Raman Shifting Crystals
Light transmitting through matter is partially scattered and some part of scattered light has frequencies shifted from frequency of incident beam by quantities equal to intrinsic vibrational frequencies of the material. The effect is called Raman scattering. If the incident beam is sufficiently intense and coherent (e.g. laser beam), it is possible to get amplification of light at5 the Raman frequencies. This is called Stimulated Raman Scattering and the effect opens the way for building new sources coherent radiation, so called Raman Shifters, expanding spectral range of conventional lasers. Lasers incorporating Raman shifters find wide application in science, spectroscopical researches, medicine etc.
Efficient stimulated Raman Scattering is observed in a number of gases, fluids and solid materials. Rapid development of all – solid state laser systems lead to building Raman shifters based on crystal materials such as Barium Nitrate or Ba(NO3)2 and Potassium Gadolinium Tungstate or KGd(WO4)2 also called as KGW.
Typical scheme of Raman coversion of Nd:YAG laser lines using Ba(NO3)2 crystal is shown below:
Main parameters of Ba(NO3)2 and KGd(WO4)2 crystals
Chemical Formula |
Ba(NO3)2 |
KGd(WO4)2 |
Symmetry |
Cubic |
Monoclinic |
Space Group |
P213 |
Ñ2h |
Lattice parameters, A |
a=8.11 |
a=8.10, b=10.43, c=7.60 |
Density , g/cm3 |
3.25 |
7.27 |
Mohs Hardness |
2.5 – 3.0 |
4 – 5 |
Solubility in water |
Solube |
Insoluble |
Thermal expansion (100 oC), 10-6/K |
a =5
|
aa=4; ab=1.6
ac=8.5 |
Thermal conductivity (100 oC), W/mK |
1.17 |
Ka=2.6; Kb=3.8
Kc=3.4 |
Transmission range (at 10% level), mm |
0.35-1.8 |
0.35-5.5 |
Damage threshold
(l =1.064 mm, t =10 ns), GW/cm2 |
> 1 |
>10 |
Refractive index
at l =0.546 mm
1.06 mm
|
1.5756
1.5551
|
np=1.978; nm=2.014
ng=2.049
|
Raman shift, cm-1 |
1047 |
901 (p[mm]p)
768 (p[gg]p) |
Raman linewidth, cm-1 |
0.4 |
5.4 (901 cm-1)
6.4 (768 cm-1) |
Gain, cm/GW (λ =1.064 mm) |
10 |
4.0 (901 cm-1)
4.4 (768 cm-1) |
Optical homogeneity, 10-6/cm |
3-7 |
4-10 |
Max. sizes of finished elements, mm
|
10õ10õ100
15õ15õ50 |
10õ10õ50
|
Typical fabrication specs:
Surface quality
Flatness
Parallelism |
20/10 S/D
λ/4
< 20’’
|
10/5 S/D
λ/8
< 20’’
|
Typical AR coatings |
R< 0.5% for pump wavelength and 1st Stoke
R< 1.5% for pump wavelength and 1st- 3rd Stoke |
|
