U.S. patent application number 09/765587 was filed with the patent office on 2001-08-02 for optical reflector and laser with external cavity incorporating such a reflector.
This patent application is currently assigned to PHOTONETICS. Invention is credited to Lefevre, Herve.
Application Number | 20010010599 09/765587 |
Document ID | / |
Family ID | 27171280 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010599 |
Kind Code |
A1 |
Lefevre, Herve |
August 2, 2001 |
Optical reflector and laser with external cavity incorporating such
a reflector
Abstract
This invention relates to an optical reflector receiving an
input beam (22) and transmitting a retroreflected reciprocal output
beam (28), comprising a beam splitter (20) giving a first secondary
beam (27) and a second secondary beam (28), parallel to one another
as well as retroreflecting means (21) each redirecting secondary
beams toward the beam splitter and forming a Sagnac
interferometer,. According to the invention, the said reflecting
means comprise a self-aligned total reflector (21) and the beam
splitter (20) is self-aligned. This beam splitter may also be
energy-unbalanced, whereas the said reflector thus transmits a
non-reciprocal output beam. The invention also relates to a laser
source with external cavity comprising an amplifier medium, a
wavelength dispersing device and such an optical reflector, whereby
the non reciprocal output provides a single output beam where the
ASE of the source is filtered spectrally.
Inventors: |
Lefevre, Herve; (Paris,
FR) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN
1941 ROLAND CLARKE PLACE
RESTON
VA
20191
|
Assignee: |
PHOTONETICS
Marly le ROI
FR
|
Family ID: |
27171280 |
Appl. No.: |
09/765587 |
Filed: |
January 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09765587 |
Jan 22, 2001 |
|
|
|
09503204 |
Feb 14, 2000 |
|
|
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Current U.S.
Class: |
359/633 |
Current CPC
Class: |
G02B 5/12 20130101; H01S
5/1071 20130101; H01S 5/14 20130101; H01S 5/143 20130101; H01S
5/141 20130101; H01S 5/142 20130101 |
Class at
Publication: |
359/633 |
International
Class: |
G02B 027/14 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 15, 1999 |
FR |
99 01788 |
Claims
What is claimed is:
1. 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 secondary beams toward the beam splitter and forming a
Sagnac interferometer, wherein the said reflecting means comprise a
self-aligned total reflector and the beam splitter is
self-aligned.
2. An optical reflector according to claim 1. wherein the
self-aligned total reflector is bidimensional.
3. An optical reflector according to claim 1, wherein the
self-aligned total reflector is unidimensional.
4. An optical reflector according to claim 1, wherein the beam
splitter is energy-unbalanced, whereas the said reflector thus
transmits a non-reciprocal output beam.
5. An optical reflector according to claim 1, wherein it comprises
a diffraction grating forming with the total reflector a
Littman-Metcalf system.
6. An optical reflector according to claim 5, wherein the
diffraction grating is located between the beam splitter and the
total reflector.
7. A laser source with external cavity comprising an amplifier
medium and a retroreflecting-dispersing device, wherein the
retroreflecting-dispersi- ng device is according to claim 5.
8. A laser source with external cavity comprising an amplifier
medium and a retroreflecting-dispersing device, wherein the
retroreflecting-dispersi- ng device is according to claim 6.
9. A laser source with external cavity according to claim 7,
wherein the amplifier medium is a wave guide and it is associated
with collimation optics that collimate the beam thereby
produced.
10. A laser source with external cavity according to claim 9,
wherein the external face of the wave guide is totally reflecting
and the non reciprocal beam is the single beam transmitted by the
source.
11. A laser source with external cavity according to claim 7,
wherein the dihedron is mobile in rotation to enable the variation
of the wavelength.
12. A laser source with external cavity according to claim 11,
wherein the dihedron is mobile in rotation and in translation to
enable continuous variation of the wavelength.
13. A laser source with external cavity according to claim 9,
wherein it 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.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] 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.
[0011] Thus, when returning, the waves corresponding to each of
these beams interfere and produce two output beams.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] According to the invention, the reflecting means comprise a
self-aligned total reflector and the beam splitter is
self-aligned.
[0019] Preferably, in various embodiments of the invention each
having their respective advantages:
[0020] the self-aligned total reflector is unidimensional;
[0021] the self-aligned total reflector is bidimensional;
[0022] the beam splitter is energy-unbalanced, whereas the said
reflector thus transmits a non-reciprocal output beam;
[0023] the optical reflector comprises a diffraction grating
forming with the total reflector a Littman-Metcalf system;
[0024] the diffraction grating is located between the beam splitter
and the total reflector.
[0025] 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.
[0026] Advantageously:
[0027] the amplifier medium is a wave-guide and it is associated
with collimation optics thereby collimating the beam that it
produces;
[0028] the external face of the wave-guide is totally reflecting
and the non-reciprocal beam is the single beam transmitted by the
source;
[0029] the dihedron is mobile in rotation to enable the variation
of the wavelength;
[0030] the dihedron is mobile in rotation and in translation to
enable continuous variation of the wavelength;
[0031] 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
[0032] The invention will be described more in detail with
reference to the appended drawings on which:
[0033] FIG. 1 represents a cube corner making up a two-dimension
self-aligned device of the prior art;
[0034] FIG. 2 represents a cat's eye making up a two-dimension
self-aligned device of the prior art;
[0035] FIG. 3 is an optical reflector according to the
invention;
[0036] FIG. 4 is a side view of a laser according to the
invention;
[0037] FIG. 5 is an above view of a laser according to the
invention;
[0038] FIG. 6 represents a blade with parallel faces usable as a
self-aligned beam splitter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] The optical reflector represented on FIG. 3 comprises a beam
splitter 20 and a self-aligned total reflector 21.
[0040] 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.
[0041] 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.
[0042] 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 .pi.-radian
phase shift and the output 27 is called a non-reciprocal
output.
[0043] 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.
[0044] Implementing the self-aligned total reflector 21 facilitates
the adjustment of the device and improves therefore its yield.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] The presence of a grating 29 in the Littman-Metcalf
configuration with the self-aligned total reflector 21, in this
laser source, enables the non-reciprocal output 27 to filter
spectrally the continuous spurious background, with ASE radiation,
and thereby to isolate the transmission line of the laser.
[0053] 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.
[0054] 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.
[0055] 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
said wave-guide can be seen from the reflecting dispersing
device.
[0056] 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'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.
[0057] 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.
[0058] Its input face 41 is partially coated with an antiglare
coating 42 and a fully reflecting coating 43.
[0059] Its output face 44 is partially coated with a partially
reflecting coating 45 and, on another area, an antiglare coating
46.
[0060] 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.
* * * * *