  The detection of the state of polarization (SOP) of light is essential for
many optical applications. However, it is a challenge for cost-effective SOP
measurement due to the complexity of conventional methods and poor
transferability of new methods. Here, we propose a straightforward, low-cost
and portable SOP measurement system based on the multimode fiber speckle.
Convolutional neural network is utilized to establish the mapping relationship
between speckle and Stokes parameters. The lowest root mean square error of the
estimated SOP on Poincare sphere can be 0.0042. This method is distinguished by
its low cost, clear structure and applicability to different wavelengths with
high precision. The proposed method is of great value in polarization-related
applications.
