HIGH EFFICIENCY– 100% FILL FACTOR
white paper
Boulder Nonlinear Systems (BNS) has been continuously developing liquid crystal spatial light modulators over the past two decades. Through this development process, there has been an improvement of SLM performance unmatched by any other company. Such performance enhancements include:
AC pixel voltage with sub‐millisecond frame loading to prevent phase ripple
100% fill factor to reduce higher‐order diffraction
Intra‐pixel‐pair modulo‐2 transitions to maximize space bandwidth product
Unique LC modulators
100% Fill Factor
The inter‐pixel gap structure associated with the reflective Liquid Crystal on Silicon SLM backplane acts as an amplitude grating that diffracts some light into higher orders. To eliminate this loss of light, BNS has developed a process for removing the grating effects due to the pixel structure. Optically, the active area of the backplane is converted into a flat dielectric mirror through a unique two‐step process. First, we deposit SiO2 onto the top of the silicon and polish it down to be both thin and optically flat. We then deposit a multilayer dielectric mirror stack on top of the polished SiO2. This SiO2 layer is unique in the industry, and is important as it allows the dielectric mirror coating to be one continuous, optically‐flat specular reflector. Following this process allows the planar dielectric mirror to eliminate the amplitude and optical path variations associated with the underlying aluminum pixel structure. The dielectric stack is kept thin to minimize any droop in electric field across the Liquid Crystal layer as shown in the figure below. In other words, there are no abrupt changes in phase modulation (such as dead zones) between pixels due to the smoothing (low pass spatial filtering) which results from separating the LC modulator from the driving electrodes.
The dielectric mirror coating is located above the silicon backplane. This is shown in Figure 1, which depicts a cross section of the SLM, showing the layer stackup. You can see the mirror is not deposited directly onto the aluminum pixels (shown in yellow), but rather onto a thin layer of polished SiO2 (shown in light blue). Typically, this mirror has very high reflectance (>90‐95%), which means almost all of the incident light is reflected. To diffract this reflected light, varying the voltage on the pixels will modify the electric field across the liquid crystal molecules. This electric field controls the orientation of the molecules, which in turn alters their optical index and hence, induced phase delay. As shown in the Reflected Wavefront on the drawing, the impact of the gap between pixel electrodes merely affects the strength of the field directly above the gap itself. This means there may be some light that is either not diffracted, or, if there is a repeated pattern, diffracted into a higher order, but the mirror itself still reflects virtually all of the light.
www.bnonlinear.com
303‐604‐0077
HIGH EFFICIENCY– 100% FILL FACTOR
white paper
For the device without the dielectric mirror coating, there are two other effects to consider, both of which are due to the “visibility” of the inter‐pixel gap, since it no longer has a high reflectance mirror coating above it. The primary effect is that the regular period of the gaps creates a fine pitch diffraction grating, which diffracts a portion of the light into both vertical and horizontal higher orders. This higher order diffracted light can clearly be seen in the image in Figure 2, as well as the decrease in this effect using an SLM with the mirror coating. The secondary loss mechanism associated with the visibility of the inter‐pixel gaps on a non‐dielectric‐mirror‐coated SLM is that some small portion of the light is in fact absorbed into the SLM backplane itself in these gaps. Most of the difference in diffraction efficiency (90% vs. 61%), however, is due to the higher order diffraction, and not from the inter‐pixel absorption.
Reflected Wavefront Coverglass
Coverglass Electrode 0 Volts
Vy
Liquid Crystal Layer
Dielectric Mirror
Polished SiO2
Aluminum Pixel Electrodes
0V
2.5V
y
Vx
5V
0V
x
Figure 1: SLM cross‐section showing planarized dielectric mirror and smoothing of the electric field eliminate most of the grating effects associated with pixellated spatial light modulators.
www.bnonlinear.com
303‐604‐0077
HIGH EFFICIENCY– 100% FILL FACTOR
white paper
Results of the dielectric mirror coating can be seen below in the zero‐order diffraction efficiency measurements. For an un‐mirrored SLM, we measure a zero‐order diffraction efficiency of 61.5%. For a dielectric mirror coated SLM, we measure a zero‐0rder diffraction efficiency (typical) of 90 – 95%.
Higher orders
SLM with mirror
Zero order
Measured zero‐order diffraction efficiency ~ 90%
SLM without mirror
Measured zero‐order diffraction efficiency ~ 61%
Laser
SLM
Power
meter
Zero‐order diffraction efficiency test setup for reflective SLMs
iris
Figure 2: Zero order versus higher order diffracted spot patters when comparing SLM’s with and without the high efficiency dielectric mirror coating.
Company Profile
Boulder Nonlinear Systems, Inc. (BNS) is an innovative technology company specializing in dynamic liquid crystal polarization control solutions for both laser‐based and imaging systems. Company strengths in scientific research and development are leveraged into
OEM and standard product offerings targeted for astronomy, biomedical, defense, microscopy, optical computing, optical storage, and telecommunications applications.
Boulder Nonlinear Systems ● 450 Courtney Way ● Lafayette, Colorado 80026
303‐604‐0077 – www.bnonlinear.com – [email protected]