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 said 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'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 said 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'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, we shall seek to obtain the self-alignment in a single dimension. We shall then use either an orthogonal dihedron instead of the trihedron of FIG. 1 or a cylindrical cat'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'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 then 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 means each redirecting the secondary beams toward the beam splitter and forming a Sagnac interferometer.
According to the invention, the reflecting means comprise 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 said 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.
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.