Infrared lasers based on ho3+:kre(wo4)2 crystals with tm3+or yb3+ as sensitizers

Identifier:  TDX:935

Handle: https://hdl.handle.net/20.500.11797/TDX935

Authors:  Jambunathan, Venkatesan

Abstract:
Eye-safe solid-state lasers that operate in the 2µm spectral range are the subject of interest in the present years because of their potential applications in the field of remote sensing, medicine and as a pump source for Optical Parametric Oscillators (OPOs). Laser transitions around 2 µm are possible in the trivalent lanthanide ions Tm3+ (Tm) (slightly below 2 µm) and Ho3+ (Ho) (slightly above 2 µm). Laser generation in Tm ions is easily achieved with comfortable diode pump sources, however, Ho lasers have usually been achieved in the past either by co-doping the active medium with Tm or by direct pumping of the Ho ions with Tm lasers. Recently, relatively cheap diodes emitting around 1.9 µm are in the market to realize Ho lasers with great potential for power scaling. Ho lasers are more suitable than Tm lasers especially for medical applications because of two reasons: The laser wavelength is slightly above 2µm, where water (main component of human tissue) shows slightly less absorption than the typical wavelength of Tm leading to a deeper penetration in human tissue. The second reason is that Ho lasers can operate in pulsed regime delivering higher energies than Tm lasers due to the longer lifetime of the emitting level 5I7 and medical applications are required to be generally in pulsed regime to avoid thermal damage of human tissue. Many oxide and fluoride crystals were shown to be suitable host for Ho, however little attention was paid to the monoclinic potassium rare earth double tungstate crystal, shortly KRE(WO4)2 or KREW, where RE= Y, Gd, Lu known to be very efficient rare earth solid state hosts for generating intermediate power levels. These anisotropic crystals exhibit very high absorption and emission cross sections when doped with lanthanide ions and especially for selected polarizations. Considering the potentialities of Ho and good properties of KREW, our work focuses in the investigation of the laser performances of a Ho doped KREW either by using Tm or Yb as sensitizers and by using in-band pump sources emitting around 1.9 µm, where the development of compact solid state infrared laser emitting at 2.1 µm for intermediate power levels is followed. Here, in this thesis, we present the results based on growth of single doped Ho:KREW, co-doped (Ho,Tm) and (Ho,Yb):KLuW crystals of several doping concentrations, their characterisation in terms of structure, composition and spectroscopy and finally dedicated for the laser generation around 2.1 µm from these materials, which was highly successful.