Firstly, you get a +rep for this question. Instead of being able to pull the knowledge off the top of my head, I actually had to go and do some research, and learnt quite a fair bit in the process.
To understand the reasoning behind the decision for the use of LBO (instead of KTP) in the Verdi, you need to understand the properties of both crystals. I've listed them out below for ya.
KTP:
- Has a relatively high damage threshold
- Can be non-critically phase matched
- (Relatively) non-temperature dependant
- Efficient (even when using a shorter crystal)
LBO:
- Needs precise temperature
- Needs to be critically phase-matched
- Longer crystals are needed for higher efficiency
- Very little problems with damage threshold
Now, in normal green lasers, it can be apparent that KTP is the better choice. High efficiency, no need for critical phase matching and no need for precision (~1C) temperature stabilization.
As a result, KTP is pretty much seen in
every 532nm laser out there.
However, at the power that the Verdi is running at, KTP will no longer suffice. Take note of the last point listed under LBO- very high damage threshold.
The diameter of the KTP needed would mean that the tight beam specs of the Verdi would no longer be achievable if KTP was used- it would not be feasible to maintain the beam diameter after SHG.
As a result, Coherent decided to use LBO, as it did not suffer from the problem with the damage threshold. Although it adds to the complexity of the system (needing precision electronics and alignment), it does mean that the high powers are still achievable while maintaining a very low beam diameter (for that power level).
Other companies such as CNI continue to use KTP in their 5W+ green lasers, however, you will notice that their beam diameters are often much larger (by up to 3 or 4 times) and that they are running in Near TEM00 or Near TEM01 modes.