Parametric amplification in large-mode-area hybrid photonic-crystal fibers

Rare-earth-doped fiber amplifiers find application in a wide range of fields, including imaging, material processing, and spectroscopy. However, certain spectral regions cannot be covered by such devices due to the lack of rare-earth dopants with emission in these regions. There is therefore a growing interest in increasing the spectral coverage of fiber amplifiers.

One approach to achieving this lies in exploiting the nonlinear response of silica for parametric amplification. If energy conservation and phase matching (PM) requirements are met, parametric amplification through four-wave mixing (FWM) in silica fibers can in principle be applied to any spectral region.¹ Management of the fiber dispersion—made up of a combination of material and waveguide dispersion—is the key to controlling PM and, therefore, to tuning the frequency components of an FWM process. Because the material dispersion in silica is intrinsic, only waveguide dispersion can be used to tailor the fiber dispersion. To achieve power scaling and good beam quality, large-mode-area (LMA) single-mode fiber designs are required.

We have developed an LMA hybrid photonic-crystal fiber (PCF) in which it is possible to tailor the core size without losing control over the fiber dispersion. A cross-sectional microscope image of the PCF is shown in Figure 1. The cladding consists of airholes and high-index germanium-doped silica rods (Ge-rods), and the core has a diameter of 36μm. The two principal fiber axes are illustrated in Figure 1(b). The slow axis lies parallel to the Ge-rods, and the fast axis is orthogonal to them. This combination of airholes and Ge-rods give rise to index guiding and photonic-bandgap (PBG) guiding, respectively.^{2, 3} Three transmission bands are visible in the white-light spectrum through a 2m section of the hybrid PCF (see Figure 2). Fiber modes near the PBG edges of the hybrid PCF are strongly affected by waveguide dispersion, thus providing a mechanism by which the fiber dispersion can be tailored.⁴ The birefringence properties of the hybrid PCF can also be exploited for PM.⁵ Furthermore, if the fiber supports more than one mode, PM through intermodal FWM may occur. There are therefore a number of PM mechanisms in the fiber that can be employed for parametric gain in the desired spectral regions.

Figure 1. (a) Microscope image of the hybrid photonic-crystal fiber (PCF). The small dark features are airholes, and the larger dark features are germanium-doped silica rods. The gray background is silica. (b) The principal axes of the fiber.

Figure 2. White light transmission spectrum through a 2m length of hybrid PCF.

We used a 40ps 1064nm fiber laser with a linearly polarized output to generate spontaneous degenerate FWM in the 2m-long hybrid PCF. Output spectra polarized along the slow and fast axes of the hybrid PCF and images of the output at selected wavelengths are shown in Figure 3.

Figure 3. Output spectra polarized along the slow and fast axes of the hybrid PCF for different pump peak powers. Images of the modes at wavelengths of 848, 995, 1145, and 1425nm are shown.

We observed Raman scattering with the same polarization as the pump laser at the first Stokes wave (near 1120nm) in the slow axis. This behavior is expected because the pump power lies above the Raman threshold. However, because the Raman gain is substantially lower for orthogonally polarized pump lasers and Stokes waves,¹ Raman scattering is not observed in the fast axis. Birefringence-assisted intramodal FWM, where the signal and idler are generated in the fundamental mode (FM), occurs at 995 and 1145nm (polarized along the fast axis). Additionally, intermodal FWM, which occurs between the pump wave in the FM and the signal and idler waves in the LP₁₁ mode, is observed at 848 and 1425nm. This behavior demonstrates that parametric amplification relies on PM via waveguide and mode dispersion in the hybrid PCF.

We achieved a fairly high conversion efficiency of 17.3% from the pump laser to the 848nm component for a pump laser peak power of 165kW, corresponding to an average output power of 1.2W at 848nm. The conversion efficiency is limited by cascaded FWM,⁵ which sets in when a considerable amount of power is transferred to the 848nm component. The onset of intermodal FWM in the fiber generates an LP₁₁ mode. This mode experiences losses because the fiber is unable to wholly confine it, causing a reduction to the output average power with respect to the input average power for higher pump peak powers.

We have achieved parametric amplification in a hybrid LMA PCF due to the unique functionalities offered by our novel cladding structure. The large design parameter space of hybrid PCFs—in terms of size and the number of airholes and Ge-rods used—allows the manufacture of a fiber that is well suited for specific amplification processes, where a spectral component at a certain wavelength is desired. This development may represent an enabling technology to extend the spectral coverage of high-power fiber light sources. In our future work, we intend to investigate other hybrid PCF designs to potentially increase the amplifier performance.