Device for measuring backscattered radiation using a frequency selective element

A device for measuring the radiation returned from a material treated by means of an optical radiation source can be inserted onto and removed from the beam path of the optical radiation source. An optical deflecting device guides the radiation of the optical radiaton source out of its beam path and subsequently guides it back onto the original beam path. The optical deflecting device has at least one element which reflects the radiation of the optical beam source and transmits the returned radiation, and a detector for detecting the returned radiaton being arranged behind the element.

BACKGROUND OF THE INVENTION 
The invention for measuring the radiation returned from a material 
irradiated by an optical radiation source, which device can be placed in 
the beam the optical radiation source. 
The returned radiation generated when a material is irradiated, for 
example, by means of a laser, has a frequency which is sufficiently 
different from that of the laser radiation, to permit its measurement in a 
very simple manner. In particular, a frequency-selective beam splitter may 
be inserted into the beam path of the laser, which transmits the radiation 
of the laser, but reflects the returned radiation (German Pat. Document DE 
37 26 466 A1). The returned radiation is coupled out of the beam path, and 
is then directed to a detector where its intensity is measured. The 
returned radiation (or the detector signal generated by it) provides 
information on the treated material, on the respective treatment state and 
the like. 
When high-power radiation sources (high-power lasers) are used, however, it 
has been found that the transmission capacity of frequency-selective beam 
splitters is too low, which results in an unacceptable heating of the beam 
splitter and a weakening of the laser beam. For this reason, a 
complementary solution was suggested in which the laser radiation itself 
is reflected on a frequency-selective beam splitter having a reflection 
layer which is designed to be penetrable by the returned radiation (German 
Pat. Document 37 39 862 C2). However, this solution has the disadvantage 
that it changes the direction of the original beam path of the laser, and 
therefore cannot subsequently be built into or removed from an existing 
laser treatment device. 
It is therefore an object of the present invention to provide a device for 
measuring the radiation returned by a material treated by an optical 
radiation, which is suitable for particularly high-power radiation and, if 
necessary, can also be inserted in existing optical treatment devices, or 
be removed from them. 
The measuring device according to invention is based on the recognition 
that the reflectivity of frequency-selective beam splitters can be 
optimized to a greater degree by corresponding dielectric layers, than can 
their transmission behavior. However, a direct application of this 
principle conflicts with the demand for a measuring device which does not 
change the beam path of the laser, and which can be retrofitted and 
removed. Thus, the above mentioned object of the invention is achieved by 
means of a combination in which an optical deflecting device deflects the 
radiation of the optical radiation source out of its original beam path 
and subsequently guides it back to the same axis, and a 
frequency-selective element separates the returned radiation from that of 
the optical radiation source. 
In order to achieve a particularly high efficiency, it is advantageous for 
the optical deflecting device to have elements which reflect only the 
radiation of the optical radiation source (for example, by means of total 
reflection). However, it is also possible to deflect the radiation of the 
optical radiation source through prisms and to provide, at a point of the 
deflected beam path, the frequency-selective beam splitter which is 
reflecting for the radiation of the optical radiation source and 
transmitting for the returned radiation. This should advantageously take 
place at a site very close to the treated material.

DETAILED DESCRIPTION OF THE DRAWINGS 
In the embodiment illustrated in FIG. 1, the radiation 2 of a laser 8 is 
aimed by a focussing device at a material 10 to be treated. Alternatively, 
the radiation 2 may first be focussed by the focussing device 9 into an 
optical transmission path (for example, optical fiber) and may then be 
aimed at the material 10. Arranged between the laser 8 and the focussing 
device 9 is a deflecting device composed of elements 4, 5 and 1, which 
deflects the radiation 2 from its original optical axis and subsequently 
guides it back to this axis. The elements 4 and 5 are ridge prisms, at the 
base and roof surfaces of which the optical radiation 2 is reflected by 
90.degree. respectively. Element 1 is a partially reflecting mirror with a 
frequency-selective coating which reflects the radiation 2 of the laser 8 
with an efficiency that is as high as possible, but transmits the 
radiation 3 returned by the material 10, which is in a different spectral 
region than the radiation 2, to be processed. The returned radiation 3, 
which may be further deflected through a prism 11 and weakened by means of 
a filter 12, is then focussed on a detector 6 by a lens system 7. The edge 
prisms 4 and 5 as well as the frequency-selective beam splitter 1 may be 
rigidly connected with one another so that the whole deflecting device is 
relatively insensitive to slight tilting in that the original beam 2 will 
then change only slightly in height. The complete deflecting and measuring 
device may be arranged in a housing provided with openings, so that it can 
be relatively easily inserted in or removed from an existing laser 
treatment device without requiring of major adjusting devices on the 
system. 
In the embodiment illustrated in FIG. 2, the radiation 22 of a laser 28 is 
not deflected rectangularly in a U-shape as in FIG. 1, but rather in a 
V-shape. This arrangement has the further advantage that only two optical 
elements are required: a rhomboid prism 24 and, a frequency-selective beam 
splitter 21 corresponding to FIG. 1. The radiation 23 returned by the 
irradiated material 20 is then guided to a detector 26 by the beam 
splitter 21. 
In the embodiment illustrated in FIG. 3, the radiation 32 of a laser 38 is 
deflected also in a V-shape, by two prisms 34 and 35 arranged 
symmetrically adjacent one another as by well as a frequency-selective 
beam splitter 31. In this embodiment, the path of the radiation is also 
V-shaped, but symmetrical with respect to the beam splitter 31. Because of 
the prism 35, and as a result of dispersion the radiation 33 returned by 
the irradiated material 30 experiences a different deflection than the 
radiation 32 and, through the beam splitter 31, reaches a detector 36. In 
this arrangement, the radiation 32 is reflected only on the beam splitter 
31; an optical refraction takes place on the remaining optical interfaces. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.