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Forum: Processes Involving Optical Photons
Re: Question Photon Trapping Fraction (matthew)
Date: 02 Mar, 2016
From: Gumplinger Peter <Gumplinger Peter>

Hi Mat,

> I'm trying to model an optical fiber with core_rindex = 1.46 and
> cladding_rindex = 1.39. This combination should produce an optical
> trapping fraction of ~ 2.4%.

What trapping fraction do you get if you don't define an G4OpticalSurface?

> In order to reproduce this spec, I've added an G4OpticalSurface between
> the cladding and the surrounding air using the following code:
> 
> // Clad-Air boundary
>     G4OpticalSurface * OpCladAirSurface = new G4OpticalSurface("CladAirSurface");
> 
>     OpCladAirSurface -> SetModel(unified);
>     OpCladAirSurface -> SetType(dielectric_dielectric);
>     OpCladAirSurface -> SetFinish(ground);
> 
>     G4LogicalBorderSurface* CladAirSurface = new G4LogicalBorderSurface("CladAirSurface",clad_phys,world_phys,OpCladAirSurface);
> 
>     const G4int NUM = 2;
>     G4double pp[NUM] = {2.038*eV, 4.144*eV};
>     G4double reflectivity[NUM] = {0.15, 0.15};
>     G4double efficiency[NUM] = {0.5, 0.7};
> 
>     G4MaterialPropertiesTable *SMPT3 = new G4MaterialPropertiesTable();
> 
>     SMPT3 -> AddProperty("REFLECTIVITY",pp,reflectivity,NUM);
>     SMPT3 -> AddProperty("EFFICIENCY",pp,efficiency,NUM);
>     OpCladAirSurface -> SetMaterialPropertiesTable(SMPT3);

It is rather unusual for you to define a "REFLECTIVITY" for a dielectric_dielectric surface type. Defining an "EFFICIENCY" certainly makes no sense for the fibers outside surface.

> My question is: Why does changing the values of reflectivity and
> efficiency have almost no effect on the number of photons I detect at
> the end of the fiber?

Please, take a careful look at Figure 5.1. Diagram of the UNIFIED Model for Optical Surfaces (courtesy A. Shankar) in the Application Developers Manual.

Your "REFLECTIVITY" is actually in this case 1 minus the absorption coefficent.

In any case, I suggest you do not specify the reflectivity and let Snell's Law determine the reflection etc. for a perfectly smooth fiber - at first. If that does not give you the expected result then you can play with the surface roughness of the fiber (for which you'll need a G4OpticalSurface) to see how it changes your result.

Peter

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1 None: Re: Photon Trapping Fraction   (matthew - 02 Mar, 2016)
(_ Feedback: Re: Photon Trapping Fraction   (Gumplinger Peter - 02 Mar, 2016)
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