It is common practice in low power polygon scanner systems to use the working beam to trigger a Start-Of-Scan (SOS) detector. The SOS trigger is essential for accurate laser scanning.
Polygon Scan Heads, used for material processing, employ high power and ultra-fast lasers. SOS detectors require a low power CW beam so SOS triggering becomes problematic with a high power or pulsing beam. It is definitely possible to use a high power CW working beam for SOS triggering but the power level, at the detector, must be low enough to avoid burning up the detector. The detector can be protected by using a beam splitter that diverts a small percentage of the energy to the SOS detector. Or it might be done by placing an ND filter in front of the detector. However, the filter may have to absorb a lot of energy which could cause thermal lensing. Then a focusing lens might be required to reduce the beam to a suitable size for the detector.
However, another problem remains when using the working beam for SOS trigger. A high power (BRIGHT) working beam will have significant reflections inside the scan head which may cause false triggering of the SOS detector. False SOS triggering from scattered light must be avoided. It is preferable to use a SOS laser of a wavelength different from the bright working beam to allow the use of filtering at the SOS detector to block the working beam reflections. Reflections will come from every optic as well as the inner walls of the scan head. Keep mind that just 0.1% of scattered or reflected light from a high powered beam is a lot of light to interfere with the Start-Of-Scan (SOS) detector.
Precision Laser Scanning offers a custom CW laser diode module designed specifically for Start-Of-Scan (SOS) trigger in high power and ultra-fast polygon scan heads. The Precision SOS Laser Diode Module has a tight 0.7 mm spot so there is no need for manual focusing or external focusing optics. The beam is visible red and eye safe which makes for easy scan head alignment. The Precision SOS Laser Diode Module is only 8mm in diameter and 30 mm long. The compact size allows easy integration into a Polygon Scan Head. The SOS laser in high power and ultra-fast polygon scan heads will be used out of plane from the working beam so the working beam does not cross the SOS detector. By using a red SOS laser when the working beam is IR, Green or UV makes it easier to filter out interference from the bright working beam.
An SOS detection system is far more accurate than a shaft mounted encoder on a polygon scanner. In most cases, a shaft mounted encoder on a polygon scanner is to give feedback to the BDC motor controller so smooth rotation can be achieved at low speeds. The encoder cannot give accurate beam location.
Here is the brochure with specifications: BROCHURE mini-SOS LASER
For design tips on Polygon Scan Heads contact Precision Laser Scanning at