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What is TDFA?

TDFA for S-band amplification

A thulium-doped fiber amplifier (TDFA) is often used for light amplification in the Near Infra-Red (NIR). Figure 1 shows major transitions for NIR emission. This article describes a TDFA used for light amplification in 1.45-1.5 µm.

Figure 1

Choice of host glass

An important factor for S-band amplification is the choice of host glass material. Silica glass – which is the most commonly used host glass for optical fiber – shows a poor emission efficiency due to its large phonon energy.

In silica glass, a Tm ion excited at the ³H₄ level rapidly relaxes to the ³H₅ level by multiphonon relaxation and thus does not emit photons efficiently. An optical fiber made by low-phonon-energy host glass, typically fluoride fiber for S-band TDFA, is needed for efficient amplification.

Pumping at 1.4 µm

One wavelength for pumping a S-band TDFA is 1.4 µm, where a pump laser diode is commercially available. The pumping scheme is shown in Figure 2. Thulium ion shows a weak Ground-State Absorption (GSA: ³H₆→³F₄) and a strong Excited-State Absorption (ESA: ³F₄→³H₄) at 1.4 µm, and amplification takes place by the following steps:

1. A small fraction of Tm ions are excited to the ³F₄ level (due to weak GSA),
2. Ions at the ³F₄ level are mostly excited to the ³H₄ level (due to strong ESA),
3. Ions at the ³H₄ level emit a photon and drop to the ³F₄level,
4. Repeat the cycle 2 and 3 without returning to the ground state.

Figure 2

Pumping at 1.0 µm

Another choice of pumping wavelength is 1.0 µm. The energy diagram and pumping scheme are shown in Figure 3. This scheme would be superior for high-power amplification than the 1.4-µm pumping, as high-powered Yb-doped fiber lasers are commercially available in this wavelength range. The efficiency is, though, fundamentally lower due to the larger energy difference between the pump and signal; the larger energy difference results in excess heat (see dotted lines in Figure 3).

Figure 3

 

FiberLabs’ TDFA Products

FiberLabs offer various kind of TDFA products. Please click here to see our product lineup.