Lasers – DGL, SHG, and Hybrid

I have no doubt that eventually all projection devices will use laser illumination due to their huge optical advantage.    But there still remain serious technical and business questions as to when they will become a major factor in displays.   A couple of years back we were in the “wild enthusiasm stage” of direct green laser (DGL) development with several companies announcing they had a direct green laser.    But since then, it has become clear that there are still some significant technical and business hurdles to clear for DGL.

The immediate effect of the DGL announcements two years ago was to make the development of second harmonic generation (SHG) and other diode pumped green lasers more difficult as it tended to make the funding and management support of developments in SHG lasers more difficult.      Since then it has become clear that while the DGL developments were very promising, they were still a very long way from being ready for production products.

Simply put, it turns out that the physics for making DGL is very difficult.  All the DGL development are based on around indium gallium nitride (InGaN).  Semiconductor today had a very good article on the subject in 2009  (note UCSB developments led to today’s Soraa).  To get to green they have had to add more indium and to try different crystal plane orientations which are less stable/yieldable.   To have a production product they have to solve simultaneously key attributes including the yield/cost, wavelength/color, stability/lifetime, output power, temperature range, and efficiency.    To date, they can only solve a few of these key attributes at the same time.    I sometimes quip, “I can get everything I want in a green laser but it is spread over 5 different parts.”

It also seems very clear that DGL are at best only going to have enough power output to support low projectors for the next several years.   The big problem is that low lumen projects have low value in the market which means that can’t afford expensive DGL.   This is creating a very big chicken or the egg problem in that there really is no significant market for early expensive, low efficiency DGL.

As reality sets in on DGL progress, there seems to be a resurgence of interest in SHG green technologies.  Getting DGL powerful enough to support the hundreds of lumens for a portable projector and the thousands and tens of thousands of lumens for conference room projector could be more than a decade away.   Companies need the optical advantages of an all laser solution to provide higher efficiency, smaller and less expensive optics, better efficiency, and long lifetimes, than traditional lamps, and they can’t wait on DGLs.

There are several techniques for getting green lasers by pumping crystals with lasers of a different wavelength.   Most commonly this is done by using an infrared laser at 808nm to pump a non-linear crystal (often periodically poled lithium niobate {PPLN}) to generate the second harmonic at 532nm green.   Spectralus   and QD Laser are two small companies with promising developments for relatively efficient SHG green lasers.

One issue with using a SGH and combining it with say direct diode blue and red lasers is that each of the lasers has a different character/beam profile and ages differently which can cause color shifts that would require constant recalibration to achieve accurate colors.   Also the direct red lasers that are available are at about 640nm which while a very deep/saturated red; it is very inefficient in terms of perceived lumens.   A more “ideal” red wavelength is in the 615nm to 620nm range but just like with trying for a 532 green, it has proven difficult to make stable/yielding red lasers in much below about 635nm.   These factors and others have caused the current large laser projector developers, such as Laser Light Engines, to use SHG for red, green, and blue even though direct diode red and blue lasers exist that could produce enough light.

Photodigm  has a more radical approach where all three colors (plus orange) could be generated from a single infrared pump source.   This could be particularly useful and very cost effective in the mid-lumen (say 100 to 3,000 lumen) projectors which use field sequential color such as DLP and LCOS.    Philips at SID 2011 also proposed a single laser only this time a blue laser to pump a crystal to provide red green blue and orange.

Another development pioneered by Casio is the “hybrid” laser projector.  In these projectors they use a red LED and blue laser for blue and use either the same blue laser through a spinning wheel or a second blue laser to drive a green phosphor to get green.    The “green” is not tightly collimated laser light and is not ideal from an optical perspective, but it is the cheapest way to get a very bright and small green light source.  Using blue lasers to stimulate a green phosphor is an admission that DGL, at least bright ones, are a ways off.    There is a so much buzz in the industry about this hybrid green approach that market analyst Insight Media has release a report on it (http://www.insightmedia.info/reports/2012hybriddetails.php)

My conclusion is that projector makers aren’t going to be waiting around for DGL to get going with laser projection.    It is clear that the hybrid/phosphor-green approach is already taking off for 2K to 3K lumens.   I also foresee the hybrid approach migrating down into the 200 to 1000 lumen markets.   But there are bigger overall advantage with smaller optics and microdisplays to be had by having an all laser solution.

2 comments

    • admin says:

      Sony was much quieter than Soraa, OSRAM and Nichia but it certainly should have been expected since they developed blue lasers for blue ray (see for example from 2007 “Sony’s Blue Laser Diodes Down to $8 – PS3 and BD Player Price Cuts Soon?”. Another company to watch would be Opnext who is a leader in red lasers and announced a blue laser diode back in January 2011. All the green laser diode developments I know of came from “stretching” their blue laser development to get green.

      With that as a preamble, to your question as to the impact of the Sony-Sumitomo announcement’s impact. I can’t see it changing anything in the short run. It looks on the spec’s to be better than other companies’ prior announcements with a 100mW, 530nm wavelength (which is about the best), and 8% WPE which when you factor in the luminous efficiency of the 530nm is about double any other announcement I have seen. Sony has a major investment in LCOS so I would suspect (I have no inside knowledge) that Sony would be looking to couple this laser with their LCOS developments.

      100mW probably only supports about a 20 lumen projector which is pretty dim except for a very dark room or a very small image. Now the bad news, a projector using an 8% WPE green laser probably probably does not have net a efficiency advantage over a good LED projector with at 20 lumens. And then we have to ask how far away is the Sony-Sumitomo green laser diode is from being a product and how much will it cost. Today a green LED to support a 20 lumen projector probably costs about $2. The big question is whether a ~20 lumen project has a value proposition today with what a laser projector will cost to make.

      Anyway, that is the way I see it,
      Karl