T 0631/91 (Raman laser/HUGHES) 27-01-1993

1. Admissibility of the appeal

The question whether the extent of the appeal within the meaning of Rule 64(b) EPC is sufficiently identified can only be decided in the context of a particular case, and the context of a particular case will normally include the contents of the decision under appeal. If an opposition has been rejected by the decision under appeal and it is simply stated in the notice of appeal that an appeal is lodged against that decision, such notice should properly be interpreted as meaning that the Appellant wishes to challenge such decision to the extent that the decision has rejected the requests made by the Appellant in his notice of oposition.

In the present case the notice of opposition includes a request to revoke the European patent in its entirety and this request is substantiated by an indication of facts, evidence and arguments against each claim of the patent in suit. Thus in the Board's view the notice of appeal in the present case should be interpreted as implicitly including a statement that the decision under appeal should be amended or cancelled to the extent that the Appellant's request for revocation of the patent in its entirety has been rejected in such decision.

Decision T 293/88 which is identified in paragraph VI(a) above was concerned with a case in which the Opponent in his notice of opposition only challenged the validity of certain specified claims of a European patent (Claims 1 to 6 and 8), and did not challenge the validity of three dependent claims (Claims 7, 9 and 10). This case is therefore quite different from the circumstances of the present case, where as explained above the patent was opposed to its full extent, and there are therefore no dependent claims which are not objected to.

For the above reasons, in the Board's judgment the notice of appeal in the present case satisfies Rule 64(b) EPC and Article 108 EPC, and is admissible.

2. Sufficiency of the disclosure for carrying out the invention

The wording of Claim 1 objected by the Appellant in paragraph V(f) refers to an alignment of the claimed combined - i.e. pump radiation transmitting and Raman radiation reflecting - output reflector 124 of the pump laser. In the Board's view, a skilled person derives from the description of the patent under appeal, column 7, lines 7 to 27, that the claimed combined reflector automatically aligns pump and reflected Raman radiation along substantially the same path from the reflector through focussing means 40 into enclosure 34 of the Raman medium, and that this closely identical path of pump and Raman radiation within Raman medium enclosure 34 causes the claimed enhancement of conversion of pump radiation into Raman radiation and the reduction of Brillouin radiation or avoidance of feedback damage. Hence, it is regarded to be a mere logical thinking of a skilled person to recognise that the claimed alignment of the three components - combined reflector, focusing means and Raman medium - reduces technically to the aim of imaging the pump light within the Raman medium in order to produce the claimed effects. Such an alignment, in the Board's view, is a mere experimental routine and moreover indicated in the description, column 4, lines 51 to 56, to be feasible by conventional means.

For the above reasons, in the Board's judgment, the ground of opposition laid down in Article 100(b) EPC does not prejudice the maintenance of the European patent.

3. Inventive step - Claims 1 and 13

3.1. Documents D5 and D7, which were cited for the first time in the grounds of appeal and in the oral proceedings respectively, have been examined by the Board according to Article 114(1) EPC with the result that they have no influence on the decision to be taken, and could therefore be disregarded under Article 114(2) EPC.

3.2. The Appellant concedes implicitly that Claims 1 and 13 are novel, in particular over document D6; see paragraph V(a). Thus, the only further substantive issue raised in this appeal is that of inventive step.

3.3. The Board agrees with the view of both parties as implicitly expressed in paragraphs V(a) and VI(b) and regards the Raman laser and method for laser wavelength conversion represented in document D6 as the nearest prior art.

Document D6 discloses in the wording of Claim 1:

"A Raman laser comprising: a pump laser (see D6, Figure 3, parts 1 to 5) for providing radiation at a first wavelength, having an output reflector (5); a Raman medium (13) disposed along the optical path of said first wavelength radiation from said pump laser for converting the wavelength of said first wavelength radiation to a predetermined second wavelength by Raman scattering processes; focusing means (9) disposed between said output reflector and said Raman medium and positioned suitably adjacent said Raman medium for focusing said first wavelength radiation into said Raman medium, said output reflector (5) being partially reflective of said first wavelength radiation so as to transmit said first wavelength radiation from said pump laser towards said Raman medium and "a Raman mirror (8)" which is substantially 100% reflective of said second wavelength radiation, said output reflector (5) being aligned relative to said focusing means (9) and said Raman medium (13)."

Document D6 discloses in the wording of independent Claim 13:

"A method for laser wavelength conversion (D6, page 1, paragraph 1) comprising the steps of exciting a lasing medium (4; page 4, lines 19 to 23) to produce pump laser radiation (18 in Figure 3) at a first wavelength; passing said pump laser radiation, along a first path, first through an output reflector (5) which is particularly reflective of said pump laser radiation, and second through a focusing means (9) which focuses said pump laser radiation to a focus within a Raman medium (13), stimulated conversion of said pump laser radiation to a second wavelength occurring in said Raman medium (page 2, lines 11 to 16 and page 5, line 32) and reinforcing said stimulated conversion of said pump laser by passing said second wavelength radiation back through said focusing means (9) along a second path; then reflecting said second wavelength radiation "from Raman mirror 8" which is substantially 100% reflective of said second wavelength radiation along a third path back through said focusing means and into said Raman medium, and aligning said output reflector (5), focusing means (9) and said Raman medium (13)."

3.4. Starting from the closest prior art as disclosed in Document D6, the objective problem underlying the present invention as claimed in Claims 1 and 13 is in the wording of these claims "to enhance desired wavelength conversion, to reduce undesired scattered radiation" and thus "to substantially avoid feedback damage to said pump laser".

3.5. This problem is solved according to Claim 1 in that not a separate Raman mirror (8) is used but the known output reflector (5) of the known pump laser (1 to 5) is additionally made "substantially 100% reflective of said second wavelength radiation so as to act as a Raman reflector for said Raman medium". In the language of independent method Claim 13 this problem is solved in that the second wavelength radiation is not reflected along a third path from a separate Raman mirror (8) but "from said output reflector which is 100% reflective of said second wavelength radiation ... so that said first, second and third paths are substantially identical".

Hence, the issue of inventive step underlying both independent claims reduces to the question whether it would be obvious for a skilled person to modify the conventional Raman laser disclosed in Figure 3 of document D6 by leaving aside conventional Raman mirror 8 and by making the conventional output reflector 5 of the pump laser reflective for Raman radiation instead in order to achieve the advantages defined in the formulation of the objective problem in paragraph 3.4 above, allowing in particular to eliminate conventional light diode 6, 7.

3.6. The Board follows the Appellant's view according to paragraph V(b) only insofar as a skilled person might expect an increase of stimulated Raman scattered (SRS) radiation in line with an improved alignment of the optical axis of the pump cavity with that of the conversion system. In order to reduce stimulated Brillouin scattering (SBS) in a Raman cell on the basis of the known experimental result disclosed in the cited prior art, a skilled person's logical thinking would lead to either a reduction of the pressure in the Raman cell (see Figure 5 of document D5) or to a reduction of the intensity of the pump radiation in the Raman cell (see the last three lines on page 409 of document D4). Hence, in the Board's view, a skilled person is not able to foresee that an improvement of the experimental parameters for an enhancement of the stimulated conversion into Raman radiation represents as well a technical measure to reduce the intensity of stimulated Brillouin scattering.

3.7. Contrary to the conclusions of the Appellant in paragraph V(c), the fact that documents D1, D2 and D3 are silent about detrimental effects of SBS-feedback to the used pump laser cannot be interpreted as suggesting that backscattered Brillouin radiation is harmless to the pump laser. Knowing that SRS- and SBS-radiation always appear simultaneously, a skilled person in the Board's view will rather ascribe the lack of particular protective measures for the pump laser used in the devices of these documents to either a negligible SBS-radiation which is due to its particular exitation conditions in the used Raman cell, or to an overdimensioned thermal capacity of the used pump laser. Moreover, document D6 states explicitly on page 5, lines 8 to 11, that light-diode 6, 7 prevents the optical elements of the pump laser from being damaged. In particular this statement keeps a skilled person from speculating that in view of the claimed kind of Raman system an elimination of SBS-radiation might be superfluous. Document D6 discloses light diode 6, 7 positioned between output reflector 5 and Raman mirror 8 as an indispensable element of the known Raman laser and thus logically excludes the technical possibility to combine output reflector 5 and Raman reflector 8 in one element as a measure of a routine simplification.

3.8. Though the pump laser output reflector in the devices of documents D2 and D3 might have a non-negligible reflectivity for Raman radiation (see paragraph V(b)), there is no statement nor a physical effect disclosed in these documents guiding a skilled person to see therein a technical means for automatic alignment. On the other hand, despite its certain amount of pump radiation reflectivity, mirror M2 of the apparatus disclosed in Figure 1 of document D3 is exclusively used as the central Raman mirror within the pump laser resonator of this intracavity arrangement. Its limited transmissibility for pump radiation is without a significant influence on the functioning of the pump laser and will be interpreted by a skilled person, in the Board's view, as an experimental limit in manufacturing a Raman mirror of narrow bandwidth; see also paragraph VI(c). The reflector corresponding to an output reflector coupling pump radiation into an extracavity type Raman cell, in the Board's view, is not mirror M2 but mirror M1 or M3 limiting the resonant cavity of the pump laser and thus forming the standing wave of the pump radiation. Hence, in the apparatus disclosed in document D3 the reflectors determining the effective propagation directions of the pump and Raman radiation are not arranged in the same plane. For these reasons, contrary to the Appellant's opinion in paragraph V(a) the Board regards a skilled person not to be able to recognise from the functioning of the device disclosed in document D3 that mirror M2, with its combined reflectivity for pump and Raman radiation might represent a technical means which would automatically align pump radiation leaving the pump laser resonator with radiation reflected from a Raman mirror arranged outside the pump laser cavity.

Furthermore, the cited prior art does not enable a skilled person to recognise in mirror M2 of document D3 a means for achieving the final technical aim of the patent under appeal, i.e. a prevention of SBS-feedback damage in the pump laser, enabling a replacement of means 5 to 8 in the apparatus of document D6 by mirror M2 of document D3. Though a reflector with the claimed physical properties was known beforehand (see also paragraph V(d)) and not difficult to manufacture (see paragraph V(e)), it was not obvious from the cited prior art to make use of such known reflector in order to solve a different technical problem as shown above; see also decision T 39/82, OJ EPO 1982, 419.

3.9. For the above reasons, the Board finds that the subject- matter of Claims 1 and 13 involves an inventive step within the meaning of Article 56 EPC.

4. Hence, it follows that granted Claims 1 and 13 are maintained. Granted Claims 2 to 12 concern particular embodiments of the Raman laser according to Claim 1 and are likewise maintained.