Praseodymium-Doped Fiber Amplifier (PDFA)

How PDFA works in optical communication

Praseodymium-doped fiber amplifier (PDFA) is one type of rare-earth-doped fiber amplifiers , which operates in the O-band. The O-band is now widely used in data centers (high-speed Ethernet), metro network, and access networks (various PON systems), whose distances are typically less than a hundred kilometers. Attenuation of conventional optical fiber in the O-band is nearly the twice of that in the C-band, and PDFA can be used to compensate the attenuation.

Figure 1: Optical telecommunication optical bands (PDFA operates in the O-band).

Fluoride fiber for PDFA

Figure 2 shows a simplified energy diagram of Pr, showing the energy levels involved in emissions at the O-band. Also shown in the figure are different energy transition behaviors of Pr ion doped in silica and fluoride glass.

Figure 2: Simplified energy diagram of Pr, and energy transitions in silica glass (left) and fluoride glass (right).

Note that decay from ³F₄ to ³H₅ level takes place by a series of phonon decays, via energy levels that are not shown in the figure.

Unfortunately, when doped in silica glass, emission from a Pr ion is very poor due to its large phonon energy. In silica glass, a Pr ion excited to the ¹G₄ level rapidly decays to the ³F₄ level by generating multiple phonons, i.e. the energy turns into multiple atomic vibrations in the glass (and does not turn into a photon). Host glass with a low phonon energy is thus required in order to efficiently amplify light in the O-band using a PDFA.

Fluoride glass has a much lower phonon energy than silica glass, and among many compositions of fluoride glass, ZBLAN is the most commonly used fluoride glass host for a PDFA. In ZBLAN, a larger number of phonons are required for such non-radiative (no photon emission) decay to take place. The probability of the decay decreases with the required number of phonons, and thus non-radiative decay is well suppressed and efficient emission can be obtained by using ZBLAN.

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