GB2256083A - Raman laser. - Google Patents

Description

2 2 5,6J1.33 1 Raman laser The invention relates to a laser device

including a Raman convertor, and particularly to an Nd:YAG laser.

Laser radiation is used in many technical areas, such as measuring distances or in radar. To avoid endangering people who are hit by direct or reflected laser beams,, operations may be performed at wavelengths in the so-called "eye-safe" range.

The term "eye-safe" is used for radiation which does not leave any damage on the human eye. Laser radiation entering the eye at wavelengths between 400 = and 1400 = damages the retina since radiation in this range is focused on the retina by the eye lens. On the other hand, radiation at wavelengths above 1400 n:m tends to be absorbed in or in the vicinity of the cornea surface such that the eye can withstand much higher radiation energy before damage due to parts of the cornea tissue beginning to be destroyed.

Raman lasers use a cell having a Raman-active medium for converting laser radiation of one wavelength into radiation of a different wavelength. The Raman-active medium is selected according to the desired wavelength of the displaced laser radiation. Methane (CH4) with a frequency shift of 2916 cm-1 enables the 1.064 gm radiation of the Nd:YAG laser, which is not safe for the eyes, to be converted to a wavelength of 1.54 gm, which is safe for the eyes.

The Raman scattering process is intensity-dependent. As a result, any alteration in the pump radiation intensity impairs the conversion efficiency at the desired wavelength.

Misalignments, tilts or curves on the Raman resonator mirrors cause deflections of the beam from the optical axis or shifts of the focal point within the Raman cell.

Furthermore, concurrent scattering processes, such as stimulated Brillouin scattering (SBS), can lead to a marked impairment of conversion efficiency. Normally SBS always occurs to a certain extent within the Raman medium, particularly when the optics are misaligned. Irrespective of the medium, Raman conversion and SBS are directly connected. The threshold condition for stimulated Raman scattering (SRS) must be below that of SBS, such that SRS occurs first and the energy of the pump laser radiation is used for the desired Raman conversion. Misaligned optics in addition lead to an increase in the SRS threshold as a result of focused pump laser beams not coinciding and reflected beams of the Raman-shifted laser. SBS is reflected back to the pump laser and can affect its beam form or even lead to destruction of optical components. This results in an optically critical adjustment and the necessity for precise mechanical construction.

Such laser apparatus is disclosed in German Patentschrift No. 31 14 815 which corresponds to European Patent No. 0 063 205. The entire Raman radiation is a single mirror, and f or this reason a half resonator is spoken of in this case. Using an optical insulator - thin film polariser and /4 plate, which is disposed in the area between the laseractive medium and the Raman cell, the back coupling of the Brillouin scattering radiation, which is frequently responsible f or damage or even destruction of the pump laser or its optical elements, is prevented. In addition, there is provided in the radiation passage direction behind the Raman cell a dichroic mirror which deflects the remaining pump laser radiation. However, this device, which in other respects is perfectly usable, is complicated to a certain extent and thereby subject to relatively great optical losses as a result of the abovementioned additional decoupling elements.

3 It is an object of the invention to make possible the provision of a Raman laser with a simpler construction and thus to avoid expenditure on the stable and precise adjustment of the optical components without impairing the conversion efficiency.

According to this invention, a laser device comprises a pump laser resonator bounded at its ends by respective reflectors, and a laseractive medium for generating within the resonator laser radiation of a first wavelength, wherein the device further comprises a Raman cell located in a radiation path between the laser-active medium and one of the reflectors so as to be embraced by the resonator, focusing elements disposed on a common optical axis and associated respectively with opposite sides of the Raman cell to focus radiation incident on the cell such that a sufficient pump laser radiation power density of the first wavelength is produced in a focus region for Raman conversion, and a beam divider for coupling out Raman- shifted radiation of a second wavelengt. With this type of laser the Raman cell is situated inside the pump laser resonator and may therefore be termed an Intracavity Backward Raman Resonator in this embodiment in laboratory jargon. It is advantageous that the entire radiation available in the pump laser resonator can be used in order to generate Raman-shifted radiation. A totally reflective mirror for the pump laser radiation on the laser-active medium side and a totally reflective mirror for the pump laser and the Raman-shifted radiation on the Raman cell side f orm a resonator f or the pump laser and a half resonator f or the Raman- shifted radiation. The radiation of the pump laser is amplified owing to the two totally reflective mirrors with high efficiency being within the laser-active medium, whilst the Raman-shifted radiation within the Raman cell is then amplified when the pump laser radiation exceeds the necessary threshold. This is achieved by means of focusing elements on either side of the Raman cell, 4 preferably with one between the cell and the beam divider and the other between the cell and the reflector on the cell side of the device. A further advantage is automatic adjustment between the pump laser resonator and the Raman half resonator such that the occurrence of SBS radiation is suppressed. These features permit the construction of a compact laser device with exceptional efficiency and high quality using simple optics.

In order now to be able to use for conversion the entire radiation produced at the first wavelength and in order to increase the desired wavelength conversion when using the device as a "Raman mirror" for the Raman medium, the reflector on the Raman cell side of the device should be 100% reflective at both wavelengths. By having the reflector on the laseractive medium side totally reflective only at the first wavelength, and having the focusing elements aligned with both reflectors, with one element between the Raman cell and the associated reflector being effective at both wavelengths, the optical paths of both focused radiations - of the first and second wavelengths within the Raman medium or between the reflector on the cell side and the Raman medium are substantially identical in order firstly to increase the desired wavelength conversion by stimulated Raman scattering and secondly to prevent damage or destruction of optical components as a result of undesired scattered radiation and competitive scattering processes. The reflector on the Raman cell side of the device and the focusing element on that side of the cell may be one and the same component in the f orm of a concave mirror, preferably bounding the Raman medium. This results in a shorter structural length with the same gas distance and in addition optical surfaces can be saved, which leads to lower losses. In this connection, the optical radiation path and thus the pump laser resonator may be constructed so as to be folded between the laser-active medium and the first focusing element by means of deflecting beam dividers, and so that the Raman-shifted radiation is coupled out of the folding region. This leads to a stable compact construction by altering the geometrical dimensions.

The dependent claims contain other optional features.

The invention will now be described by way of examples with reference to drawings in which:- Figure 1 is a schematic illustration of a laser device in accordance with the invention; Figure 2 is an illustration of a device similar to that of Figure 1 but with intersecting pentaprisms as reflectors; is Figure 3 is an illustration of another device in accordance with the invention, with a collapsed or folded pump laser resonator and a hollow mirror in place of one of the reflectors of the devices of Figures 1 and 2; and Figure 4 is another schematic illustration, showing a device similar to that of Figure 1, but with a so-called unstable resonator.

Referring to Figure 1, a Raman laser 1 in accordance with the invention has a Raman medium 14 which is inside a pump laser resonator 3 such that the entire radiation 7 of a first wavelength generated in the pump laser 2 is available for Raman conversion. For the sake of clarity, in the following embodiment an Nd:YAG laser which operates at a wavelength of 1. 064 gm is considered, although in other embodiments it is possible to use other laser media and thus to generate other primary radiation wavelengths without departing from the scope of the invention.

In this case. then, the pump laser 2 has an Nd:YAG laseractive medium 4 which is positioned between two totally 6 reflective reflectors 5 and 6 which, together with the optical elements lying therebetween, f orm the pump laser resonator 3. In this construction the entire radiation 7, 71 is retained between the two reflectors. The reflectors can each be an optical surface with a reflective coating or a polished mirror, a totally reflective penta or triple prism, or some other reflector unit of known type.

In order to achieve high radiation intensity in the pump laser 2, it is preferable to have an optical Q-switch 8 inside the pump laser resonator 3. The Q-switch 8 can be a saturable or fadable fluid or foil, a saturable crystal or some other known unit which f ades optically in order to achieve permeability with a given energy density or optical intensity. Likewise an electro-optical Q-switch such as a Pockel cell, Kerr cell, etc. may be used. High inversion can thus build up until the Q-switch is optically transparent and at this point the resonator quality is high such that a high-powered giant pulse is generated.

The 1.064 gm radiation 7 generated in the pump laser resonator 3 passes through the focusing elements 9 and 10 lenses in this case - inside which there is a Raman cell 11 with windows 12 and 13, which contains highly pressurised methane gas (CHO as the Raman medium 14. In order to generate high power density for Raman conversion, the radiation path does not pass any decoupling mirror which partially reflects the pump laser radiation.

The efficiency with which the 1.064 gm wavelength radiation is converted into radiation at 1.54 gm as a result of the scattering process by the molecules of the Raman medium 14 within the Raman cell 11 depends on the power density of the incident 1.064 gm radiation, the amplification of the Raman medium and the length of the conversion range in the Raman medium. Below a given threshold the radiation inside a focus range 15 of the cell 11 is not efficiently converted 7 into radiation at the new 1.54 tm wavelength. This threshold can be reduced by a longer conversion range. For this reason the mirror 6 reflects the radiation at both the 1.064 gm and the 1.54 gm wavelengths such that the stimulated Raman scattering 16 (SRS) thereby produced is amplified by back SRS of the incident pump laser radiation 7. By using an aligned common mirror which is totally reflecting at 1.064 jim and 1.54 4m, and common focusing elements, the optical paths of radiation at the two wavelengths 1.064 [im and 1.54 tm within the Raman cell 11 coincide, and undesired leakage radiation and competitive scattering processes such as SBS radiation are suppressed in favour of the desired conversion.

An amplified Raman-shifted beam 17 is coupled out as a Raman laser initial beam 19 by means of a dichroic beam divider 18 disposed between the totally reflective mirrors 5 and 6 as well as between the laser- active medium 4 and Raman medium 14.

In this Raman laser 1 conversion is brought about substantially by socalled backward SRS within the Raman medium 14; this scattering is increased by a back coupling of the Raman-shifted radiation 16 scattered forwards. This convertor may also be understood as a type of "half resonator" convertor in which only one mirror is used to back couple the Raman-shifted radiation.

By coupling the pump laser resonator 3 with the Raman half 30 resonator via the common mirror 6 totally reflecting the 1.064 gm and the 1.54 gm radiation, high quality is achieved for the two resonators owing to the pulse compression of the pump laser and the Raman-shifted radiator such that the initial energy and pulse width of the Raman laser are highly stable.

8 In order to render the Raman laser 1 insensitive to possible tilting of the totally reflective mirrors 5 and 6 and thus to achieve particular stability, two intersecting ridge or pentaprisms 51 and 61 maybe used as reflectors, as shown in Figure 2. Owing to the different refractive indices at the 1.064 gm and 1.54 jim wavelengths, in this case achromatic lenses 91 and 101 are used as focusing elements, these being located on either side of the Raman cell 11. In the case of a further embodiment which is not illustrated, these lenses can also be replaced by the windows 12 and 13 of the Raman cell 11. This insensitivity to misalignments inter alia demonstrates that the Raman laser is particularly stable with respect to energy and radiation quality even when the resonator is tilted.

Referring to Figure 3, a combination of a diffraction optical element and an intersecting pentaprism 61 may be used instead of achromatic lenses or lenses with the focusing element 9 to form a concave mirror 20. In addition, in the region optically between the first focusing element 9 and Nd:YAG medium 4 the beam path and thus the pump laser resonator 3 is doubled by means of two beam dividers 22 disposed at 45' relative to the optical axis. The diverging Raman-shifted radiation is collimated here by means of the lens 9 and additional lenses 23 disposed behind each respective beam divider 22. The amplified, Ramanshifted beam 19 can be coupled out alternatively via the two paths I or II.

When laser oscillation in the pump laser and Raman resonator 3 and 11 respectively is restricted to a transverse fundamental mode only a small volume of the laser-active medium 4 and of the Raman medium 14 is generally used, as a result of which the pump laser and Raman laser energy is restricted. Referring to Figure 4, a Raman laser 1 with an "instable resonator" has a first mirror 21 totally reflecting the pump laser radiation 71, a second mirror 20 9 totally reflecting the pump laser and Raman-shifted radiation, and a lens 9. The cross-section of the pump laser and Raman laser beam is thus not restricted by the resonator but only by the outer surface of the laser- active medium 4. As a result, the entire active volume of the laser- active medium 4 and thus also a larger active volume of the Raman medium 14 can also be used for the transverse f undamental mode. This is manifested by higher initial energy and improved divergence compared with a "stable resonator".

In all cases the primary radiation at the 1.064 gm wavelength and the Raman-shifted 1.54 gm radiation on the side of the Raman-active medium 14 always takes the identical closed path such that automatic alignment of the optics with an optimally aligned pump laser resonator 3 is ensured. In this case the Raman process is highly efficient and dominates the SBS. Thus, since the SBS radiation is only very slight or is reflected back in the direction of the laser-active medium 4, destruction of the optics is prevented.

Since the majority backward direction, the pump laser beam the focus area 15) energy is permitted occurs in the focus area.

of Raman scattering occurs in the (in this way, however, it is precisely 7 which is weakened before it reaches laser activity with relatively high before electrical spark breakthrough In practice a Q-switched, flash-lamp pumped Nd:YAG laser is used as the pump laser which produces a Raman-shifted initial beam with 45 mi per pulse and a pulse width of 4 ns with pressurized methane gas (CHO as the Raman medium. The pump energy for the Nd:YAG laser is then 8.5 J. In comparison to known Raman lasers this signifies a marked improvement in efficiency.

To sum up it can be stated that the proposed Raman laser operates with considerably improved efficiency and perfected automatic alignment between pump laser resonator 3 and Raman resonator. As a result, complexity of alignment is reduced and a compact uniform structure possible. The optical stability of this Raman laser is demonstrated in a pulse-topulse stability of 3%.

In practice a gaseous Raman medium 14, such as methane, is preferred in the described structure. Nevertheless the Raman medium used can also be one of the many gases, liquids or solids which generate SRS radiation at a desired wavelength. Examples of other Raman media of this type are CO, H2, D2, NH3 and a large number of glasses. The medium used is determined especially by the desired wavelength, pump laser wavelength and power requirements. The methods and structures described above enable the Raman laser to be used for a great number of Raman- and laser-active media for the pump laser.

Many modifications and variations of the above described devices, in particular with regard to multi-wavelength systems, are conceivable.

Claims (20)

1. A laser device comprising a pump laser resonator bounded at its ends by respective reflectors, and a laser- active medium f or generating within the resonator laser radiation of a first wavelength, wherein the device further comprises a Raman cell located in a radiation path between the laser-active medium and one of the reflectors. so as to be embraced by the resonator, focusing elements disposed on a common optical axis and associated respectively with opposite sides of the Raman cell to focus radiation incident on the cell such that a sufficient pump laser radiation power density of the first wavelength is produced in a focus region for Raman conversion, and a beam divider for coupling out Raman-shifted radiation of a second wavelength.

2. A device according to claim 1, wherein the pump laser resonator includes a Q-switch.

3. A device according to claim 1 or claim 2, wherein the Raman cell comprises a Raman medium and is bounded by inlet and outlet windows, the focusing elements are situated on opposite sides of the cell, and the beam divider is situated in an optical path between the cell and the laser-active medium.

4. A Raman laser device having:- a) a laser-active medium for generating laser radiation of a first wavelength within a pump laser resonator which is bounded at its end opposite the radiation direction by a totally reflective mirror and optionally uses a Q-switch; b) a Raman cell which follows in the radiation passage direction, is bounded by inlet and outlet windows, and comprises a Raman medium which can be energized by focused radiation of the laser-active medium; c) two focusing elements disposed on a common optical axis, within which the Raman cell is disposed such that in 12 the f ocus area the high power density of the pump laser radiation of the f irst wavelength required f or the Raman conversion is produced; and d) an optical element between the laser-active medium and Raman resonator in order to couple out undesired radiation, wherein:

e) a second totally reflective mirror is disposed in the radiation passage direction after the last focusing element or coincides with the latter and the Raman cell is thereby also embraced by the pump laser resonator; and f) the Raman-shifted wanted radiation of a second wavelength reflected by the second mirror can be coupled out by means of a beam divider.

5. A device according to claim 4, wherein the second mirror is constructed so as to be substantially 100% reflective for both wavelengths.

6. A device according to claim 4 or claim 5, wherein a second focusing element common to both wavelengths is provided in the area between the Raman cell and the second mirror.

7. A device according to any of claims 4 to 6, wherein the totally reflective mirrors are in the form of pentaprisms, triple mirrors, mirrors with a reflective coating or phase- conjugating mirrors.

8. A device according to claim 5, wherein the second mirror is in the form of a focusing concave mirror with a reflective coating and is provided instead of the second window of the Raman cell.

9. A device according to any preceding claim, wherein the focusing elements are convex lenses or achromatic lenses or lenses in the form of.a diffraction-optic element.

13

10. A device according to any preceding claim, wherein the optical coupling out element is a dichroic beam divider or a polarization coupling out device after which a collimator is connected if necessary in the coupling out direction.

11. A device according to claim 10, wherein the dichroic beam divider is constructed so as to be substantially reflective for one of the two wavelengths and permeable for the other wavelength respectively.

12. A device according to claim 11, wherein the first dichroic beam divider has a transmission of more than 99.5% for the first wavelength and a reflection of more than 98.5% for the second wavelength or the second dich ' roic beam divider has a transmission of more than 98.5% for the second wavelength and a reflection of more than 99.5% for the first wavelength.

13. A device according to any of claims 4 to 12, wherein the first mirror is formed so as to be totally reflective merely for the first wavelength and the focusing elements are aligned with both mirrors.

A device according to any one of the preceding claims, 25 wherein the optical radiation path and thus the pump laser resonator are constructed so as to be folded between the laser-active medium and the first focusing element by means of deflecting beam dividers, and so that the Ramanshifted radiation is coupled out of the folding region.

15. A device according to claim 2 or any of claims 4 to 14 including a Qswitch which is a saturable or fadable fluid or foil, a saturable crystal or an electro-optical Q-switch in the form of a Pockel or Kerr cell, or an acousto-optical modulator in the form of a Bragg cell.

16. A method of generating laser radiation comprising:

14 generating laser radiation of a first wavelength in a pump laser resonator having a laser active medium and being bounded at each of its ends by respective reflectors, arranging f or the radiation of the first wavelength to be incident on a Raman cell located in an optical path between the laser active medium and one of the reflectors, which reflector acts to reflect radiation passing through the cell and emanating from the cell back into the cell, focusing elements arranged on a common optical axis and on opposite sides of the cell being used to focus radiation entering the cell into a focus region for stimulating Raman scattering in the cell, and deflecting Raman shifted radiation of a second wavelength out of the resonator using a beam divider.

17. A method according to claim 16, wherein the Raman shifted radiation is coupled out of an optical path between the laser-active medium and the Raman cell.

18. Method of converting laser waves using a device according to any of claims 4 to 15, wherein:- a) the pump laser radiation of the first wavelength is converted, without previously passing through a coupling out mirror which is partially reflective for the pump laser radiation, by stimulated Raman scattering in the forwards direction and by means of the second mirror also in the rearward direction relative to the pump laser radiation as soon as the necessary threshold has been exceeded; b) the f orwards-scattered Raman-shif ted laser radiation is amplified by means of the back scattering of the Ramanshifted radiation using the totally reflective second mirror common to both wavelengths in each case and the focusing element and c) the Raman shifted laser radiation of the second wavelength is coupled out by means of the beam divider in the area between the laser-active medium and Raman medium.

is

19. A laser device constructed and arranged substantially as herein described and shown in the drawings

20. A method of generating laser radiation substantially as 5 herein described with reference to the drawings.