WO2003010576A1

WO2003010576A1 - Fibre optical grating

Info

Publication number: WO2003010576A1
Application number: PCT/SE2002/000941
Authority: WO
Inventors: Sten Helmfried; Bengt Sahlgren
Original assignee: Proximion Fiber Optics Ab
Priority date: 2001-07-25
Filing date: 2002-05-17
Publication date: 2003-02-06
Also published as: SE0102613D0

Abstract

This invention relates to a fibre optical grating for use in an optical communication system, said fibre optical grating comprising an optical fibre having a core region (1, 11), extending along a longitudinal axis of said optical fibre, and a cladding region (1, 12), essentially encompassing said core region (1, 11) along said fibre, whereby said optical fibre core region (1, 11) at least partly is manufactured from a photo-sensitive material. Furthermore, a plurality of channels (3a, 3b; 13) is defined in the photosensitive region of the fibre, said channels extending along said longitudinal axis of the fibre. This invention also relates to an optical fibre being usable for production of a fibre optical grating as described above.

Description

FIBRE OPTICAL GRATING

Field of the invention

This invention relates to a fibre optical grating comprising an optical fibre having a core region, extending along a longitudinal axis of said optical fibre, and a cladding region, essentially encompassing said core region along said fibre, whereby said optical fibre core region at least partly is manufactured from a photosensitive material.

This invention also relates to an optical fibre being usable for production of a fibre optical grating as described above .

Background of the invention

The need for capacity in optical communication net- works is currently increasing rapidly. One widespread method of achieving increased transfer capacity in existing optical fibre networks is wavelength division multiplexing (WDM) . In WDM, a single optical fibre is used for the transmission of several channels, each channel having being associated with a specific wavelength.

Due to the current development within the field of WDM, new optical network devices are constructed. In many of those devices wavelength selective fibre-optical gratings play an important role. A fibre-optical grating of this kind usually comprises a core and a cladding, together forming an optical fibre. The material is a photosensitive material, whereby a grating is formed in the fibre by exposing the fibre to radiation that is periodically modulated. As the radiation is absorbed by the pho- tosensitive material of the fibre, a permanent, dose- independent refraction index increase is induced in the fibre . The transmission spectrum in a uniform fibre grating is theoretically a narrow dip around the so-called Bragg wavelength, being defined by:

where n is the average of the refractive index and Λ is the grating period of the fibre optical grating.

In practice there are usually a few side lobes around the main resonance that may be do to abrupt termination of the grating envelope. These side lobes are supress- able, by smoothing the profile of the fibre grating. This process is also referred to as apodisation. However, on the short side of the Bragg wavelength, there are addi- tional resonances at a distance of a few nanometers that may not be reduced or suppressed by the above-mentioned method. These are induced by coupling to so-called cladding modes, and results in an over-all power loss in the fibre. These cladding mode losses may cause a severe deg- radation of the performance in many different WDM-based devices, since the main resonance for one channel may overlap with a cladding resonance resulting from another channel. Consequently, there is a need to further suppress the formation of such cladding modes, in order to achieve a better performance in WDM applications.

In order to obtain a good coupling between different modes, two conditions need to be fulfilled. First, the phase of the light that is coupled from a first mode must match the phase of a second mode, propagating along the interaction path. In other cases, interference between them will be destructive. This is referred to as a longitudinal phase matching condition. Secondly, the transverse profile of the distortion that induces coupling - i.e. in our case the profile of the refractive-index shift of the grating multiplied by the amplitude distribution of the exciting mode- must match the profile of the mode to which it is coupled. This is referred to as a transverse phase-matching condition. If the coupling is induced by a transversally uniform index shift, a mode can only couple to the corresponding counter-propagating mode, as all other co- and counter-propagating modes are orthogonal and have zero overlap.

Currently, two types of fibres, in which the influence of cladding modes has been reduced, are available, namely cladding-mode suppressed fibres and cladding-mode offset fibres. In cladding-mode offset fibres, the longitudinal phase matching condition is manipulated. The longitudinal phase matching in fibre gratings is best illustrated in a vector diagram, see fig la and lb. The magnitude of the wave vector k of an optical mode corresponds to the phase shift per unit length when the mode is propagating. The wave vector k is proportional to the so-called effective index of the mode and inversely proportional to the wavelength of the propagating light. In fig la and lb, the wave vector for the input mode is denoted ki_n and the wave vector for the reflected mode is denoted k_out • At the

Bragg wavelength, where phase matching occurs and the coupling is at a maximum, the wave vector of the reflected mode is equal to the vector sum of the wave vectors of the incoming mode and of the grating. The grating wave vector K is inversely proportional to the grating pitch Λ. If the incoming light is detuned to the shorter wavelengths, the vector sum will not add up. However, the cladding modes experience a lower effective index than ^"the guided modes, and it is, therefore, always possible to find a cladding mode that is longitudinally phase matched. The basic idea in a cladding-mode offset fibre is to use a small core, and have a high index step between the core and the cladding. This enhances the effective index of the guided mode. Since the cladding modes always experience a lower effective index than the actual refractive index in the cladding, the effective-index difference becomes larger than in a standard photosensi- tive optical fibre and the detuning from the Bragg wavelength for longitudinal phase-matching becomes bigger. In this way, it is possible to push the lossy region away from the main resonance. The resulting shift may be as large as 10 nm, or even larger.

However, in some applications, it may not be enough to shift the lossy region to shorter wavelengths, since the spectral region of interest for the application is too large. In those cases, cladding-mode suppressed fi- bres may be utilised. In this kind of optical fibre, the transverse phase-matching condition is manipulated in order to completely eliminate the coupling to cladding modes. In a standard grating fibre, only the core is made of a photosensitive material. Consequently, the product between the profile of the exciting mode and the index shift in the grating phase fronts is proportional to the mode profile inside the core, but drops abruptly to zero in the cladding, as shown in fig 2a. Hence, the transverse match to the backward propagating guided mode is not perfect, and coupling to other modes is also possible. If the photosensitive area of the fibre is extended slightly outside the core, so that the phase fronts are wider than the propagating mode, as seen in fig 2b, the index shift in the grating may be regarded as constant in the transverse direction, and the matching to the backward propagating mode is perfect. Since all other modes are orthogonal, there is no coupling to other cladding modes .

However, the dopant, or dopants, that is added in or- der to provide the cladding with photosensitive properties, also affect the refractive index of the cladding. Consequently, in order to obtain the same guiding properties of the fibre, it is necessary to further add index- depressing dopants in the doped cladding region. Hence, the chemical composition of the core and the photosensitive part of the cladding are never identical. At the same time, it is necessary that the photosensitivity of the core and the cladding are identical, in order to maintain a good cladding-mode suppression. In practice, this is problematic to achieve, in particular as the saturation properties of the dopants may vary. Conse- quently, if the suppression is ideal for one particular grating strength, it may be inadequate for lower and/or higher exposure .

Therefore, an object of this invention is consequently to achieve an optical fibre grating having a sup- pressed index in the cladding, without altering the chemical composition of the cladding material.

Yet another object of the invention is to achieve a fibre grating, having a suppressed index structure, being generated by an alternative method of production, whereby the use of multiple dopant fibres is avoided.

Summary of the invention

These and other objects are achieved in accordance with the invention by a fibre optical grating as de- scribed by way of introduction, being characterised in that a plurality of channels is defined in the photosensitive region of the fibre, said channels extending along said longitudinal axis of the fibre. Thereby, coupling to cladding-modes, having wavelengths shorter than the Bragg wavelength, may be suppressed.

Suitably, a mode propagating wave-guide is defined in a central region of said core region, said plurality of channels being provided outside said central region. Consequently, a mode being fed into the central region of the fibre will propagate along the fibre within this region, only a tail of said mode propagating within the channel-provided region of the fibre.

Said channels are suitably filled with a material having a lower refractive index than the surrounding op- tical fibre material, and preferably said channels are filled with air, having a comparatively low refractive index. In accordance with a preferred embodiment of this invention, said channels are essentially regularly distributed over the entire cross-section of the fibre, i.e. in both the core and cladding regions of said optical fibre, with an exception for said central region. Thereby, by choosing the transverse distribution of said channels in a certain way, only a guided mode that is confined by the photonic bandgap effect can propagate.

In accordance with a second embodiment of this inven- tion, said plurality of channels essentially is arranged within said photosensitive core region. Preferably, said channels are densely distributed in a radially extending cylinder, extending longitudinally with the fibre axis, in the outer part of the photosensitive region, thereby defining a waveguide structure in the centre of the optical fibre, in which an optical mode may propagate by means of mode guiding .

Further, said fibre optical grating preferably comprises a chirped Bragg grating, in order to attain multi- pie wavelength reflection in said fibre optical grating.

The above mentioned objects are also achieved by an optical fibre, said fibre having a core region, extending along a longitudinal axis of said optical fibre, and a cladding region, essentially encompassing said core re- gion along said fibre, whereby said optical fibre core region at least partly is manufactured from a photosensitive material, whereby a plurality of channels is defined in the photosensitive region of the fibre, said channels extending along said longitudinal axis of the fibre, said optical fibre being usable for production of a fibre optical grating as described above.

Brief description of the drawings

For exemplifying purpose, the invention will herein- after be described in closer detail, with reference to embodiments thereof illustrated in the attached drawings, wherein: Fig la shows a wave vector diagram in accordance with the prior art for longitudinal phase matching between counter-propagating waves in a fibre grating.

Fig lb shows a wave vector diagram in accordance with the prior art for cladding-mode coupling at wavelengths shorter than the Bragg wavelength.

Fig 2a shows a transverse profile of counter- propagating modes in a standard fibre grating in accordance with prior art . Fig 2b shows coupling in a fibre grating, written in a cladding-mode suppressed fibre in accordance with the prior art .

Fig 3 shows a cross-section of a photonic bandgap fibre in accordance with the invention, said fibre having a photosensitive core region.

Fig 4 shows an alternative embodiment of the invention, being a cross-section of a fibre having an inner cladding being defined by densely distributed small channels .

Detailed description of preferred embodiments

A fibre grating, as described by way of introduction, basically comprises an certain length of optical fibre, having a core 1, 11 and a cladding region 2, 12. The core is manufactured from a photosensitive material, e.g. a silica material being doped with germanium. The cladding is for example manufactured from an undoped silica material .

The basic idea of the present invention is to sup- press the refractive index in the cladding, without altering the chemical composition of the material. In accordance with the invention, as seen in fig 3 and 4 this is achieved by defining a wave guide in the centre of the fibre by arranging a plurality of channels in the fibre, said channels extending along the longitudinal axis of the optical fibre. Furthermore, in accordance with the invention said cladding mode suppression may be accom- pushed in two different ways, having the same fundamental mechanical feature with longitudinal channels.

A first embodiment of the invention is disclosed in fig 3. In this first embodiment the photonic bandgap ef- feet is used for guiding the lowest-order mode along the optical fibre. The fibre is provided with channels, 3a, 3b extending longitudinally along the entire fibre length. The channels are regularly distributed over essentially the entire cross-section of the fibre. However, in a centre area of the fibre, no channels are arranged, in order to provide a central waveguide for mode propagation. The transverse distribution of said channels is chosen in such a way that a guided mode be confined by the photonic bandgap effect. The physical reason for this is know in the prior art, and could be briefly explained as a correspondence to a semiconductor bandgap. One possible design of a photosensitive cladding-mode suppressed photonic bandgap optical fibre is shown in fig 3. This fibre construction, as seen in cross section, comprises a photosensitive core region 1 and a cladding region 2. A plurality of channels 3a, 3b is regularly distributed over the entire cross-section of the fibre, with an exception for the centre of said core region 2, as described above. The channels are filled with a material having a refractive index that is lower than the refractive index of the surrounding material of the fibre. In the present case the fibre is manufactured from silica, and the photosensitive core is doped by germanium, making the material photosensitive as well as inducing a refrac- tive index shift, resulting in a refractive index of the core of about 1.5. The channels are filled with air, having a refractive index of about 1. Since the channels 3a, 3b are regularly distributed over the entire cross- section of the fibre, with an exception for the centre part, a first group of said channels 3a are confined within the photosensitive region in the centre of the fibre. In this way, the mode is more or less completely confined to the photosensitive area, in the case when the mode is fed into the fibre in the area of the channel distribution asymmetry, i.e. in the central core region. Due to the fact that channels are arranged within the photosensitive region of the fibre, a small reduction of the average photosensitivity in this region will occur, due to the absence of photosensitive materials in said channels. However, this reduction is completely independent of the UV-radiation dose applied when manufacturing the fibre-optical grating. Furthermore, an important aspect of the invention is the dimensions of the channels, since the channel distribution as well as their diameter and size have a direct effect on the fibre bandgap properties, since this determines the spatial extension of the optical mode, when propagating in said fibre. Consequently, this must be taken into consideration when designing the optical fibre, but this will not be closer described herein.

A second embodiment of the invention is shown in fig 4. In this embodiment a wave guiding structure is defined in the centre of the fibre, i.e. in the photosensitive core region 11 of the fibre, by arranging a plurality of densely distributed channels 13, extending longitudinally along the entire fibre length. The channels 13 are dis- tributed in the outer part of the photosensitive region 11, as shown in fig 4. Together, said plurality of densely distributed channels 13 are confined within a uniform "ring" or "cylinder" more or less encapsulating the core region of the fibre. By arranging said plurality of channels 13 in the photosensitive region of the fibre, the average refractive index of this region is reduced, due to the following. In the present case the fibre is manufactured from silica, and the photosensitive core is doped by germanium, making the material photosensitive as well as inducing a refractive index shift, resulting in a refractive index of the core of about 1.5. The channels are filled with air, having a refractive index of about 1. The reduction of the photosensitivity due to the removal of doped host material is, however, dose independent, as described above, and furthermore it may be made small. In fact an index reduction of a few tenth of a percent is sufficient for the present application. In this embodiment, there are no requirements on the periodicity of the channel distribution in order to keep the mode intact, since the mode are held together by means of total reflection instead of the above described photonic bandgap effect.

The optical fibre, as described in any of said embodiments, may thereafter be exposed to a periodically modulated UV-radiation or the like, in order to produce a fibre grating, as described in the prior art. The above-described optical fibre may be manufactured on per se known manner, by drilling openings in the fibre preform, before the fibre is drawn in a tower in an optical fibre manufacturing plant .

Fibre optical gratings as described above, may also be incorporated as chirped Bragg gratings, having a varying grating period, thereby enabling reflection of a plurality of different wavelengths, in fact resulting in a broadband reflector.

It shall be noted that, although the present inven- tion is described above with reference to presently preferred embodiments, other embodiments of this invention are possible within the scope and spirit of this invention, as defined by the appended claims. For example, the above-described channels may be filled with a material other than air in order to elaborate with the effective refractive index of the fibre. It might also be possible to fill different channels with different materials, having diverging refractive indexes. It shall also be noted that the invention is equally applicable to fibres not having a sharp transition between its core and cladding, but rather a smooth index transition, in accordance with prior art . Furthermore, although the above-described embodiments are preferred at this time, other configurations of the channels are possible. For example, in fig 3, it is possible to reduce the number of channels in the periphery of the optical fibre. In fact, the channels do only need to be provided to a radial extent such that one is certain that the tail of the signal is insignificant at that radius. Regarding the embodiment shown in fig 4, the "ring" of cylinders encircling the core region, need not be provided in the edge portion of the photosensitive region, as shown in fig 2, but might have a smaller radius, or extend into the cladding region of the optical fibre, depending on desired properties.

Claims

1. A fibre optical grating comprising an optical fibre having a core region (1, 11) , extending along a lon- gitudinal axis of said optical fibre, and a cladding region (1, 12) , essentially encompassing said core region (1, 11) along said fibre, whereby said optical fibre core region (1, 11) at least partly is manufactured from a photo-sensitive material, c h a r a c t e r i s e d in that a plurality of channels (3a, 3b; 13) is defined in the photosensitive region of the fibre, said channels extending along said longitudinal axis of the fibre.

2. A fibre optical grating as in claim 1, wherein a mode propagating wave-guide is defined in a central region of said core region (1,11), said plurality of channels (3a, 3b; 13) being provided outside said central region.

3. A fibre optical grating as in claim 1 or 2 , wherein said channels (3a, 3b; 13) are filled with a material having a lower refractive index than the surrounding optical fibre material .

4. A fibre optical grating as in claim 1, 2 or 3 , wherein said channels (3a, 3b; 13) are filled with air.

5. A fibre optical grating as in any one of claims 1- 4, wherein said channels (3a, 3b) are essentially regu- larly distributed over the entire cross-section of the fibre, i.e. in both the core and cladding regions (1, 2) of said optical fibre, with an exception for said central region.

6. A fibre optical grating as in any one of the claims 1-4, wherein said plurality of channels (13) essentially is arranged within said photosensitive core re- gion ( 11 ) .

7. A fibre optical grating as in claim 6, wherein said channels (13) are densely distributed in a radially extending cylinder, extending longitudinally with the fibre axis, in the outer part of the photosensitive region (11) .

8. A fibre optical grating as in any of the above claims, wherein said fibre optical grating comprises a chirped Bragg grating.

9. A fibre optical grating as described in anyone of the above claims, for use in an optical communication system.

10. An optical fibre, said fibre having a core region (1, 11), extending along a longitudinal axis of said optical fibre, and a cladding region (1, 12) , essentially encompassing said core region (1, 11) along said fibre, whereby said optical fibre core region (1, 11) at least partly is manufactured from a photo-sensitive material, whereby a plurality of channels (3a, 3b; 13) is defined in the photosensitive region of the fibre, said channels extending along said longitudinal axis of the fibre, said optical fibre being usable for production of a fibre optical grating as claimed in any one of the claims 1-9.