strongly guided fiber?
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Posted by psvanvic on May 1, 2007, 2:14 pm
Please log in for more thread options came up in doing my homework on standard step index smf fiber. Many of the texts I have referenced and done the standard weakly guided solutions for fibers all use the "weakly guided assumption" and the dispersion relationship to search for modes... What happens if a fiber is strongly guided? I know the modeling breaks down for that type of solution technique...but what happens in reality with a strongly guided fiber? Why are they not desirable in industry..for telecom etc? I am a mechanical engineer doing stress sensor research...so I didn't have an EM class. Any help would be appreciated..this is just a question that I have, not really related to my research. Thanks, Pat. | ||||||||||||||||||||||
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Posted by Phil Hobbs on May 1, 2007, 2:34 pm
Please log in for more thread options The long answer: In any lossless optical system, the product of the area and projected solid angle of any single mode is always lambda**2/2. Stronger guiding equals a wider angular acceptance, which increases the projected solid angle. To keep a fibre single-mode with stronger guiding, you have to decrease the core diameter. This has the following disadvantages: 1. Tighter mechanical tolerances--if you halve the diameter, you have to align twice as accurately. 2. Higher losses at splices--you can't readily make the cladding index very much lower than that of fused silica (~1.46), so you have to increase the core index, which leads to higher reflection losses. 3. Higher losses due to microbending--roughness at the core-cladding boundary causes much higher scattering when the index discontinuity is large and the field strength is high, as in small-diameter, high-index fibre. All of these things increase the cost of transmission systems, which is why very weak guiding is used. Strong guiding is good for lots of things too: 1. More light per unit area--in a multimode fibre, you can pack more modes (and hence more light from an incoherent source) into the same fibre diameter with stronger guiding. 2. Lower bending loss--stronger guiding means you can turn tighter corners without losing light. Silicon-core waveguides used in optical ICs (at 1.55 micron wavelength) are about 0.45 microns wide and 0.2 microns tall, and you can make a 2-um radius bend with low loss. For fibre it isn't usually that big a deal, because bend radii are also limited by mean time to fracture (especially if the fibre can become wet). 3. Stronger interaction with perturbations, e.g. in sensors. This is the flip side of the increased microbend loss. Sometimes you _want_ increased sensitivity to what's going on at the surface. The modelling thing can be fixed up to work with strongish guiding--though it gets much harder with silicon guides. Cheers, Phil Hobbs Strongly guided fibre is good for multimode, because you can get more light into the fibre--more modes for a given diameter. | ||||||||||||||||||||||
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Posted by Phil Hobbs on May 1, 2007, 2:51 pm
Please log in for more thread options Phil Hobbs wrote:
>
solid angle of any single mode is always lambda**2/2. Stronger guiding
> In any lossless optical system, the product of the area and projected equals a wider angular acceptance, which increases the projected solid angle. Minor correction: the invariant is n**2*A*Omega-prime. Forgot the n-squared. Cheers, Phil Hobbs | ||||||||||||||||||||||
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Posted by mechpat on May 1, 2007, 3:22 pm
Please log in for more thread options Thanks for a great explanation.
I have one theoretical question then. If you could dope the clad with Fluorine or something that could drop the index a bunch, keep the core the same size...you could have a strongly guided fiber without most of the problems you mentioned though right? Thanks, Pat. Phil Hobbs wrote: > Phil Hobbs wrote:
> >
> > In any lossless optical system, the product of the area and projected > solid angle of any single mode is always lambda**2/2. Stronger guiding
> equals a wider angular acceptance, which increases the projected solid > angle. > > Minor correction: the invariant is n**2*A*Omega-prime. Forgot the > n-squared. > > Cheers, > > Phil Hobbs | ||||||||||||||||||||||
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Posted by Phil Hobbs on May 1, 2007, 8:13 pm
Please log in for more thread options mechpat wrote:
> Thanks for a great explanation.
> > I have one theoretical question then. If you could dope the clad with > Fluorine or something that could drop the index a bunch, keep the core > the same size...you could have a strongly guided fiber without most of > the problems you mentioned though right? > The lowest-index material that isn't water soluble is magnesium fluoride, with N=1.38 or so. Thus you can't really go that low by this method. You'd win a little bit that way, since the microbending loss wouldn't be quite as bad, due to the larger diameter that this method could afford. The index contrast at the core-cladding boundary would still be large, though, so the scattering would still be strong. Cheers, Phil Hobbs | ||||||||||||||||||||||
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> came up in doing my homework on standard step index smf fiber. Many
> of the texts I have referenced and done the standard weakly guided
> solutions for fibers all use the "weakly guided assumption" and the
> dispersion relationship to search for modes...
>
> What happens if a fiber is strongly guided? I know the modeling
> breaks down for that type of solution technique...but what happens in
> reality with a strongly guided fiber? Why are they not desirable in
> industry..for telecom etc?
>
> I am a mechanical engineer doing stress sensor research...so I didn't
> have an EM class. Any help would be appreciated..this is just a
> question that I have, not really related to my research.
>
> Thanks,
>
> Pat.
>