We have made crosstalk measurements on our U-groove isolated photodiode array by two independent techniques, an electrical measurement method and a laser scanning method. These two methods are shown to corre-late with each other. The crosstalk between two adjacnt pixels of the U-groove isolated array was below 5 X 10-4, which compared favorably to the value of 10-1 for the conventional diffusion junction photodiode array. Also, the effective diffusion length of the photogenerated carriers was measured to be 10µm in the N-region and 6 um in the top P-region. The advantages of the detector isolation technique for optical signal processing are presented.