Using the DMD with coherent light
result in diffraction and consequently these effects must be taken into account, but
the diffraction also provides advantages in many applications. Some of these
advantages are:
- At large wavelengths (IR):
- The blaze condition is
fairly insensitive to production variation in tilt angle.
- The on and
off state blaze orders are separated on the diagonal of the
2D Sinc envelope giving an excellent extinction ratio from on to
off.
- Because light is
separated into discrete orders, optics can easily be made that gather
only the order of interest.
- At small wavelength (UV):
- Tuning can usually be
accomplished by changing incident angle less than +/- 2 degrees
- If the input beam is collimated
then the individual orders are also collimated (that is these orders retain the
characteristics of the input beam).
There are also some special
considerations when using the DMD with coherent light that are a result of the
nature of diffraction:
- For a given wavelength and pixel
pitch, orders are fixed in direction space by the incident angle alone
(switching the mirrors does not move them).
- The intensity envelope of the
orders is determined by the shape of the mirrors (square for the DMD) so that
individual orders cannot be arbitrarily extinguished without affecting all the
other orders.