Optical fibers for optical amplifiers

An amplifying optical fiber wherein Tm ions of .alpha. ppmw are contained in a core of said fiber, where .alpha. is the amount of doped Tm ions, said fiber has a confinement factor .eta. of light confined in the core having wavelength of 1.65 .mu.m, Tb ions are doped in a clad of said fiber employing (1.8.times..alpha.)/(1-.eta.) (ppmw) as an upper limit, where the confinement factor .eta. is represented by

• .eta.=1-exp (-(a/w).sup.2)
• where w=a.times.(0.46+1.619 V.sup.-1.5 +2.879 V.sup.-6),
• when the specific refraction index difference of said optical fiber is .DELTA.n, the core diameter is a, the refraction index of the core glass is n.sub.1, the wavelength of input light is .lambda., and a normalized frequency V=2 .pi.n.sub.1 a(2.DELTA.n).sup.1/2 /.lambda..

An amplifying optical fiber, comprising:

• a core containing Tm ions as activation ions which provide an absorption spectrum having 1.65 .mu.m as a center wavelength and a spontaneous emission spectrum having 1.8 .mu.m as a center wavelength;
• and
• a clad containing at least one of Tb ions, Eu ions, Ho ions, Dy ions, Sm ions and Nd ions as ions directly absorbing said spontaneous emission spectrum having 1.8 .mu.m as the center wavelength.

An amplifying optical fiber comprising a core composed of more than two layers and a clad surrounding said core and having a refraction index lower than that of said core, and

• wherein Tm is contained in more than one of said more than two layers of said core to provide a spontaneous emission spectrum having 1.8 .mu.m as a center wavelength and an absorption spectrum having 1.65 .mu.m as a center wavelength through a stimulated emission transition by pump light, and
• wherein at least one of Tb ions, Eu ions, Ho ions, Dy ions, Sm ions and Nd ions is contained in more than one of said more than two layers in which Tm is not contained, as ions directly absorbing said spontaneous emission spectrum having 1.8 .mu.m as the center wavelength.