A coherent planar lightwave circuit (PLC) optical mixer chip is described comprising a matched pair of symmetrical four-coupler interferometers for in-phase and in-quadrature detection of two polarization components of light using a polarization diversity arrangement. The waveguide structure of the chip is symmetrical about two orthogonal central axes of the chip, whereby the effect of temperature gradients on the mixer is reduced. A light traveling from an input to an output of any of the two interferometers is cross-coupled by one of the two couplers it passes through, and is bar-coupled by the other of the two couplers, so as to compensate for polarization, wavelength, and manufacturing process dependence of bar-coupling with that of cross-coupling.