have a pulse energy from 1 J to 200 J.

Claims

1. A passively Q-switched solid-state laser, comprising a resonator including an active laser material and a decoupling end mirror configured to decouple laser pulses that have a pulse duration of less than 1 ns from the resonator, an optical fiber configured to receive the laser pulses decoupled from the decoupling end mirror, a chirped volume Bragg grating configured to reflect the laser pulses after they have passed through the optical fiber for shortening the pulse duration such that the pulse duration after reflection on the chirped volume Bragg grating is less than 30 ps, the active laser material arranged in the resonator is Nd:YAG and a saturable absorber that is formed from Cr:YAG and has a transmission in the unsaturated state of less than 50% is also arranged in the resonator, and a length of the resonator is from 1 mm to 10 mm and the laser pulses decoupled at the decoupling end mirror of the resonator have a pulse energy from 1 J to 200 J.

2. The solid-state laser according to claim 1, wherein the optical fiber has a core diameter of greater than 10 m.

3. The solid-state laser according to claim 2, wherein the optical fiber is a polarization-maintaining fiber.

4. The solid-state laser according to claim 1, wherein a length of the optical fiber is from 0.5 m to 5 m.

5. The solid-state laser according to claim 1, wherein sides of the active laser material and the saturable absorber directed toward each other are at a Brewster angle relative to an axis of the laser beam, and are inclined in a same direction relative to an orientation at a right angle to the axis of the laser beam.

6. The solid-state laser according to claim 1, wherein an incident angle and emergent angle of the laser beam at the chirped volume Bragg grating is 0.

7. The solid-state laser according to claim 6, further comprising /4 plate and a polarizer, and a decoupling of the laser pulses after the reflection on the chirped volume Bragg grating is realized by the polarizer, and the /4 plate is arranged between the chirped volume Bragg grating and the polarizer.

8. The solid-state laser according to claim 1, further comprising a laser diode configured for pumping of the active laser material through an end mirror of the resonator opposite the decoupling end mirror.

9. The solid-state laser according to claim 8, wherein the pumping of the active laser material is continuous.

10. The solid-state laser according to claim 1, wherein an oscillation build-up of at least essentially only a single longitudinal mode is realized in the resonator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Additional advantages and details of the invention will be explained below with reference to the accompanying drawing. Shown therein are:

(2) FIG. 1 a schematic representation of a first embodiment of the invention, and

(3) FIG. 2 a somewhat modified embodiment of the invention.

DETAILED DESCRIPTION

(4) The laser has a resonator 1, in which Nd:YAG as active laser material 2 is arranged. Cr:YAG, more precisely Cr.sup.4+:YAG, is arranged in the resonator 1 as saturable absorber.

(5) The resonator 1 has an end mirror 4, through which the pump radiation 5 indicated by dashed lines is emitted. The end mirror 4 is highly reflective for the laser mode in the resonator 1, while it is highly transmissive for the pump radiation.

(6) On the side opposite the end mirror 4, the resonator 1 has a decoupling end mirror 6. This is used to decouple the laser beam 7. The decoupling end mirror 6 is thus partially reflective for the laser mode in the resonator 1.

(7) The resonator 1 is a linear resonator (=standing wave resonator). The length a of the resonator 1 (=geometric length or structural length from end mirror to end mirror) is in the range between 1 mm and 10 mm, preferably in the range between 2 mm and 6 mm.

(8) The embodiment is an optically stable resonator, as this is preferred.

(9) The resonator 1 is tuned by its length a so that a longitudinal mode of the laser radiation is right at the amplification maximum of the active laser material 2 of 1064 nm. Due to the shortness of the resonator 1, the free spectral range is so large that the adjacent longitudinal modes are so far outside of the amplification maximum that these essentially do not cause oscillation build-up (i.e., their energy is less than 10% of the dominant mode), in particular, do not cause oscillation build-up at all (i.e., the laser threshold is not exceeded). Thus, while operating, the laser has at least essentially only a single longitudinal mode. Advantageously, while operating, the laser also has at least essentially only a single transverse mode.

(10) The saturable absorber 3 forms a passive Q-switch for the laser. The transmission of the saturable absorber 3 in its unsaturated state is less than 50%, preferably less than 30%, and can be in the range of 10%, for example.

(11) Due to the shortness of the resonator 1 in connection with the high absorption of the saturable absorber 3 in the unsaturated state, laser pulses can be generated with a short pulse duration that is preferably less than 500 ps.

(12) In the embodiment, the active laser material 2 and the absorber 3 are each cut and polished in a so-called flat-Brewster configuration. The active laser material 2 and the absorber 3 are each at the Brewster angle relative to the axis 8 of the laser beam 7 on their sides directed toward each other, wherein these sides are at least essentially parallel to each other and the opposite sides are at a right angle to the axis 8 of the laser beam. The flat side is preferably coated with an anti-reflection coating for the laser wavelength (optionally also for the wavelength of the pump radiation). The Brewster surfaces are not necessarily coated. The two Brewster surfaces let the p-polarization through unhindered, but produce losses for the s-polarization. In this way, the resonator is forced to run in the p-polarization, so that the s-polarization at least essentially does not cause oscillation build-up (i.e., the energy is less than 10%, preferably 1%, of that of the fundamental mode of the p-polarization), in particular, it does not cause oscillation build-up at all (i.e., the laser threshold is not exceeded).

(13) To avoid an etalon effect for the s-polarization, which might reduce or switch off the losses for the s-polarization, the distance between the two Brewster surfaces must be selected exactly, so that there is not high transmission of the s-polarization for the wavelengths of any s-polarized modes that are close to the amplification maximum of the active laser material 2. Alternatively or additionally, a mutual slight tilting of the Brewster surfaces can also be provided.

(14) The absorption can have a variable construction for such a flat-Brewster configuration of the absorber 3 through a displaceability of the absorber 3 at a right angle to the axis 8 of the laser beam 7.

(15) On the other hand, the resonator 1 could also have a monolithic construction.

(16) For pumping the laser, a laser diode 9 can be used. The pump power is preferably in the range of 1-10 W, for example, approximately 5 W. The pump radiation has, for example, a wavelength of 808 nm. The pump beam is focused onto a diameter of less than 200 m in the active laser material in the embodiment.

(17) Preferably, the pumping is continuous.

(18) For example, the pump radiation output by the laser diode can be injected into a light guide 10. The emitted pump radiation output by the light guide 10 is collimated by a lens 11 and irradiated by the end mirror 4 into the active laser material 2.

(19) The laser beam 7 formed by laser pulses and decoupled at the decoupling end mirror 6 is injected by at least one lens 12 into an optical fiber 13, in particular, glass fiber. Due to the high intensity of the laser radiation in the optical fiber 13, the wavelength spectrum of each laser pulses is broadened by self phase modulation (SPM), which results in spatial spreading of the various spectral components. The length of the optical fiber 13 is preferably in the range from 0.5 m to 5 m, advantageously in the range from 1 m to 3 m, for example, 2 m. In particular, it is a large mode area fiber (LMA fiber). The core diameter can be, for example, in the range from 15 m to 20 m.

(20) Ideally, the fundamental mode of the fiber is excited.

(21) For avoiding back reflection, the entry facet of the optical fiber 13 is preferably cleaved so that it is at an angle to the orientation perpendicular to the axis of the laser beam, for example, at an angle of 8.

(22) The laser beam decoupled from the fiber is collimated by at least one lens 14 and is reflected at a chirped volume Bragg grating 17 after passing through a polarizer 15 and a /4 plate 16, wherein the pulse duration of each laser pulse is shortened. The reflected laser beam passes, in turn, through the /4 plate 16 and is then reflected at the polarizer 15, because the polarization direction is rotated by 90 relative to the first passage through the polarizer 15.

(23) The pulses of the emerging laser beam have a pulse duration that is less than 30 ps, preferably less than 20 ps. The pulse energies of the emerging pulses are preferably greater than 1 J. A downstream amplification, in order to use the emerging laser beam for micromaterial processing, thus can be eliminated.

(24) The pulse repetition rate is preferable in the range from 1 kHz to 100 kHz.

(25) In the embodiment, the laser beam is incident on the chirped volume Bragg grating 17 at a right angle, thus the incident angle and the emergent angle is 0. Incidence at an angle not equal to 0 would also be possible. The reflected laser beam would then be separated locally by the reflection of the incident laser beam. The polarizer 15 and the /4 plate 16 could also be eliminated in such a construction.

(26) A second, somewhat modified embodiment is shown in FIG. 2. The difference with the first embodiment consists in that, here, the Brewster angle is not applicable at the active laser material 2 and at the saturable absorber 3. A selection of the desired polarization of the laser beam could then be performed in a different way. For this purpose, e.g., an element of the resonator could have a grating structure, whose transmission differs for different polarization directions. For example, one of the end mirrors 4, 6 could be provided with a coating having such a grating structure. Through an appropriate selection of the orientation of the crystals of the active laser material 2 and the saturable absorber 3 and/or through selected application of a mechanical pressure on the active laser material 2 and/or the saturable absorber 3, such a polarization selection can also be achieved. It would also be conceivable and possible to eliminate such a polarization selection. The decoupling of the laser beam after the reflection on the chirped volume Bragg grating 17 could then be realized in a different way by a polarizer, for example, by an incidence of the laser beam on the chirped volume Bragg grating 17 at an angle not equal to 0.

LEGEND TO THE REFERENCE SYMBOLS

(27) 1 Resonator

(28) 2 Active laser material

(29) 3 Saturable absorber

(30) 4 End mirror

(31) 5 Pump radiation

(32) 6 Decoupling end mirror

(33) 7 Laser beam

(34) 8 Axis

(35) 9 Laser diode

(36) 10 Light guide

(37) 11 Lens

(38) 12 Lens

(39) 13 Optical fiber

(40) 14 Lens

(41) 15 Polarizer

(42) 16 /4 plate

(43) 17 Chirped volume Bragg grating