  The aim of this study is to probe the sub-mJy polarized source population
with LOFAR. We present the method used to stack LOFAR polarization datasets,
the resulting catalog of polarized sources, and the derived polarized source
counts. The ELAIS-N1 field was selected for a polarimetric study at 114.9-177.4
MHz. A total area of 25 deg2 was imaged at 6"- resolution in the Stokes Q and U
parameters. Alignment of polarization angles was done both in frequency and in
Faraday space before stacking datasets from 19 eight-hour-long epochs. A search
for polarized sources was carried out in the final, stacked dataset, and the
properties of the detected sources were examined. The depolarization level of
sources known to be polarized at 1.4 GHz was quantified. A one-sigma noise
level of 19 {\mu}Jy/beam was reached in the central part of the field after
stacking. Twenty-five polarized sources were detected above 8\sigma, five of
which had not been detected in polarization at any other radio frequencies
before. Seven additional polarized components were found by lowering the
threshold to 6\sigma at positions corresponding to sources known to be
polarized at 1.4 GHz. In two radio galaxies, polarization was detected from
both radio lobes, so the final number of associated radio continuum sources is
31. The detected sources are weakly polarized, with a median degree of
polarization of 1.75% for the sample of sources detected in polarized emission.
The sources previously detected in polarization at 1.4 GHz are significantly
depolarized at 150 MHz. The catalog is used to derive the polarized source
counts at 150 MHz. This is the deepest and highest-resolution polarization
study at 150 MHz to date.
