def diffraction_limited_spot_gaussian(wavelength=1e-10,focal_length=10,fwhm=100e-6):
    """
    returns FWHM of focused beam
    https://www.rp-photonics.com/focal_length.html
    formula is w_focus = λ*f/(π*w)
    w = 2*σ = 2/2.35*FWHM = 0.85*FWHM
    so the formula becomes
    0.85*FWHM_focus = λ*f/(π*0.85*FWHM) →
    FWHM_focus = λ*f/FWHM * 1/(π*0.85*0.85) = 0.44*λ*f/FWHM
    """
    return 0.44*wavelength*focal_length/fwhm