Optical waveguide coupler

## Keywords

optical, bidirectional

## Ports

Name | Type |
---|---|

port 1 | Optical Signal |

port 2 | Optical Signal |

port 3 | Optical Signal |

port 4 | Optical Signal |

## Properties

### General Properties

Name | Default value | Default unit | Range |
---|---|---|---|

Defines the name of the element. |
Waveguide Coupler | - | - |

Defines whether or not to display annotations on the schematic editor. |
true | - | [true, false] |

Defines whether or not the element is enabled. |
true | - | [true, false] |

Defines the element unique type (read only). |
Waveguide Coupler | - | - |

A brief description of the elements functionality. |
Optical waveguide coupler | - | - |

Defines the element name prefix. |
C | - | - |

Defines the element model name. |
- | - | - |

Defines the element location or source in the library (custom or design kit). |
- | - | - |

Defines the local path or working folder $LOCAL for the element. |
- | - | - |

An optional URL address pointing to the element online help. |
- | - | - |

### Standard Properties

Name | Default value | Default unit | Range |
---|---|---|---|

Defines the bidirectional or unidirectional element configuration. |
bidirectional | - | [bidirectional, unidirectional |

### Waveguide Properties

Name | Default value | Default unit | Range |
---|---|---|---|

Defines the insertion loss (attenuation). |
0 | dB | [0, +∞) |

Defines whether to provide the power coupling coefficient, measurements, or the cross over length. |
coupling coefficient | - | [coupling coefficient, length, table |

The length of the waveguide. |
0 | m | [0, +∞) |

Defines whether or not to load measurements from an input file or to use the currently stored values. |
false | - | [true, false] |

Defines whether to use the conjugate of the cross-coupling coefficients or not. |
false | - | [true, false] |

### Waveguide/Mode 1 Properties

Name | Default value | Default unit | Range |
---|---|---|---|

The first identifier used to track an orthogonal mode of an optical waveguide. For most waveguide, two orthogonal identifiers '1' and '2' are available (with the default labels 'TE' and 'TM' respectively). |
1 | - | [1, +∞) |

The label corresponding to the first orthogonal identifier. |
TE | - | - |

The power coupling coefficient corresponding to the first orthogonal identifier. |
0.5 | - | [0, 1] |

The cross over coupling length corresponding to the first orthogonal identifier. |
1 | m | [0, +∞) |

The file containing the frequency dependent power coupling coefficients. |
- | - | - |

The table containing the frequency dependent power coupling coefficients. |
<2> [193.1e+012, 0.5] | - | - |

### Waveguide/Mode 2 Properties

Name | Default value | Default unit | Range |
---|---|---|---|

The second identifier used to track an orthogonal mode of an optical waveguide. For most waveguide, two orthogonal identifiers '1' and '2' are available (with the default labels 'TE' and 'TM' respectively). |
2 | - | [1, +∞) |

The label corresponding to the second orthogonal identifier. |
TM | - | - |

The power coupling coefficient corresponding to the second orthogonal identifier. |
0.5 | - | [0, 1] |

The cross over coupling length corresponding to the second orthogonal identifier. |
1 | m | [0, +∞) |

The file containing the frequency dependent power coupling coefficients. |
- | - | - |

The table containing the frequency dependent power coupling coefficients. |
<2> [193.1e+012, 0.5] | - | - |

### Numerical/Digital Filter Properties

Name | Default value | Default unit | Range |
---|---|---|---|

Defines whether or not to use a single tap digital filter to represent the element transfer function in time domain. |
false | - | [true, false] |

Defines the method used to estimate the number of taps of the digital filter. |
fit tolerance | - | [disabled, fit tolerance, group delay |

Defines the mean square error for the fitting function. |
0.001 | - | (0, 1) |

Defines the window type for the digital filter. |
rectangular | - | [rectangular, hamming, hanning |

Defines the number of coefficients for digital filter. |
256 | - | [1, +∞) |

Defines the number of coefficients for digital filter. |
4096 | - | [1, +∞) |

Defines the time delay equivalent to a number of coefficients for digital filter. |
0 | s | [0, +∞) |

Defines whether to use the initial input signal to initialize filter state values or to set them to zero values. |
false | - | [true, false] |

### Diagnostic Properties

Name | Default value | Default unit | Range |
---|---|---|---|

Enables the frequency response of the designed filter implementation and the ideal frequency response to be generated as results. |
false | - | [true, false] |

The number of frequency points used when calculating the filter frequency response. |
1024 | - | [2, +∞) |

## Results

Name | Description |
---|---|

diagnostic/response #/transmission | The complex transmission vs. frequency corresponding to the ideal and designed filter. |

diagnostic/response #/gain | The gain vs. frequency corresponding to the ideal and designed filter. |

diagnostic/response #/error | Mean square error comparing the frequency response of the designed filter implementation with the ideal frequency response. |

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## Implementation Details

The “Waveguide Coupler” (C) element represents a 2 x 2 directional waveguide coupler. Signals input into the ports are output to the two ports on the opposite side of the element. There is no reflection back out of the input port. The total power of the output signals is equal to the input power minus the “insertion loss”. The ratio between the power of the two output signals is determined by the power coupling coefficient. This coupling coefficient defines the fraction of the total output power that is output by the cross port.

The C element can be defined using several different methods, selected using the “input parameter” property:

- “coupling coefficient”: The coupling coefficient is directly defined as a single number.
- “length”: The coupling coefficient is defined based on the “length” and “cross over length” properties. The coupling coefficient is equal to sin(length/cross over length).
- “table”: The coupling coefficient is defined as a function of frequency with a table of values.

If the “table” option is selected there are two ways to define the table. If “load from file” is set to “false”, the coupling coefficients and frequencies can be modified in the matrix editor by clicking on the “Value” field of the “coupling coefficients 1”/”coupling coefficients 2” property in the Property View.

If “load from file” is set to “true”, a table from a text file can be imported. The required file format consists of a column of frequency values and a column of the corresponding power coupling coefficients, separated by tabs or spaces. An example is shown below:

1.930e+14 0.4

1.931e+14 0.5

1.932e+14 0.6

Once the table has been imported from a file, “load from file” can be set to “false” and the imported table can be modified in the matrix editor. However, if “load from file” is set to “true” when running the simulation, the file will be reloaded and any changes to the table will be overwritten by the values in the file.

A pi phase shift can be applied to the signal output from the cross port by setting “conjugate” to “true”. If “conjugate” is set to “false”, no phase shift is applied to the output signals. The C element can be changed between bidirectional and unidirectional operation with the “configuration” property.

## Use in ring resonator circuits

The C element is commonly used for the coupling region of ring resonator circuits, as in the (ring resonator AG example). For sample mode transient simulations with these circuits it is best to use a single value for the coupling coefficients (in other words, a point coupler). A frequency dependent value defined using a table will introduce artificial delays to the circuit, which will lead to inaccurate results for the properties of the ring resonator.