Fiber Amplifiers

In simple cases, with negligible parasitic losses of the active fiber and with negligible ASE (see part 4), the power conversion efficiency of the amplifier does not depend on the pump direction:

• The overall gain depends only on the average excitation density along the fiber.
• The total pump absorption efficiency also depends only on that average.
• The same holds for the amount of power lost via fluorescence (spontaneous emission).

All this holds also for quasi-three-level laser transitions, and of course also for bidirectional pumping.

If the fiber has substantial parasitic losses, backward pumping is more efficient. This is because the signal is strong near the pump input end, holding the excitation density down in that region, so that the pump light is efficiently absorbed by the ions before parasitic losses can get it. In certain situations, bidirectional pumping may be even more efficient. However, most amplifiers are only a couple of meters long, and parasitic losses within that length are not very important.

A more important difference for the power conversion efficiency can arise from ASE and is discussed next:

We have already seen in part 4 that without a signal input, ASE can be much stronger in backward direction. Not surprisingly, that asymmetry does not vanish if we also have a signal input, and the amount of ASE – which constitutes a power loss – can substantially depend on the relative direction of signal and pump light.

As an example, let us consider the same ytterbium-doped fiber amplifier as in part 3 and 4. We have already seen how the powers evolve for forward pumping at 940 nm and 1 mW of signal input power at 1030 nm on the left side:

Now we change to backward pumping:

We see that ASE is now negligibly weak, and we obtain 346 mW of signal output power instead of only 290 mW. The ASE spectrum (not displayed here) only shows some weak ASE around 1030 nm and much less around 975 nm. Positive net gain at 975 nm occurs only in a short region of the fiber around 2 m from the left side.

These findings are quite typical. Generally, quasi-three-level fiber amplifiers have lower ASE losses for backward pumping, and therefore a higher power conversion efficiency in that configuration, if ASE losses would otherwise be significant. For higher input signal powers, causing stronger gain saturation, the difference between the two directions is smaller.

Another aspect is that ASE which propagates together with the signal constitutes a broadband noise for that signal. For backward pumping, the ASE propagating in the same direction as the signal is generally stronger for quasi-three-level systems – even if the total power loss by ASE is lower! So for low-noise operation, forward pumping is better, despite the possibly lower efficiency.

If we amplify short pulses, nonlinear effects are often detrimental. This will be discussed in part 7, but here it should be mentioned already that backward pumping is often much better. The reason is that for backward pumping, the pulse energy and peak power initially rises more slowly, so that the spatially integrated peak power gets lower for a given output peak power.

Go to Part 6: High-power Fiber Amplifiers with Double-clad Fibers or back to the start page.

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