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, articles on TDFAs at 0.8 µm and 1.65-2.1 µm will be added in the near future.

 

Figure 1: Major NIR emission wavelengths from thulium ion.

 

TDFA for 1.45-1.5 µm (S-band)

Thulium (Tm) shows a broad emission centered at 1.47 µm, and the spectral range covers the S-band; therefore TDFA has been used in optical communication using the S-band.

Pumping scheme

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: 3H63F4) and a strong Excited-State Absorption (ESA: 3F43H4) 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 3F4 level,
  4. Repeat the cycle 2 and 3 without returning to the ground state.

 

Figure 2: Energy diagram and amplification using 1.4-µm pumping.

 

Another choice of pumping wavelength is 1.05 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: Energy diagram and amplification using 1.05-µm pumping.
(The dotted line indicates heat radiation due to multiphonon relaxation.)

 

Choice of host glass

Another 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 a photon. An optical fiber made by low-phonon-energy host glass, such as fluoride fiber or tellurite fiber, is needed for efficient amplification in the S-band.

 

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