No abstract

We demonstrate a Tm-doped fiber laser which operates in three self-sweeping regimes, depending on pump power: (1) a single-frequency self-sweeping operation with a normal scanning direction; (2) a multi-frequency operation with a reverse direction, and; (3) wavelength stopping. In the last case, the wavelength can be stopped at an arbitrary value in the range from 1912 nm to 1923 nm, depending on the prehistory of spectral dynamics of the laser. The wavelength fluctuations in the case of wavelength stopping are below our measurement resolution of 50 pm within 5 min. As a result, we have manually controlled the laser wavelength with pump power adjustment. This result can extend the potential practical applications of self-sweeping lasers.

An operation of a linearly polarized Raman fiber laser with random distributed feedback based on a polarization-maintaining twin-core fiber (TCF) is demonstrated for the first time, to the best of our knowledge. The results indicate that the TCF allows one to obtain laser generation with a linewidth that is about five times smaller than that for the random laser based on a conventional fiber with similar parameters. The reasons for narrowing include both the weakening of nonlinear effects due to the power density reduction and the spectrally selective properties of the TCF.

A near infrared spectrometer based on the self-scanning Tm-doped fiber laser is developed. The spectrometer is applied to measurements of the water nuclear spin isomer abundances using water absorption lines in the vibrational band ν 2 + ν 3 near 1.91 µm. The ortho/para ratio of water nuclear spin isomers is measured to be equal to 3.06 ± 0.02, which is close to the theoretical prediction of 3 at room temperature.