The radiation of rather powerful multimode laser diodes (LD) has two key features:
- The beam quality along the fast and slow axes differs a lot.
- The divergence along the fast axis is so great that the beam cannot be considered paraxial.
The first key feature leads to the fact that the focal spot obtained in any optical system that does not introduce great aberrations will be either considerably elongated or will have very different focal depth for two transverse directions. The second feature leads to purely technical difficulties, since ordinary lenses when working with non-paraxial beams introduce considerable aberrations.
To focus laser diode beams einzel lenses, even aspheric, are rarely used. Usually, the collimator-focuser scheme is introduced. Sometimes, a prismatic or cylindric telescope (glass) is placed between the collimator and focuser to correct astigmatism. The collimator should be able to intercept and collimate the laser diode beams with wide divergence along the fast axis. As for the focuser (focusing object-glass), its look depends on the given assignment. If maximum focusing is not required an ordinary spherical lens can play the role of an object-glass. Otherwise, an object-glass with a great numeric aperture is needed. For example, one more collimator can be used as a focuser.
There are different types of modern collimators:
1. Aspherics.
On the market, there is a wide choice of aspheric lenses from different manufacturers to collimate the laser diode radiation. Usually, their numerical aperture does not exceed 0.6, but, practically, this is enough, though sometimes a small amount of the radiation is not intercepted. The specific feature of aspherics is their rather narrow spectral range where spherical aberrations are suppressed – about 10 nm.
Links to the lenses available on the market:
from China:
https://www.amazon.com/Collimating-lens-Focusing-Laser-Diodes/dp/B00SECAP1A
There are no details. We think these are aspherics. The price is 8 USD.
From other countries:
https://optlasers.com/collimators/collimator-400-700-nm Poland
framed aspherics, F=4.02 mm, coated for 400-700 nm, NA=0.6, 22 USD
2. A packaged collimator of a number of spherical lenses.
The collimators for laser diodes of this type are also widely represented on the market. Most often, they consist of three lenses (so-called triplets) or of four lenses, although, more complicated costructions are also present, such as the one below:
Fig. 1. A focusing/collimating object-glass of 8 lenses with a focus of 5 mm and numeric aperture of 0.8. it is designed for work in a wide spectral range of 620 – 680 nm.
Market:
China:
https://optlasers.com/collimators/collimator-445-nm
this is a framed triplet for 445 nm, F=8 mm, NA=0.6, 18 USD
https://aliexpress.ru/item/32623759824.html?gatewayAdapt=glo2rus&sku_id=59387289110
This is also a framed triplet, F=8 mm, 1 107,45 RBL.
eBay stuff, a company, not a private person is the seller. 7.5 USD.
2. The same, but two aspheric cylindric collimators are used. One for a fast axis and one for a slow one.
Some time ago cylindric micro lenses were used for the fast axis collimation. Often, micro lenses were just tiny pieces of optic fiber with a diameter of 1 mm. But cylindric lenses when collimating off-axis beams introduce significant aberrations. That’s why, today for fast axi scollimation, they use aspheric cylindric lenses, i.e. lenses similar to regular cylindric ones, the refractive surface shape of which is slightly changed to correct aberrations.
As for the slow axis collimation, regular cylindric lenses can be used here, although acylindric ones are also used. In the future transition to free-form optics is also possible.
Lenses on the market:
fast axis collimators ( FAC ):
China:
https://webfile.focuslight.com/web/image/20211231/Rev03%20FAC570_FS%20Datasheet%20EN.pdf
These are short-focused lenses that fit only to be installed inside the diode package. There are no other lenses for 445 nm on their site.
Other countries:
https://www.fisba.com/sites/default/files/2021-02/FISBA_White_Paper_FastAxisCollimators_final_14-06-2017.pdf this is an international company headquatered,as it seems, in Switzerland. They sell lenses for 44 nm with 1.7 mm focus.
https://www.ebay.com/itm/393038845532 LIMO lenses
Slow axis collimators (SAC):
China:
https://www.focuslight.com/site/product?id=77
Other countries:
https://www.fisba.com/en/products/slow-axis-collimator-lenses-sacs
3. One-piece collimators
This is an optical element to collimate both the fast and slow axes at a time, making the divergence along the fast and slow axes approximately equal. Therefore, it is enough to place a focusing lens behind such a collimator, in the focus of which an approximately round spot will be obtained. At least, that is what is claimed.
The Chinese FocusLight ( https://www.focuslight.com/site/product?id=79 ) manufactures such collimators. This is what it looks like:
It is installed at a distance of 1.2 mm from the laser diode crystal end (so, it is possible to place it outside the diode package without opening it).