WO2001035503A1

WO2001035503A1 - Fibre laser

Info

Publication number: WO2001035503A1
Application number: PCT/FR2000/003103
Authority: WO
Inventors: David Pureur; Benoît Cadier; Brice Kerrinckx; Jean-François Bayon
Original assignee: Highwave Optical Technologies
Priority date: 1999-11-09
Filing date: 2000-11-08
Publication date: 2001-05-17
Also published as: AU1400701A; FR2800926A1; FR2800926B1

Abstract

The invention concerns a device forming a fibre power laser emitting a controlled transverse monomode radiation. The invention is characterised in that it comprises: a pump source (40), a clad amplifying optical fibre section (10) consisting of a double-clad fibre comprising a core (12) enclosed in two successive sheaths (15, 18), whereof the refractive indices are suited for defining a multimode optical structure, such that the pump wave is guided in the multimode structure of the fibre formed by the two sheaths (15, 18), two reflectors (20, 25) arranged on the ends of said section (10), consisting of diffraction gratings selected in the group comprising Bragg grating or reflecting dielectric filters, and a bandstop filter (30) capable of emitting a laser radiation centred at about 980 nm in the section.

Description

FIBER LASER

The present invention relates to the field of power fiber lasers. More specifically, the present invention aims to develop a power fiber laser capable of emitting transverse single mode radiation at 980 nm.

The field of power fiber lasers has already given rise to an abundant literature. By way of nonlimiting examples, reference may be made to the references

[1] L. Zenteno, "High power double-clad fiber lasers", Journal of Lightwave

Optical Technology, 11, 1993.

[2] G. Mitchard et al. "Double-clad fibers enable lasers to handle high power", Laser Focus World, Jan. , 1999.

[3] R. Paschotta et al., "Ytterbium-doped fiber amplifiers", IEEE Journal of

Quantum Electronics 33, 1997.

[4] B. J. Ainslie et al., "The absorption and fluorescence spectra of rare earth ions in silica-based monomode fiber", Journal of Lightwave optical Technology 6, 1998.

[5] A. Bertoni et al., "A model for the optimization of double-clad fiber laser operation", Applied Physics B66, 1988.

Lasers, in particular at 980 nm, have already been produced in ytterbium-doped single-mode fibers with a power of a few tens of milliwatts [3]. However, the small size of the fiber core and its low numerical aperture do not allow the energy delivered by power pump diodes to be collected. Double-clad fiber technology was developed in the 1990s to improve the available power of fiber lasers (up to several tens of Watts). In this configuration, the pump wave is guided in the multimode structure of the fiber while the laser wave is generated only in the single-mode core. The laser radiation thus has all the properties of spatial coherence of a single-mode fiber laser however with high power due to multimode pumping. Such power lasers exist in the emission band [1.01 μm-1.162 μm] of the ytterbium ion but not in the second band [0.975 μm, 0.985 μm].

In fact, despite the numerous researches carried out in the field, to the knowledge of the Applicant, so far we have not been able to develop power fiber lasers emitting radiation centered around 980nm.

The object of the present invention is to remedy this difficulty.

This object is achieved in the context of the present invention, thanks to a device comprising:

- a pump source,

- a section of sheathed amplifying optical fiber consisting of a double cladding fiber comprising a core surrounded by two successive claddings, the refractive indices of which are suitable for defining a multimode optical guide, so that the pump wave is guided in the multimode structure of the fiber formed by the two sheaths,

- two reflectors arranged on the ends of this section, made up of diffraction gratings chosen from the group comprising Bragg gratings or reflective dielectric filters, and - a band-cut filter capable of emitting laser radiation centered around 980 nm in said section .

Other characteristics, aims and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings, given by way of nonlimiting example and in which:

FIG. 1 represents the general structure of a device according to the present invention forming a fiber laser cavity at 980 nm using a double clad fiber,

FIG. 2 represents a schematic view in cross section of a double-clad fiber comprising a multimode rectangular guide according to a first variant of the invention,

- Figure 3 shows a similar schematic view in cross section of a double clad fiber comprising an undoped core surrounded by a doped ring in accordance with another alternative embodiment of the present invention,

FIG. 4 represents the general structure of a device according to the present invention corresponding to a variant of FIG. 1 in which reflectors framing the laser cavity are located outside the double-clad fiber,

FIG. 5 represents the spectrum of the fluorescence of the fibers illustrated in FIGS. 2 and 3 in the spectral band [876 nm, 1076 nm], and

- Figure 6 shows the emission spectrum of the laser device according to the present invention emitting radiation at 978 nm.

FIG. 1 shows the general structure of a device according to the present invention.

We can see in this figure 1:

a section 10 of double-clad fiber (single-mode core 12 and polymer sheath 18, the structure of which will be described in more detail below),

at least one or more reflectors 20, 25 disposed respectively on the ends of this section 10 of fiber,

- a filter 30 forming a band cut beyond 1030 nm integrated in the fiber 10 between the reflectors 20, 25, and - a system of pump diodes 40 emitting from 915 nm to 930 nm placed opposite a first end of said fiber 10.

An injection device 50 can be placed between this pump diode system 40 and the fiber 10. The injection device 50 can be longitudinal or transverse relative to the axis of the fiber. The radiation at 980 nm coming from the fiber laser cavity thus formed can be recovered for example in a single mode fiber 60 at 950 nm welded at 62 on the second end of the sheathed fiber section 10.

In the case of a fiber laser with 3 energy levels (case of rytterbium at 980 nm), the expressions of the linear gains at pump and signal frequencies take the following forms:

g _P = [- σ _a ^p (1-x)] r _p NL (1) g _s = [σ _e ^s x- σ _a ^S (1-x)] r _s NL (2) where g denotes the linear gain (g = In (P ₀ u _t / Pin) with P _in and P _or t the input and output powers), σ _a and σ _e the absorption and emission cross sections , r the overlap factor between the optical wave and the doped region, N the density of ytterbium ions, L the fiber length and x the fraction of excited ions (or population inversion rate). The indices ^p and ^s differentiate the pump wave (915 nm) from the emitted laser wave (980 nm). In the case of a fiber laser, x is expressed by the relation: x = - {ln (Rι R ₂ ) ^1/2 } / {(σ _e ^s + σ _a ^s ) T _s NL} + {σ _a ^s / (σ _e ^s + σ _a ^s )} (3) where Ri and R ₂ denote the reflection coefficients of the two reflectors which close the cavity at each fiber end. The Pseuil laser threshold is written:

P _se uii - ANL x / τ for long cavity lengths (4) τ is the lifetime of the upper level of the laser transition.

The fluorescence spectrum of a ytterbium-doped fiber 10 optically pumped by a laser 40 emitting radiation at 915 nm is shown in FIG. 5. The competition between the absorption σ _a and emission σ _e cross sections gives fluorescence important centered at 980 nm and 1030 nm. The shape of this spectrum depends on the formers and modifiers of the vitreous matrix such as germanium and aluminum. It also depends on the length of amplifying fiber 10. A high concentration of aluminum will, for example, smooth the fluorescence peak at 1030 nm without modifying the spectrum at 980 nm. Similarly, a long length of fiber 10 will decrease the inversion of average population x and shift the spectrum towards the long wavelengths. The configuration of the laser in accordance with the present invention makes it possible to obtain a single mode guided laser emission from a few milliwatts to several hundred milliwatts around 980 nm, with a system of multimode pump diodes 40 delivering a power of 1 to 10 Watts towards 915. nm. Two geometries of laser fiber 10 (respectively with a heart or a doped ring) have been developed by the Applicant with ytterbium ion concentrations, mode overlays and optimal guided structures.

As seen in Figures 2 and 3, the fiber 10 used in the context of the present invention comprises a core 12 (simple according to Figure 2, surrounded by a ring 14 according to Figure 3) placed in a multimode sheath 15 , itself surrounded by a polymer sheath 18.

The multimode optical sheath 15 has an asymmetrical shape (rectangular with small rounded or non-rounded sides) to eliminate the helical modes. These modes indeed have a spatial distribution in minimum intensity in the region of the core and maximum on the edges of the multimode fiber. To obtain a rectangular shape, the preform is first machined before being fiberized and then protected by the polymer sheath 18.

According to the first geometry illustrated in FIG. 2, the core 12 of the fiber 10 is doped with germanium or aluminum and ytterbium ions. It should be noted that this is not true for commercially available double clad fibers, exclusively doped with aluminum which, by virtue of its trivalent structure, makes it possible to incorporate a high concentration of ytterbium.

In this configuration, the guided propagation theory in a single-mode fiber gives an overlap factor r _s (between the laser wave and the doped core) of the order of 0.8. On the other hand, the recovery factor r _p (pump wave / doped heart) is written as a first approximation and in multimode mode: r _p = B / A ~ 0.002 (5) with A the area of the rectangle corresponding to the section of the multimode sheath 15 and B the area of the doped zone (here the heart 12). Such an approximation is justified by the models used which demonstrate a speckle structure (uniform distribution of the intensity perpendicular to the axis of the fiber) after only a few millimeters of propagation. r _p is therefore 400 times lower here than in single-mode propagation regime. To compensate for the low value of r _p and keep sufficient absorption (g _p ) (see (1)), it is in principle necessary to increase the terms N and L. Nevertheless, increasing one or the other of these 2 terms tends to disturb the linear gains g _s at 1030 nm and at 980 nm (see (2)). The laser threshold at 1030 nm is thus reduced while that at 980 nm is increased, which is detrimental for the application intended in the context of the present invention. The present invention nevertheless makes it possible to produce a laser at 980 nm by placing an attenuating filter 30 at 1030 nm on the fiber section 10, in order to eliminate the disturbing fluorescence. Such an intra-cavity component 30 can be a dissipative Bragg grating. The network type with lines inclined with respect to the axis of the fiber. Such a network couples the energy from the guided mode in the heart to the radiative modes. The fiber then has a transmission drop at the desired wavelength.

According to the second configuration illustrated in FIG. 3, the heart 12 of the fiber 10 is doped only with germanium while a ring 14 centered around the heart 12 is itself doped with ytterbium. The vitreous matrix, at the location of the ring 14, is codoped with either germanium or aluminum (depending on the concentration of ytterbium ions). In this configuration, r _p is almost unchanged (r _p = 0.002) while r _s (overlapping laser wave centered in the heart / doped ring) is reduced (r _s <0.1). The second doping profile thus makes it possible to keep a linear gain g _s at 1030nm lower than at 980nm while increasing the fiber length to obtain sufficient absorption.

The polymer or silicone material 18 which covers the optical sheath 15 is a product whose refractive index (n {1.4) makes it possible to have a multimode optical guide with a large digital aperture. The power emitted by the diode 40 is thus more effectively captured by the guiding structure. The parameters of the core 12 of the fiber are further adapted so that the diameter of the fundamental mode is identical to that of a standard single-mode fiber at 980 nm.

More specifically, the double-clad fiber illustrated in FIGS. 2 or 3 preferably meets the following characteristics:

Dimension of the rectangular optical sheath 15: axb ≈ (from 80 to 180) x (from 50 to 120) μm ² Ring size 14: external diameter R _M ≈ 4 to 30 μm and internal diameter R _m ≈ 4 to 15 μm (in the case of Figure 3) sheath / polymer numerical aperture: from 0.35 to 0.45 polymer index 18: between 1.36 and 1.40 measured at 1 μm diameter of the core 12 of the fiber: from 4 to 6 μm difference in index core / cladding: ≈ from 5 to 10.10 ^"3 numerical aperture heart / cladding: from 0.1 to 0.2 concentration of the core in germanium or aluminum: a few molar% concentration of the core or the ring in ytterbium: ≈ several thousand ppm loss in the single mode part at 1.3 μm: ≈ 1 to 20 dB / km

The laser cavity can consist of two Bragg gratings 20, 25, a few meters apart, photoinscribed in the single-mode core 12 of the double clad fiber. Photo-registration is made possible with UV radiation at 244 nm thanks to doping of the heart 12 with germanium. Indeed, with doping of the aluminum core 12, photo-registration at 244 nm is made impossible. A standard photo-registration bench consisting of a UV laser and an interferometric device can be used for this purpose. To increase the photosensitivity of the fiber, it is preferably placed in a hydrogen tank under high pressure. Saturated reflectors have thus been manufactured in order to obtain a cavity with a high quality coefficient and therefore a low laser threshold. Dielectric layers deposited at the end of double-clad fiber can also be used with fibers doped with aluminum. The spectral selectivity of the reflectors makes it possible to obtain widths of laser line ranging from a few gigahertz up to several nanometers.

At the outlet of the cavity, the standard single-mode fiber 60 at 950 nm is welded to the double-clad fiber 10 with losses less than 0.4 dB. The laser emission spectrum is presented in Figure 6. FIG. 4 illustrates another configuration of cavity which includes reflectors 20, 25 produced respectively on sections of a standard single-mode fiber 64, 66 (and not on the double clad fiber as described for FIG. 1) . These pieces of fiber 64, 66 are then welded to each end of the double sheath fiber 10 at 63, 65.

The structure of the device illustrated in FIG ^. 4 is for the rest in accordance with the architecture illustrated in FIG. 1. FIG. 4 indeed shows: the pump diode 40 associated with an injection device 50 and a fiber 10 with double sheath comprising a band cutting filter 30.

The laser cavity obtained in the context of the present invention has the following characteristics:

Reflectivity Ri for the reflector 20 = 99.9% Reflectivity R ₂ for the reflector 25 = from 4 to 90% Length of the cavity: a few meters

Pump wavelength: from 915 to 930 nm Available pump power: from 0.001 to 10 Watts Preferably, the diode 40 is mounted on a Peltier device. The dimensions of the mechanical housing grouping the entire system, namely the double gain fiber 10, the reflectors 20, 25, the filter 30, the pump diode 40 and the system 50 for injection into the multimode part of the fiber are typically of the order of 100 * 50 * 30 mm ³ . The output fiber 60, 66 can be a standard Flexcor 1060 single mode fiber at 950 nm. A version of the device according to the invention may contain a fiber isolator at 980 nm located at the output of the laser diode and an integrated modulator.

Such a single mode power laser is particularly intended for the telecommunications market. In particular, it makes it possible to pump erbium-doped fiber amplifiers with greater efficiency than previous systems. These amplifiers are the key elements of optical transmission and are currently limited by the low power (200 mW maximum) of the pump laser diodes at 980 nm. Higher power laser diodes are also available on the market but at much higher cost. Another application of such power lasers is Raman amplification. This type of optical regeneration does not require population inversion and is therefore not limited by the absorption bands accessible to rare earth ions. Raman amplification, which is currently booming, makes it possible to widen the bandwidth of optical telecommunications towards low wavelengths (up to 1455 nm). The industrial outcome of these lasers is thus compatible with telecommunications but also more generally with industrial control techniques by laser, medical, and research. Of course the present invention is not limited to the particular embodiment which has just been described, but extends to all variants in accordance with its spirit.

Claims

1. Device forming a power fiber laser emitting controlled transverse monomode radiation, characterized in that it comprises:

- a pump source (40),

a section (10) of sheathed amplifying optical fiber constituted by a double cladding fiber comprising a core (12) surrounded by two successive claddings (15, 18), the refractive indices of which are suitable for defining a multimode optical guide, so that the pump wave is guided in the multimode structure of the fiber formed by the two sheaths (15, 18),

- two reflectors (20, 25) arranged on the ends of this section (10), made up of diffraction gratings chosen from the group comprising Bragg gratings or reflective dielectric filters, and - a strip cut filter (30) suitable for emit laser radiation centered around 980 nm in said section.

2. Device according to claim 1, characterized in that the section (10) of sheathed optical fiber consists of a double clad fiber doped with ytterbium ions.

3. Device according to one of claims 1 or 2, characterized in that the section (10) of sheathed optical fiber comprises a doped core (12).

4. Device according to one of claims 1 or 2, characterized in that the section (10) of sheathed optical fiber comprises a ring (14) doped centered around the heart (12).

5. Device according to one of claims 1 to 4, characterized in that the concentration of dopant ytterbium is of the order of several thousand ppm.

6. Device according to one of claims 1 to 3 and 5, characterized in that the heart (12) of the fiber (10) is doped with germanium or aluminum and ytterbium ions.

7. Device according to one of claims 1, 2, 4 and 5, characterized in that the heart (12) of the fiber (10) is doped only in germanium while the ring (14) centered around the heart is doped with ytterbium.

8. Device according to claim 7, characterized in that the vitreous matrix of the fiber (10), at the location of the ring (14), is codoped with either germanium or aluminum.

9. Device according to one of claims 1 to 8, characterized in that the filter (30) is formed of a Bragg grating with lines inclined with respect to the axis of the fiber.

10. Device according to one of claims 1 to 9, characterized in that the reflectors are photo-registered or deposited on the ends of the section (10) of sheathed optical fiber.

11. Device according to one of claims 1 to 9, characterized in that the reflectors (20, 25) are photo-registered or deposited on fiber sections (64, 66) attached and welded to the ends of the section (10) sheathed optical fiber.

12. Device according to one of claims 1 to 11, characterized in that the sheathed optical fiber section (10) comprises a multimode optical sheath (15) of asymmetrical shape, preferably of generally rectangular section.

13. Device according to one of claims 1 to 12, characterized in that the band-cutting filter (30) is an attenuating filter at 1030 nm.

14. Device according to one of claims 1 to 13, characterized in that the band-cutting filter (13) is formed by a dielectric filter or a dissipative Bragg grating.

15. Device according to one of claims 1 to 14, characterized in that the section (10) of sheathed optical fiber comprises a multimode optical sheath (15) of generally rectangular section whose dimensions are of the order of 80 to 180 μm on 50 to 120 μm.

16. Device according to one of claims 1 and 2 and 4 to 15, characterized in that the section (10) of sheathed optical fiber comprises a doped ring (14) surrounding the heart (12) whose external diameter is included between 4 to 30 μm and the internal diameter is between 4 to 15 μm.

17. Device according to one of claims 1 to 16, characterized in that the digital aperture multimode sheath (15) / polymer forming external sheath (18) is of the order of 0.35 to 0.45.

18. Device according to one of claims 1 to 17, characterized in that the index of the polymer forming the outer sheath of the section (10) of sheathed optical fiber is of the order of 1.36 to 1.40, measured at 1 μm.

19. Device according to one of claims 1 to 18, characterized in that the diameter of the core (12) of the fiber (10) is of the order of 4 to 6 μm.

20. Device according to one of claims 1 to 19, characterized in that the difference in index between the heart (12) and the multimode sheath (15) which surrounds it is of the order of 5 to 10.10 ^{" 3} .

21. Device according to one of claims 1 to 20, characterized in that the digital aperture core (12) / multimode sheath (15) is of the order of 0.1 to 0.2.

22. Device according to one of claims 1 to 21, characterized in that the concentration of the core (12) in germanium or aluminum is of the order of a few molar%.

23. Device according to one of claims 1 to 22, characterized in that the length of the cavity defined between the two reflectors (20, 25) is of the order of a few meters.

24. Device according to one of claims 1 to 22, characterized in that the pump wavelength emitted by the source (40) is of the order of 915 to 930 nm.