  Developments in quantum technologies lead to new applications that require
radiation sources with specific photon statistics. A widely used Poissonian
statistics are easily produced by lasers; however, some applications require
super- or sub-Poissonian statistics. Statistical properties of a light source
are characterized by the second-order coherence function g^(2)(0). This
function distinguishes stimulated radiation of lasers with g^(2)(0)=1 from
light of other sources. For example, g^(2)(0)=2 for black-body radiation, and
g^(2)(0)=0 for single-photon emission. One of the applications requiring
super-Poissonian statistics (g^(2)(0)>1) is ghost imaging with thermal light.
Ghost imaging also requires light with a narrow linewidth and high intensity.
Currently, rather expensive and inefficient light sources are used for this
purpose. In the last year, a superluminescent diode based on amplified
spontaneous emission (ASE) has been considered as a new light source for ghost
imaging. Even though ASE has been widely studied, its photon statistics has not
been settled - there are neither reliable theoretical estimates of the
second-order coherence function nor unambiguous experimental data. Our computer
simulation clearly establishes that coherence properties of light produced by
ASE are similar to that of a thermal source with g^(2)(0)=2 independent of pump
power. This result manifests the fundamental difference between ASE and laser
radiation.
