Repeatable Nonlinearity in Frequency Swept DFB Diode Laser and Its Compensation in Fiber Sensors

Nonlinearity in optical frequency-sweeping of a DFB diode laser is investigated and proved to be repeatable. Using a fiber interferometer, the nonlinearity is observed as changes in the instantaneous beat frequency of its interferogram. Comparison of nonlinearity extracted from measurements taken with a few-month gap shows a very small difference of 0.03% between them. The one-time extraction of the repeatable nonlinearity permits reducing its own effect in a fiber sensor without the need of using an auxiliary reference interferometer. Once measured for a particular DFB diode laser, the equidistant resampling mesh can be used for reducing negative effects of nonlinearity in another interferometric fiber system. The nonlinearity reduction is implemented in a fiber refractometer, resulting in one order of magnitude improvement in the resolution of the sensing system, from
$7\times 10 ^{-4}$
to
$7\times 10 ^{-5}$
. The reported method can be easily implemented in different interferometric sensing systems.