siph_dircoupler :
An optical directional coupler.

The siph_dircoupler primitive represents an optical directional coupler, which splits the incident wave entering the port in into two waves leaving the ports thru and cpl. In typical optical directional couplers, the port thru is the opposite-side port on the same waveguide with the port in and the port cpl is the opposite-side port on the coupled waveguide. Both the waves experience the propagation delay through the waveguides, which is equal to the optical path length of each waveguide (specified by the parameter oplength) divided by the speed of light in free space (c0=2.99792458e8 meters/sec).

                 in  ---+             +--- thru
                         \           /
                          +---------+
                          +---------+
                         /           \
      (isolated) iso ---+             +--- cpl (coupled)
  

The parameter gain_thru specifies the transmission gain from the port in to port thru and the parameter gain_cpl specifies the coupling gain from the port in to port cpl. The wave propagating from the port in to port cpl may also experience an additional phase shift specified by the parameter phase_cpl (default: -M_PI/2). The siph_dircoupler primitive is a reciprocal element that can receive incident waves from any port and propagate waves with the symmetrical transfer characteristics.

The parameter oplength specifies the optical path length of each waveguide in meters, which is equal to the waveguide’s propagation delay multiplied by the speed of light in free space (c0). For instance, a quarter-wavelength directional coupler for a 1550nm laser would have an optical path length of 1550nm/4=387.5nm. Note that this primitive does not model the dependencies of the gains gain_thru and gain_cpl on the light’s wavelength or waveguide’s length.

The parameter Zrel specifies the coupler’s characteristic impedance relative to a reference impedance (Z0) of your choice. Note that the use of this relative impedance allows one to determine the amount of reflections at impedance discontinuities without having to know the absolute values of the characteristic impedances. While the choice of the reference impedance (Z0) can be arbitrary, it must be consistent across all the silicon-photonic components used in a system. In other words, all the equivalent circuit elements describing the impedance relationships at the optical ports must use the impedance values relative to the same Z0.

Input/Output Terminals

Name	I/O	Type	Description
in	input	xreal	input port
thru	input	xreal	thru port
cpl	input	xreal	coupled port
iso	input	xreal	isolated port

Parameters

Name	Type	Default	Unit	Description
Zrel	real	1.0	None	relative characteristic impedance
oplength	real	387.5e-9	meters	optical path length
gain_thru	real	0.7071	None	transmission gain (in-to-thru)
gain_cpl	real	0.7071	None	coupling gain (in-to-cpl)
phase_cpl	real	-M_PI/2	radians	coupling phase shift (in-to-cpl)