Page 361 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 32 1

strongly. Absorption coefficients also depend strongly on purity of the crystal and
the growth conditions. As such, the absorption coefficients can vary significantly
from vendor to vendor and can also vary as a function of the date of purchase
even if the crystals are from the same vendor. For many commercially available
nonlinear crystals. absorption coefficients are on the order of 1.0 m-I [2J]. With
absorption coefficients on this order, average power limits on the order of several
lvatts appear feasible. However, optical materials with larger commercial demand
can have significantly lower absorption coefficients. Because the heating parame-
ter depends on the product of the average absorption coefficient and the average
power, an order of magnitude decrease in the absorption implies an order of mag-
nitude increase in the average power. Although absorption effects can impose
practical limits, they can be mitigated through nonlinear crystal selection and
crystal growth development efforts.
Pulse repetition frequency (prf) does not enter into the preceding estimate of
the average power limit. As defined, the absorbed power which creates a thermal
gradient large enough to limit the effective volume of the nonlinear crystal is
estimated If absorption of the pump power is the primary contribution to the
heating, then the average power of the pump rather than the prf per se is the pri-
mary factor. However. if the absorption of the signal or idler is the primary con-
tribution to the heating, then the prf can have more of an effect. With a constant
average pov’er and a high prf. the pump energy per pulse decreases. If this in
turn decreases the conversion efficiency, less heating can occur. As such, as the
prf increases, the average power heating decreases. However, the signal and idler
power still decrease because of the lower conversion efficiency of even the ide-
ally phasematched interaction.
If even higher average power is required, the nonlinear crystal can be fabri-
cated into a series of thin plates. The thin plates could be cooled by flowing gas
between them. In essence. this decreases the thermal gradient by increasing the
surface to volume ratio of the nonlinear crystal [25]. For a geometry like this, the
longitudinal heat extraction technique is appropriate. While this technique will
work, antireflection coatings on the surfaces will be required. A practical limit
on the thickness of the plates will be set by the fabrication process.

7. NONLINEAR CRYSTALS

Many good nonlinear crystals are currently available for optical parametric
oscillators and amplifiers and new nonlinear crystals are being developed con-
stantly. In the early days of the development of optical parametric oscillators and
amplifiers. oiily a relatively few nonlinear crystals were available. In addition.
the available nonlinear crystals had limited utility, either because of fundamental
reasons or because of limited size and optical quality. Lack of good nonlinear
crystals limited development of practical devices utilizing nonlinear crystals in
these situations. Since then, many more nonlinear crystals have been discovered