Abstract:
Optical reflector for receiving an input beam and transmitting a retro-reflected reciprocal output beam. The optical reflector includes a beam splitter for splitting the input beam into first and second secondary beams. The first and second secondary beams being parallel to one another. A reflecting system is also included for redirecting the first and second secondary beams toward the beam splitter. The reflecting system including a total reflector. The beam splitter and the reflecting system are self-aligned.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical reflector intended for receiving an input beam and transmitting a reverse-direction and parallel output beam, optionally superimposed on the input beam. 
     BACKGROUND OF THE INVENTION 
     It is well known that the alignment of the optical components is decisive for the quality of the devices that are fitted with the components. Therefore, any self-alignment, i.e. any assembly in which the properties of the output luminous flux are little sensitive to the orientation or to the position of one or several components, is required. 
     Among the self-aligned retroreflecting systems known for a long time, the following can be mentioned for exemplification purposes: the cube corner illustrated on FIG. 1 with which an incident beam  1 ,  1 ′ on a reflecting orthogonal trihedron  3  produces a parallel output beam  2 ,  2 ′, whatever the angle of incidence with respect to the diagonal  5  of the cube and the position of the point of incidence  4 . 
     The so-called ‘cat&#39;s eye’ assembly is also well known, consisting of a convergent optical system  8  with optical axis  9 , in the focal plane of which is located a mirror  10 , substantially perpendicular to the axis  9 . A collimated incident beam  11 ,  11 ′ converges onto the mirror  10 , is reflected on the mirror  10  and then diverges in return onto the optical system  8  that produces an output beam  12 ,  12 ′, also collimated and parallel to  11 . Such a cat&#39;s eye is represented on FIG.  2 . 
     Both these systems described previously offer self-alignment of the direction of the output beam  2 ,  2 ′ and  12 ,  12 ′ on the input beam, respectively  1 ,  1 ′ and  11 ,  11 ′ in two dimensions, i.e. in all the planes parallel to the direction of the input beams. In certain systems, one can seek to obtain the self-alignment in a single dimension. In this case, one can use either an orthogonal dihedron instead of the trihedron of FIG. 1 or a cylindrical cat&#39;s eye, i.e. a lens or a cylindrical optical system instead of the spherical optical system  8  in the case of FIG.  2 . The dihedron ensures self-alignment in the plane perpendicular to its edge and the cylindrical cat&#39;s eye in the plane perpendicular to the generatrix of its cylindrical lens. In the parallel plane, both these systems behave like a mirror. 
     Self-aligned separating optical components or devices, which produce two emerging beams, parallel to one another, from a single incident beam, are also known. These may be for instance a periscope or a blade with parallel faces one of which is partially metallized, in order to modify its reflection coefficient on a portion of its surface. 
     Conventional dihedra and trihedra allow however only total reflection of the beam whereas, in certain applications, a second partial output is required in addition to the first retroreflected output. 
     SUMMARY OF THE INVENTION 
     The invention implements an interferometric device of the Sagnac interferometer type, capable of producing two beams, respectively identified as a reciprocal beam and a non-reciprocal beam, in relation to the number of reflections affecting each of the two interfering beams to make the output beams. 
     The Sagnac interferometers are well known. They consist of a beam splitter and a ring, i.e. a closed optical beam beginning and ending at the beam splitter. 
     This ring and the beam splitter are arranged so that an input beam is split into two secondary beams, each circulating in the ring, respectively in opposite directions. 
     Thus, when returning, the waves corresponding to each of these beams interfere and produce two output beams. 
     This ring often consists of three independent mirrors and it is well known that a reciprocal output is thus produced for which the interfering waves have been subjected to the same number of reflections during their circulation in the ring and a non-reciprocal output for which these waves have been subjected to a number of different reflections. 
     By ‘matched phase front’, we mean the state of interference obtained when the orientation of the mirrors is adjusted to spread the interferences fringes down to their complete elimination. 
     The aim of the invention is to provide such a Sagnac interferometer comprising an optical reflector, that benefits from the advantages of a self-aligned reflecting device and that would be then particularly simple to adjust and whose long-term stability is improved. 
     Another advantage of the invention is to allow the possible construction of wavelength tuneable laser, possibly continuously, that would enable extracting the output luminous beam in optimised conditions, with minimum losses. 
     According to another embodiment of the invention, it is possible to obtain a laser whose background noise resulting from the ASE (Amplified Spontaneous Emission) is filtered spectrally and that exhibits therefore better efficiency at the emission wavelength of the laser. 
     The invention also relates to an optical reflector receiving an input beam, transmitting a retroreflected reciprocal output beam and comprising a beam splitter giving a first secondary beam and a second secondary beam, parallel to one another as well as retroreflecting systems each redirecting the secondary beams toward the beam splitter and forming a Sagnac interferometer. 
     According to the invention, the reflecting system comprises a self-aligned total reflector and the beam splitter is self-aligned. 
     Preferably, in various embodiments of the invention each having their respective advantages: 
     the self-aligned total reflector is unidimensional; 
     the self-aligned total reflector is bidimensional; 
     the beam splitter is energy-unbalanced, whereas the reflector thus transmits a non-reciprocal output beam; 
     the optical reflector comprises a diffraction grating forming with the total reflector a Littman-Metcalf system; 
     the diffraction grating is located between the beam splitter and the total reflector. 
     The invention also relates to a laser source with external cavity comprising an amplifier medium and a retroreflecting dispersing device comprising an optical reflector formed by a Sagnac interferometer with a total reflector and a Littman-Metcalf configuration grating. 
     Advantageously: 
     the amplifier medium is a wave-guide and it is associated with collimation optics thereby collimating the beam that it produces; 
     the external face of the wave-guide is totally reflecting and the non-reciprocal beam is the single beam transmitted by the source; 
     the dihedron is mobile in rotation to enable the variation of the wavelength; 
     the dihedron is mobile in rotation and in translation to enable continuous variation of the wavelength; 
     the laser source comprises several amplifier guides that are offset at an angle with respect to the retroreflecting-dispersing device and enabling the transmission of the source over several wavelengths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described more in detail with reference to the appended drawings on which: 
     FIG. 1 represents a cube corner making up a two-dimension self-aligned device of the prior art; 
     FIG. 2 represents a cat&#39;s eye making up a two-dimension self-aligned device of the prior art; 
     FIG. 3 is an optical reflector according to the invention; 
     FIG. 4 is a side view of a laser according to the invention; 
     FIG. 5 is an above view of a laser according to the invention; and 
     FIG. 6 represents a blade with parallel faces usable as a self-aligned beam splitter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The optical reflector represented on FIG. 3 comprises a beam splitter and a self-aligned total reflector  21 . 
     The self-aligned splitter  20  divides the input beam  22  into two parallel beams: i.e. a first split beam  23  and a second split beam  24  propagating through free space. The first split beam  23  is reflected by the self-aligned reflector  21  and forms a reflected split beam  23 ′ that is directed to the beam splitter  20 , that reflects it partially and transmits it partially. The transmitted beam  23 ″ is sent back in parallel and in the reverse direction of the input beam  22 . 
     The beam reflected by the splitter  20  is a beam  23 ″ superimposed and of reverse direction to the input beam  22 . Similarly, the beam  24  generated from the input beam  22  by reflection onto the splitter  20 , is itself reflected by the total reflector  21  and forms the beam  24 ′ that is sent back to the splitter  20  that divides it into two beams, respectively  24 ″ and  24 ′″, which interfere with the beams  23 ″ and  23 ′″ thus generating two output beams, respectively  27  and  28 , parallel to one another. The beam  28  is produced by interference of the beams  23 ′″ and  24 ′″ that have each been exposed to a single reflection on the splitter  20 . This so-called reciprocal beam  28  is superimposed to the input beam  22  and of reverse direction. 
     Conversely, the beam  27  is generated by the interferences of the beam  23 ″ that has not been exposed to any reflection on the splitter  20  and of the beam  24 ″ that has been exposed to two reflections on the same splitter. This difference in the number of reflections to which each beam is exposed, provides a π-radian phase shift and the output  27  is called a non-reciprocal output. 
     The device according to the invention is therefore an optical reflector that, from the input beam  22 , generates two beams, respectively reciprocal  28  and non-reciprocal  27 , parallel to one another and self-aligned on the input beam. The reciprocal beam  28  is superimposed to the input beam  22  whereas the non-reciprocal beam  27  is offset. 
     Implementing the self-aligned total reflector  21  facilitates the adjustment of the device and improves therefore its yield. 
     When the splitter  20  is a 50/50 splitter, the intensity of the split beams  23  and  24  is equal and during their recombination, the whole energy is gathered on the reciprocal output into one beam  28 , whereby the beam  27 , further to the phase shift between the waves of the beams  24 ″ and  23 ″, has a zero energy, i.e. the phase shift does not exist. 
     It is possible to use an energy-unbalanced splitter  20 , enabling the distribution of the incident energy between both output beams  27  and  28 . R and T being respectively the coefficients of energy reflection and transmission of the splitter  20 , the incoming energy can be found at the non-reciprocal output (1-4 RT). For instance with R=90% and T=10%, we obtain (1-4 RT)=64% at the non-reciprocal output. 
     The self-aligned splitter  20  can be advantageously a periscopic splitter comprising a splitting interface  20 ′ and two mirrors  25  and  26  that are parallel to the former. 
     The use of a diffraction grating  29 , on the path of the luminous beam enables to spread geometrically the spectrum of the luminous fluxes at output and possibly to select a portion of the former. 
     This diffraction grating  29  is advantageously located in a Littman-Metcalf configuration between the self-aligned beam splitter  20  and the self-aligned total reflector  21 . 
     FIGS. 4 and 5 represent respectively the side view and the above view of a laser source according to the invention and, on these figures, the elements common to those of FIG. 3 have been kept with the same numeric references. 
     An amplifier medium, preferably an amplifier wave-guide  30 , whose internal extremity  30 ′ is placed at the focus of the collimation lens  31  of centre  31 ′, generates the collimated input beam  22 . The external face  30 ″ of this amplifier wave-guide  30  is entirely reflecting, and the splitter  20  is unbalanced. Thus, a laser cavity is formed between the entirely reflecting face  30 ″ and the self-aligned total reflector  21  through the reciprocal output where the beam  28  is superimposed to the input beam  22 . The offset non-reciprocal output  27  makes up the laser output and forms therefore the transmitted beam. 
     The presence of a grating  29  in the Littman-Metcalf configuration with the self-aligned total reflector  21 , in this laser source, enables the nonreciprocal output  27  to filter spectrally the continuous spurious background, with ASE radiation, and thereby to isolate the transmission line of the laser. 
     The transmission wavelength adjustment can be obtained either by rotating the grating or by rotating the total reflector  21  or still by rotating the assembly formed by the grating  29  and the total reflector  21 , whereas the filtered non-reciprocal beam  27  remains stable since it is parallel to the input beam. This beam can be coupled optionally in a monomode optic fibre. 
     A rotation and/or translation coordinated movement of the reflecting dihedron  21  with the movement of the grating  29  enables to provide a continuous tuneable laser source. Such a coordinated movement is for example disclosed in French patent FR-2.724.496. 
     Such a laser source can also be made with several amplifier media or wave-guides  30  located in the focal plane of the lens  31 . This allows us to provide a multiwavelength source, formed by the superimposition of several laser fluxes, each corresponding to a wave-guide and whose wavelength depends on the angle from which the wave-guide can be seen from the reflecting dispersing device. 
     The figures and the description have been made while using as a self-aligned reflector, a cube corner or a dihedron, but similar results can be obtained by using a cat&#39;s eye. The cube corner and the dihedron can be formed by plane mirrors, but also made out of a full trihedron and a rectangular isosceles prism, operating in total internal reflection. 
     It has been mentioned above that the self-aligned splitter  20  can be a periscopic splitter. It can also be a blade with parallel faces. Such a blade  40  is represented on FIG.  6 . 
     Its input face  41  is partially coated with an antiglare coating  42  and a fully reflecting coating  43 . 
     Its output face  44  is partially coated with a partially reflecting coating  45  and, on another area, an antiglare coating  46 . 
     Thus, an incident beam  47  is partially transmitted at  48  and the residual flux is transmitted, after two reflections at  49 . The required function is thereby achieved.