A group from the Prokhorov General Physics Institute of the Russian Academy of Sciences, Dianov Fiber Optics Research Center (Moscow, Russia), and the G.G. Devyatykh Institute of Chemistry of High Purity Substances of the Russian Academy of Sciences (Nizhny Novgorod, Russia) has been working on modifying light passing through an optical fiber by adding a structure separated from the core, but inside the cladding. First, the researchers came up a mode filter by making the added structure absorbing—and now, they have created a resonant structure that acts as a spectral filter for the fiber. The group has experimentally verified the results.
The idea is to use a high-refractive-index rod or rods placed a distance S away from the core to resonantly couple with the core, absorbing certain spectral regions while leaving others unaffected. This simple idea is especially appealing because it can be used as a filter on single-mode fibers—for example, suppressing undesired lasing frequencies in fiber lasers, or suppressing stimulated Raman scattering.
The researchers first modeled the technique using COMSOL Multiphysics software, as well as using software they wrote themselves. In the fiber design, the core and each of the high-index rods support propagation of one mode; thus, a fiber with three rods in addition to the core would support four modes. The fiber core diameter for all modeled fibers was 10 μm and the index difference of the core was 0.002. In one example, a single rod with a diameter of 1.14 μm and an index difference of 0.01 was placed a distance from the core (see figure).
Distance between rod and core is important
The model shows that the mode propagating mostly in the core at wavelengths shorter than the resonant wavelength is then mostly localized in the high-index rod at wavelengths longer than the resonant wavelength, thus spectrally separating the light (at the resonant wavelength, the power is split equally between core and rod modes). The resonance bandwidth changes depending on the distance between the core and the rod, with smaller distances producing a larger bandwidth. The ratio between the loss at the wavelength to be suppressed and the loss at the operating wavelength is higher for narrower resonant peak bandwidths. Bending the fiber results in a shift of the resonant wavelength.
To enhance mode suppression, the researchers also studied a configuration with three high-index rods equally angularly spaced around the fiber axis. In this case, bending the fiber did not much shift the center wavelength of the suppression, but broadened it while reducing the peak suppression at the center of the spectral band.
The researchers fabricated a single-mode silica fiber with a high-index rod designed to resonantly suppress light at around the 1030 nm wavelength, which is the spectral range of unwanted amplified spontaneous emission of a ytterbium (Yb)-doped laser operating at 977 nm. The configuration suppressed the signal near 1027.5 nm by about 20 dB through a 1-m-long section of fiber.
The researchers note that the measured intensity of the loss peak is higher than what would be expected simply from the absorption coefficient of the high-index rod; they hypothesize that the (desirable) additional loss is due to bending or microbending losses.
1. S. Aleshkina et al., Opt. Lett. (2021); https://doi.org/10.1364/ol.412429.