TI white paper: Laser Power Handling for DMDs

Technical documentation doesn’t present the same kind of splashy headlines that make it jump to the front-page of a website but, from an engineer’s standpoint, it’s always interesting to see new publications within the field. TI recently released this white paper, inconspicuously placed within the Technical Documents section of TI’s website describing the thermal effects of using lasers with a DLP microdisplay. This is the type of document you’d like to reference if you’re building a laser-based DLP projector so you don’t destroy the MEMS mirrors with too much laser power. TI provides a very logical, step-by-step approach to the analysis which provides some very valuable information for those working with laser projectors. From Alces’ perspective there is a couple of interesting details presented within the paper:


Points of interest

• “Laser applications use continuous and pulsed mode operation. One of the many advantages of pulsed operation is that during the pulse very high peak powers can be reached with relatively low average power consumption.”
• “…it is desirable to keep pixel surface temperature below a critical temperature of 150˚C.” This value includes the ambient temperature, so TI gives the example: “typical consumer projection systems can easily reach 50-55˚C…” which means that the temperature rise associated with laser illumination should not exceed 95-100˚C. From a MEMS perspective, the 150˚C critical temperature is most likely from the sensitivity of the aluminum (or aluminum alloy) to high temperatures. As thin-film aluminum is heated the induced stress can cause the aluminum surface to degrade through a process called “hillocking”. Another possible source for a 150˚C limitation may also be from TI’s anti-stiction film which is in place to prevent the landing springs from sticking to the anchor points.
  
• TI describes the average areal optical power specification as 25 W/cm² at 8% absorbed (92% aluminum reflectivity). It’s unclear what this value means because TI goes on to describe if a short-pulse, high peak-power, laser source is used, and even if that average power was ~25W/cm² , then the temperature rise would still exceed the critical temperature.
  
• This is somewhat of a side note but a table is presented which includes a “pixel size” of 5.4um, which is interesting because we were unaware of any DLP chip with pixel sizes less than 7.6um…

TI presented a good analysis of thermal effects associated with pulsed-laser sources on a DLP. At Alces, we’ve done some similar analysis as it relates to our 1D MEMS arrays. The different MEMS structure and characteristics of the laser display require some additional considerations which we’ll present in future, but overall thermal management plays a very important role in the design of a laser projector and requires making careful design choices.