Start-Of-Scan detector for Polygon Scan Heads

The high speed and high laser damage threshold of polygon scanners are required for many high power and Ultrafast / Ultra Short Pulse (USP) material processing applications.  With pulse lasers operating at MHz rates, traditional galvanometer scanners are not fast enough.  Polygon scanners are 10+ times faster than galvanometer (galvo) scanners and polygon scanners have a much higher laser damage threshold.

A Start-Of-Scan (SOS) detector is needed to accurately scan with a polygon scanner.  The SOS detector tells the system exactly where the beam is prior to the beam entering the work zone.  The polygon is rotating at a constant speed so a system clock is used to determine where beam is during the scan.  The SOS detector is triggered by a low power red CW laser.  With a high powered (very bright) working laser within the polygon scan head, stray light can interfere with the SOS detector causing false SOS detection. 

The new Precision SOS Detector™ solves this interference problem in two stages.  A detachable opaque tube gives the detector tunnel vision blocking stray light outside the field of view.  The SOS detection system operates out of plane from the working beam but in situations where scattered light of the working beam is within the detector’s field of view, the detector tube can be fitted with a 12.5 mm filter to band pass the red SOS laser or to notch the frequency of the bright working beam.

In high power Polygon Scan Heads, the working beam and / or the hot working environment can elevate the temperature inside the scan head.  This can cause thermal drift of conventional SOS detectors.  Thermal drift will cause the starting position of the working beam to vary with temperature.  The new Precision SOS Detector uses a temperature compensating bi-cell sensor to eliminate thermal drift accuracy error.

The new Precision SOS Detector is powered by 5VDC.  The SOS output trigger signal has a fall time of < 7 ns and is compatible with 50Ω coax.  There are four screw terminals to attach +5V, Power Ground, Shield Ground and SOS Trigger Signal out.

The new Precision SOS Detector board may be installed without the light shield to allow visual alignment of the low power red SOS laser to the detector.  Once the alignment is complete, the light shield can be installed.  This is done my screwing the tube into the threaded ring on the circuit board.  When the tube lightly makes contact with the circuit board, back tube off one full turn and use the locking ring on the tube to tighten against the ring on the board.  The SOS laser must scan across the detector in the direction of the arrow.  The photosensitive area is 2.5 mm tall so there is margin to align a 0.7 mm beam to the detector.

There are two multi-turn potentiometers, one to adjust each of the two cells in the detector to the brightness of the SOS laser.  The board ships with the near minimum sensitivity for using an unfiltered 1mW red laser with a .7mm spot.  Should you use a filter to block the working beam it will diminish the SOS laser beam at the detector but the detector circuit gain can be increased by adjusting the pots.  If you see a double trigger pulse that may be a result of too much gain.  In that case, back off the gain.

Other features include:
–    Reverse polarity protection.
–    Over voltage protection with a self-resetting fuse.
–    One mounting hole has plating so the PCB ground may be connected to chassis ground.  There is a jumper to isolate grounds if necessary.
–    Overall size of the board is minimized by using a detector with integrated electronics.
–    Conventional SOS detectors are not designed for high power Polygon Scan Heads.  The Precision SOS Detector will work equally well in both high power and low power polygon scanning applications.

For more information: Precision SOS Detector

Precision Laser Scanning also offers an optional SOS laser matched to the SOS detector for power and spot size.  It is a compact red laser diode module that does not require focusing so it is easy to align within a Polygon Scan Head.

For more information: Precision Laser Diode Module

Ordering both the Precision SOS Detector and the Precision Laser Diode Module
gives you a Plug-N-Play SOS detection system.  This will save the time and expense of designing and implementing a custom SOS detection system.