The distance between the bars within a stack of laser diode bars or “pitch”, is only nominal value of the average diode laser bar spacings and by no means a constant precise value. Typically, the tolerance on bar spacing is about 50 um.
The term pitch somehow implies the parallelism of the bars, but the measurements reveal that this is not the case. Therefore, making cylindrical lens array for the fast axis collimation with constant pitch and good parallelism between the lenses might not be the best solution after all. The mismatch between the real-world laser diode bar stack and the cylindrical lens array with parallel lenses and constant pitch would lead to increased overall beam divergence.
To solve this problem, Doric Lenses Inc. has developed a procedure of making FAC lens arrays that match 3D geometry of specific laser diode bar stack. The pattern of emitters that makes up the laser diode bar stack is measured and digitalized data is used to fabricate corresponding lens holder.
Typically, we use standard doric™ gradient-index cylindrical lenses to populate the lens array holder as they are least sensitive to lateral misalignment and focusing errors, but other doric™ acylindrical lenses could be used as well. The irregularities in the geometry of the laser diode stack are replicated within the fast axis lens array, and consequently, the mismatch between the stack and lens array is significantly reduced. With this new technology, initially developed for space applications, it is possible to align each fast axis collimating lens with corresponding diode bar position in a single step.
The holder is designed to be glued on the stack, or to be bolted to the heatsink. Material of the holder is selected to match thermal expansion of the heatsink material and therefore to provide passive athermal collimation.
NOTE : A paper about our LD stack collimation technique have been presented by M. Roy McBride of Power Photonic. A copy is available on our website at : www.doriclenses.com/lire/64.html