I tried to plot beam intensity using a function that evaluates a numerical integral, but it didn't work.

Here is my code, which did not produce a result.

a1 = 0.6328*10^-6; 
k = (2*π)/a1;
w = 0.02;
c = 0.02; 
m = 1; 
a = 800; 
z = 1000;

M1[x_, r1_, r2_] = 
    Sum[k^2*z^(-2)*BesselJ[m, a *r1]*BesselJ[m, a* r2]*
       BesselJ[b, k* r1* x/z]*BesselJ[b, k* r2* x/z]*
       BesselI[m + b, 2*r1*r2*c^(-2)]*
       Exp[-(c^(-2) + w^(-2))*(r1^2 + r2^2) + (I*k) / 
         (2*z)*(r2^2 - r1^2)]*r1*r2, 
       {r1, 0, ∞}, {r2, 0, ∞}], 
    {b, -∞, ∞}];
a1 = Table[{x, Abs[M1[x]]}, {x, -0.02, 0.02, 0.001}];
ListPlot[a1, PlotRange -> All] TimeUsed[]

Here is how appears in my notebook:

enter image description here

  • $\begingroup$ Welcome on Mathematica.StackExchange. Please always provide copyable code in InputForm. This can be done by: (i) marking the code to copy (ii) right click (iii) selecting "Copy As" -> "Input Text". $\endgroup$ – Henrik Schumacher Jan 12 at 9:04
  • 1
    $\begingroup$ Your first problem is that you define M1[x_,r1_,r2_]= but then you use M1[x]. I am guessing you want to change your function definition to M1[x_]:= so that your r1 and r2 don't disagree with your r1 and r2 variables of integration. $\endgroup$ – Bill Jan 12 at 9:15
  • $\begingroup$ I'm sorry ,I have changed the error. $\endgroup$ – qb.Suo Jan 12 at 12:05

(Not an answer, extended comment.)

Please experiment with your sum, say, like this:

 Block[{b = 1000, s = 100}, 
    Table[k^2*z^(-2)*BesselJ[m, a*r1]*BesselJ[m, a*r2]*
      BesselJ[b, k*r1*x/z]*BesselJ[b, k*r2*x/z]*
      BesselI[m + b, 2*r1*r2*c^(-2)]*
      Exp[-(c^(-2) + w^(-2))*(r1^2 + r2^2) + (I*k)/(2*z)*(r2^2 - 
           r1^2)]*r1*r2, {r1, 0, s}, {r2, 0, s}]]

 (* {0.35228, 0. + 0. I} *)

I get zeroes for all 5-6 values of b I tried.

  • $\begingroup$ Thank you.this expression was from a paper,and I think the author may do some approximation. $\endgroup$ – qb.Suo Jan 13 at 2:32

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