Gaussian Radius of curvature in an argon ion laser

[lauraboyleify]

Hi, having a bit of trouble with this question
"In an argon ion laser ( λ = 514nm) the minimum beam waist is 1.0mm and is close to the plane mirror. Calculate the radius of curvature of the beam at the output mirror. 1.15m away"

Attempt at a solution:

θ = 2λ/pi W0²
R = Z + Z_R²/Z
Z_R = piW₀² / λ
I used
W0 = min beam waist = 1.00 mm
Z= distance from beam waist to mirror = 1.15m
And found Z_r with the formula and used it in the Radius of curvature, R formula but got the worng answer. I got R = 33.5m
The answer given is R = 3.08m

 

[DeShark]

Beam width isn't beam radius!