Ultraviolet fiber lasers are highly desired in different fields like lithography, laser processing, optical communications, optical storage, biomedicine, etc. On the other hand, all-fiber frequency conversion technology is of great significance in scientific and practical aspects, as it provides an alternative to the current solutions based on nonlinear crystals. The development of special optical fibers with both suitable mechanical performance and conversion efficiency, and reducing the difficulty of their preparation, are the key challenges in bringing this novel technology into practical application. In this paper, we design three step-index optical fibers of simple structure, namely, a conventional single-cladding fiber (CSCF) with high numerical aperture, a microfiber (MF) and a W-type double-cladding fiber (WDCF), and study the third-harmonic generation in ultraviolet band with them respectively. The fundamental (pump) wavelength used in this work is 1064 nm and the third harmonic is at 355 nm.
In order to achieve good transmission in the ultraviolet band, the core of all three optical fibers are designed to be made of pure silica glass, and the core diameters are determined according to the phase matching condition for the fundamental wave and the third harmonic, by solving the eigenvalue equations. The cladding of CSCF is fluorine-doped silica glass, and the cladding of MF is air; for WDCF, the inner and outer claddings are fluorine-doped silica glass and fluoroplastics, respectively. Both CSCF and WDCF have solid cladding, and their core diameters can be greater than 2 μm, so they have adequate mechanical properties. In comparison, due to the air cladding and thin core the diameter of which has to be less than 1 μm for phase matching, the MF is fragile in structure and thus its mechanical performance is rather poor.
The conversion efficiency from these three fibers is investigated in detail, by solving numerically the coupled mode equations for the pump and the third harmonic with the Runge-Kutta method. The effect of random fiber roughness (i.e. core diameter fluctuation) and enhancement in conversion efficiency by cascading the fibers are also analyzed. The results show that the conversion in MF is the most efficient, which gives efficiency of 2% with a single MF segment 5 mm in length and more than 20% when the MFs are cascaded; however, MF requires strict fabrication accuracy, and the tolerance of core diameter is only ±0.3 nm. CSCF has the lowest conversion efficiency, which is 0.1% for a single segment 50 mm long and in the level of about 1% after cascading, and the tolerance of core diameter is ±1 nm. The conversion efficiency of WDCF is between those of CSCF and MF, nearly 2% with a 50 mm-long segment and about 16% when four such segments are cascaded; WDCF bears core diameter tolerance of ±3 nm, which is 3 times that of CSCF and 10 times that of MF.
Therefore, the W-type double-cladding fiber WDCF actually integrates the advantages of conventional single-cladding fiber CSCF and microfiber MF, showing both satisfactory mechanical performance and conversion efficiency, as well as reduced fabrication difficulty, which provides a promising solution for all-fiber third-harmonic generation in the ultraviolet band.