Introduction

I received an email from David Kessler, an expert in optics and who has designed several laser beam scanning displays. In that email, David wrote that my speculation Hololens 2 (HL2) might be using the pupil expansion method shown in their US10,025,093 patent (right) was incorrect. David said that the HL2 was not using a screen-like pupil expander (EPE 306).

Microsoft’s Hololens 2 announcement video, a still from which is on the left, showed the scanning engine with mirror optics. More recently, there was a teardown with a video by a Microvision stock investor “u/s2upid” user on Reddit who tore apart a Trimble version HL2 just to find the Microvision logos inside.

I thought that optics in the 10,024,093 patent might be hiding behind the sizeable slow-scan mirror in the figure from the video above to expand the pupil, but this proves not to be the case. I went back to search the Microsoft patents the explanation of how the HL2 pupil expansion optics works

After going through the HL2’s pupil expansion patent, I will discuss some other tops, including the size of the engine and some display artifacts.

Microsoft’s “Increasing Pupil Size In A Display System” Patent Application

Patent Application US2019/0278076, “Increasing Pupil Size In A Display System,” matches the optics in the video and the teardown quite closely. Below are some still frames from the teardown video and the key figure in the patent application. Dimensions have been added (in blue).

As the patent explains, the pupil of the fast scan mirror must be small to enable the fast-scan mirror to be small enough that it can faster without distorting. Quoting the application in reference to FIG. 5-2 above:

As the fast scan arc 338 increases, the forces on the fast scan mirror 310 necessary to move through the fast scan arc 338 with a frequency greater than 10 kHz, 20 kHz, or 30 kHz may begin to distort the fast scan mirror 310. Decreasing the size of the fast scan mirror 310 through magnification of the pupil size may limit and/or prevent distortion of the fast scan mirror 310.

Figure 6 shows a simplified side view of the optics in figure 5-2. The figure has been rotated 90 degrees from the patent to orient it the same as both Fig. 5-2 and in the HL2 with the laser output shooing up.

On the HL2, there are three sets of converging optical elements (440). The laser module (right) is on the opposite side of the teardown pictures above. Each of the red, green, and blue colors has dual lasers with shared converging optics (440 in application Fig. 6 above). The lasers go to a conventional dichroic color combiner (not shown in FIG 6) and then to a turning prism with mirror surfaces to direct the light upward toward the fast scan mirror (410/310 above).

Fig. 7 from the patent application (below) shows how the pupil size changes as it goes through the optics of figures 5-2 and 6 above. Note that the output pupil is taller than it is wide. Going back to the teardown pictures, the output port on the optics is also taller than it is wide, suggesting that the display image is also being anamorphically compressed in the horizontal direction.

Pupil Shaping and Anamorphic Image Distortion

On the left is a front view of the laser engine attached to the waveguide from the teardown with added dimensions in blue. Note that the engine is roughly a rectangular solid that is 24.4mm by 29mm by 19mm (approximately an inch on a side) or about 13.4 cubic centimeters, very large for an AR/MR headset display engine.

Note in the picture of the HL2 waveguide that the output in the waveguide is taller than it is wide. You might also notice the somewhat triangular-shaped left (cyan reflected light) and right (blue-reflected light) intermediate DOE expander’s inputs are similarly taller than they are wide.

I have not seen the anamorphic distortion of the image itself being discussed in the Microsoft patents/applications. It appears that the HL2’s butterfly waveguide ends up roughly doubling the width of the image, as about half of the image propagated down each side of the diffractive optical elements (DOEs), as shown in Microsoft’s US2017/0363871 application (below). Figures 13 and 14 from the patent are combined with the parts of figure 14 that were different in figure 13 in red.

Figure 16 from the same patent shows how the left and right portions combine, in effect, roughly doubling the resultant image width. While not explicitly discussed in the patents that I could find, there appears to be an anamorphic horizontal stretching inherent in the butterfly waveguide. The horizontal stretching by the waveguide then driven the need horizontally to compress the output from the laser engine. And thus the taller than wide output window as seen in the pictures above.

HL2 Optics are Large for AR/MR

As shown in the pictures above, the laser combining and pupil expansion optics results in a large optical engine of about 13.4 cubic centimeters.

For comparison below, I have included a few recent AR headset display engines for diffractive waveguides. Compound Photonics has an engine with less than a quarter the volume with a similar FOV and much higher resolution using LCOS. Waveoptics has a DLP based engine with slightly better resolution but a somewhat smaller FOV that is about 70% the volume. At CES this year, Vuzix and JadeBird (using Waveoptics) had prototypes of green (only) MicroLEDs with engines that are about 1cc (~10mm on a side) that they claim can be much smaller in production to show the future potential of MicroLEDs

Trapezoidal Bright Area Possible Explanation

Looking back and the patent’s combined figures 13 and 14 something interesting to note is how the light progresses diagonally through the left and right intermediate DOE.

The diagonal propagation seems to explain the trapezoidal shape I noted in Hololens Display Evaluation (Part 3: Color Uniformity) as seen in the picture (left).

Thin, 1 or 2 Scan Lines wide, Diagonal Lines

High-resolution pictures such as the one on the left, show diagonal lines in the display. In the last article, I speculated that they might be a screen-like exit pupil expander (EPE), but the evidence above, shows there is not an EPE “screen.”

The figure below shows crops from the left, center, and right side of the display as indicated by the rectangles in the picture of the whole display.

1. The lines slant from right to left, going top to bottom on the left side of the display and right to left on the bottom (see left and right above).
2. As shown, the lines transition from slanting left to slanting right toward the middle of the display (see center image below).
3. In some areas, the lines are spaced apart horizontally by about the same distances as two scan lines. The thickness of the lines varies.
4. In some areas, the lines are spaced apart by the width of a single scan line. In a transition area, the lines will get thinner, and then a line in between the two lines will start. (left)
5. The slope of the lines is about 60 degrees.
6. The lines are “sharp” with longer exposures (8+ fields), suggesting they don’t move.
7. The lines are even evident in “black” parts of the image suggesting they are picking up scattered light (see above right)
8. While the image has chroma aberrations (see left), the lines don’t, this suggests that the lines are optically near the output (perhaps the output grating).
9. The lack of chroma aberrations suggests that whatever is causing the lines is near or on the output grating.
10. If the lines are from the output grating, then they are on the order of 12 to 13 microns for more tightly spaced lines and 24 to 26 microns for the line with lower periodicity (~854 scan lines per 29 degrees vertical FOV and the waveguide about 20mm from the eye). So it does not seem like the gratings themselves, which should be nearer 1 micron (wavelength of light times about 1.4x).

Conclusions

The HL2 is using lenses to aim the beam onto a small, fast-moving mirror. It then uses other mirror optics to shape the pupil for entrance into the waveguide. The use of mirrors to shape the pupil is likely much more energy-efficient than using a screen-like pupil expander, but like most mirror optics, it takes up a larger volume. It’s not clear how much this type of pupil expander could shrink.

I think there is an answer for the trapezoidal area in the HL2’s display that is brighter and more uniform. It is where both the left and right DOEs contribute to the output image as a result of the diagonal traveling of light.

The thin roughly 60 degree diagonal lines remain a bit of a mystery. If you have a reason or even a functional theory, let me know.