Optical Fibres With Depressed Claddings For Suppression of Coupling into Cladding Modes in Fibre Bragg Gratings

A new method for suppression of coupling from guided optical modes into cladding modes in an optical fibre Bragg grating using a fibre with a strongly depressed cladding is proposed. Strong suppression of the coupling has been demonstrated both theoretically and experimentally. Optical fibre Bragg gratings have attracted much attention in recent years due to their numerous applications such as reflectors for fibre lasers, filters, and sensors. They can be used to build add/drop filters for use in wavelength-division-multiplexing (WDM) systems [l]. Chirped gratings are also used for dispersion compensation in optical fibre links and for pulse shaping [2]. In a fibre grating with a photosensitive core, the guided LPO1 mode, however, does not only reflect into the LPO1 mode itself but also into cladding modes and radiation modes which are eventually absorbed by the high refractive polymer coating. This coupling causes a series of loss bands at the short wavelength side of the main Bragg band. This loss can be quite severe in strong gratings (more than 10 dB has been measured in some gratings) and restricts the use of these gratings in a WDM system. One proposed method to counter this problem is based on suppression of the normalised refractive index modulation for this coupling by having a uniform photosensitive region across the cross section plane of the optical fibre [3]. From the orthogonality principle of the modes, overlap of the modal fields and the grating index modulation would be zero in this case. The LPol mode will therefore not couple into any of the cladding modes. Since the LPO1 mode only has field distribution over the core and the part of the cladding immediately next to the core, it is usually sufficient to have only this part of the optical fibre photosensitive. Although it is possible to introduce a photosensitive cladding around a photosensitive core, it is, however, very difficult to make the same photosensitivity over both cladding and core. Even if such a fibre could be made, writing a grating with a uniform refractive index change over the whole photosensitive area would be difficult because the writing beam is strongly attenuated as it penetrates into the thick photosensitive region. The second proposed method is to use a high NA fibre [4]. The use of the high NA fibre increases the gap between the main grating band and the next cladding mode coupling band, so it leaves a useful operation band. However, such a band is only -7 nm wide in a high NA fibre (0.25) and this is much less than what is desired in most applications. In our proposed method, a depressed cladding is added between the photosensitive core and the normal cladding. Such a depressed cladding is very effective in reducing the cladding mode field strength over the core region of the optical fibre and therefore reduces the coupling strength between the guided mode to the cladding modes. By introducing a depressed cladding with appropriate index and thickness, substantial suppression of the coupling into the cladding modes can be achieved. This method can also be combined with the photosensitive cladding method to achieve a further suppression of the coupling. Most of the state of the art technology for silica optical fibre manufacture is based on a chemical vapour deposition process which allows a depressed cladding to be easily introduced by doping silica with fluorine or boron. The effective index modulation for coupling between the guided LPO1 and LP,, modes serves as a good measure of the coupling strength, where An(r,cp) is the refractive index modulation of the grating, and ~or(r,~) and ymn(vp) are the respectively. normalised optical field distributions of the LPO1 mode and LP,, mode We studied a fibre which has a step index profile and a core with uniform photosensitivity for simplicity. In practice, an optical fibre does not have a step index profile and An(r,cp) is not constant over the core. This, however, will only introduce a quantitative error in the analysis and does not affect the general conclusion. Since An(r,cp) is uniform over the fibre core, i.e. An(r,cp) = An when rep, An(r,cp) = 0 elsewhere, where p is the core radius, it can thus be taken out of the integral in equation (1). We are comparing coupling strength between modes with the same grating strength, i.e. the same An. We can therefore drop An out of equation (1) and just use the remaining integration part in our analysis, which becomes This is the normalised overlap integral of the two modes over the core region. Two sets of modes, LPO” and LPI, modes, are studied in our analysis. vo”(r,cp) has a radial symmetry and does not depend on cp, i.e. v(r,cp) = w(r), however v,“(r,cp) is radially asymmetric and changes sign when cp changes by 180°, i.e. v(r,cp) = -v(r,cp+x). Therefore, for the LPI, modes, NOI in equation (2) equals to zero. There is, therefore, no coupling between the LPO, mode and the LPI, modes in a grating with a uniform index modulation over the core. If a grating is blazed or has a non-uniform refractive index change over the core, coupling between the LPO, mode and the LPI, modes can occur. We consider the worst case in our analysis where An(r,cp) = -An(r,cptx), hence the product An(r,cp) v(r,cp) in equation (1) will not change sign as cp changes. This is equivalent to replacing vIn(r,q) with only the positive part of vl”(r,cp) in equation (2). In the analysis, we used the following parameters for the fibre core: a core radius of 2.8 pm, and a core refractive index of 0.01 above that of the silica cladding. We have also assumed that air with a refractive index of 1 surrounds the optical fibre. Figure 1 shows the NOI of the LPO1 mode and the first nine modes of the LPon modes over the core region at different depressed index depths of the depressed cladding. The thickness of the depressed cladding used is 13.4 pm. As a depressed cladding with increasing depressed index depth is introduced, the guided LPO1 mode is more confined to the core and the increase of the field strength of the LPO1 mode over the core leads to an increase in the NOI between the LPO, mode and the LPO, mode over the core. As expected from the reduction of the field strength of all the other LPm modes over the core, the NOI of the LPO, mode with all the other LPon modes over the core decreases with an increase in the depressed index depth. For the higher order LPon modes, e.g. the LPoa and LP 09 modes, the decrease is only significant if a depressed index depth of more than 0.003 is used. In general, a suppression of more than 20 dB can be achieved for the LPon cladding modes in figure 1 if a depressed depth of more than 0.01 is used. Figure 2 shows the NOI of the LPO1 mode and the first nine modes of the LPI, modes over the core at different depressed index depths of the depressed cladding. For the lower order LPI, modes! the NOI over the core decreases as a depressed cladding with an increasing depressed depth is introduced. For the higher order LPI, modes, e.g. the LP17, LPI8 and LPI9 modes, an increase of NOI over the core is seen with an increase in the depressed index depth when the depressed index depth is small [5]. However a reduction of NOI over the core for all the LPI, modes in figure 2 can be obtained when a depressed index depth of more than 0.008 is used. A near 20 dE3 suppression for all the modes in figure 2 can be achieved with a depressed index depth of more than 0.012. In general, the coupling into the LPI” modes can be suppressed by reducing the blaze angle of the grating and having a uniform refractive index modulation over the core. These methods, however, do not suppress the coupling from the LPO, mode into the LPO” modes. Figure 3 shows the NOI over the core between the LPO, mode and the first nine modes of the LPO” modes at different thickness of the depressed cladding. A depressed index depth of 0.012 is used for this analysis. The NOI between the guided LPO, mode and the LPO1 mode over the core increases slightly with an increase in the thickness of the depressed cladding when the thickness is small and remains more or less constant when the thickness is large. The NOls between the LPO1 mode and all the other LPO” modes in figure 3 decrease with an increase in the thickness of the depressed cladding. The decrease in NOI for the higher order modes of the LPO” modes, however, slows down at larger cladding thickness. Figure 4 shows the NOI of the LPO, mode and the first nine modes of the LPI, modes over the core at different thickness of the depressed cladding. For the lower order LPI” modes, the NOI over the core decreases with an increase in the thickness of the cladding. For the higher order LPI, modes , e.g. LPI8 and LPI9 modes, an increase of NOI over the core is seen with an increase in the thickness of the cladding when the thickness is large. This shows the existence of an optimum cladding thickness for the suppression of these LPI” modes. In this case, the optimum thickness is -12 ,urn where a suppression of more than 10 dB is obtained for the LP19 mode. We have fabricated an optical fibre with a depressed cladding to demonstrate the suppression of the coupling from the guided modes into cladding modes. The fibre has a core radius of 2.8 ,um, an effective index difference of 0.01 between the core index and the index of the silica cladding, a depressed index depth of 0.009 and a depressed cladding thickness of 13.4 pm. The depressed cladding was achieved by boron-doping and the core was doped with germanium. The fibre has a LPI1 mode cut-off of 1.4 urn. Figure 5 shows the transmission spectrum of a 15 mm long grating with a refractive index modulation of 0.001 in (a) a step index fibre with a photosensitive core with a NA of 0.15 and radius of 3.98 pm, and (b) the fibre fabricated with a depressed cladding. The strong suppression of the coupling of the guided LPO, mode into cladding modes in the fibre with a depressed cladding can be seen in figure 6b. The small peaks in figure 6 b are due to coupling into higher order LPI, modes by a blaze in the grating. To conclude, we have demonstrated an effective method for suppression of coupling from a guided mode to cladding modes in a fibre grating. Although an optical fibre having only a photosensitive core is used, an optical fibre with both photosensitive core and cladding will offer further suppression of the coupling from guided modes into cladding modes.