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RP Fiber Power – Simulation and Design Software for Fiber Optics, Amplifiers and Fiber Lasers

Power Form: Continuous-wave Fiber Laser and Amplifier

This Power Form allows one to set up models for a continuous-wave fiber laser plus a multi-stage fiber amplifier. A single quasi-monochromatic signal (amplified laser output) is considered.

Basic Features of the Model

• Laser: We first simulate a fiber laser, where the laser wavelength can be either input by the user or calculated.
• Amplifier stages: We simulate a fiber amplifier with up to five different stages. Each stage may be activated or deactivated; in the latter case, the light is not passed through it.
• We assume continuous-wave operation, i.e., with constant pump and signal powers. We investigate only the steady-state operation, not e.g. the turn-on behavior.
• Fiber parameters: for each amplifier stage, one can select a fiber parameter set - from a commercially available fiber, for example. One can also override various parameters.
• Pump sources: the laser and each amplifier stage can have up to 5 quasi-monochromatic or broadband forward or backward pump inputs. It is assumed that no pump light can ever get from one amplifier stage to another stage.
• Single or double pass amplification: normally, the signal light would get from the signal source through some number of amplifier stages, each one passing in just one direction (called forward direction). However, one can have a wavelength-dependent reflector at the end of a stage to realize a double-pass amplifier stage; in that case, the signal is propagating back through the same fiber and then separated from the input with a Faraday circulator before being sent to the next stage.
• Coupling losses: after each amplifier stage, and after the input signal source, we can introduce coupling losses, which can be wavelength-dependent.
• Amplified spontaneous emission (ASE) in the amplifier stages can also be considered. See the encyclopedia article on ASE. ASE may get from one stage to the next one, suffering the above-mentioned wavelength-dependent losses (e.g. from a bandpass filter), but it is assumed that it cannot get to the previous stage (e.g. due to a Faraday isolator preventing that).

The Laser

You can select a fiber data set for the active fiber, determine its length and the number of longitudinal and radial numerical steps.

The laser wavelength may be directly input, or is calculated automatically when checking Use calculated. That automatic calculation depends on the given end mirror reflectivities, which may be wavelength-dependent.

The reflectivities of the output coupler mirror and the back mirror can be specified. These may be wavelength-dependent: you can enter an expression which contains the wavelength variable l (for lambda). (The wavelength dependence can be essential for automatically calculating the laser wavelength.) The units for the reflectivities can be selected using the drop-down menu (%, dB or 0-1 = zero to one).

Amplifier Stages

The Amplifier stages section allows the definition of up to 5 amplifier stages. Each one contains an active fiber as its central piece and some pump source(s).

There are tabs for five amplifier stages. All these settings are the same as for the Power Form for continuous-wave amplifiers and are thus not explained again here.

Output Area

After executing the Power Form, various outputs can be seen in the Output area on the right-hand side of the user interface:

Here, you can see in detail e.g. how much of the forward and backward pump power of the laser or any amplifier stage is left after passing through the active fiber, how the signal power evolves, and how much signal gain each amplifier stage has.

Diagrams

In addition to various numerical outputs in the form and in the output area on the right side, the form offers a large choice of diagrams for displaying various kinds of results:

Just select those which you need, and configure certain options. For example, most diagrams offer the option of using a dBm scale instead of a linear vertical scale. In some cases, you may show graphs for all individual input signals, or alternatively only a graph for the total signal power.

The diagrams are grouped into

• Input diagrams: used mainly for sanity checking your settings for the input signals
• Output diagrams: display various outputs based on the form settings
• Variation diagrams: produce diagrams where one of the system parameters is varied in a certain range. For example, you may vary a pump wavelength to see what effect that has on the amplifier performance.

Each diagram has the option to add some script code, which will then be executed directly after other code for generating that diagram. This allows the user to add any plots, lines or annotations, for example, without modifying the script code of the form. One may also use such code to display additional numerical items in the output area. Another possibility would be code to write certain data to a file. In case that you need help, call our technical support.

Diagrams for an Example Case: Yb Fiber Laser and Amplifier

The following screenshots show you some of the diagrams which can be made with this simulation model. Here, we simulate a laser/amplifier system with the following details:

• The laser is based on a single-mode fiber. We assume that the output coupler has a Gaussian spectral reflectance profile with a peak at 1080 nm and 10 nm FWHM. The automatically calculated laser wavelength is then 1079.34 nm – a bit shorter than 1080 nm as the gain is higher at shorter wavelengths.
• We use a single backward-pumped double-clad fiber amplifier. We assume that ASE from the laser is filtered out so that it cannot get into the amplifier.

One of the diagrams shows the powers and excitation density along the active fiber of the laser:

Interestingly, the maximum Yb excitation occurs not at the left end, where the pump intensity is highest, but somewhat more inside because of backward ASE.

Another diagram shows the power evolution in the whole system, i.e., including the amplifier:

You see the forward and backward powers in the laser, as well as the rising power in the amplifier fiber.

It is also interesting to inspect the ASE output spectra on both fiber ends of the amplifier:

We see that the ASE maximum is at somewhat shorter wavelengths than the gain maximum, since ASE powers depend not only on the gain but also on spontaneous emission, which is stronger at shorter wavelengths.

The last diagram shows the evolution of ASE inside the amplifier fiber:

Case Study

The following case study is available, where we used this Power Form:

See also: overview on Power Forms