METHOD AND DEVICE FOR PRODUCING AN OPTICAL FIBER

Description

technical field

[0001] The present invention relates to a method for manufacturing an optical fiber. It also relates to a device for manufacturing an optical fiber. Prior art

[0002] EP 1 319 636 describes a partially diffusing optical fiber and a manufacturing process.

[0003] EP 0 649 007 describes a process for manufacturing optical fibers.

[0004] GB 2 213 142 describes a fiber optic-based sensor and a manufacturing method.

[0005] The article by PJ Roberts et al. published in OPTICS EXPRESS 236, January 10, 2005, Volume 13, No. 1, describes hollow-core photonic crystal fibers.

[0006] US 5,992,181 describes a method for randomly irradiating an optical fiber core with a laser.

[0007] Today, the limit of optical fiber transmission performance is set by diffusion losses.

[0008] Hollow core optical fiber has seen phenomenal progress since its conception almost 20 years ago. This progress is reflected in its manufacturing techniques and transmission performance, as well as in the applications that have emerged following its development, covering diverse and varied fields such as telecommunications, nonlinear optics, lasers, micromachining, cold atom physics, high-resolution spectroscopy, and sensors.

[0009] In this context, the state of the art for transmission losses is approximately 0.1 dB / km at 1550 nm for conventional solid fibers and around 2 to 10 dB / km for hollow fibers depending on the type of fiber and the spectral range.

[0010] The aim of the present invention is to propose a method and device for manufacturing optical fiber which improves the transmission performance of said fibers and / or has reduced losses relative to the current state of the art. Description of the invention

[0011] This objective is achieved with a method for manufacturing an optical fiber according to claim 1.

[0012] Preferably, the oven raises the temperature of the preform and the working area is preferably located relative to the oven so that the working area of ​​the preform is in the viscoelastic phase.

[0013] The modulation frequency is preferably less than 10 kHz.

[0014] Each laser beam is preferably modulated between a maximum and a minimum power. Preferably, the minimum power value is between 95% of the maximum power and 0% of the maximum power.

[0015] Each laser beam can be modulated over a range from maximum power down to at least 50% of that maximum power.

[0016] Each laser beam can be modulated over a range from maximum power down to at least 10% of that maximum power.

[0017] Each laser beam is preferably power modulated according to a modulation frequency greater than 100Hz.

[0018] The at least one laser beam preferably comprises several intersecting laser beams over the work area.

[0019] Each laser beam preferably has a wavelength greater than 200 nm and / or less than 12 µm.

[0020] Each laser beam is preferably collimated on the work area. The preform is preferably a glass preform.

[0021] The working area of ​​the stretched preform preferably includes a structure composed of walls with a thickness of less than 10µm, preferably less than 3µm, preferably less than 1µm.

[0022] The work area is preferably located outside the oven.

[0023] The oven preferably includes: an enclosure through which the preform passes, and a heat source disposed in the enclosure and raising the temperature inside the enclosure, the working area being located at a distance of less than 10 cm from the heat source.

[0024] The working area is preferably at a temperature above 700°C, preferably above 1000°C, preferably above 1500°C.

[0025] According to yet another aspect of the invention, a device for manufacturing an optical fiber is proposed according to claim 15.

[0026] The modulation frequency is preferably less than 10 kHz.

[0027] The modulation means are preferably arranged so that each laser beam is modulated between a maximum and a minimum power. The minimum power value is preferably between 95% of the maximum power and 0% of the maximum power.

[0028] The modulation means are preferably arranged so that each laser beam is modulated over a range from maximum power down to at least 50% of that maximum power.

[0029] The modulation means are preferably arranged so that each laser beam is modulated over a range from the maximum power down to at least 10% of that maximum power.

[0030] The emission means are preferably arranged so that at least one laser beam comprises several intersecting laser beams over the work area.

[0031] The emission means are preferably arranged so that each laser beam has a wavelength greater than 200 nm and / or less than 12 µm.

[0032] The emission means are preferably arranged so that each laser beam is collimated on the work area.

[0033] The work area is preferably located outside the oven.

[0034] The oven preferably includes: an enclosure through which the preform passes, and a heat source disposed in the enclosure and arranged to raise the temperature inside the enclosure, the working area being located at a distance of less than 10 cm from the heat source.

[0035] The emission means and the furnace are preferably arranged so that the working area is at a temperature above 1000°C, preferably above 1500°C. Description of the figures and methods of implementation

[0036] Other advantages and features of the invention will become apparent upon reading the detailed description of implementations and embodiments, which are by no means limiting, and the following attached drawings: [ Fig. 1 ] there figure 1 is a schematic cross-sectional view of a first embodiment of device 101 according to the invention, through which a preform 1 passes, [ Fig. 2 ] there figure 2 is a schematic top cross-sectional view of preform 1 at the level of a working area 4, [ Fig. 3 ] there figure 3 is a schematic profile view of preform 1 at the level of working area 4, [ Fig. 4 ] there figure 4 is an enlargement of part 10 of the figure 3 and is a schematic cross-sectional profile view of a wall 7 of the preform 1 at the working area 4 with ( figure 4a ) and without ( figure 4b) implementation of a process according to the invention, [ Fig. 5 ] there figure 5 is a schematic profile view of the preform 1 at the level of the working area 4 in a process variant according to the invention, and [ Fig. 6 ] there figure 6 is a diagram of a capillary wave at the interface between a liquid or viscoelastic medium and a gaseous medium.

[0037] We will first describe, with reference to figures 1 to 5 , a first embodiment of device 101 for manufacturing an optical fiber according to the invention.

[0038] Device 101 includes a furnace 2. Furnace 2 is, for example, a fiber drawing furnace at Nextrom.

[0039] The furnace 2 includes an inlet 21 arranged to insert a preform 1 (typically a glass rod) into the furnace 2. The device 101 further includes on the side of the inlet 21 retaining means (not shown) arranged to retain a first end of the preform 1 on the side of the inlet 21. These retaining means consist for example of a mandrel placed on a translation plate.

[0040] Device 101 includes means for stretching the preform 1 through an outlet 3 of the furnace 2, along a stretching direction 30. These means, not shown, typically include two facing belts arranged on the side of the outlet 3 of the furnace 2, and where the stretched fiber is placed between the two belts for stretching with an adjustable speed.

[0041] Inlet 21 and outlet 3 are located at two opposite ends of furnace 2 along direction 30.

[0042] The device 101 includes emission means arranged to subject, after exit 3 from the oven 2, a working area 4 of the preform 1 thus stretched to at least one laser beam 5. These emission means typically include one or more laser sources, and optionally complementary optical means for shaping and / or dividing and / or grouping one or more of the at least one laser beam 5.

[0043] The emission means are arranged so that at least one laser beam 5 comprises several laser beams 5 intersecting on the work area 4.

[0044] The emission means are arranged so that each laser beam has a wavelength greater than 200 nm and / or less than 12 µm.

[0045] The emission means are arranged so that each laser beam has a maximum power greater than 10 W.

[0046] The emission means are arranged so that each laser beam is collimated on the working area 4.

[0047] The emission means are arranged so that the entire part 8 of the wavefront of each laser beam 5 coming into contact with the stretched fiber 1 (more precisely with the working area 4) has, at each given instant, a spatial power density between a maximum at that instant t and 80% of that maximum.

[0048] Thus, at each instant t, each laser beam 5 has a spatially homogeneous power density on the stretched fiber 1 (more precisely on the working area 4).

[0049] In the example illustrated in figure 1 , we use four YRL reference continuous laser sources from IPG and arranged to emit four laser beams 51, 52, 53, 54.

[0050] Device 101 further includes modulation means arranged to modulate each laser beam 5 in power according to a modulation frequency. These modulation means are either already provided in the laser or by an intensity modulator via electro-optical and / or acousto-optical means and / or by a rotating toothed wheel.

[0051] The modulation frequency is: greater than 10Hz, preferably greater than 100Hz and less than 1000 kHz, preferably less than 10 kHz.

[0052] Thus, each laser beam 5: is spatially homogeneous or substantially homogeneous in power density at each time t considered on the stretched fiber 1 (more precisely on the working area 4), that is to say has on the stretched fiber 1 (more precisely on the working area 4) at each time t considered a spatial power density between a maximum (specific to this time t) and 80% of this maximum, but is temporally modulated in power according to a modulation frequency.

[0053] The modulation means are arranged so that each laser beam 5 is modulated between the maximum power 6 and a minimum power such that the minimum power value is between 95% of the maximum power and 0% of the maximum power, preferably between 95% of the maximum power and 10% of the maximum power. More precisely, the modulation means are arranged so that each laser beam is modulated over an interval 9 from the maximum power 6 down to 50% or less of that maximum power. More precisely, the modulation means are arranged so that each laser beam is modulated over an interval 9 from the maximum power 6 down to 10% or less of that maximum power.

[0054] Work area 4 is located outside of oven 2.

[0055] Work area 4 is located on the side of exit 3 of oven 2.

[0056] Oven 2 includes: an enclosure 31 through which the preform passes, and a heat source 32 disposed in the enclosure 31 and arranged to raise the temperature inside the enclosure 31.

[0057] The work area 4 is located at a distance 43, measured along direction 30, typically less than 10 cm from the heat source 32.

[0058] The distance between the laser working zone 4 (ZL) and the heating zone 32 of the thermal furnace (ZF) is arranged so that the temperature of the drawn preform shows a temperature higher than that of the softening temperature of the preform 1, i.e. so that the preform 1 is viscoelastic at the level of the working zone 4. For a thermal furnace operating at temperatures between 1800 and 2000 °C, and for drawing speeds of 10 to 100 m / min, this distance can typically be between 5 cm and 10 cm.

[0059] Each laser beam 5 is positioned so that the stretched preform is in a viscoelastic phase at the working zone 4. In other words, the distance between the laser and the furnace (here furnace is defined as the heating zone) must be close enough so that, at zone 4, the preform (i.e. the glass) has not yet cooled to the point of solidifying, and far enough from the furnace to guarantee optimal operation (i.e. temperature distribution and amplitude comparable to the standard furnace).

[0060] The oven raises the temperature of the preform and the working zone 4 is located relative to the oven so that the working zone of the preform is in the viscoelastic phase.

[0061] Each 5 laser does not create material defects on the preform.

[0062] The emission means and the furnace 2 are arranged, in particular by the relative positions of the furnace 2 and at least one beam(s) 5, so that the working zone 4 is at a temperature above a softening or glass transition temperature of the material composing the preform 1, preferably above 1000°C, preferably above 1500°C.

[0063] The glass transition temperature is, for example: 150°C for Chalcogenide GeSbTe, 245°C for Chalcogenide AsGeSeTe, 235°C for Fluoride Glass ZBLAN, 280°C for Tellurium dioxide, 400°C for Fluoroaluminate, 1200°C for Fused Quartz.

[0064] Thus, the invention is a method and device for an opto-thermal furnace enabling the reduction of the diffusion effect due to the inhomogeneity of the density of the glass by photo-absorption effect.

[0065] The preform 1 is tricked into the thermal furnace 2, typically resistive or inductive. A section of the preform 1 first passes through the heating zone 31 of the furnace 2 and then, by flow effect, into the optical section where the heated section 4 of the preform 1 is excited by at least one laser beam 5.

[0066] We will now describe, with reference to figures 1 to 5 , a first embodiment of the process according to the invention implemented by device 101.

[0067] In this embodiment of the optical fiber manufacturing process: The preform 1 is inserted into the oven 2, the preform 1 is stretched through the outlet 3 of the oven 2, and after the outlet 3 of the oven 2, the working area 4 of the preform 1 thus stretched is subjected to at least one laser beam 5, each laser beam 5 being modulated in power according to the modulation frequency.

[0068] The modulation frequency is: greater than 10Hz, preferably greater than 100Hz and less than 1000 kHz, preferably less than 10 kHz.

[0069] Each laser beam 5 is modulated between the maximum power 6 and a minimum power such that the minimum power value is between 95% of the maximum power and 0% (or preferably 10%) of the maximum power. Preferably, each laser beam is modulated over an interval 9 from the maximum power 6 down to 50% or less of that maximum power. Preferably, each laser beam 5 is modulated over an interval 9 from the maximum power 6 down to 10% or less of that maximum power.

[0070] The at least one laser beam 5 comprises several laser beams 5 intersecting on the work area 4.

[0071] Each laser beam 5 has a wavelength greater than 200 nm and / or less than 12 µm.

[0072] Each laser beam 5 has a maximum power 6 greater than 10 W.

[0073] Each laser beam 5 is collimated on the working area 4.

[0074] Preferably, at least one bundle 5 comprises N bundles 5 distributed around the area 4 in a plane perpendicular to the direction 30 with regular separation angles of 2n / N (N being a natural number).

[0075] Depending on the variant considered, at least one bundle 5 can therefore include: a single beam 5, 51 ( figure 2 ) two beams 51, 52 ( figure 3 ) preferably facing each other three beams 51, 52, 53 ( figure 5 ) preferably distributed around zone 4 with regular angles of 2n / 3 four beams ( figure 1 ) preferably distributed around zone 4 with regular angles of n / 2, or even more beams 5, the greater the number of beams 5, the better the homogeneity of illumination of zone 4.

[0076] Preform 1 is a glass preform, for example of the silica type.

[0077] The working area 4 of the stretched preform 1 is a microstructured optical fiber.

[0078] The working area 4 of the stretched preform 1 comprises a structure (for example, of the Kagome type) composed solely of walls 7 (of glass, illustrated in white on a black background on the figures 2 And 5 ) whose thickness is less than 10µm, preferably less than 3 µm, preferably less than 1 µm.

[0079] The entire portion 8 of the wavefront of each laser beam 5 entering contact with the stretched fiber 1 (more precisely, the working area 4) has, at any given instant, a spatial power density between a maximum at that instant t and 80% of that maximum. Thus, at any instant t, each laser beam 5 has a spatially homogeneous power density along the stretched fiber 1 (more precisely, along the working area 4).

[0080] Work area 4 is located outside oven 2, on the side of exit 3.

[0081] The work area 4 is located at a distance 43 less than 10 cm from the heat source 32.

[0082] The working zone 4 is at a temperature above the softening or glass transition temperature of the material composing the preform 1, preferably above 1000°C, preferably above 1500°C.

[0083] The invention makes it possible to limit the local density fluctuation for conventional solid optical fibers and the surface roughness for hollow core microstructured fibers, due to a hydrodynamic phenomenon during fiber drawing and which induce losses by light scattering and therefore a limit on the transmission performance of optical fibers.

[0084] During optical fiber fabrication, preform 1 is inserted into furnace 2 and heated to a temperature that causes the glass to undergo a phase transition and become viscoelastic. This "liquid" glass is then drawn into an optical fiber. During this liquid phase, the glass experiences mass or density fluctuations that can be described as a stochastic composition of phononic waves. During this drawing process, the glass undergoes a second phase transition from liquid to solid, and the aforementioned fluctuations are "frozen." In the case of conventional solid fiber, these frozen fluctuations take the form of nanometric inhomogeneities in the glass matrix. These inhomogeneities induce guided light scattering and impose a limit on transmission losses of approximately 0.1 dB / km at 1550 nm, which varies with wavelength.In the case of hollow fibers (optical fiber guiding in a hollow core), frozen fluctuations result from surface waves, called capillary waves, and take the form of a surface roughness illustrated on the . figure 4.b In the absence of implementation of the process according to the invention, i.e., without the beams 5, this roughness also induces light scattering, the amplitude of which is proportional to the light overlap rate in the core with the glass contour and to the surface roughness amplitude. Currently, this scattering is a limitation of the state of the art for hollow fibers.

[0085] On the contrary, after implementation of this process according to the invention, as illustrated in the figure 4.a a reduction of surface roughness by modulation of the index and mass relative to a surface reference, the beams 5 exciting the surface or matrix to be treated and thus modulating the index and / or density by photo-absorption effect.

[0086] THE figures 4.a and 4.b are only schematic. In reality, the roughness (i.e., the "bumps") is less high and smoother or less chaotic on the figure 4.a compared to the figure 4.b

[0087] Studies have shown that for hollow fibers guided by Coupling-Inhibited, Kagome or tubular type, losses can be as low as 10⁻² to 10⁻³ dB / km in the spectral range from Visible to UV if the surface roughness is greatly reduced.

[0088] Similarly, for conventional fibers (e.g., fiber for telecom), reducing density fluctuation can lower the lowest current losses of 0.1 dB / km by a factor of 10.

[0089] The invention therefore addresses a key problem for hollow core optical fiber or conventional solid fibers.

[0090] In a specific example: the preform is made of hollow-core silica with a microstructured Kagome-type sheath and has an external diameter of 40 mm at the inlet 21 the furnace enclosure 2 is heated to 1900°C the working area is at a temperature above 1200°C the preform 1 is stretched, on the outlet side 3, at a speed of 15 m / min each laser beam 5 has a wavelength of 1060 nm, with a modulation frequency of 220 Hz between a maximum power of 80 W and a minimum power of 30 W.

[0091] The resulting optical fiber has excellent transmission loss.

[0092] The invention makes it possible to obtain a hollow fiber with lower losses than the current state of the art and will consolidate its applications in sectors such as: 1. Laser micromachining: The fiber obtained according to the present invention allows for flexible and secure delivery of high-flux laser beams. 2. Laser pulse compression: The fiber obtained according to the present invention allows for the compression of laser pulses by simply optimizing the fiber length and selecting the gas to be introduced into the fiber core. 3. Surgery: The fiber obtained according to the present invention allows for flexible and secure delivery of laser beams for endoscopic or LASIK-type operations. 4. Cell treatment: The fiber obtained according to the present invention allows for flexible and secure delivery of ultrashort laser beams to destroy cancer cells in a targeted manner and without heat deposition. 5. Gas laser: The fiber obtained according to the present invention, the core of which is filled with an active gas, can be used as a laser gain medium. 6.Frequency standard: The fiber obtained according to the present invention, the core of which is filled with a gas (e.g., acetylene, Rb or Cs vapor), can be used as a frequency standard. 7. THZ imaging: The THZ waveguide obtained according to the present invention allows for flexible and secure transmission and collection of THZ radiation for imaging. 8. Waveguide for THZ and microwave: The waveguide obtained according to the present invention allows for unimodal THZ and / or microwave guiding with low transmission losses. 9. Low-latency telecommunications in data centers.

[0093] It is noted that in all the embodiments just described, each beam 5 is not a pulsed laser beam but a continuous power-modulated laser beam.

[0094] The power of each laser beam is 5, P , is given as a function of time t by: P = P 0 + P m sin 2 πν mod t Here, v modis the frequency at which the laser power is modulated. P 0 represents the continuous part or DC. P m is the modulated power portion.

[0095] The expression for P as a function of the total (or maximum) power can also be written in the form P = P T 1 − P m P T 1 − sin 2 πν mod t The ratio P m P T represents the modulation depth.

[0096] Transmission in optical fiber 1 is limited by light scattering induced by density fluctuations in the glass. These fluctuations are due to the "freezing" of thermal waves as fiber 1 exits furnace 31, where the material constituting fiber 1 transitions from a viscoelastic phase (similar to a highly viscous liquid phase) inside furnace 31 to a solid phase outside. Here, the thermal waves are surface waves and are called capillary waves (CW).

[0097] For the rest of this text, we will preferably use the abbreviation CW for "capillary wave".

[0098] These frozen CWs are a source behind the formation of surface roughness on the walls of a hollow-core PCF crystal fiber (HCPCF). This surface roughness in HCPCFs is a limiting factor in the transmission of these fibers through the phenomenon of surface diffusion transmission losses.

[0099] Thus, in the present embodiments of the invention, capillary waves are controlled by restructuring their amplitude, frequency and damping by the generation of other capillary waves via a photoelastic phenomenon.

[0100] A laser is used to excite the photoelastic phenomenon while the fiber is in the "liquid" phase, and to create destructive interference with capillary waves of thermodynamic origin and / or to reshape the surface according to the desired profile.

[0101] There figure 6 This diagram represents surface roughness. It is represented by the elevation of the height (as a function of the spatial coordinates x and z defining a plane) of a liquid or viscoelastic medium. h ( x, z ) = h 0 + Σ i ζ i ( x , z ) e - i ( q i z - ω i t )< , induced by capillary waves. Here h 0 is the static height of the medium, and the second term on the right-hand side of the equation represents a linear combination of several capillary waves. Each CW is determined by its amplitude. ζ i ( x, z ), its oscillation frequency ωi , and its wave vector qi in the direction z . Frequency

[0102] For a membrane of thickness h very thin, the pulse ω ( q) (oscillation frequency to within a factor of 2n) and the wave vector q are related by the following dispersion equation: ω q = γh ρ q 2 , with γ And ρ where is the surface tension and is the density of the membrane, respectively. Here we have neglected the contribution of the gas (e.g., air) whose air density and surface tension are low relative to those of the membrane.

[0103] This expression is valid for the case of a thin membrane, where the effect of gravity can be neglected compared to the surface tension. Therefore, the wave vector amplitude belongs to an interval given by the following inequalities: q v = ρg γ ≪ q ≪ q h = h − 1 ,

[0104] Here, γ is the surface tension, and g is the acceleration due to gravity. In the case of glass, we have qv ~271 m -1< (i.e. wavelength λ v ~ 2.3 cm ) And qh ~ 10 6< m - 1< (i.e. wavelength λ h ~1µm (which is determined by the membrane thickness) for a 1µm thick glass membrane. This gives the following range for the oscillation frequencies of a CW in a glass membrane: 2 π × 8 Hz ≪ ω ≪ 2 π × 2 MHz , This calculation can obviously be applied to other materials. Amplitude

[0105] As for the amplitude of each mode of the CW, it is given by the following approximate expression, for thermodynamic excitation (i.e. induced by the thermodynamic fluctuation of temperature): ζ i q i ω i = k B T γq i 2 + ρg ≈ k B T γq i 2 ,

[0106] For silica, the working temperature T is between 1734°C and 2354°C. kB is the Boltzmann constant. Therefore, the amplitude of a CW lies within the following intervals: 0.7 pm < < ς v q v ω v < < 0.8 m , For the lower limit of the wave vectors, and 0.0002 pm ≪ ζ h q h ω h ≪ 0.0003 pm , for the upper limit of the wave vectors.

[0107] The results show that modes with low frequencies have the strongest amplitudes. Therefore, to obtain an order of magnitude of the surface elevations at a given point induced by these CWs, we must integrate ζ i ( qi , ω i ) on a part of the interval [ qv, qh ] . This results in an average elevation of Δ h CW = k B T 4 πγ ln q h q v + 1 ≈ 200 pm for T = 1734°C, and Δ h CW ≈ 250 pm for T=2354°C. These values ​​are typical of measurements obtained on the surfaces of HCPCF. This calculation can obviously be applied to other materials. Depreciation

[0108] Another important property of CWs is their decay rate. In fact, depending on the excitation conditions or boundary conditions, CWs are either propagating or purely evanescent (no oscillations). In both cases, a CW is damped with the following relaxation time: τ q = ρ 2 η 1 q 2

[0109] Here ηis the viscosity of the medium (membrane). This depends on the temperature in the case of molten glass. For example, for silica glass, η = 10 3< Not for T=2354°C and η = 10 6.2< Not for T=1734°C. Using the same method as below, we find the following intervals for τ q : 0.02 μs ≪ τ v ≪ 30 μs , for low-frequency modes, and 1 fs ≪ τ h ≪ 2 ps , for high-frequency modes.

[0110] This calculation can obviously be applied to other materials.

[0111] Given these ranges of amplitudes and relaxation times of CWs, low-frequency modes dominate in roughness formation. Consequently, the following table of typical values ​​for wavelengths, frequency, amplitude, and relaxation time can be constructed, by fixing the useful range of wave vectors. q has [ q v 100 q v]: Typical properties of a CW in a 1µm thick silica membrane and for temperatures between 1734°C and 2453°C Wavelength Oscillation frequency amplitude Amortization period 0.2-23mm 10Hz-8kHz 01pm-0.1pm 3ns-30µs The values ​​above (the calculation of which can obviously be repeated for other materials) were obtained for silica, using the thermomechanical values ​​from the table below: Thermomechanical properties of silica at 1734°C - 2453°C Density ( ρ ( kg. m -3< )) 2201 Coefficient of thermal expansion ( α th (K -1< )) 4.8x10 -7< Thermal conductivity ( κ (Wm -1< K -1< )) 2.7 Heat capacity (Cp (J / Kg.K)) 1052 Viscosity( η (Not)) 10 6.2< @1734°C, Surface tension ( γ ( J . m -2< )) 10 3< @2453°C 0.3 @1231°C

[0112] The dynamics of CWs are generated by thermal noise. CWs can be controlled if a judiciously chosen external excitation (of each beam 5) is applied. This can be shown through the time evolution of a CW in the presence of an excitation F ( t ) described in the following equation: ∂ ζ i x z ∂ t + ζ i x z τ q i = T x z + F t Here, T ( x, z ) represents the force induced by the thermodynamic noise mentioned above. If we consider conditions such that T ( x, z ) « F ( t ), this equation reduces to that of the oscillation of a forced damped harmonic. With an excitation of F ( t ) ∝ Fcos ( ωt + φ ), with the right choice of, F , pulse ω and phase φ , we can control the oscillatory movement to accelerate / decelerate its slowing down, greatly reduce its amplitude or restructure the surface profile.

[0113] τ qi is the relaxation time of the capillary wave i for the material considered, for example silica.

[0114] Similarly, the boundary conditions of the CW can be controlled or modified with the profile and spatial extent of the external excitation, and consequently give a particular profile to the roughness.

[0115] In particular, by judiciously dimensioning the spatial extent of the excitation, its amplitude Fand its modulation frequency, the surface can be structured in such a way that a CW dominates the other capillary waves and the final position of the excitation corresponds to the trough of the dominant CW's amplitude. The result is a surface flatter than that of a surface generated by thermodynamic noise.

[0116] It is these two control levers, namely the spatial extension of the excitation and its amplitude and frequency of modulation, that the invention can exploit.

[0117] According to the invention, to generate the external excitation for controlling CW (coefficient of vibration) within or on the surface of a viscoelastic medium, one can rely on the combination of two physical phenomena. The first is the photothermal effect, and the second is the thermoelastic effect. It should be noted that these phenomena differ from those associated with the inscription of a Bragg grating in fibers (e.g., photosensitivity), which involves electronic restructuring. What contrasts with the photothermal and photoelastic phenomena is that the latter do not involve a change in the refractive index or electronics of the medium. For example, the medium must exhibit a level of viscoelasticity capable of achieving the desired CW control; this is a major difference compared to Bragg grating inscription techniques.

[0118] The photothermal effect involves a thermodynamic temperature fluctuation induced by photoabsorption. This fluctuation is caused by the intrinsic fluctuation of light (i.e., "shot noise"). In parallel, the thermoelastic effect involves a geometric deformation (e.g., change in length) induced by a temperature change, via the phenomenon of thermoelasticity, itself induced by photoabsorption of thermal expansion. In other words, we have Δ h / h = α th Δ T , with, α th being the coefficient of expansion, Δ T the temperature change and Δ h / h the relative change in the length of the medium. Practically, the temperature variation is a temperature modulation, which can be achieved by photoabsorption of a power-modulated laser. This can be formulated as a modulation of the length change: Δ h / h ∝ α th P abs cos(2 πv mod t ), Or P abs is the power of the laser absorbed by the medium. The amplitude of the surface deformation induced by the photothermal effect in the case of a collimated (non-focused) laser, power-modulated at the frequency v mod and at normal incidence on the surface, can be quantified by its response function (in units of length), given below: h PT ν = 2 π α th κ 1 + σ P abs νc ν si ν ≫ νc P abs si ν ≪ νc Here are the quantities α th , κ And σ are respectively the coefficient of expansion, the thermal conductivity and the Poisson ratio of the medium (e.g. glass) respectively. νc = 2 π − 1 κ / ρCw 0 2 , with ρ and C being respectively the density and heat capacity of the heated medium and w 0 is the effective radius of the laser. 5. For silica, this cutoff frequency is written as νc = 2 Hz 100 μm w 0 , i.e., a cutoff frequency of 2Hz for a laser with an effective diameter of 200 µm . 〈 P abs〉 is the average laser power absorbed by the stretched preform. This is determined by both the absorption coefficient of the medium at the laser wavelength, and parameters of P 0, P m P T cited above. Here, the characteristic times of photo-absorption are much faster to consider that the modulation frequency is not affected by the value of 〈 P abs 〉 . This gives us P abs = α abs P O 2 + P m 2 2 We note that in the case of maximum modulation depth (i PO = 0, and P m = PT ) , we have P abs 〉 = α abs P m / √2 . Whereas for a small modulation P m P T ∼ 0 , we have 〈 P abs 〉 = α abs P 0 . Consequently, in the case of non-modulation, we have a constant and "static" deformation of h 0 ≈ S PT ν → 0 ≈ 1 π α th κ 1 + σ α abs P 0 In order for this deformation to dominate that induced by the CW, we must have h 0 ≥ Δ h CW This inequality imposes constraints on the laser power given by P 0 ≥ πκ α th 1 + σ α abs Δ h CW . Returning to the case of a silica glass membrane given above, we find P 0 ≥ 45 W , For an absorption coefficient of 100 ppm. When the laser is power modulated at frequencies higher than vc , the deformation is written Δ h ≈ h 0 νc ν 1 + P m P T 2 1 − P m P T − 2 The expression below illustrates the role of frequency. v and the modulation depth P m P T , in surface control. Finally, we see that the impact of the modulation frequency value is determined by vc. The frequency vc is related to the thermal relaxation time of an excitation induced by the photoelastic phenomenon, or to the thermal diffusion length. τ c = 2 πν c − 1 This allows time for the silica to relax. τ c ≈ 3 s w 0 100 μm This formula demonstrates the role of laser beam size in the relaxation of thermal waves generated by photoelasticity. Note that the scattering length is independent of w0, as shown by the formula below: L th = κ ρC ν = w 0 ν c ν The diffusion length L th can only be controlled by the modulation frequency. For silica glass we have L th ~30 µm for a modulation frequency of 1 kHz, and L th ~300 µm for a modulation frequency of 10Hz. Therefore, the modulation frequency can be used as a means of control L th (i.e., the spatial extent of the generated CW) without affecting the capillary wave damping. The following table shows the impact of the laser parameters on the surface structuring. The impact of laser parameters on surface profile control Power continue ( P 0 ) 1. Primary surface profile (long span) Modulation frequency (v) 1. Selective excitation of short-range CWs 2. Control of diffusion length Modulation Depth 1. Control of the amplitude of short-range CWs Beam size 1. Control of CW relaxation time 2. Control of boundary conditions of CWs (i.e., control of CW phases) Laser-furnace distance 1. Adjusting with respect to the viscoelastic state of the CWs induced by thermodynamic noise in the thermal furnace. 2. Control of boundary conditions of CWs (i.e., control of CW phases) We reach a point where we can identify the operating conditions for shaping the surface of a glass subjected to thermal noise-induced CW (generalizable to a volume). Designing the invention to structure the surface can be achieved by establishing a hierarchy between the characteristic times (i.e., equivalently: the characteristic lengths) associated with the different underlying effects of the generation of CWs by thermal noise in the furnace and their control by photon excitation. Below, a table summarizing the characteristic times in The impact of laser parameters on surface profile control Characteristic time Associated phenomenon Expression Interval Cooling time ( τ cool ) The cooling time of a glass heated after being removed from the furnace ρC p κ V A 2 w 0 1ms-60ms Laser-matter interaction time ( τ int ) The interaction time with the laser of a glass (e.g., fiber) moving uniformly at a speed vd 2 w 0 v d 100µs-10ms Laser-matter modulation time ( τ las ) The time associated with the laser power modulation 1 ν mod 1ms-10s Thermal relaxation time ( τ therm ) The cooling time of a glass heated by photo-absorption ρC p κ w 0 2 8ms-3s Correlation time of a CW ( τ CW ) The time associated with the correlation length of a CW. γ / gρ c s 0.7µs-1 µs V And A volume and longitudinal area of ​​glass heated by the thermal furnace. cs speed of sound of heated glass. vd Glass drawing speed at the exit of the thermal furnace. See text for other parameters. Below are the different operating conditions: Operating conditions for surface shaping Description of the condition Condition on device parameters The dominant CW amplitude induced by photoelasticity must be greater than the dominant CW amplitude caused by thermal noise. This condition is required to be able to shape the surface photoelastically. P 0 ≥ 45 W The distance between the furnace and the laser beam, d , must be short enough so that the temperature of the fiber at the furnace outlet, T h , remains above the softening temperature. A condition for having the CWs induced in the furnace remain dynamic and not solidify (or "frozen"). d ≤ v d τ cool ln T h T soft For silica d ≤ 10 mm For vd = 10 m / min The time spent in the laser beam by the fiber / preform must be long enough for the photoelastic phenomenon to be established over the entire volume of the glass "tricked" in the laser beam. τ int ≥ max ( τ las , τ therl ) One strategy for damping the amplitude of thermal noise-induced CWs in the furnace is to "neutralize" them by interfering with photoelastic CWs. This implies that the frequency range of the photoelastically induced CWs is the same as that of the thermally noise-induced CWs. This, in turn, determines a range for the laser modulation frequency. ν mod ∈ 2 π − 1 g ρh γ , γ ρh 3 For silica v mod ∈ [10 Hz, 8 kHz ] Another strategy for flattening roughness is to exploit the wave properties of CWs and their boundary conditions to fix their q cw × 2 w 0 = (2 n + 1) π For the dominant CW amplitude at zero by superimposing the position of the valley of a CW with the edge of the laser beam area. 2 w 0 ρg γ = 2 n + 1 π (here n an integer) This condition can be achieved by controlling the diameter of the laser (see opposite) and / or its position. For silica 2 w 0 = (2 n + 1) × 11 mm All these calculations in the entire description can obviously be repeated for other materials.

[0119] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

[0120] In one variant, preform 1 is not hollow or microstructured but can be a solid glass fiber.

[0121] Of course, the different features, forms, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive.

Claims

1. Method for manufacturing an optical fibre, in which: - a preform (1) is inserted into a furnace (2), - the preform is drawn via an outlet (3) of the furnace, and - at least one laser beam (5) is applied to a working zone (4) of the thus-drawn preform, each laser beam being power-modulated according to a modulation frequency characterized in that: - each laser beam is power-modulated according to a modulation frequency greater than 10 Hz and / or less than 1000 kHz - each laser beam has a power greater than 10 W - the working zone is at a temperature above a softening or vitreous transition temperature of the material constituting the preform - the part (8) of the wavefront of each laser beam coming into contact with the working zone has, at each given instant, a spatial power density comprised between a maximum and 80% of this maximum.

2. Method according to claim 1, characterized in that the modulation frequency less than 10 kHz.

3. Method according to claim 1 or 2, characterized in that each laser beam is modulated between a maximum power (6) and a minimum power such that the minimum power value is comprised between 95% of the maximum power and 0% of the maximum power.

4. Method according to claim 3, characterized in that each laser beam is modulated over an interval (9) ranging from the maximum power to at least 50% of this maximum power.

5. Method according to claim 4, characterized in that each laser beam is modulated over an interval ranging from the maximum power to at least 10% of this maximum power.

6. Method according to any one of the preceding claims, characterized in that each laser beam is power-modulated according to a modulation frequency greater than 100 Hz.

7. Method according to any one of the preceding claims, characterized in that the at least one laser beam comprises several laser beams crossing one another on the working zone.

8. Method according to any one of the preceding claims, characterized in that each laser beam has a wavelength greater than 200 nm and / or less than 12 µm.

9. Method according to any one of the preceding claims, characterized in that each laser beam is collimated onto the working zone.

10. Method according to any one of the preceding claims, characterized in that the preform is a glass preform.

11. Method according to any one of the preceding claims, characterized in that the working zone of the drawn preform comprises a structure composed of walls (7) the thickness of which is less than 10 µm, preferably less than 3 µm, preferably less than 1 µm.

12. Method according to any one of the preceding claims, characterized in that the working zone is situated outside the furnace.

13. Method according to any one of the preceding claims, characterized in that the furnace comprises: - an enclosure through which the preform passes, and - a heat source arranged in the enclosure and raising the temperature inside the enclosure, the working zone being situated at a distance less than 10 cm from the heat source.

14. Method according to any one of the preceding claims, characterized in that the working zone is at a temperature above 1000°C, preferably above 1500°C.

15. Device for manufacturing an optical fibre, comprising: - a furnace (2) - an inlet for inserting a preform (1) into the furnace (2), - means for drawing the preform via an outlet (3) of the furnace, and - emission means arranged to apply to a working zone (4) of the thus-drawn preform, at least one laser beam (5), and modulation means arranged to power-modulate each laser beam according to a modulation frequency characterized in that: - the modulations means are arranged so that each laser beam is power-modulated according to a modulation frequency greater than 10 Hz and / or less than 1000 kHz - the emission means are arranged so that each laser beam has a power greater than 10 W - the emission means and the furnace are arranged so that the working zone is at a temperature above a softening or vitreous transition temperature of the material constituting the preform, - the emission means are arranged so that the part of the wavefront of each laser beam coming into contact with the working zone has, at each given instant, a spatial power density comprised between a maximum and 80% of this maximum.