Laser ignition system

Abstract

A laser ignition system, in particular for an internal combustion engine, including a vertical emitter and a laser-active crystal, the laser-active crystal being doped in at least some areas using ytterbium, the ytterbium-doped area having a length of 200 m to 7000 m. The monolithic laser is based on a YAG or LuAG host crystal having 3 differently doped areas: a laser-active ytterbium-doped area, an undoped area which determines the resonator length and therefore the pulse duration, and a chromium-doped or vanadium-doped area for the passive Q-switch. The resonator is delimited by 2 mirrors.

Claims

1. A laser ignition system for an internal combustion engine, comprising: a vertical emitter used as a pump light source; and a laser-active crystal having three areas, the laser-active crystal being doped in at least some of the three areas using ytterbium; wherein the laser-active crystal is at least in areas at least one of: i) a YAG host crystal, ii) a LuAG host crystal, iii) a GGG host crystal, and iv) a GSGG host crystal, wherein a first area is an ytterbium-doped area, wherein a second area of the laser-active crystal is essentially undoped, wherein a third area of the laser-active crystal is doped using chromium (Cr.sup.4+) or vanadium (V.sup.3+), wherein the area which is doped using ytterbium has an ytterbium doping of 3 at % to 30 at %, wherein the third area is a passive Q-switch and has an initial transmission of not less than 30% for the wavelength generated in the laser-active material, wherein the vertical emitter is able to emit a wavelength in a range from at least one of: i) 935 nm to 940 nm, and ii) 965 nm to 985 nm, wherein the laser ignition system has a resonator which is delimited by a first resonator mirror and a second resonator mirror, wherein the first resonator mirror is located in the form of a coating on a front end of the laser-active crystal facing toward the pump light source, wherein the second resonator mirror is located as a coating on a front end of the laser-active crystal facing away from the pump light source, wherein the first area has a length in the range of about 200 m to about 7000 m, wherein the second area has a length in the range of about 4 mm to about 49 mm, wherein the third area has a length in the range of about 50 m to about 150 m, wherein the laser-active crystal, excluding the first resonator mirror and the second resonator mirror, has a total length of 5 mm to 50 mm, and wherein the Yb-doping concentration is selected to be sufficiently high so that the absorption length of the pump light source is 200 m to about 7000 m long and corresponds to approximately half of a Rayleigh length of the pump light source.

2. The laser ignition system as recited in claim 1, wherein the ytterbium-doped area has a length of 300 m to 5000 m.

3. The laser ignition system as recited in claim 1, wherein at least three areas of the laser-active crystal have the same host crystal.

4. The laser ignition system as recited in claim 1, wherein the third area has a length of greater than 100 m.

5. The laser ignition system as recited in claim 1, wherein the laser ignition system is configured as a laser spark plug having at least one of an electrical line, an optical line, and a thermal line.

6. The laser ignition system as recited in claim 1, wherein the laser ignition system is configured as a laser spark plug having at least one of: i) an arrangement for installation on an internal combustion engine, and ii) an arrangement for optical imaging of an emitted laser beam into a combustion chamber of an internal combustion engine.

7. The laser ignition system as recited in claim 1, wherein the laser ignition system is configured as a laser spark plug having a line for a cooling liquid for cooling at least one of the pump light source and the laser active crystal.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows an example of a laser ignition system according to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(2) FIG. 1 shows a schematic illustration of laser ignition system 1 according to the present invention, which includes a pump light source and a solid-state laser 3. The associated electrical lines for the power supply of pump light source 2, and details of the structural embodiment of laser ignition system 1 as a laser spark plug, which are described, for example, in European Patent No. EP 1 519 038 A1, are not shown. Means are optionally provided for the optical transmission of laser beam 11 or pump light 10. Furthermore, a thermal line, in particular for cooling liquid, may be provided for cooling pump light source 2 and/or solid-state laser 3 and/or other components. Possible means for installing laser ignition system 1 on an internal combustion engine are also not shown. Furthermore, for example, the joint arrangement of pump light source 2 and solid-state laser 3 in a housing and/or at least one means for optical imaging of emitted laser beam 11 in the combustion chamber of an internal combustion engine and/or a combustion chamber window, which separates laser ignition system 1 and the means for optical imaging from the combustion chamber, are not shown.

(3) In this exemplary embodiment, a vertical emitter is used as pump light source 2 for solid-state laser 4. The vertical emitter emits pump light 10 having the wavelength of 940 nm or in the range from 970 nm to 980 nm. The specifications of the wavelength are to be understood to include the uncertainties typical in laser technology. Pump light 10 is focused by a pump light lens 4 onto solid-state laser 3. Pump light lens 4 has an antireflective coating for the pump light wavelength used.

(4) Solid-state laser 3, which is shown in this exemplary embodiment, includes a laser-active crystal, which has three differently doped, i.e., partially doped and partially undoped, areas 5, 6, 7, and two resonator mirrors 8, 9. The monolithically formed laser-active crystal is doped using ytterbium in first area 5. Second area 6 is essentially undoped, essentially meaning that the host crystal was not intentionally doped and only has the typical manufacturing-related contaminants. Third area 7 is doped using Cr.sup.4+ or V.sup.3+.

(5) A YAG crystal is typically used as the host crystal. In an alternative exemplary embodiment, a LuAG crystal may instead also be used as the host crystal.

(6) First area 5 has an ytterbium-doping of 3 at % to 30 at % in a YAG crystal or a LuAG crystal. Length 12 of first area 5 is in the range from 200 m to 7000 m.

(7) Second area 6 has a length in the range of 4 mm to 49 mm.

(8) Third area 7 is used as a passive Q-switch. The initial transmission of the passive Q-switch is not less than 30%. The initial transmission is preferably not greater than 98%.

(9) Length 13 of third area 7 is longer than 50 m, in particular longer than 100 m. In an alternative exemplary embodiment, length 13 of third area 7 may also not be longer than 150 m.

(10) Total length 14 of laser-active crystal 3, without resonator mirrors 8, 9, is in the range from 5 mm to 50 mm.

(11) First resonator mirror 8 is located in the form of a coating on the front end of the laser-active crystal facing toward pump light source 2. First resonator mirror 8 is highly transmissive for the wavelength of pump light source 2 and highly reflective for the wavelength of solid-state laser 3.

(12) Second resonator mirror 9 is also located as a coating on the front end of the laser-active crystal facing away from pump light source 2. Second resonator mirror 9 is highly reflective for the wavelength of pump light source 2 and partially reflective for the laser wavelength of solid-state laser 3. The reflection of second resonator mirror 9 is in the range from 8% to 90%.

(13) One alternative embodiment provides only one resonator mirror 8 on the side of the laser-active crystal facing toward the pump light source. Second resonator mirror 9 is omitted, so that no further reflection takes place except at the interface between the side of the laser crystal facing away from pump light source 2 and the air. A laser ignition system 1 having improved robustness and simplified construction results. The reflectivity at the interface of, for example, a YAG crystal (n=1.82) to air (n=1) is approximately 8%.