Efield[r_?NumericQ, z_?NumericQ, [Alpha]_?NumericQ] := (
  2 [Pi])/([Lambda]laser z)*w0*w[z]*
  NIntegrate[
   Exp[-x^2]*
    Exp[I*[Pi]*(a [z] x^2 - x/[CapitalDelta][[Alpha], z])]*
    BesselJ[0, [Beta] [r, z] x]*x, {x, 0, [Infinity]}]

Intensity[r_?NumericQ, z_?NumericQ, [Alpha]_?NumericQ] :=
 Abs[Efield[r, z, [Alpha]]]^2

PowerLens =
 NIntegrate[(Intensity[r, distance, conv*20])*
   r*2 [Pi], {r, 0, 1}]

nm = 10^-6 mm;
mm = 10^-3;
[Mu]m = 10^-3 mm;
[Lambda]laser = 532 nm;
klaser = (2 [Pi])/[Lambda]laser;
w0 = 5 mm;
flens = 50 mm;
ng = 1.5;
conv = (2 [Pi])/360;
Gaussian beam parameters

zR = [Pi] w0^2/[Lambda]laser // N
w[z_] = w0*Sqrt[1 + z^2/zR^2]
Rc[z_] = z*(1 + zR^2/z^2)
Guoy[z_] = ArcTan[z/zR]

GaussianBeamField[[Rho]_, z_] = (w0/w[z])*Exp[-[Rho]^2/w[z]^2]*
  Exp[I*(klaser z - Guoy[z] + (klaser [Rho]^2)/(2 Rc[z]))]

GaussianBeamIntensity[[Rho]_, z_] =
 Abs[GaussianBeamField[[Rho], z] ]^2 // ComplexExpand

Normalisation[z_] :=
 Integrate[2 [Pi]*[Rho]*GaussianBeamIntensity[[Rho], z], {[Rho],
   0, [Infinity]}, Assumptions -> {Element[{[Rho], z}, Reals]}]

GaussianBeamIntensityNormalised[[Rho]_, z_] = (1/Normalisation[0])*
  GaussianBeamIntensity[[Rho], z]

b[[Alpha]_] = (2 [Pi] (ng - 1) )/[Lambda]laser Tan[[Alpha]]
[CapitalDelta][[Alpha]_, z_] = [Pi]/(b[[Alpha]]*w[z])
[Beta][r_, z_] = (2 [Pi] w[z] r)/([Lambda]laser z)
a[z_] = w[z]^2/[Lambda]laser*(1/z + (1/Rc[z] - 1/flens))

Efield[r_?NumericQ, z_?NumericQ, [Alpha]_?NumericQ] := (
  2 [Pi])/([Lambda]laser z)*w0*w[z]*
  NIntegrate[
   Exp[-x^2]*Exp[I*[Pi]*(a [z] x^2 - x/[CapitalDelta][[Alpha], z])]*
    BesselJ[0, [Beta] [r, z] x]*x, {x, 0, [Infinity]}]

Intensity[r_?NumericQ, z_?NumericQ, [Alpha]_?NumericQ] :=
 Abs[Efield[r, z, [Alpha]]]^2


p1 = Plot[GaussianBeamIntensityNormalised[r, flens], {r, 0, 15 mm}]

p2 = Plot[IntensityNormalised[r, flens, 20*conv], {r, 0, 15 mm}]

Show[p1, p2]

PowerLens =
 NIntegrate[Intensity[r, flens, conv*20]*r*2 [Pi], {r, 0, [Infinity]}]