EP1454388A2

EP1454388A2 - Surface-free ring cavity optical resonator, corresponding communication and/or video projection apparatus

Info

Publication number: EP1454388A2
Application number: EP02801130A
Authority: EP
Inventors: Patrice Feron; Raymond Le Bras
Original assignee: Universite de Rennes 1
Current assignee: Universite de Rennes 1
Priority date: 2001-12-12
Filing date: 2002-12-12
Publication date: 2004-09-08
Also published as: US20050068611A1; FR2833417A1; AU2002364837A1; WO2003050925A3; WO2003050925A2; FR2833417B1

Abstract

The invention concerns a surface-free ring cavity optical resonator (250, 450), the resonator comprising a plurality of modules, each of the modules itself including polarizing splitter means (251, 470, 471) for an incident beam in accordance with a polarization base ((x,y), (x,z)), first and second arms, one of the arms including means for redirecting and switching (214, 213, 234, 233) an output beam, said resonator further comprising in at least one of the modules, means for amplifying (215, 225, 205) at least one output beam located on the path of the beam(s) to be amplified, between the polarizing splitter means and the redirecting and switching means of the arm concerned.

Description

Optical ring resonator without surface, corresponding communication and / or video projection device.

The present invention relates to the field of lasers. More specifically, the invention relates to ring laser components.

A laser is an optical oscillator. Like all oscillators, it is composed of an amplifier and a suitable feedback loop.

In the case of a laser, the amplifier consists of a medium capable of amplifying (by stimulated emission) the spontaneous emission. This means that if a light brush crosses such a source, it leaves it with a greater intensity than it had when entering.

The feedback loop consists of a resonant optical cavity. This consists of mirrors arranged in such a way that the light circulates between them and stays there as in a tank. The laser effect and its characteristics (intensity, polarization, emission wavelength, line width ...) are the result of the adequacy between these two main elements which are the amplifying medium and the optical resonator.

Lasers are known comprising optical cavities used of the “two mirrors” type (for example, a Fabry-Perot interferometer). Such a resonator leads to the generation of a standing wave inside the cavity. The first consequence of this will be to bring non-uniform saturation to the amplifying medium (periodicity λ / 2 where λ represents the wavelength of the amplified light signal). This effect is conventionally known by the English term "spatial hole burning". It has drawbacks, in particular, a reduction in the performance of the oscillator both in terms of intensity and in terms of transmission stability (amplitude noise, phase noise, partition noise corresponding to mode jumps).

The solution to this problem is traditionally provided by using a ring resonant cavity presented opposite Figures la and lb. Figure la shows a ring cavity without an optical diode. This cavity is formed by three mirrors 100 to 102, placed at the top of an equilateral triangle and oriented so that a light beam 103 is successively reflected by the three mirrors and passes through an amplifying medium 106 located between the mirrors 101 and 102. According to the embodiment of FIG. La, the light beam 103 can follow the two possible directions of travel of the cavity and two beams 107 and 108 emerge from the mirror Ms 102.

Figure lb illustrates a variant of a cavity which includes the same elements as the cavity of Figure la and, in addition, an optical diode 109 placed, for example, between the amplifying medium 106 and the mirror 112 and promoting a sense of course of the cavity (traveling wave inside the resonator). According to this variant, a single emerging beam 117 of total intensity equal to the sum of the intensities of the two beams 107 and 108 is obtained at output 112.

The introduction of the "optical diode" is possible due to the geometric size given to the amplifying medium. Indeed, to optimize the gain of the amplifier, it is sized at Brewster angle in order to promote a rectilinear polarization axis of the laser light. In other words, a quantification axis is fixed for the amplifying medium.

However, these ring cavities have several drawbacks, in particular:

- They have an extended cavity which is detrimental to the compactness of these structures and their stability;

- In addition, the configuration of the resonator only makes it difficult to pump the amplifying medium longitudinally. Resonators are also known in monolithic microchip rings which have the advantage of greater compactness.

However, the latter have the drawback of allowing a modest transmitted power (around 100 mW). In addition, for most of them, it is impossible to introduce elements into the cavity for the purpose of selection or for an active purpose (generation of harmonics or impulse operation thanks to a passive Q-switch).

We also know a resonator described in the documents written by M. Mac Donald, T. Graf, J. Balmer and H. Weber entitled respectively "Configuration Q-Switching in a diode pumped multirod Variable configuration resonator" (or literally, in French, "Switch of Q in a variable configuration resonator", Q being a quality factor of the resonator also called overvoltage factor) published in the review "IEEE Journal of Quantum Electronics", Vol. 34, No. 2 of February 1998 and a "surfaceless" ring resonator illustrated in the document "Efficient polarized output from end-pumped multirod resonators" (or "polarized efficient output of pumped multiple rod resonators" in French) published in the journal "Optics Communications", Vol. 160 (1999) of February 15, 1999 and edited by Elsevier. The cavity described in the first of these documents and illustrated with reference to FIG. 1c is a traveling wave cavity resting on a cavity comprising:

- a polarization splitter 180; three mirrors 152, 162 and 172 each associated with a quarter-wave plate respectively 154, 164 and 174, a laser crystal respectively 153,

163 and 173 being inserted between each mirror and the associated quarter-wave plate;

- an outlet 182; and

a switch 181 placed between the separator 180 and the outlet 182. The mirrors 152 and the outlet 182, on the one hand, and the mirrors 162 and 172, on the other hand, form the branches of a cross having for intersection the separator 180.

Furthermore, pumps 151, 161 and 171 supply the cavity via the mirrors 152, 162 and 172 respectively. This configuration does not include a quarter-wave plate between the separator 180 and the output 182. Thus, this article does not disclose a ring resonator. On the other hand, according to the variant described in the second article cited above, a λ / 4 blade is introduced. Thus, this arrangement makes it possible to obtain a "ring" cavity (ring without surface). It should be noted that this structure does not in any way require the insertion of an optical diode (association of a Faraday rotator and a crystalline plate provided with optical activity) in order to favor a direction of travel to obtain a maximum gain on one of the journeys. Indeed, this component is necessary in the case of a traditional ring laser where the beams corresponding to the two possible paths do not allow the superimposition of the emerging beams.

However, this technique has the drawback of allowing operation with good efficiency, only with isotropic amplifying media. The invention according to its different aspects aims in particular to overcome these drawbacks of the prior art.

More specifically, a first objective of the invention is to provide a resonator allowing a high transmission power, for example of the order of

1 Watt, while having reduced noise. In particular, an objective of the invention is to provide a resonator suitable for transmitting continuously or quasi-continuously a high power of the order, for example, of several watts.

Another objective of the invention is to implement a resonator allowing a wide variety of emission wavelengths especially in narrow band if the cavity is extended and in wider band if the cavity is compact.

The invention also aims to provide a laser resonator allowing short length cavities.

An objective of the invention is also to allow the introduction and optimal use of optically anisotropic media, whether amplifying media or non-linear crystals for generation. harmonics (these harmonics are not necessarily superior, since they can be added or subtracted).

An additional objective of the invention is to provide a resonator which can be used for various applications. In addition, the invention also aims to allow an optimized configuration for longitudinal pumping.

In addition, the invention aims to allow a resonator having a compact structure.

For this purpose, the invention provides an optical ring resonator allowing at least one optical beam to circulate inside the resonator by forming a ring of zero surface, the resonator comprising a plurality of modules, each of the modules comprising itself even:

- polarization separation means, along a polarization base, of an incident beam entering the module; - a first arm; and

- a second arm; the separation means being adapted to separate:

- A first component of the incident beam oriented in a first direction of the polarization base by forming a first output beam emitted towards the first arm; and

- a second component of the incident beam oriented in a second direction of the polarization base by forming a second output beam emitted towards the second arm; at least one of the first and second arms comprising means for redirection and tilting of the first or second output beam respectively, the redirection and tilting means being adapted to redirect the first or second output beam to the separation means and to changing the direction of polarization of the first or second output beam to form an incident beam entering another of the modules; the resonator being remarkable in that it further comprises, in at least one of the modules, means for amplifying at least one of the first and second output beams, said at least one beam to be amplified, the means for amplification being adapted to be associated with longitudinal or transverse pumping means and being located, on the path of the at least one beam to be amplified, between the polarization separation means and the redirection and tilting means belonging to the arm concerned.

Thus, the invention can be implemented with longitudinal pumping which allows good efficiency (for example of the order of 30 to 40%) in terms of optical balance and efficiency. In addition, the configuration of the resonator is optimized for longitudinal pumping of the different active media. In this way, the invention makes it possible to easily obtain powers greater than one watt.

In addition, the amplifying medium is traversed in one direction by a wave polarized rectilinearly and on the return by a wave of the same nature but polarized perpendicularly. Consequently, the amplifying medium will present an optimum gain for each of these directions of travel.

The resonator also allows an implementation with a short cavity, which has the consequences: - a possibility of fairly large laser line width, which is favorable for a pumping type use of a Raman laser having a fairly wide emission ; and - a small number of longitudinal modes, which associated with the absence of HSB will give a reduced partition noise, hence a simpler stabilization than with a standing wave laser structure.

The resonator obtained according to the invention can also have a compact structure: outside the pumping system, the solid laser structure can in particular fit in the volume of a packet of cigarettes.

Preferably, the total length of the cavity of the ring resonator (and therefore the length of the arms) is adapted to the desired wavelength. According to a particular characteristic, the resonator is remarkable in that the separation means comprise a polarization separator cube.

The polarization splitter cube can be specified "wideband", that is, it will separate the polarizations in a wide spectral range. Such a “broadband” polarization splitter cube may advantageously be used in the case of a resonator with multiple wavelengths.

According to a particular characteristic, the resonator is remarkable in that the separation means comprise a semi-transparent plate with polarization separation. It should be noted that the semi-transparent blade used in particular consists of a film system.

According to a particular characteristic, the resonator is remarkable in that the separation means are common to all of the modules.

Thus, the invention allows an implementation very compact and simple to implement.

According to a particular characteristic, the resonator is remarkable in that the set of modules comprises at least two subsets, all the modules of the same subset of modules sharing common separation means. Thus, the resonator can be implemented in the form of a cascaded structure to allow, in particular, greater transmission power.

According to a particular characteristic, the resonator is remarkable in that the polarization redirection and tilting means comprise:

- a mirror positioned to reflect the first or second output beam; and

- means for phase shifting the first or second output beam; so that the first or second output beam is:

- first phase shifted by an angle equal to π / 2 radians by the phase shift means; - then reflected by the mirror; and - finally again phase shifted by an angle equal to π / 2 radians by the phase shifting means.

Thus, one pumps along two orthogonal axes in the amplification means which improves the gain. Here, the mirror preferably comprises:

- a cut and polished substrate (support) with a suitable radius of curvature; and

- a metallized layer or a dielectric stack allowing total reflection of a light beam at the wavelength of the resonator. The substrate can be separated from the phase shifting means.

In order to obtain greater compactness of the resonator and / or greater emission power, the phase shifting means and the mirror can form a monolithic optical element, the substrate forming part of the phase shifting means: the monolithic optical element thus formed is, for example, produced by depositing a dielectric stack on a quarter-wave plate or a Fresnel cobra.

In order to optimize the efficiency of the resonator, in particular in the case of longitudinal pumping, the external face (pump side) of the mirror may be subjected to an anti-reflection treatment. According to a particular characteristic, the resonator is remarkable in that the phase shifting means and the amplification means comprise the same material which is not doped in the phase shifting means and doped in the amplification means and are joined so that the first or second output beam passes from amplification means to phase shift means and vice versa without changing the medium.

In this way, a resonator structure in which the phase shifting means comprising a doped crystal to obtain gain and an amplification medium comprising the same undoped crystal are placed side by side (that is to say placed side by side without space separating them) avoids losses linked to the change of environment and due to the change of index. According to a particular characteristic, the resonator is remarkable in that the phase shifting means comprise a quarter wave plate.

According to a particular characteristic, the resonator is remarkable in that the phase shifting means comprise a Fresnel rhombohedron. According to a particular characteristic, the resonator is remarkable in that the mirror is concave.

According to a particular characteristic, the resonator is remarkable in that the mirror is planar.

Thus, a mirror implemented in the resonator can be: - concave with a radius of curvature making it possible to guarantee good stability, in particular for large cavities; or plan for an implementation particularly well suited to resonators of small cavity size, in particular of total length less than 1 cm. According to a particular characteristic, the means for polarization redirection and tilting comprise:

- a mirror positioned to reflect the first or second output beam; and means for rotating an angle equal to π / 4 radians of the first or second output beam.

Thus, after a round trip in the rotation means, a beam will have undergone a rotation of π / 2 radians which in particular makes it possible to improve the gain by amplifying the beam along two orthogonal axes in the amplification means. The rotation means are for example a Faraday rotator which in particular makes it possible to widen the bandwidth of the rotation means, or more generally a medium endowed with a magnetic rotary power associated with a magnetic field.

According to a particular characteristic, the resonator is remarkable in that the amplification means comprise an anisotropic material having own polarization axes corresponding to the directions of the polarization base.

Thus, the invention allows great transmission powers and good efficiency, the optical beams passing through the anisotropic amplifying medium whose own axes of polarization correspond to the directions of polarization respectively in each of the two directions of propagation of the optical signal.

According to a particular characteristic, the resonator is remarkable in that the anisotropic material belongs to the group comprising: anisotropic crystals; and - glasses showing dichroism.

According to a particular characteristic, the resonator is remarkable in that the anisotropic material belongs to the group comprising:

- Nd type materials: YAP; Nd type materials: YVO4; and - Er: YAP type materials.

According to a particular characteristic, the resonator is remarkable in that the isotropic material is of the Ho, Tm: YAG type.

Thus, the resonator coupled to such an amplifying medium can in particular emit in the infrared and can be applied advantageously to laser anemometry, vibrometry, telemetry and more generally to remote measurements exploiting a coherent detection. In addition, a good amplifying medium (in particular of the Ho, Tm: YAG type) advantageously coupled with the qualities of the resonator allows in particular great stability and is suitable for providing high power in particular in continuous or quasi-continuous emission. According to a particular characteristic, the resonator is remarkable in that the amplification means comprise an isotropic material.

According to a particular characteristic, the resonator is remarkable in that the isotropic material belongs to the group comprising:

- isotropic crystals; and - the co-doped Er: Yb phosphate glasses. According to a particular characteristic, the resonator is remarkable in that the isotropic material is of the Nd: YAG type.

Thus, the invention authorizing a large choice of amplifier materials compatible with longitudinal and / or transverse pumping makes it possible to optimize the resonator as a function of the desired application and in particular makes it possible to choose one or more emission wavelengths.

For example, in the particular case of using an Nd: YAP crystal (with a potential application for pumping Raman lasers), depending on the size of the crystals two wavelengths 1064 and 1079 nm can be emitted. By arrangement and orientation of the amplifying media and the use of Fresnel rhombohedra (equivalent to a broadband quarter-wave plate), a laser with two wavelengths can be implemented.

Generally, the material used in the isotropic or anisotropic amplification means is doped (in particular by rare earth ions) to allow amplification of the medium. In addition, the resonator used as a source can be configured so as to meet the eye safety criteria of a user. Thus, it could, for example, be doped with rare earth ions of the Ho type.

According to a particular characteristic, the resonator is remarkable in that the resonator further comprises, in at least one of the modules, means for promoting a direction of propagation of the first and second output beams in the arm concerned.

Thus, one can promote a direction of propagation, for example, by implementing in a resonator according to the invention include an optical diode which provides high stability of the resonator.

According to a particular characteristic, the resonator is remarkable in that the resonator further comprises, in at least one of the modules, non-linear crystals capable of generating a beam having harmonics, from one of at least one optical beam crossing non-linear crystals. Thus, since the non-linear crystals are sensitive to polarization, the invention allows generation of harmonics (for example for a frequency doubling or tripling resonator) simple and effective to implement, the beam emitted being unidirectional. Of course, in this case, the output mirror (s) are transparent to the harmonic (or harmonics) generated.

According to a particular characteristic, the resonator is remarkable in that the resonator further comprises, in at least one of the modules, an element belonging to the group comprising:

- Fabry-Perot interferometers; - the Fabry-Perot standards;

- modulators; and

- saturable absorbents.

Thus, it is possible to insert into the cavity of a resonator according to the invention (in particular into an arm which would be devoid of amplifying medium) various elements adapted to certain applications, in particular:

- a Fabry-Perot interferometer, for example, in case of proven need to make the laser single mode longitudinal; non-linear crystals for different applications, for example, for the intra-cavity generation of harmonics; electro-optical or acoustico-optical modulators allowing, many applications in particular in the field of telecommunications or the production of pulsed lasers in triggered mode or in blocked mode.

- of a saturable absorbent in particular for a passive “Qswitch” type switch.

According to a particular characteristic, the resonator is remarkable in that it comprises polarization separation means, along a polarization base, of an incident beam and four arms, the separation means being adapted to separate the components of a incident beam coming from one of the arms so that: - The incident beam is reflected by the separation means, to be emitted towards a first arm among the arms when the polarization of the incident beam is oriented in a first direction of the polarization base; and - the incident beam is transmitted without undergoing reflection through the separation means to be emitted towards a second arm among the arms, the second arm being distinct from the first arm, when the polarization of the incident beam is oriented in a second direction of the polarization base; each of the arms comprising means for redirecting the incident beam coming from the separation means, the redirection and tilting means being adapted to redirect the incident beam towards the separation means and to change the direction of the polarization of the incident beam, the resonator further comprising, in at least three of the arms, means for amplifying the incident beam known as the beam to be amplified, the amplification means being adapted to be associated with longitudinal or transverse pumping means and being located on the path of the beam to be amplified, between the polarization separation means and the redirection and tilting means belonging to the arm concerned. The invention also relates to a telecommunication device, remarkable in that it comprises a resonator as described above.

Such a resonator (if, in particular, it is powerful) could in particular be used for pumping optical fiber amplifiers, for example, of Raman amplifier or Raman laser type as well as for Erbium doped fiber amplifiers. The choice of amplifier depends on the wavelength delivered by the resonator.

In addition, the invention relates to a video projection device, remarkable in that it comprises a resonator as described above.

Thus, a video projection device can be equipped with laser resonators, according to the invention, of red, green and blue color respectively (corresponding video primary colors) compact. In order to obtain a good chromaticity rendering, each fundamental color component defined by red (610 to 630 nm), green (520 to 540 nm) and blue (450 to 460 nm) is covered. These colors can be obtained using a resonator comprising amplifying materials and ad-hoc doublers corresponding to the desired colors. Thus, it is possible to use anisotropic amplifier materials (for example of the Nd: YVO4 type which make it possible to obtain a blue line at 456 nm by doubling the line at 912 nm) or isotropic (for example of the Nd: YAG type which allow to obtain a green line at 532 nm by doubling the line at 1064 nm) Such a device equipped with resonators delivering approximately

1.5 Watt per color can thus allow high-quality projection on a cinema-type screen.

The power of the video projection device can also, according to the invention, be much less than 1.5 watts or on the contrary reach several watts. The advantages of telecommunication and video-laser devices are the same as those of the optical resonator, they are not described in more detail.

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given by way of simple illustrative example and not. limiting, and. annexed drawings, among which:

- Figures la to the present optical resonators known in themselves;

- Figure 2 illustrates a block diagram of an optical resonator with four arms according to the invention according to a particular embodiment; - Figure 3 illustrates a block diagram of a polarization splitter cube and an arm used in the resonator of Figure 2; and FIG. 4 describes a variant of a resonator comprising several polarization splitter cubes and six arms, in accordance with the invention according to a particular embodiment. The general principle of the invention is based on the implementation of a resonator in which a wave or two counter-propagating waves and of rectilinear polarizations perpendicular to one another, the optical beam being divided into two paths fictitious optics forming a ring without surface.

The resonator comprises one or more polarization separation means, for example of the cube-type or semi-transparent polarization separator plates and arms, some of which themselves include polarization redirection and tilting means, amplification means being inserted between the polarization separation means and the polarization redirection and tilting means.

Thus, for example, an incident beam enters a separator cube, its polarization being in particular such that it crosses the cube. At the exit of the separating cube, the polarized optical beam is amplified by amplification means (anisotropic crystal materials comprising own polarization axes oriented along the polarization base of the cube for maximum efficiency or other isotropic materials) before be reflected and undergo a perpendicular tilting of its polarization on the redirection and tilting means. The reflected beam is then amplified again in the amplifying means before entering the separator cube. Its polarization having been tilted perpendicularly to the incident beam, it will be reflected in a direction imposed by the cube, which, for example, makes an angle of 90 ° relative to the incident direction.

The mechanism of crossing the cube or reflecting on the cube, amplification and redirection / tilting of polarization is reiterated inside the structure. The beam is therefore amplified.

The embodiment of the resonator allows a large choice of amplification materials which can be isotropic or anisotropic with polarization axes depending on the polarization base of the polarization separation means, the materials being, for example, chosen according to the desired gain. FIG. 2 illustrates a block diagram of optical resonator 250 according to the invention.

The resonator 250 having the shape of a cross comprises:

- a polarization separator cube 251 placed in the center of the cross; and - four arms 206, 216, 226 and 236 forming the branches of the cross.

The cube 251 defining a polarization base (Λ:) OR (X, Z) (the vectors, here are represented in bold and in italics) is oriented so that an incident beam entering the cube with a polarization:

- in the plane (defined by axes y, z according to the polarization base) according to the diagram is reflected in a direction forming an angle of

90 ° with the incident beam; and

- in a direction perpendicular to the plane according to the diagram (defined by an axis x according to the base of polarization) crosses the cube 251.

The vector space of the polarization states is of dimension 2. It is always in the plane orthogonal to the direction of propagation. Thus, two directions of propagation are represented with regard to FIG. 2:

- one "vertical", corresponding to a propagation along the z axis in a positive or negative direction (back and forth), the associated polarization base being (ΛΓ) with a beam polarized in the direction x passing through the cube and a beam, polarized in the direction reflected therein; and

- the other “horizontal” corresponding to a propagation along the y axis, the associated polarization base being (x) with a beam polarized in the direction x passing through the cube and a beam polarized in the direction z reflected

The first three arms 206, 216 and 226 each comprise, successively placed along one of the corresponding axes or z, starting from the point closest to the cube 251:

- an amplification zone or bar with active amplifying medium 205, 215 and 225 respectively; - a quarter wave plate 204, 214 and 224 respectively; and

- a mirror 203, 213 and 223 respectively.

Each of the amplification zones 205, 215 and 225 comprises an isotropic material (for example glass (in particular of the codoped phosphate type Er: Yb), polymer or isotropic crystal (in particular Nd: YAG or Ho, Tm: YAG)) or a material anisotropic (in particular anisotropic crystal (for example, of type Nd: YAP, ND: YVO4 or Er: YAP) or glass exhibiting dichroism) having axes of polarization which are clean or said to be privileged with respect to the direction of propagation of light combined: - with the x and v axes if the propagation is along the z axis; and

- with the x and z axes if the propagation takes place along the y axis. Each of the amplification zones 205, 215 and 225 can be pumped transversely or longitudinally (in this case, as shown in FIG. 2, the pumps 200, 210 and 220 (for example of the laser diode type) associated respectively with the zones of amplification 205, 215 and 225 and whose wavelengths are adapted to the amplifying media are external to the resonator 250, the pump signal passing through the mirrors 203, 213 and 223 respectively according to techniques well known to those skilled in the art).

Note that mirrors 203, 213 and 223 have reflection coefficients equal to 100% for the intracavity signal and transmission coefficients of 100% for the pump signal.

The fourth arm 236 comprises placed successively along the axis z:

- a free zone 235; and

- a quarter wave plate 234; and - a partially transparent output mirror 233 allowing the emission of an emitted beam 237. According to a first variant not shown, the quarter-wave plates 204, 214, 224 and 234 can be replaced by Fresnel rhombohedra.

According to a second variant, not shown, the cube can be replaced by a semi-reflecting plate placed diagonally along the y and z axes and parallel to the axis Λ ;, so that it lets pass a beam polarized along the axis Λ: and reflects at an angle of 90 ° a beam polarized along one of the axes or z.

According to a third variant not shown, the free area 235 comprises: - an amplifying medium pumped transversely, allowing a greater gain of the resonator;

- a Fabry-Perot interferometer to, for example, render the longitudinal single mode laser;

- non-linear crystals allowing the generation of harmonics; - electro-optical or acoustico-optical modulators; and / or a saturable absorbent, in particular for a type switch

Passive "Qswitch".

For the generation of intra-cavity harmonics, for example, an amplifying medium couple Nd: YAG associated with potassium niobate (KNbO3) is used. This crystal once thermostate is well suited because it has a high damage threshold and high efficiency.

In the case of a doubling, called type I where the double frequency emerges polarized perpendicular to the polarization of the signal at the fundamental frequency which gave birth to it, KNbO3 offers the advantage of having an index according to the direction of the polarization fundamental which is almost equal to that of Nd: YAG which will have the effect of reducing losses.

It will be noted that in this type of doubling, there is necessarily a rectilinearly polarized fundamental wave, hence the advantage of introducing such a doubling device into a portion of cavity where rectilinear polarizations prevail, which is the case in the resonator 250.

Also shown in Figure 2, an optical beam on a direction of travel. For reasons related to the good understanding of the assembly presented with reference to FIGS. 2 and following, the path of the signal composed of two counter-propagating waves and rectilinear polarizations perpendicular to one another is divided into two optical paths fictitious positioned arbitrarily on either side of the real optical axes 207 and 227 of the cavity. In reality, these fictitious optical paths are confused with the real axes.

A rectilinear polarization perpendicular to the plane of the figure, that is to say along the axis x has been noted using a cross 241 in a circle while an arrow 240 corresponds to a rectilinear polarization parallel to the plane of the figure.

The direction of propagation of the rectilinear polarized electromagnetic waves noted and represented in FIG. 2 was chosen arbitrarily. This does not matter because no "optical diode" device (possible insertion in the arm 236) is present. As a result, two counter-propagating waves are present. Only the polarizations are to be taken into account (although in total coherence with the chosen direction).

The closed space (optical plane) materialized by the end mirrors 203, 213, 223 and 233 brought back on these real optical axes 207 and 227 defines the overall path (in cross) of the two progressive waves maintained in the resonator. In view of the above, this particular configuration lets appear the concept of configuration called "ring without surface").

Given the polarization properties of the different elements of the resonators 250, an optical beam follows, in a particular direction, the following path: - starting, for example, from the blade 204, a first beam 201 oriented along the axis z and polarized along the x axis is amplified by the medium 205 pumped by the pump 200; - Then, it crosses the cube 251 to be amplified by the medium 215 pumped by the pump 210; - Then, it crosses the blade 214, acquires a circular polarization and is reflected by the mirror 213, crosses again the blade 214, its polarization then being rectilinear along the y axis and the beam being referenced 211; - The beam 211 again amplified by the medium 215 entering the cube 251 is reflected along the y axis and acquires a rectilinear polarization along the z axis (beam 225);

- Then, the beam 225 is amplified by the medium 225 pumped by the pump 220, crosses the blade 224, acquires a circular polarization and is reflected by the mirror 223, crosses again the blade 224, its polarization then being rectilinear according to the z axis and the beam being referenced 221;

- The beam 221 again amplified by the medium 225 crosses the cube 251 and the free area 235;

- then, it acquires, by crossing the blade 234, a circular polarization, part of the beam then being emitted towards the outside of the resonator

250 and another part being reflected by the mirror 233;

- The reflected beam then acquires a rectilinear polarization along the z axis by crossing the plate 234 (beam 231);

- the beam 231 then crosses the free area 235, is reflected by the cube

251 along the z axis and with a polarization along the y axis (beam 202);

-. the beam 202 is then amplified by the medium 205, passes through the plate 204, acquires a circular polarization, is reflected by the mirror 203 and passes through the plate 204 again to acquire a rectilinear polarization along the x axis; we then find the starting point of the route (beam 201). FIG. 3 illustrates the cube 251 and the arm 216 of the resonator 250 which notably comprises the amplifying medium 215, the blade 214 and the mirror 213.

The polarization splitter cube 251 (or CSP also known by the acronym PBS from the English “Polarizing Beam Splitter”) has a dual function, namely:

- it imposes a privileged base of orthogonal rectilinear polarizations, one corresponds to the direction x symbolized by a point or a cross in Figures 2 and following and the other in the direction perpendicular y (in the plane of the figure for Figures 2 and following); and the cube 251 plays the role of a switch for these two polarizations. Indeed, let us consider for incident beam a wave propagating according to the direction of the increasing z and any polarization P. The polarization of this wave breaks down as a linear combination of the two privileged polarizations, in other words P is equal to ax + βy. The component along x will cross the cube 251 while the component β following y will, in turn, be reflected and therefore directed in another direction along z (reflection at 90 ° in Figures 2 and 3). Whatever the input face of the device, we have the same behavior, the polarization along x crosses the device and the polarization along v is reflected on the surface equivalent to a mirror noted 300. The point

304 on the CSP 251 symbolizes the "passing" axis of this device (rectilinear polarization perpendicular to the plane of the figure).

We will call in the following “π / 2 phase shifters” the quarter wave plates or the Fresnel rhombohedra inserted in the assembly. The group composed of a π / 2 phase shifter and the associated mirror also has a double function: it operates a change of direction on the incoming beam; and - it switches the polarization of the beam by an angle equal to 90 °. ;

Thus, after correct orientation with respect to the polarization base imposed by the CSP 251 (proper axes at 45 ° or 135 ° from the axes of the CSP), the π / 2 phase shifter, for example, 214 allows the transformation of a rectilinear polarization along the x axis of the go 201 in right circular polarization (go 301). The reflection on the mirror 210 of circularly polarized waves is a determining element. Thus, a right circular polarized wave corresponding to the go 301 is transformed into a left circular wave by reflection (to form the return 311) (conversely a left circular is transformed into a right circular). The phase shifter 214, then allows the transformation of a left circular polarization (return 311) into rectilinear polarization (return 211) along the perpendicular y axis to the x axis. In other words, the go 201 and the return 211 of a rectilinear polarized wave through the assembly consisting of the phase shifter π / 2 214 and the mirror 210 will have the role of returning a rectilinear polarized wave perpendicular to the incident polarization. This role is identical to that of a phase plate called λ / 2 which would be oriented at 45 ° from the privileged axes of the CSP, with the difference that the tilting action of the polarization is accompanied by a change in the direction of propagation. .

According to the embodiment described, the mirrors 203, 213, 223 are curved in order to improve the stability of the resonator 250. The radius of curvature of each of these mirrors is optimized as a function of the size of the cavity according to known methods of a person skilled in the art who can refer to the work "process physics in coherent optical radiation generators" written by L. Tarassov and published by MIR Moscow in 1985 and more particularly in chapter 2 and in paragraphs 2.4 and subsequent . According to a variant particularly suitable for small cavities, the mirrors of the resonator are planar.

The proposed configuration is such that the amplifying medium (active medium for the laser) respectively 205, 215 and 225 is placed between the CSP 551 and the group consisting of the phase plate respectively 204, 214 and 224 and the mirror 203, 213 and 223. This arrangement is the only one which can ensure the function of the ring-shaped path of the cavity in the case of the use of an anisotropic crystal as an amplifier.

Indeed, an anisotropic medium has the particular property of having a single base 320 of orthogonal rectilinear polarizations for each direction of propagation of light passing through it (for example, along the axes x and v for the arm shown opposite FIG. 2 ). These polarization directions are called “eigen polarizations” and they are associated with different propagation speeds (linked to the “eigen” indices n _x and n ₂ associated with these two “eigen” directions), we denote them in the following u- , and u ₂ and are associated with the phase velocities c / n ₁ and c / n ₂ respectively. So, only a polarization straight parallel to M OR u ₂ will not see its polarization state transformed at the output of the crystal. Any other polarization will be transformed into a new elliptical polarization (a linear combination of the type a _l u ₁ + a ₂ e ^lδ u ₂ ).

Because of this property of anisotropic media, we see here appear a fundamental difference in design with the resonator cavity described in the documents of Pr. Weber cited above and illustrated with reference to the figure. Indeed, in the schematic diagram of the cavity of Pr. Weber, the active media respectively 153, 163 and 173 are inserted between the λ / 4 plates respectively 154, 164 and 174 and the mirrors respectively 152, 162 and 172. In other words, in the cavity of Pr. Weber , the active media are traversed by two counter-propagating fields, one of right circular polarization, the other of left circular polarization. It is necessary that its polarizations are preserved so that the A / 4 plate ensures the transformation into waves of rectilinear polarizations and that the CSP can play its role of referral as a function of the polarization. Only the use of an isotropic active medium can ensure this in this configuration. In the case of an anisotropic active medium, this configuration would give a medium traversed by two counter-propagating waves of different elliptical polarizations and the plate λ / 4 would not allow the transformation into rectilinear polarizations such that the CSP can play its role . In fact at the output of the plate λ 4 (as oriented on the diagram), the polarization would be an elliptical polarization still different from that of the wave traversing the active medium as a result of which the CSP would give rise to two beams of orthogonal rectilinear polarizations in different arms which would not allow a ring path of the cavity and would make this cavity lose its interest. According to the invention, the configuration proposed with the anisotropic crystal present in the amplifying medium 205, 215 (with its own axes coinciding with the x and y axes of the CSP 251) and 225 (with its own axes coinciding with the axes Λ: and z of CSP 251) respectively and inserted between CSP 251 and the group consisting of the phase shifter π / 2 204, 214 and 224 respectively (axes oriented at 45 ° axes of the CSP 251) and of the mirror 203, 213 and 223 respectively, makes it possible to ensure the circulation of a rectilinear polarized wave in the ring.

This configuration is moreover excessively simple as regards the orientations to be given to the various elements: - the axes of the CSP 251 are fixed by construction;

- The axes of phase shifters 204, 214 and 223 are similarly known during construction and identified; and

the laser crystal is cut according to specifications (specific to obtaining the laser effect) according to particular orientations of the crystalline axes of the materials, the own axes Uj and u ₂ are also easily identified and noted on the crystal as soon as it is cut. It follows an easy assembly of these various components in terms of orientation.

Depending on the type of amplifying medium chosen for the application envisaged, it will be noted that such a configuration leaves free choice as to the use of isotropic or anisotropic active media:

- if the medium is isotropic the orientation of the medium could be arbitrary;

- if the medium is anisotropic the orientation of the crystal must be taken into account so as to preserve the proper axes imposed by the CSP 251.. .

Furthermore, the resonator 250 obtained can be relatively small. Considering the use of standard components, one could, for example, implement a resonator 250 comprising:

- a cube with a side of 10 mm; - amplifying media (optimized for longitudinal pumping) of 5 or 7 mm in diameter and of length equal to or less than 10 mm;

- λ / 4 blades with a thickness of 2 mm; and

- concave mirrors on thick substrate with a total thickness of 5 mm. This gives a vertical overall length (arms with the media 215 and 205) of 44 mm. Of course, the various adjacent parts of the resonator 250 (in particular the cube, the amplifying media, the λ / 4 plates, the mirrors) may or may not be joined.

According to a variant of the invention, the amplifying media and the λ / 4 plates are joined and produced in the same anisotropic material, this material being doped for the amplifying media in order to obtain a gain and not doped for the λ / 4 plates . Thus, an optical beam passing between a plate and an adjacent amplifying medium does not suffer any loss during the passage from the plate to the amplifying medium and vice versa. FIG. 4 represents a resonator 450 comprising two polarization splitter cubes according to a variant of the invention. The 450 resonator is particularly well suited to high power emissions. The resonator 450 comprises: two separator cubes 470 and 471; - five amplifier arms 403, 413, 423, 433 and 443, each of these arms being supplied by a pump 400, 410, 420, 430 and 440 respectively and comprising an amplifying medium placed between a cube associated with the arm and a phase shift blade λ / 4 itself associated with a mirror; - an arm 463 connecting the two cubes 470 and 471. comprising a free area and not including blades λ 4; and an output arm 453 associated with the cube 471 and comprising a free zone, a λ / 4 plate and a semi-transparent mirror allowing the emission from the internal beam towards the outside of the resonator 450 by forming a beam 480.

The arms 403, 413, 423, 433 and 443 are very similar to the arms 216, 226 and 227, they are not described more fully.

Likewise, the outlet arm 453, similar to the arm 236 illustrated previously, is not described more fully. In addition, cubes 470 and 471 are similar to cube 251 illustrated with reference to FIGS. 2 and 3.

However, care must be taken to orient the above elements appropriately so that the resonator 450 forms a ring resonator without surface, a beam following, for example, the following path, starting from the blade of the arm 403 according to a polarized wave following an x axis perpendicular to the plane of the figure:

- cube 470 (go 401), arm 413;

- cube 470 (return 411 polarized along an axis y in the plane of the figure), arm 423 (go 421 polarized along an axis z in the plane of the figure, perpendicular to the axis v);

- cube 470 (return 422 polarized along the axis Λ :), arm 463, cube 471, arm 453 the beam being partly emitted outwards, the other part being reflected; - cube 471 (return 451 polarized along the z axis), arm 433 (go 431 polarized along the v axis);

- cube 471 (go 432 polarized along the x axis), arm 443; cube 471 (return 441 polarized along the y axis), arm 463 (go 461 polarized along the z axis), cube 470; and - arm 403 (return 40 polarized along the y axis).

Thus, the cubes 470 and 471 are oriented so that they are passing (respectively reflecting with an angle of 45 °) when an incident wave enters one of these cubes according to a polarization the x axis (respectively the v or z axis). In addition, the blades λ / 4 and the mirrors present in each of the arms 403,

413, 423, 433, 443 and 453 cause a change of direction of the incident beam and a tilting of the polarization (a polarization along the x axis (respectively one of the y or z axes) tilting towards a polarization according to one y or z axes (x respectively). Of course, the first, second and third variant illustrated with reference to the description of FIG. 2 can also be implemented within the framework of the embodiment described with reference to FIG. 4.

Thus, for example, according to the third variant illustrated above, the free area present in the arms 453 and 463 can include:

- an amplifying medium pumped transversely;

- a Fabry-Perot interferometer;

- a Fabry-Perot stallion;

- non-linear crystals for the intra-cavity generation of harmonics;

- electro-optical or acoustico-optical modulators; and / or a saturable absorbent.

According to a variant not shown, the resonator comprises several polarization cubes similar to cubes 470 and 471, each of the sides of each cube being associated with:

- a link arm to another cube similar to the arm 263;

- one of the amplifier arms similar to arms 206, 216 and 226; or

- an output arm similar to arm 453.

Thus, such a resonator structure can comprise a large number of amplifier arms, which makes it possible to obtain a large transmission power and a great flexibility of implementation (the arms and constituting the elementary entities being able to be easily combined as a function needs of the intended application).

According to a variant not shown, two neighboring cubes are joined without being connected by an arm, the arm comprising a free area being optional. This embodiment notably allows a resonator structure with at least two relatively compact cubes.

Of course, the invention is not limited to the embodiments mentioned above. In particular, the person skilled in the art can make any variant in the structure of the ring resonator without surface, in particular in the types of the constituent elements, in particular:

- in the polarization separation means (PBS cube, polarization plate, ...);

- in the beam redirection and polarization tilting means (λ / 4 plate or Fresnel rhombohedron associated with a mirror); and or

- In the amplification means inserted between the polarization separation means and the beam redirection and tilt polarization means. According to the invention, the polarization separation means can also separate a polarized beam in two non-orthogonal directions (for example at an angle of 30 °) for, for example, an implementation of the resonator requiring a particular shape or structure .

According to yet another variant, the cube or cubes are chosen so that the arms are not all in the same plane.

It should also be noted that the invention is not limited to the case where the resonator is pumped transversely but extends to any type of longitudinal pumping. It should also be noted that the amplification means according to the invention comprise amplifying media of various types (anisotropic, isotropic crystals, polymers, glasses, etc.) to obtain emission wavelengths in ranges themselves varied.

Claims

1. Ring optical resonator (250, 450) allowing at least one optical beam to circulate inside said resonator by forming a ring of zero surface, said resonator comprising a plurality of modules, each of said modules itself comprising: polarization separation means (251, 470, 471), along a polarization base ((x,), (Λ;, z)), of an incident beam (201, 231) entering said module; - a first arm (216); and

- a second arm (206); said separation means being adapted to separate: a first component of said incident beam oriented in a first direction of said polarization base ((xy)) by forming a first output beam emitted towards said first arm (216); and a second component of said incident beam oriented in a second direction ((Λ:, z)) of said polarization base by forming a second output beam emitted towards said second arm (206); at least one of the first and second arms comprising redirection and tilting means (214, 213, 234, 233) of said first or second output beam respectively, said redirection and tilting means being adapted to redirect said first or second beam output to said separation means and to change the direction of polarization of said first or second output beam to form an incident beam entering another of said modules; characterized in that said resonator further comprises, in at least one of said modules, amplification means (215, 225, 205) of at least one of said first and second output beams, said at least one beam to be amplified, said amplification means being adapted to be associated with longitudinal (210, 220, 200) or transverse pumping means and being located, on the path of said at least one beam to be amplified, between said polarization separation means and said redirection and tilting means belonging to the arm concerned.

2. Resonator according to claim 1, characterized in that said separation means comprise a polarization separator cube (251).

3. Resonator according to claim 1, characterized in that said separation means comprise a semi-transparent plate with polarization separation.

4. Resonator (250) according to any one of claims 1 to 3, characterized in that said separation means are common to all of said modules.

5. Resonator (450) according to any one of claims 1 to 3, characterized in that the set of said modules comprises at least two sub-sets, all the modules of the same sub-set of modules sharing means of common separation.

6. Resonator according to any one of claims 1 to 5, characterized in that said polarization redirection and tilting means comprise:

- a positioned mirror (203, 213, 223, 233) for reflecting said first or second output beam; and

- phase shift means (204, 214, 224, 234)) of said first or second output beam; so that said first or second output beam is:

- first phase shifted by an angle equal to π / 2 radians by said phase shift means;

- then reflected by said mirror; and

- finally again phase shifted by an angle equal to π / 2 radians by said phase shift means.

7. Resonator according to claim 6, characterized in that said phase shift means and said amplification means comprise the same material not doped in said phase shifting means and doped in said amplification means and are coupled so that said first or second output beam passes from said amplification means to said phase shifting means and vice versa without change of medium.

8. Resonator according to any one of claims 6 and 7, characterized in that said phase shift means comprise a quarter wave plate.

9. Resonator according to any one of claims 6 and 7, characterized in that said phase shifting means comprise a Fresnel rhombohedron.

10. Resonator according to any one of claims 6 to 9, characterized in that said mirror is concave.

11. Resonator according to any one of claims 6 to 9, characterized in that said mirror is plane.

12. Resonator according to any one of claims 1 to 11, characterized in that said amplification means comprise an anisotropic material having own polarization axes corresponding to the directions of said polarization base.

13. Resonator according to claim 12, characterized in that said anisotropic material belongs to the group comprising:

- anisotropic crystals; and - glasses showing dichroism.

14. Resonator according to any one of claims 12 and 13, characterized in that said anisotropic material belongs to the group comprising:

- Nd type materials: YAP;

- Nd type materials: YVO4; and - Er: YAP type materials.

15. Resonator according to any one of claims 1 to 11, characterized in that said amplification means comprise an isotropic material.

16. Resonator according to claim 15, characterized in that said isotropic material belongs to the group comprising: - isotropic crystals; and - Er: Yb codoped phosphate glasses.

17. Resonator according to any one of claims 15 and 16, characterized in that said isotropic material is of the Nd: YAG type.

18. Resonator according to any one of claims 15 and 16, characterized in that said isotropic material is of the Ho, Tm: YAG type.

19. Resonator according to any one of claims 1 to 18, characterized in that said resonator further comprises, in at least one of said modules, means for promoting a direction of propagation of said first and second exit beams in said arm concerned.

20. Resonator according to any one of claims 1 to 19, characterized in that said resonator further comprises, in at least one of said modules, non-linear crystals capable of generating a beam having harmonics, from a said at least one optical beam passing through said non-linear crystals.

21. Resonator according to any one of claims 1 to 19, characterized in that said resonator further comprises, in at least one of said modules, an element belonging to the group comprising:

- Fabry-Perot interferometers;

- the Fabry-Perot standards; - modulators; and

- saturable absorbents.

22. Optical resonator (250) according to any one of claims 1 to 21, characterized in that it comprises polarization separation means, along a polarization base, of an incident beam and four arms, said means of separation being adapted to separate the components of an incident beam coming from one of said arms so that:

- Said incident beam is reflected by said separation means, to be emitted towards a first arm among said arms when the polarization of said incident beam is oriented in a first direction of said polarization base; and - Said incident beam is transmitted without undergoing reflection through said separation means to be emitted towards a second arm among said arms, the second arm being distinct from said first arm, when the polarization of said incident beam is oriented in a second direction of said polarization base; each of said arms comprising means for redirecting said incident beam from said separation means, said redirection and tilting means being adapted to redirect said incident beam to said separation means and to change the direction of polarization of said incident beam; said resonator further comprising, in at least three of said arms, means for amplifying said incident beam said beam to be amplified, said amplification means being adapted to be associated with longitudinal or transverse pumping means and being located, on the path of said beam to be amplified, between said polarization separation means and said redirection and tilting means belonging to the arm concerned.

23. Telecommunication device, characterized in that it comprises a resonator according to any one of claims 1 to 22.

24. Video projection apparatus, characterized in that it comprises a resonator according to any one of claims 1 to 22.