In the http://imaginary_nematode.home.comcast.net/papers/Gagnon_et_al_1988.pdf" experiment, two waveguides are driven on one end by the same 40.16GHz sine-wave source, the two waveguides run parallel to one another along the negative z-axis, and the waves reflect back and forth within the waveguides at different pitch angles with the result that the waves emerging at the opposite ends of the two waveguides have traveled along paths having substantially different geometric length. We describe the total phase-angle traversed by a wave traveling through a first waveguide as k * z where k is the wave-number of the first waveguide, and z is the length of the first waveguide. The term "wave number" refers to the number of complete wave cycles of an electromagnetic field that exist in one meter of linear space, but because the waves in the waveguides reflect back and forth within the waveguide, there can be many more wavelengths stored within the waveguide than if the wave traveled along a straight line through the waveguide. Similarly, we describe the total phase-angle traversed by a wave traveling through a second waveguide as k' * z where k' is the wave-number of the second waveguide, and z is also the length of the second waveguide layed along the negative z-axis.
When the two waves emerge at the far end of the waveguides, a detector circuit is used to measure the
differential phase of the two waves. To do this, the common-mode phase of the two signals is "rejected". On page 1769 of the Gagnon paper under section III. Experimental Analysis, the authors describe their detector circuit:
Gagnon p. 1769 said:
Signals from the two waveguides are recombined in a balanced mixer which gives a dc output proportional to their phase difference when the waveguide outputs are set near phase quadrature.
clj4, I figured out that the missing term in your fifth equation is just a sign. You could correct that equation by swapping the order of E_z and E'_z. The "dc output proportional to their phase difference..." that Gagnon refers to in the quote above is proportional to sin((k-k')z/2), where "there phase difference" refers to (k-k')z. The term that you refer to
incorrectly as "the resultant phase measured by Gagnon" is actually the
common-mode phase shift that is rejected by the balanced mixer. To demonstrate this, I have modeled a balanced mixer circuit in SPICE using the Micro-Cap 8.1.1.0 Evaluation Version which is freely available for download on the web. Anyone can download this circuit simulator free of charge and reproduce my results independently.
The attached file "BalancedMixerSchematic.jpg" shows a screen-shot of the schematic of the circuit used to simulate the output traces discussed below. The schematic shows three identical balanced mixer circuits driven by dual sine wave sources at a frequency of 40.16KHz (for convenience, I have simulated a 40.160KHz carrier wave rather than 40.160GHz; the principles are the same). The first circuit is driven by sine sources WG1 and WG2 with phase parameters PH=840 and 40 radians respectively. The blue trace on the "BalancedMixerOutput.jpg" plot corresponds to the simulated output of this circuit over a period of one second. The traces in this plot all start out on the left side at higher than normal voltages because the circuit parameters are undefined prior to T=0, and then they settle to nearly flat dc signals as time goes by. These voltages would settle a bit further if the simulation were run for a longer time.
The second (lower left) circuit is driven by sine sources WG3 and WG4 with phase parameters PH=840.7854 and 40.7854 radians respectively. The red trace on the "BalancedMixerOutput.jpg" plot corresponds to the simulated output of this circuit. The phases of the two sine sources have both been offset by +\pi/4. Since this is a common-mode phase shift, we see that the red trace settles to the same value of the blue trace so that there is no difference between the outputs of the two circuits once they have settled. This is what is expected, and if the simulation were run for a longer period the two traces would continue to converge on the same value. Conclusion: common-mode phase shifts do
not affect the output of a balanced mixer circuit, by design.
The third (center right) circuit is driven by sine sources WG5 and WG6 with phase parameters PH=840.7854 and 39.2146 radians respectively. The green trace on the "BalancedMixerOutput.jpg" plot corresponds to the simulated output of this circuit. The phases of the first sine source has been offset by +\pi/4 (relative to PH=840), and the phase of the second sine source has been offset by -\pi/4 (relative to PH=40). Since this is a
differential-mode phase shift, we see that the green trace settles to a substantially different value from the red and the blue traces. This is what is expected. Conclusion: differential-mode phase shifts
do affect the output voltage of a balanced mixer circuit, by design.
GGT predicts
common-mode phase shifts for the two waveguides, and this is simply
not observable by means of the experiment described by Gagnon
et al..