Optical fiber evaluation method and optical fiber evaluation apparatus
An optical fiber testing method is presented for measuring the change amount for the wave number k of a Brillouin Frequency Shift ν in stimulated Brillouin scattering generated in the same acoustic mode with respect to each target propagation mode. In this way, the ratio of the change amount measured at each propagation mode is acquired as the group delay ratio between the modes.
1. An optical fiber testing method for evaluating a group delay ratio between modes of an optical fiber to be measured, the method comprising:
measuring a change amount for wave number in a Brillouin Frequency Shift for each two propagation modes having same acoustic mode excited when stimulated Brillouin scattering occurs; and
calculating a ratio of the change amount for each of the propagation modes as the group delay ratio between the modes.
2. The optical fiber testing method according to claim 1 , wherein when the change amount is measured, the method comprises:
allowing probe light and pump light having a given wave number to enter the optical fiber to be measured for each of the propagation modes, thereby obtaining a Brillouin Frequency Shift amount;
allowing probe light and pump light having a different wave number different from the given wave number to enter the optical fiber to be measured for each of the propagation modes, thereby obtaining a Brillouin Frequency Shift amount; and
determining a difference between the Brillouin Frequency Shift amount for the given wave number and the Brillouin Frequency Shift amount for the different wave number for each of the propagation modes, as the change amount.
3. The optical fiber testing method according to claim 1 , wherein when the ratio of the change amount is calculated, the ratio is calculated by Expression C1,
[
Math
.
C1
]
τ
m
τ
n
=
dv
Bm
/
dk
dv
Bn
/
dk
(
C1
)
wherein τ m and τ n represent group delay times of the modes per unit length in the optical fiber to be measured in the propagation modes of LPm and LPn, respectively;
τm/τ n represents the ratio of the change amount; and v Bm and v Bn represent the Brillouin Frequency Shift amounts generated in the propagation modes of LPm and LPn, respectively.
4. An optical fiber testing device for evaluating a group delay ratio between modes of an optical fiber to be measured, the device comprising:
a measuring instrument for measuring a change amount of wave number in a Brillouin Frequency Shift for each two propagation modes having the same acoustic mode excited when stimulated Brillouin scattering occurs; and
a calculator for calculating a ratio of the change amount for each of the propagation modes as the group delay ratio between the modes.
5. The optical fiber testing device according to claim 4 , wherein the measuring instrument
allows probe light and pump light having a given wave number to enter the optical fiber to be measured for each of the propagation modes, thereby obtaining a Brillouin Frequency Shift amount;
allows probe light and pump light having a different wave number different from the given wave number to enter the optical fiber to be measured for each of the propagation modes, thereby obtaining a Brillouin Frequency Shift amount; and
determines a difference between the Brillouin Frequency Shift amount for the given wave number and the Brillouin Frequency Shift amount for the different wave number for each of the propagation modes, as the change amount.
6. The optical fiber testing device according to claim 4 , wherein the calculator calculates the ratio of the change amount by Expression C1,
[
Math
.
C1
]
τ
m
τ
n
=
dv
Bm
/
dk
dv
Bn
/
dk
(
C1
)
wherein τ m and τ n represent group delay times of the modes per unit length in the optical fiber to be measured in the propagation modes of LPm and LPn, respectively;
τ m /τ n represents the ratio of the change amount; and v Bm and v Bn represent the Brillouin Frequency Shift amounts generated in the propagation modes of LPm and LPn, respectively.