The absorption line according to the 2 F7/2(1) → 2 F5/2(1') transition of Yb 3+ ions in the crystal fiber is shown in the inset of Figure where the half-width of the transition line is indicated by horizontal arrows. The absorption line of the 2 F7/2(1) → 2 F5/2(1') transition being measured at 300K appears symmetrical and exhibits a peak at a frequency of νmax = 10246 cm -1 . The full width of the transition at half height, Δfull (300 K), was determined to be 30 cm -1 by approximating the line with a Voigt distribution function . Lowering the temperature to 77 K causes the absorption line to narrow to a value of Δ′(77K) = 17 cm -1 at the instrumental function of RA = 2 cm -1 . The absorption lines of the 4 F(1) → 4 F5/2(1') transition were fitted using a Gaussian distribution function. Using the equation derived in in the work , the value of the homogeneous broadening of the 4 F7/2(1)