In the gravitational lensing of gravitational waves,the wave optics should be used instead of the geometrical optics when the wavelength A of the gravitational waves is longer than the Schwarzschild radius of the lens mass M_L.For the gravitational lensing of the chirp signals from the coalescence of the super massive black holes at the redshift z_S approx 1 relevant to LISA,the wave effects become important for the lens mass smaller than approx 10~8M_(sun).For such cases,we compute how accurately we can extract the lens mass from the lensed signal.We consider two simple lens models:the point mass lens and the SIS(Singular Isothermal Sphere).We find that the lens mass can be determined within approx 0.1% [(S/N)/10~3]~(-1)for the lens mass larger than 10~8M_(sun),where(S/N)is the signal to noise ratio of the unlensed chirp signals.We find that the lensing cross section is an order of magnitude larger than that for the usual strong lensing of light.
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