  Optical intensity interferometry, developed in the 1950s, is a simple and
inexpensive method for achieving angular resolutions on microarcsecond scales.
Its low sensitivity has limited intensity interferometric observations to
bright stars so far. Substantial improvements are possible by using avalanche
photodiodes (APDs) as light detectors. Several recent experiments used APDs in
single-photon detection mode; however, these either provide low electronic
bandwidths (few MHz) or require very narrow optical bandpasses. We present here
the results of laboratory measurements with a prototype astronomical intensity
interferometer using two APDs observing an artificial star in continuous
("linear") detection mode with an electronic bandwidth of 100~MHz. We find a
photon--photon correlation of about $10^{-6}$, as expected from the ratio of
the coherence times of the light source and the detectors. In a configuration
where both detectors are on the optical axis (zero baseline), we achieve a
signal-to-noise ratio of $\sim$2700 after 10 minutes of integration. When
measuring the correlation as a function of baseline, we find a Gaussian
correlation profile with a standard deviation corresponding to an angular
half-width of the artificial star of $0.55''$, in agreement with the estimate
by the manufacturer. Our results demonstrate the possibility to construct large
astronomical intensity interferometers using linear-mode APDs.
