Discovery 4000 Energy Efficiency

Jinyang Liang said:

     (1)Diffraction pattern

I noticed that central orders of the diffraction pattern have similar intensity. I think this indicates that the diffraction grating (DMD) doesn't operate at Litthrow condition. This becomes the dominant factor for energy loss. Is it possible that I can make DMD working at the Litthrow condition? How should I adjust the DMD or incident light?

You are wanting a "blaze" condition rather than the special blaze condition called a "Littrow Blaze".  It sounds like you are very near an"off-blaze" condition.  you can try moving the incident angle through a range and observe the effect on the pattern.  Ideally you want a condition where a single order is much much brighter than all others (i.e. a "blaze")

Jinyang Liang said:

     (2) Second Diffraction Pattern

What is more, we also observed the second set of the diffraction pattern. Previously, when we use DMD Discovery 1100 with visible window, our conclusion was this second diffraction order is due to the high reflection at the inner surface of the cover mirror. But, it is harder to explain it because the reflectivity of the NIR window is about 5% now. Did you observe this pattern before? What is your explanation?

Even At 5% you may observe a second diffraction pattern but it should be dimmer than when you had the visible window.  There is also some angular dependance for the window reflectance.

Jinyang Liang said:

     (3)    Mirror Reflectivity

 Does mirror reflectivity depend on wavelength? Do you have any document regarding this issue?

Yes, but it should be getting slightly better toward the IR.

The mirror gap is on the order of 0.7 - 1 um, but that is not exact.  That is included in the 13.68

I hope this is helpful.

Fizix

Fizix,

The "blaze" condition is the center of sinc function matches with the reflection angle, right? but when you move the incident angle, the sinc function as well as the reflection angle move together. So, how will you achieve the blaze condition??

Thanks and merry Xmas,

Jinyang

Jinyang,

The center fo the sinc envelope does follow the reflection angle, but the location of the orders does not.  As you move the incident angle the location of the nonzero orders relative to the envelope do move which allows the envelope to catch up with an order.

Have a great new Year.