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The intensity distribution of an outgoing beam from a linearly tapered grating coupler is studied on the basis of the first-order perturbation theory by dividing the grating region into short subsections. The resultant intensity distribution of the outgoing beam is compared with that calculated by means of the step approximation in which the linearly tapered grating is assumed to consist of a sequence of uniform gratings with amplitudes varying stepwise from one grating to the next. This comparison verifies the validity of the step approximation for investigating the profile of the outgoing beam from the tapered grating coupler. With this verification for the step approximation, the beam profiles from tapered grating of various types are actually investigated by means of the step approximation. It is found that the beam profile of an outgoing beam can be further made to approach a Gaussian shape of the laser beam by using a tapered grating with a Gaussian shape instead of a linearly tapered grating.