Fiber Amplifiers and Lasers

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Overview

Optimization of a 12-pump Fiber Raman Amplifier

Description

The utilization of high-gain bandwidth in Fiber Raman Amplifiers (FRA) requires multiple pumps as well as a careful adjustment of their wavelengths and powers. Due to the large number of variable parameters, this is only feasible with the help of special optimization algorithms. It is essential that such algorithms consider not only pump-pump and pump-signal Raman interactions but also other physical effects, whose impact is a priori unknown but can significantly affect the performance of the FRA. Here, an automatic optimization of 12-pump Raman wavelengths and powers is demonstrated, taking into account the signal-signal Raman interactions. The FRA provides 10dB net gain with 0.2dB ripple over 80nm bandwidth. The optimization can be performed for uni- and bi-directional pumping, for a flat or non-flat (with pre-emphasis) gain goal and for homogeneous or heterogeneous fibers (consisting of just one or multiple spans with different parameters, respectively).

Typical Results

The simulation setup is shown in Figure 1. The WDM signals are input to the Raman pump optimizer which initially guesses the pump configuration and adds the Raman pumps to the input signal. The combined signal is sent to the fiber. After fiber propagation the signal returns to the optimizer, where the actual signal gain is calculated and compared with the user-specified goal. The difference between them, as well as the distribution of the signal power within the fiber provides information for the iterative optimization algorithm to correct the pumps at the next iteration. The iterations repeat until the goal gain is achieved with the desired accuracy.
Figure 2 demonstrates the wavelength optimization of the Raman pumps using a Heuristic Genetic Algorithm. At this stage only the main contribution to the Raman gain due the pump-signal interaction is taken into account (Approximate Gain). Pump wavelengths are searched iteratively (generations) by the crossover and mutation operations to minimize the deviation of the Approximate Gain from the goal value. The found wavelengths are further used for final optimization of the launch pump powers, taking into account other physical effects contributing to the signal gain (Actual Gain) and possible constraints on the maximum and minimum pump powers. The optimized pump configuration is shown in Figure 3. The gain and noise figure spectra of the FRA are shown in Figure 4. Note the difference between the Actual and the Approximate Gain due to the Raman tilt caused by the signal-signal interaction.

Keywords

Fiber Raman Amplifier (FRA), Raman Pump Optimization, Wavelength Optimization, Power Optimization