  We study the polarization properties of light emitted by quantum dots that
are embedded in chiral photonic crystal structures made of achiral planar GaAs
waveguides. A modification of the electromagnetic mode structure due to the
chiral grating fabricated by partial etching of the wave\-guide layer has been
shown to result in a high circular polarization degree $\rho_c$ of the quantum
dot emission in the absence of external magnetic field. The physical nature of
the phenomenon can be understood in terms of the reciprocity principle taking
into account the structural symmetry. At the resonance wavelength, the
magnitude of $|\rho_c|$ is predicted to exceed 98%. The experimentally achieved
value of $|\rho_c|=81$% is smaller, which is due to the contribution of
unpolarized light scattered by grating defects, thus breaking its periodicity.
The achieved polarization degree estimated removing the unpolarized nonresonant
background from the emission spectra can be estimated to be as high as 96%,
close to the theoretical prediction.
