Using a Spectra-Physics mod. 177, 488 nm argon ion laser for personal biological R&D (I'm working on phototherapy by laser light ...), I had recently to build a special simple device to count bacterias using scattered light measurement (90°) through a rectangular quartz tube containing a bacterial culture broth.
As the bacteria concentration for my experiment is low, I need a very stable laser source to improve S/N ratio.
This argon laser is rated for +/- 1% power stability.
The laser tube is new.
The laser beam delivery is performed via a kineFlex 3 m mono mode fiber system.
Unfortunately, during the tests, the laser output was very unstable and I worked on that issue during two months ! (well, during my free time ...).
I finally understood that this laser was very sensitive to back reflexions …
In fact, the back reflexions were involved at two locations : at the input fiber coupler and on the test tube input wall.
I solved the input coupler issue simply by revolving the coupler by a 90° turn. So the polarized back reflexions were not able to re-enter the Brewster window of the laser cavity.
To solve the back reflexions issue due to the test tube (even through the 3 m fiber !), I imagine at first to purchase an optical isolator … a quite expensive device.
Finally, I solved the problem in lowering the laser output power from 40 to 20 mW.
So accordingly, I have to work with a more concentrated bacteria broth, I have to dilute after my measurement for use.
So, be aware of back reflexions : in some situations, they can even damage the laser ...
