  We study the photoresponse of graphene field effect transistors using
scanning photocurrent microscopy in near and far field configurations, and we
find that the response of graphene under a source-drain bias voltage away from
the contacts is dominated by the bolometric effect caused by laser induced
heating. We find no significant change in the photocurrent with the optical
modulation frequency upto 100 kHz. Although the magnitude of the bolometric
current scales with bias voltage, it also results in noise. The frequency
dependence of this noise indicates that it has a 1/f character, scales with the
bias voltage and limits the detectable bolometric photoresponse at low optical
powers.
