U bend loss in POF

[Saptarshi Mitra]

I am trying to simulate a U shape bend in a POF with the hope of gaining an insight into the RI profile, modal field and finally the evanescent field with change in the external media. I plan to use this fiber in a liquid detection sensor. I did a structure with air as the waveguide and the pof as an optical fiber embedded in it. However, I can’t figure out if it is possible to implement the input source only into one end of the fiber. Also when I try to see the ref index profile, I can’t see the fiber RIs at all(this may be due to the 12000*1100*15500 size of the waveguide, but wanted a confirmation). The design file is attached and I am new to optical simulations, so any suggestions are welcome.

[Steve Dods]

The Beam Propagation Method (BPM) is a paraxial method. This means it applies only in the case where there is an optical axis. The U bend is not such a case. On the other hand, accurate results have been demonstrated when BPM is used together with a conformal mapping. (see, for example, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 10, pg 899-909 OCTOBER 2007)

OptiBPM has a conformal mapping region. Please use the conformal mapping region of OptiBPM to calculate the details of optical propagation in a U bend.

[Saptarshi Mitra]

Hi Steven,

I figured that out after posting this. Did it using the conformal mapping. Thanks for your reply though. I actually found one of your old posts with some sample files.:)
On a related note, when I am using the OptiFiber to get the Macrobend losses for the POF, it is not showing me any results for the required wavelength and fiber. Attaching the file for reference.
P.S: Not sure if it is ok to post this in BPM forum, but as you used OptiFiber in your example, I suppose you are the right person to ask.

[Steve Dods]

I’m afraid OptiFiber will not help with this simulation. OptiFiber uses modal analysis, but this method won’t work well for this sample. Your sample fibre has a radius of 980 µm, wavelength .65 µm, core and cladding refractive index 1.49 and .13725. The V number is therefore 5,494. The number of modes for high V number is estimated from 0.5*V^2, which is more than 15 million. I’m afraid that is too many modes! OptiFiber can perform multimode analysis. It will work reliably even if there are dozens of modes, but 15 million is just too many!

You can probably get meaningful results from a propagation kind of simulation, like BPM, but modal analysis is simply the wrong method for this problem.

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