WO1997008791A1

WO1997008791A1 - Optical fibre for improved power coupling

Info

Publication number: WO1997008791A1
Application number: PCT/US1996/013184
Authority: WO
Inventors: Tanya Oleskevich; Peter Gottfried Berrang
Original assignee: Sdl, Inc.
Priority date: 1995-08-31
Filing date: 1996-08-16
Publication date: 1997-03-06
Also published as: US6073465A; US5790735A

Abstract

An optical fibre having a single mode absorptive core (10) whose position, relative to the cross-sectional plane of the inner multimode cladding (12), varies along the length of the fibre. The periodicity and the magnitude of the relative displacement of the core (10) with respect to the cladding (12) are such that the transfer of radiation from the inner multimode cladding (12) to the core (10) is substantially improved over conventional fibres.

Description

OPTICAL FffiRE FOR IMPROVED POWER COUPLING

TECHNICAL FIELD OF THE INVENTION This invention relates to optical fibres. In particular, the invention relates to an improved structure for double-clad optical fibres designed to maximize the coupling of modes propagation in the multimode cladding into the core.

BACKGROUND ART

A number of applications, such as biomedical and thermal printing applications, require high power laser sources (>1 Watt) in the near infrared region, as well as practical, flexible beam delivery systems. The maximum lasing power which can be extracted from a fibre laser source depends ultimately on the amount of pump radiation which can be coupled into the fibre laser cavity.

The large emitting area and extreme aspect ratios (100 μm x 1 μm) of high power stripe diodes available as pump sources typically result in unacceptably low coupling efficiencies of 10% into single mode fibre.

Double clad optical fibres, consisting of a single mode core, an inner multimode cladding and a further cladding, offer improved coupling efficiency as the dimension of inner cladding can be chosen to optimize coupling from the intended pump source. Up to 90% coupling efficiency has been achieved with double-clad fibres. In a typical double clad fibre, a lasant-doped single mode core, having index of refraction N_l5 is surrounded by a large inner multimode cladding with index of refraction n₂<n This inner multimode cladding is then surrounded by a further layer, with index refraction n₃<n₂, which confines radiation in the inner cladding and prevents scattering losses due to accumulation of dirt and humidity on the guiding interface. The pump radiation is transferred from the modes in the inner cladding to the absorptive, single mode core as they propagate along the length of the fibre. In this way, several watts o pump power can eventually be completely absorbed by the lasant-doped single mode core and thus single mode lasing at much higher powers can be achieved using double-clad fibres. The double-clad fibre geometry essentially acts as a brightness converter on the order of the area ratio of the inner cladding to the core, which may be typically 400:1. The overall lasing efficiency of fibre lasers made using double-clad fibre is not as high, however, as in conventional single-clad devices. The absorption of the pump light guided in the cladding is, in a first approximation, governed by Beer's law wherein the absorption coefficient of the doped core is reduced by the cladding/core area ratio. Experimental results have shown that the effective absorption coefficient of concentric circular double-clad fibre is only 5% of that predicted from the geometric area ratio of Beer's law. The reason for this is thought to be as follows. The strength of the absorption of a given mode depends on the field distribution of the mode at the position of the absorptive core. The higher the order of modes, the lower the field distribution at the center of the inner cladding. For a double-clad fibre optimized for pumping by a broad stripe diode laser (typically lOOμm wide), the number of modes in the inner cladding is on the order of IO⁵' and only a small fraction of the field is propagating in the center of the inner cladding and can thus be absorbed by the doped core. The radiation transfer from the multimode cladding to the single mode core is reduced by the number of modes propagating in the cladding which do not exhibit a local maxima at the location of the core. Typically, the cladding/core area ratio can be up to 400:1 and in this instance only 3% of the modes are absorbed by the single mode core (Bedό et al. in Optics Communications, 1993, pages 331-335). It has also been shown by Bedo et al. that the absorption coefficient of double- clad fibre is non-linear with fibre length and may be up to three times larger for short lengths versus long lengths of fibre. This can be understood if one considers the modes propagating in the cladding as two ensembles, those that are absorbed by the core and those that are not, which are inter-related by a mixing parameter. The effective absorption coefficient for the double-clad fibre is then a function of the absorption coefficient, the mixing or power coupling coefficient, and the fraction of the ensemble of non-absorbing modes to the total power in the fibre. The small fraction of modes in the multimode cladding which initially do have local maxima at the position ofthe core are quickly depleted in a short distance by the highly absorptive core. This has the effect of increasing the relative fraction of non-absorbing modes, and thus reduces the effective absorption coefficient in the remaining length of fibre. The effective absorption of the double-clad fibre, after a characteristic length, is in fact, lower than that predicted by considering only the geometric ratio of the areas of the core and cladding regions. It has been recognized that introduction of perturbations in the multimode cladding modes induces mode mixing and increases the radiation transfer from the inner cladding to the core. The conventional approach to achieving this effect has been to induce bends in the fibre. However, it is often the case that as many modes as are subsequently coupled into the ensemble of absorbed modes are also coupled into non-guided modes in the cladding and so are lost. The net effect is an only marginal increase in device performance. It has also been suggested to design a fibre with a constant offset of the single mode core from the geometric center of the multimode cladding. This approach has had some success, however, it is still much less efficient than the 100% transfer efficiency inherent in traditional single-clad single mode fibre lasers in which the pump radiation and lasing mode are both guided in the same waveguide structure.

There remains a need to improve the overall efficiency of devices incorporating double-clad fibre geometries such as fibre lasers and amplifiers. It is therefore an objective of the invention to provide a structure for a double-clad optical fibre wherein there is improved coupling of the modes in the cladding to the core and thereby improve the performance of such devices.

SUMMARY OF THE INVENTION The invention provides a double-clad fibre structure comprising a substantially single mode lasant-doped core, an inner multimode cladding surrounding the core, and an outer cladding surrounding the inner multimode cladding wherein the position of the core relative to the cross sectional plane of the inner cladding varies continuously, or "wanders", along the length of the fibre. As a result, a substantial amount of radiation in the cladding is coupled into the single mode core, and a continually renewed distribution of undepleted high order modes propagating in the inner cladding are coupled into the single mode core along the length of the fibre. Substantially all of the energy launched into the inner cladding may be absorbed in a shorter characteristic length of fibre in this wandering core configuration than in conventional double-clad fibre geometries. BRIEF DESCRIPTION OF DRAWINGS The invention may be more fully appreciated by reference to the detailed description of the preferred embodiment which follows and to the drawings, shown in exaggerated detail in which: Fig. IA to ID is a series of four cross-sectional views of the fibre according to the preferred embodiment of the invention, with each view being taken at different lengths along the fibre;

Fig. 2 is a longitudinal sectional view of a length of fibre according to the preferred embodiment of the invention; Fig. 3 is a longitudinal sectional view of a length of preform according to a first method of making fibre according to the preferred embodiment of the invention;

Fig. 4A to 4D are a series of four cross sectional views of a first alternative embodiment of the invention, taken at different lengths along the fibre; Fig. 5 is a side view of a preform for making fibre according to the first alternative embodiment of the invention wherein the preform has been machined with a helical groove and showing the inner cladding in ghost outline;

Fig. 6A to 6C are a series of three cross sectional views of a second alternative embodiment of the invention taken at different lengths along the fibre; Fig. 7A and 7A are a longitudinal sectional view and a cross-sectional view of a preform for making fibre according to the second alternative embodiment of the invention, with Fig. 7B being a cross sectional view taken from 7B-7B in Fig. 7A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, the optical fibre of the preferred embodiment of the invention has a single mode core 10 surrounded by an inner cladding 12 having an index of refraction less than the index of refraction of the core 10. The core 10 may be, for example, a fused silica core, doped with an active lasant material. The core may also be co-doped with a suitable element such as germanium or boron to increase the photosensitivity of the core to ultra violet radiation to facilitate the writing of Bragg gratings in the core. The inner cladding 12 may be formed of fused silica, and acts as a multimode waveguide. This inner cladding may be doped with trace elements, for example, fluorine or phosphorus, to modify the index of refraction to allow more flexibility in the fibre design parameters such as the numerical aperture (NA) and the dimensions of the core and claddin regions. The ratio of the area of the inner cladding to that of the core in the invention is substantially in the range of approximately twelve to four hundred to one (12 - 400:1). The inner cladding 12 is surrounded by an outer cladding 14 whose index of refraction is less than that of the inner cladding 12.1. A final jacket 15 is applied to protect and strengthen the fibre.

As illustrated in Fig. IA to ID and Fig. 2, the position of the core 10 in relation to the inner cladding 12 wanders along the length of the fibre. In each o the figures, the relative size and wander of the core has been exaggerated for clarity. In the progression of views in Fig. IA to ID, the position of the core with respect to the cross sectional plane of the inner cladding is shown to vary readily along the length of the fibre.

In the preferred embodiment, the variation of the position of the single mode core is dictated by the absorption length of the double-clad fibre. For the case of four level laser system, such as a double-clad fiber laser in which the core is doped, for example, with ytterbium (Yb), the length, 1, of fibre which absorbs a power, P_a, is given by

where x is the ratio ofthe outer to inner core areas, N is the total number density of ytterbium ions and P(0) is the launched pump power, P_s, is the saturation power and is given by hv A P_s = (2)

where σ_ep and σ_ap are the pump emission and absorption cross-sections, respectively, τ is the upper level lifetime and A is the area of the inner cladding and hv_p is the photon energy of the pump radiation. If we define a characteristic absorption length, l_abs, to be that which absorbs half of the launched pump power, then l_ahs is derived from equation (2) and gives

In the preferred embodiment the single mode core wanders a minimum distance equivalent to the diameter of the single mode core within a length of fibre given by 1^. but may wander from this minimum up to 0.9r, where r is the radius of the inner cladding in the case of circular geometry.

Fig. 3. illustrates a preform for making fibre according to the preferred embodiment of the invention. A preform 16 having a single mode core 10 and a multimode cladding 12 is provided. As illustrated in Fig 3, V-grooves 18 are machined into opposing sides of the preform and are offset from the opposing V- groves by an amount equal to one-half of the width of the V-groove. The fibre is drawn from the preform according to methods known to those skilled in the art of drawing preforms, but so as to result in a fibre in which the core is substantially single mode, the inner cladding is substantially multimode and the ratio of the areas of the cladding to the core is 12 to 400:1. As a result of the V-grooves 18 in the preform, the position of the core 10 relative to the cross sectional plane of the cladding 12 varies along the length of the pulled fibre, alternating from one side of the cladding 12 to the other. As an example, an 8 mm diameter machine preform with V-grooves which are 2 mm deep and 2 mm wide may produce a fibre with a total core wander of 60 μm within a ten-meter length of drawn fibre. An outer cladding 14, which may be a clear plastic, is applied after the fibre leaves the drawing furnace and a further protective jacket 15 is applied over this.

It will be appreciated by those skilled in the art of pulling fibre that the grooves need not necessarily be in the shape of a V. Other shape of grooves are contemplated, as for example square grooves. However, V-grooves are preferred rather than square grooves, to provide a more gradual displacement of the core in the drawn fibre to reduce guiding losses in the single mode core. The displacement of the core need not necessarily be a radial displacement and any pattern of substantial displacement of the core with respect to the cross sectional plane of the inner cladding which promotes the transfer of radiation from the cladding to the core is within the scope of the invention. However, the periodicity should not be so short nor the displacement so great as to induce substantial guiding loss in the single mode core thereby negating the improvement achieved by the invention. In a fibre laser or fibre amplifier device, the longitudinal period of the wander should be less than or equal to the length of the fibre required in the device.

An alternative embodiment of the Fig. 4A to 4D. It may be fabricated in much the same way as the preferred embodiment except that instead of V- grooves, a helical or spiral groove 21 is created in the preform 22 as illustrated in Fig. 5, of sufficient width and depth that the displacement of the core 10 of the fibre drawn form this preform describes a corkscrew pattern along the length of the fibre, the resulting core displacement in the pulled fibre is illustrated in the succession of figures from Fig. 4A to 4D. In this embodiment, the core displacement may be characterized as angular as opposed to radial.

A second alternative embodiment of the invention is shown in Fig. 6A to 6C. Again, it may be fabricated in much the same way as the preferred embodiment except that the preform 24, shown in Fig. 7A and Fig. 7B, is first polished to provide a shape having a substantially rectangular cross-section. V-groves 26 are then formed into opposing sides of the preform, and the fibre is drawn.

The fibre, according to the invention, may be used as an optical gain medium for fibre amplifiers or fibre lasers. In the former, the pump radiation from, for example a large area stripe diode is launched into the inner cladding with high coupling efficiency and subsequently absorbed in the doped core creating a high gain environment at a wavelength characteristic of the active dopant. Radiation at the signal wavelength is launched into the core and is amplified during a single pass through the fibre.

In the case of a fibre laser, reflective media at each end of the fibre provide optical feedback thereby creating a resonant cavity. Lasing is achieved in the doped core when the intensity of the pump radiation creates sufficient gain to overcome the intrinsic loss of the cavity The reflective media may consist of, for example, bulk dielectric mirrors abutted directly against the end facets of the fibre, or fused-tapered wavelength de-multiplexers (WDMs) or reflective Bragg gratings.

In both devices the numerical aperture (NA) of the inner cladding should be substantially in the range 0.2 - 0.4 to allow efficient coupling to high power, broad stripe laser diode pump sources, which typically have high NA's. The geometry and dimensions of the inner cladding may also be matched to that of the pump source to further maximize coupling efficiency. The diameter of the core should be chosen to be as large as possible, within the fibre design restrictions necessary for substantially single mode behavior in the core, to provide a large an area as possible to intercept radiation propagating along the inner cladding. Fiber, according to the invention, is also advantageous for fibre lasers incorporating double-doped double-clad fibre as described in U.S. Patent No. 5,291,501 to Hanna. In this fibre design, both the core and the inner cladding are doped with active lasant material, different from each other. Appropriate pump radiation is launched into the inner cladding and lasing is induced in the inner cladding. This lasing emission then serves as the pump radiation for the optical fibre core which then also lases. The wandering core configuration of the inventive fibre would greatly enhance the transfer of radiation from the inner cladding to the fibre core and thus improve the device performance. It will be appreciated from the foregoing that optical fibre according to the invention provides improved efficiency in coupling multimode radiation into a single mode core. It will also be appreciated that variations to the preferred and alternative embodiments described herein may be practiced without departing from the scope of the invention.

Claims

CLAIMS What is claimed is:

1. An optical fibre comprising a substantially single mode core doped wit an active element and having at least one cladding within wliich said core i embedded, the position of said core within said cladding relative to a centra region of said cladding varies continuous along at least a portion of the length o the fibre for increasing the efficiency of coupling radiation out of said claddin into said core.

2. The optical fibre of claim 1 wherein said core position varies transversel relative to said cladding central region.

3. The optical fibre of claim 2 wherein said core position varies radiall relative to said cladding central region.

4. The optical fibre of claim 2 wherein said core position varies angularl about said cladding central region.

5. The optical fibre of claim 1 wherein said core position varies sinusoidall relative to said cladding central region along said fibre portion.

6. The optical fibre of claim 1 wherein said positional variation of said cor within inner cladding is periodic along said fibre portion.

7. The optical fibre of claim 1 wherein said fibre has a circular cross sectional configuration.

8. The optical fibre of claim 1 wherein said fibre has a four-sided cross sectional configuration.

9. The optical fibre of claim 8 wherein said four-sided configuration is rectangular.

10. The optical fibre of claim 1 wherein said fibre is formed from a pulled perform of the fibre having grooves formed in at least a portion of the surface o said fibre perform.

11. The optical fibre of claim 10 wherein said grooves are formed at least o opposite sides of said fibre preform after which said fibre preform is pulled.

12. The optical fibre of claim 11 wherein said grooves are V-shaped.

13. The optical fibre of claim 10 wherein said grooves are formed as a helical groove along said fibre portion after which said fibre preform is pulled.

14. The optical fibre of claim 13 wherein said grooves are V-shaped.

15. The optical fibre of claim 10 wherein the ratio of cross sectional area of said cladding to said core is in the range of about 12 to 400:1.

16. The optical fibre of claim 1 wherein said core is doped with a rare-earth species.

17. The optical fibre of claim 16 wherein said core is co-doped a plurahty of rare-earth species.

18. The optical fibre of claim 1 wherein at least one Bragg grating is formed in said fibre core.

19. The optical fibre of claim 1 wherein the fibre is utilized as an optical gain medium.

20. The optical fibre of claim 19 wherein said optical gain medium is fibre laser or fibre amplifier.

21. The optical fibre of claim 1 wherein said core position varies radially relative to said cladding central region, the magnitude of variation, 1^., of said core position along said fibre portion is given by:

where x is the ratio ofthe outer to inner core areas, N is the total number density of active element particles, P(0) is the launched pump power and P_s is the saturation power.

22. The optical fibre of claim 21 wherein P_s is given by: hv A

P =

(σ_ep + σ_ap)τ where σ^ and σ_ap are the pump emission and pump absorption cross-sections, respectively, τ is the upper level lifetime, A is the area of said cladding and hv_p is the photon energy of the pump radiation.

23. The optical fibre of claim 1 wherein said fibre is a double clad fibre comprising a substantially single mode core having a refractive index n_α embedded in a multimode inner cladding having a refractive index n₂ embedded in an outer cladding having a refractive index n₃ where n₃<n₂<n

24. A method of fabricating an optical fibre having a body with core embedded within a cladding wherein the position of the core within the cladding relative to a central region of the cladding varies continuous along the length of the fibre, comprising the steps of: forming a fibre preform having grooves along at least a portion of its body; and pulling the fibre preform under elevated temperature conditions to bring about said positional continuous variation.

25. The method of claim 24 wherein the step of forming the grooves comprises the step of forming a plurality of grooves along opposing sides of the fibre body.

26. The method of claim 25 wherein the grooves are V-shaped.

27. The method of claim 24 wherein the step of forming the grooves comprises the step of forming a helical groove along the fibre body.

28. The method of claim 27 wherein the helical groove is V-shaped.

29. The method of claim 24 wherein said the ratio of cross sectional area of the cladding to the core is in the range of about 12 to 400:1.

30. The method of claim 24 comprising the further step of applying a further cladding over the pulled fibre body.

31. The method of claim 24 wherein said fibre is a double clad fibre comprising a substantially single mode core having a refractive index n_x embedded in a multimode inner cladding having a refractive index n₂ embedded in an outer cladding having a refractive index n₃ where n₃<n₂<n₁.

32. The method of increasing the coupling efficiency between a core and a cladding surrounding the core of an optical fibre body comprising the step of forming the fibre so that the positional relationship of the fibre core relative a central region of the cladding varies continuously along at least a portion of the length of the fibre.

33. The method of claim 32 wherein the step of forming the continuous variance in positional relationship is accomplished by the step of pulling a preform of the fibre body.

34. The method of claim 33 wherein the step of pulling the fibre preform is preceded by the step of forming grooves in at least portions of the fibre body portion.

35. The method of claim 34 wherein the step of forming the grooves comprises the step of forming a plurality of grooves along opposing sides of the fibre body portion.

36. The method of claim 35 wherein said grooves are V-shaped.

37. The method of claim 34 wherein the step of forming the grooves comprises the step of forming a helical groove along the fibre body portion.

38. The method of claim 37 wherein said groove is V-shaped.