Element doped in fibre amplifiers for optical fibre communication (erbium)

The same question, asked 2 times: 10 May 2024 S1, 13 Apr 2026 S1. Every wording below, oldest first.

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Question 1
10 May 2024 S1
75 $Which of the following elements is doped to obtain fiber amplifiers for optical fiber communication system?$
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Question 2
13 Apr 2026 S1
34 $Which of the following elements is doped to a fiber amplifier in an optical fiber communication system?$
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