## Modes dialog box

The “Modes” dialog box allows you to:

- Select the types of modes you are interested in
- Select the range of mode numbers you are interested in
- Find the spectrum of supported modes at the specified wavelength with their mode numbers and effective refractive indices
- Select a mode or group of modes for subsequent calculations.
- Preview the mode pattern of a selected mode in a number of ways.
- “Compose” a beam by the interference of an arbitrary group of supported modes.
- “Decompose” in input beam into an arbitrary group of supported modes (calculate input coupling efficiencies.)

To access this dialog box, do one of the following steps:

Step | Action |

1 | Select “Modes” on the “Simulation” menu, or |

2 | Click the “Modes” icon in the “Navigator” pane |

The elements and controls of “Modes” dialog box options are described below.

**LP Modes (Finite Difference Method)**

Check this option to calculate fiber propagation modes in the Linear Polarization (LP)

approximation (scalar modes). LP(m,n) modes are conventionally labeled using two

numbers: the azimuthal number m (the Order), and the radial number (mode number)

n. You can select the range of Order and the maximum n number for the solver.

**LP Modes (Matrix Method)**

This option finds LP modes as above, but by an exact method that uses radial transfer

matrices. See Appendix A – Fiber Mode Solvers by Matrix Method for details. This

method sometimes runs more slowly, especially if the profile has many layers.

However, the far field has the correct asymptotes, and this method is more accurate,

especially in the calculation of dispersion and other derivatives with wavelength.

**Vectorial Modes**

Check this option to calculate fiber propagation modes without the scalar

approximation. Conventionally, the vector modes have subsets of the hybrid HE and

EH modes, the Transverse Electric (TE) modes, and the Transverse Magnetic (TM)

modes. Hybrid (m,n) modes are conventionally labeled using the numbers: the

azimuthal number m (Order), and the radial number n. The vector modes are found

by a 4×4 transfer matrix method, see Appendix A – Fiber Mode Solvers by Matrix

Method for details.

**Wavelength**

This is the wavelength the calculated spectrum of modes relates to. It is independent

of the wavelength specified in the “Fiber Profile” dialog box.

**Recalculate**

Press the “Recalculate” button to start the mode solver.

**List of modes**

The results found by mode solver are displayed in the list of modes. The modes are

labeled according to the calculation option, with the modal index beside the label.

**Modal Index Export**

Export the contents of “List of modes” to a text file.

**Mode field preview**

Mode field can be previewed on the small display, where you can select the display

color mode and contrast.

**Composition**

Starts the “Composition” dialog box allowing to calculate arbitrary interference

combinations of the currently selected group of modes.

**Decomposition**

Starts the “Decomposition” dialog box allowing to calculate the coupling efficiencies

of an arbitrary input beam into each mode of the currently selected group of modes.

## Composition dialog box

This dialog box controls the calculation of the superposition/interference pattern of an

arbitrary group of modes each of them having arbitrary power and input phase.

To use this feature, do the following steps:

Step | Action |

1 | In the “Modes” dialog box, select a group of modes by using Ctrl-, Shift- and the left mouse button. |

2 | Press the “Composition” button. In the “Composition” dialog box, enter the weighting coefficient for each mode. |

3 | Press “OK” |

The elements and controls of “Composition” dialog box options are described below.

**Weighting coefficients table**

Enter the complex coefficients assigned to each participating mode in the three-column

table (grid) in the upper left part of the dialog box

**Preview display area**

The graphical window in the lower right part of the dialog box shows a color-coded

topological view of the current pattern.

**Calculate**

Find the current interference pattern as defined by the set of modes and the

coefficients.

**Define the Mesh**

Increase/Decrease the resolution of the preview screen.

**Norm Coef.**

Normalizes the weighting coefficients. After normalization, the sum of the squared

magnitudes of all of the weighting coefficients is equal to 1.

**Load Coef.**

Load the weighting coefficients to a text file.

**Save Coef.**

Save the weighting coefficients to a text file.

**Zoom**

Check this option to zoom the details of the speckle pattern. Useful for fibers with

narrow core and wide cladding.

**Power/Field**

Display the intensity or the amplitude of the composed field.

**Save field**

Saves the calculated field to a text file. The saved file has the complex BCF3DCX 3D

file Format, described in the “Technical Background”.

**Propagation Length**

When this field is 0.0, the preview is the sum of the modes with amplitudes and

phases as indicated in the list. Enter a distance in microns in this field, and press

Calculate, to see the field as it would appear after a propagation of this distance.

## Linear decomposition of a given field distribution

This dialog box controls the calculation of the per-mode coupling efficiencies (the

linear decomposition) of an user defined input beam into an arbitrarily selected group

of supported modes.

To use this feature, do the following steps:

Step | Action |

1 | In the “Modes” dialog box, select a group of modes by using Ctrl-, Shift- and the left mouse button. |

2 | Press the “Decomposition” button. In the “Decomposition” dialog box, define the parameters of the input beam. |

3 | Press “OK” |

The elements and controls of “Decomposition” dialog box options are described

below.

**Calculate**

Find the decomposition coefficients for the current set of modes and the current input

beam.

**Weighting coefficients table**

Contains the calculated field (with their real and imaginary parts) and power

decomposition coefficients. The values in the “Power” are the squared sum of the real

and imaginary parts and have the meaning of the usual coupling coefficients.

**From File**

Read the input beam from a user defined file having the BCF3DCX complex 3D file

Format (please see the description in “Technical Background”).

**Gaussian**

The input beam is modeled as Gaussian field defined by the several parameters,

FWHM Width and polar angle.

Gaussian for Decomposition

**Center**

The center position x_{0} of the input Gaussian beam.

**Width**

The width w_{0} of the input Gaussian beam.

**Mesh**

The number of point of the internal calculations. Higher value means better accuracy

of the coefficients at the expense of computating time.

**Angle**

Enter the polar angle between the input beam and the fiber axis.

**Save Coef.**

Export the coefficients to a text file.

## Properties of Fundamental Mode dialog box

The “Properties of Fundamental Mode” dialog box controls the powerful parameter

scanning routines of OptiFiber related to the fundamental mode. It also allows the

user to select which characteristics of the fundamental mode to be calculated.

To access this dialog box, take one of the following steps:

Step | Action |

1 | Select “Fundamental Mode” on the “Simulation” menu |

2 | Click the “Fundamental Mode” icon in the “Navigator” pane |

In the case of a simple step-index fiber it look as shown:In order to use this dialog the user should have defined a legitimate profile in the

“Profile” dialog box.

The elements and controls of the “Properties of Fundamental Mode“ dialog box are

described below.

**Scan**

- The upper pane lists what is subjected to scanning:
- The wavelength within a spectral region.
- The profile as a whole within user-specified limits, for example

between 0.8 and 1.2 of the original width. This means that all spatial

dimensions of the original design are proportionally shrunk/expanded

at regular steps from 0.8L to 1.2L, where L is the initial width. - The single spatial regions of the profile.

- The lower pane lists the various technological parameters of the current region

that are available for scan calculations.

When “Wavelength” or “Profile Width” is selected in the upper pane, then there is no

other parameter to scan. When a region is selected, then all region parameters

available for scan calculations are displayed in the lower pane. Their number and

physical meaning depends on the type of the region. The available parameters may

include function parameters for function-defined regions. For example:

- If a simple “constant” region is selected, then for scanning are available:
- its spatial width and
- maximal refr. index

- If a “Gaussian function” region is selected, then for scanning are available:
- its spatial width
- maximal refr. index
- FWHM and
- central position

**Parameters**

The “Parameters” section refers to the range of parameter being scanned and the

number of steps. “From” – Initial value of the scan parameter “To” – Final value of the

scan parameter “Steps” – Number of calculation steps in the scan calculations

**Calculate**

The “Calculate” section allows you to select the fundamental mode characteristics to

be calculated versus the scan parameter.

The options are:

- Group Delay and Dispersion
- Mode Field Diameter
- Bending Loss
- Splice Loss
- Effective nonlinear coefficient

The Material Loss option always refers to scanning over the wavelength parameter.

You start the calculations by pressing the “Calculate” button.

## Properties of Higher Order Modes dialog box

The “Properties of Higher Order Modes” dialog box controls the powerful parameter

scanning routines of OptiFiber related to a higher order mode or a group of modes. It

also allows the user to select which modal characteristics to be calculated.

To access this dialog box, take the following steps:

Step | Action |

1 | Select “Higher Order Modes” on the “Simulation” menu, or |

2 | Click the “Higher Order Modes” icon in the “Navigator” pane |

In order to use this dialog the user should have:

- Defined a legitimate profile
- Solved for the spectrum of modes in the “Modes” dialog box and found that the

fiber is multi-mode at that wavelength. - Selected in the “Modes” dialog box
- a higher order mode, or
- a group of modes

If some of these conditions are not fulfilled, the user is automatically sent to the

respective place of OptiFiber to alter the design, to solve for modes, etc.

The logic of the scanning routines depends alternatively on the number of modes that

have been selected in the “Modes” dialog box. In the case of one selected higher

mode the “Properties of Higher Order Modes” works in the same way as the

“Properties of Fundamental Mode” dialog – it allows the defined characteristics of that

mode to be calculated versus some scanned parameter.

In the case of multiple selected modes the “Properties of Higher Order Modes”

calculates the defined characteristics of each mode. An option is offered to sort and

display the results:

- according the sequential number of the modes in the selected group.
- according the effective refr. index of the modes in descending order.

The elements and controls of “Properties of Higher Order Modes” are almost the

same as those of the “Properties of Fundamental Mode” dialog box and will not be

described separately.

**Note**: The calculations are performed for the mode or modes previously selected

in the “Modes” dialog box.

## Cutoff dialog box

The “Cutoff” dialog box finds the spectrum of supported modes at a specified

wavelength, to select a set of the supported modes, and to find their cutoff

wavelengths. For explanation of the methods used to estimate the cutoffs, please

refer to the section “Cutoff Wavelengths” in the “Technical Background”.

To access this dialog box, take the following steps:

Step | Action |

1 | Select “Cutoff” on the Simulation menu, or |

2 | Click the “Cutoff” icon in the “Navigator” pane |

3 | Enter wavelength and click Recalculate. |

The elements and controls of the “Cutoff” dialog box are described below.

**LP Modes (Finite Difference Method)**

Check this option to calculate fiber propagation modes in the Linear Polarization (LP)

approximation (scalar modes). LP(m,n) modes are conventionally labeled using two

numbers: the azimuthal number m (the Order), and the radial number (mode number)

n. You can select the range of Order and the maximum n number for the solver.

**LP Modes (Matrix Method)**

This option finds LP modes as above, but by an exact method that uses radial transfer

matrices. See Appendix A – Fiber Mode Solvers by Matrix Method for details. This

method sometimes runs more slowly, especially if the profile has many layers.

However, the far field has the correct asymptotes, and this method is more accurate,

especially in the calculation of dispersion and other derivatives with wavelength.

**Vectorial Modes**

Check this option to calculate fiber propagation modes without the scalar

approximation. Conventionally, the vector modes have subsets of the hybrid HE and

EH modes, the Transverse Electric (TE) modes, and the Transverse Magnetic (TM)

modes. Hybrid (m,n) modes are conventionally labeled using the numbers: the

azimuthal number m (Order), and the radial number n. The vector modes are found by a 4×4 transfer matrix method, see Appendix A – Fiber Mode Solvers by Matrix

Method for details.

**Wavelength**

The wavelength at which the initial spectrum of modes is calculated. If the fiber is

single-mode at the specified wavelength – decrease it, in order to be able to select a

mode for the calculations.

**Recalculate**

Press the “Recalculate” button to start the mode solver.

**List of modes**

The results found by mode solver at the specified wavelength are here. The modes

are labeled according to the calculation option. The mode effective index of each

modes is given in parenthesis.

**Lists of cutoff values**

Displays the estimated cutoff wavelength values for the selected modes.

Two lists are available:

- “Theoretical values” and
- “Estimated ITU-T values”

in accordance with the two cutoff definitions. A cutoff wavelength is not defined for the

fundamental mode and therefore is marked ‘N/A’ (although occasionally, for quite inappropriate combination of parameters, the fundamental mode could be unsolvable for, a condition called “a numerical cutoff”).

**Export**

Click here to make an ascii text file that contains the modes listed in the columns

Theoretical values and Estimated ITU-T values.