Optical fiber, device for improving the nonlinear properties of an optical fiber, and method for improving the nonlinear properties of an optical fiber

Abstract

Multimode optical fiber (F) comprising a core (FC) in a first material (M1) and a cladding (FG) surrounding said core in a second material (M2), a first transverse profile (P1) of a fictitious temperature of said core having a first variation between a center (C) of the core and an edge of the core, called first variation, between A% and B% / greater than X% / less than Y%.

Description

Title of the invention: Optical fiber, device for improving the nonlinear properties of an optical fiber, and method for improving the nonlinear properties of an optical fiber. Technical field

[0001] The present invention relates to the field of optical fibers for nonlinear conversion and more particularly to optical fibers for nonlinear conversion in which a periodic longitudinal nonlinearity array is inscribed. Prior art

[0002] It is known to achieve second harmonic generation in crystals exhibiting a 2nd order nonlinearity (or susceptibility) denoted In order to obtain high conversion efficiency, it is necessary to obtain a phase match between the fundamental wavelength and the second harmonic during their propagation in the nonlinear medium.

[0003] Periodic polishing in crystals is a technique for obtaining a quasi-phase matching of nonlinear conversions in these crystals (PA Franken and JF Ward, “Optical harmonies and nonlinear phenomena”, Rev. Mod. Phys. 35, 23 (1963)). It involves a process that generates a periodic inversion of the domain orientation in a nonlinear crystal such that the sign of the nonlinear coefficient also changes.

[0004] It is also known to perform so-called optical polling in fibers comprising a core made of a centrosymmetric glassy material (Stolen, RH, & Tom, HWK (1987). Self-organized phase-matched harmony generation in optical fibers. Optics letters, 12(8), 585-587). Indeed, in their initial state, these centrosymmetric materials do not allow the generation of a second harmonic. Optical polling consists of inscribing, in the optical fiber, a second-order nonlinearity z(2) periodically longitudinally by optical means. This inscription is artificially created by the beat of a fundamental frequency with its second harmonic, inducing a static electric field that breaks the centrosymmetry of the core material. It is then possible to generate the second harmonic in a quasi-phase-matched regime thanks to the periodic longitudinal network of second-order nonlinearity

[0005] Figure 1 schematically illustrates a prior art device 10 adapted for optically polishing an optical fiber 1. A pulsed laser source 2 is adapted to generate a beam 3 at a fundamental frequency. This beam is frequency-doubled using a nonlinear crystal 4 per second generation. harmonic. The beam 5 exiting crystal 4 therefore comprises two frequencies: the fundamental frequency and the second harmonic at frequency û?2 = 2. This beam 5 is injected into optical fiber 5 which has a graded-index core in a glassy material, for example germanium-doped silica.

[0006] For beam powers 5 of approximately 1 MW at the first frequency and 100 W at the second harmonic, an injection of the beam 4 into the optical fiber 5 with a typical duration of several tens of minutes allows for a longitudinal periodic inscription of a second-order nonlinearity j(2\). This inscription step is called "optical polling". This network persists for a certain time in the fiber after the beam 4 is interrupted and allows for quasi-phase matching for the generation of the second harmonic when an additional beam at the first frequency is injected into the optical fiber, forming a beam 6 at the frequency

[0007] It should be noted that cepoling can be carried out by methods other than that of [Fig. 1]. In particular, cepoling can be carried out by thermal effect or by electrical effect by locally and periodically longitudinally modifying the phase matching conditions of a nonlinear conversion.

[0008] However, it remains necessary to better control and improve the non-linear conversion efficiency of this type of fiber with polling.

[0009] To this end, an object of the invention is an optical fiber comprising a core with a transverse profile of a fictitious temperature exhibiting a first variation between a center of the core and an edge of the core of less than 105%. Indeed, the aforementioned variation of the fictitious temperature ensures high nonlinear conversion efficiency by quasi-phase matching in the fiber after polishing. Summary of the invention

[0010] For this purpose, an object of the invention is an optical fiber comprising a core in a first material and a sheath surrounding said core, a first transverse profile of a fictitious temperature of said core having a maximum located substantially at said center of the core and having a first variation between a center of the core and an edge of the core, said first variation f less than 105%, preferably less than 60%.

[0011] Preferably, the optical fiber has a transverse profile of a fictitious fiber temperature, called the second profile, having a variation between the center of the core and an outer edge of the cladding, called the second variation ATf, of less than 125%, preferably less than 105%.

[0012] Preferably, the heart exhibits a transverse profile of non-constant index variation with a maximum located substantially at the center of the heart

[0013] Preferably, the first material is silica or a glass of which a The rate of structural relaxation is approximately equal to that of silica.

[0014] Preferably, the optical fiber comprises a coating surrounding the cladding in a material opaque to visible radiation or to visible-UV radiation.

[0015] Another object of the invention is a device for improving the nonlinear properties of an optical fiber comprising: a. an optical fiber according to the invention, b. a quasi-phase matching generation system adapted to inscribe a longitudinal periodic array of a nonlinearity of order two or more in the optical fiber allowing a quasi-phase matching for a nonlinear conversion of a first wavelength in the optical fiber.

[0016] According to an embodiment M1 of the device for improving the nonlinear properties of an optical fiber, wherein the quasi-phase-matching generation system comprises a laser system adapted to generate at least one incident beam having a first wavelength and a coupling system adapted to inject said at least one incident beam into said optical fiber for a predetermined duration, the laser system and the coupling system being further adapted so that said at least one incident beam has sufficient intensity during its propagation within the fiber to inscribe said longitudinal periodic lattice.

[0017] Preferably, in embodiment M1, the laser system, the coupling system and the optical fiber are adapted so that said longitudinal periodic grating is a grating of second-order nonlinearity, said nonlinear conversion of the first wavelength in the optical fiber being a second harmonic generation.

[0018] Preferably, in embodiment M1, said laser system comprises a laser adapted to generate a first beam having the first wavelength and a nonlinear conversion element adapted to generate a second beam having a second wavelength 22 obtained by generating the second harmonic of the first wavelength of the first beam, said first beam and said second beam forming said at least one incident beam inscribing said longitudinal periodic grating.

[0019] Preferably, in embodiment M1, the laser system and the coupling system are adapted so that the peak power of the first and second beams is respectively between 100 W and 10 MW and between 10 W and 10 MW at the time of their injection into the optical fiber

[0020] Preferably, in embodiment M1, the predetermined duration is greater than 5 minutes.

[0021] Preferably, in embodiment M1, the first or second variation and the predetermined duration and intensity of the incident beam during their propagation in the optical fiber are adapted for a second harmonic generation efficiency in the fiber greater than 1%, preferably 3%.

[0022] According to an embodiment M2 of the device for improving the nonlinear properties of an optical fiber, the quasi-phase matching generation system comprises electrodes arranged longitudinally along the fiber and adapted to generate an electric field periodically along the length of the optical fiber in order to inscribe said longitudinal periodic network. Preferably, the electrodes are adapted to generate an electric field greater than 10 kV / mm, preferably 20 kV / mm.

[0023] Alternatively, according to an embodiment M3 of the device for improving the nonlinear properties of an optical fiber, the quasi-phase matching generation system includes heating elements arranged longitudinally along the fiber and adapted to heat said core in order to inscribe said longitudinal periodic network.

[0024] Another object of the invention is a method for improving the nonlinear properties of an optical fiber comprising the following steps:

[0025] A- select an optical fiber comprising a core in a first material and a cladding surrounding said core in a second material, a first transverse profile of a fictitious temperature of said core having a maximum located substantially at said center of the core and a first variation between a center of the core and an edge of the core, said first variation less than 105%, preferably less than 60%,

[0026] B- inscribing a longitudinal periodic network of a nonlinearity of order two or more in the optical fiber allowing a quasi-phase match for a nonlinear conversion of a first wavelength in the optical fiber. Brief description of the drawings

[0027] Other features, details and advantages of the invention will become apparent from the description given with reference to the accompanying drawings provided by way of example, which represent, respectively:

[0028] [Fig. 1], a schematic view of a prior art device adapted to perform optical polling of an optical fiber.

[0029] [Fig.2], a schematic representation of the transverse profile along an xy plane of an optical fiber according to the invention,

[0030] [Fig.3A], a schematic representation of a device for improving the non-linear properties of an optical fiber according to the invention

[0031] [Fig. 3B], a schematic representation of an embodiment M1 of the device improvement of the non-linear properties of an optical fiber of the invention

[0032] [Fig.3C], a schematic representation of the longitudinal periodic lattice of non second order linearity obtained in the optical fiber of the invention after optical polishing using the device of [Fig.3B].

[0033] [Fig.4A], a curve showing the evolution of the average power of the second harmonic generated at the optical fiber output in the device of Figure 3B, as a function of the injection duration of the incident beam. The curve in Figure 4A is obtained for a first ATf variation of 59%. For comparison, Figure 4B shows the same evolution curve with identical injection parameters (power, frequency, repetition rate, pulse duration, etc.) obtained with the setup of Figure 1 with an optical fiber 1 that does not have a first ATf variation below 105%.

[0034] [Fig. 4B], the same evolution curve as in Figure 4A with parameters identical injection but with an optical fiber 1 not including a first variation Æf less than 105%.

[0035] [Fig. 5], a schematic representation of a preferred embodiment of the fiber of the invention

[0036] [Fig.6], a schematic representation of a method for improving the nonlinear properties of an optical fiber adapted to be implemented by the device of [Fig.3A].

[0037] In the figures, unless otherwise indicated, the elements are not to scale. Description of the implementation methods

[0038] Figure 2 schematically illustrates the transverse profile along a plane of an optical fiber F according to the invention. As will be explained later, this fiber F is specifically adapted to exhibit a high nonlinear conversion efficiency Ef, after the inscription of a longitudinal periodic grating IP with a nonlinearity of order two or higher. In the remainder of this document, the step of inscribing this nonlinearity grating is referred to as "poling."

[0039] The optical fiber F of the invention comprises a core FC made of a first material M1 and a cladding FG surrounding the core made of a second material M2. The invention is of particular interest for centrosymmetric glassy materials M1. More precisely, the material M1 can be a glass or an antiglass. Indeed, due to their symmetry, these materials do not allow nonlinear conversion in the initial state. The material M1 can also be a birefringent material.

[0040] In the optical fiber F of the invention, the core FC has a transverse PI profile of fictitious temperature, hereinafter referred to as the "first PI profile". This first PI profile has a maximum located substantially at the center C of the core and a first variation ATf between the center C and an edge B of the core of less than 105%. By "localized" "approximately at the center C", we mean that the profile P\ has a maximum at the center C or within an interval of ± 10% of the radius of the core, centered at the center C. This first variation ^T fl is preferentially less than 60%.

[0041] It is recalled here that the fictitious temperature is defined as the temperature at which the glassy material would be in equilibrium if it were suddenly moved from its given state. For a more detailed explanation of the fictitious temperature, reference should be made to the document Narayanaswamy, O. (1971). “A model of structural relaxation in glass”, Journal of the American Ceramic Society, 54(10), 491-498 or to the document Tool AQ, “Relation between inelastic deformability and thermal expansion of glass in its annealing action”, Journal of the American Ceramic Society, 29(9), 240-53 (1946). In the context of the invention, the fictitious core temperature thus provides information on the local structure of the material M1. This fictitious temperature is controlled by the manufacturing process of the fiber F, in particular by the applied load and its quenching rate as a function of the core and cladding diameters.

[0042] In a manner known per se, the fictitious temperature profile of a fiber can be indirectly determined by a 2D Raman micro-imaging system. Documents describe the determination of fictitious temperature by point Raman micro-spectroscopy (see, in particular, Martinet, C. et al. M. (2008) “Radial distribution of the fictitious temperature in pure silica optical fibers by micro-Raman spectroscopy”, Journal of Applied Physics 103(8), 083506).

[0043] Following numerous experiments, the inventors identified that the second harmonic generation efficiency varied considerably in apparently identical glass-core optical fibers in which optical polling was performed with identical injection parameters (power, frequency, repetition rate, pulse duration, etc.). By "apparently identical fibers," we mean fibers having the same chemical profile, the same refractive index profile, the same core doping, and the same core and cladding diameters. Given the apparent similarity of the fibers, this variability was extremely surprising.

[0044] Following extensive research on fiber structure, the inventors unexpectedly identified that the fictitious temperature variation ATf in the core was the intrinsic fiber characteristic that directly influenced the nonlinear conversion efficiency Ef f of the optical fiber after polishing. In effect, a high fictitious temperature is similar to immobilizing the structural entities forming the core in a metastable state, a state that can be modified by temperature annealing. This allows the local stresses to be relaxed in a final position with a more stable organization corresponding to a minimum electronic potential. A local modification of the structural organization allows a local modification of the index and the creation of a second-order non-linearity, periodic or not.

[0045] This relationship between the variation ATf and Ef f had not been identified in art previous and its determination allowed inventors finer control of the nonlinear properties of an optical fiber after polishing.

[0046] Thus, a first AT / variation of the core less than 105% ensures a satisfactory non-linear conversion efficiency Eff in the optical fiber F after polishing. In particular, the second harmonic generation efficiency in optical fiber F after polling is improved by selecting a fiber exhibiting the first ATf variation less than 105%.

[0047] Compared to an AT / variation of less than 105%, selecting an AT / variation of less than 60% significantly increases the nonlinear conversion efficiency in the optical fiber F after polishing. In particular, the second harmonic generation efficiency can be more than quadrupled by selecting a first AT / variation of less than 60% compared to an AT / variation of less than 105%, for identical injection parameters.

[0048] Indeed, through experiments and fiber cross-section analyses by 2D Raman microspectroscopy, the inventors determined that the second harmonic generation efficiency in a fiber after polishing, E / , was related to the first variation ATf by the following relation: r_1 H is understood that 7 2 — ” 3.82 \ 84.71 / This relationship depends on the polling generation parameters. As a non-limiting example, this relationship is obtained for a graded-index silica fiber with parabolic germanium doping, an index difference between the core and cladding of An = 0.015, a core diameter of 50 µm, and a cladding diameter of 125 µm. In this example, the polling is generated using a mode-locked laser delivering pulses at 1064 nm with a duration of 740 ps and a repetition rate of 27 kHz, with a peak power of 1 MW.

[0049] The PI profile and the first ATf variation are shown in [Fig. 2]. By way of non-limiting example, the PI profile of fiber F in [Fig. 2] is Gaussian. According to another embodiment, the PI profile of fiber F is chosen from a group comprising a parabolic profile, a super-Gaussian profile, a triangular profile, a Lorentzian profile, a multi-lobe profile, and a square hyperbolic secant profile.

[0050] Experimental results illustrating the relationship between the ATf variation of fictitious temperature in the core and the nonlinear conversion efficiency of the optical fiber after poling are presented further in Figures 4A and 4B.

[0051] Another way of characterizing the nonlinear conversion efficiency of the F fiber After polling, the next step is to measure a fictitious temperature transverse profile P2 of the fiber between the core center and an outer edge of the cladding. This fictitious temperature transverse profile is called the "second transverse profile P2" to differentiate it from the first transverse profile PI.

[0052] By the same experiments and measurements as previously mentioned, the inventors determined that a variation AT / 2 of the second profile P2, between the center of the core and an outer edge (referenced B in Figure 2) of the cladding, of less than 125%, ensured satisfactory nonlinear conversion efficiency in the optical fiber F after polishing. The variation AT / is called the "second variation" to distinguish it from the first variation ^Tf

[0053] Similarly, compared to a second AT variation less than 125%, selecting a second AT variation less than 105% significantly increases the nonlinear conversion efficiency in the optical fiber F after polishing. In particular, the second harmonic generation efficiency can be more than tripled by selecting a second AT variation less than 105% compared to an AT variation less than 125%, for identical injection parameters.

[0054] Indeed, the inventors determined that the second harmonic generation efficiency in a fiber after polishing, Ejf^, was related to the second variation ATf by the following relation pff _ 1 This relation was J 2 “ 3.47 45.75 J obtained with the same parameters as those used to determine the relationship linking W2etAT / f

[0055] It is understood that the relationship linking T / and AT / is dependent on the parameters of the fiber F and in particular on the diameter of the core FC and the cladding FG. Therefore, the choice of an ATf variation of less than 125% (respectively less than 105%) is preferentially associated with a fiber F with a core diameter between 0.5 pm and 500 pm, and a cladding thickness greater than 0.5 pm and less than 0.6 pm in order to ensure a satisfactory (respectively high) nonlinear conversion efficiency.

[0056] The P2 profile and the second variation ETj are shown in [Fig. 2]. By way of non-limiting example, the P2 profile of the fiber in [Fig. 2] is Gaussian. According to another embodiment, the P2 profile is chosen from a group comprising a parabolic profile, a super-Gaussian profile, a triangular profile, a Lorentzian profile, a multi-lobe profile, and a square hyperbolic secant profile.

[0057] Preferably, the F fiber is a multimode fiber with a non-constant transverse index variation profile and a maximum located substantially at the center of the core. For example, the fiber is a graded-index fiber ( GRIN (Graded INdexfiber in English) exhibiting a parabolic index profile with a maximum centered at point C, as illustrated in [Fig.2]. Alternatively, the core exhibits a transverse index profile chosen from a Gaussian profile, a super Gaussian profile, a triangular profile, a Lorentzian profile, a multi-lobe profile, and a hyperbolic secant squared profile.

[0058] Anmax denotes the maximum refractive index difference between the core and the cladding. This refractive index profile is typically obtained by varying the concentration of a doping material in the first core material Ml. This doping material is, for example, germanium, aluminum, and / or phosphorus, or ions, preferably rare-earth ions.

[0059] A multimode graded-index F-fiber enables continuous spatial cleaning by Kerr effect of the energy distributed in different spatial modes towards the fundamental mode (provided that the peak power of the guided beam is sufficient). The resulting output beam has improved brightness and its power is primarily supported by the fundamental mode. The same spatial relocalization effect is obtained during nonlinear conversion in the F-fiber after polling. Therefore, a multimode F-fiber after polling makes it possible to obtain an almost single-mode output beam with the wavelength(s) obtained by nonlinear conversion from a fundamental wavelength.

[0060] In addition, these fibers can have a large core diameter (up to 600 / / / 71) allowing the injection and guidance of higher peak power pulses.

[0061] Preferably, the first material Ml of the optical fiber F is silica or a glass whose structural relaxation rate is substantially equal to that of silica. By "structural relaxation rate is substantially equal to that of silica," we mean a glass whose structural relaxation rate is equal to that of silica ± 25%. This relaxation rate is preferred because it allows for the inscription of the periodic longitudinal lattice with satisfactory nonlinearity.

[0062] Figure 3A schematically illustrates a perspective view of a device D for improving the nonlinear properties of an optical fiber comprising: - the optical fiber F according to the invention, - a quasi-phase matching generation (SG) system adapted to inscribe a longitudinal periodic IP array of second-order or higher nonlinearity in the optical fiber, enabling quasi-phase matching for nonlinear conversion of a first wavelength in the optical fiber. This longitudinal periodic IP array is not shown in [Fig. 3A] but is visible in [Fig. 3C].

[0063] Following this inscription, the improved optical fiber F therefore exhibits a non-linear conversion efficiency Ef fv / cxaccrbcc thanks to the judicious choice of the first variation ^Tf (or the second variation ATle if applicable).

[0064] By injecting a beam at the first wavelength with sufficient intensity into the optical fiber F after polling, we then obtain a beam at the output of the fiber having one or more wavelengths 2NL generated by efficient nonlinear conversion of the first wavelength / qvia a quasi-phase match all along the fiber thanks to the longitudinal periodic network IP.

[0065] Various SG generation systems for performing optical fiber polishing are known to those skilled in the art, and an exhaustive description of all these systems is beyond the scope of the invention. However, it is specified that the SG generation system is suitable for performing this polishing step by thermal effect, electrical effect, optical effect, or a combination of several of these methods.

[0066] Thus, according to an embodiment M1 of [Fig. 3A] illustrated in [Fig. 3B], the SG generation system is adapted to generate the longitudinal periodic nonlinearity grating by optical polling. For this purpose, the SG generation system comprises an LS laser system.

[0067] The laser system LS which includes a laser L adapted to generate a first beam Fl having the first wavelength 2j = 2 / rc / aq.

[0068] The laser L is preferably a pulsed source in order to deliver pulses with a peak power sufficiently high to allow the inscription of the longitudinal periodic grating IP. By way of example, the laser L is a passively triggered Nd:YAG laser source using CR4+:YAG crystals delivering pulses at λ = 1064 nm with a duration of 740 ps and a repetition rate of 27 kHz.

[0069] The LS laser system further comprises a nonlinear conversion element (CNL) adapted to generate a second beam F2 at the frequency w2 — 2m j obtained by generating the second harmonic of the first beam. The CNL element is an element known to those skilled in the art. By way of example, the CNL element is a KTP crystal (potassium titanyl phosphate), or PPKTP (periodically poled KTP), or lithium triborate (LiB3O5 — or LBO), or even BBO (beta barium borate), and can also be the doped fiber itself, the centrosymmetry of which is modified by the implantation of ions.

[0070] The beams Fl and F2 form an incident beam FI injected for a predetermined duration into the optical fiber via a coupling system SC such that the FI beam inscribes the longitudinal periodic grating IP. By way of non-limiting example, the coupling system SC of Figures 3B and 3D comprises a linear polarizer such that the incident beam FI exhibits a predetermined linear polarization upon its injection into the fiber F, and one or more lenses adapted to focus the incident beam FI into the fiber F.

[0071] Preferably, as illustrated in Figure 3B, the laser system, the coupling system and the optical fiber are adapted so that the longitudinal periodic grating IP is a grating of a second-order nonlinearity. Thus, the injection of a fundamental beam FF at the first wavelength = 2jtc / with a sufficient peak power (typically greater than 100 MW) into the optical fiber F after polling allows the generation of an output beam FS at a frequency o?2 — 2^.

[0072] The creation of a second-order nonlinearity network rather than another order is particularly favored by seeding the second harmonic beam F2 simultaneously with the injection of the beam Fl at the fundamental frequency into the fiber F. Indeed, this beam F2 accentuates the beat of the fundamental frequency with its second harmonic, increasing the static electric field creating the second-order nonlinearity of the fiber (see Stolen, R. et al (1987). “Self-organized phase-matched harmony generation in optical fibers. Optics letters” 12(8), 585-587).

[0073] Figure 3C schematically illustrates the second-order nonlinearity longitudinal periodic IP grating obtained in fiber F after optical polishing using device D in Figure 3B. The period P of the IP grating is: p = 2 / r / AÆ, with AÆ--k( the phase mismatch vector between the fundamental wave at the frequency and the wave at the second harmonic 2.ms.

[0074] Furthermore, on [Fig.3C], the generation of the second harmonic from the fundamental beam FF injected into the fiber F after optical polishing has been schematically represented.

[0075] This optical polling is only possible if the IF beam is injected into the optical fiber for a sufficient duration (typically greater than 5 minutes, preferably greater than 30 minutes) with sufficient peak power. In order to ensure a sufficiently high second harmonic generation efficiency in the optical fiber after polling, it is preferable that the peak power of the first and second beams be between 100 W and 10 MW and between 10 W and 10 MW, respectively, at the time of their injection into the optical fiber.

[0076] Alternatively, according to an embodiment different from that illustrated in Figure 3B, the CNL element is adapted to generate a second beam F2 at the frequency 0^2 — ko\ with k > 2, so as to promote the creation of a longitudinal periodic IP array of nonlinearity to an order k > 2. In this embodiment, the injection of a fundamental beam FF at the first wavelength 2 j = 2jtc / aq with sufficient peak power into the optical fiber F after polling allows the generation of an output beam FS at a frequency «2 = ko)^

[0077] Alternatively, the periodic variation of the second-order nonlinearity can also induce a variation of the third-order nonlinearity with an identical period.

[0078] The CNL element is an optional element that can be omitted from the device in the figure 3B. Its use is preferred, however, because it allows the generation of the second beam F2, which itself significantly reduces the predetermined duration for which the incident beam FI must be injected into the fiber F to generate the longitudinal periodic IP array. Furthermore, the F2 beam allows the "selection" of the order k of the nonlinearity x^ created or amplified in the longitudinal periodic IP array via the seeding of the F2 beam at the frequency

[0079] Figure 4A shows a curve of the average power of the second harmonic generated in the FS beam by the device D of Figure 3B, as a function of the injection duration of the incident beam FI. The curve in Figure 4A is obtained for a first variation ATf of 59%. For comparison, Figure 4B shows the same curve with the setup of Figure 1 using an optical fiber 1 with a first variation &Tf not less than 105% and with injection parameters (power, frequency, repetition rate, pulse duration, etc.) identical to those of [Fig. 4A]. As a non-limiting example, the curves in Figures 4A and 4B are obtained from pulses at 1064 nm with a duration of 740 ps and a repetition rate of 27 kHz, with a peak power of 1 MW.

[0080] More specifically, curves 4A and 4B are obtained by the following protocol: - The FI beam (respectively beam 5) is injected into the F fiber (respectively fiber 1) for a predetermined injection time with the same injection parameters - The CNL element (respectively crystal 4) is removed - We measure the average power of the second harmonic generated at the fiber output.

[0081] Furthermore, optical fiber F of Figure 4A and optical fiber 1 of Figure 4A have an identical transverse chemical profile (same chemical profile, same refractive index profile, same core doping, same core and cladding diameters). By way of non-limiting example, the fibers in Figures 4A and 4B are graded-index silica fibers with parabolic germanium doping and a refractive index difference between the core and cladding of An = 0.015. These fibers have a core diameter of A 50 kim and a cladding diameter of 125

[0082] In [Fig. 4A], it can be observed that the average power of the second harmonic generated in the FS beam at the output of optical fiber F increases with the injection duration of the FI beam up to 150 minutes of injection duration, and then essentially plateaus after this duration. The average power of the second harmonic increases by a factor of approximately 10 for 150 minutes of injection duration and by a factor of approximately 5 for 50 minutes of injection duration compared to an injection duration of 0 minutes—that is, compared to fiber F without optical polishing.

[0083] By comparison, in Figure 4B, the average power of the second harmonic generated in the beam 6 at the output of the optical fiber 1 is constant as a function of the injection time of the beam 5. Thus, without a first variation &Tf as mentioned above, the polling treatment does not produce any improvement of the nonlinear properties of the fiber 1. The second harmonic generation efficiency Eff of the fiber F after polling with an injection time of 150 min is about ten times greater than that of the fiber in [Fig.4B].

[0084] Thus, thanks to the judicious choice of the first variation &Tf of the F fiber, the non-linear properties of the F fiber can be considerably exacerbated by polling.

[0085] To ensure satisfactory enhancement of the nonlinear properties of the F fiber, the predetermined injection time of the FI beam is greater than 5 minutes. It is understood that this time can be significantly modified depending on the various FI beam injection parameters, the fiber materials, and the fiber core diameter. However, generally, a peak power of the first and second beams greater than 100 kW and 10 kW, respectively, at the time of their injection into the optical fiber, and a predetermined injection time greater than 5 minutes, allows for a second harmonic generation efficiency in the fiber greater than 1%, thanks to a first ATf variation of less than 105%. Similarly, with the same parameters, a predetermined injection time greater than 50 minutes allows for a second harmonic generation efficiency in the fiber greater than 3%.

[0086] According to an embodiment M2 of [Fig. 3A], the quasi-phase-matching generation system SG comprises electrodes arranged longitudinally along the fiber F. These electrodes are adapted to perform electrical polling of the fiber F. That is, the electrodes are adapted to generate an electric field periodically longitudinally within the optical fiber in order to inscribe the longitudinal periodic IP array. Indeed, as is known per se, applying an electric field to the core FC in a given direction modifies the susceptibility of the core material Ml, which locally changes the phase-matching conditions between the primary and secondary photons generated by nonlinear conversion.

[0087] For example, in the case of a centrosymmetric material M1, the local application of a sufficiently intense electric field makes it possible to generate locally a second-order susceptibility x^ with a value substantially proportional to the intensity of the electric field. By placing the electrodes at regular intervals along the fiber (for example with a period = 2tt / A&, with AÆ — k( ) - k( ) ) it is possible to generate a quasi-phase match between the primary photons and the secondary photons generated by second harmonic.

[0088] Thus, a laser beam at the first wavelength and injected with sufficient intensity into the fiber F will induce the generation of a beam at the fiber output having a wavelength kNL generated by nonlinear conversion in the fiber F with a nonlinear conversion efficiency Eff exacerbated by the polling electrical. It is understood that the electrodes (via the intensity of the delivered field) can be adapted to create a longitudinal periodic IP network of nonlinearity of order higher than two.

[0089] Preferably, the electrodes are adapted to generate an electric field greater than 10 kV / mm, preferably greater than 20 kV / mm within the fiber to achieve sufficient intensity in the fiber to create the longitudinal periodic IP network inducing satisfactory nonlinear conversion efficiency.

[0090] Embodiment M2 can be combined with embodiment M1 of [Fig. 3B]. That is to say, device D can comprise both the laser system LS and the electrodes in order to inscribe the longitudinal periodic grating IP by both optical and electrical polling.

[0091] According to an embodiment M3 of Figure 3A, the quasi-phase-matching generation system SG comprises heating elements arranged longitudinally along the fiber and adapted to heat the core in order to inscribe the longitudinal periodic IP lattice. Indeed, as is known per se, a variation in the local core temperature induces a local variation in the core index and its nonlinear coefficients, which locally changes the phase-matching conditions between the primary and secondary photons generated by nonlinear conversion. By placing the thermal elements at regular intervals along the fiber (for example, with a period = 2Φ / Ak, with AΦ = k(2M) - k(Φ)), it is possible to generate quasi-phase-matching between the primary and secondary photons.

[0092] Embodiment M2 can be combined with embodiment M1 and / or embodiment M2 of [Fig. 3B]. That is to say, device D can comprise both the laser system LS and the electrodes in order to inscribe the longitudinal periodic grating IP by thermal and / or optical and / or electrical polishing.

[0093] Figure 5 illustrates a preferred embodiment of the fiber of the invention in which the fiber F comprises an RE coating surrounding the cladding formed from a material opaque to visible or visible and UV radiation. Thus, this RE coating makes it possible to limit the harmful influence of external radiation on the longitudinal periodic IP lattice inscribed during the polling process. Indeed, without this RE coating, the in Interactions between external radiation RP and the longitudinal periodic nonlinearity grating are likely to significantly decrease the nonlinear conversion efficiency EffNL of the F- fiber

[0094] The embodiment of [Fig. 5] is particularly relevant for protecting a non-centrosymmetric index grating inscribed by optical polling, for example with the device of [Fig. 3A]. Indeed, in the case of optical polling, the incident beam IF is typically interrupted after generating the non-centrosymmetric index grating IP. The longitudinal periodic grating IP is maintained and generated by the beat of the fundamental wave and its second harmonic generated by the photo-inscribed grating. In this case, degradation of the longitudinal periodic index grating IP is more detrimental because the longitudinal periodic grating IP is not continuously regenerated.

[0095] Figure 6 schematically illustrates a method for improving the nonlinear properties of an optical fiber adapted for implementation by the device in Figure 3A. The method comprises the following steps:

[0096] A- select the optical fiber F with the first transverse profile PI exhibiting the first AT / variation between the center C of the core and the edge B of the core less than 105%

[0097] B- inscribing the longitudinal periodic IP array of nonlinearity of order two or more in the optical fiber allowing quasi-phase matching for the nonlinear conversion of the first wavelength in the optical fiber.

[0098] Following step D, we therefore have an improved optical fiber F exhibiting enhanced nonlinear conversion efficiency thanks to the judicious choice of the first ATf variation (or the second ATf ​​variation, as appropriate). By injecting a beam at the first wavelength with sufficient intensity into the improved optical fiber F, we then obtain an output beam with a wavelength generated by efficient nonlinear conversion of the first wavelength into quasi-phase matching along the entire length of the fiber thanks to the longitudinal periodic IP array.

Claims

Claims

1. Device (D) for improving the non-linear properties of an optical fiber comprising: - an optical fiber (F) comprising a core (FC) in a first material (Ml) and a cladding (FG) surrounding said core, a first transverse profile (PI) of a fictitious temperature of said core having a maximum located substantially at said center of the core and having a first variation between a center (C) of the core and an edge (B) of the core, called first variation &TJ less than 105%, preferably less than 60%, - a quasi-phase matching generation system (SG) adapted to inscribe a longitudinal periodic grating (IP) of a non-linearity of order two or more in the optical fiber allowing quasi-phase matching for a non-linear conversion of a first wavelength in the optical fiber.

2. Device according to the preceding claim, in which the optical fiber (F) comprises a transverse profile (P2) of a fictitious temperature of the fiber, called the second profile, has a variation between the center of the core and an external edge of the cladding, called the second variation ATf less than 125%, preferably less than 105%.

3. Device according to any one of the preceding claims, in which the core of the optical fiber (F) has a transverse profile of non-constant index variation with a maximum located substantially at the center of the core.

4. A device according to any preceding claim, wherein the first material is silica or a glass having a structural relaxation rate substantially equal to that of silica.

5. Device according to any one of the preceding claims, in which the optical fiber (F) comprises a coating (RE) surrounding the sheath in a material opaque to visible radiation or visible-UV radiation.

6. A device according to any preceding claim, wherein said quasi-phase matching generation system comprises electrodes arranged longitudinally along the fiber. and adapted to generate an electric field periodically longitudinally within the optical fiber in order to inscribe said longitudinal periodic network (IP).

7. Device according to the preceding claim, in which the electrodes are adapted to generate an electric field greater than 10 kV / mm, preferably 20 kV / mm.

8. A device according to any preceding claim, wherein said quasi-phase matching generation system comprises heating elements arranged longitudinally along the fiber and adapted to heat said core in order to inscribe said longitudinal periodic grating (IP).

9. Device according to any one of the preceding claims, in which said quasi-phase matching generation system comprises a laser system (LS) adapted to generate at least one so-called incident beam (FI) having a first wavelength and a coupling system (SC) adapted to inject said at least one incident beam into said optical fiber for a predetermined duration, the laser system and the coupling system being further adapted so that said at least one incident beam has sufficient intensity during its propagation within the fiber to inscribe said longitudinal periodic grating (IP).

10. Device according to the preceding claim, in which the laser system, the coupling system and the optical fiber are adapted so that said longitudinal periodic grating (IP) is a grating of a second order non-linearity, said non-linear conversion of the first wavelength in the optical fiber being a generation of a second harmonic.

11. Device according to the preceding claim, in which said laser system comprises a laser (L) adapted to generate a first beam (F1) having the first wavelength (Q) and a non-linear conversion element (CNL) adapted to generate a second beam (F2) having a second wavelength (Q) obtained by generation of a second harmonic of the first wavelength of the first beam, said first beam and said second beam forming said at least one incident beam inscribing said longitudinal periodic network (IP).

12. Device according to the preceding claim, in which the laser system and the coupling system are adapted so that a peak power of the first and second beams is respectively between 100 W and 10 MW and between 10 W and 10 MW when they are injected into the optical fiber.

13. Device according to the preceding claim, in which the predetermined duration is greater than 5 minutes.

14. Device according to one of claims 9 to 13 in combination with claim 10, in which the first variation or the second variation and the predetermined duration and the intensity of the incident beam during its propagation in the optical fiber are adapted for an efficiency of generation of second harmonic in the fiber is greater than 1%, preferably 3%.

15. Method for improving the non-linear properties of an optical fiber comprising the following steps: A- selecting an optical fiber (F) comprising a core (FC) in a first material (Ml) and a cladding (FG) surrounding said core in a second material (M2), a first transverse profile (PI) of a fictitious temperature of said core having a maximum located substantially at said center of the core and a first variation between a center (C) of the core and an edge of the core, called first variation &Tf less than 105%, preferably less than 60%, B- writing a longitudinal periodic grating (IP) of a non-linearity of order two or more in the optical fiber allowing quasi-phase matching for a non-linear conversion of a first wavelength in the optical fiber.

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