  Speckle patterns have been widely confirmed that can be utilized to
reconstruct the wavelength information. In order to achieve higher resolution,
a varies of optical diffusing waveguides have been investigated with a focus on
their wavelength sensitivity. However, it has been a challenge to reach the
balance among cost, volumes, resolution, and stability. In this work, we
designed a compact cylindrical random scattering waveguide (CRSW) as the light
diffuser only by mixing TiO2 particles and ultra-violate adhesive. The speckle
patterns are generated by the light multiple scattering in the CRSW.
Importantly, the thin layer of upconversion nanoparticles (UCNPs) were sprayed
on the end face of the CRSW. This allows the near infrared (NIR) light to be
converted to the visible light, breaking the imaging limitation of visible
cameras in the NIR range. We further designed a convolution neural network
(CNN) to recognize the wavelength of the speckle patterns with good robustness
and excellent ability of transfer learning, resulting in the achievement of a
high resolution of 20 kHz ( 0.16 fm) at around 1550 nm with temperature
resistance of 2 celsius. Our results provide a low-cost, compact, and simple
NIR wavemeter in particular with the ultra high resolution and good temperature
stability.
