Optical heterodyne mixers providing image-frequency rejection

DETAILED DESCRIPTION An arrangement of an optical heterodyne mixer providing image-frequency rejection in accordance with the present invention is shown in FIG.
1.
There, a received optical signal and an optical local oscillator (L.
O.
) signal are received at separate inputs and directed at a beam splitter 30 which adds the received signal to the L.
O.
signal.
It is to be understood that one of the input signals, either the received signal or the L.
O.
signal, is linearly polarized, while the other input signal is circularly polarized.
Additionally, it should be understood that the received signal can comprise one or more optical signals at different frequencies, where each optical carrier is modulated by a separate information signal.
The merged beam at the output of beam splitter 30 comprises components of both the received and the L.
O.
signals and enters a polarizing beam splitter 31 which functions to separate the horizontal and vertical polarization components of the merged beam.
It is to be understood that polarizing beam splitter 31 should be oriented with its polarization axes at 45 degrees relative to the polarization direction of the input signal which is linearly polarized.
Such orientation permits the power of the linearly polarized input signal to be equally divided between the transparent and reflective outputs of polarizing beam splitter 31, such that equal components of the linearly polarized input signal in the merged beam enter branches 32 and 33 at the output of beam splitter 31.
It is to be understood that polarizing beam splitter 31 also causes equal components of the circularly polarized input signal in the merged beam to enter branches 32 and 33 concurrent with the above-mentioned components of the linearly polarized input signal.
The arrangement of FIG.
1 provides the means to maintain the required phase relationships between the in-phase and quadrature signal components independently of the optical path dimensions of the circuit because the L.
O.
signal and the received signal follow the same optical paths