FR2773227A1 - Parametric oscillator using a nonlinear cascade to produce continuous optical radiation - Google Patents

Abstract

A radiation source generates light of wavelength lambda p. This is fed through input mirror Me into a resonant cavity. The cavity contains a cascade of materials possessing nonlinear optical properties, one of which is the tendency to reradiate at wavelength lambda s. The result is that the oscillator converts the majority of the input into radiation which is transmitted through the output mirror Ms at wavelength lambda s.

Description

OPTICAL PARAMETRIC OSCILLATOR WITH CASCADE EFFECT

  The field of the invention is that of optical power sources, capable of generating an optical wave of wavelength As from an optical wave of wavelength λp and this by exploiting the

  2nd order linearity of some optically nonlinear materials.

  Indeed, the frequency conversion operations allowed by the nonlinear optics, widen very significantly the range of spectral ranges accessible to laser sources. Thus, from laser sources perfectly controlled but whose emission spectral range remains reduced, it is possible by non-linear effect to convert the emitted radiation to inaccessible wavelength bands by conventional means. However, to be truly usable, these operations must have a significant conversion energy efficiency. But in many cases, this return is fundamentally limited

  by the nonlinear interaction itself.

  Thus, using parametric fluorescence, it is possible to generate a long beam Xs called signal, from a pump much shorter wavelength Xp The generation of the signal is accompanied by the generating a complementary wave called idler and wavelength Xi. The conservation of energy imposes the following relation between the three wavelengths: h h h 27r p 27 + 27c i:

+

  Xp xs Xi For the energy conversion of the pump to the signal and the idler to be effective, it is imperative that there is a phase agreement between the nonlinear polarization generated by the pump and the signal and idler waves, this is to avoid destructive interference. This condition is summed up by the following formula: np = ns and Xp xs xi o np, ns etni are the refractive indices of the non-material

  linear respectively for the pump, the signal and the idler.

  If we exclude the special case of degeneracy (Xs = 2Xp) and if we consider that only the signal is of interest, the conversion energy efficiency can not exceed the limit imposed by the ratio Xp / Xs less than 1. Therefore, if one seeks, for example, to emit a wave towards 10 pm from a beam located in the near infrared (2 pm for example), the energetic efficiency will be typically limited to 20%, knowing that this upper limit is rarely reached experimentally. Better pull bet of incident photonic energy, remains

  therefore a problem to solve in this type of conversion.

  To alleviate this problem, the applicant has proposed the method

  described below and which has been the subject of a published application.

  According to this method, the quantum yield of the conversion Xp to Xks can be multiplied by two if it is possible to make the following two processes P1 and P2 coexist: P1: Xp gives s and ki1 (1 / s + 1 / Xi1, = 1 / kp) P2 Xil 'gives Xks and Xi2 (1 / ks + 1 / Xi2 = 1 / k) In the second interaction, the idler photons created during the first interaction are used to generate new signal photons. This second interaction, involving different wavelengths, assumes phase-matching conditions different from the first one. Still according to the same scheme, if the difference between the pump wavelength λp and the wavelength targeted is justified, a greater number of processes can be used. In general, and using angular frequencies, n processes can be used to convert Op photons into photons O s such that: P1: oep- es + Oei (Oep = Cs + iC) P2: c'i - - + Os + ei2 (Oi, 1 = COs + i, 2) Pn: ein-1 -> s +0 in () in-_ l = s + (in)

  with n = E (0 p / (s), where E denotes the integer part.

  This amounts to "cutting" the pump photons into n photons signal. The ultimate photon yield is then n times that given for a single interaction. Figure 1 shows the energetic diagram corresponding to this method, the n '' interaction being at the degeneracy (xs = Xin = 2 in-1) The wave with co p will be called the primary pump and the O i1 with c in -1 the

secondary pumps.

  Whatever the configuration envisaged, each parametric process implies an interaction with conditions of agreement of

distinct phase.

  For non-linear birefringent materials with perfect phase tuning, this amounts to associating several crystals in series whose orientation and / or temperature are adjusted to satisfy the

  phase agreement conditions specific to each interaction (Figure 2).

  Applying these methods to QAP materials may lead to the use of a single crystal and simplify the system. Indeed, the phase tuning is in this case achieved by a periodic inversion of the sign of the nonlinear coefficient, the pitch depending on the dispersion of the material and the nonlinear interaction envisaged. Thus, to realize several distinct frequency conversions in a single QAP material is simply to use a structure

multi-periodic (Figure 3).

  Such periodic structures can be obtained by flipping the ferroelectric domains under the influence of an electric field for ferroelectric materials such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). These materials being absorbent

  for wavelengths longer than 5 pm, if it is desired to transmit

  Beyond this, it is possible to use semiconductor materials. In the case of GaAs, for example, the solid periodic structure can be obtained by welding strips, of a thickness equal to one or three lengths of

  coherence, and alternate orientations (see French Patent No. 2,704,953).

  Consider, for example, a pumped assembly at 2.13 μm and emitting a signal wave at 9.6 μm. The sequence of interactions, the quantum limits (n) and the necessary periods (A) at each step in the case of GaAs at QAP are: P1: 2.131rm gives 9.6grm and 2.74pm (? <22%) ( A1 = 82km) P2: 2.74pm gives 9.61zm and 3.82pm (q <44%) (A2 = 127p.m) P3: 3.82t.m gives 9.6pm and 6.70pm (R <66% ) (A3 = 197tm) P4: 6.70im gives 9.6m and 14.4pm (ir <88%) (A4 = 146pm) Nevertheless, the configurations presented in the aforementioned patent application, are applicable as they in the case where the gain in single pass is high enough to induce a significant conversion of the primary and secondary pumps in each of the interactions. Often, the intensities involved do not achieve such a gain in single pass. It is therefore necessary to insert the amplifying medium into a resonant cavity to form a parametric oscillator. Depending on the nature of the optical pumping sources of the nonlinear medium in which the parametric interactions take place, there may be

  optimized configurations of parametric oscillator.

  The subject of the invention is an optical parametric oscillator configuration, particularly adapted to a continuous or quasi-continuous pump optical source by means of which the oscillator

  optical parametric can operate in stationary or near-steady state

  stationary. More specifically, the subject of the invention is an optical parametric oscillator comprising a pump source delivering a wave

  of pump with the length XPl, n operating in continuous or quasi-steady state

  continuous, a first cavity defined by at least one input mirror and an output mirror, and a non-linear medium in the cavity, capable of converting the pump wave at the wavelength Pln into a signal wave at the wavelength ks, characterized in that: the nonlinear medium comprises a succession of n domains with in the first domain means for converting the pump wave to XP1, n into waves at Xi1, n and; , ..., in the first domain means for converting the wave to ip i 1n into waves at Xiin and Xs, .... in the domain of means for converting the wave to.Pn, n in waves to; inn and, s; - the input mirror having a maximum reflection coefficient at the wavelength; s; - the output mirror having a reflection coefficient adapted to the wavelength ks of to optimize the output energy to S. Advantageously, the pump source may be a laser or a

  primary parametric oscillator delivering pump wave at XP1 n.

  In classical parametric oscillator structures where a single interaction takes place within the oscillator, on the two waves generated, the shorter wavelength radiation is usually resonated, that is to say the one called here idler. The small difference between the wavelengths pump and idler confers on these two radiations, divergence properties close to o better coverage of the two beams inside the cavity. In addition, the realization of broadband dielectric mirrors is better controlled at shorter wavelengths. In the context of the invention, with n successive interactions involved, if we resonate the low wavelengths, that is to say the set of secondary pumps (COi1 ao in-1) to beginning of the second interaction (P2) and for those that follow, the three waves to which parametric amplification will be applied are initially present. However, the direction of conversion of the nonlinear interaction, that is to say from the pump to the

  signal and the idler or the opposite, depends on the relative phase of the three waves.

  In this case, it is imperative to perfectly control the phase of each wave present in the oscillator, which seems difficult to achieve in practice, if only for reasons of random phase introduced at the mirrors. In addition, resonate the set of secondary pumps XPin with 2 <i <n requires the use of very broadband or multiple band mirrors difficult to achieve in practice, especially in the infrared. Finally, such a configuration increases the number of waves present inside the cavity with a high intensity, and therefore the risk of parasitic interactions can reduce the

  performance (doubling of frequency, summation etc ...).

  In the context of the invention, at the beginning of each interaction, there exist only the signal and the primary pump or the secondary pump generated in the previous interaction. Under these conditions, the phase of the generated idler automatically adapts to maximize the conversion. Thus since only one wave is reflected in the cavity, the realization of the mirrors is identical to that encountered for optical parametric oscillators. According to a first variant of the invention, the non-linear medium comprises a succession of n crystals whose orientation and / or temperature for each of the crystals, are adjusted so as to satisfy phase-matching conditions between the lengths

of wave Xpi, n, iin and As.

  According to a second variant of the invention, the nonlinear medium comprises n periodic modulations of period Ai of a physical parameter of the nonlinear medium, for example a sign of the nonlinear coefficient corresponding to the domain, so as to satisfy the quasi-phase agreement conditions between waves at wavelengths

XPi ,, s and Xiin.

  To ensure the tunability of the signal wave, the nonlinear medium may comprise k sequences of parallel sequences of n periodic modulations of period Aij of a physical parameter of the nonlinear medium,

with 1 <i n and l j <k.

  Advantageously, the length Li of each of the domains can be defined as follows: Li = Zoin = [soXp.in'Xs-.Mi'n.nPin.ns.niin] 1 2z iOn 47deff in (tWn) 1/2 with z n_ = K (yin) [1-r 11/2 upl, n (O) i + (n-1) r UPln (O) = nr wP + - rc s 1-r 1-r -1/2 Yin = i + (n-1) rJ r: the product of the reflection coefficients of the mirrors x parameter including the

discrete losses of the cavity.

  Wn = IP1, n (0) 1 + 1 - s) I -nr e p '10 and. K (y) a complete elliptic integral So the dielectric permittivity of the vacuum nPin, ns nin the refractive indices of the nonlinear medium at wavelengths XPin,% s. iin IPln (0) the intensity of the pump wave at XPn deffin the effective nonlinear coefficient of the i * m nonlinear interaction cp: the pulsation of the signal wave oe p = 27r 2 C (os: the pulsation of the signal wave co = C The invention will be better understood and other advantages

  will appear on reading the description which will follow, given as a non

  FIG. 1 illustrates the energy diagram relating to the cutting of photons in a process with n parametric interactions for frequency converters; FIG. 2 illustrates a first example of a device implementing the process with n parametric interactions; FIG. 3 illustrates a second example of a device implementing the process with n parametric interactions; FIG. 4 illustrates an optical parametric oscillator according to the invention comprising a linear resonant cavity; FIG. 5 illustrates an optical parametric oscillator according to the invention comprising a ring resonant cavity; FIG. 6 illustrates an example of parametric oscillator

  optical system comprising a succession of k almost

  phase tuning so as to be able to vary the signal wave at xs. In general, the optical parametric oscillator according to the invention comprises a non-linear medium MNL pumped by a source of

  pump, the medium being inserted into a resonant cavity.

  According to a variant of the invention illustrated in FIG. 4, the resonant cavity is defined by an input mirror Me having a maximum reflection coefficient at the wavelength Xset by an output mirror Ms having an optimized reflection coefficient to Xs so as to let out of

  the cavity, photons at the wavelength Xs.

  The non-linear medium MNL comprises i domains, where Di is the seat of i optical parametric interactions, from the photons of

  pump u p, from the pump source LP.

  According to another variant of the invention, illustrated in FIG. 5, the resonant cavity is a ring cavity defined by four mirrors Me2, Me3, Me4 andMs2, the mirrors Me2, Me3, Me4 having maximum reflection coefficients at X. mirror Ms2 having an optimized reflection coefficient at As. The advantage of such a configuration lies in the fact that the waves pass through the nonlinear medium only in the sense of amplification, which makes it possible to limit the influence of the losses due to the medium

non-linear.

  In general, the nonlinear medium may comprise a succession of n nonlinear crystals or advantageously a single nonlinear material with a multi-periodic structure of n periods A1 ..., An, in which all the conditions of quasi are realized.

  phase agreement (QAP).

  In this case, it is possible to place side-by-side k sets of interactions perfectly matched to each other, each of the sets being able to generate a particular range of specific lengths in order to ensure the tunability of the oscillator

  parametric, as shown in Figure 6.

  We will describe in more detail the operation of the optical parametric oscillator also called OPO of the invention and this

  in the context of a non-linear material with a multi-periodic structure.

  From the point of view of the dynamics of the oscillator, one can

  break down its operation into three distinct temporal phases.

  First, the phase corresponding to the rise time of the OPO where the signal and the idler are amplified from the value of the quantum noise with a depopulation almost zero of the primary pump. This phase

  corresponds to a loss of energy since very little conversion takes place.

  The smaller the cavity length, the shorter the round-trip time of the resonant wave and the more amplifications over a given time. The length of the cavity thus influences the rise time of the OPO. Thus, for the same length of crystal amplifier, the cavity

  is short, more rise time is reduced.

  The second phase begins when the amplification has been large enough in the first interaction so that the generated idler can serve as a pump in the second. This time, the pump and the signal are both present at the input of the periodicity zone A2, the conversion is therefore extremely fast: the signal will be amplified while

  that the pump of the third interaction will be generated.

  This results in a cascaded amplification process until all secondary pumps have been created and the OPO has reached a stationary state, which is the third phase of its operation. From a system point of view, the structure of the OPO can be broken down into two entities. The first is constituted by the crystal zone corresponding to the first interaction which makes it possible to initiate the oscillation. The second groups the areas of periodicity A2 to An seats processes P2 Pn supplied by the secondary pumps in order to amplify the signal generated by the process P1. This second entity is of interest only when a stationary or quasi-stationary regime can be established, that is to say when the duration of action of the pump is large.

  before the rise time of the oscillator.

  This is obviously the case in continuous pumping, but a number of configurations with a pulse pump to use the system described above, as is explained below: a Quasi-continuous pumping When the duration of the pulse pump is typically greater than the microsecond, that is to say, much greater than the rise time of the OPO which is of the order of a few nanoseconds to a few tens of nanoseconds, it installs a quasi-stationary regime during the duration of the pulse to obtain a significant increase in yield. a Synchronous pumping

  This is a pumping where the impulses are very short

  ie of the order of the picosecond or the femtosecond, but with a very high repetition rate around a few tens of megahertz to a few gigahertz. The length of the cavity is adjusted so that the time interval between two pump pulses is equal to the travel time of the wave resonating in the cavity. Thus, at each passage, the impulse of the signal wave is regenerated by a new

  pump impulse, o establishment of a quasi-stationary regime.

  o Intracavity Oscillator The laser material that will make it possible to generate the pump with the parametric oscillator and the nonlinear material are placed in the same cavity. The mirrors in the laser cavity are at maximum reflection for the oap primary pulse pump, transparent for the secondary pumps and optimally reflective for the signal. At first, the laser material is pumped and the pump energy that is generated at o p in the cavity decreases very slowly due to the high reflection coefficients of the mirrors. Once the pump is created, the parametric oscillation can take place insofar as the characteristic life of the pump in the cavity, which is related to the high overvoltage coefficient at Cop, is

  greater than the rise time of the OPO.

  The laser cavity thus constitutes a reservoir of energy until the signal and the idler of the first nonlinear interaction have reached an intensity sufficient to initiate the depopulation of the primary pump and

cascading conversions.

  In the context of the invention, the lengths of the different domains Di here constituted by periodic structure, of period Ai, can be optimized as is explained below: The signal always being the wave which one seeks to amplify by cascade effect, so-called quantum efficiency, the ratio of the number of photons to so. on the number of photons at o p, this yield increases when the

  number of interactions increases, as described in the introduction.

  To evaluate the gain in efficiency when the number of interactions increases and the amplifying medium is placed in a cavity, it is important to note that the parametric amplification is a cyclic process: there is first conversion of the pump to the signal and the idler then, once the depopulation of the pump is total, there is conversion of the signal and the idler towards the pump. The greater the intensity of the waves involved, the shorter the interaction length required for the depopulation-reconversion cycle. Therefore, at each intensity corresponds an optimum length for each of the domains Di which induces a total depopulation of the corresponding pump while avoiding its reconversion. It follows that the more the number of domains increases, the more the intracavity intensity of the signal increases because of the successive amplifications and therefore, the longer the lengths of each of the interactions must be.

  be reduced to avoid reconversion.

  In addition, at the start of the oscillator, only the incident pump is present, the signal and the idler of the first interaction have an intensity equal to that of the quantum noise, that is to say extremely low. Since the pump of the second interaction has not yet been generated in the first one, the conversion is zero for processes with indexes greater than or equal to 2. It is therefore only the first interaction that makes it possible to perform the conversion process. and it is she who will condition the threshold of the optical parametric oscillator. If the length of the first interaction is too small, the gain in single pass will be less than the losses and the oscillator will never be able to start. Thus, when the total number of interactions increases, it may be necessary to reduce the length of the first set of period A1, so that the threshold is no longer reached. It is then necessary to increase the length of this interaction to exceed the threshold, this at the cost of a slight reconversion of the pump when the parametric optical oscillator will have reached its steady state. This partial reconversion of the incident pump is therefore responsible for a

  slight reduction in the expected yield.

  In the configuration studied here, an analytical model of OPO operation was developed to predict the lengths required for each interaction. This model is based on the resolution of the nonlinear coupling equations discussed in the article by Baumgartner and Byer entitled "Optical Parametric Amplification" (IEEE Journal of Quantum Electronics, Vol.EQ-15, No. 6, June 1979). In the formulas that follow, the value of any physical quantity A corresponding to the interaction on a total number of n interactions is noted Ain. The 0in represent the normalized lengths allowing to obtain an eme

  complete depopulation of the interaction.

  O K (Yin) | 1-r (1) uPi% (0) i + (n-1) ro K (y) is a complete elliptic integral referenced in the "Handbook of Elliptic Integrals for Engineers and Physicists" by PF Byrd and MD Friedman, Ed. Springer, Verlag / Berlin - Gottinger - Heidelberg, and o uPin (0) and Yin have the following form: up2n (0) = n (2) p + lO r Osnr 1Ip - r 2 _ 1-r in i + (ni) r 3) n is the total number of nonlinear interactions, and r is equal to the product of the mirror reflection coefficients of the cavity for the signal and can possibly account for absorption and reflection losses on the faces of the non-linear material . up2n (O) represents the intensity of the primary pump at the beginning of the first interaction, which is proportional to a number of photons per unit of time and area. The coefficient y, meanwhile, accounts for the proportion of pump photons relative to the number of signal photons at the beginning of the image.

lo interaction.

  Finally, the real Zoin lengths of the Di domains which maximize the conversion, are related to the normalized lengths 01, n by the following relation 4 ZOi n = Eso Pn l Xii, n nyn ns nii, n (4) 47: deMn lcWn Eo is the dielectric permittivity of the vacuum, Xp, Xs, Xi, the wavelengths of pump, signal, idler and np, ns, nor the refractive indices of the material at the wavelengths considered. deff is the effective nonlinear coefficient of the nonlinear interaction. Finally, Wn is a constant related to energy conservation, equal to the sum of the intensities of the three waves involved in each of the interactions. In the case of the complete depopulation of each of the pumps and in the absence of absorption, the energy is kept interacting interaction, Wn then takes the following form: Wn = iP1, n (0) (1+) 1rr cos (5) 1-r 0pJ o IPl, n (O) is the intensity of the primary pump at the input of the

first interaction.

  According to these calculations, it is then possible to optimize

  numerically the Li lengths in the vicinity of the ZOin lengths.

  As explained above, if the first interaction length L1 thus calculated is less than the threshold length, it becomes necessary to increase it to cross the threshold and thus start the oscillator. The threshold length is defined more precisely, as the length for which the gain is equal to the losses on a round trip. Is

  the length defined in equation 6.

1 E OC; Ls Xil'n nPl'n 'ris' i'.

  Lseuil = Arg Ch [1 i. 11, fl npl, n ns. (6) r deffln 8x 2Pn () So if Z01, n <Lseuil

  we impose Z01, n = (1 + x) Lseuil, with typically x of the order of 0.1.

  Example of an optical parametric oscillator comprising a lithium niobate crystal emitting at 4.1 .mu.m Using a pump source emitting at XPln = 1.0641m, it is desired to produce an optical parametric oscillator emitting around a

wavelength ks of 4.1 μm.

  The peak intensity IP1, n (0) of the pump can typically be equal

at 0.2 MW / cmn2.

  The resonant cavity is made with a mirror Me of reflection coefficient Re = 1 and with a mirror Ms of reflection coefficient Rs = 0, 95. The nonlinear coefficient of the material used is deff = 20pm / V. The 3 following interactions are necessary: 1) 1 / Xp1,3 = 1 / Xi1,3 + 1 / Xks with X; Pl3 = 1,064gm kil, 3 = 1,441m 2) 1 / Xpz3 = 1 / Xi2,3 + 11 / s with Xp2, 3 = 1.441gm i2.3 = 2.21gm 3) 11Xp3, 3 = 1I / i3.3 + 1 / Xs with Xp3,3 = 2.21gm i3.3 = 4.8pm The Li lengths of the domains are as follows: L1 = 13.2 mm L2 = 14.1 mm L3 = 23.4 mm The overall quantum yield is 2.65. This efficiency is defined by numerical analysis by taking the parameters of the optical parametric oscillator and associating them with the evolution equations of the

  fields, described by Baumgartner (reference cited above).

  The length of the whole of the nonlinear medium is of the order of cm which is commonly carried out on substrates of lithium niobate. The example of lithium niobate is not limiting since serial interactions can be obtained with other quasi-phase-coherent or birefringent materials such as LiTaO3, KTP, KTA orKNbO3.

  To reach higher wavelengths one can use GaAs but also other materials III-V and II-VI and crystals

  birefringents such as AgGaSe, ZnGeP2, CdSe, etc ...

  Example of an Optical Parametric Oscillator Comprising GaAs Emitting at 9.6 μm Using a pump source emitting at 2.13 μm, it is desired to reach wavelengths around 9.6 μm, with an intensity of

pump Ipl, n (0) = 1 MW / cm 2.

  As in the previous example, the cavity is formed of

  Me and Ms mirrors with Me = 1 and Ms = 0.95 for the Xs wavelength.

  The effective nonlinear coefficient deff = 108pm / V. The following 4 interactions are necessary: 1) 1 / XP1,4 = 1 / kil, 4 + 1 / Xs with XPl, 4 = 2,1 3gm it, 4 = 2, 74tm 2) 1 / XPZ4 = 1 / Xi2,4 + 1I / s with XP24 = 2,74p.m i2,4 = 3,82gm 3) 1 / LXP3,4 = 1 / i3,4 + 1 /; Is with Xp3,4 = 3,82.m L3,4 = 6,701m 4) 1 / lXP4,4 = 1 / L4,4 + 1 / s 1 with P4,4 = 6,70m Ki4,4 = 14.4gm The lengths Li of the domains are as follows: L1 = 3.9 mm L2 = 3.95 mm L3 = 5.96 mm 4 = 12.95 mm

  The overall quantum yield is 3.47.

Claims (14)

  An optical parametric oscillator comprising a pump source delivering a pump wave at length k.lP1n operating in a continuous or near-continuous mode, a first laser cavity defined by at least one input mirror and an output mirror, and a nonlinear medium in the cavity, capable of converting the pump wave at the wavelength XP1 n into a signal wave at the wavelength; s, characterized in that: - the nonlinear medium comprises a succession of n domains with, in the first domain, means for converting the pump wave to XP1, n into waves at, n and Xs, in the second domain of means for converting the wave to Pi, n = ii - 1, n in waves at Xiin and Xs, in the field of means for converting the wave at XPnn into waves at kinrn Xs; the input mirror having a maximum reflection coefficient at the wavelength λs; the output mirror having a reflection coefficient adapted to the wavelength λs so as to optimize the extraction energy at; s.   2. Optical parametric oscillator according to claim 1, characterized in that the nonlinear medium comprises a succession of n crystals whose orientation and / or the temperature of the crystals are adjusted so as to satisfy phase agreement conditions between waves at wavelengths XPi ,, s, Xin.   3. Optical parametric oscillator according to claim 1, characterized in that the nonlinear medium comprises n periodic modulations of period Ai of a physical parameter of the nonlinear medium, in the i6th field so as to satisfy quasi-linear conditions. agreement   phase between the waves at wavelengths X1Pi ,, s and Xii, n.   4. Optical parametric oscillator according to claim 3, characterized in that it comprises k successions put in parallel of n periodic modulations of period Aij of a physical parameter of the medium.   nonlinear with 1 <i <n and 1 <j <k.   Optical parametric oscillator according to one of the claims   1 to 4, characterized in that the length Li of each of the i domains is defined by the relation: Li = ZOn [s Pin.Xs. kiin.npin.ns.niin] 1/2 Zin 47: deff in (7Wn) 1/2 K (Ln) F 1- 1/2 with ZOi, n -   u Pn (O) i + (n-1) rJ UP1, n (0) = nr nr (op + os 1 -r 1 -r) / 2 Yi'n = i + (n-1) r] Wn = Ipln (0 ) 1 n) r and. K (y) a complete elliptic integral 15. so the dielectric permittivity of the void * nPin, ns niin the refractive indices of the nonlinear medium at the wavelengths XPi, ns, Xiin IPln (O) the intensity of the wave of XPin pump deffin the effective nonlinear coefficient of the i th 'non linear interaction op: pulsation of the pump wave At o s: pulsation of the pump wave 6. The optical parametric oscillator according to claim 5, characterized in that: 25. if Zo1, n> Lseuii i1 / 21 C.OGXs. 1rn nPwn fls ni1i with Lseuil = Arg Ch 1 deffl1n. {8c c slnnPlnns'nil) i rl / 2 8n21p (o) 1i, and L1 = Zo1, n if Z01, n <Lseuii L1 = (l + x) Lseuil with x > 0   Optical parametric oscillator according to one of claims 1 to   6, characterized in that it comprises a stationary wave cavity, defined by at least two mirrors.   1 0 Optical parametric oscillator according to one of claims 1 to   6, characterized in that it comprises a ring cavity, defined by at least three mirrors which have a maximum reflection coefficient at the AXs wavelength.   Optical parametric oscillator according to one of claims 1 to   8, characterized in that the pump source is a laser.   Optical parametric oscillator according to one of claims 1   at 8, characterized in that the pump source is an oscillator parametric primary.   11. Optical parametric oscillator according to one of claims 1   at 10, characterized in that the pump source continuously transmits the pump wave.   Optical parametric oscillator according to one of claims 1   10, characterized in that the pump source outputs pulses of   duration greater than or equal to the microsecond.   Optical parametric oscillator according to one of claims 1   at 10, characterized in that the pump laser emits pulses of very short duration less than one picosecond, with a high repetition rate higher than a few megahertz allowing synchronous pumping of the parametric oscillator.   Optical parametric oscillator according to claim 9, characterized in that the pump laser comprises a second cavity, the   first cavity being located inside the second cavity.