Light intensity distribution measurement method and light intensity distribution measurement device
A light intensity distribution measurement apparatus is presented that is capable of accurately measuring the intensity of light in each mode at each position of an optical fiber through which light is propagated in a plurality of modes. With the light intensity distribution measurement apparatus, a gain coefficient matrix is acquired in advance, which is constituted by Brillouin gain coefficients of propagation modes with predetermined optical frequency differences measured using a reference optical fiber that exhibits the same properties as a measurement-target optical fiber and that does not cause mode coupling, and the intensity distribution of light in each propagation mode in a lengthwise direction of the measurement-target optical fiber is calculated based on the gain coefficient matrix and a difference in light intensity before and after Brillouin amplification of the probe light emitted in a predetermined propagation mode at a predetermined optical frequency difference measured using the measurement-target optical fiber.
1. A light intensity distribution measurement method for measuring an intensity distribution of light in a plurality of propagation modes, in a lengthwise direction of a fiber under test, the method comprising:
preparing a reference optical fiber that has the same number of propagation modes as the fiber under test and that does not cause mode coupling;
inputting pump light and probe light that have a given optical frequency difference, to the reference optical fiber, for each of combinations of the pump light in all of the propagation modes and the probe light in given one mode of the propagation modes;
acquiring a Brillouin gain coefficient generated for the probe light, for each of the combinations of the propagation modes;
acquiring Brillouin gain coefficients respectively corresponding to the propagation modes, while varying the given optical frequency difference for each of the propagation modes;
generating a gain coefficient matrix of the Brillouin gain coefficients for the propagation modes and the optical frequency differences;
inputting pump light and probe light that have the optical frequency difference with which the gain coefficient matrix has been generated, to the fiber under test, for the combinations of the propagation modes;
acquiring Brillouin amplification components generated for the probe light, for each of the optical frequency differences and each of the combinations of the propagation modes, as distribution in a lengthwise direction of the fiber under test; and
calculating, based on the distribution, a light intensity of the pump light in each of the propagation modes at a given point of the fiber under test by multiplying a vector of the Brillouin amplification components at the given point of the fiber under test by an inverse matrix of the gain coefficient matrix.
2. The light intensity distribution measurement method according to claim 1 , further comprising:
calculating a ratio between light intensities of the pump light in the propagation modes.
3. A light intensity distribution measurement apparatus comprising:
a light input unit that inputs pump light and probe light that have a given optical frequency difference, to an optical fiber, in all combinations of propagation modes;
a light receiving unit that measures a light intensity of the probe light in given one propagation mode of the propagation modes propagated through the optical fiber; and
a control calculation unit that controls the light input unit and calculates light intensity distribution of light in each of the propagation modes in a lengthwise direction of the fiber under test, based on the light intensity measured by the light receiving unit,
wherein the control calculation unit:
causes the light input unit to input pump light and probe light that have a given optical frequency difference, to a reference optical fiber that has the same number of propagation modes as the fiber under test and that does not cause mode coupling, for each of combinations of the pump light in all of the propagation modes and the probe light in given one mode of the propagation modes;
acquires a Brillouin gain coefficient generated for the probe light, for each of the combinations of the propagation modes, based on the light intensity measured by the light receiving unit;
causes the light input unit to vary the given optical frequency difference for each of the propagation modes;
acquires Brillouin gain coefficients respectively corresponding to the propagation modes, while varying the given optical frequency difference for each of the propagation modes, based on the light intensity measured by the light receiving unit; and
generates a gain coefficient matrix of the Brillouin gain coefficients for the propagation modes and the optical frequency differences, and
the control calculation unit further:
causes the light input unit to input pump light and probe light that have the optical frequency difference with which the gain coefficient matrix has been generated, to the fiber under test, for the combinations of the propagation modes;
acquires, based on the light intensity measured by the light receiving unit, Brillouin amplification components generated for the probe light, for each of the optical frequency differences and each of the combinations of the propagation modes, as distribution in a lengthwise direction of the fiber under test; and
calculates, based on the distribution, a light intensity of the pump light in each of the propagation modes at a given point of the fiber under test by multiplying a vector of the Brillouin amplification components at the given point of the fiber under test by an inverse matrix of the gain coefficient matrix.
4. The light intensity distribution measurement apparatus according to claim 3 ,
wherein the control calculation unit further calculates a ratio between light intensities of the pump light in the propagation modes.