you can see transfer function of X coupler in below image:
https://drive.google.com/open?id=0Bxkt-d32S34gUm5nUVBUTGNEbWc
X coupler option transfer function is related to conjugated disabled or enabled…
where p is the signal of the c is coupling coefficient and a is the additional loss. If the parameter Conjugate is disabled, p is positive (value = 1), and the coupler will use the definition of [1], otherwise p is negative (value = -1) and the coupler will use the definition of [2]
for mach-zehnder parallel with xcoupler conjugate disabled….
https://drive.google.com/open?id=0Bxkt-d32S34gNFF1OTh2dlZMeWM
for mach-zehnder parallel with xcoupler conjugate enabled….
https://drive.google.com/open?id=0Bxkt-d32S34geHc5NmtqZ1drS28