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KBBF

Crystal KBBF is capable of producing DUV and even vacuum-UV coherent light by direct second harmonic generation, but a major problem is its plate-like growth nature that makes it very difficult to grow samples thicker than4 mmalong the c-axis, and KBBF crystal is cleaves easily, so it is impossible to cut the crystal along the phase-matching angle.


With a directly incident pump, the external incident angle will increase to 90° for a second harmonic wavelength of 235 nm. To solve the problem of producing coherent laser light below 235 nm, a special prism-coupling technique ( PCT )  was developed to avoid cutting the crystals along the phase-matching angle.

A KBBF crystal is sandwiched between two prisms made of UV-fused silica, which has almost the same refractive indices as KBBF in the visible to UV spectral region.


Using PCT devices, the maximum aperture is about 6×6 mm, the thickness of KBBF is about1.5mm.


Advantages

4th harmonic generation of Nd:YAG laser at 266nm

(1) Advantages of KBBF over BBO and CLBO

●Much higher damage threshold

●Much smaller two-photon absorption

●No photorefractive effect

●No moisture

(2)Disadvantages of KBBF over BBO and CLBO

   ● Deff (KBBF) = 1.53(KDP)

   ● Deff (BBO) = 2.72(KDP)

   ● Deff (CLBO) = 1.97KDP) at 266nm

Targets the KBBF can realize:

● Watt-level 193nm output power with 4th HG of Ti:sapphire Laser

● 100-500mw QCW 177.3nm output power with 6th HG of Nd:YAG Laser

● Wide tunable 2.0mw coherent light output from 220nm-175.0nm with 4th HG of tunable Ti:sapphire Laser

● average 7.8 watt-level 266nm output power

stable mw-level 165-167nm output power


Optical Properties

Deff :

KBBF crystal belongs to the space group R32, so from the point group symmetry of D3 and the IEEE/ANSI standard definition of the dij coefficient, it has only two nonzero dij coefficients, namely, d11 and d14. The matrix form of the coefficients can be written as follows:


where d14 is very small according to both theoretical calculation and experiments. On the other hand, the effective deff coefficients of KBBF are:


deff=d11cosqcos3f (type I)

deff=d11cos2qsin3f (type  = 2 \* ROMAN II)

By comparing the fringe envelope of the d11 coefficient with the d36 coefficient in KDP, the d11 of KBBF is deduced to be (0.47+0.01) pm/V (d36(KDP)= 0.39 pm/V).

Transparency range

The cutoff wavelength on the UV side is located at 150 nm, and on the IR side around3.5mm. There is virtually no absorption in this region.

Sellmeier coefficients

Sellmeier equations as follows: (l is in units of mm)

no2=2.1738+0.00764λ2-0.01641-0.01174λ2

ne2=1.9319+0.00623λ2-0.00400-0.00356λ2


Damage threshold

It has been measured and compared with that of BBO under the same laser pump conditions (1064 nm, 1 Hz, 10.5 ns), The value is 35.51 J/cm2 (3.38 GW/ cm2) for the former, which is more than two times larger than the 13.47 J/cm2(1.28 GW/ cm2) of the latter; With a 200 fs pulse width and 1 kHz repetition rate at 390 nm, the damage threshold of KBBF was found to be as high as 60 GW/ cm2.