cannot obtain the same transmission spectrum as that of reported paper

[Lizzie Li]

Hi,

I want to simulate the transmission spectrum reported in T. Erdogan’s paper [1] by using OptiGrating. After setting the same parameters as the reference paper, however, the figure obtained using OptiGrating shows a big difference with the corresponding figure in the paper.

I’m wondering where it is wrong?

Reference
[1] Turan Erdogan, Cladding-mode resonances in short-and long-period fiber grating filters, JOSA A, Vol. 14, Issue 8, pp. 1760-1773 (1997)

[Damian Marek]

Which figure are you trying to reproduce?

Regards

[Lizzie Li]

Hi, Marek
I want to obtain the Fig. 10(b) in the reference paper using OptiFiber. But the simulated transmission spectrum shows a big difference with Fig. 10(b).

Thanks.

[Steve Dods]

I tried to reproduce results from this paper. I got stuck at equation (28), where the grating is defined. The problem I had with (28) is the parameter “m”, the induced-index fringe modulation. The paper says it is 0 <= m <= 1.0, but I could not find the value for this parameter that is actually used in the paper. Its value will certainly influence the results. You cannot reproduce Fig. 10 b without it. But I could not find its value specified anywhere.

[Lizzie Li]

Thanks Steve,

According to the paper reported on the LPFGs, the default value of parameter “m” is 1 in the literature. In addition, I do not know where the parameter “m” can be defined in the OptiGrating.

[Steve Dods]

The parameter “m” is not defined in OptiGrating. However, you can define it yourself in the User Defined option of the Grating Shape function. To get m = 1 you can write 1 + cos(2*pi*x/Period) in the definition box. However, I wasn’t able to reproduce Fig. 10 either. The description specifies some of the grating parameters, but it isn’t clear what the fibre parameters are. I tried using the parameters in Section 2 of the paper. Then used that fibre to calculate what the grating period should be to get a resonance at 1.55 um from an input wave in LP(0,1). The paper says the period is 570 um. I don’t find that to be the grating period for coupling to any of the cladding modes, so it is clear the fibre of Fig. 10 is not the same as that in Section 2. I can’t determine what fibre was used for Fig. 10.

[Lizzie Li]

Hi, Steve

I have reproduced Fig. 10 by using matlab based on the numerical method reported by E. Anemogiannis, et al. The fiber parameters are: core radius 2.5μm, cladding radius 62.5μm, core refractive index 1.4573, cladding refractive index 1.45, index modulation amplitude 3.6e-4, grating length 25mm and grating period 570μm. However, the transmission spectrum obtained by using the formulas provided by T. Erdogan presents a large wavelength shift if compared with Fig. 10 (see the figure attached below). Using OpitGrating still cannot get any resonances.

After checking the document of OptiGrating, I think the definition of the grating index profile (see page 26) is very different from that of the paper reported by T. Erdogan, which may be the reason why OptiGrating cannot get the same transmission spectrum as Fig. 10. However, I don’t know how to define the same index profile in OptiGrating as that of Fig.10.

Thanks.

Appendix
grating index profile defined in OptiGrating (page 26): n(x,y,z)=n0(x,y)+Δn0(x,y,z)+Δn⋅P(x,y)⋅A(z)⋅f[Λ(z)⁄cosθ,z]

References
E. Anemogiannis, E. N. Glytsis, and T. K. Gaylord. Journal of Lightwave Technology, Vol. 21, Issue 1, 218-227 (2003)

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