Raman gain from waveguides inscribed in KGd(WO4)2 by high repetition rate femtosecond laser

Abstract

Articles you may be interested in 1.84 m emission of Tm 3 + sensitized by Yb 3 + ions in monoclinic K Gd ( W O 4 ) 2 single crystalsWe report the formation of waveguides in Raman-active KGd͑WO 4 ͒ 2 with a focused, high repetition rate femtosecond laser. Parallel guiding regions, formed to either side of the laser-induced damage track, supported TE and TM modes that coupled efficiently to optical fiber at telecom wavelengths. Micro-Raman spectroscopy of the guiding regions revealed the preservation of the characteristic 768 and 901 cm −1 Raman mode intensities. Raman gain with 6% efficiency was demonstrated for the 768 cm −1 Raman line by pumping the waveguide with an infrared 80 ps source, the first time Raman gain has been reported in laser formed waveguides.

High-Resolution Refractive Index and Micro-Raman Spectroscopy of Planar Waveguides in KGd(WO$_{4}$)$_{2}$ Formed by Swift Heavy Ion Irradiation

“…This strong Raman shift in the waveguide core region can have an impact on the output wavelength of SRS-based devices. This was found previously in Raman gain experiments performed on waveguides fabricated by compressive stress through femtosecond laser writing, where a similarly shifted 768-cm mode in these waveguide cores produced SRS output at wavelengths which correlated strongly with these measured Raman mode shifts [10]. Overall, although strong waveguides are formed using oxygen irradiations as seen by the refractive index profiles, the Raman performance in the guiding regions is significantly reduced, suggesting that damage to the crystal lattice and thus device performance is expected to decrease for Raman conversion applications.…”

“…The shifts of the Raman modes reveals some interesting characteristics, where the 901-cm mode is found to shift by cm in the guiding region, while the 768-cm mode shifts by approximately cm . This suggests this 768-cm mode to be a sensitive stress indicator for KGW, as was also reported in [10]; however, here the presence of lattice damage can also be a factor in producing frequency shifts in addition to stresses. This strong Raman shift in the waveguide core region can have an impact on the output wavelength of SRS-based devices.…”

“…The cross-sectional area is also seen to be recovered in the guiding region, and thus the Raman steady-state and transient gains are expected to be preserved for waveguiding devices using SRS processes. There is a slight negative Raman frequency shift of about cm observed in the guiding region for the 901-cm mode, suggesting a tensile stress following previous results on tungstate crystals [10], [23]. Annealing may help eliminate these stress effects, which will otherwise cause an output wavelength shift in SRS-based devices.…”

“…Stress measurements were performed on PbWO crystals, where its similar 902-cm mode was found to shift to higher frequencies for compressive stress and lower frequencies for tensile stress [23]. This behavior was also found for channel waveguides fabricated in KGW using femtosecond laser processes that use compressive stress to form core waveguiding regions [10]. In addition, the measurement of Raman frequency shifts in the waveguide core region through either stress or damage will directly indicate the subsequent shifts of the output Stokes wavelength in SRS-based devices.…”

“…Waveguides have been fabricated in double tungstate crystals using different techniques such as light ion implantation [5], [6], liquid-phase epitaxy [7], and femtosecond laser writing [8]- [10]. Swift heavy ion irradiation (SHI) is becoming increasingly popular as a method of strongly modifying the refractive index of optical crystals in well-defined regions [11]- [14].…”